Intelligent Vehicles Technology, Introduction 627 Intelligent Vehicles Technology, of the 3D stereo vision technique. The common characteristic points clustering problem is discussed Introduction and a model-based scene ﬂow method is applied toFEI-YUE WANG determine the positions and instantaneous motionInstitute of Automation, Chinese Academy of Sciences, states of the vehicles.Beijing, China “▶ Dynamic Environment Sensing Using an Intel- ligent Vehicle” studies the 3D environment reconstruc- tion problem. Not only moving vehicles but alsoThe basic goal of intelligent vehicle research is to stationary objects (e.g., trees, walls, etc.) are recognizedprovide a safe and comfortable ride for drivers and and modeled in such applications. Similarly, “▶ Driverpassengers. To reach this goal, an intelligent vehicle Assistance Systems, Automatic Detection and Siteneeds to dynamically sense the surrounding environ- Mapping” discusses how to reconstruct objects inment, receive the correct command from drivers, and the environment, particularly when driving inmake appropriate movements in real time. constructing sites. In the 15 chapters in this book, 11 address how to LIDAR (Light Detection And Ranging) sensor issense the environment via vision sensors, 3 study how to another frequently used vision sensor and often servesmonitor drivers’ status and understand their commands, as a supplement to ordinary vision sensors. It canand 1 discusses vehicle motion control. The basis and measure the distance between objects and the vehiclecontent of these chapters are described below. and provides depth information that cannot be To obtain richer and more accurate visual informa- directly collected via ordinary vision sensors. Intion, researchers are trying to design more powerful “▶ Vehicle Detection, Tightly Coupled LIDAR andvision sensors. One important direction combines Computer Vision Integration for,” a calibration algo-different vision sensors to achieve better image quality. rithm was developed to map the geometry transforma-For example, how to obtain true color night vision tion collected via a multi-plane LIDAR system intovideo was studied in the chapter “▶ True Color Night camera vision data. Both simulations and real-worldVision Video Systems in Intelligent Vehicles.” The experiments have been carried out to show itsproposed night vision cameras sense the near-infrared reliability. “▶ Active Pedestrian Protection System,radiation of the car’s own headlights to extend the Scenario-Driven Search Method for” studies pedes-range of forward vision. The near-infrared and visible trian detection problem via the fusion of LIDAR andspectrum can then be combined into natural-looking camera systems.color images for the driver to read. Another type of approach combines camera vision Adjusting the focus of high-resolution video systems and radar systems. Some related discussion iscameras and directing them to the regions of interest presented in “▶ Night Vision Pedestrian Warning incan provide better image data of certain objects. Intelligent Vehicles.” The pedestrian detection problem“▶ Active Multifocal Vision System, Adaptive Control is studied as an example to prove the merit of radar/of ” presents methods for obtaining active pointing camera fusion systems.and gaze stabilization camera systems via an adaptive Since occlusion is generally unavoidable, the visionmulti-focal vision system. Sliding mode control has been capability of a single vehicle is always limited. Oneshown to be an effective control strategy to reach this goal. straightforward way to solve this difﬁculty is to employ Stereo vision sensors have become more popular vision information that is collected from differentbecause they can recover more three-dimensional vehicles in a small area. Clearly, this objective requiresinformation about nearby objects via stereo video us to share the understandings of surroundingdata. In “▶ 3D Pose Estimation of Vehicles Using Ste- environments in an efﬁcient way. As an early attempt,reo Camera,” neighboring vehicles pose a detection “▶ Cooperative Group of Vehicles and Dangerousproblem and are discussed as a special application Situations, Recognition of ” discusses how to deﬁneM. Ehsani et al. (eds.), Transportation Technologies for Sustainability, DOI 10.1007/978-1-4614-5844-9,# Springer Science+Business Media New York 2013Originally published inRobert A. Meyers (ed.) Encyclopedia of Sustainability Science and Technology, # 2012, DOI 10.1007/978-1-4419-0851-3
628 Intelligent Vehicles Technology, Introduction the objects around several vehicles and share their The ﬁnal chapter on driver assistance is “▶ Driver position information. Characteristics Based on Driver Behavior.” It presents The ﬁnal chapter in vision sensory ﬁeld, “▶ Driving a classical method to model driver characteristics, the under Reduced Visibility Conditions for Older Adults,” driver behavior questionnaire. If well designed, a driver discusses how to enhance the gathered vision informa- behavior questionnaire can tell us how drivers make tion in reduced visible conditions and transform it suf- their decisions under certain conditions. This method ﬁciently for older adults. This chapter can be viewed as is not novel but may still useful to driver modeling. an interdisciplinary study that crosses the boundaries of The only chapter on vehicle motion control vision sensor system design and driver assistance is “▶ Unscented Kalman ﬁlter in Intelligent Vehicles.” systems. It covers vehicle tire friction monitoring. Because The ﬁrst chapter on driver assistance, “▶ Driver a vehicle’s motion is primarily determined by the Assistance System, Biologically inspired,” is hard to friction forces transferred from the road via the tires, categorize. It discusses how drivers pay attention to tire friction features receive attention. This chapter certain changes in vision and how we can learn from illustrates how researchers consider the connection drivers’ response. It points out that advanced driver between vehicle motion dynamics and tire friction assistance systems may be designed to alert drivers in features. It also presents application of the Unscented way that ﬁt their biological characteristics. However, Kalman Filter (UKF) to realize tire feature monitoring. a detailed notiﬁcation method is yet to be developed. There is an additional chapter related to intelligent “▶ Driver Behavior at Intersections” provides vehicles. As mentioned above, sharing information a relatively mature approach that identiﬁes drivers’ between vehicles and infrastructures is a hot topic in turning features at intersections based on the sampled recent studies. In “▶ Vehicular ad hoc networks, vehicle motion records. Following a stylized procedure, enhanced GPSR and Beacon-assist Geographic it ﬁrst introduces how to gather the required driving Forwarding in,” utilization of real-time vehicle location data. Then a decision model of drivers’ side turn behav- information to accelerate vehicle ad hoc communica- ior is proposed and the gathered driving data are used tion is discussed. to calibrate the model. This chapter can be viewed as a representative of related reports.
Internal Combustion Engines, Alternative Fuels for 629 Internal Combustion Engines, for both spark-ignition (SI) and compression-ignition (CI) applications. Alternative in this context describes Alternative Fuels for a fuel that can be used in place of a conventional fuelTHOMAS WALLNER1, SCOTT A. MIERS2 such as gasoline or diesel. Alternative fuels are not1 Energy Systems Division, Argonne National equivalent but to a wide extent include renewableLaboratory, Argonne, IL, USA fuels since renewable only suggests a fuel can be created2 Mechanical Engineering, Michigan Technological from resources that are never used up or can beUniversity, Houghton, MI, USA replaced by new growth. The most common alternative fuels for SI engines are ethanol in blends with gasolineArticle Outline as well as compressed natural gas (CNG), liqueﬁedGlossary natural gas (LNG), and liqueﬁed petroleum gasDeﬁnition of the Subject and Its Importance (LPG). Biodiesel, synthetic diesel, green diesel, and DMEIntroduction are the most common alternative fuels for CI engines. InLimitations light of increasing energy demand worldwide, limited fossilLegislation fuel reserves partially located in politically unstable regions,Overview of Alternative Fuels growing environmental awareness and concern related toFuel Properties air pollutants as well as greenhouse gas emissions, alterna-Alternative Fuels for Spark-Ignition (SI) Engines tive fuels have transitioned into social and political focus.Alternative Fuels for Compression-Ignition (CI) Legislation and mandates were put in place specifying Engines required amounts of alternative fuels in the fuel mix inFuture Directions Europe, Asia, and the United States. This article highlightsBibliography some of the dominant alternative fuels, their market share, properties as well as their implication on engine andGlossary vehicle design, engine efﬁciency, and emissions.Advanced biofuel Renewable fuel not produced from food crops such as cellulosic biofuel and biomass- Introduction based diesel. Although gasoline and diesel are nowadays consideredAlternative fuel A fuel that can serve or be used in place of another or an unconventional fuel choice. conventional fuels, early engine concepts were designed to use other fuels. Combustion engines date back as earlyDiesel gallon equivalent (DGE) The amount of alter- as 1807 with Francois Isaac de Rivaz of Switzerland and ¸ native fuel required to match the energy content of his combustion engine that used a mixture of hydrogen 1 gal of diesel fuel. and oxygen for fuel . The idea of alternative fuels is asGasoline gallon equivalent (GGE) The amount old as engines themselves with Rudolf Diesel including of alternative fuel required to match the energy considerations for use of gaseous, liquid and solid fuels content of 1 gal of gasoline. in “Theory and Construction of a Rational HeatMixture caloriﬁc value The amount of energy Motor” in 1894 . The ﬁrst mass produced vehicle, contained per volume of fresh charge typically at stoichiometric conditions that can be introduced Ford Model T, was designed to run on ethanol. Economic growth and prosperity are closely linked into the cylinders of an internal combustion engine.Renewable fuel A fuel created from resources that with availability and cost of energy. A large portion of worldwide energy demand and the majority of energy are never used up or can be replaced by new growth. used for transportation are covered through fossil resources. The two most dominant fuels for transporta-Definition of the Subject and Its Importance tion applications are gasoline in spark-ignition enginesAlternative fuels for internal combustion engines and diesel in compression-ignition engines. Announce-include a wide range of liquid and gaseous chemicals ments of newly discovered crude oil reserves andM. Ehsani et al. (eds.), Transportation Technologies for Sustainability, DOI 10.1007/978-1-4614-5844-9,# Springer Science+Business Media New York 2013Originally published inRobert A. Meyers (ed.) Encyclopedia of Sustainability Science and Technology, # 2012, DOI 10.1007/978-1-4419-0851-3
630 Internal Combustion Engines, Alternative Fuels for uncertainty on size of known reserves result in a large included in this article. The interested reader is referred error band in prediction of the time span in which fossil to  for detailed well-to-wheel analysis of alternative fuels will be depleted. However, the fact that fossil fuels fuel pathways. are ﬁnite is indisputable and with increasing worldwide Before an attempt is made to classify alternative fuels energy demand the need for viable and sustainable it is important to deﬁne alternative fuels. The word alternative fuels becomes ever more pressing. alternative in the context of fuels suggests that a fuel Today more than ever, society is concerned with the can serve or be used in place of another and might also availability of energy which has become a matter of suggest an unconventional choice. However, alternative national security. The transportation sector alone fuels are not to be confused with renewable fuels since accounted for 70% of the total US oil consumption in renewable suggests that the resources used to create the 2007. Foreign sources continue to outpace domestically fuel are never used up or can be replaced by new growth. produced fuel in the United States as well as Europe Alternative fuels and renewable fuels are neither mutu- and Asia, leading to increased dependence on oil from ally exclusive nor mutually inclusive; although the vast politically unstable regions. Alternatives, either majority of renewable fuels are also considered alterna- from domestic reserves or locally produced have been tive fuels, by far not all alternative fuels are renewable. identiﬁed as a critical factor in ensuring national Methanol is just one example for a fuel that is renewable security and also as a means to stabilizing fuel prices if produced, e.g., from biomass, and since not widely in times of ﬂuctuating crude oil prices. used also an alternative fuel. Ethanol is a renewable fuel As third-world countries develop and further if produced from corn or sugarcane, but since it is growth of other countries continues, the amount of commonly used it may or may not be considered an transportation-borne pollution increases. This devel- alternative fuel in the sense of an unconventional choice. opment poses a threat due to local pollution resulting For our discussion ethanol is still considered an alterna- from emissions of regulated constituents and air toxics tive since it is used in place of other fuels. Natural gas is as well as the global impact of increased greenhouse gas just one example for an alternative fuel that, if fossil emissions. Alternative fuels can play a major role reserves are used, is not a renewable fuel. in reducing regulated emissions through cleaner combustion, and renewable fuels in particular can Legislation help reduce greenhouse gas emissions. There are several potential solutions to these issues Recently introduced legislation worldwide mandates from reduced consumption, increased production, the increased use of alternative fuels. Directive 2009/ and/or alternative sources of energy. It is this last 28/EC of the European Parliament and of the Council option that is the focus of this article and how alterna- of 23 April 2009 on the promotion of the use of energy tive fuels and their properties impact engine as well as from renewable sources and amending and subse- vehicle operation. quently repealing Directives 2001/77/EC and 2003/30/ EC states the need to increase energy efﬁciency as part of the triple goal of the “20-20-20” initiative for 2020, Limitations which means a saving of 20% of the European Union’s This article is focused on technical implications of alter- primary energy consumption and greenhouse gas native fuels on engines and vehicles, their performance emissions, as well as the inclusion of 20% of renewable and emissions characteristics with particular focus on energies in energy consumption. automotive applications. The characteristics of the most Japan issued a New National Energy Strategy dominant alternative fuels are deﬁned and compared to in 2007 targeting to expand the domestic biofuel conventional gasoline and diesel fuel. The discussion on production to cover approximately 10% of the fuel engine and fuel efﬁciency is strictly limited to the effects consumption by 2030 mainly through increased of alternative fuels on engine performance. Differences bioethanol and biodiesel utilization . of alternative fuels in production pathways, production US legislation through the Renewable Fuel Stan- efﬁciency and challenges for their distribution are not dard (RFS) mandates a fourfold increase in renewable
Internal Combustion Engines, Alternative Fuels for 631 40 35 Biomass-based Diesel Renewable fuel production [Billion gallons] Advanced (non-cellulosic) biofuels 30 Advanced (cellulosic) biofuels Conventional biofuels 25 20 15 10 5 0 YearInternal Combustion Engines, Alternative Fuels for. Figure 1Mandated biofuel production in the United Statesfuel production from approximately 9 billion gallons in alternative fuels, including their market share, proper-2008 to 36 billion gallons in 2022. The RFS also limits ties, and impact on engine efﬁciency and emissions asthe amount of conventional biofuels to 15 billion well as other renewable fuel options with considerablegallons . Figure 1 shows the mandated biofuels market share either globally or in local markets.production in the United States which regulatescontribution by conventional biofuels, advanced Classification of Alternative Fuelscellulosic and non-cellulosic and biomass-based diesel. As mentioned earlier, different alternative fuels requireDespite the rapid growth, US biofuels consumption more or less signiﬁcant changes to vehicle fuel systemremains a small contributor to US motor fuels, design, engine calibration, and safety equipment. Somecomprising about 4.3% of total transportation fuel of these changes will be addressed in detail whenconsumption (on an energy-equivalent basis) in 2009 discussing individual alternative fuels. In general,which is expected to grow to about 7% with the a classiﬁcation can include dedicated vehicles, ﬂexiblemandated 36 billion gallons by 2022 . fuel vehicles, and multi-fuel vehicles. Dedicated vehicles only operate on a single (alternative) fuel andOverview of Alternative Fuels are therefore typically purpose-built and optimized forAlternative fuels for internal combustion engines this speciﬁc fuel. Dedicated vehicles are typically builtinclude fuels used in blends with conventional fuels for applications with fuels that cannot be stored insuch as ethanol which is used in low level gasoline/ conventional fuel tanks, such as compressed gaseousethanol blends (e.g., E10) or biodiesel in low level fuels or liqueﬁed fuels. On the other hand, blendedblends with diesel (e.g., B5) as well as fuels requiring operation describes usage of two or more fuels of thea dedicated fuel system design such as compressed same type (liquid or gaseous) at varying mixture ratios.natural gas (CNG) or liqueﬁed petroleum gas (LPG). Vehicles capable of operating on varying blends ofThe following chapters focus on these mainstream gasoline and ethanol are commonly referred to as
632 Internal Combustion Engines, Alternative Fuels for FlexFuel vehicles. Finally, multi-fuel vehicles are vehicles which includes all types of vehicles such as deﬁned here as vehicles that can operate on two light-duty, medium-duty, and heavy-duty vehicles or more fuels and feature multiple fuel storage and as well as busses. delivery systems. Considering all vehicle types CNG is the dominant According to their most common use, alternative alternative fuel for dedicated vehicles with a share of fuels can also be classiﬁed as alternatives for spark- 1.27% which amounts to a total of more than 11 ignition engines or compression-ignition engines. million NG vehicles worldwide. However, distribution This very generic classiﬁcation is used for the following and share of CNG vehicles is not uniform and strongly discussion although there are examples of alternative depends on local fuel supplies. Figure 2 shows the fuels that can be used in both applications. Gasoline is absolute number of vehicles for all countries with used as the baseline fuel for spark-ignition engine a total CNG vehicle population in excess of 10,000 applications. For compression-ignition engine applica- units. In addition the fraction of natural gas vehicles tions diesel is used as the reference. compared to the countries’ total vehicle population is presented. The leading nations in terms of total CNG population are Pakistan (2.25 million units), Argentina Market Share of Alternative Fuels (1.8 million units), Iran (1.7 million units), and Brazil Worldwide approximately 5% of the automobile ﬂeet (1.6 million units). Following India with approxi- are alternative fuel or advanced technology vehicles . mately 700,000 units is Italy with 677,000 units as These vehicles can be grouped into dedicated vehicles the largest European CNG vehicle ﬂeet. The CNG that are designed and built for a certain type of fuel on vehicle ﬂeet in the United States is approximately the one hand and vehicles that are capable of operating 100,000 vehicles, which accounts for only 0.04% of on alternative or conventional fuels on the other hand. the entire vehicle population. The number of CNG Total vehicle population is an estimated 884 million vehicles available in the United States dropped from 2.5 100 2 80 Fraction of NG vehicles of total vehicle count [%] Number of NG vehicles Number of vehicles [Mio] Fraction of total vehicles 1.5 60 1 40 0.5 20 0 0 Pakistan Argentina Iran Brazil India Italy China Colombia Ukraine Banglad… Thailand Bolivia Egypt Armenia Russia USA Germany Peru Bulgaria Uzbekistan Malaysia Japan Korea Sweden Myanmar Venezuela France Canada Tajikistan Internal Combustion Engines, Alternative Fuels for. Figure 2 NG vehicles worldwide (Based on data supplied via NGVA Europe and the GVR)
Internal Combustion Engines, Alternative Fuels for 633eight models in MY2004, to ﬁve in MY2005, and one in that are actually fueled with gasoline/ethanol blendsMY2006 . was around 450,000 units in 2008, still making it the In terms of market share of alternative fuel technol- dominant alternative fuel in the United States. Metha-ogy, CNG is only exceeded by ethanol and gasoline/ nol as an alternative fuel was fairly popular in the 1990sethanol ﬂexible fuel (FlexFuel) vehicles. FlexFuel with a peak of more than 21,000 units in 1997.vehicles are vehicles that can run on gasoline and any Although the number of hydrogen powered vehiclesgasoline/ethanol blend up to 85 vol % (E85). Since has been steadily increasing since 2003, their markettheir introduction in 1979, more than 12.5 million share with slightly over 300 vehicles in 2008 was lessethanol and FlexFuel vehicles have been sold in Brazil than 0.05% of all alternative fuel vehicles.. In the United States, the FlexFuel ﬂeet in 2009 The United States consumed approximately 43accounted for approximately 8.35 million vehicles billion gallons of petroleum diesel fuel in 2005. Europe. Although the number of FlexFuel vehicles exceeds consumed 59.4 billion gallons, and Asia (excluding thethat of natural gas–fueled vehicles, the total number of middle east) consumed 48.5 billion gallons in 2005. Invehicles likely operated on ethanol is lower than that of 2009, Europe produced the largest percentage of world-CNG. In the United States, the number of vehicles wide biodiesel followed by the United States and Asiaactually operated on E85 is only 450,000 . Figure 3 Paciﬁc. Figure 4 shows the distribution of biodieselshows the development of alternative fuel vehicles in production for EU member states from 1998 to 2010use in the United States since 1995. While LPG was the . Overall biodiesel production has been decliningdominant alternative fuel in 1995 with more than in Europe since 2001, while it is increasing in the170,000 units, this number dropped to approximately United States and Asia Paciﬁc.150,000 units in 2008. In the same time frame, the Since 2000, US production of biodiesel hasnumber of CNG vehicles increased from approximately increased by a factor of 350, as shown in Fig. 5.50,000 units in 1995 to more than 110,000 units in Government-imposed incentives combined with the2008. As mentioned earlier, the number of E85 vehicles requirements of the Renewable Fuel Standard have 800,000 700,000 600,000 H2 LNG M85 Number of vehicles [-] 500,000 CNG LPG E85 400,000 300,000 200,000 100,000 0Internal Combustion Engines, Alternative Fuels for. Figure 3Alternative fuel vehicles in use in the USA 
634 Internal Combustion Engines, Alternative Fuels for 10,000 9,000 Biodiesel production [1000 tons] 8,000 7,000 6,000 Germany France Spain Italy 5,000 Other EU Total EU 4,000 3,000 2,000 1,000 0 2002 2003 2004 2005 2006 2007 2008 2009 Internal Combustion Engines, Alternative Fuels for. Figure 4 EU biodiesel production [12, 13] 800 700 Estimated US Biodiesel Production [Mio Gal] 600 500 400 300 200 100 0 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Internal Combustion Engines, Alternative Fuels for. Figure 5 Estimated US Biodiesel Production by year 
Internal Combustion Engines, Alternative Fuels for 635been the primary reasons behind the expansion of here to simplify the discussion regarding energy con-biodiesel production and associated utilization in the tent of alternative fuels. In a similar way, diesel gallonUnited States. equivalent (DGE) is deﬁned as the amount of alterna- In 2009, the United States produced 17.7% of the tive fuel required to match the energy content of 1 galworld’s biodiesel, making it the second largest producer of diesel fuel. GGE is used for alternative fuels that arebehind Europe. The biodiesel market is expected to mainly used in spark-ignition engines and thereforegrow in the United States over the next several years displace gasoline whereas DGE is used for alternativeand is predicted to reach $6,500 million liters (1,717 fuels used in compression-ignition engines with themillion gallons) by 2020. Biodiesel is considered potential to substitute diesel fuel. The conversion factora biomass-based diesel source, and a minimum of between GGE and DGE is approximately 0.88.3.79 billion liters (1 billion gallons) must be producedby 2012. This is a 50% increase over 2010 production. Alcohol FuelsThe biodiesel market in Germany is not anticipatinggrowth but rather a decline in production. Taxes Table 1 summarizes the fuel properties of the mosthave made the biodiesel too expensive for consumers common alcohol fuels and compares them to gasolinecombined with export subsidies for US producers. as a baseline. Characterized by a hydroxyl functionalThe European Union has imposed import tariffs on group (-OH) bound to a carbon atom the most com-US biodiesel to promote and protect European mon alcohols are methanol, with only a single carbonproduction. atom, ethanol (C2) as well as butanol, a four carbon alcohol. The negative impact of the oxygen in the hydroxyl group on the energy content decreases withFuel Properties increasing carbon count. While the lower heating valueAn understanding of alternative fuels and their impact of methanol is less than 47% of gasoline, it increaseson vehicle design as well as engine performance and with carbon count to approximately 63% for ethanolemissions characteristics requires knowledge of fuel and 78% for butanol. Since the density of these fuels isproperties. For this purpose the fuel properties of the in a similar range, the ratio of volumetric energy con-most common liquid as well as gaseous fuels are tent is similar to the lower heating value withdiscussed in detail and compared to conventional a signiﬁcant advantage of gasoline compared to thefuels as a baseline. The alternative fuels are grouped alcohols in general and the short chain alcohols ininto alcohol fuels including methanol, ethanol as well particular. The lower volumetric energy content isas iso-butanol as the most promising butanol isomer; also reﬂected in the GGE values for alcohol fuels.liqueﬁed fuels including liqueﬁed natural gas (LNG) Approximately 2 gal of methanol contain the sameand liqueﬁed petroleum gas (LPG), and gaseous fuels energy as 1.5 gal of ethanol, 1.2 gal of butanol, orincluding methane/CNG and hydrogen. Since these 1 gal of gasoline. A major advantage of these mostfuels are mainly used in spark-ignition (SI) engines, common alcohol fuels compared to gasoline is theirgasoline as the most common SI engine fuel is used as higher knock resistance expressed as octane number,the baseline for comparison. Biodiesel and synthetic which is around 100 for methanol as well as ethanoldiesel fuel produced through the Fischer–Tropsch pro- and 98 for iso-butanol compared to approximately 90cess, green diesel as well as dimethyl ether (DME) are for gasoline. Although concerns have been voiced thatcompared to diesel fuel for CI engine applications. laboratory engine Research and Motor octane rating In order to compare the energy content of alterna- procedures are not suitable for use with neat oxygen-tive fuels to a conventional fuel, gasoline gallon equiv- ates , the presented values are still indicators toalent (GGE) is introduced. GGE refers to the amount of evaluate knock resistance. The difference in oxygenalternative fuel required to match the energy content of content is once again apparent when comparing the1 gal of gasoline. Although this measure is somewhat stoichiometric air demand. It increases from 6.45 forcontroversial due to the variability in the energy con- methanol to 9 for ethanol and 11.2 for butanol com-tent of gasoline as well as alternative fuels , it is used pared to 14.7 for gasoline. Finally, the solubility of
636 Internal Combustion Engines, Alternative Fuels for Internal Combustion Engines, Alternative Fuels for. Table 1 Comparison of fuel properties – gasoline versus common alcohol fuels [16, 18, 19] Parameter Unit Gasoline Methanol Ethanol iso-Butanol Chemical formula C4 – C12 CH3OH C2H5OH C4H9OH Composition (C, H, O) wt.% 86, 14, 0 37.5, 12.6, 49.9 52, 13, 35 65, 13.5, 21.5 Lower heating value MJ/kg 42.7 20 26.9 33.1 3 Density kg/m 720–780 792 789 805 Volumetric energy content MJ/l 31.6 15.8 21.2 26.5 Vol. energy content relative to gasoline % 100 50 67 84 GGE gal 1 2.01 1.5 1.19 Research octane number (RON) – 92–98 106–114 107–111 105 Motor octane number (MON) – 80–90 91–95 89–94 91 AKI 90 100 100 98 Stoichiometric air/fuel ratio – 14.7 6.45 9.0 11.2 Solubility in water @ 20 C ml/100 ml H2O 0.1 Miscible Miscible 7.6 Boiling temperature C 25–215 64.7 78.4 107–108 Flashpoint C –43 12 17 28 Ignition limits vol % 1.0–7.6 4–75 3.5–15 1.2–10.9 l 0.4–1.4 Latent heat of vaporization MJ/kg 0.305 1.103 0.84 0.69 ethanol in water is known to present a challenge for Liquefied Fuels transporting gasoline/ethanol blends in pipelines . Table 2 compares the properties of liqueﬁed natural gas A critical factor for liquid fuels is their evaporation (LNG) and liqueﬁed petroleum gas (LPG) to those of characteristics. These characteristics affect engine gasoline. Natural gas, after it has been cooled down to behavior, in particular at cold start, as well as evapora- its liquid state at a temperature of –161 C, is referred tive vehicle emissions. Vapor pressure curves for alco- to as liqueﬁed natural gas (LNG). Since the liquefaction hol fuels as a function of blend level are shown in Fig. 6 requires the removal of certain components that could based on data from Anderson et al. . The vapor form solids during the process, LNG typically contains pressure of alcohols is lower than that of gasoline. approximately 95% methane . Liqueﬁed petroleum When alcohols are mixed with gasoline, they form an gas (LPG), also called autogas, is a mixture of propane azeotropic mixture with some components of the gas- (C3H8) and butane (C4H10) with changing composi- oline which results in an asymmetric relation between tion depending on the region as well as seasonal differ- vapor pressure of the blended fuel and the volumetric ences. In the United States, Canada, and Japan, LPG blend ratio . Evaporation of ethanol produces 4.6 consists mainly of propane, whereas the butane content times as much cooling of the charge as iso-octane, in other areas of the world can be signiﬁcant and a major component of gasoline, while methanol pro- typically increases for countries with warmer climate duces 8.4 times as much cooling. Charge cooling (butane content in Greece is 80%). The actual compo- reduces the tendency of the end gas to autoignite and sition of LPG signiﬁcantly affects its properties. Figure 7 therefore, further decreases the tendency for combus- shows the vapor pressure of propane and butane, the tion knock .
Internal Combustion Engines, Alternative Fuels for 637 90 80 70 60 Vapor pressure [kPa] 50 40 Methanol 30 Ethanol iso-Butanol 20 10 0 0 20 40 60 80 100 Alcohol content [Vol-%]Internal Combustion Engines, Alternative Fuels for. Figure 6Vapor pressure of alcohol fuels blends based on data from Internal Combustion Engines, Alternative Fuels for. Table 2 Comparison of fuel properties of gasoline with LNG andLPG  Parameter Unit Gasoline LNG LPG (USA) Chemical formula C4 – C12 CH4 (typ. 95%) C3H8 (maj) C4H10 (min.) Lower heating value MJ/kg 42.7 46.7 46 3 Density kg/m 715 – 765 430–470 508 Volumetric energy content MJ/l 31.6 21 23.4 Vol. energy content relative to gasoline % 100 66 74 GGE gal 1 1.5 1.35 Octane number ((R + M)/2) – 90 120 105 Stoichiometric air/fuel ratio – 14.7 17.2 15.8 Boiling temperature C 25–215 –162 –42 Physical state for storage – Liquid Cryogenic liquid Pressurized liquid Ignition limits vol % 1.0–7.6 5.3–15 2.1–9.5 l 0.4–1.4 0.7–2.1 0.4–2two main constituents of LPG as a function of temper- is due to the lower density of the two alternative fuelsature. Although the lower heating value of both LNG compared to gasoline and results in a volumetric energyand LPG is higher than that of gasoline, the volumetric content of LNG of only 66% and 74% for LPG com-energy content of both fuels is signiﬁcantly lower. This pared to gasoline. The resulting GGE values of LNG and
638 Internal Combustion Engines, Alternative Fuels for 16 Internal Combustion Engines, Alternative Fuels for. 14 Table 3 Typical composition of natural gas  12 Methane CH4 70–90% Vapor pressure [bar] 10 Ethane C2H6 0–20% 8 Propane Propane C3H8 Butane 6 Butane C4H10 4 Carbon dioxide CO2 0–8% 2 Oxygen O2 0–0.2% 0 Nitrogen N2 0–5% –40 –30 –20 –10 0 10 20 30 40 Hydrogen sulfide H2 S 0–5% Temperature [°C] Rare gases A, He, Ne, Xe Trace Internal Combustion Engines, Alternative Fuels for. Figure 7 Vapor pressure as a function of temperature for propane and butane that is several orders of magnitude lower than that of gasoline, it takes 912 gal of methane or 3,075 gal of LPG are 1.5 and 1.35, respectively. The octane rating of hydrogen to achieve the energy equivalent of 1 gal of LNG is approximately 120 and signiﬁcantly higher than gasoline. The comparison is more favorable on a weight gasoline (90) and LPG with an octane rating of 105. basis, suggesting that 1 kg of hydrogen contains the same energy as 2.5 kg of methane and 2.86 kg of Gaseous Fuels gasoline. As mentioned earlier, LNG typically contains more Biodiesel and Synthetic Diesel than 95% methane (CH4) since other impurities are removed for the liquefaction process. Although com- The average properties of petroleum diesel fuel and pressed natural gas (CNG) also mainly contains meth- biodiesel are shown in Table 5. In addition, to accen- ane, its range of composition is wider than that of LNG tuate the differences in properties between biodiesel and varies geographically. In addition to methane it and other alternative diesel fuels, the properties of typically includes signiﬁcant amounts of ethane, pro- a typical synthetic diesel fuel produced through the pane, and butane as well as other constituents in Fischer–Tropsch process are included in Table 5 as well. smaller amounts, as shown in Table 3. Diesel fuel properties are controlled by ASTM Given that methane is by far the dominating con- D975. Hundred percent biodiesel has been speciﬁed stituent of CNG, the comparison of fuel properties of as an alternative fuel to diesel and has its own standard, gaseous fuels shown in Table 4 uses the properties of ASTM 6751. For blends between 6% and 20% biodie- methane as representative for CNG. Although not sel, a third standard is used, ASTM D7467. Before neat widely used for transportation applications at this biodiesel can be blended with petroleum diesel, both point, the fuel properties of hydrogen as another prom- fuels must meet their respective ASTM standards and ising alternative fuel are included in this comparison. then be analyzed as a blend using D7467 if the biodiesel The mass-speciﬁc lower heating value increases from content is between 6 and 20 vol %. gasoline to methane to hydrogen starting at 43.5– For example, if the cetane number falls too low, 50 MJ/kg and 120 MJ/kg respectively. It is interesting it can lead to increased ignition delay, combustion to note that the lower heating value increases with pressure, and noise, along with increased smoke during increasing hydrogen content in the mass-speciﬁc fuel cold start. composition. However, since both methane and hydro- Increased viscosity has been shown to increase fuel gen are gaseous at ambient conditions with a density spray penetration resulting in cylinder wall wetting and
Internal Combustion Engines, Alternative Fuels for 639Internal Combustion Engines, Alternative Fuels for. Table 4 Comparison of fuel properties of gasoline and gaseousfuels (methane and hydrogen) [18, 19] Parameter Unit Gasoline Methane Hydrogen Chemical formula C4 – C12 CH4 H2 Composition (C, H, O) wt.% 86, 14, 0 75, 25, 0 0, 100, 0 Lower heating value MJ/kg 43.5 50 120 3 Density gaseous kg/m – 0.72 0.089 liquid 730–780 430–470 71 GGE (by volume) gal 1 912 3075 GGE (by weight) kg 2.86 2.5 1 Stoichiometric air demand – 14.7 17.2 34.3 Boiling temperature C 25–215 –162 –253 Ignition limits vol % 1.0–7.6 5–15 4–75 l 0.4–1.4 0.7–2.1 0.2–10 Minimum ignition energy mJ 0.24 0.29 0.02 Self-ignition temperature C approximately 350 595 585 Diffusion coefficient mm/s – 1.9 Â 10 –6 8.5 Â 10–6 Quenching distance Mm 2 2.03 0.64 Laminar flame speed cm/s 40–80 40 200Internal Combustion Engines, Alternative Fuels for. Table 5 Fuel properties for diesel, biodiesel, and synthetic diesel[25, 27] Parameter Unit Diesel Biodiesel Synthetic diesel Chemical formula C8–C25 C12 – C22 Composition (C, H, O) wt% 87, 13, 0 77, 12, 11 85–87, 13–15, 0 Lower heating value MJ/kg 42–44 36–38 43.9 3 Density kg/m 810–860 870–895 770–780 Volumetric energy content MJ/l 34–37.8 31–34 33.8–34.2 Vol. energy content relative to diesel % 100 93 94.4–95.5 DGE gal 1 1.074 1.05–1.06 Cetane number – 40–55 45–65 73–80 Viscosity @ 40 C mm /s 2 2.8–5.0 2–6 2.0–3.5 Flash point C 60–78 100–170 59–109 Cloud point C –24 – –10 –5–5 –19–0 Stoichiometric air/fuel ratio – 14.7 13.8 Boiling range/temperature C 180–340 315–350 160–350 Ignition limits vol % 0.6–6.5 0.3–10 Lubricity Baseline Higher Lower
640 Internal Combustion Engines, Alternative Fuels for impingement. Biodiesel has a slightly higher viscosity diesel fuel with high cetane, good cold-ﬂow and oxida- than petroleum diesel, while synthetic diesel is similar tive stability properties, and ultra low sulfur content. to petroleum diesel. In addition, the atomization of the The Fischer–Tropsch process was ﬁrst commercialized fuel is reduced as fuel viscosity is increased. Too low of by Sasol in South Africa. They produce over 165,000 viscosity will increase pump wear due to reduced barrels/day of transportation fuel from coal. However, lubricity. Biodiesel has been utilized as a lubrication the process is on the order of three times more additive recently. expensive to produce fuel than conventional petroleum Sulfur content has previously been utilized for production processes . lubrication in fuel injection systems but increased emissions controls have forced the reduction of sulfur Green Diesel and DME in diesel fuel. The sulfur can be converted into sulfur dioxide and sulfur trioxide, poisoning the cata- Hydrogenated-derived renewable diesel fuel or “green” lyst and sensors in the exhaust stream. In addition, diesel is an alternative compression-ignition engine small droplets of sulfuric acid and other sulfates can fuel that is produced by utilizing the conventional become nucleation sites for particulate matter emis- processing method for petroleum diesel fuel, fractional sions. In the United States, the sulfur content of distillation. The process is feedstock tolerant and unlike on-road diesel fuel was mandated to not exceed transesteriﬁcation, the properties of the fuel do not 15 ppm sulfur as of 2006. Biodiesel and synthetic diesel change signiﬁcantly when the feedstock varies from do not contain sulfur. vegetable oil to waste grease. However, there are signif- The cloud point is the temperature at which crys- icant differences in properties compared to biodiesel tals begin to form in the fuel and prevent the ﬂow of and even petroleum diesel. Green diesel has a higher fuel through ﬁlters and pumps. A signiﬁcant challenge cetane number, typically 75–90 compared to 40–50 for for biodiesel fuels is overcoming the signiﬁcant petroleum diesel. The pour point and energy content increase in cloud point temperature compared to can be nearly the same as petroleum diesel, alleviating petroleum diesel fuel. Additives, tank heaters, and the negative aspects of biodiesel. Green diesel is not various blends of petroleum and biodiesel have all oxygenated so the oxidative stability is very good. The been experimented with to improve the cold-weather process of producing green diesel is more energy inten- operation of biodiesel. sive and requires higher temperatures and pressures Synthetic diesel fuel whether produced from bio- compared to transesteriﬁcation. The majority of mass (BTL), coal (CTL), or natural gas (GTL) does not research is currently focused on increasing the output vary widely in properties based on the feedstock due to of fuel and scaling up production facilities. the consistent processing techniques employed. If the Dimethyl ether (DME) is a two-carbon, oxygenated Fischer–Tropsch process is employed, the feedstock is molecule that has unique properties in the liquid state ﬁrst converted to a synthesis gas (syngas) consisting of as an alternative diesel fuel. DME is primarily produced hydrogen and carbon monoxide. This process involves by converting hydrocarbons such as natural gas to combustion of the feedstock in a reduced oxygen envi- synthesis gas (syngas). The syngas is then converted to ronment. Once the hydrogen and carbon monoxide are methanol in the presence of a copper-based catalyst. An produced, the effect of the feedstock composition is additional dehydration process of methanol using removed from the ﬁnal fuel properties. The remaining a silica-alumina catalyst results in the production of process to produce liquid fuel (Fischer–Tropsch) basi- DME. This two-step process is the most widely utilized cally receives the same input regardless of feedstock. currently, but a one-step production process is cur- The other method of producing synthetic diesel fuel is rently being researched. Because DME is in the vapor to ﬁrst convert the biomass to bio-oil through pyrolysis state at standard temperature and pressure, it must be (zero oxygen environment) and then convert the bio- compressed, like liquid propane, and stored in tanks oil to fuel. Again, the pyrolysis step removes the effect for transportation and consumption. Table 6 shows of feedstock composition, thus producing a consistent some of the primary differences between green diesel fuel. Both processes produce a high quality alternative and DME compared to petroleum diesel fuel.
Internal Combustion Engines, Alternative Fuels for 641Internal Combustion Engines, Alternative Fuels for. Table 6 Green diesel and DME fuel properties compared topetroleum diesel [26–29] Unit Petroleum diesel Green diesel DME Chemical formula C8–C25 C2H6O Lower heating value MJ/kg 42–44 44 28 3 Density (+20 C) kg/m 810–860 780 675 Volumetric energy content MJ/l 34–37.8 34.3 18.9 Vol. energy content relative to diesel % 100 94 52 DGE – 1 1.06 1.92 Cetane number – 40–55 98–99 65 2 Viscosity @ 40 C mm /s 2.8–5.0 3.0–3.5 0.21 Flash point C 52 –41 Auto ignition temperature C 250 350 Cloud point C –24 – –10 –15 n/a Lubricity Baseline Similar to baseline Poor Stoichiometric air/fuel ratio – 14.7 8.99 Ignition limits vol % 0.6–6.5 3.4–28Effect of Feedstock Composition on Biodiesel Pro- It is well known that cetane number decreases withperties Unlike ethanol whose properties are relatively decreasing chain length and increasing branching.unaffected by the feedstock due to the processing tech- As well, oxidative stability increases with degree ofnique employed (distillation), biodiesel properties have saturation. Table 8 shows the range of cetane number,been shown to vary widely depending on the feedstock heat of combustion, cloud point or cold ﬁlter pluggingproperties (fatty acid proﬁle) and catalyst employed point (CFPP), and kinematic viscosity for several bio-(typically methanol or ethanol) during the fuel diesel fuels.production process. Figure 8 shows the effect of 33 different feedstocks The fatty acid proﬁle of the feedstock directly inﬂu- on fuel oxidative stability, measured in hours. This dataences the properties of the biodiesel. The primary fea- shows that as the number of double bonds increasestures of interest for the fatty acid include chain length, (increased degree of unsaturation), the oxidative sta-degree of unsaturation, and branching of the chain. bility decreases signiﬁcantly. A test of iodine number ofThe properties of the biodiesel that are most directly the biodiesel provides a measure of the degree ofimpacted by changes in the fatty acid proﬁle include unsaturation of the fuel.cetane number, heat of combustion, cold-ﬂow, oxida- One of the signiﬁcant advantages of biodiesel is thetive stability, viscosity, and lubricity. Table 7 shows the ability to improve the lubricity of the fuel with onlyfatty acid proﬁles for the primary feedstock options in a small quantity of ester. As shown in Fig. 9, the lubricitybiodiesel and the wt% of each fatty acid contained in improves signiﬁcantly for the ﬁrst 1% of methyl esterthe feedstock. Note that C18:1 refers to an 18 carbon added to the baseline diesel fuel. Lubricity is measuredchain molecule with one double bond. Increasing using the ASTM D6078 Scufﬁng Load Ball-on-Cylindernumbers of double bonds reduce the saturation of the Lubricity Evaluator (SLBOCLE) test rig. The higher themolecule. If no double bonds exist, the molecule is said value, the higher the load before scufﬁng occurs whichto be fully saturated. translates to a fuel with improved lubricity.
642 Internal Combustion Engines, Alternative Fuels for Internal Combustion Engines, Alternative Fuels for. Table 7 Fatty acid profile for the most common biodiesel feed- stocks [30, 31] Vegetable oil C16:0 C18:0 C18:1 C18:2 C18:3 Castor 1 1 3 4 Coconut 7.5–10.5 1–3.5 5–8 1–2.6 Cottonseed 22–26 2–3 15–22 47–58 Palm 40–47 3–6 36–44 6–12 Peanut 6–14 2–6 36.4–67.1 13–43 Rapeseed (Canola) 2–6 4–6 52–65 18–25 10–11 Soybean 10–12 3–5 18–26 49–57 6–9 Sunflower 5–7 3–6 14–40 48–74 Internal Combustion Engines, Alternative Fuels for. Table 8 Fuel properties of biodiesel fuels from various feedstocks  Cetane Vegetable oil number Heat of combustion [KJ/kg] Cloud point or CFPP [ C] Kinematic viscosity [40 C, mm2/s] Coconut ethyl 67.4 38,158 5 3.08 Cottonseed 51.2 –5 (pour point) 6.8 (21 C) Palm ethyl 56.2 39,070 8 4.5 (37.8 C) Rapeseed 48–56 37,300–39,870 –3; CFPP –6 4.53 (Canola) Soybean 48–56 39,720–40,080 from –2 to 3 4.0– 4.3 Sunflower 54–58 38,100–38,472 0–1.5 4.39 The impact of biodiesel on cetane number can be content of fuel/air mixtures. It speciﬁes the amount of clearly identiﬁed in Fig. 10. As a general rule regardless energy contained per volume of fresh charge typically at of feedstock, increasing quantity of biodiesel volume stoichiometric conditions that can be introduced into increases the cetane number and improves the over- the cylinders of an internal combustion engine. The all combustion quality. The data shown in Fig. 10 uti- mixture caloriﬁc value is not only used to describe the lized a baseline petroleum diesel fuel meeting ASTM fuel properties of a speciﬁc fuel and mixture formation D975 speciﬁcations. strategy, but also allows estimation of the theoretical power density of a speciﬁc alternative fuel. The brake mean effective pressure (BMEP) or engine torque can be Comparison of Fuels Based on Mixture Calorific calculated based on the volumetric efﬁciency (VE) of a Value speciﬁc engine, the brake thermal efﬁciency (BTE), and In addition to the fuel properties highlighted in the mixture caloriﬁc value (HG). Under the (theoreti- Tables 1–9, the mixture caloriﬁc value for different cal) assumption that volumetric efﬁciency and brake fuels and mixture formation strategies can be calculated thermal efﬁciency remain unchanged when switching and presents a measure for the volume-speciﬁc energy from one fuel to another, any change in mixture
Internal Combustion Engines, Alternative Fuels for 643 36 16 Oxidation stability [h] 12 8 4 3 h min. 0 Perilla Seed Choice White Grease Linseed Palm Fish Evening Primrose Rice Bran Tung Hemp Sunflower Used Cooking Oil Castor Mustard Hepar, Low IV Camelina Hepar, High IV Beef Tallow Borage Soybean Corn, Distiller’s Jatropha Moringa oleifera Yellow Grease Neem Canola Coffee Algae 1 Cuphea viscosissima Lesquerella fendleri Poultry Fat Algae 2 Babassu CoconutInternal Combustion Engines, Alternative Fuels for. Figure 8Dependence of biodiesel oxidative stability on feedstock  7,000 6,000 5,000 SLBOCLE [grams] 4,000 Soy Canola 3,000 Lard Edible tallow 2,000 High FFA yellow grease 1,000 0 0% 1% 2% 3% 4% 5% Methyl ester quantity [%]Internal Combustion Engines, Alternative Fuels for. Figure 9Lubricity improvement with methyl ester quantity 
644 Internal Combustion Engines, Alternative Fuels for 66 62 Soy Canola Lard Cetane number [-] 58 Edible tallow High FFA yellow grease 54 50 46 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Methyl ester quantity [%] Internal Combustion Engines, Alternative Fuels for. Figure 10 Effect of methyl ester on cetane number  Internal Combustion Engines, Alternative Fuels for. Table 9 Comparison of methyl ester and ethyl ester properties with a soybean feedstock Kinematic Acid Iodine Peroxide Glycerol- free/ Water/ Cetane Density Oxygen viscosity Ester No. No. No. Glycerol-bound Sediment No. g/cm3 wt% 40 C 100 C Methyl soy 0.15 121 340 0.007/0.223 0 52.3 0.8836 11.44 4.03 1.64 Ethyl soy 3.02 122 123 0.003/0.031 0 47.3 0.8817 11.55 4.33 1.74 caloriﬁc value is directly reﬂected in corresponding fuel/air mixture (rMixture), relative air/fuel ratio (l), change in (maximum) brake mean effective pressure, and stoichiometric air demand (LSt) and is deﬁned as: in other words, maximum torque. LHVrMixture HG ¼ ð2Þ BMEP ¼ VE Â BTE Â HG ð1Þ lLSt þ 1 For air-aspirating operation the density of air (rAir) For mixture-aspirating engines (carbureted or port is used instead of the fuel/air mixture density and the injected/external mixture formation) the mixture cal- value is deﬁned as: oriﬁc value is deﬁned relative to 1 m3 of mixture and for air-aspirating engines (direct injection/internal LHVrAir HG ¼ ð3Þ mixture formation) to 1 m3 of air. The mixture caloriﬁc lLSt value for mixture-aspirating operation is calculated Figure 11 shows the mixture caloriﬁc value for based on lower heating value (LHV), density of the various alternative fuels both for air-aspirating and
Internal Combustion Engines, Alternative Fuels for 645 5 4.8 Mixture calorific value HG, HG [MJ/m3] 4.6 Air-aspirating 4.4 Mixture-aspirating – 4.2 Methanol 4 Ethanol Butanol Hydrogen 3.8 Diesel Gasoline Propane 3.6 Methane Biodiesel 3.4 3.2 3 5 10 15 20 25 30 35 Stoichiometric air/fuel ratio [-]Internal Combustion Engines, Alternative Fuels for. Figure 11Mixture calorific value for conventional and alternative fuel optionsmixture-aspirating operation as a function of stoichio- considered an alternative fuel. The main productionmetric air/fuel ratio. To achieve high power density, the routes for methanol all use natural gas as a feedstock.mixture caloriﬁc value should be as high as possible. Methanol can be used to make methyl tertiary-butylWith liquid fuels the difference in the mixture caloriﬁc ether (MTBE), an oxygenate that was used in blendsvalue between air- and mixture-aspirating operation is with gasoline as an octane booster. MTBE productionminimal since the liquid fuel due to its high density and use has declined in recent years because it has beentakes up only a small volume. However, for gaseous found to contaminate ground water. As an enginefuels the difference between the mixture caloriﬁc value fuel, methanol has similar chemical and physicalin air-aspirating and mixture-aspirating operation characteristics to ethanol.increases signiﬁcantly with decreasing gas density A limited study of low level methanol gasoline(rPropane = 1.83 kg/m3, rMethane = 0.72 kg/m3, rHydrogen blends up to 15 vol % on a four-stroke, single-cylinder,= 0.089 kg/m3). The impact is most pronounced for variable compression ratio engine with a displacementhydrogen as a fuel, because it displaces approximately volume of 582 cm3 suggests an approximately 10%30% of the aspirated air at stoichiometric conditions increase in torque at the highest blend level which iswith external mixture formation. Even for liquid fuels attributed to the improved volumetric efﬁciency due tothere is noticeable difference in mixture caloriﬁc the higher heat of vaporization . An experimentalvalues that should be considered when comparing evaluation of engine cold start behavior with gasolinealternatives. as well as 10 vol % and 30 vol % blends of methanol in gasoline on a small three-cylinder engine came to the conclusion that the addition of methanol improvesAlternative Fuels for Spark-Ignition (SI) Engines combustion stability, indicated mean effective pressure (torque), and reduces misﬁres. In addition a 70%Alcohol Fuels (Ethanol, Butanol, Methanol) reduction in CO emissions as well as a 40% reductionMethanol As per the Energy Policy Act of 1992, in HC emissions were also observed. These positiveMethanol (CH3OH), also known as wood alcohol, is effects of methanol addition were attributed to higher
646 Internal Combustion Engines, Alternative Fuels for vapor pressure, lower boiling temperature as well as the containing up to 2 vol % of MTBE could subsequently high oxygen content of methanol . A detailed ther- be blended with 10 vol % of ethanol . Only recently modynamic evaluation of alcohol fuels including EPA granted a partial waiver to allow gasoline that their effect on engine efﬁciency and NOx emissions contains greater than 10 vol % ethanol and up to 15 concluded that the use of methanol could result in vol % ethanol (E15) for use in model year 2007 increase in engine brake thermal efﬁciency of up to and newer light-duty motor vehicles, which includes 1% regardless of engine load and speed. This increase passenger cars, light-duty trucks, and sport utility was attributed to slightly reduced pumping losses vehicles (SUV) . because of the increased amount of fuel vapor at Several markets also promote higher ethanol constant load compared to the iso-octane baseline. blends, with E85 being the most prominent one with The study further found that a 25% NOx emissions existing distribution networks in several countries reduction can be expected with methanol compared including the United States and Sweden. to the baseline fuel which is mainly caused by reduced Due to its large domestic production capabilities of process temperatures . The high octane rating ethanol from sugarcane Brazil has a special role in the of methanol also makes it well suited for knock-free worldwide ethanol market with a distribution network operation at high compression ratios. A study that was for both, Type C gasoline as well as neat hydrous performed on single-cylinder, four-stroke, naturally ethanol ($5 vol % water content). Type C Gasoline aspirated, high-compression direct-injection stratiﬁed describes a 25 Æ 1 vol % anhydrous ethanol blend with charge spark-ignition engine revealed the potential of gasoline  and is referred to as gasohol in Brazil. methanol for high engine efﬁciencies. Performing A critical aspect potentially limiting the maximum optimization of injection timing and spark timing at blend level of any alcohol fuel with gasoline is the vapor the research engine with a compression ratio of 16:1 at pressure of the fuel blend since it is critical for full load conditions at 1,600 RPM resulted in cold startability. Figure 6 shows the vapor pressure for a maximum indicated efﬁciency of approximately several alcohol fuels as a function of volumetric blend 51% . level. Vapor pressures below 45 kPa  are known to Due to challenges with methanol such as toxicity as potentially cause cold start problems which effectively well as safety related issues due to invisible methanol limits the maximum ethanol blend level without taking ﬂames widespread use of methanol as a transportation additional measures to approximately 75% during fuel is rather unlikely. winter months. Other limitations when increasing the amount of ethanol in the fuel might be inferred due to Ethanol Ethanol is probably the most widely used incompatibility of the blend with metals, plastics, and liquid alternative fuel and is typically used in blends elastomers in the fuel system. Despite these direct with gasoline. The two main variables when using effects, higher ethanol blends potentially also limit the gasoline/ethanol blends are the blend level and whether use of existing fuel infrastructure, in particular pipe- hydrous or anhydrous ethanol is used. Typically gaso- lines ; however, these limitations are beyond the line is blended with anhydrous ethanol whereas neat focus of this article. ethanol when used in vehicles is hydrous (E100). A more detailed overview of vehicle components Although attempts are made to get higher blends that are affected by ethanol addition as a function of approved, the most commonly used blend throughout blend level are shown in Fig. 12 . Blend levels below the world is E10, a blend of 90 vol % gasoline with 10 5 vol % do not require any changes to the vehicle vol % anhydrous ethanol. US legislation limits the regardless of vehicle age. Typically, modiﬁcations are amount of oxygen content in blends of gasoline with also not required at blend levels up to 10 vol % for alcohol fuels. In 1991, the maximum oxygen content most vehicles less than 15–20 years of age. Only older was increased from 2 wt.% to 2.7 wt.% for blends of vehicles might require changes to the engine carbure- aliphatic alcohols and/or ethers excluding methanol tors. Although studies suggest that most fuel system . To ensure sufﬁcient gasoline base was available components are not negatively affected at a blend level for ethanol blending, the EPA also ruled that gasoline of 20 vol % [44–46], modiﬁcations to the fuel system
Internal Combustion Engines, Alternative Fuels for 647 Vehicle component Fuel pressure device Evaporative system Catalytic converter Cold start system Exhaust system Intake manifold Ignition system Fuel injection Basic engine Carburetor Fuel pump Fuel filter Fuel tank Motor oil ≤ 5% For any vehicle Ethanol 5–10% For vehicles up to15–20 years old blend 10–25% 25–85% For specially designed vehicles ≥ 85% - Not necessary - Probably necessaryInternal Combustion Engines, Alternative Fuels for. Figure 12Required vehicle modifications with ethanol blend level (Based on )including injectors, pumps, regulators and ﬁlters, the direct fuel injection concluded that at full load atignition system as well as the catalytic converters are 2,000 RPM a 9% improvement in efﬁciency as well ascommon for blend levels up to 25 vol %. At higher maximum torque could be achieved while reducingblend levels up to 85 vol % additional modiﬁcations exhaust gas temperatures by 60 C . The Saabinclude the base engine, engine oil as well as intake and Biopower, a turbocharged sedan built to operate onexhaust system. Only for blend ratios beyond 85 vol % gasoline as well as E85 utilizes the advantageous fueland neat ethanol, a separate cold start system might properties of the ethanol blend. Rated at a maximumbe required. power of 110 kW at 5,500 RPM and a peak torque of As mentioned earlier, speciﬁc engine designs are 240 Nm at 1,800 RPM with gasoline, the same enginetypically only found for engines that are intended to achieves a maximum power of 132 kW at 5,500 RPMrun on higher ethanol blends in excess of 25 vol %. and a peak torque of 280 Nm at 2,400 RPM with E85Experimental results with port fuel injection on a 1.8 L . With a compression ratio of 9.3:1 the 2.0 L enginefour-cylinder engine converted to single cylinder oper- takes advantage of the higher octane rating of theation revealed the efﬁciency and emissions potential of ethanol blend by operating the turbo engine at higherhigh ethanol blends. In particular at moderate engine combustion pressures, producing more power withoutspeed of 1,500 RPM torque could be improved signif- risk of engine knock. Independent vehicle dynamome-icantly due to the increased knock resistance resulting ter testing of the Saab Biopower showed that the vehiclein a 20% increase with E100 compared to gasoline, acceleration time with E85 was 1 s faster than withwhile improving indicated thermal efﬁciency from gasoline, and that the car also met US Tier 2, Bin 532% to 40%. At the same time a reduction in oxides emissions levels on both gasoline and E85, which isof nitrogen emissions by approximately 125 ppm signiﬁcant since Europe, where the car is certiﬁed,together with a 2 C reduction in exhaust temperature does not require emissions certiﬁcation on E85 andper 10% increase in liquid volume fraction of ethanol applicable Euro 4 emissions standards are less strin-in the fuel could be observed which was attributed to gent. In addition, a detailed exhaust speciation revealedlower adiabatic ﬂame temperatures . A theoretical that ethanol and aldehyde emissions were higher onas well as experimental study comparing performance, E85 while hydrocarbon-based hazardous air pollutantsefﬁciency, and emissions of gasoline and E85 with were higher on gasoline . A detailed study of the
648 Internal Combustion Engines, Alternative Fuels for impact of alcohol blends on regulated emissions and air engine differs in the direct injector operating pressure, toxics with direct injection and ethanol blend levels of piston shape and compression ratio to efﬁciently run up to 50 vol % also concluded that ethanol addition with E100 and gasoline. The E100 version operates at resulted in an increase in aldehyde emissions, in par- an increased injection pressure (300 bar vs 200 bar) and ticular acetaldehyde, whereas formaldehyde emissions an increased compression ratio (13:1 vs 9:1). Improve- remained almost constant with ethanol addition. The ments in power and torque vary from 20% to 28% over same study also concluded that with E50 oxide of the range of engine speed, while the fuel conversion nitrogen emissions were reduced by approximately efﬁciency increases 17% to 23% with a peak efﬁciency 15% compared to gasoline operation, which was attrib- of approximately 40% . The report acknowledges uted to a temperature reduction as a result of direct that cold start issues exist for E100 engines and indi- injection and the increased in-cylinder cooling effect of cates that fuel is injected after intake valve closing ethanol compared to gasoline. Speciﬁc emissions which helps to insure complete vaporization of the trends for oxides of nitrogen as well as relevant air ethanol and prevents or minimizes wall wetting, how- toxics at a typical part load engine operating point ever, no speciﬁc measures to ensure proper cold start are summarized in Fig. 13 . These trends hold behavior are discussed. true independent of engine load, the magnitude Measures to mitigate the cold start issue with E100 changes slightly depending on engine speed and load include fuel choice, e.g., E85 where the 15 vol % of conditions. highly volatile gasoline are typically sufﬁcient to A study targeted at designing an engine speciﬁcally improve cold startability, sometimes combined with for neat ethanol application suggested that a combina- intentional overfueling to further improve the cold tion of direct injection and turbocharging at increased start behavior. However, it would be preferable to use compression ratios is ideally suited for operation on neat ethanol to fully utilize its fuel properties and also E100. The proposed direct injection, turbo charged avoid overfueling in order to reduce cold start 180 1,500 RPM 2.62 bar BMEP 160 140 Gasoline Emissions relative to gasoline [%] E10 E50 120 100 80 60 40 20 0 Oxides of nitrogen (NOx) Formaldehyde (CH2O) Acetaldehyde (C2H4O) Internal Combustion Engines, Alternative Fuels for. Figure 13 Influence of ethanol content on oxides of nitrogen and air toxics 
Internal Combustion Engines, Alternative Fuels for 649emissions. Technical solutions that have been devel- performed with gasoline as well as 10 vol % ethanoloped to mitigate the cold start problem include heated in gasoline (E10) and 10 vol % n-butanol in gasoline onfuel injectors , heated fuel-rails as well as installa- a 2.2 L Direct-injection engine suggests that enginetion of a second tank, ﬁlled with highly volatile gasoline efﬁciency and emissions characteristics are almostto run the engine for a few seconds before switching to identical throughout a majority of the engine operatingethanol . regime. Apparent differences were only found at the Ethanol currently is one of the most dominant alter- highest speed and load conditions which could benative fuels which is mainly due to its favorable proper- attributed to differences in the knock resistance of theties and production pathways. The properties and different fuels . Consequently the study waschallenges of ethanol as a fuel are well understood and extended to include iso-butanol in addition tomost technical hurdles for use of ethanol in internal n-butanol and blend levels were increased up to 85combustion engines have been overcome. It is likely vol % with similar ﬁndings as for the low level blends.that ethanol will remain a dominant alternative fuel for Even at the highest blend levels for ethanol, n-butanol,both, low and mid-level blends with gasoline for the and iso-butanol, the engine efﬁciency and emissionsmajority of applications as well as high level blends and characteristics are almost identical to the gasoline base-neat ethanol for local markets. Further development is line. However, at high load conditions the engine couldexpected in dedicated engines for high level blends and be run with signiﬁcantly earlier spark timing with eth-neat ethanol, however, the main area for advancement anol and iso-butanol compared to n-butanol and gas-will have to be in renewable production pathways which oline. The earlier combustion phasing due to thecreate ethanol from biomass rather than food crops. increased knock resistance of ethanol and iso-butanol compared to gasoline resulted in an increase in brakeButanol Butanol, a four carbon alcohol that exists in thermal efﬁciency in excess of 1% . Due to its lowerseveral isomers, is currently under investigation as octane rating n-butanol does not exhibit the advan-a potential alternative fuel. Butanol is particularly tages in terms of knock resistance that ethanol andattractive compared to ethanol due to its higher volu- iso-butanol show compared to gasoline. In fact, n-metric energy content and its lower afﬁnity to water butanol was found to behave very similar to regular(see Table 1). In a joint venture BP and DuPont gasoline in terms of combustion knock . A study offounded Butamax™ Advanced Biofuels LLC to market gasoline-butanol blends with up to 80 vol % alcoholand promote biobutanol, which consists mainly of iso- content on a port injected spark-ignition enginebutanol. In addition to 135 cars, including 1995 – 2009 suggests that the emissions characteristics remainmodel years, that have been tested to date with 1.5 similar with increasing blend ratio, however, improvedmillion miles driven, a retail demonstration was com- combustion stability with increasing butanol waspleted in 2009 in the UK. A total of 10 million liters of observed which could be attributed to shorter ignitionbiobutanol were blended and supplied to ten retail sites delays . An issue that was raised speciﬁcally withas EN228 compliant gasoline. Over the course of the the use of alcohol fuels is the amount of aldehydes anddemonstration approximately 250,000 vehicle ﬁlls were other air toxics as a result of the combustion. A study ofperformed and 80 million miles driven suggesting regulated and non-regulated emissions as a result ofbiobutanol can be treated as a normal fuel component combustion of various blends of gasoline with ethanol,and used without special procedures . Since buta- n-butanol, and iso-butanol suggests that aside fromnol has a lower oxygen content than ethanol, a 16 vol % a consistent reduction in engine-out emissions ofblend of butanol with gasoline contains the same oxides of nitrogen regardless of blending agent, bothamount of oxygen, approximately 3.7 wt.%, than a 10 formaldehyde and acetaldehyde emissions increasedvol % blend of ethanol with gasoline (E10). with addition of butanol to the fuel blend, whereas In addition to vehicle tests performed and publi- formaldehyde did not increase signiﬁcantly with addi-cized by BP and DuPont, few engine performance and tion of ethanol. Figure 14 shows oxides of nitrogenefﬁciency tests have been performed on iso-butanol as as well as formaldehyde and acetaldehyde emissionswell as other butanol isomers. A comparative study for n-butanol as well as iso-Butanol gasoline blends at
650 Internal Combustion Engines, Alternative Fuels for 250 1,500 RPM 2.62 bar BMEP Emissions relative to gasoline [%] 200 Gasoline n-But16 iso-But16 150 n-But83 iso-But83 100 50 0 Oxides of nitrogen (NOx) Formaldehyde (CH2O) Acetaldehyde (C2H4O) Internal Combustion Engines, Alternative Fuels for. Figure 14 Influence of butanol content on oxides of nitrogen and air toxics  blend levels of 16 and 83 vol % . Since the load at –42 C, isobutane –10 C, and n-butane 1 C. Special point is identical and the butanol blends with 16, and measures have to be taken to keep liqueﬁed fuels 83 vol % contain the same amount of oxygen as ethanol from evaporating at ambient conditions. As mentioned gasoline blends at 10 and 50 vol %, these results are earlier, LPG composition differs depending on regional directly comparable to those for ethanol blends shown supplies which affect vapor pressure and storage in Fig. 13. Although the increase in air toxics in engine- conditions (see Fig. 7). Typically LPG is stored in out emissions is noticeable, further studies will have to a steel or composite tank at moderate pressure around clarify the effect on tailpipe emissions. 10 bar. The two most common mixture formation strat- Butanol appears to be a promising alternative fuel egies for LPG are vapor fumigation and LPG injection with certain fuel properties superior to ethanol such as into the intake. More recently work has also been volumetric energy content and afﬁnity to water. Several reported on LPG direct injection. For vapor fumigation studies suggest that butanol is well suited as a fuel for the LPG supply to the engine is typically controlled spark-ignition internal combustion engines. However, by a regulator or vaporizer, followed by a mixer in the butanol nowadays is produced mainly as an industrial intake manifold. For a typical vaporizer system a power chemical rather than a transportation fuel. Since feed- density that is up to 15% lower than a comparable gaso- stock and production pathways for butanol are similar to line engine can be expected [60, 61]. LPG injection ethanol, the higher alcohol also faces the same challenges systems supply the liquid phase to the intake manifold, in terms of economy of production from celluloses. where a phase change occurs once the fuel is injected. Since this phase change results in increased charge density due to the cooling effect of the evaporation, Liquefied Fuels (LPG, LNG) LPG injection systems are superior compared to fumi- Liquefied Petroleum Gas At normal ambient pres- gation systems in terms of power density and also sure the boiling point of methane is –162 C, propane is reduce the tendency for backﬁring . An increase
Internal Combustion Engines, Alternative Fuels for 651 120 115 Peak torque relative to gasoline [%] 110 105 100 95 90 Gaseous LPG PFI Liquid LPG PFI 85 LPG DI 80 2,300 2,400 2,500 2,600 2,700 2,800 2,900 3,000 Engine speed [RPM]Internal Combustion Engines, Alternative Fuels for. Figure 15Relative peak torque of LPG mixture formation concepts compared to gasoline (Based on )in power density of LPG injection systems of approxi- characteristics with direct injection of LPG and gaso-mately 5–10% compared to the gasoline baseline has line concluded that the particulate emissions in LPGbeen reported [60, 61]. Most recently LPG engines operation were lower by a factor of 100 compared tohave also been operated with direct injection, and gasoline . Finally, the higher octane rating of LPGa performance advantage in excess of 10% compared compared to gasoline indicates that a dedicated LPGto a gasoline baseline has been reported . Figure 15 engine could possibly run at a higher, more efﬁcientshows the maximum engine torque as a function of compression ratio. A computational study that focusedengine speed for gaseous LPG port injection, liquid on developing an engine design for direct injection ofLPG port injection, and LPG direct injection relative CNG and LPG proposed 14.2:1 as a suitable compres-to gasoline. sion ratio for LPG operation . Since the fuel prop- A comparative emissions study of a bi-fuel vehicle erties of propane were used for the study, the actualcapable of operating on gasoline or LPG using compression ratio for real world LPG with a highera vaporizer system showed signiﬁcant emissions advan- butane content might be lower. Nonetheless the pro-tages for LPG. Depending on the drive cycle posed compression ratio is still signiﬁcantly highera reduction in CO emissions of 10–30%, 30–50% in than that of typical gasoline engines with compressionHC, and 40–77% in NOx was reported . The same ratios below 10.5:1 for port injection and less than 12:1study also concluded that CO2 emissions were reduced for gasoline direct injection .by approximately 10% when operated on LPG com- Liqueﬁed petrol gas (LPG) is well established as anpared to gasoline regardless of drive cycle. Given that alternative fuel in certain markets and the combustionthe CO2 emissions factor per unit energy for LPG is properties of LPG are well understood. However, theapproximately 12% lower than that of gasoline , market share compared to mainstream fuels is small inthe previously mentioned reduction in CO2 emissions most markets limiting dedicated vehicle development.suggests that the two fuels are equivalent in fuel econ- Besides the challenges due to the liqueﬁed state of theomy. Due to the rapid vaporization of LPG a signiﬁcant fuel, additional issues with infrastructure limitationsadvantage also in terms of particulate emissions can be and safety concerns appear to further limit the interestrealized. A study of combustion and emissions in large scale deployment.
652 Internal Combustion Engines, Alternative Fuels for Liquefied Natural Gas LNG is stored on-board of Vehicle range and weight of the fuel storage system vehicles as a cryogenic liquid in super-insulated storage are challenges for natural gas vehicles. At constant tanks. The typical operating pressure of these tanks is in range a gaseous storage system operating at 200 bar the range of approximately 5 bar but can reach up to requires approximately four times the storage volume 15 bar. Although LNG is stored in its liquid form, it is and outweighs the gasoline system by a factor of 3. typically evaporated before use. The quality of the Lightweight materials, increased storage pressures, evaporated gas differs depending on whether natural and improved engine efﬁciencies are expected to boil-off gas or forced boil-off gas is used. The gaseous improve the volumetric ratio to 1:2.9 and the gravi- phase in the top portion of LNG tanks is called natural metric ratio to 1:1.4 with a vehicle range of up to boil-off gas, consists mainly of methane and some 600 km . nitrogen, and has a high knock resistance. If LNG is The dominant mixture formation strategy for nat- extracted from the liquid phase and evaporated sepa- ural gas in automotive applications is intake manifold rately, it is considered forced boil-off gas which could injection. The higher knock resistance of natural gas have a different knock resistance than the natural boil- compared to gasoline allows for dedicated NG engines off gas. In addition to the potential difference in knock to operate at higher compression ratios resulting in resistance the lower heating value of natural boil-off higher thermal efﬁciencies on the one hand and higher gas is approximately 33–35 MJ/m3 and signiﬁcantly NOx emissions on the other. The lower mixture calo- lower than that of forced boil-off gas with an LHV of riﬁc value of natural gas with external mixture forma- 38–39 MJ/m3 . tion (see Fig. 11) compared to gasoline causes reduced Regardless of which type of gas from an LNG tank is power density with the gaseous fuel which can be used, the fuel is delivered to the engine in its gaseous further compromised when a lean burn strategy is state. Therefore, an engine combustion system designed employed. The increased NOx levels with NG also and marketed for operation on LNG is in fact designed to warrant the use of exhaust gas recirculation (EGR), run on natural gas or methane and does not necessarily however, EGR is limited due to combustion instabil- differ from CNG engines [69, 70]. There are, as outlined ities and misﬁres which occur at increased rates. above, signiﬁcant differences in the fuel storage, supply, The reduced power density with injection into the and conditioning systems. For engine design, efﬁ- intake manifold can be compensated with ciency, performance, and emissions the same consider- supercharging or turbocharging with intercooling. ations as for compressed natural gas apply. More recently, attempts have also been made to employ direct injection (DI) of natural gas but efforts are hampered since high pressure gaseous injection hard- Gaseous Fuels (Natural Gas, Hydrogen) ware is not available on the open market . Compressed Natural Gas Compressed natural gas is A comparative study of gasoline and CNG on typically stored on-board a vehicle in pressure cylin- a four-cylinder spark-ignition engine showed signiﬁ- ders, with single steel cylinders mainly used in the past cant differences between the two fuels at identical oper- which are more and more replaced by several smaller ating conditions. At wide open throttle operation at composite cylinders. The gas is typically stored at pres- speeds between 1,500 and 5,000 RPM the engine pro- sures around 200–270 bar with the additional cylinders duced between 8% and 16% less torque with sequential adding around 60 kg of extra weight to the average CNG injection relative to gasoline port fuel injection vehicle . which was mainly attributed to the reduced mixture The ﬁrst CNG model of the Honda Civic released in caloriﬁc value. The increased reduction in engine 1998 had a natural gas storage capacity of approxi- torque at higher speeds was further attributed to the mately 8 GGE at a fuel economy of 24 MPG City and lower ﬂame speeds of CNG compared to gasoline. 34 MPG Highway . The 2011 model still has the Similar results in terms of reduction in maximum same storage capacity (7.8 GGE) at reported fuel econ- torque had been reported on the same engine when omy numbers of 24/36 MPG allowing for a vehicle operated with a mixer type CNG system instead of the range of up to 450 km (280 miles) . sequential fuel injection. However, the fuel conversion
Internal Combustion Engines, Alternative Fuels for 653efﬁciency with intake manifold injection was reported lean limit of natural gas engines. Results on a 1.6 Lto be an average of 13% higher compared to gasoline single cylinder engine with hydrogen addition rates ofwhereas the carburetor system only showed an average up to 19% showed an extension of the lean limit from3% improvement over the gasoline baseline. Both the a relative air/fuel ratio around 1.8 without hydrogencarburetor and the sequential injection system showed addition to almost 2 with 19% hydrogen. A simulta-emissions reductions of up to 80% for CO, 8–20% for neous increase in indicated efﬁciency which can beCO2, and an average reduction in hydrocarbons of attributed to the increased burn rate with hydrogenapproximately 50%, while NOx emissions were addition was also reported .increased by around 33% with the carburetor system Energy security and domestic production in addi-compared to gasoline and no NOx emissions were tion to natural gas being considered a clean-burningreported for the sequential injection system [76, 77]. fuel are the main drivers for renewed interest in natural The previously mentioned results are representative gas in the United States . Natural gas has beenof stoichiometric engine operation just like conventional widely used for stationary power supply at outstandinggasoline engines. However, the relatively wide ignition engine efﬁciencies and emissions levels. However, mostlimits of natural gas suggest that a lean burn strategy natural gas vehicle applications rely on conversionwhich is beneﬁcial for engine efﬁciency be applied. from conventional gasoline engines for light-duty Experiments on a 1.3 L four-cylinder engine with applications or diesel natural gas dual fuel applicationsone cylinder operated with port fuel injection of natu- for heavy duty applications. Future development forral gas showed that lean operation with relative air/fuel natural gas applications is expected to result in dedi-ratios up to 1.3 is feasible without a dramatic decrease cated engines optimized to cater to the speciﬁc prop-in combustion stability. The lean burn strategy also erties of the gaseous fuels. In terms of storageresulted in an up to 75% reduction in NOx emissions technology both compressed natural gas as well ascompared to the stoichiometric case with only liqueﬁed natural gas storage will have their marketa moderate increase in HC and CO emissions . share since either one is well suited for certainModern stationary natural gas engines operate at applications.relative air/fuel ratios around 1.7 which is close tothe misﬁre region which results in a further Hydrogen Although the situation for hydrogen inincrease in engine efﬁciency while minimizing NOx terms of fuel storage is similar to that of natural gasemissions . with compressed and cryogenic storage as the two Further extension of the lean combustion limits can dominant techniques, hydrogen vehicles are not typi-be achieved with stratiﬁcation of the fuel/air mixture cally classiﬁed based on their storage system. Since, aswhich is frequently accomplished by employing direct for natural gas, the fuel supply to the engine is almostinjection. Research on a single-cylinder 0.9 L engine exclusively in gaseous form, engine technology doeswith direct injection during the compression stroke not depend on the storage system. Attempts havesuggests that the lean limit could be expanded to been made to utilize the low temperature of gaseousapproximately 1.8 also identifying the injection timing hydrogen from liquid storage to increase the poweras a critical parameter to inﬂuence combustion and density of hydrogen port injection engines .emissions characteristics . A comparison of early Hydrogen storage is a signiﬁcant challenge for theinjection, late injection, and split injection on a 1 L development and viability of hydrogen-poweredsingle-cylinder research engine further showed that vehicles. On-board hydrogen storage in the range ofstratiﬁed mixtures resulting from late and split injec- approximately 5–13 kg is required to enable a drivingtion can extend the lean limit up to relative air/fuel range of greater than 300 miles for the full platform ofratios in excess of 2.5, eliminating NOx emissions. light-duty automotive vehicles using fuel cell powerHowever, decreased combustion stabilities and expo- plants or hydrogen internal combustion engines. Cur-nential increase in hydrocarbon emissions were rent on-board hydrogen storage approaches involvereported for these conditions . Finally, hydrogen compressed hydrogen gas tanks, liquid hydrogenaddition has also been examined as a tool to extend the tanks, cryogenic compressed hydrogen, metal hydrides,
654 Internal Combustion Engines, Alternative Fuels for high-surface-area adsorbents, and chemical hydrogen 10,000 storage materials. Storage as a gas or liquid or storage in metal hydrides or high-surface-area adsorbents consti- 8,000 NOx emissions [ppm] tutes “reversible” on-board hydrogen storage systems because hydrogen regeneration or reﬁll can take place 6,000 on-board the vehicle. For chemical hydrogen storage approaches (such as a chemical reaction on-board the 4,000 vehicle to produce hydrogen), hydrogen regeneration is not possible on-board the vehicle and thus, these spent 2,000 materials must be removed from the vehicle and regenerated off-board. On-board hydrogen storage sys- 0 tem performance targets were developed through the 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fuel to air equivalence ratio Φ [–] FreedomCAR and Fuel Partnership, a collaboration among DOE and the US Council for Automotive Internal Combustion Engines, Alternative Fuels for. Research, and the major energy and utility companies. Figure 16 The targets developed are system-level targets and are NOx emissions trends for hydrogen fuelled engines customer-driven based on achieving similar perfor- mance and cost levels as competitive vehicles. The storage system includes the tank, storage media, A study of hydrogen mixture formation concepts in safety system, valves, regulators, piping, mounting comparison to gasoline performed on an automotive brackets, insulation, added cooling capacity, and any single-cylinder engine conﬁrmed the superior power other balance-of-plant components. In order to density with hydrogen direct injection. A 15% achieve system-level capacities of 1.8 kWh/kg system improvement in torque at 2,800 RPM with hydrogen (5.5 wt.% hydrogen) and 1.3 kWh/L (0.040 kg hydro- direct injection compared to gasoline port injection gen/L) in 2015 and the ultimate targets of 2.5 kWh/kg could be achieved; the relative torque improvement of system (7.5 wt.% hydrogen) and 2.3 kWh/L (0.070 kg direct injection compared to hydrogen port injection hydrogen/L), the gravimetric and volumetric capacities exceeded 75% . of the material alone must clearly be higher than the In terms of engine efﬁciency hydrogen internal system-level targets [85, 86]. combustion engines have been shown to be superior A primary classiﬁcation of mixture formation strat- to other, more conventional fuels. Peak efﬁciency num- egies of hydrogen engines can be done based on the bers reported by several research groups around the location of mixture formation or the location of the globe suggest that brake thermal efﬁciencies in the hydrogen dosing devices. External mixture formation range of 45% can be achieved with hydrogen direct refers to concepts in which hydrogen and air are mixed injection [90–93]. outside the combustion chamber, whereas internal Since hydrogen is the only fuel that does not con- mixture formation refers to concepts with hydrogen tain any carbon, its combustion does not create any being introduced directly into the combustion cham- carbon-based emissions constituents except for traces ber. Some researchers have also proposed combined that are expected to result from lube oil combustion. concepts with a combination of external and internal The major challenge for hydrogen combustion engines mixture formation . As indicated in Fig. 11, the is their NOx emissions which are a result of high tem- mixture formation strategy, especially in hydrogen peratures during the combustion process (Fig. 16). operation, has signiﬁcant impact on the theoretical A trade-off between relative air/fuel ratio and NOx power output of the engine. The dramatic difference emissions for homogeneous hydrogen combustion in theoretical power output is mainly caused by the low resulting from port injection or early direct injection density of hydrogen, resulting in a signiﬁcant decrease has been reported by several researchers [89, 94]. Com- in mixture density when external mixture formation is bustion of lean hydrogen–air mixtures with fuel-to-air employed . equivalence ratios of less than 0.5 (l 2) results in
Internal Combustion Engines, Alternative Fuels for 655extremely low NOx emissions. Due to the excess air For non-methane hydrocarbon (NMHC) emissionsavailable in the combustion chamber, the combustion the cycle-averaged emissions are actually 0 g/mile,temperatures do not exceed the NOx critical value of which require the car to actively reduce emissionsapproximately 1,800 K . Exceeding the NOx critical compared to the ambient concentration .equivalence ratio results in an exponential increase in The properties of hydrogen make it well suited foroxides of nitrogen emissions, which peaks around internal combustion applications with excellent enginea fuel-to-air equivalence ratio of 0.75 (l $ 1.3). At efﬁciency potential and inherently low emissionsstoichiometric conditions, the NOx emissions are at signatures for carbon based emissions. Furtheraround 1/3 of the peak value. The highest burned gas development of injection equipment will be requiredtemperatures in hydrogen operation occur around a fuel- for production applications of advanced mixtureto-air equivalence ratio near 1.1, but at this equivalence formation concepts. The main challenges for a wide-ratio oxygen concentration is low so the NOx concentra- spread utilization of hydrogen in transportationtion does not peak there . As the mixture gets remain the lack of a fuel infrastructure as well asleaner, increasing oxygen concentrations initially offset sufﬁcient on-board storage density.the falling gas temperatures, and NOx emissions peakaround a fuel-to-air equivalence ratio of 0.75 (l 1.3). Alternative Fuels for Compression-Ignition (CI)Since hydrogen engines, due to the efﬁciency advan- Enginestages, are generally operated with fuel lean conditions,the above mentioned NOx emissions trade-off with Operation of compression-ignition engines on non-air/fuel ratio also limits the power output. petroleum fuel is not a new concept. In fact, Rudolf If NOx emissions have to be avoided, hydrogen Diesel, one of the pioneers of the diesel engine, origi-engine operation with port injection is frequently lim- nally designed the compression-ignition engine to runited to air/fuel ratios below 0.5 and supercharging is on a variety of fuels, including straight vegetable oil.employed to increase power density [96, 97]. With In 1900, at the World Exhibition in Paris, he demon-hydrogen direct injection the mixture homogeneity strated his engine running on pure peanut oil.can be inﬂuenced through tailored design of injection Operation of today’s modern, highly sophisticatedstrategy including injector location and nozzle design compression-ignition engines on straight vegetable oilas well as injection strategy. Parameter studies with is not recommended due to the higher viscosity andhydrogen direct injection revealed that the NOx emis- increased contaminants in the oil. While short-termsions characteristics signiﬁcantly depend on the injec- operation may be possible, long-term effects includetion strategy. A consequent optimization of injection build-up of engine deposits, ring sticking, lube oiltiming with conventional hydrogen direct injection gelling, and other signiﬁcant maintenance problemsallowed a NOx tailpipe emissions reduction below the that can reduce engine life.level of a comparable gasoline engine; with a multiple The primary alternative fuels for compression-injection approach NOx emissions could be further ignition engines include biodiesel and synthetic diesel.reduced to a level of less than 25% compared to Emerging fuel technologies include hydrogenated-derivedsingle-injection strategies . diesel fuel (“green” diesel) and dimethyl ether (DME). A recently released hydrogen internal combustionengine vehicle has been pushing the envelope in terms Biodieselof emissions levels achievable with combustionengines. A BMW Hydrogen 7 Mono-Fuel demonstra- Biodiesel is the most widely used alternative diesel fueltion vehicle that was tested for fuel economy as well in the world. Biodiesel is deﬁned as, “a fuel comprisedas emissions on the Federal Test Procedure FTP-75 of mono-alkyl esters of long chain fatty acids derivedcold-start test as well as the highway test achieved emis- from vegetable oils or animal fats, designated B100, andsions levels that were only 3.9% of the Super Ultra Low meeting the requirements of ASTM D 6751” .Emissions Vehicle (SULEV) standard for nitric oxide The properties of biodiesel can vary widely, depending(NOx) and 0.3% for carbon monoxide (CO) emissions. on the feedstock and processing technique employed.
656 Internal Combustion Engines, Alternative Fuels for Biodiesel is produced from a fat or oil by reacting it One theory as to why NOx increases with increasing with an alcohol like methanol or ethanol in the pres- biodiesel content relates to the reduction in PM. As the ence of a catalyst such as sodium or potassium hydrox- combustion event occurs, the formation of soot and ide. The transesteriﬁcation process produces esters particulate matter is reduced due to the presence of (biodiesel) and glycerin. Typically, the alcohol is sup- oxygen in the fuel. Typically, this mass in the cylinder plied in excess to ensure a quick reaction and it can be would increase the speciﬁc heat of the burned-gas reused. The processing temperatures are around 65 C mixture and lower the overall temperature. With the and the pressures rarely exceed 140 kPa. The conversion reduction in particulate matter mass, the temperature efﬁciency, from oil to methyl ester exceeds 98%. If the rises and the production of NOx increases . alcohol is methanol, then the biodiesel is called a fatty A comparison of unregulated, gaseous toxic emis- acid methyl ester (FAME). If the alcohol is ethanol, sions for two biodiesel blend ratios is shown in Fig. 18. then the biodiesel is called a fatty acid ethyl ester Except for Styrene and Toluene, all toxic emissions (FAEE). The two “biodiesels” have signiﬁcantly differ- levels reduced as the biodiesel blend quantity increased ent properties, as shown in Table 9 for a biodiesel from 20% to 100%. Even for the 20% blend compared derived from a soybean feedstock. As the properties of to a base petroleum diesel fuel, all but three toxic biodiesel change, there is a direct impact on fuel econ- emissions constituents decreased measurably. omy, emissions, performance, and engine efﬁciency. Because of the lower energy density compared to Experimental testing has shown that particulate petroleum diesel fuel, biodiesel is associated with matter, CO, and HC emissions can be signiﬁcantly a decrease in fuel economy. As the blend volume reduced with relatively low levels of biodiesel blend, increases, the impact on fuel economy becomes greater. while NOx emissions have been shown to increase. One would expect up to a 15% reduction in fuel econ- Figure 17 shows a typical trend in emissions as the omy due to the reduction in lower heating value. To quantity of biodiesel is increased. maintain equivalent power, additional fuel must be 20 10 0 NOx –10 Change in emissions [%] PM CO –20 HC –30 –40 –50 –60 –70 –80 0 20 40 60 80 100 Biodiesel content [Vol-%] Internal Combustion Engines, Alternative Fuels for. Figure 17 Impact of emissions on biodiesel percentage 
Internal Combustion Engines, Alternative Fuels for 657 80% Average change compared to base fuel [%] 60% 20% biodiesel 100% biodiesel 40% 20% 0% –20% –40% –60% –80% Acetaldehyde Acrolein Benzene 1,3-Butadiene Ethylbenzene Formaldehyde n-Hexane Naphthalene Styrene Toluene XyleneInternal Combustion Engines, Alternative Fuels for. Figure 18Comparison of gaseous, toxic emissions at two biodiesel blend ratios injected when operating on biodiesel. This is achieved this fuel, the properties are very similar to petroleumthrough an increase in injection pressure or injection diesel and it blends well with conventional fuel. Theduration. It is advantageous to increase injection pres- fuels are straight chain parafﬁnic hydrocarbons that dosure to not only increase the fuel delivery rate but also not contain aromatics, oxygen, or sulfur. Neste claimsimprove atomization of the liquid fuel. However, ﬁeld that their green diesel, named NExBTL results in andata has not shown the signiﬁcant reduction in fuel 18% reduction in NOx and almost a 30% reductioneconomy as expected. The oxygenated fuel is also asso- in PM, compared to petroleum diesel fuel . Thisciated with an increase in combustion efﬁciency up to data was the result of a delayed injection event5% at blend levels below 25% . The positive from a lower bulk modulus compared to petroleumimpact on combustion efﬁciency reduces as the blend diesel. It has been found that the bulk modulus onlevel increases beyond 30%. The higher cetane number pump-line-nozzle injection systems is a sensitiveof biodiesel reduces the ignition delay, resulting in parameter in affecting the start of injection. Therefore,reduced noise and vibration from the engine . each fuel system needs to be custom tuned for differentThis also reduces the premixed combustion event of alternative fuels to ensure optimum performance anddiesel operation, known to have a signiﬁcant inﬂuence minimal emissions. This requires advanced sensors andon the production of NOx emissions. signiﬁcantly more involved calibration to enable ﬂex- fuel operation in the ﬁeld. In another study using a common-rail injectionHydrogenated-Derived Renewable Fuel, system, the impact of 30% and 100% HVO on emis-Hydrogenated Vegetable Oil (HVO), or Green Diesel sions and fuel consumption was studied. Figure 19Green diesel refers to the diesel-like fuel product pro- shows the signiﬁcant reduction in emissions and evenduced from renewable feedstocks utilizing the conven- gravimetric fuel consumption when no changes weretional distillation process for petroleum fuel. Because made to the injection system. Additional beneﬁts wereconventional distillation methods are used to produce realized by modifying the injection timing depending
658 Internal Combustion Engines, Alternative Fuels for SFC SFC THC NOX CO FSN (gravimetric) (volumetric) 10 100% HVO 5 30% HVO / 70% Diesel petroleum-based diesel fuel [%] 0 Relative change compared to –5 –10 –15 –20 –25 –30 –35 –40 Internal Combustion Engines, Alternative Fuels for. Figure 19 Impact of green diesel (HVO) on emissions and fuel consumption  on the parameter of interest. For example, when spe- due to the varying energy densities between the base- ciﬁc fuel consumption (SFC) was held constant for all line D2 diesel fuel and SunDiesel. Depending on the three fuels, the 100% HVO reduced NOx by 16% parameter of concern, a signiﬁcant reduction in NOx and smoke by 23% compared to petroleum diesel emissions at the same BSEC level was achievable with fuel . the synthetic diesel fuel . A signiﬁcant reduction in the number of soot par- ticles in the exhaust stream was measured when the Synthetic Diesel engine was operated on SunDiesel, as shown in Fig. 21. Synthetic diesel fuel is characterized by high cetane There is also a slight shift toward smaller particles for number, low pour point, low sulfur content, and high the synthetic diesel fuel compared to the petroleum energy density. These characteristics provide a high based fuel. This modiﬁcation of the particulate matter quality alternative diesel fuel that is capable of blending size and quantity can have implications on the partic- with petroleum diesel at any ratio. In one study utiliz- ulate ﬁlter efﬁciency and regeneration. ing a synthetic diesel fuel derived from biomass feed- In a comprehensive look at GTL, BTL, and CTL stock (BTL), emissions and fuel consumption were fuels produced using the Fischer–Tropsch process, Gill positively impacted when operating the engine on the et al.  concluded that the higher cetane number BTL fuel. The engine was a 1.7 L, four-cylinder, and lower aromatic content were the primary fuel direct injected diesel (Euro II) with intercooled properties that directly affected the emissions. The turbocharging. A single speed/load point was utilized higher cetane number reduced the ignition delay, lead- for all the testing, which simulated a typical cruise ing to a reduction in NOx emissions. Typically, injec- point for this engine. The fuel was produced by Choren tion timing is retarded to reduce NOx emissions but at Industries under the brand name SunDiesel. The the expense of fuel consumption (higher BSFC). With impact on NOx and BSEC is shown in Fig. 20. Brake a higher cetane number and thus lower ignition delay, speciﬁc energy consumption (BSEC) was utilized the injection timing can be delayed with almost no instead of brake speciﬁc fuel consumption (BSFC) impact on the BSFC. The shorter ignition delay also
Internal Combustion Engines, Alternative Fuels for 659 12,000 3°BTDC BSEC [kJ/kW-hr] 11,500 SunDiesel ULSD 11,000 10,500 12°BTDC 10,000 2 3 4 5 6 NOx [g/kW-hr]Internal Combustion Engines, Alternative Fuels for. Figure 20Brake-specific energy consumption versus specific NOx emissions for D2 and SunDiesel  20,000 Speed: 2,500 rpm 3° Load: 50% rated EGR: 10% 16,000 Sundiesel 6° D2 Number of particles [–] 12,000 9° 3° 8,000 12° 6° 9° 4,000 12° 0 10 100 1000 Mobility diameter [nm]Internal Combustion Engines, Alternative Fuels for. Figure 21Soot particle size and distribution for D2 and SunDiesel 
660 Internal Combustion Engines, Alternative Fuels for provides additional time for the oxidation of soot par- DME has poor lubricity and high compressibility com- ticles and subsequently reduced the engine-out partic- pared to diesel fuel and thus requires modiﬁcation to ulate matter. The reduced aromatic content of the XTL the fuel delivery and storage system. To achieve com- fuels reduced the formation of soot by removing soot parable mileage to a petroleum diesel vehicle, the fuel nucleation sites . EGR is another effective method storage capacity for a DME-powered vehicle would of reducing NOx emissions but typically increases the have to be doubled . PM production. XTL fuels with their lower aromatic As shown in Fig. 22, DME has shown signiﬁcant content have a higher tolerance to EGR, producing reductions in HC, CO, and NOx emissions compared less PM emission for the same EGR rate as petroleum to petroleum diesel. Smoke emissions for DME were diesel fuel. too low to be measured. Of interest is the associated reduction in BSFC, though perhaps a BSEC compari- son would be more meaningful due to differences in Dimethyl Ether (DME) energy density. DME is characterized by high cetane (55) and low DME can be produced from the conversion of nat- emissions characteristics. The oxygenated fuel has been ural gas, coal, oil residues, and bio-mass . DME is shown to reduce PM, NOx, and THC emissions com- an effective energy carrier and reforming of DME to pared to petroleum diesel fuel and engine noise is produce hydrogen-rich fuel-cell streams is currently typically reduced as well. The absence of carbon– being researched. In fact, hydrogen yields have been carbon bonds combined with high oxygen content found to be equivalent to methanol at comparable ($35 wt.%) and rapid evaporation are the primary operating temperatures . reasons for the emissions reduction with DME. This When comparing alternative fuels on a well-to- makes it an attractive alternative fuel for compression- wheels analysis for efﬁciency and GHG production, ignition engines and provides additional means to DME combined with existing engine technology is reduce NOx emissions through injection timing with- quite favorable. For example, compression-ignition out a signiﬁcant increase in PM emissions. However, engines operating on DME has the highest well-to- 6 Road load emission and fuel consumption 5 4 3 Petroleum diesel DME w/oxidation catalyst 2 1 0 ] ] ] ] ] -h -h -h SN -h /kW [g/ kW [g/ kW e[ F [g/ kW [kg ok x FC HC CO Sm NO BS Internal Combustion Engines, Alternative Fuels for. Figure 22 Road load emissions and fuel consumption for DME and conventional petroleum diesel 
Internal Combustion Engines, Alternative Fuels for 661wheel efﬁciency of all non-petroleum–based fuel utiliz- A comparative study of methanol diesel and ethanoling conventional, hybrid-electric, and fuel processor diesel blends at blend levels of 5 and 10 vol % on a single-fuel cell technologies (excluding natural gas) . cylinder engine concluded that trends for both alcoholsWhen compared against existing engine technology, were similar. The results suggest a consistent reductionDME produces the lowest amount of well-to-wheel in soot and CO emissions as well as increased speciﬁcGHG emissions compared to FT diesel, biodiesel, fuel consumption with alcohol addition. However, themethanol, and ethanol . reported trends of decreasing HC emissions and increas- ing NOx emissions with addition of ethanol and meth- anol are inconsistent with earlier studies. All reportedAlcohol/Diesel Blends trends did not show any signiﬁcant difference betweenVarious drivers including energy security and potential addition of ethanol and methanol .for emissions reduction of diesel engines have triggered A study was performed using a four-cylinderattempts to blend diesel with alcohol fuels. A variety Mercedes Benz C220 turbo-diesel vehicle and compar-of factors including blend stability, viscosity and lubric- ing the emissions for conventional diesel fuel, 20 vol %ity, materials compatibility, energy content, cetane and 40 vol % n-butanol. Tests were performed for thenumber, safety and biodegradability have to be consid- cold-start UDDS, hot-start UDDS, and HWFET driveered if alcohol is to be added to conventional diesel fuel cycles as well as several steady-state points. The resultsfor engine applications. Although all factors require showed that for the urban drive cycle, both total hydro-attention, the safety aspect is particularly emphasized carbon (THC) and carbon monoxide (CO) emissionsat this point. Diesel fuel has a ﬂashpoint of 64 C and is increased as larger quantities of butanol were added totherefore classiﬁed as a Class II or combustible liquid the diesel fuel. Oxides of nitrogen (NOx) were notaccording to NFPA guidelines. Alcohol fuels as well as signiﬁcantly affected by the 20% butanol blend andgasoline with ﬂashpoints below 37.8 C (see Table 1) are decreased with the 40% butanol blend. Drivability ofclassiﬁed as Class I or ﬂammable liquids requiring the vehicle decreased noticeably for the 40% butanolmore stringent storage requirements such as greater blend, especially for the cold-start urban drive cycle.distance in location of storage tanks from property Fuel consumption increased and thus fuel economylines, buildings, and other tanks. Studies have shown decreased (mpg) as the blend ratio of butanolthat blends of 10, 15, and 20 vol % of ethanol in diesel increased, as shown in Fig. 23 due primarily to theexhibit combustion safety characteristics essentially lower energy density of butanol compared to diesel.identical to those for pure ethanol. Essentially any For the steady state tests, a signiﬁcant reduction inblend of alcohol with diesel fuel has to be stored and ﬁlter smoke number (FSN) with increasing butanolhandled like gasoline rather than diesel [111, 112]. quantity was observed, as shown in Fig. 24 . Blends of diesel and ethanol, also referred to as A performance and emissions study performed onE-Diesel, generally show increased speciﬁc fuel con- a 6.0 L bus engine at several steady state operating pointssumption due to the lower energy content of ethanol reached similar conclusions. At the tested blend levels ofcompared to diesel. However, engine efﬁciencies 8 and 16 vol % of n-butanol with diesel, smoke, NOx,remain constant or improve slightly with addition of and CO emissions were consistently reduced withethanol. It is also accepted that the addition of ethanol increasing butanol content. At the same time increasingto diesel fuel has a beneﬁcial effect in reducing the PM butanol content also resulted in increased HC emis-emissions at least. The amount of improvement varies sions as well as increased speciﬁc fuel consumptionfrom engine to engine and also within the working despite a simultaneous increase in engine efﬁciencyrange of the engine itself . A heat release and .emissions study of 5, 10, and 15 vol % ethanol/diesel Although the technical advantages of alcohol addi-blends also concluded that soot, NOx, and CO tion to diesel have been demonstrated other limitingemissions decreased with increasing blend levels factors such as blend stability, materials compatibilitywhile hydrocarbon emissions showed an opposite and, especially, safety have to be critically evaluated.trend . Given that even addition of small amounts of alcohol
662 Internal Combustion Engines, Alternative Fuels for 60 55 0% butanol 50 20% butanol 45 40% butanol 40 Fuel economy [MPG] 35 30 25 20 15 10 5 0 UDDS cold-start UDDS hot-start HWFET Internal Combustion Engines, Alternative Fuels for. Figure 23 Impact on fuel economy for butanol/diesel blends over three drive cycles  3 2.5 0% butanol 20% butanol 2 40% butanol Smoke [FSN] 1.5 1 0.5 0 0 2 4 6 8 10 Road load grade [%] Internal Combustion Engines, Alternative Fuels for. Figure 24 Effect of n-butanol in diesel fuel on smoke emissions 
Internal Combustion Engines, Alternative Fuels for 663changes the ﬂammability dramatically raises the ques- will require a combination of improved transportationtion on whether a transition to alcohol diesel blends is efﬁciency and increased use of alternative fuels. Anyeconomically viable. efﬁciency beneﬁt a new engine technology can provide is also desirable when burning alternative fuels along the lines of “Using alternative fuels is desirable, havingFuture Directions to use less even more so.”Although alternative fuels can be grouped according totheir fuel properties or suitability for certain engine types, Bibliographytheir diversity requires dedicated engines to fully utilizetheir unique characteristics. Due to the wide range of Primary Literatureengine applications with their speciﬁc requirements such 1. Dutton K (2006) A brief history of the car. New Ideas 1as emissions limits or efﬁciency targets and the variety of 2. Diesel R (1894) Theory and construction of a rotational heatalternative fuel options with speciﬁc advantages and lim- motor. Spon Chamberlain, Londonitations it is likely that a future transportation scenario 3. MacLean H, Lave LB (2003) Evaluating automobile fuel/propulsionwill rely on an increased range of fuels. Meeting current system technologies. Prog Energy Combust Sci 29:1–69and future engine and vehicle efﬁciency as well as emis- 4. Tanaka R (2007) Biofuels in Japan. Report by the British Embassy in Tokyosions targets will likely also require advanced combustion 5. Section 201-202 Renewable Fuel Standard (RFS) Energy Inde-concepts to be implemented which in turn might beneﬁt pendence and Security Act of 2007 (Pub.L. 110-140, originallyfrom introduction of alternative fuels. To this extent named the CLEAN Energy Act of 2007)a stronger link between engine development and fuel 6. Schnepf R, Yacobucci B (2010) Renewable Fuel Standard (RFS):development is desirable which should result in opti- Overview and issues. CRS Report for Congress, Order Code R40155mized utilization of certain fuel properties in dedicated 7. Sperling D, Gordon D (2009) Two billion cars: driving towardengines. This would put an end to the currently applied sustainability. Oxford University Press, New York, pp 42–43.approach of developing fuels that best mimic the prop- ISBN 978-0-19-537664-7erties of gasoline or diesel which themselves are far from 8. Yacobucci BD (2008) Natural gas passenger vehicles: availabil-optimal for certain engine concepts. ity, cost, and performance. CRS Report for Congress. Order Further, an isolated evaluation of fuels based on Code RS22971 9. Brazilian Automotive Industry Association – ANFAVEA,their efﬁciency potential or emissions characteristics Brazilian Automotive Industry Yearbook 2010. Available onlinewill likely yield a suboptimal solution. Assessment of at http://anfavea2010.virapagina.com.br/anfavea2010/alternative fuel options must include technical merits 10. Energy Information Administration (2008) Annual survey ofand downsides as well as economical aspects. While alternative fuel vehicle suppliers and users, “as reported in”technical considerations have been discussed here, an Alternatives to traditional Transportation fuels 1998–2008 reports (Tables 14 or S1 depending on year of report). www.economical assessment including the entire vehicle and eia.doe.gov/cneaf/alternate/page/atftables/afv-atf2008.pdffuel life cycle known as “well-to-wheel” analysis has to 11. US Energy Information Administration (EIA) (Apr 2010) EIA’sbe performed. These assessments have to include fac- Alternatives to traditional transportation fuels, Table V1. www.tors such as fuel production, storage and distribution eia.doe.gov/cneaf/alternate/page/atftables/afv-atf2008.pdfoptions as well as secondary factors such as water use, 12. European Biodiesel Board (2009) http://www.ebb-eu.org/recycling, or land usage. Furthermore, ﬁndings and stats.php 13. International Energy Agency (IEA) Statistics division (2007)recommendations of such studies might vary dramat- Energy balances of OECD countries (2008 edition)–Extendedically depending on regional factors such as climate as balances and energy balances of Non-OECD countries (2007well as typical driver behavior. To this extent a link edition)–Extended balances. IEA, Paris. http://data.iea.org/between engine and fuel related aspects and vehicle ieastore/default.asprelated aspects will have to be established which will 14. http://www.biodiesel.org/buyingbiodiesel/plants/ 15. Boyd RA (2009) Proposed method of sale and quality speciﬁ-clarify the impact of engine and fuel choices for differ- cation for hydrogen vehicle fuel. Summary of current informa-ent powertrain conﬁgurations such as hybrid vehicles. tion. Standards Fuel Speciﬁcations Subcommittee (FSS). U.S. Overall improving energy security while reducing National Work Group for the Development of Commercialharmful pollutants as well as greenhouse gas emissions Hydrogen Measurement
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