Effect of hydrogen enrichment on emissions and performance of a diesel engine

Effect of hydrogen enrichment on combustion
characteristics, emissions and performance of
a diesel engine
Yasin Karag€oz a,**
, _Ilker Gu¨ler b
, Tarkan Sandalcı a
, Levent Yu¨ksek a
,
Ahmet Selim Dalkılıç c,*
a
Automotive Division, Department of Mechanical Engineering, Mechanical Engineering Faculty, Yıldız Technical
University, Yildiz, Besiktas, Istanbul, 34349, Turkey
b
Tu¨rk Trakt€or ve Ziraat Makineleri A.S‚., Sakarya, Turkey
c
Heat and Thermodynamics Division, Department of Mechanical Engineering, Mechanical Engineering Faculty,
Yildiz Technical University, Yildiz, Besiktas, Istanbul, 34349, Turkey
a r t i c l e i n f o
Article history:
Received 5 July 2015
Received in revised form
18 September 2015
Accepted 21 September 2015
Available online xxx
Keywords:
Hydrogen
Diesel engine
NOx
THC
CO
Smoke
a b s t r a c t
In this study, hydrogen fuel was injected into intake manifold using an LPG-CNG injector
that is controlled by a self-developed ECU, whereas diesel fuel was directly injected into
cylinder using diesel injector. Different hydrogen energy fractions are used in a diesel-
fueled CI engine at 1100 rpm constant engine speed and full load. The effect of 0% (pure
diesel), 22%, and 53% hydrogen addition of total fuel energy (hydrogen þ diesel fuel) on CO,
THC, smoke, and NOx emissions, engine performance (BSFC and brake thermal efficiency),
and combustion characteristics (in-cylinder pressure, heat release rate etc.) were experi-
mentally investigated. According to obtained results, a great improvement was provided
with increasing percentage of hydrogen on CO (67.3% and 69.2%, for 22% and 53% hydrogen
enrichment, respectively) and smoke emissions (43.6% and 58.6%, for 22% and 53%
hydrogen enrichment, respectively). Even though a slight raise was observed on THC
emissions, it is below emission regulations and can be ignored. On the other hand,
although a slight increase (almost the same value) was observed with 22% hydrogen
addition, a dramatic increase could not be prevented with 53% hydrogen addition in NOx
emissions compared with pure diesel fuel (0% hydrogen). Also, peak-in-cylinder pressure
values increased by 7.81% and 36.2% with 22% and 53% hydrogen addition, respectively, in
comparison to pure diesel fuel. Furthermore, a 25.77% increase in peak heat release rate
was obtained with 22% hydrogen addition and a great increase of 110.94% was acquired
with 53% hydrogen enrichment.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
* Corresponding author. Tel.: þ90 212 383 2821; fax: þ90 212 383 2765.
** Corresponding author. Tel.: þ90 212 383 2901; fax: þ90 212 383 2765.
E-mail addresses: ykaragoz@yildiz.edu.tr (Y. Karag€oz), dalkilic@yildiz.edu.tr (A.S. Dalkılıç).
Available online at www.sciencedirect.com
ScienceDirect
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
http://dx.doi.org/10.1016/j.ijhydene.2015.09.064
0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Karag€oz Y, et al., Effect of hydrogen enrichment on combustion characteristics, emissions and
performance of a diesel engine, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.064

Introduction
The fast depletion of fuels and increase in prices of conven-
tional fossil fuels in recent years force researchers to find a
new eco-friendly, renewable, and alternative fuel [1,2]. On the
other hand, it is mandatory to find a new, clean, and alter-
native fuel to reduce negative effects on environment and on
human beings, which often are generated by combustion of
fossil fuels [3e5]. Most of the energy demand in transportation
is still provided by the petroleum based fuels. Even though the
measures have followed the Kyoto Protocol, the increase in
CO2 emissions could not be prevented, and a 27% increase in
total CO2 emissions and 37% increase in transport related to
CO2 emissions between 1990 and 2004 were obtained [6]. Also,
stringent emission regulations in regulated emissions (NOx,
CO, THC, PM) force researchers to investigate alternative fuels
[7e10]. For this reason, some researchers [1,11e13] have been
studying with alternative fuels such as ethanol, LPG, CNG,
LNG, producer gas, biogas, hydrogen, etc. Especially, hydrogen
is an outstanding alternative fuel among these fuels, since it
emits neither greenhouse gases (GHG) nor toxic pollutants
[14,15]. Furthermore, hydrogen is a long-period renewable fuel
[7,8,16]. Hydrogen can be acquired by several methods like
water electrolysis, biomass, chemical wastes, etc. [17,18].
Hydrogen can be an alternative for electric energy because it
can be assumed as an energy carrier [1].
Using hydrogen in spark ignition engines is more conve-
nient than in compression ignition engines since it improves
lean operation region, combustion stability, brake thermal
efficiency, carbon monoxide, and unburned hydrocarbons
[19e22]. Although some researchers have assessed hydrogen
in homogeneous charge compression ignition (HCCI) engines
[10,23,24], it is difficult to commercialize them due to their
high level of cyclic variations, narrow engine operation
ranges, and difficulties in ignition period.
Hydrocarbons, carbon monoxide, nitrogen oxides, and
particulate matter are the basic regulated emissions that are
emitted from petroleum-based fuels [8]. IC engines will be
used in transport in the near future [25]. Diesel engines are
widely used in the transport sector, and the percentage of
diesel engines have been increasing. Diesel engines have
some advantages such as high thermal efficiency, low CO2,
and THC emissions compared with spark ignition engines.
However, diesel engines produce high levels of NOx and
smoke emissions [4,26e28]. Hydrogen could not be directly
used as diesel fuel due to its high self-ignition temperature of
576
C [1]. Therefore, an ignition source is required for using
hydrogen in CI engines [7]. To use hydrogen directly as diesel
fuel, a glow plug or heating intake air is required. The high
self-ignition temperature and high ignition delay are the most
important barriers beyond using hydrogen directly as diesel
fuel [29].
Ikegami et al. [30] directly used hydrogen in a CI engine as
diesel fuel thanks to a glow plug. Antunes et al. [29] directly
used hydrogen in a single-cylinder, naturally aspirated, direct-
injection compression-ignition engine instead of diesel fuel
heating intake air to 80
C and a self-developed high-pressure
gas injector was used to inject hydrogen into cylinder. Ac-
cording to their obtained results, a 14% power increase,
an increase in thermal efficiency between 28% and 43%, and a
20% decrease in NOx emissions were acquired.
Using hydrogen as additional fuel in compression ignition
engines is possible. According to this notion, hydrogen is used
as additional fuel, and ignition can be provided by pilot diesel
injection. In this dual fuel mode, three basic methods are
applied in the literature: timing hydrogen manifold injection,
continuous hydrogen injection, and direct hydrogen injection
[7]. Continuous hydrogen injection causes some combustion
problems such as backfiring, and engine operating range is
limited. Also, both continuous hydrogen injection and timing
hydrogen manifold injection methods lead to low volumetric
efficiency and poor power output since hydrogen displaces
intake air [7]. By using direct hydrogen injection, these
handicaps can be overcome. However, direct hydrogen injec-
tion method requires a specially designed high-pressure
injector that resists high in-cylinder temperature, pressure,
and modification on cylinder head. Consequently, timing
hydrogen manifold injection is the most appropriate method
for hydrogen þ diesel dual fuel [8,16,31].
Hydrogen has unique combustion properties. For instance,
the flammability range of hydrogen is wider than other fuels
and the flame speed of hydrogen is higher than other fuels
[18,32,33]. On the other hand, the quenching distance of
hydrogen is shorter than other fuels. Thus, using hydrogen in
diesel fuels as additional fuel improves CO, CO2, HC, and PM
emissions since it does not contain any carbon atoms [22].
Bose and Maji [1] sent 0.15 kg/h hydrogen into intake
manifold on a single cylinder, and water compression ignition
engine and partial load conditions were tested. Also, 10% and
20% EGR were applied to prevent an increase in soot emis-
sions. According to their results, CO, CO2, THC, and soot
emissions were reduced with hydrogen addition, and NOx
emissions were dropped by means of EGR. Liew et al. [22]
investigated the effect of hydrogen on the combustion char-
acteristics of a four-stroke, six-cylinder diesel engine experi-
mentally. According to their results, the combustion duration
was shortened and the peak rate of heat release value
increased. Bari and Esmaeil [3] tested the effect of
hydrogen þ oxygen gas blend as supplementary fuel that is
produced by an alkaline electrolyzer in a four-cylinder, direct-
injection diesel engine at different engine loads. They found
that HC, CO, and CO2 emissions were improved, but NOx
emissions increased with hydrogen þ oxygen gas mixture
addition. K€ose and Ciniviz [7] studied on a four-cylinder,
water-cooled, direct-injection diesel engine to investigate ef-
fect of different percentage of (2.5%, 5%, and 7.5% on volume
basis) hydrogen on the engine performance. They obtained
that the increase in NOx emissions could not be prevented and
that HC, CO, and O2 emissions reduced with hydrogen
enrichment. Saravanan et al. [8] studied a single-cylinder,
water-cooled, direct-injection compression-ignition engine,
and 10 lpm constant hydrogen flow rate was supplied. In
addition, diesel fuel was injected into cylinder at 23
before
top dead center. Three hydrogen-injection duration (30
CA,
60
CA and 90
CA) and five hydrogen-injection advance
(À5
BTDC to 15
ATDC) were experimentally studied. 5
ATDC
hydrogen injection advance and 90
CA hydrogen injection
duration show the best emission and engine performance.
Miyamoto et al. [26] worked on hydrogen enrichment on a

single-cylinder, common-rail system in CI engine that has EGR
system. They controlled engine noise and NOx emission with
two different strategies. These strategies are low-temperature
combustion in which diesel fuel injection advance was
delayed until 2
after top dead center and EGR method.
Stringent emission regulations and fast depletion of fossil
fuels force researchers to study on a new, clean, and renew-
able alternative fuel to use in ICEs. Also, using after-treatment
systems on road vehicles becomes obligatory to meet strin-
gent emission regulations. However, the cost of after-
treatment systems is close to engine cost because of high
expense of catalyst materials. For this reason, hydrogen,
which has unique combustion properties, is an attractive
alternative fuel. It is an eco-friendly fuel, since neither carbon-
based pollutants nor GHG emittes are formed with combus-
tion of hydrogen. In a previous study, Sandalcı et al. [34]
investigated the effect of 0%, 16%, 36%, and 46% hydrogen
enrichment (energy basis) on performance and emission
characteristics of a compression ignition engine at 100% en-
gine load and 1300 rpm constant engine speed, which is equal
to C engine speed of European Stationary Cycle (ESC). In this
study, the effect of 0%, 22%, and 53% hydrogen addition on
energy basis is investigated at 1100 rpm constant engine
speed, which is equal to A engine speed of ESC. Our aim is to
investigate the effect of a different level of hydrogen addition
at low engine speed since compression ignition engines usu-
ally have emissions (especially smoke emission). However,
increasing amounts of hydrogen level can overcome this
problem; the formation of oxides of nitrogen can be affected
negatively. For this reason, investigation of effect of different
levels of hydrogen addition (0%, 22% and 53%) on performance
and emissions (especially on smoke and NOx emissions) at full
load and 1100 rpm engine speed has been performed in the
current study. Because of the combustion problems after 53%
hydrogen enrichment (especially backfire and pre-ignition),
this level could not be exceeded. Furthermore, the effect of
hydrogen addition on combustion characteristics is investi-
gated elaborately.
Experimental setup
An experimental study was performed on the emissions,
performance, and combustion characteristics of a hydrogen-
enriched diesel engine at 1100 rpm constant engine speed.
All of the tests were done at the IC Engines Laboratory at Yildiz
Technical University. The test cell and the compression igni-
tion engine were adapted to work with hydrogen in the
laboratory.
Test engine and dyno
A single-cylinder, four-stroke, naturally aspirated, water-
cooled, CFR (cooperative fuel research) engine was operated.
It can be operated as both SI (spark ignition) and CI
(compression ignition) engine by means of its variable
compression ratio. The CFR engine was operated on CI mode,
and the compression ratio was set to 19:1. The engine was
coupled with a DC dynamometer, and the engine torque was
measured by a load cell. Details about the test engine and DC
dyno are presented in Table 1. A TLF 23 NewFlow thermal
mass flow meter and a VZ 0.2 AL-S Sika miniature turbine-
type positive-displacement flow meter were used to mea-
sure hydrogen flow rate and diesel flow rate, respectively. An
inclined tube manometer was used to measure pressure dif-
ference in air where air is introduced into an air tank and
passes into an orifice. Air flow rate was calculated with ob-
tained pressure difference in orifice. Engine coolant temper-
ature was measured with K-type thermocouples. Schemetic
view of the experimental setup is illustrated in Fig. 1. The
diesel fuel consumption, hydrogen fuel consumption, the
load-cell, AVL Dicom 4000 exhaust gas analyzer, and AVL 415S
exhaust analyzer were connected via a USB-type data acqui-
sition card (NI 6215) and it was connected to a personal
computer. Using Labwiev software, a program interface was
developed to collect data.
H2 fuel line
A schematic diagram of hydrogen fuel line was given in Fig. 2.
Hydrogen fuel was stored in a high-pressure tank, and the
pressure of hydrogen was dropped using a double-stage
pressure regulator (SS 316). A shut-off valve was installed
online to prevent a possible backfiring. A flame arrester and a
check valve were installed before the hydrogen was sent into
the intake manifold to prevent an explosion. A relief valve was
set into the fuel line to avoid overpressure, and a discharge
line was set, which purged overpressure outside the labora-
tory. A line regulator was used to fix line pressure to 4 bar. A
rotameter and a thermal mass flowmeter were used, which
were calibrated to hydrogen. A Keihin LPG-CNG gas injector
was used to inject hydrogen into an intake manifold. A self-
developed electronic control unit was used to control gas
injector. The material of hydrogen line is stainless steel, and
the line was durable up to 250 bar.
Tail-pipe emission measurement
CO, THC, and NOx emissions were measured by AVL Dicom
4000 exhaust gas analyzer, and the smoke emission was
measured by AVL 415S smoke analyzer. However, AVL Dicom
4000 measures CO and THC emissions as %vol.; NOx emissions
as ppm, respectively, and AVL 415S measures as FSN or mg/
m3 all were converted into g/kWh according to VDMA Exhaust
Emission Legislation to obtain more general results.
Table 1 e The test engine and dyno specifications.
Engine manufacturer Ferryman 4-stroke
CFR engine
Aspiration Natural
Number of cylinders 1
Bore Â stroke [mm] 90 Â 120
Cylinder volume [cm3
] 799
Compression ratio 19 (adjusted for this study)
Scavenge volume [cc] 765
Speed range minemax [rpm] 600e2000
Number of intake exhaust
valves
1 1
Cooling Water cooled
Dyno type power [kW] DC 7.5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0 3

Experimental procedure
A single-cylinder, 4-stroke, water-cooled CFR engine was
coupled with a DC dynamometer, and all the experiments
were carried out with this test bench. The hydrogen fuel that
was used in the tests has 99.99% purity. Diesel fuel met EN 590
standard, and it was injected into cylinder 22oBTDC (before
top dead center) during compression stroke. In addition,
hydrogen fuel was injected into intake manifold at TDC (top
dead center) during intake stroke. All of the tests were per-
formed at the ESC (European stationary cycle) at 1100 rpm
engine speed for test engine and 100% engine load (full load)
condition. Three different hydrogen energy fractions, which
are 0%, 22%, and 53% of total fuel, were applied and compared
with each other. Firstly, the test engine was warmed up until it
reached regime temperature (coolant inlet and coolant outlet
temperature), and after steady-state operating conditions
were provided, the tests were conducted. This procedure was
repeated for all hydrogen energy fractions (0%, 22%, and 53%).
The engine brake power value was fixed at the same value
(3.07 kW). Details about experimental procedure are shown in
Table 2. Any combustion problems such as backfiring, pre-
ignition, or knock were not seen during experimental tests.
The accuracies and calculated uncertainties according to
Kline and McClintock method [35] are given in Table 3.
Results and discussion
Brake thermal efficiency
The variation of brake thermal efficiency versus hydrogen
energy fraction is presented in Fig. 3. The brake thermal effi-
ciency decreased with increasing hydrogen amount. The
brake thermal efficiency is obtained as 29.43 at 0% hydrogen
energy fraction (pure diesel), as 27.06 at 22% hydrogen energy
fraction, and as 23.04 at 53% hydrogen energy fraction. The
brake thermal efficiency is reduced to 8.02% and 21.69% with
22% and 53% hydrogen addition on energy basis compared
with pure diesel (0% hydrogen), respectively. Normally,
hydrogen flame speed is much higher than diesel and other
Fig. 1 e Schematic view of the experimental setup.
Fig. 2 e Schematic diagram of the hydrogen fuel line.
Table 2 e Experimental conditions.
Engine
brake
power
Engine
speed
Start of
diesel
injection
H2 energy
fraction
Diesel
energy
fraction
3.07 kW 1100 rpm 22
BTDC 0% 100%
3.07 kW 1100 rpm 22
BTDC 22% 78%
3.07 kW 1100 rpm 22
BTDC 53% 47%

petroleum-based fuels [36,37]. For this reason, hydrogen-
enriched diesel fuel burns more completely and peak-in-
cylinder pressure increases with hydrogen addition [3].
Nevertheless, Varde and Frame [38] found that hydrogen
addition can decrease brake thermal efficiency. This is
because of the position of the piston during peak-in-cylinder
pressure. The diesel injection advance should be modified to
prevent this inefficiency. Moreover, Owston et al. [39] showed
that heat flux of hydrogen is higher than other conventional
fuels. The thermal loss increases with hydrogen addition. The
results of this study are consistent with the results of Varde
and Frame [38] and Zhou et al. [40] in terms of brake thermal
efficiency.
Brake-specific fuel consumption
The variation of brake-specific fuel consumption (BSFC)
versus hydrogen energy fraction is depicted in Fig. 4. The BSFC
is found according to diesel fuel, and consumed hydrogen fuel
value was converted into diesel fuel according to LHVs (lower
heating values) of fuels. Then, the equivalent diesel value of
consumed hydrogen and consumed diesel fuel was added.
The BSFC value was increased with hydrogen addition as
shown in Fig. 5. With 0%, 22%, and 53% hydrogen addition of
total fuel on energy basis, the BSFC values were obtained as
287.8, 312.8, and 367.5, respectively. Increases of 8.72% and
27.71% were calculated for 22% and 53% hydrogen addition
according to pure diesel (0% hydrogen). In this study, the BSFC
is negatively affected from increasing percentage of hydrogen,
since injection advance of diesel injector was not modified
according to hydrogen enrichment, and combustion period is
not appropriate for the piston top dead center as acknowl-
edged in the work of Varde and Frame [38]. Furthermore, with
an increasing amount of heat flux with hydrogen addition, the
thermal efficiency is affected negatively [40]. Similar results
regarding BSFC were found in a study by Zhou et al. [40] and
Varde and Frame [38].
Table 3 e Accuracies and the calculated uncertainties.
Measured
parameter
Measurement
device
Accuracy
Engine torque Load cell ±0.05 Nm
Engine speed Incremental encoder ±5 rpm
Diesel flow rate Sika VZ 0.2 ±1% (of reading)
H2 flow rate New-Flow TLF 23 ±1% (F.S.)
CO AVL DiCom 4000 0.01% Vol.
CO2 AVL DiCom 4000 0.1% Vol.
HC AVL DiCom 4000 1 ppm
NOx AVL DiCom 4000 1 ppm
Smoke AVL 415S 0.4% Vol.
Calculated results Uncertainty
Power ±0.43%
BSFC ±1.08% (0% H2)
±1.59% (22% H2)
±1.38% (53% H2)
Hydrogen energy fraction [%]
]%[ycneiciffelamrehtekarB
0
10
20
30
40
0 22 53
Fig. 3 e Effect of hydrogen enrichment (0%, 22% and 53% on
energy basis) on the brake thermal efficiency at 1100 rpm
constant engine speed and 3.07 kW constant engine
power.
]hWk/g[cfsB
0
100
200
300
400
0 22 53
energy basis) on the brake specific fuel consumption at
1100 rpm constant engine speed and 3.07 kW constant
engine power.
]hWk/g[OC
0,0
0,5
1,0
1,5
2,0
2,5
3,0
0 22 53
energy basis) on CO emission at 1100 rpm constant engine
speed and 3.07 kW constant engine power.

Carbon monoxide
Carbon monoxide emissions versus hydrogen energy content
is illustrated in Fig. 5. CO emission was measured as 2.52 g/
kWh under 0% hydrogen, 0.82 g/kWh under 22% hydrogen,
and 0.77 g/kWh under 53% hydrogen energy fraction. The
improvement in CO emissions may be higher, but AVL Dicom
4000 exhaust analyzer could not measure values lower than
0.01% by volume. Therefore, the improvement in CO emis-
sions is more than obtained results. However, 67.2% and 69.2%
improvements were obtained with 22% and 53% hydrogen
enrichment compared with neat diesel, respectively. The
diffusion coefficient of hydrogen is higher than other con-
ventional fuels, and it causes a decrease in heterogeneity of
combustible mixture. The availability of oxygen increases
with hydrogen enrichment [41]. According to the results, only
a small amount of carbon monoxide is emitted from the en-
gine because of pilot diesel fuel and lube oil [1]. Bari and
Esmaeil [3], Bose and Maji [1], and Zhou et al. [40] obtained
similar results with this study in terms of carbon monoxide.
Total hydrocarbon
Hydrocarbons are formed because of the incomplete com-
bustion of fuel, and they are organic-based compounds. The
unburned or partially burned hydrocarbons (HC) in tail pipes
are usually known as total hydrocarbon (THC) [4]. Fig. 6 shows
the variation of total hydrocarbons versus hydrogen energy
fraction. The THC emissions are measured as 0.025 g/kWh,
0.032 g/kWh, and 0.046 g/kWh with 0%, 22%, and 53% hydrogen
enrichment, respectively. The accuracy of AVL Dicom 4000
exhaust analyzer is 1 ppm, and it could not measure differ-
ences below 1 ppm. For this reason, although THC emissions
were increased with increasing percentage of hydrogen
enrichment, the differences of THC values were very small and
could be ignored. In addition to this, the measured THC values
are below the regulation limits. Zhou et al. [40] found that
hydrocarbon emissions increased at some partial engine loads
with hydrogen addition. Moreover, the effect of hydrogen
enrichment for high-engine loads on THC emissions can be
ignored, according to their study. The results of Zhou et al. [40]
are consistent with results of this study.
Smoke
The variation of smoke emissions versus hydrogen energy
fraction is given in Fig. 7. A great improvement was achieved
with hydrogen enrichment. The smoke emission level
decreased to 1.29 g/kWh from 2.91 g/kWh with 22% hydrogen
enrichment on energy basis. The smoke value was decreased
to 0.94 g/kWh from 2.91 g/kWh with 53% hydrogen enrich-
ment. A reduction of 43.6% and 58.6% was obtained with 22%
and 53% hydrogen addition, respectively, in comparison to
neat diesel (0% hydrogen). Only water is emitted from
hydrogen fuel combustion, and it does not reveal any partic-
ulate matter [1]. Moreover, the improvement in smoke emis-
sions was achieved because hydrogen fuel does not contain
any carbon element [8]. Also, higher diffusion coefficient of
hydrogen in air provides a decrease in the heterogeneity of the
mixture, the oxygen availability is improved, and conse-
quently combustion efficiency is increased with hydrogen
addition [24,45]. For this reason, smoke emissions decreased
with an increase in hydrogen percentage. Zhou et al. [40] and
Bose and Maji [1] found similar results with this study in terms
of smoke emissions.
Oxides of nitrogen
Effect of hydrogen addition (0%, 22%, and 53% on energy basis)
on the NOx emission at 1100 rpm constant engine speed and
3.07 kW constant engine power is depicted in Fig. 8. With
increasing percentage of hydrogen, specific NOx emissions
increase. However, only a slight increase was measured with
22% hydrogen addition on energy basis, a dramatic rise
observed with 53% hydrogen addition compared with neat
diesel (0% hydrogen). Specific oxides of nitrogen emissions
(NOx) increased to 3.72 g/kWh from 3.33 g/kWh with 22%
hydrogen addition according to neat diesel. Specific NOx
]hWk/g[CHT
0,00
0,01
0,02
0,03
0,04
0,05
0 22 53
energy basis) on THC emission at 1100 rpm constant
engine speed and 3.07 kW constant engine power.
]hWk/g[ekomS
0,0
0,5
1,0
1,5
2,0
2,5
0 22 53
energy basis) on the smoke emission at 1100 rpm constant

emissions increased to 11.23 g/kWh from 3.33 g/kWh with 53%
hydrogen addition. According to obtained results, an 11.73%
and 237.47% increase was calculated in NOx values with 22%
and 53% hydrogen addition compared with neat diesel,
respectively.
NOx emissions are basically dependent on reaction dura-
tion, gas temperature, and oxygen availability [7]. Normally,
hydrogen addition causes an increase in cylinder gas tem-
perature; therefore, it is assumed that NOx formation in-
creases with hydrogen addition [3]. Despite this, White et al.
[42] found that NOx formation is related to excess air ratio and
that it may decrease if there is not appropriate excess air ratio
value. Also, Rortveit et al. [43] showed that N2, He, and CO2
gases dilute in-cylinder combustible gas mixture, and they
found that NOx emissions may decrease significantly if the
level of gases exceeds critical value. Frassoldati et al. [44]
found that NOx emissions can be limited with dilution of
combustible mixture. Moreover, it is known that there are lots
of studies that use N2 to simulate EGR in internal combustion
engines (ICEs) [45,46].
On the other part, the adiabatic flame speed of hydrogen is
higher than other petroleum fuels [36,37]. The ideal thermo-
dynamic cycle is more reachable with hydrogen enrichment
thanks to unique combustion properties of hydrogen.
Furthermore, the LHV of hydrogen is about three times as
diesel fuel. Therefore, peak gas temperature value and gas
pressure value increase with increasing percentage of
hydrogen [36,37]. NOx emission could be increased with
hydrogen addition because increasing cylinder gas tempera-
ture with hydrogen addition is one of the most important
reasons for NOx formation. However, if the effects of hydrogen
(both dilution effect and peak cylinder gas temperature) are
considered together, a slight increase was seen in NOx emis-
sions with 22% hydrogen addition and a dramatic increase
was observed that with 53% hydrogen addition. According to
obtained results, the percentage of hydrogen has a huge effect
on NOx emissions. The results of this study are consistent
with the results of Pan et al. [47] and Zhou et al. [40].
Cylinder gas pressure
The effect of a different percentage of hydrogen enrichment
on cylinder gas pressure related to crank angle at 1100 rpm
engine speed and full engine load is depicted in Fig. 9. It is
shown that peak-in-cylinder pressure value increases pro-
portionally with an increasing amount of hydrogen. Results
show that peak in cylinder rises to 59.82 bar from 55.48 bar
with 22% hydrogen addition and peak in cylinder increases to
75.57 bar from 55.48 bar with 53% hydrogen enrichment on
energy basis at 3.07 kW engine power and 1100 rpm constant
engine speed. According to acquired results, 7.81% and 36.20%
increases were obtained with 22% and 53% hydrogen addition,
respectively. High flame speed of hydrogen provides rapid
combustion of combustible mixture, and peak-in-cylinder
temperature rises with an increasing amount of hydrogen,
especially with 53% hydrogen enrichment. The results of this
study are similar to those of the study by Christodoulou and
Megaritis [48].
Heat release rate
Fig. 10 shows the effect of different percentage of hydrogen
enrichment on heat-release rate related to crank angle at
1100 rpm engine speed and full engine load. With an
increasing amount of hydrogen, peak heat-release-rate value
increases proportionally. With hydrogen addition, explosive
type combustion is observed instead of classical diesel com-
bustion [8]. High flame speed of hydrogen increases the causes
to premixed combustion-phase-dominated heat-release rate.
The released energy value in premixed combustion phase is
increased and released energy value in mixing controlled
combustion phase is reduced with increasing hydrogen level.
However, combustion problems such as knock, backfiring,
and pre-ignition may happen with this type of explosive
combustion. For this reason, the hydrogen energy fraction did
not exceed 53% since knock problem was seen time to time
after this hydrogen level. The peak heat release rate increases
to 35.20 J/o from 27.99 J/o with 22% hydrogen enrichment.
NOx]hWk/g[
0
2
4
6
8
10
12
0 22 53
Fig. 8 e Effect of hydrogen addition (0%, 22% and 53% on
energy basis) on the NOx emission at 1100 rpm constant
Crank angle [degree]
250 300 350 400 450
In-cylinderpressure[bar]
0
20
40
60
80
0% hydrogen
22% hydrogen
53% hydrogen
Fig. 9 e Effect of different percentage of hydrogen
enrichment on cylinder gas pressure related to crank angle
at 1100 rpm engine speed and full engine load.

Also, peak heat release rate increases to 59.04 J/o from 27.99 J/o
with 53% hydrogen addition. The increases in heat release rate
are 25.77% and 110.94% with 22% and 53% hydrogen enrich-
ment compared with neat diesel fuel, respectively. It is clear
that an explosive type combustion period occurred with
hydrogen addition due to hydrogen injection into intake air.
The results of Saravanan and Nagarajan [49] are consistent
with the results of this study in terms of heat-release rate.
Cylinder gas temperature
Effect of different percentage of hydrogen enrichment on in-
cylinder gas temperature related to crank angle at 1100 rpm
engine speed and full engine load is illustrated in Fig. 11. At
least 100 cycles were measured in terms of cylinder pressure,
and average values were used to calculate cylinder gas tem-
perature. First of all, using a Kistler 6061B pressure sensor
cylinder gas pressure values were obtained, then cylinder gas
pressure was found using PV ¼ mRT equation. The obtained
maximum cylinder gas values were 1360
C, 1355
C, and
1453
C for 0%, 22%, and 53% hydrogen enrichment, respec-
tively. The maximum cylinder temperature was reduced to
0.39% with 22% hydrogen addition compared with pure diesel
situation. Also, the maximum cylinder temperature increased
to 6.85% with 53% hydrogen enrichment according to neat
diesel. The radicals of O and OH accelerate the reaction with
hydrogen addition, and the combustion duration may be
shortened; thus, local cylinder gas temperature and pressure
values increased [22]. Furthermore, higher flame speed of
hydrogen than petroleum-based fuels shortens combustion
duration. On the other hand, the exhaust losses and cooling
losses may be improved with hydrogen addition and, thus,
increased cylinder gas temperature with hydrogen addition is
not a surprise [6]. However, a slight decrease was obtained
with 22% hydrogen addition. This unimportant decrease in
cylinder gas temperature is related to the position of the pis-
ton in cylinder during the combustion period. Since injection
advance of diesel injector was not modified and optimized
according to dual fuel combustion, a slight decrease occurred.
Indicator diagram
Fig. 12 shows the effect of different percentage of hydrogen
enrichment on cylinder gas pressure related to cylinder vol-
ume at 1100 rpm engine speed and full engine load. According
to obtained results, an instantaneous and explosive type
combustion period is shown with hydrogen enrichment on
PeV diagram (pressureevolume diagram). On the other hand,
exhaust losses and incomplete combustion losses improved
with hydrogen addition thanks to unique combustion prop-
erties of hydrogen. Higher diffusion coefficient of hydrogen
compared to diesel fuel provides more homogenous
combustible mixture. Also, higher flame speed of hydrogen
compared to fossil fuels causes a more complete combustion.
Similar results were obtained in the study by Christodoulou
and Megaritis [48].
300 320 340 360 380 400 420 440
Heatreleaserate[J/oCA]
-20
0
20
40
60
80
0% hydrogen
22% hydrogen
53% hydrogen
enrichment on heat release rate related to crank angle at
1100 rpm engine speed and full engine load.
0 100 200 300 400 500 600 700
In-cylindertemperature[oC]
0
200
400
600
800
1000
1200
1400
1600
0% hydrogen
22% hydrogen
53% hydrogen
enrichment on cylinder gas temperature related to crank
angle at 1100 rpm engine speed and full engine load.
enrichment on cylinder gas pressure related to cylinder
volume at 1100 rpm engine speed and full engine load.

Conclusion
According to the study, the engine performance (BSFC and
brake thermal efficiency), emissions (CO, THC, smoke, and
NOx) were tested, and combustion characteristics (in-cylinder
pressure, heat release rate, in-cylinder temperature and in-
dicator diagram) was analyzed on a single-cylinder CFR en-
gine at 1100 rpm constant engine speed, 3.07 kW constant
brake engine power, and different hydrogen levels (0%, 22%,
and 53%).
Obtained test results are listed below:
a. The brake thermal efficiency is decreased with an
increasing amount of hydrogen. The brake thermal effi-
ciency is obtained as 29.43 at 0% hydrogen energy fraction
(neat diesel), as 27.06 at 22% hydrogen energy fraction, and
as 23.04 at 53% hydrogen energy fraction. The brake ther-
mal efficiency is reduced to 8.02% and 21.69% with 22% and
53% hydrogen addition, respectively, on energy basis
compared with neat diesel.
b. The CO emission was measured as 2.52 g/kWh under 0%
hydrogen, as 0.82 g/kWh under 22% hydrogen and 0.77 g/
kWh under 53% hydrogen energy fraction. The improve-
ment in CO emissions may be higher, but AVL Dicom 4000
exhaust analyzer could not measure values lower than
0.01% by volume. Therefore, the improvement in CO
emissions is supposed to be much more than obtained
results.
c. THC emissions are measured as 0.025 g/kWh, 0.032 g/kWh,
and 0.046 g/kWh with 0%, 22%, and 53% hydrogen enrich-
ment, respectively. The accuracy of AVL Dicom 4000
exhaust analyzer is 1 ppm, and it could not measure dif-
ferences below 1 ppm. However, THC emissions were
increased with an increasing percentage of hydrogen
enrichment. The differences of THC values are very small,
and they could be ignored. Also, the measured THC values
are below the regulation limits.
d. A great improvement was achieved with hydrogen
enrichment on smoke emissions. The smoke emission
level dropped to 1.29 g/kWh from 2.91 g/kWh with 22%
hydrogen enrichment on energy basis. The smoke value
decreased to 0.94 g/kWh from 2.91 g/kWh with 53%
hydrogen enrichment. A reduction of 43.6% and 58.6% was
obtained with 22% and 53% hydrogen addition, respec-
tively, according to neat diesel.
e. A remarkable result was obtained on oxides of nitrogen
formation with hydrogen enrichment. According to ob-
tained results, an 11.73% increase on NOx emissions was
found with 22% hydrogen addition, and a dramatic in-
crease (237.47%) was obtained with 53% hydrogen enrich-
ment compared with neat diesel fuel.
f. Maximum cylinder gas pressure values increased by
7.81% and 36.20% with 22% and 53% hydrogen addition,
according to pure diesel fuel, respectively. Peak heat
release rate values with hydrogen addition increased by
25.77% and 110.94% with 22% and 53% hydrogen enrich-
ment in comparison to those with neat diesel,
respectively.
Acknowledgement
This research was supported by the Yıldız Technical Univer-
sity Scientific Research Projects Coordination Department.
Project Number: 2011-06-01-YULAP01. Also, the authors are
indebted to Tu¨ rk Trakt€or ve Ziraat Makineleri A.S‚ . for test
apparatus and equipment donation.
Nomenclature
BSFC Brake specific fuel consumption
CFR Cooperative fuel research
CI Compression ignition
CNG Compressed natural gas
CO Carbon monoxide
CO2 Carbon dioxide
ECU Electronic control unit
GHG Greenhouse gases
H2 Hydrogen molecule
HC Hydrocarbons
HCCI Homogenous charge compression ignition
He Helium
IC Internal combustion
ICE Internal combustion engine
LPG Liquefied petroleum gas
N2 Nitrogen
NOx Oxides of nitrogen
O2 Oxygen molecule
SI Spark ignition
THC Total unburned hydrocarbons
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