Mahle paper published in springer on Advanced Piston Technologies for Gasoline and Diesel Engine Applications to Meet EU6 Emission Norms

123
Smart Innovation, Systems andTechnologies 65
Amaresh Chakrabarti
Debkumar Chakrabarti Editors
Research into Design
for Communities,
Volume 1
Proceedings of ICoRD 2017

Advanced Piston Technologies
for Gasoline and Diesel Engine
Applications to Meet EU6 Emission Norms
M. Selvaraji, K. Pramod, S. Sathishbabu, M. Rajkumar, Qi Li,
Shaobo Zeng and Haixiao Chai
Abstract The Indian government announced that the country would adopt Bharat
Stage VI (Euro VI) norms by 2020, skipping Euro V. In order to meet EU6
emission targets, the technologies which are available for Gasoline vehicles are;
Gasoline Direct Injection (GDI) engine, Three-Way Catalytic Converter (TWC),
Exhaust Gas Recirculation (EGR) and Universal wide range oxygen sensors. The
vehicles with diesel engines having Common Rail Direct Injection systems (CRDI)
with variable fuel timing and metering strategies and Injection pressure is up to
2100 Bar, Variable Geometry Turbocharger (VGT), cooled high-pressure EGR,
Diesel Oxidation Catalysts (DOC), diesel particulate Wall-flow type filter (DPF),
Lean-NOx Traps (LNT) technology (<3.0 L) [Sanchez et al., Estimated Cost of
Emission Reduction Technologies for Light-Duty, 2012, 4], Selective Catalytic
Reduction (SCR) technology (>3.0 L) are used in combination to control the
emissions. For gasoline applications with high peak cylinder pressures (PCP) and
high specific power output (kW/L), the pistons can include a cooling gallery and/or
top ring groove reinforcement with Hard Anodising or Ring carrier. To handle the
high loads and particulate matter that are being re-circulated into the engine
cylinder due to exhaust gas recirculation system to control NOx formation. Diesel
engine pistons include features to enhance bowl rim life like TopCast or Bowl rim
remelting is applied to refine the micro structure to achieve higher fatigue strength
M. Selvaraji (&) Á K. Pramod Á S. Sathishbabu Á M. Rajkumar
MAHLE Engine Components India Pvt. Ltd, Chennai, India
e-mail: selvaraji.muthu@in.Mahle.com
K. Pramod
e-mail: pramod.kawade@in.Mahle.com
S. Sathishbabu
e-mail: sathishbabu.sankarachary@in.Mahle.com
M. Rajkumar
e-mail: rajkumar.mani@in.Mahle.com
Q. Li Á S. Zeng Á H. Chai
MAHLE Technologies Holding (China) Co., Ltd, Shanghai, China
© Springer Nature Singapore Pte Ltd. 2017
A. Chakrabarti and D. Chakrabarti (eds.), Research into Design
for Communities, Volume 1, Smart Innovation, Systems
and Technologies 65, DOI 10.1007/978-981-10-3518-0_68
789
selvaraji.muthu@in.mahle.com

at elevated temperatures, and selective area Hard anodising. For very highly loaded
applications, MAHLE Monotherm, Monoweld or Monolite Steel pistons are used.
Keywords Gasoline and diesel piston technology Á GDI Á CRDI Á Steel piston Á
PVD Á BS6 (EU6) emission norms
1 Introduction
As per the latest announcement, BS-VI will be compulsory for all new vehicles in
all states of India by April 2020. At present India has enforced BS-IV in NCR and
thirteen major cities while rest of the India are still having norms below BS-IV. The
major difference in fuel to go from BS-IV to BS-VI is in its Sulphur content [4].
Major OEMs have the technology for BS-VI ready as they are producing EU-VI
vehicles for the European countries. The challenge lies in developing the vehicles
for Indian operating conditions in the given short period, as such design changes
require rigorous testing. In this paper we will be discussing briefly about the dif-
ferent technologies used by OEMs for EU-6 compliant vehicles. The technologies
used in piston assembly design for the EU-VI compliant vehicles will be discussed
in detail. The paper will conclude with the feasibility of using the technologies
related to piston assembly design for BS-VI. The Bharat Stage norms have been
derived from Euro norms styled to suit specific needs and demands of Indian
conditions.
Figure 1 shows the implementation of EU norms and BS norms by year.
Currently in Europe, EU6c norms are being used. In most of the India, BSIII norms
are applied, with some cities having BSIV norms. At present India is not going to
apply the CO2 emission levels and vehicle efficiency that is being applied in
Europe. RDE is the Real World Driving Emissions, which is the vehicle emissions
at actual running conditions, and is being developed in Europe for implementation
in the coming years. Looking at the trend of emission regulation in Europe, India
will be following the same in coming years. Till now India is using Modified Indian
Driving Cycle (MIDC) which is equivalent to the New European Driving Cycle
(NEDC) based testing. With implementation of EU6 norms, India will be incor-
porating the worldwide harmonized Light Vehicle Test Cycle (WLTC) procedure.
1.1 Engine Exhaust System
The Exhaust system of any automobile in general consists of the components like
(i) Exhaust Manifold, (ii) Catalytic converter, (iii) Resonator, (iv) Muffler and
(v) Tail Pipe [5]. The additional devices provided in between engine and tail pipe
such as the Catalytic Converter and Muffler, performs the function of removal of
hazardous elements from the exhaust and noise reduction respectively. Limitations
790 M. Selvaraji et al.

on fuel cleanliness and the combustion process leaves some unwanted emissions
from the engine that are hazardous and cannot be left untreated. The major function
of the Catalytic converter is to ﬁlter and treat engine emissions like CO, NOx and
HC [6].
1.2 Emission Norms
Governments all over the world are implementing stringent norms to have cleaner
air standards. Bharat stage standards are the adoption of the EURO standards. The
complete guideline for BS VI norms has not been laid down by the country but it is
pretty obvious that it is going to follow the EU VI norms.
It is clear from Table 1 that while moving from EU IV to EU VI, for gasoline
vehicles NOx emissions decrease by 25% and other emission levels are the same.
For diesel vehicles, CO emissions reduce by 68%, NOx by 82% and HC emission
remains the same. In addition to the above, a limit on PM emissions of 0.005 and
0.009 is imposed on gasoline and diesel vehicles respectively.
Fig. 1 Evolution of emission norms
Table 1 Emission levels for EU4 versus EU6 [4]
Emission
targets in
g/km
EU IV EU VI % reduction
CO NOx HC PM CO NOx HC PM CO NOx HC PM
Gasoline 1 0.08 0.1 − 1 0.06 0.1 0.005 0 25 0 −
Diesel 0.25 0.025 0.5 0.05 0.08 0.005 0.5 0.009 68 82 0 82
Advanced Piston Technologies for Gasoline and Diesel … 791

2 Vehicle Emissions and Control Strategies
The ideal combustion of fuel inside the combustion chamber should have CO2 and
H2O as the by-product. However, due to the complex combustion process and
presence of other gasses and contaminates, it produces undesirable gases like CO
and NOx. In addition to CO and NOx, Hydrocarbons (HC) and Particulate matter
(PM) is also present in engine exhaust gas as a result of incomplete combustion [3].
The particulate matter is present in all diesel engines and in Gasoline Direct
Injection (GDI) engines. NOx is produced as a result of a reaction between oxygen
and nitrogen at high temperature. In other words we can say that a lean mixture
provides enough oxygen for complete combustion of hydrocarbons but it causes
high temperature in the combustion chamber and also makes extra oxygen available
for NOx formation. The emission control from an engine is a trade off between HC
and NOx. The method to reduce exhaust emission from a vehicle includes both
In-cylinder control and after treatment [4]. Both these controls needs to be designed
as a single unit, to work in synchronization with each other to obtain a desired and
economic pollution control.
If we look at the in-cylinder combustion controls, NOx and CO2 are the
by-products of combustion and can be reduced by controlling the combustion
parameters like air management, ignition timings, fuel injection control etc. One of
the major sources of hydrocarbon emission is the un-burnt fuel left in the gap
between top land and cylinder. Also the lubrication oil carried by oil rings into the
combustion chamber gets burned and causes pollution. The blow-by which is the
leakage of hot combustion gases into the crank case contributes to the crankcase
hydrocarbon emission which is a major concern in any automobile [2]. This shows
that the design of a piston assembly which consists of a piston, piston rings,
gudgeon pin, circlip and conrod is of utmost importance if we are looking for
emission control. Besides the emissions of NOx, CO and HC, the design of the
piston assembly is also responsible for some part of the fuel economy of the
vehicle, as the friction in the engine is one of the major sources of energy loss.
There are two ways to control the emission produced by any engine In-cylinder and
after-treatment control.
2.1 In-Cylinder Control
Gasoline Engine
The total In-cylinder emission control methods work as a system which is designed
to meet emission regulations while maintaining performance of the engine. Most of
the in-cylinder control strategies have the objective of reducing the temperature and
extra oxygen inside the combustion chamber. The various technologies used today
are namely, (i) Electronic Control Unit (ECU), (ii) Exhaust Gas Recirculation
(EGR), (iii) Heated Oxygen Sensor (HO2S), (iv) Variable Valve Timings (VVT),

(v) Engine Downsizing, (vi) Gasoline Direct Injection (GDI) and (vii) Multi Point
Fuel Injection (MPFI) [4].
Diesel Engine
Diesel engine In-cylinder emission control is more complex compared to the
gasoline engine and this is mainly because of the difference in fuel ignition
methods. In order to have a complete combustion, fuel should mix properly with the
compressed air. In order to achieve this, the combustion bowl design and fuel
injection system have a major role to play. The various technologies available for
diesel engines are, (i) Turbo-charging (Variable Geometry Turbine VGT),
(ii) Cooled EGR, (iii) Electronically Controlled IDI or DI, (iv) Common Rail Fuel
Injection and (v) Variable Fuel Injection Timings [3].
2.2 After-Treatment Control
Gasoline Engine
The market trend of going towards direct injection has made a use of Gasoline
Particulate Filter (GPF) and CC catalyst mandatory in many engines. Some of the
after-treatment technologies are, (i) Three-Way Catalytic Converters,
(ii) Closed-Coupled catalyst, (iii) Optimized Wash-Coats design and (iv) Gasoline
Particulate Filters [4].
Diesel Engine
A Diesel oxidation catalyst (DOC) is used to convert incomplete combustion
products (CO and HC) into CO2 and H2O. The particulate matter from a diesel
engine is ﬁltered using a Diesel Particulate Filter (DPF). Nitrogen traps are used to
remove NOx from an exhaust. The after treatment emission control methods for
diesel engine are not independent of In-Cylinder emission control systems. They
have to be synchronised. For example, the DOC needs to warm up fast in order for
it to be effective similarly the DPF needs to be regenerated at certain intervals. For
this purpose, fuel injection strategies need to be controlled. Some of the
After-treatments for diesel engines are, (i) Diesel Oxidation Catalyst, (ii) Diesel
Particulate Filter and (iii) Lean Nitrogen Trap [6].
2.3 Applicability of Different Technologies for Gasoline
Engines
Table 2 shows the applicability of different technologies for different Euro norms.

2.4 Applicability of Different Technologies for Diesel
Engine
Table 3 shows the various technologies used in EU6 compatible vehicles. From
In-cylinder emission control Common Rail Injector System, Variable Geometry
Turbocharger, Variable fuel timing and metering strategies, High fuel injection
pressure, Cooled EGR are the some of the technologies used [7].
Table 2 Gasoline engine technologies
<EU3 EU4 EU5 EU6
Three-Way catalytic Converter (TWC )
single O2 sensor
Multipoint fuel injection (MPFI)
Combustion improvements through engine
Incremental improvements in air-fuel
After-treatmentsystems.
Universal wide range oxygen sensors
underfloor (UF) catalyst
close-coupled (CC) catalyst for cold start
Exhaust Gas Recirculation (EGR)
more responsive heated oxygen sensors
secondary oxygen sensors after the catalyst
Technology Matrix Norms
electromechanical distributors
Electronic ignition
In-cylindercontrol
Gasoline direct injection (GDI)
Table 3 Diesel engine technologies
<EU3 EU4 EU5 EU6
<900 1600 1900 2100
Without inter-cooling
With inter-cooling
Variable Geometry Turbocharger (VGT)
Mechanically activated EGR circuits
Cooled EGR
Cooled, electronically controlled and
solenoid-Operated EGR.
Cooled EGR with a DC motor actuator
Cooled high-pressure EGR.
DPF with active regeneration.
Wall-flow DPF
Technology
Matrix
Norms
Indirect fuel injection
Common-rail or unit-injector systems
Injection pressure. Bar
Electronic fuel timing
in-cylindercontrol
Mechanical rotary pump fuel injection systems
Electro-mechanical cam-controlled fuel injection
Rotary fuel injection system
Variable fuel timing and metering strategies
Naturally Aspirated (NA)
Turbocha
rging
Electronic assistance for fuel metering
Selective Catalytic Reduction (SCR ) (> 3.0 L).
ExhaustGas
Recirculation
(EGR)
After-treatment
systems.
DPF.
Diesel oxidation catalysts DOC
Lean-Nox Traps (LNT) (< 3.0L)

3 Piston Assembly
A piston assembly consists of a Piston, Piston rings, Gudgeon pin and Circlips. The
function of a piston assembly is to transfer the pressure force of the hot gasses to the
crankshaft, where it gets converted into the rotary movement of the crankshaft. The
function of sealing the combustion chamber is done by the top and second com-
pression ring. The second ring also does the function of scrapping extra oil off the
cylinder surface. The third ring is called the oil control ring and does the function of
retaining the oil in the sump and lubricating the cylinder wall. The temperature of a
piston can reach as much as 350 °C dependent on the engine type, capacity, ignition
timing and piston design etc. [2]. The piston with rings, pin and a part of the conrod
which is towards the small end altogether becomes an oscillating mass.
3.1 Piston Design Considerations
Design of a piston is mainly a trade off between weight and strength. The piston
also forms a variable combustion chamber so its design is also influenced by the
emission requirements. The major parameters of the design are the compression
height, ring land height, cooling of piston, combustion bowl or piston crown and
pin boss. Similarly, a reduction in top land height of the piston results in reduced
HC emission from the engine. Reduced hydrocarbons emissions demands the
minimum possible height of the top land, but as we reduce the height of the top land
it causes an increase in temperature of the ring grooves and pin bore, also it reduces
the strength of the crown. Higher temperatures in the first ring groove causes
burning of lube oil in the ring groove and land area and deposits carbon inside the
groove resulting into clogging of the groove and soot emission. To counter the
problem of high temperature, a cooling gallery can be provided. The gallery is
designed in such a way that it absorbs heat from the piston in an optimised way. If
the heat removed by the cooling oil in the gallery is too high, it will result in a high
temperature gradient in the piston crown and can result in high thermal stresses and
reduced life. In some of the pistons where specific power is high or fuel quality is
affecting groove wear, a ring carrier is used to avoid micro welding etc. between the
piston groove and ring.
The effective force transferred to crank reduces with increasing inertial force.
This indicates that as we move to higher operating speed, the oscillating mass
reduction becomes necessity for engine performance. Figure 2 shows the different
types of MAHLE gasoline pistons. The EvoLite is the latest light weight design,
and can be designed with Ring carrier and/or cooling gallery. The latest EvoLite
design is 38% lighter than the Box type of design with increased specific power [2].
The major weight reduction is achieved through reducing overall wall thickness and
improving the structural stiffness of the piston in combination with advanced
materials like M174+ which is stronger than M124. Figure 3 shows the temperature

distribution in a typical Evotec2 and EvotecSC (cooling gallery type) gasoline
piston.
For diesel applications, TopCast pistons are made using a special casting method
combined with a heat treatment to produce a ﬁne granular structure in the piston
bowl rim resulting in improved performance of the piston. The ﬁne distribution of
grains in the bowl rim of TopCast pistons compared to standard cast pistons results
in the improved life of the piston for the same level of stress amplitude. The highest
temperature in the piston for diesel piston is at the bowl rim which is the failure
prone area. By careful design of the bowl rim area, including TopCast, elevated
cooling galleries (eSC), applying selective Hard anodising, and inspecting the rim
for oxides with Eddy current inspection the life of the piston can be enhanced.
Figure 4 shows the effect of MAHLE’s eSC2 technology in reducing the temper-
ature by almost 50 K. Figure 4 shows the comparison of temperature between a
Fig. 2 Mahle piston evolution
Fig. 3 Effect of SC on temperature distribution [2]

standard cooling gallery and eSC2 cooling gallery design. This allows the top land
height to be reduced.
For very high loaded applications, the latest trend is to move towards the use of
steel pistons. Steel pistons allow reductions in compression height due to their
higher strength and thermal stability. This results in engine downsizing as well as
improved piston life. The disadvantage of the steel piston is its weight and cost.
3.2 Piston Ring Design Considerations
Piston rings are the metallic seals used to seal the combustion chamber. They also
provide the function of transferring heat from piston to cylinder. The rings also
prevent lubrication oil passing from crank case to combustion chamber. The radial
contact with cylinder is achieved through the spring force of the ring and it is
assisted by the gas pressure on the 1st and 2nd compression rings. The contact
pressure exerted by the ring on to the cylinder surface is a function of tangential
force of the ring which in turn is a function of open gap of the ring is given by the
below equation,
P ¼
2Ft
dh
ð1Þ
where, Ft is the tangential load of the ring, d is the bore diameter and h is the axial
height. Also one of the important parameter while designing piston ring is the
conformability, which is the ability of the piston ring to be in contact with deformed
cylinder bore. Gas pressure keeps the ring sitting on the piston ring groove, hence
assisting the sealing. Piston rings are one of the major unavoidable sources of
friction in a piston assembly. The blow-by gasses from the combustion chamber
ﬂows to the crankcase and is one of the primary cause of crank case emissions [4].
Sealing provided by the rings prevents the oil in the crank case from entering into
the combustion chamber where it would get burnt. Two things to look for while
designing a ring pack is the tangential force and surface coatings of the rings. In
passenger car piston assemblies the ring pack consists of three rings namely,
(i) First compression Ring, (ii) Second compression ring and (iii) Oil ring. The
major function of the ﬁrst compression ring is to seal the combustion chamber
during the combustion stroke. The second compression ring also has the function of
Fig. 4 Salt core and elevated salt core technologies

partially sealing the combustion chamber, and in addition it also helps by scrapping
the excess oil from the cylinder wall. The main function of oil control ring is to
retain the lubricating oil in the sump, and it also lubricates the cylinder wall for the
compression rings.
Piston Ring Coatings
The surface of the ring that faces the cylinder wall is called the running surface.
First rings are usually made from steel wire, and different surface treatments and
coatings applied to achieve the engine performance goals. If lower friction and
increased wear resistance is needed, the ring face can be coated with two layer or
three layer ion coatings. Double layered ion coatings applied by PVD (Physical
Vapour Deposition) involve a ﬁrst layer of chromium called as a chromium
interlayer followed by a CrN layer. The thickness of the coating can be 10–20 μ.
The rest of the surfaces of the rings remain as a nitride layer. The second ring is
generally a Napier ring or a scrapper ring as it scrapes and removes the excess oil
from the cylinder surface. The recess provided below the scrapping edge facilitates
effective removal of oil. The face of the Napier ring can be coated with chrome
plating in order to provide enhanced wear resistance if needed. The relative radial
wear reduces by 70% due to chrome plating when compared with a base material of
pearlitic grey cast iron.
The Oil ring can be a two piece ring or the three piece design. A two piece
design has an I-shaped ring and a spring behind the ring which provides the
additional radial force required for sealing. The i-shaped ring of a two piece design
generally has a series of slots or holes provided on the inner surface to allow oil to
pass. A three piece oil ring design has a spring sandwitched between two side rails.
The function of an oil ring is to remove the excess oil from the cylinder surface, but
provide an optimum amount for lubrication of the compression rings. Figure 5
shows the effect of total ring pack tension on CO2.
Fig. 5 Guidelines for selecting ring pack for diesel engine [4]

4 Conclusions
• Improved structural stiffness has helped reduce piston mass by 38 percent from
conventional Box type to Evolite design.
• The compression height can be reduced by 10% by using Evolite design
compared to a Box type piston.
• For gasoline pistons, a drop in temperature of 25 K can be achieved by pro-
viding a cooling gallery to an Evotec design piston.
• For diesel pistons, a temperature drop can be is achieved by enhanced cooling
with MAHLE’s eSC2 cooling gallery design.
• The durability of diesel aluminium alloy pistons is enhanced by a TopCast
designed piston by up to 5 times compared to a conventional diesel piston.
• MAHLE’s range of steel pistons, including Monotherm, Monoweld and
Monolite has made it possible to meet high power, long oil change and high
durability requirements for diesel engines.
• Ring pack frictional losses can be reduced by 20–50% with optimised rings in
terms of tangential load and high wear resistant coatings (PVD and DLC).
References
1. DieselNet. (n.d.).: Emission Standards India, Cars and Light Trucks. DieselNet: https://www.
dieselnet.com/standards/in/ld.php. Accessed May 2016
2. Mahle GmbH.: Pistons and Engine Testing. In: Mahle. (ed.) Vol. XIII. Vieweg + Teubner
Verlag (2012)
3. MECA: Emission Control Technologies for Diesel-Powered Vehicles. Manufacturers of
Emission Controls Association, Washington (2007)
4. Sanchez, P., Bandivadekar, A., German, J.: Estimated Cost of Emission Reduction
Technologies for Light-Duty. The International Council on Clean Transportation (ICCT),
Washington (2012)
5. Scharp, R.: Heavy Duty Composite Piston for Euro 6 and Beyond, vol. 70. Mahle GmbH,
Stuttgart (2009)
6. Schnitzler, J.: Particulate Matter and NOx Exhaust Aftertreatment Systems. FEV
Motorentechnik GmbH (2006)
7. Wieske, P., Lüddecke, B., Ewert, S., Elsäßer, A., Hoffmann, H., Taylor, J.: Optimization of
Gasoline Engine Performance and Fuel Consumption through Combination of Technologies.
Mahle GmbH, Stuttgart (2009)

Mahle paper published in springer on Advanced Piston Technologies for Gasoline and Diesel Engine Applications to Meet EU6 Emission Norms

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