Downloaded from SAE International by Suramya Naik, Thursday, July 06, 2017

2017-26-0056
Published 01/10/2017
Copyright © 2017 SAE International
doi:10.4271/2017-26-0056
saeeng.saejournals.org

Achieving Bharat Stage VI Emissions Regulations While Improving Fuel

Economy with the Opposed-Piston Engine
Suramya Naik, David Johnson, Laurence Fromm, John Koszewnik, Fabien Redon,
Gerhard Regner, and Neerav Abani
Achates Power, Inc.

ABSTRACT
The government of India has decided to implement Bharat Stage VI (BS-VI) emissions standards from April 2020. This requires OEMs
to equip their diesel engines with costly after-treatment, EGR systems and higher rail pressure fuel systems. By one estimate, BS-VI
engines are expected to be 15 to 20% more expensive than BS-IV engines, while also suffering with 2 to 3 % lower fuel economy.
OEMs are looking for solutions to meet the BS-VI emissions standards while still keeping the upfront and operating costs low enough
for their products to attract customers; however traditional engine technologies seem to have exhausted the possibilities. Fuel economy
improvement technologies applied to traditional 4-stroke engines bring small benefits with large cost penalties.

One promising solution to meet both current, and future, emissions standards with much improved fuel economy at lower cost is
the Opposed Piston (OP) engine. Recently, there has been surge in developing highly efficient OP engine architecture to
modernize it using today’s analytical tools, high pressure fuel system and manufacturing technologies to meet emissions, while
reaping the fuel economy advantage.

As the company pioneering the OP engine technology, Achates Power Inc. (API) has been publishing technical papers in recent years,
including a paper describing inherent efficiency benefits of OP engines, multi-cylinder steady state and transient results for medium
duty truck and light duty applications. This technical paper provides detailed performance and emissions results measured on API’s
4.9L multi-cylinder OP 2-stroke diesel engine configured specifically to meet BS-VI emissions standards for commercial truck
application. The results include:

• Measured performance and emissions data for emissions test cycles.

• After-treatment details and confirmation to meet tailpipe emissions for BS-VI standards.
• Details of API’s multi-cylinder test engine’s indicated thermal efficiency, friction and pumping losses.
• Comparison with 4-stroke diesel engine.

CITATION: Naik, S., Johnson, D., Fromm, L., Koszewnik, J. et al., "Achieving Bharat Stage VI Emissions Regulations While Improving
Fuel Economy with the Opposed-Piston Engine," SAE Int. J. Engines 10(1):2017, doi:10.4271/2017-26-0056.

INTRODUCTION Table 1. Tailpipe emissions standards for India.

The Indian government is introducing Bharat Stage VI (BS-VI)

emissions standards (equivalent to Euro VI standards) from 2020,
completely by-passing Stage V standards. For commercial vehicles
with diesel engines, these standards will reduce the NOx emissions
by 88% and Particulate Matter (PM) emissions by 66% from current
BS IV standards. Table 1 shows tailpipe emissions and test cycles for
BS IV and BS VI emissions standards [1].

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18 Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017)

While these steps will help reducing pollution from vehicles, it will MULTI-CYLINDER OP 2-STROKE
require costly additional after-treatment devices such as Diesel RESEARCH ENGINE
Particulate Filters (DPF) for trapping exhaust particulate matters and
Details of API’s 4.9 L 3-cylinder OP 2-Stroke engine are shown in
Selective Catalytic Reduction (SCR) for treating engine-out NOx
Table 2 below.
with aqueous urea solution. Additionally, the engine will require
Exhaust Gas Recirculation (EGR) system (EGR valve, cooler etc.)
Table 2. Multi-cylinder Achates Power OP 2-Stroke engine specification.
for reducing engine-out NOx with upgraded turbocharger. The Diesel
fuel system will also have to be upgraded for higher injection
pressures to reduce engine out particulates. As per one estimate done
by International Council for Clean Transportation, the additional
hardware required to upgrade 2.5 L 4-cylinder light duty diesel
engines from Euro IV to Euro VI is expected to increase the cost of
the engine by $1134 [2]. For 12 L truck engine, this expected cost is
$2740 [3]. Even with all cost cutting measures, this increased cost
translates into 15 to 20% more expensive engines for BS VI vehicles.

Not only the costs of the engines will increase significantly for API’s 4.9 L 3-cylinder engine has been designed and developed
meeting BS VI emissions, the fuel consumption will also get internally for carrying out research and developing OP engine
adversely affected because of the following reasons: technology before developing production engines with customers.
Therefore, it is designed with higher safety margin components to
• Increased exhaust back pressure resulting from more restrictive allow investigations for different applications. It is also designed to
after-treatment system together with high intake manifold disassemble quickly and is built with modular components that are
pressure requirements for BS VI engine increase pumping losses. switchable. Moreover, this engine has off-the-shelf components
• Higher EGR requirements also increase pumping losses without customization, primarily because this engine is not
especially as the recirculated exhaust gas has to pass through production intent and parts customization costs were unnecessary. All
restrictive coolers. of these factors however, result in higher friction and pumping loss
• Higher fuel injection pressure requirements result in increased penalties than will be measured on optimized and customized
power loss to the fuel pump. production OP engines.
• Increased Peak Cylinder Pressures (PCP) due to higher air and
The air system layout together with on-engine measurement sensors
EGR requirements increase friction penalties.
for this particular configuration of the API multi-cylinder engine is
described in figure 1.
Because of the above mentioned reasons, the BSFC is expected to
increase at least 2 to 3% without using additional fuel-saving
technologies for BS VI engines in comparison to BS IV engines [4].
With 4-stroke fuel saving technologies proving to be less cost effective
in providing improved fuel economy [3][4], there is a serious need for
the industry to search for fundamentally better engines. Opposed
Piston (OP) engines have been historically more fuel efficient and
have potential for reducing engine cost because of simpler architecture
and less number of parts [5][8]. These engines are now being
investigated by major OEMs around the world as a solution for
reducing fuel consumption at lower cost for modern vehicles [6].

Achates Power, Inc. (API), a US based company has been working

since 2004 towards developing OP engine technology using today’s
analytical and manufacturing technologies. Through numerous
technical papers, API has explained advantages of its OP engines
such as reduced heat losses; leaner, faster and earlier combustion; and
higher turbulent kinetic energy at the start of injection with its
proprietary piston bowl and two opposing injectors in each cylinder
[7][8][10]. API has also explained practical considerations for various
applications [8] and demonstrated improved fuel economy while Figure 1. API 4.9L research engine air system configuration & sensor layout.
meeting strict engine-out emissions on steady state basis on its
As seen from the figure 1, API’s 3-cylinder inline OP 2-Stroke engine
multi-cylinder OP 2-stroke research engine [9][10]. This paper
research has turbocharger and a supercharger. It has high pressure
describes further investigations on API’s multi-cylinder research
EGR system with one intercooler between turbo and supercharger,
engine for BS VI emission standards.
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Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017) 19

and two small air coolers downstream of supercharger. There are also
provisions for supercharger recirculation, 2-speed supercharger drive
and wastegate for the fixed geometry turbocharger. Main advantage
of this air system include lower pumping losses, faster transient
response and improved cold start & warm-up performance [8][10].

Figure 3. API 4.9L research engine torque curve for truck application with
ESC and WHSC points.

As seen from figure 3, the WHSC cycle points are heavily weighted
in the low speed and low load region of the torque curve compared to
the ESC points. This justifies using of different turbocharger to
improve BSFC at lower load and lower speed region for BS VI
emissions. However, the engine data was measured for both ESC 13
mode and WHSC points on same air system described earlier.

Figure 2. Rear view of API 4.9L research engine on cart. API has developed control strategy for addressing the challenges of
the OP 2-stroke engines. As seen from figure 1, supercharger 2-speed
Figure 2 shows the rear view of API’s 4.9 L research engine on the
drive and supercharger recirculation valve are two main actuators for
cart ready to be tested on engine dynamometer. More details of the
controlling air flow; while EGR valve is used for controlling EGR
API OP research engine hardware and test cell configuration has been
flow. Air massflow is accurately measured with MAF sensor located
published before in literature [9].
upstream of the compressor, while EGR massflow is measured with
venturi and deltaP sensor in the EGR path. The M470 rapid
A standard diesel after-treatment system for heavy duty engine with
prototyping open ECU from Pi Innovo has been programmed to
Diesel Oxidation Catalyst (DOC), Diesel Particulate Filters (DPF),
allow for firing two injectors simultaneously in one cylinder.
Selective Catalytic Reduction (SCR) and Ammonia Slip Catalyst
(ASC) has been assumed for meeting BSVI emissions standards. The
The detailed results of the steady state measurements are shown in
SCR is assumed to have NOx conversion efficiency of 90% and
the Appendix A. The results show OP engine’s high indicated thermal
therefore the engine out NOx target for WHSC is less than 4 g/kWh.
efficiency over the entire engine map. The friction loss for the 4.9L
The engine out soot target for WHSC cycle is set to be less than
research engine is higher than production version engines as
0.025 g/kWh to allow for passive regen of particulate filter during
explained earlier. Pumping losses over the engine map are reasonable
real world driving with low pressure drop. It is assumed that BSIV
even with off-the-shelf air system components.
engine may only have Particulate Oxidation Catalyst (POC) like
device in the after-treatment and therefore the engine out NOx for
Summary of steady state cycle averaged results are shown in table 3
ESC cycle is same as vehicle out (less than 3.5 g/kWh) - which turns
below. With optimized air system for better BSFC at lower load and
out to be only slightly lower to engine out NOx requirements for
speed operating conditions - as required for the WHSC cycle - the
BSVI engine with full after-treatment.
cycle averaged BSFC can be reduced about 2 to 4 g/kWh.

STEADY STATE TEST RESULTS Table 3. Summary of ESC and WHSC cycle results.

The API 3-cylinder research engine torque curve for truck application
together with ESC and WHSC points are shown in the figure 3.
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20 Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017)

Figure 4 shows BSFC map of API’s 4.9 L research engine from these supercharger and fixed geometry turbo with 2-speed supercharger for
steady state measured results. transient response for 1 second torque ramps at two different engine
speeds is shown in table 4 below.

Table 4. Transient response comparison of VTG turbocharger and single speed

supercharger Vs FG turbocharger and 2-speed supercharger.

As seen from the results, the supercharger 2-speed drive is improving

the transient response of the OP 2-stroke engine significantly. With
developed transient controls, the 4.9 L research engine was put on
test for transient emissions cycle.
Figure 4. BSFC map from steady state measured data on API research engine.

TRANSIENT CONTROLS AND TEST RESULTS

Compared to the controls software for steady state calibration, the
transient operation of the engine need strategy for limiting smoke
during acceleration; and for faster actuator response for driving air
and EGR. The 4.9L research engine controls strategy was improved
with a smoke limiter algorithm and feed-forward controllers for air
and EGR actuators.

The smoke limiter algorithm essentially is limiting the amount of fuel

that can be injected in the cylinder during acceleration as the air
handling devices (turbocharger and supercharger) respond slower
than the fuel system. Rail pressure modifier was also implemented for
increasing rail pressure during transient.

For increasing airflow during acceleration, EGR valve is closed to Figure 5. US heavy-duty FTP together with European ETC and WHTC cycles
allow for more massflow through turbine for reducing turbo lag. For operating points on API 4.9L research engine torque curve.
supercharger, first the recirculation valve is closed; if the airflow
Figure 5 shows engine operating conditions for three transient cycles
demand is still not met (or in conditions where the recirculation valve
- US heavy duty FTP, WHTC and ETC plotted with the torque curve.
is already fully closed for the starting point), the supercharger
2-speed drive is switched to higher drive ratio. With smoke limiter
As seen from the figure 5, the ETC cycle operates heavily around
implemented and higher supercharger drive ratio, the engine was able
1800 rpm for API 4.9L engine (on the higher engine speed region
to achieve the full load torque from 25% load at constant speed
similar to ESC cycle), while the WHTC cycle is weighted more in the
within 1.5 seconds with minimal NOx and soot spikes. The torque
region of 1500 rpm (relatively lower engine speed region similar to
response time and emissions results for different supercharger drive
WHSC cycle). Though the engine operates more in the speed range
ratios for OP engine have been discussed in detailed earlier [13].
of 1700 to 2200 rpm for the heavy duty FTP cycle for US 2010
emissions, this transient cycle has wider speed range and larger speed
The 4.9 L research engine was also investigated for transient response
gradients. Also, it is designed for both city as well as highway driving
with Variable Turbine Geometry (VTG) turbocharger and single gear
conditions as seen in the figure 6 with New-York Non-Freeway
ratio supercharger. A comparison of VTG turbo with single drive
(NYNF), Los Angeles Non-Freeway (LANF) and Los Angeles
Freeway (LAFY) segments. Therefore heavy duty FTP was selected
for testing transient capabilities of the 4.9 L research engine.
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Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017) 21

Figure 6. US heavy-duty FTP cycle with NYNF, LANF and LAFY segments [16].

During the transient testing, engine speed, torque and power

Figure 7. Johnson Matthey’s SCRT® aftertreatment system [12].
requirements were appropriately matched with the targets to meet the
heavy duty FTP cycle requirements. Statistically, the R2 values of the This system was sized for reasonable space velocities through its
measured speed, torque and power compared to the targets were 0.97, components and simulated with chemistry models by Johnson
0.94 and 0.98 respectively - within the range specified by the Matthey engineers to check its performance for 13 mode steady state
regulations. The heavy duty transient cycle averaged values of BSFC, engine out exhaust from API’s 4.9L research engine. Full details of
BSNOx and BSSoot were measured to be 217.3 g/kWh, 4.3 g/kWh and the study have been published in 2016 emissions conference paper
0.056 g/kWh respectively. When compared with the BSFC map data [12]. The details of the various ATS components size and structure
generated from the steady state measurements, the transient BSFC is are shown below in figure 8.
only 2.1 g/kWh higher - suggesting that the controls strategies are
working decently as required for such application. Detailed description
and results of the US 2010 heavy duty transient test for API’s 4.9L
engine have been published in 2016 SAE paper [11].

Additional to transient controls, API has also developed warm-up

strategies for catalyst light off, details of which have been published
in other papers [10][13].

Figure 8. Simulated ATS components volume, CPSI/wall thickness and PGM

CONFIRMING TAILPIPE EMISSIONS loading [12].

API teamed up with Johnson Matthey - a leading after-treatment To check for possibilities of passive regeneration of CSF, the DOC
supplier to check if the tailpipe emissions of its 4.9L OP engine meet was simulated for two cases -
stringent BSVI standards. Johnson Matthey has developed a patented
SCRT® aftertreatment system (ATS) which allows for passive Case 1. Low Pt:Pd (2:1) ratio aged at 7800C/10h and
regeneration of particulate filter using higher engine out NOx, and
Case 2. High Pt:Pd (5:1) ratio aged at 7800C/100h [12].
SCR to reduce the NOx [12].
The 100 repetitions of 13 mode steady state cycle results show that
The system has DOC with platinum group metals (PGM) as first
for case 2, the DOC would remove THC and CO 92 and 100%
component to oxidize HC and CO, also to convert NO to NO2 that
respectively [12]. CSF would go through sufficient passive
helps with passive regen of particulate filter. Second component is
regeneration to stabilize at 1.3 g/L soot loading after 100 ESC tests
Catalyst Soot Filter (CSF) for removing PM. Urea is injected after
[12]. The NOx conversion efficiency of 96% can be achieved with
CSF and before SCR to remove NOx emissions. And finally, ASC is
the SCR [12]. The maximum pressure drop through this ATS for the
used to oxidize access NH3. Figure 7 shows schematic of Johnson
steady state cycle simulations is 15 kPa for 0 g/l soot in CSF and 16.5
Matthey’s patented SCRT® aftertreatment system.
kPa for 3 g/l soot loading in CSF. Table 5 below show cycle averaged
tailpipe emissions for 13 points of the ESC test [12].
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22 Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017)

Table 5. Summary of Johnson Matthey ATS simulation results on API’s 4.9L These ATS system simulations for steady state and transient cycles
engine out cycle-averaged emissions for 13 points of ESC cycle [12]. with measured engine out emissions and exhaust temperatures
confirm that API’s 4.9 L engine will meet BSVI tailpipe emissions.

COMPARISON WITH 4-STROKE DIESEL

ENGINE
The data published in this paper so far is with API’s 4.9 L research
engine that has high friction penalties as seen in the data table in
Results of steady state cycles operating points listed in Appendix A Appendix A. When the design is optimized for production, API’s 4.9 L
show that the turbine out temperatures of the exhaust for all of the year 2020 engine has been predicted to achieve best BTE of 48.5% and
WHSC points except idle range between 250 to 3680C which is ESC 12 mode cycle average BTE of 46.6% (180 g/kWh cycle averaged
similar to the range of 236 to 3570C seen on the ESC cycle points. BSFC) [9] while meeting US 2010 emissions (comparable to BSVI
Therefore, even though the aftertreatment simulations were carried emissions standards). These data can be compared with 6.7 L Ford
out on 100 cycles of the 13-modes of ESC test, simulating the WHSC Power-stroke V8 engine [14] and 6.7 L inline 6 Cummins ISB engine
operating points with after-treatment should also be able to meet the (data published in one report by the International Council of Clean
tailpipe emissions targets. Transportation (ICCT) [15] and in another by SouthWest Research
Institute (SWRI) [17]). Table 7 below shows comparison of API’s 4.9L
The same aftertreatment system was also simulated by Johnson OP engine with Ford Power-stroke and Cummins ISB specifications.
Matthey engineers for heavy-duty transient FTP cycle data measured
on 4.9 L research engine. Figure 9 below show space velocities Table 7. Specifications of comparable 2-stroke OP and 4-stroke engines
through different ATS components and inlet temperature for the meeting US 2010 emissions standards (comparable to BSVI) .
heavy-duty FTP cycle.

Figure 9. Space velocities through different ATS components and inlet

temperature for the heavy-duty FTP cycle.
The SAE paper with Ford Power-stroke [14] and SWRI report [17]
As seen from figure 9, the ATS inlet gas temperature varies between have full steady state BSFC data allowing comparison for steady state
160 to 3000C. The maximum pressure drop of the entire ATS for the cycles. The SWRI report has also predicted the BSFC of improved
heavy-duty FTP cycle simulation was 10.4 kPa for 0 g/l soot Cummins ISB engine in year 2019 with reduced friction, improved
loading and 12.1 kPa for 3 g/l soot loading. Table 6 below show turbocharger and reduced combustion duration [17]. This data can be
emissions conversion efficiencies achieved during the heavy-duty compared with API’s 4.9L production engine for 2020.
FTP cycle simulations.
Figure 10 show comparison between various medium duty
Table 6. Summary of Johnson Matthey aftertreatment system simulation application engines that meet US 2010 emissions - which are similar
results on API’s 4.9L engine out exhaust for heavy-duty FTP transient cycle. to BSVI emissions standards. As seen from the figure 10, API’s 4.9 L
research engine measured data show 20.7% and 10.7% fuel economy
advantage over 2010 Ford Power-stroke and 2012 Cummins ISB
engines respectively when compared for 12 operating modes of the
ESC cycle excluding idle. With flat fuel map for API’s OP engine [8]
[9], higher available torque at lower speeds and improved part load
fuel economy over 4-stroke engines, the vehicle fuel economy
advantage of OP engine in real world driving can be significantly
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Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017) 23

higher. When API’s production 4.9L engine predicted performance is • API has successfully developed and implemented controls
compared with 2019 Cummins ISB predicted by SWRI [17], the OP strategies for the engine to run it effectively on steady state and
engine show 16.2% fuel economy advantage. transient emission cycles.
• When simulated with Johnson Matthey sized conventional
diesel after-treatment system, API’s OP engine can meet Bharat
Stage VI tailpipe emissions standards.
• API’s current 4.9L research engine is showing 10 to 21% fuel
economy improvement over comparable conventional medium-
duty 4-stroke engine. This fuel economy advantage is expected to
increase with API’s lower friction optimized production engine.

Thus, Opposed Piston engines are capable to address the challenges

faced by Indian OEMs to meet Bharat Stage VI emissions standards
with reduced cost and offer improved fuel economy to the end users.

REFERENCES
Figure 10. 12 mode cycle averaged BTE results comparison shown together 1. Heavy-Duty emissions standards for India; Retrieved from Dieselnet
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CONTACT INFORMATION
Suramya Naik
Chief Engineer and Program Manager
Achates Power, Inc.
naik@achatespower.com

ACKNOWLEDGMENTS
We acknowledge and thank Johnson Matthey for carrying out
after-treatment sizing and simulation activities and for allowing us to
publish the results.

DEFINITIONS/ABBREVIATIONS
API - Achates Power Inc.
OP - Opposed-Piston.
BSFC - Brake Specific Fuel Consumption
BSNOx - Brake Specific Nitrogen Oxides
BSHC - Brake Specific Hydrocarbons
BSCO - Brake Specific Carbon Monoxide
PM - Particulate Matter
THC - Total Hydrocarbon
ESC - European Steady-state Cycle
ETC - European Transient Cycle
WHSC - World Harmonized Steady-state Cycle
WHTC - World Harmonized Transient Cycle
BTE - Brake Thermal Efficiency
VTG - Variable Turbine Geometry
ATS - After-treatment System
DOC - Diesel Oxidation Catalyst
DPF - Diesel Particulate Filter
SCR - Selective Catalyst Reduction
ASC - Ammonia Slip Catalyst
CSF - Catalyst Soot Filter
PGM - Platinum Group Metals
CPSI - Cells per square inch
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APPENDIX
Appendix A: Measured steady state data on API’s 4.9L OP 2-Stroke research engine
ESC Cycle
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26 Naik et al / SAE Int. J. Engines / Volume 10, Issue 1 (February 2017)

WHSC Cycle

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