Monday, 19 February 2018

2018-02-21, BRUSSELS

Dr. Hua Lu Karlsson

Expert Engineer

PN EMISSIONS FROM HEAVY DUTY CNG ENGINE AND CNG PEMS PN ISSUE

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� PN emissions from HD diesel

� Oil carryover from HD combustion engine

� PN emissions from HD CNG

� HD CNG PN emissions, SwRI study

� HD CNG PN emissions during real world driving

� Reduction of PN emissions from HD CNG

CONTENTS

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� Typical HD diesel engine emits 1014 #/kwh (WHTC). Diesel PN emissions are mostly

caused by fuel combustion, particle emissions are dominated by soot.

� Soot builds ‘soot cake’ in DPF, soot cake is essential in order to efficiently remove

particles from diesel exhausts.

� Regeneration of DPF will be activated either passively or actively to remove the

accumulated soot from the filter to avoid high DPF back pressure.

� Ashes will not be removed by regeneration. Off-vehicle ash cleaning is needed

periodically to prevent high back pressure.

PN EMISSIONS FROM HD DIESEL ENGINE

ENGINE LUBE OIL CARRYOVER IN HD COMBUSTION ENGINES

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� Lube oil carryover is when engine oil escapes from the crankcase through the piston rings into the combustion chamber.

� Engine oil carryover is vital for lubrication of the piston/bore interface, therefore carryover cannot be removed completely and optimisation of carryover amount under all conditions is time consuming and challenging.

� The carryover lube oil is combusted in the cylinder and forms ash and soot i.e. particles, the higher oil carryover, the higher PN emissions.

� The lube oil carryover is especially high when the crankcase pressure is higher than the cylinder pressure, the pressure difference pushes the oil from the crankcase into the cylinder

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� Typical CNG emits 1011 #/kwh (WHTC). Unlike diesel engine, PN emissions from spark ignition CNG engine are mostly originated from oil combustion due to the oil carryover process, oil carryover occurs more profoundly in CNG engine than in diesel engine.

� Spark ignition engines (CNG) use a throttle as primary control of air intake amount, consequently fuel amount, to regulate power output. Throttling, especially during motoring (overrun*)and idling, leads to under pressure in the cylinder, thus causes increased oil carryover.

� Power output of diesel engine is controlled directly by controlling the fuel injection amount, i.e. not via throttling of air intake, thus oil carryover is much less severe than in spark ignition gas engines.

* driver takes foot off the gas pedal, closing the throttle =>engine brakes

PN EMISSIONS FROM HD CNG ENGINE

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CNG engine has higher sub-25 nm and > 25 nm PN emissions than DPF diesel engine, but within same magnitude, significantly lower than diesel engine without DPF. PM emissions from CNG are significantly lower than the limits.

HD CNG and diesel PN emissions (SwRI, 45th PMP meeting presentation)

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Ash from HD CNG ( SwRI):Example of high PN emissions caused by idling

• confirmed that oil combustion is the main source of CNG PN emissions.

• FTP cycle with more idle emits more ash

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• High PN emissions are produced at high acceleration after long period of idling.

Example of high PN emissions caused by idling (SwRI)

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Ash profile follows PN profile, confirming the PN emissions related to oil combustion (SwRI)

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HD CNG PN EMISSIONS DURING REAL WORLD DRIVING

• PN emissions from CNG are mainly caused by oil combustion, thus is closely related to oil carryover. Consequently CNG vehicle driving conditions and payload will significantly influence the PN emissions.

• This is not the case for diesel equipped with DPF since DPF will remove most particles from combustion independent of driving conditions.

• We do observed such phenomena during ACEA HD PEMS PN exercise.

REDUCING PN EMISSIONS FROM HD GAS ENGINES

� PN emissions from CNG engine can be reduced by reducing lube oil

carryover. This can be achieved by

�Lowering the crankcase pressure (no pressure difference can drive oil

carryover)

�Tribological optimization of piston design, piston rings and liner design

that gives less lube oil carryover without negative wear effects.

� Development work to reduce oil carryover is ongoing at OEMs, one of the

most time consuming engine R&D activities specially considering needs to

verify durability of any new solutions. Minimize oil carryover without

jeopardizing engine lifetime is an extremely challenging task, since

mistakes could lead to fatal damaging of the engine.

REDUCING PN EMISSIONS FROM HD CNG/LNG/DUEL FUEL ENGINES

� CPF (CNG particulate filter) for HD gas engines (CNG/LNG, dual-fuel):

�The engine out PN emission of a gas engines are one to two order of

magnitude lower than those of diesel engines.

�Therefore conventional diesel particulate filter couldn’t be used directly

in a gas engine, since a ‘soot cake’ cannot be formed easily.

� ‘Ash cake’ will different density than ‘soot cake’, higher density will lead

to high backpressure.

�High ash contents lead to another requirement for maintenance/service

of the CPF, since ash can not be removed by regeneration.

�CPF for serial application is not commercially available at the moment

�R&D of special CPF is initiated by our suppliers and at OEM level

CNG PN Measurement

� We notice the PMP PN method was developed based on diesel engine

technology. HD-CNG was not considered in the PMP pilot validation.

� JRC did separately some basic investigations with gas engines/vehicles.

Validation of HD-gas technology was intensively discussed in

WP.29/GRPE informal group PMP (“PN sub 23nm program”).

� ACEA HD OEMs have decided to support PMP program with gas engines

and test cell capacity. Discussion of detailed PMP test program on CNG

will be soon initiated between JRC/PMP and ACEA.

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SUMMARY

� PM emissions from HD CNG are low. But together with the lube oil derived ash particles

CNG PN emission level is higher than those for DPF equipped diesel engines (incl. dual

fuel).

� The EURO VI PN limit was set at that time based barely on PMP diesel engine results. HD

CNG has never been investigated in the PMP program.

� PN limit was adopted for PI engines in Euro VI stage B, 21 months after introducing the

PN limit for diesel (stage A).

� Ever since beginning of this decade OEMs have been fighting to minimize the PN emission

with the difficult target of meeting the WHTC PN limit.

� Even more challenging is to minimize CNG PN emissions during real world driving without

jeopardizing engine life time

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HD GAS FUEL ENGINES, NICHE PRODUCT

For all HD OEMs, gas has < 3% of diesel production volume, it is a niche product for

sustainability.

With current gas technology (CNG/LNG and dual fuel) we have certain difficulties to demonstrate

in-service conformity for PN under all defined conditions, severity depending 0n on CF target to

be defined by commission.

We kindly ask Commission to start dialog in order to seek an appropriate solution for OEM

gaining lead time to develop countermeasures to further reduce particle number from gas

engines (CNG/LNG and dual-fuel).

ACEA COMMERCIAL VEHICLE MEMBERS

@ACEA_eu

www.ACEA.be

THANK YOU FOR YOUR ATTENTION