Troels Dyhr Pedersen.indd - Solid Mechanics

ABSTRACT
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2010-01-1489
Controlling the heat release in HCCI combustion of DME with
methanol and EGR
Troels Dyhr Pedersen, Jesper Schramm
Technical University of Denmark
Tadanori Yanai, Yoshio Sato
National Traffic Safety and Environment Laboratories, Japan
The effects of methanol and EGR on HCCI combustion of dimethyl ether have been tested separately in a diesel
engine. The engine was equipped with a common rail injection system which allowed for random injection of
DME. The engine could therefore be operated either as a normal DI CI engine or, by advancing the injection
timing 360 CAD, as an HCCI engine. The compression ratio of the engine was reduced to 14.5 by enlarging the
piston bowls.
The engine was operated in HCCI mode with DME at an equivalence ratio of 0.25. To retard the combustion
timing, methanol was port fuel injected and the optimum quantity required was determined. The added
methanol increased the BMEP by increasing the total heat release and retarding the combustion to after TDC.
Engine knock was reduced with increasing quantities of methanol. The highest BMEP was achieved when the
equivalence ratio of methanol was around 0.12 at 1000 RPM, and around 0.76 at 1800 RPM.
EGR was also used to retarding the timing. With a moderate amount of EGR the effect on the combustion was
not notable, but as the equivalence ratio approached unity the combustion was increasingly delayed and the rate
of reaction reduced. Engine knock seized entirely as the EGR ratio was increased above 60 %. The BMEP gain
was however moderate, since lower cylinder pressures at higher EGR quantities counteracted the positive
effects of combustion timing.
INTRODUCTION
HCCI combustion is considered to be an efficient alternative to part load SI and CI operation. The main
advantage of HCCI combustion is a high efficiency compared to SI part load, and much lower emissions of NOx
and PM than DI.
The efficiency of part load HCCI combustion is usually higher than SI part load since there are no throttling
losses. The part load efficiency of HCCI can be comparable to DI CI engines, which has the advantage of a
higher compression ratio but also the disadvantage of a slow combustion.
Very low emission levels of NOx and negligible amounts of particulate matter are inherent effects of the lean
and premixed combustion. NOx formation is kept low due to a uniform low combustion temperature and PM
formation is avoided due to absence of fuel rich zones.
CO and HC levels are high and comparable to untreated emissions from SI operation. CO is believed to
originate from flame quenching near the combustion chamber walls, and from running the combustion very