Troels Dyhr Pedersen.indd - Solid Mechanics

2010-01-1487
Reduction of HCCI combustion noise through piston crown design
Copyright © 2010 SAE International
ABSTRACT
Troels Dyhr Pedersen, Jesper Schramm
Technical University of Denmark
Seven shapes of piston crowns have been evaluated for their ability to reduce HCCI knock and transmission of
combustion noise to the engine. The performance of each piston crown was evaluated with measurements of
cylinder pressure, engine vibration and acoustic sound pressure measured one meter away from the engine. The
experiments were conducted in a diesel engine that was run in HCCI combustion mode with a fixed quantity of
DME as fuel.
The results show that combustion knock is effectively suppressed by limiting the size of the volume in which
the combustion occurs. Splitting the compression volume into four smaller volumes placed between the
perimeter of the piston and the cylinder liner increased the noise to a higher level than that generated with a flat
piston crown. This was due to resonance between the four volumes. Using eight volumes instead decreased the
noise. The noise was further reduced with another piston crown where eight cylindrical volumes were drilled
into the piston crown, so that the cylinder liner was not exposed directly to the combustion. A configuration
with seven hemispherical volumes was less silent in operation, but still better than the flat piston crown. The
largest and most consistent reduction in noise level was however achieved with a diesel bowl type piston.
The increased surface area as well as the larger crevice volumes of the experimental piston crowns generally
resulted in lower IMEP than the flat piston. While the crevice volumes can be reduced, increased heat transfer
can not be avoided. Thus, the use of alternative piston crown geometries designed to split the combustion may
not be viable means of avoiding HCCI engine knock. The traditional diesel bowl type piston is therefore
possibly the best alternative to the flat piston in terms of noise reduction as well as heat losses.
INTRODUCTION
Homogeneous charge compression ignition (HCCI) combustion has a number of advantages such as very low
emissions of NOx and particulate matter, high indicated efficiency and the ability to use a wide range of fuels.
HCCI combustion may be used as a part load option in normal SI and DI engines where low emissions are
important. The combustion process is limited to part load primarily due to the reaction rate becoming
excessively high when load is increased. As the reaction rate and hence the pressure rise rate increases, the
noise level from the engine also increases.
The noise from the engine is caused by large amplitude resonance patterns in the chamber, when reflections of
powerful pressure waves transfer energy to the cylinder liner. The energy is transmitted through the engine
structure and a small fraction is dissipated as noise, while the rest is dissipated as heat in the structure.
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