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.