I've been told that because diesels are detonation engines (more properly spontaneous ignition) the shape of the combustion chamber is irrelevant. The RJL 40 head contrapiston has a shallow bubble and was said to idle better. When we replaced the contrapiston after it broke, the piece of aluminum we used happened to have a shallow cone in the center. Both contrapistons ran fine for my purpose, which does not include idling. Most diesels have either a flat crown piston and a flat contra piston, or a slight cone on the piston and a matching cone in the contrapiston. We know that combustion chamber shape is important in a glowplug engine because combustion originates at the glowplug. Any thoughts on diesel?

Jim

I bet a squish band might be worth trying with a small combustion chamber. good luck Erik

Please excuse me for jumping on this but I've seen it several times and I would like to suggest a correction to the terminology used in describing a diesel engine. Properly described it is a compression ignition engine. The terms detonation and spontaneous combustion are both inaccurate and classically incorrect. Detonation indicates an explosion which is not really the case in any internal combustion engine though there are certainly conditions under which more rapid propagation of the flame front in the combustion chamber result in near detonation. Spontaneous combustion is not the case either. A captive fuel air mixture is compressed and the resulting rise in temperature cause the mixture to ignite, compression ignition. Spontaneous comnbustion has various conditions underwhich it will occur. Diesel ignition is "timed" by the compression ratio which determines at what point ignition takes place, over-compressed is too "advanced", undercompressed causes "burping" as the fuel/air ratio changes between "burps". There is a narrow range in which the engine will run steadily but it is possible to have it too far "retarded" (under compressed) and thereby not produce optimal power.

As far as combustion chamber shape goes I am sure there are better shapes yet to be developed as diesels are being developed for throttled operation. The truth is that diesels have mostly been used at fixed speeds ( I know, there have been throttled diesels for years, but they are not prolific) and the few variations of chamber shape that have been used have not really shown any appreciable differences.