Originally Posted by Chris W

Hi Lou,
long time since I have commented in this thread but since many of the responders are still around I don't feel so bad.
Question, define "silliness."

Pascals law (which seems to be the crutch that holds this one up) only applies well if the fluid is incompressable and static, and with fluid in a model engine that compresses and expands in cycles measures in the thousands per minute, applying this thinking is flawed.
If flow is contured or flowing in any way and especially if its cyclic, then pressures within the bounded area will not be the same everywhere at any time.
Where there is a baffle, pressure differences can and will happen, where gas is forced to expand and move around a shape, such as a conical moving piston you will suffer a loss as a result of an action.
A conical crown will angle an expanding charge away from the direction of travel into the contact area between the cylinder wall and the piston creating a loss, far better to have it only directed parrallel to the direction of travel.

Maybe the effect is only slight in practice but I believe that any convex shaping of the combustion chamber, and this includes the crown, is adverse to any HCCI engine that endevours to ignite the charge all at once. In contrast concave shaping seems to be beneficial to this process due to the reasons given above.

Cheers.