Recycled Flyer

I can see that shaping the combustion chamber away from the pistons line of travel (degrees above or below it being flat) should result in the charge forces following a 90 degree from angle of that shape.

In other words I see the ideal line of force from combustion being straight towards BDC and this is how a flat topped piston, and flat head would resolve the forces of combustion - you want all of the force acting against the angle and shape of the chamber.
But with an angled surfaces the forces will resolve at angles perpendicular to those surfaces just like in shaped charge explosives.

So a cone head piston will resolve the pressures against it with a partial inwards pinch crushing against the piston crown itself and waste some of the energy otherwise useful for power generation - or the more angle you have towards the pistons line of travel the more (in effect) sidewall you will have.

Hence why flat surfaces at 90 degrees to the piston travel are the best for rapid detonation engines - they simply transfer more energy into the correct direction.

And I can well imagine that a lot of attention has been given has been given to model diesel combustion chamber shape but its been belied by its simplicity.