A piston in a engine at TDC gets forced down by particle momentum after the ignition of the fuel air mixture. The particles hit the piston and transfer force to the piston and move the piston downwards. <-- Is that the current explanation for the process of the reaction and it is momentum based? vs A piston in a engine at TDC gets forced down by the reaction of the ignited fuel air mixture. The ignition reaction causes the particles to change their fields/forces, the particles convert some of their mass to field/force strength from the thermal reaction. The excess field/force strength causes the particles/atoms to repel each other, forcing the piston to move down via forced displacement. <-- This would be field/force repulsion from interacting fields/forces.
The former is correct. The ignition causes the gases to heat up which results in the expansion of the gas. The increased pressure causes the piston to be forced down. On a microscopic scale higher pressure means more atoms per unit time are 'hitting' the walls and piston.
Oh I see! I did not give the reply you wanted so you are throwing a little tantrum. Got it. My bad, I thought you really wanted a discussion.