Question

In a car engine, where does the combustion reaction occur?

Answer 1

the combustion happens when the gas hits the spark plugs in the the chamber where the heads are.

Answer 2

 As the piston reaches the top of its stroke or TDC the spark plug fires igniting the mixture. Due to the high compression of this mixture (typically around 190 PSI in a typical engine) it is very volatile and it explodes when the spark is introduced. This pushs the piston downward and produces power.