When should the combustion process be completed if the fuel/air ratio is proper and ignition timing is correct?

The combustion process in a reciprocating engine should ideally be completed just after top dead center (TDC) at the beginning of the power stroke. This timing ensures that the maximum pressure from the combustion gases occurs as the piston begins its downward travel, optimizing the engine's efficiency and power output.

When the fuel/air mixture ignites at the correct moment, the energy from the combustion can effectively convert to mechanical work as the piston moves down. If combustion occurs too early (before TDC), it can lead to engine knock or pre-ignition, which can cause damage. Conversely, if combustion is delayed until after TDC, then the piston has already begun its descent and the potential energy from combustion isn't fully utilized for effective power generation.

Thus, completing combustion just after TDC provides the best timing for harnessing the combustion pressure and maximizing engine performance.