In a four-stroke engine, combustion theoretically occurs at which condition?

In a four-stroke engine, combustion theoretically occurs at constant volume during the compression and power strokes. This is particularly the case during the ignition phase when the fuel-air mixture is compressed in the cylinder. The process leading up to ignition is characterized by a rapid increase in pressure and temperature within a fixed volume, as the piston rises and compresses the mixture before the spark plug ignites it.

At this stage, the volume remains constant while the pressure increases until the ignition point, resulting in a significant rise in temperature. After ignition, a rapid combustion process takes place, resulting in an increase in pressure as well.

Understanding that combustion occurs at constant volume helps in grasping the thermodynamics involved in engine cycles and their efficiency. Constant pressure and constant temperature conditions pertain to different thermodynamic processes and are not applicable to the specific mechanics of combustion in four-stroke engines. Constant velocity does not describe a thermodynamic state relevant to combustion processes either.