During the power stroke of a 4-stroke engine, what action takes place?

During the power stroke of a 4-stroke engine, the correct action is that fuel is ignited to produce power. This event is the primary purpose of the power stroke in the cycle, which generates the energy needed to push the piston downwards.

In a 4-stroke engine, the power stroke occurs after the compression stroke, when the air-fuel mixture is compressed into the combustion chamber. At the start of this stroke, the spark plug ignites the compressed mixture, resulting in a rapid expansion of gases that push the piston downward. This downward movement is what ultimately produces the power output of the engine, transferring energy to the crankshaft.

While it is true that the piston moves downward during this stroke, and the crankshaft is at its lowest point after the power stroke is completed, these actions are not as specific to the power stroke itself as the ignition of the fuel. Similarly, the opening of the exhaust valve or intake valve happens in other phases of the engine cycle and is not relevant to the function of the power stroke. The ignition is the critical event that characterizes this phase, leading to the conversion of chemical energy into mechanical energy.