On which two strokes are both valves closed?

The power and compression strokes in a four-stroke engine are critical to its operation. During the compression stroke, the piston moves upward in the cylinder, compressing the fuel-air mixture with both valves closed. This closure is essential for maintaining the pressure and temperature necessary for proper combustion.

Once the compression stroke is complete, the engine enters the power stroke. At this stage, as the mixture ignites, the force from the combustion pushes the piston downward, again with both valves closed. This closure during the power stroke ensures that the pressure from the combustion does not escape through the valves, maximizing the energy transferred to the crankshaft.

In contrast, during the intake stroke, the intake valve is open to allow the fuel-air mixture to enter the cylinder, while in the exhaust stroke, the exhaust valve is open to expel the combustion gases. Therefore, during both the compression and power strokes, it is crucial that both valves remain closed to facilitate effective compression and energy production.