FAA Powerplant Written Practice Test

During the power stroke of a 4-stroke engine, what is the correct position of the valves?

1. Both valves open

2. Both valves closed

3. Intake valve open, exhaust valve closed

4. Exhaust valve open, intake valve closed

The correct answer is: Both valves closed

During the power stroke of a four-stroke engine, the correct position of the valves is both valves closed. This stroke occurs after the ignition of the fuel-air mixture, resulting in a rapid expansion of hot gases that push the piston down, converting thermal energy into mechanical work. Having both valves closed during this phase is essential to maintain pressure within the combustion chamber, allowing the engine to effectively harness the energy produced from combustion. If either valve were open during this stroke, it would lead to a loss of pressure and decreased efficiency, as the expanding gases could escape prematurely. The design of the four-stroke cycle emphasizes the importance of timing for valve operation, ensuring that each stroke functions optimally within the cycle. Thus, keeping both valves closed during the power stroke is vital for the effective operation of the engine.