FAA Powerplant Written Practice Test

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

1. Both open

2. Both closed

3. Intake open, exhaust closed

4. Exhaust open, intake closed

The correct answer is: Both closed

During the power stroke of a 4-stroke engine, the position of both valves is crucial for ensuring that the engine operates efficiently. During this phase, the piston moves downward from the top dead center (TDC) to the bottom dead center (BDC) after the combustion process has taken place. At this point, the intake valve should be closed to maintain the pressure in the combustion chamber and to prevent any backflow of gases. Simultaneously, the exhaust valve must also be closed to allow the combustion gases to be contained within the cylinder without escaping. Keeping both valves closed during the power stroke allows for maximum pressure to be exerted on the piston due to the force of the expanding gases from the combustion process. This pressure is what generates the power needed to turn the crankshaft and ultimately propel the vehicle. Both valves being closed ensures that the engine effectively converts the chemical energy of the fuel into mechanical energy during this critical phase. Thus, understanding the sequence and timing of valve operations is key in the functioning of a 4-stroke engine, which is designed to have specific stages where each valve's position changes. During the power stroke, maintaining both valves closed is essential for achieving optimal performance and power output.