FAA Powerplant Written Practice Test

On which stroke are both valves open in a four-stroke cycle reciprocating engine?

1. Power stroke

2. Exhaust stroke

3. Intake stroke

4. Roughly between intake and exhaust

The correct answer is: Power stroke

In a four-stroke cycle reciprocating engine, both valves are open during the exhaust stroke. This occurs at the end of the power stroke as the piston moves upward, pushing the burned gases out of the cylinder. The design allows for optimal exhaust flow, aiding in the scavenging process, which is essential for the efficient operation of the engine. Additionally, the slight overlap in the timing of the opening and closing of the intake and exhaust valves, which occurs as the piston approaches the top of the exhaust stroke and the bottom of the intake stroke, facilitates a better mixture of incoming air-fuel and enhances the engine's overall efficiency. While the intake stroke includes the opening of the intake valve for drawing in the air-fuel mixture, the correct understanding of the question focuses on the specific timing during the exhaust phase that allows for both valves to be open momentarily, reflecting optimal valve timing as per the engine’s design.