Certified Flight Instructor - Flight Instructor Airplane Practice Exam

During which stroke of a reciprocating engine is the gaseous mixture expanding within the cylinder?

1. Power.

2. Intake.

3. Compression.

4. Exhaust.

The correct answer is: Power.

The power stroke of a reciprocating engine is the phase during which the gaseous mixture within the cylinder expands. During this phase, combustion has occurred, resulting in a significant increase in temperature and pressure within the cylinder. As a result, the hot gases push the piston downward, converting thermal energy from the combustion into mechanical energy. This expansion is crucial as it is what ultimately drives the engine and propels the aircraft or vehicle. In contrast, during the intake stroke, the piston moves down the cylinder, creating a vacuum that draws in the air-fuel mixture; this is when the mixture enters the cylinder rather than expanding. The compression stroke follows this, during which the piston moves upward to compress the mixture before ignition. Lastly, the exhaust stroke occurs after the power stroke, where the piston moves upward again to expel the combustion byproducts from the cylinder. Each of these strokes has distinct roles in the engine’s operation, but the expansion of the gas mixture occurs solely during the power stroke.