FAA Powerplant Written Practice Test

How is the compression ratio of an engine defined?

1. The volume of the cylinder at TDC divided by the volume at BDC

2. The volume of the cylinder with the piston at BDC divided by the volume at TDC

3. The difference in volume between TDC and BDC

4. Its maximum volume during the intake stroke

The correct answer is: The volume of the cylinder with the piston at BDC divided by the volume at TDC

The compression ratio of an engine is defined as the volume of the cylinder with the piston at the bottom dead center (BDC) divided by the volume of the cylinder with the piston at the top dead center (TDC). This ratio is a critical parameter in an engine's performance as it directly impacts the engine's thermal efficiency and power output. When the piston is at BDC, the cylinder contains the maximum volume of the air-fuel mixture, referred to as the displacement volume. Conversely, when the piston is at TDC, the cylinder's volume is at its minimum, as the piston has compressed that mixture. The resulting ratio indicates how much the volume of the air-fuel mixture is reduced during the compression stroke. A higher compression ratio typically improves engine efficiency but also requires higher octane fuel to prevent knocking or pre-ignition. Thus, this definition of the compression ratio is fundamental to understanding engine operation and design.