FAA Powerplant Written Practice Test

What does the compression ratio of an engine equal?

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

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

3. The sum of the volumes at TDC and BDC

4. The difference between the volumes at TDC and BDC

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

The compression ratio of an engine is a crucial parameter that defines how much the air-fuel mixture is compressed within the engine cylinder before ignition. It is calculated by dividing the volume of the cylinder when the piston is at the Bottom Dead Center (BDC), which represents the maximum volume of the combustion chamber, by the volume of the cylinder when the piston is at the Top Dead Center (TDC), which represents the minimum volume. This measurement indicates the extent of compression that the engine's cylinders can achieve during operation. A higher compression ratio typically leads to greater efficiency and more power output, as it allows for a more thorough combustion process. Consequently, the correct understanding of this relationship is fundamental for evaluating engine performance, fuel efficiency, and potential modifications. While other choices may mention relationships between TDC and BDC volumes, they do not correctly reflect the definition of compression ratio as understood in engineering principles. The accurate ratio uniquely emphasizes the ratio of these two specific volumes, making the selection of the second choice not only the correct one but essential for a proper understanding of engine mechanics.