FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician is calculating the compression ratio of a cylinder that has a swept (displacement) volume of 85 cubic inches and a combustion chamber clearance volume of 10 cubic inches. What is the compression ratio?
- A8.5:1
- B8.0:1
- C9.0:1
- D9.5:1
Show answer & explanationAnswer & explanation
Correct answer: D. 9.5:1
Compression ratio = (swept volume + clearance volume) / clearance volume = (85 + 10) / 10 = 95/10 = 9.5:1.
Why the other options are wrong
- A. Incorrect division, does not use total volume correctly.
- B. Uses swept volume alone divided incorrectly.
- C. Close but understates the total cylinder volume.
Compression Ratio
The ratio of total cylinder volume (piston at BDC) to clearance volume (piston at TDC), indicating how much the fuel-air charge is compressed.
- CR = (swept volume + clearance volume) / clearance volume
- Higher CR increases thermal efficiency but risk of detonation
- Typical aircraft reciprocating engines run 7:1 to 10:1
Memory trick: Big volume over small volume tells the squeeze story