FAA Aviation Mechanic General (AMG)Physics, Math and Fluid LinesMedium
An engine cylinder has a total volume of 88 cubic inches with the piston at the bottom of its stroke, and a clearance volume of 8 cubic inches with the piston at the top of its stroke. What is the engine's compression ratio?
- A10:1
- B8:1
- C88:1
- D11:1
Show answer & explanationAnswer & explanation
Correct answer: D. 11:1
Compression ratio equals total cylinder volume divided by clearance volume: 88 ÷ 8 = 11. This is expressed as 11:1, meaning the fuel-air mixture is compressed to 1/11 of its original volume.
Why the other options are wrong
- A. This is close but results from a rounding/division error.
- B. This is simply the clearance volume value, not the ratio.
- C. This is the total volume value, not divided by clearance.
Compression Ratio
The ratio of total cylinder volume (piston at bottom) to clearance volume (piston at top), indicating how much the fuel-air mixture is compressed.
- CR = Total volume ÷ Clearance volume
- Higher CR generally increases engine efficiency and power
- Excessive CR can cause detonation without proper fuel
Memory trick: 'Big volume squeezed into small volume' gives the ratio.