FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A gear-driven supercharger has an impeller drive ratio of 8:1. If the engine crankshaft is turning at 2,500 RPM, what is the impeller speed?
- A25,000 RPM
- B22,500 RPM
- C17,500 RPM
- D20,000 RPM
Show answer & explanationAnswer & explanation
Correct answer: D. 20,000 RPM
Impeller speed is found by multiplying crankshaft RPM by the gear drive ratio: 2,500 RPM × 8 = 20,000 RPM. This high step-up ratio is necessary because the impeller must spin extremely fast to compress the induction air effectively.
Why the other options are wrong
- A. 25,000 would result from multiplying by 10, not 8.
- B. 22,500 would result from multiplying by 9, an incorrect ratio.
- C. 17,500 would result from multiplying by 7, not the given 8:1 ratio.
Supercharger Gear Ratio Calculation
The impeller speed of a gear-driven supercharger is found by multiplying the crankshaft RPM by the supercharger's step-up gear ratio.
- Formula: Crankshaft RPM × Gear Ratio = Impeller RPM
- Superchargers use high step-up ratios (commonly 6:1 to 12:1) since the crank turns much slower than the impeller needs to
- High impeller speeds increase induction air pressure and density
Memory trick: Multiply crank RPM by the gear ratio to find how fast the impeller spins.