A mechanic is comparing supercharging methods on two reciprocating engines. Engine A uses a gear-driven supercharger powered directly by the crankshaft, while Engine B uses a turbocharger. What is a key operational difference between these two systems?
- AThe gear-driven supercharger's boost is proportional to exhaust gas energy, while the turbocharger's boost is proportional to engine RPM
- BThe gear-driven supercharger requires a wastegate to control boost, while the turbocharger does not
- CThe gear-driven supercharger draws power directly from the crankshaft, reducing usable engine power, while the turbocharger is driven by exhaust gases that would otherwise be wasted
- DThe gear-driven supercharger only operates at altitude, while the turbocharger only operates at sea level
Show answer & explanationAnswer & explanation
Correct answer: C. The gear-driven supercharger draws power directly from the crankshaft, reducing usable engine power, while the turbocharger is driven by exhaust gases that would otherwise be wasted
A gear-driven (engine-driven) supercharger takes mechanical power from the crankshaft to compress intake air, which reduces the net power available for propulsion. A turbocharger, by contrast, uses exhaust gas energy that would otherwise be exhausted overboard, making it more efficient in terms of parasitic power loss.
Why the other options are wrong
- A. This reverses the relationship; gear-driven boost follows RPM, turbocharger boost relates to exhaust energy.
- B. Wastegates are used on turbochargers to control boost, not gear-driven superchargers.
- D. Both systems can operate across altitude ranges; this statement is factually incorrect.
Gear-Driven Supercharger vs Turbocharger
A gear-driven supercharger is mechanically driven by the crankshaft, consuming engine power, while a turbocharger is driven by exhaust gas energy, recovering otherwise wasted energy.
- Gear-driven supercharger reduces net crankshaft power output
- Turbocharger uses exhaust-driven turbine to drive compressor
- Turbochargers generally more efficient due to using waste exhaust energy
Memory trick: Gear-driven steals crank power; turbo recycles exhaust power