Superchargers & turbochargers: forced induction
Forced induction compresses intake air before the cylinders, to hold manifold pressure as altitude climbs.
| Supercharger | Turbocharger | |
|---|---|---|
| Drive | Air compressor gear-driven off the engine | Compressor spun by turbine on exhaust gas |
| Power cost | Uses engine power | None: uses wasted exhaust energy |
| Induction manifold (carries mixture to cylinders) | Sits between fuel metering device and manifold | Air pressurised into the manifold |
Waste gate: adjustable butterfly valve in the exhaust.
- Closed: exhaust goes through the turbine, more boost.
- Open: exhaust bypasses the turbine.
Critical altitude: highest altitude where a turbocharged engine still makes full rated sea-level power.
- Above it: power falls like a normally aspirated engine.
- Waste gate fully closed at this point.
- Supercharged engine holds power until its critical altitude (PHAK ch.11).
Overboost: MAP above the engine limit.
- Manual waste gate: pilot watches MAP gauge. Gate left closed after descent causes overboost, risks severe detonation from the leaning effect of denser air.
- Automatic waste gate: can still overboost on takeoff power with cold oil.
- Advance throttle cautiously to stay under MAP limit.
Trap: tell apart by power source, not effect. Supercharger: engine-driven, costs power. Turbocharger: exhaust-driven.
References
- PHAK ch.7
- PHAK ch.11