Supercharging & Turbocharging
Restoring manifold pressure with altitude, engine-driven superchargers vs exhaust-driven turbochargers, wastegate and critical altitude.
Supercharging & turbocharging, summary notes
- Supercharging/turbocharging compresses the induction air to restore the manifold pressure (and hence power) lost as the aircraft climbs and the air thins, or to boost it above ambient at low level.
- A SUPERCHARGER is a compressor driven mechanically from the crankshaft (through gearing): instant response and no lag, but it consumes engine power to run.
- A TURBOCHARGER drives its compressor from a turbine spun by the otherwise-wasted EXHAUST gas: it takes no direct shaft power (better efficiency) but has some spool-up lag; a wastegate regulates boost by diverting exhaust around the turbine.
- 'Critical altitude' is the highest altitude at which the system can still maintain the rated manifold pressure; above it, MAP and power fall off. An intercooler cools the compressed charge to raise density and reduce detonation risk.
- 'Turbo-normalising' merely restores sea-level MAP at altitude; full 'turbo-charging' boosts MAP above ambient for extra power, demanding careful management to avoid detonation and overboost.
- ⚠ Exam trap: a supercharger is driven by the ENGINE (crankshaft), a turbocharger by the EXHAUST gas; the wastegate controls a turbocharger's boost by bypassing exhaust around the turbine, it does not drive a supercharger.
What is the fundamental difference in how a supercharger and a turbocharger are driven, and what practical trade-off does it create?
A supercharger is geared to the crankshaft and driven mechanically, so it responds instantly but consumes engine power. A turbocharger is driven by the exhaust-gas turbine, using energy that would otherwise be wasted, so it is more efficient and doesn't sap shaft power, but it has some lag before the turbine spools up and its boost must be regulated by a wastegate.
Supercharging & turbocharging
Boosting concept map
Supercharging & Turbocharging
Supercharging & turbocharging quiz
Supercharging & Turbocharging, quiz
1. A supercharger is fitted to a piston engine to:
Restore or increase manifold pressure, maintaining power at altitudeReduce fuel consumption at sea levelCool the cylinders2. A turbocharger differs from a mechanically driven supercharger because it is driven by:
Exhaust gas energyThe crankshaft through gearsAn electric motor only3. An intercooler (charge cooler) is fitted after the compressor to:
Cool the charge, raising its density and reducing detonation riskWarm the charge for better burningFilter the air4. Ambient pressure is 20.6 inHg at 10 000 ft and the turbocharger holds a pressure ratio of 1.7. The manifold pressure available is:
35.0 inHg22.3 inHg12.1 inHg5. The purpose of supercharging/turbocharging is to:
Restore/boost manifold pressure lost with altitudeCool the exhaustReduce compression ratio6. A supercharger is driven by the:
Engine crankshaft (mechanically)Exhaust gasBattery7. A turbocharger is driven by the:
Exhaust gas (a turbine)Crankshaft directlyOil pump8. A turbocharger's boost is regulated by the:
Wastegate (bypassing exhaust around the turbine)Mixture controlCowl flaps