As an aircraft climbs, the air gets thinner. A normally aspirated piston engine breathes less air with every thousand feet, so it produces less power. Two devices solve this problem in different ways: the turbocharger and the supercharger.
Why piston engines lose power with altitude
Engine power depends on how much air (and therefore oxygen) reaches the cylinders. Air density falls with altitude, so a normally aspirated engine typically loses a significant share of its sea-level power by the time it reaches medium altitudes. Compressing the intake air before it enters the engine restores that density.
Turbocharger: powered by exhaust gas
A turbocharger has two wheels on one shaft. Hot exhaust gas spins a turbine, and the turbine drives a compressor that squeezes the intake air. Because it uses energy that would otherwise be wasted out of the exhaust, it is efficient.
- A wastegate controls how much exhaust passes through the turbine, and therefore how much boost is produced.
- Turbo-normalised engines use the turbo only to maintain sea-level power at altitude; turbosupercharged engines can produce more than sea-level pressure.
- The critical altitude is the highest altitude at which the engine can still produce its rated power, with the wastegate fully closed.
- Compressing air heats it, so many systems add an intercooler.
Watch: the difference in 60 seconds
Supercharger: driven by the engine
A supercharger's compressor is driven mechanically, through gears or a belt from the crankshaft. It responds instantly because it does not wait for exhaust flow to build up. The trade-off is efficiency: the engine spends some of its own power driving the supercharger. Superchargers were widely used on large radial engines in the piston-airliner and wartime era.
Side by side
| Turbocharger | Supercharger | |
|---|---|---|
| Driven by | Exhaust gas turbine | Engine crankshaft (gears or belt) |
| Efficiency | High: uses waste energy | Lower: takes engine power |
| Response | Slight lag while boost builds | Immediate |
| Complexity | Hot section, wastegate control | Mechanically simpler |
Pilot considerations
- Move the throttle smoothly to avoid over-boosting the engine.
- Watch manifold pressure and temperatures, especially turbine inlet temperature.
- Allow a cool-down period before shutdown on turbocharged engines, so the turbo's bearings are not damaged by heat soak.
Quick revision
- Both compress intake air to keep power at altitude.
- Turbo: exhaust-driven, efficient, wastegate-controlled.
- Super: engine-driven, instant response, costs engine power.
- Critical altitude: the highest altitude at which rated power is still available.
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