A ground boosted turbocharger increases manifold pressure only during take-off.
An altitude boosted turbocharger increases inlet manifold pressure once the aircraft has reached 10 000 ft.
An aircraft fitted with an altitude-boosted turbocharger is fitted to allow the aircraft to achieve higher altitudes.
Ground boosted turbochargers are switched to OFF once the aircraft has achieved a safe altitude.
Refer to figure.
Learning Objective 021.10.10.01.03: Power-augmentation devices: explain the requirement for power augmentation (turbocharging) of a piston engine.
Learning Objective 021.10.10.01.05: Explain the difference between an altitude-boosted turbocharger and a ground-boosted turbocharger.
Piston Engines can have their power increased in three main ways, by increasing the size of the engine (displacement), increasing the RPM, or increasing the pressure of the air/fuel mixture (called the Manifold Absolute Pressure, MAP). This last case is why we might use a turbocharger in our car engines, so we can have a small, efficient engine, with extra power available from a turbocharger (which compresses the intake air) when needed.
Turbochargers are arguably even more useful in aircraft, where piston engines become less powerful in the thinner air at higher altitudes. This is where turbochargers can compress that air, to make the engine operate with the same power output it would do at a much lower altitude. We call this an "altitude boosted" turbocharger.
They are more useful to us by keeping the same power up to higher altitudes, rather than giving us lots more power on the ground, which would put a lot of strain on the engine, which would be a "ground boosted" turbocharger.
Ground-boosted turbochargers will not just provide extra power at take-off though, and they will allow for higher MAP (than a naturally aspirated engine) at all altitudes, but do cause more stress on an engine due to this higher MAP.
Turbochargers (and any superchargers, really) increase the amount of air (and therefore fuel) we can ignite in the cylinders, so if we used full turbocharger at take-off, for instance, the forces on the engine would be enormous, and would put too much strain on an engine. We do this in road cars, because we can make engines stronger/heavier, and if they fail it is not life threatening, but with aircraft engines, we cannot afford to gain the extra power for fear of failure, so we mostly use altitude boosted turbochargers to gain extra altitude without causing extra stress on the engine.
A turbocharger works by using the excess speed and pressure of the exhaust gasses to spin a turbine, which then directly spins a compressor, which compresses the fresh intake air ready for entry into the cylinders. All superchargers compress fresh intake air like this, but turbochargers are special types of superchargers that use the excess energy leftover in the exhaust gasses that we would not otherwise use.
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