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Which of the following statements is correct regarding aircraft engine turbochargers?
  • A
    The waste-gate is situated in the inlet manifold.
  • B
    When the waste-gate is fully open the turbocharger is running at full speed.
  • C

    The waste-gate actuator is controlled by the waste-gate.

  • D
    The waste-gate diverts exhaust gas to drive the turbine.

Refer to figure.
Learning Objective Power-augmentation devices: explain the requirement for power augmentation (turbocharging) of a piston engine.
Learning Objective Describe the function and the principle of operation of the following main components of a turbocharger: turbine; compressor; waste gate; waste-gate actuator.

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.

Turbochargers (and any superchargers, really) increase the amount of air (and therefore fuel) we can ignite in the cylinders, and are measured by the MAP; the higher the MAP, the more air/fuel mixture we can burn in each power stroke.

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 mixing with fuel and then entry into the cylinders (or sent straight into the cylinders where fuel injection takes place). 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.

The turbochargers we talk about are controlled by waste-gates, which are situated in the exhaust manifold, and decide how much exhaust gas to divert to the turbine; if the MAP is too low (for what has been asked for), then the waste-gate will send more air to the turbine to get a faster spinning compressor, and vice-versa. The movement of the waste-gate is powered by the oil pressure on the waste-gate actuator which is cleverly controlled by a system that measures the MAP and makes changes to the oil pressure accordingly, thereby moving the waste-gate actuator, and the waste-gate position.

To go a little deeper, the waste-gate (as per its name) actually controls how much exhaust gas is allowed to bypass the turbine, and by closing the waste-gate and letting no gas escape would mean that all the gas would go through the turbine (to cause a high MAP) and vice versa (open waste-gate for slow turbine/compressor for lower MAP).

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