Due to the reduction in air density by gaining altitude, lower air pressure or higher ambient temperatures an aircraft engine loses some of its rated power. At about 10000 ft the air pressure has dropped around 25% compared to sea level. Thus the engine gets 25% less air per intake stroke and therefore is reduced in power output. As a results aircraft performance suffers in climb ratio's, cruise speed and more. Even the pilot in an unpressurized cabin suffers and must be using extra oxygen when staying longer than thirty minutes above 10000 ft.
But carrying oxygen bottles for the engine is not very practical considering the amount of air it needs per minute. We therefore need a different solution. This is done in the form of an exhaust turbocharger or a geared supercharger and they compress the air before feeding it to the engine. And on these pages we will discuss both systems.
Turbo's are used more often than geared superchargers, but they place certain demands on the aircraft and pilot skills alike. Handling a turbo engine requires a bit more thought and training for the pilot.
With the turbocharger, the impeller (compressor) is linked together to a turbine driven by exhaust gas. The compressor is a radial or centrifugal flow type and the turbine is usually a radial flow, sometimes an axial flow turbine is used. The turbo uses engine oil for lubrication of the bearings. See the image to the right.
These devices are almost as old as the engines themselves. The one major obstacle was the difficulty in operating the turbine wheel in the white hot exhaust gasses. New metallurgy was needed to overcome these problems. The fact that the RPM of a turbocharger (over 80000 RPM) was so high, also placed some extra demands on the bearings and lubrication oil for the turbo.
The turbocharger uses the energy left in the exhaust gas (heat and pressure), it is more efficient than the geared supercharger which is mechanically driven by the engine crankshaft. The only drawback maybe is that the turbo creates some back pressure which reduces overall efficiency a bit. But of these two systems turbocharging is the more efficient one.
If all exhaust is directed to the turbine then with increasing engine RPM the turbo would spin faster and faster, possibly until destruction. At the same time the compressor would try to do its best to compress intake air fed to the engine. The pressure and temperature in the intake manifold could be more than the engine could handle leading also to its destruction.
To control this process, a valve is placed in parallel with the turbine and the exhaust and when it is closed, almost all gasses are led to the turbine. This way it is possible to regulate the amount of gas to the turbine and so the intake manifold pressure.
The waste gate can be fixed or automatically / manually controlled to regulate the manifold pressure within its limits. With a fixed waste gate there is nothing a pilot can do but to retard throttle if anything goes awry. When applying power on take off, care is needed not to overboost the engine.
An automatically controlled waste gate uses some kind of logic (or a membrane connected to the inlet manifold) to control the amount of exhaust gasses to bypass the turbine. Leading to a better efficiency and easier control.
Engine charging can be turbo-normalizing, ie maintaining sea level pressure (~30" MAP) in the intake manifold until the waste gate is fully closed, this way the engine will keep its rated sea level power up to the altitude where the waste gate is fully closed. From that point on power output will drop as can be expected.
With turbocharging the inlet manifold is pressurized to about 40" (or even higher in some radial engines) putting more air into the cylinders. And more air in the cylinder means that more fuel can be burned thus more power is developed by the same engine. Although some strengthening is usually needed. To help avoid detonation most turbo engines have lower compression end values as intake air will already be under pressure.
Personally I would go for the turbo-normalized system as this will lead to the least amount of added stress in the engine and thus increases reliability and prolongs engine life so that it can run up to its specified TBO and even beyond.
The Bombardier Rotax 914 and 915iS are turbocharged versions of the normally aspirated Rotax 912(iS), power went up from 80 to 115/100 (135) bhp and they can maintain 100/127 bhp up to 16000/23000 ft. But equally, or maybe more, the complexity of the engines increased too.
These engines have an electronic control box and numerous sensors for temperature and pressure to maintain and control the turbo, its inherently complex and the pilot needs to handle the turbo with care regarding cool down time after landing and when applying take-off power.
We have an AMT article for those interested in more information on turbochargers.