Engine Turbocharger with Waste Gate

Turbo- Supercharging Aircraft Engines

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 25%. Thus the engine gets 25% less air per intake stroke and therefore is reduced in power output. Aircraft performance suffers in climb ratio's, cruise speed and more. Even the pilot in an unpressurized cabin suffers and must be on 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 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 this page we will discuss both systems.

Induction and Exhaust

Air needed for combustion is fed through the intake filter, carburettor (not on a diesel) inlet manifold to the combustion chamber and leaves via the exhaust manifold. In this process and due to the design on the air inlet, pressure is lost and the engine can not get its full volumetric efficiency. Climbing reduces the air pressure around the aircraft and that is not helping either. A normally aspirated engine thus develops full rated power only on sea level with the throttle wide open and under ISA conditions.

So what is needed is that we compensate for the loss of air density by compresssing the intake air before it enters the combustion chamber and this will make up for the loss of air pressure. By doing this we can reach a couple of objectives: regain power lost by climbing by keeping sea level pressure set in the air intake until a certain altitude or increase sea level power by increasing the inlet pressure to higher than sea level pressure.

Avoiding detonation

The maximum amount of supercharging is limited by engine design and by the uninevitable rise in temperature due to compression of air which could cause detonation. Intercoolers, lowering the compression of the engine, higher octane fuel all help reducing the risk of detonation. On the pilot side: avoiding too high MAP with a too low RPM (for example: 35" MAP and 1800 RPM) will help too.

Ram air

Some aircraft use ram air to reduce the loss of ambient air pressure during climb. Air is taken in at the front of the engine so that the air is pressurized by the speed of the aircraft. Lancair uses a special valve so that the pilot can select either filtered air for ground operations and unfiltered ram air when flying at altitudes higher than 1000 ft. DynAero uses an air intake in the topcowling with a coarse prefilter, so the engine always has filtered ram air.

For maximum performance and the least amount of loss in the intake system: keep it as short as possible, use wide diameter tubes, no sharp bends and make use of ram air by installing the air intake into the airstream pointing forward for good use of airspeed.

We have two methods available of supercharging: by a geared (single or multiple stages) supercharger or by an exhaust driven turbocharger. Both principles will be discussed below.

Geared Superchargers

These were used on the larger piston aero engines of bygone days. Think about the Merlin engines in the Mustang, Hurricane and/or Spitfire aircraft. These superchargers were mechanically connected to the crankshaft via a single, dual or multistage gearbox. Either operated by the pilot or automatically.

Today you can find these superchargers (or roots blower types) on some two stroke aero diesel engines for starting and low power operation. A number of car manufacturers still use them too, Mercedes to name one.

Intake air is fed through the carburettor and to the impeller of the supercharger, an big advantage is that the air/fuel mixture will be thoroughly mixed and atomized before being fed to the engine, resulting in a more efficient combustion.

As engines needed more power superchargers grew bigger. But this added weight and running the supercharger also costs engine power thus the net result was not as high as could be possible. And, out from the exhaust pipes came high temperature, high velocity gasses which still had a lot of energy left in them. Thus the exhaust driven turbocharger invention was closeby.

Exhaust Turbochargers

Engine Turbocharger

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.

These devices are almost as old as the engines themselves. The one major obstacle was the difficulty in operating the turbinewheel in the white hot exhaust gasses. New metalurgy 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 backpressure which reduces overall efficiency a bit. Of these two systems turbocharging is the more efficient one.

Waste Gate

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 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 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.

Turbo-normalizing and charging

Engine charging can be turbo-normalizing, ie maintaining sea level pressure (~30" MAP) intake pressure until the waste gate is fully closed, or turbo-charging where inlet manifold is pressurised upto about 40" (or even higher in some radial engines). And more air in the cylinder means that more fuel can be burned thus more power is developed by the same engine. Although some strenghtening is usually needed.

The Bombardier Rotax 914 is turbocharged version of the normally aspirated Rotax 912, power went up from 80 to 115/100 bhp. But equally, or maybe more, the complexity of the engine.

We have an AMT article for those interested in more information on turbochargers.