Engine Turbocharger with Waste Gate
Turbo & Supercharging, I
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 some 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 found in the form of an exhaust turbocharger or a geared supercharger and they are driven by the engine compressing the air before feeding it to the engine. And on these pages we will discuss both systems and see how they operate.
Induction & Exhaust
Air needed for combustion is fed through the intake filter (which cleans the air from any damaging particles like vulcanic ash or other dust), to the carburettor (not so on a diesel) into inlet manifold and in to the combustion chamber, afterwhich it leaves via the exhaust manifold. In this process and due to the design on the air inlet (mainly drag), pressure is lost and the engine can not get its full volumetric efficiency.
Climbing to altitude also reduces 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 or better conditions.
So we need to compensate for the loss of air density by compressing the intake air before it enters the combustion chamber and this proces will make up for the loss of air pressure. By doing this we can reach a couple of objectives: regain lost power due to climbing by keeping sea level pressure to a preset value in the air intake upto a certain altitude or increase sea level power by increasing the inlet pressure to higher than sea level pressure.
The maximum amount of supercharging is limited by engine design and by the inevitable 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.
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 top cowling 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 tubing 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 pressure.
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 and on the next page.
These were used on the larger piston aero engines of bygone days. Think about the Merlin engines in the P51 Mustang, Hurricane and/or Spitfire aircraft. These superchargers were mechanically connected to the crankshaft via a single, dual or multistage gearbox. Either can be operated by the pilot or automatically for convenience.
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 Benz to name one.
Intake air is fed through the carburettor and to the impeller of the supercharger. The big advantage is that the air/fuel mixture will be thoroughly mixed and atomized before being fed to the engine, resulting in a much more efficient combustion and a bit more power.
As airplanes grew larger this meant that engines needed more power too, so superchargers also 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.