SuperchargingChapter 8

Aim

Understand the principals of operation of superchargers

Objectives1.Discuss the purpose of supercharging the engine2.State the principals of operation of the internal

(geared) supercharger3.State the principals of operation of the

turbocharger4.Describe operating limitations of supercharged

engines

1. Purpose of SuperchargingNaturally Aspirated Engines

The induction system for a naturally aspirated engine consists of:• An air intake• Ducting• Carburettor or fuel control unit

Air intakes are usually forward facing to take advantage of ram air, improving volumetric efficiency and performanceRam air is often unfiltered therefore most engines incorporate an alternate, filtered air sourceTo improve efficiency the intake air should be as cold as possible, carburettor engines use carb heat to prevent icing, this is typically unfiltered air from around the exhaust manifold

1. Purpose of SuperchargingReasons for Supercharging

With a naturally aspirated engine, maximum power is produced at sea levelAs the aircraft climbs air density reduces therefore the density of the charge reduces, reducing the power output.Eventually the aircraft will reach an altitude where full throttle is required to maintain straight and level flight (absolute ceiling)In order to compensate for this reduction of density at altitude and increase overall power, we can increase the pressure in the intake manifold by the use of a superchargerThere are two types of superchargers in use:

• Geared (or internal) supercharger• Exhaust driven turbocharger

2. Geared SuperchargerSupercharger

Geared supercharging systems are typically only found on old, large piston engines such as the Rolls Royce Merlin.An engine driven gear train drives an impellor which accelerates the mixture outwards where it enters the diffuserThe diffuser slows down the mixture, converting kinetic energy into pressure, from here the mixture will enter the induction manifoldThe swirling action induced on the mixture improves mixing of the fuel and airAs air is compressed its temperature increases, this temperature increase aids in vaporisation

2. Geared SuperchargerSupercharger

Superchargers can be classified as single-stage, two-stage or multi-stage depending on the number of impellor stages used to increase pressure of the chargeHere we can see the two-stage supercharger on the back of a Rolls Royce Merlin engine

3. TurbochargerTurbocharger

More commonly found on modern piston aircraft is the exhaust-driven supercharger, known as the turbochargerIn a turbocharged system the impeller is coupled with a turbine which is powered by the exhaust gas as it departs the engineCompared to the supercharger, the turbo charger is far more efficient as it utilizes energy in the exhaust gases that would otherwise be wastedThe penalty comes from backpressure created in the exhaust systemTurbochargers were not used in early piston engines due to limitations on the material used in turbine manufacture. The turbine must be able to handle extremely high temperatures, typically up to 1000⁰C in light piston aircraft

3. TurbochargerTurbocharger

The principal of operation of the impeller is the same as a supercharger. The system shown uses a fuel injection system rather than a carburettor, so instead of the charge being compressed, air alone is compressed and mixed with fuel in the inlet manifoldThe turbine is placed in the exhaust efflux, control of the turbine depends on the position of the waste gateThe waste gate can either be fixed or variable:

• In the fixed system the waste gate is set on the ground to produce optimum power at a specific altitude. This limits the amount of boost available at higher altitudes and may lead to overboosting at lower altitudes

3. TurbochargerTurbocharger• The variable waste gate allows the pilot to adjust the amount of boost

being produced in the air. There are three systems that can be used for this: 1. A mechanical lever that can be set by the pilot. This will usually be located

next to the throttle2. Direct coupling with the throttle,

as the throttle is increased the waste gate will close

3. With an automatic control system (as shown in the diagram), the pilot sets the required manifold pressure and through use of an aneroid capsule the waste gate actuator moves automatically to maintain the selected manifold pressure

4. Operating LimitationsOver boosting

Over boosting occurs when maximum manifold pressures are exceededCorrectly adjusted controllers and proper throttle use will prevent over boostingOver boost can still occur when the engine oil has not yet reached operating temps and the throttle is rapidly advanced for take off or when a pilot on short final quickly advances the throttle for a go-around and the prop governor lags or the waste gate sticksOver boost can also occur at full throttle and below critical altitude when exhaust gases expelled are more than capable of driving an uncontrolled compressor to engine damaging speeds

4. Operating LimitationsTurbo lag

Turbo lag is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating from idle as compared to a naturally aspirated engine.Inertia, friction, and compressor load are the primary contributors to turbo lag.Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.

4. Operating LimitationsThrottle control

Extra care must be taken to operate the throttle smoothly to prevent engine surgeThe throttle is very sensitive and due to turbo lag it requires time for the turbine to ‘catch up’ to any throttle increases. The turbine itself can reach speeds of up to 120,000 RPMAny reduction in power must be done with care to prevent shock cooling and allowing enough time for the turbine to spin down. Often the aircraft flight manual will prescribe minimum times for ground operation before shutdown to aid this

Questions?