Supercharging Chapter 8

Aim Understand the principals of operation of superchargers

Objectives Discuss the purpose of supercharging the engine State the principals of operation of the internal (geared) supercharger State the principals of operation of the turbocharger Describe operating limitations of supercharged engines

1. Purpose of Supercharging Naturally 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 performance Ram air is often unfiltered therefore most engines incorporate an alternate, filtered air source To 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 Supercharging Reasons for Supercharging With a naturally aspirated engine, maximum power is produced at sea level As 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 supercharger There are two types of superchargers in use: Geared (or internal) supercharger Exhaust driven turbocharger

2. Geared Supercharger Supercharger 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 diffuser The diffuser slows down the mixture, converting kinetic energy into pressure, from here the mixture will enter the induction manifold The swirling action induced on the mixture improves mixing of the fuel and air As air is compressed its temperature increases, this temperature increase aids in vaporisation

2. Geared Supercharger Supercharger 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 charge Here we can see the two-stage supercharger on the back of a Rolls Royce Merlin engine

3. Turbocharger Turbocharger More commonly found on modern piston aircraft is the exhaust-driven supercharger, known as the turbocharger In a turbocharged system the impeller is coupled with a turbine which is powered by the exhaust gas as it departs the engine Compared to the supercharger, the turbo charger is far more efficient as it utilizes energy in the exhaust gases that would otherwise be wasted The penalty comes from backpressure created in the exhaust system Turbochargers 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. Turbocharger Turbocharger 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 manifold The turbine is placed in the exhaust efflux, control of the turbine depends on the position of the waste gate The 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. Turbocharger Turbocharger 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: A mechanical lever that can be set by the pilot. This will usually be located next to the throttle Direct coupling with the throttle, as the throttle is increased the waste gate will close 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 Limitations Over boosting Over boosting occurs when maximum manifold pressures are exceeded Correctly adjusted controllers and proper throttle use will prevent over boosting Over 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 sticks Over 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 Limitations Turbo 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 Limitations Throttle control Extra care must be taken to operate the throttle smoothly to prevent engine surge The 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 RPM Any 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?