1. Supercharging
Chapter 8

2. Aim
Understand the principals of operation of superchargers

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

13. 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

14. Questions?