Supercharging

Supercharging/Turbosupercharging

Purpose

To increase the mass airflow to an engine above that available from ambient air pressure

The manifold pressure of a normally aspirated engine is limited to just below the value of ambient atmospheric pressure

A supercharger is an air pump that “packs” extra air molecules into the induction system

Supercharging/Turbosupercharging

The purpose of supercharging is to increase volumetric efficiency

This deals with how much fuel/air mixture can actually be placed into the cylinder during the induction stroke using an air pump

The mechanism by which the air pump is driven can be broken into two groups

• Supercharging

• Turbo supercharging

Supercharging

A supercharger air pump is driven via the engine crankshaft

Superchargers usually compress the fuel/air mixture after it leaves the carburettor

The amount of supercharging that can be done is restricted by the temperatures produced, to avoid the problems of pre-ignition and detonation

A super charger may have one or several levels of pressure increase called stages

Supercharging

The possible pressure increase will be limited to the physical structure of the engine

If 29.92” Hg manifold pressure is ambient, 40” Hg manifold is feasible for an engine to handle

As altitude increases supercharger speed may be increased.

Turbo-Supercharging

Turbochargers, like superchargers deliver compressed air to the engine

Turbochargers derive their power from the energy of the engine exhaust gases directed against a turbine wheel

The turbine wheel is connected by a shaft to a compressor which then packs air into the inlet manifold

Turbo-Supercharging

To derive energy to operate a supercharger some power is removed from the crankshaft which is now not available to the propeller

Using engine exhaust to power a turbocharger is almost like getting something for nothing but there are some disadvantages

The turbine and compressor routinely rotate at very high speeds and the turbine which drives the entire unit is subject to extremely high engine exhaust temperatures

Turbo-Supercharging If exhaust gases operate the compressor there needs to be a system to control the free wheeling turbine and the resultant air pressure at the inlet manifold

This is accomplished with a wastegate which controls the amount of exhaust that hits the turbine rotor

Opening the waste gate directs the exhaust gases away from the turbocharger turbine wheel and no extra air pressure is obtained

Closing the waste gate directs the exhaust gases to the turbocharger turbine wheel and the energy derived form the wheel is converted to energy in the inlet manifold

Turbo Supercharger

Turbo Supercharger

Turbo Supercharger

A significant problem with turbochargers is the compressed air being pumped into the intake manifold being hot Hot air both destroys engines (detonation) and robs them of power

To reduce the extent of the heat problem intercoolers are placed between the compressor discharge and the intake manifold to cool the hot compressed air before it goes into the engine

The disadvantages of using intercoolers are increased weight, and drag (use of ram air)

Turbo Supercharger Operation

The control of power provided by a turbocharger is via control of the wastegate

The wastegate is either controlled manually or automatically

Manual control is usually accomplished with a vernier control in the cockpit connected by cable directly to the wastegate

During climb the wastegate would be progressively closed as the outside air density decreased; and as manifold pressure reduced, intake pressure could be made up back to the desired value

Turbo Supercharger Operation

Automatic wastegate control is accomplished by regulating the wastegate between the fully open and fully closed positions to maintain a constant power output

The position of the wastegate is controlled by oil pressure acting on the wastegate valve

When oil pressure is increased on the piston the wastegate valve moves towards the closed position and intake pressure increases

Conversely when oil pressure is decreased (or oil pressure is lost) the wastegate valve moves to the open position and power decreases

Turbo Supercharger Operation

Every power setting has a critical altitude

Critical altitude is where an increase in height results in a power loss because the wastegate has gone to the fully closed position

Modern turbochargers have over boost protection but do not rely on it as wastegates are known to jam

Supercharging of an aircraft engine increases its power output by

A. Increasing the mass of mixture entering the cylinders during each cycle

B. Allowing a briefer compression stroke

C. Increasing the mass of mixture entering the cylinders during each stroke

D. Increasing the temperature of the fuel-air mixture

……. and the answer is ………….

Whilst increasing the volumetric efficiency of an engine, supercharging

A. increases the density and decreases the temperature of the fuel-air mix

B. increases the density and temperature of the fuel- air mix

C. increases the temperature and decreases the potential energy of the charge

D. increases the pressure of the charge by dropping its temperature

……. and the answer is ………….

What effect does turbo supercharging have on compression ratio?

A. none

B. increases the compression ratio as the wastegate is closed

C. increases the compression ration as the wastegate is opened

D. increases the compression ratio whenever power is increased regardless of wastegate operation

……. and the answer is ………….

If there is an appreciable leak in the exhaust pipe downstream of the turbocharger wastegate the result would be

A. loss of power output

B. excessive power output

C. no effect on power output

D. fluctuating power output

……. and the answer is ………….