A turbo-charger consists of a turbine (hence turbo) which drives a compressor to charge or force more air into the engine's inlet manifold. The turbine is driven by exhaust gases from the exhaust manifold and so runs at a high temperature - it is coloured red in the cut-away turbocharger (below right). The compressor (blue) runs much cooler although the very act of compressing air raises its temperature. The turbine and the compressor are invariably (?) of the single stage centrifugal type.
Hot air is less dense than cold air so many turbocharged engines include an intercooler "radiator" which cools the compressed air, further increasing its density. For maximum power you want to get a large mass of air into the engine, not necessarily a large volume. The intercooler can either be of the air-to-air or the air-to-water type. The former is simpler and lighter.
The turbine and compressor are connected by a short shaft which generally runs on plain bearings. The speed of revolution is very high, of the order of 100,000 rpm so good lubrication is crucial. The shaft conducts heat from the turbine and is largely cooled by oil flow to the bearings. For this reason some manufacturers specify that the engine should not be turned off immediately after heavy work, but should be allowed to idle for a minute or two. Many turbocharged engines require higher quality oils and more frequent oil changes and these conditions should be strictly adhered to. A very few turbochargers are water cooled for improved cooling.
The turbine extracts waste energy from the exhaust gases. Its speed rises as the throttle opens often with a characteristic turbo "lag" or delay. The turbo lag can be minimised by having a small light turbocharger which will spin up quickly and by minimising the volume of air ways to be pressurised.
The amount of boost depends on turbo speed and thus throttle opening, giving most boost at wide openings and high engine revs - just what you want. However, if there is acceptable boost in the mid range the boost at the top end might be too high, causing engine damage or rendering the car hard to drive. A waste-gate senses the pressure in the inlet manifold and opens a valve to allow some exhaust gases to bypass the turbine, thus limiting the maximum boost pressure. The black rubber tube (above right) taps into the air outlet of the turbocharger and feeds to an actuator which opens the waste gate.
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