A turbocharger is an exhaust gas driven turbine driving a compressor unit. The compressor unit compresses the incoming air. The compressed air is then used in the engine to combust fuel. By using compressed rather than uncompressed air the efficiency of the engine is improved. Most of the work done to compress the air would otherwise have been wasted, so with a little tradeoff the efficiency of the whole engine is improved.
When air is compressed it generates a lot of heat. The compressed air from the turbo must be cooled before it is introduced to the engine. This cooling is done with something called an 'after-cooler'. This is also known as an 'Intercooler', or a 'charge air cooler'. These are fancy names for something that is really just a type of radiator. This cooling process is important to prevent fuel pre-ignition and engine 'knocking'.
The cooled compressed gas then passes into the inlet manifold where it is ready for use in the cylinders to burn the fuel. The increased mass of oxygen in a fixed volume of compressed air allows more fuel to be burned, or for a much cleaner, cooler burn of the normal amount of fuel.
As the turbo spins very fast (10,000 to 100,000 rpm depending on size, weight and design), care must be taken in maintaining it. A turbo 'letting go' and shedding its blades is not a pretty sight, as well as being expensive. This speed also causes problems for standard ball bearings, which would explode in a turbo. All but the most expensive turbo-chargers use a fluid bearing. The fluid bearing of a turbo is a flowing layer of oil which suspends and cools the moving parts. More expensive turbochargers use incredibly precise ball bearings because they offer less friction than a fluid bearing. This lower friction in turn allows the turbo shaft to be built with lighter materials, which reduces something called 'lag'.
Lag is felt by the driver of a turbocharged vehicle. Lag is the time delay between pushing on the accelerator pedal, and feeling the turbo 'kick-in'. This lag is due to the fact the turbine needs to get up to speed before it can perform its magic.
Lag can be reduced by reducing the rotational inertia of the turbine. Using lighter parts is one way to allow the spin-up to happen more quickly, and in this way the lag is reduced. Another way to reduce lag is to change aspect ratio of the turbine so that the diameter is reduced and the width is increased. Lag is also reduced by using a precision bearing rather than a fluid bearing, but this last one is to do with reducing friction rather than rotational inertia.
As long as the oil supply is clean and the exhaust gas doesn't get too hot, a turbocharger is very reliable. Regular cleaning of both the exhaust driven turbine side and the air compressor side of the turbo is recommended to remove any build-up of soot and dust.
A turbocharger is related to a supercharger in that both compress air for combustion. Superchargers are spun using energy directly from the engine as opposed to using energy from engine exhaust.
Turbocharging is very common on Diesel engines, both for conventional automobiles and also for truck, marine, and heavy machinery applications. In fact, for current automotive applications non-turbocharged diesel engines are becoming increasingly rare. Diesels are particularly suitable for turbocharging for several reasons:
- naturally-aspirated diesels have inferior power-to-weight ratios to gasoline engines, turbocharging can address much of this deficit.
- Diesel engines require more robust construction because they already run a very high compression ratio, so they generally require little additional reinforcement to be able to cope with the addition of the turbocharger. Gasoline engines often require extensive modification for turbocharging.
- Diesel engines have a narrower band of engine speeds at which they operate, thus making the operating characteristics of the turbocharger over that "rev range" less of a compromise than on a gasoline-powered engine.