As has been stated previously, a turbocharger
is a two sided device primarily comprised of a turbine
wheel fed by exhaust air
connected to a separate compressor
wheel via a spinning shaft that compresses intake air. More air in the cylinder allows more fuel to be burned by power stroke, producing more power.
It is a common misconception that the exhaust turbine of a turbo is driven purely by the kinetic energy of the exhaust gases. While the kinetic energy of the exhaust flow does contribute to the work performed, the vast majority of the energy transfered comes from gas expansion. As the exhaust pulse exits the cylinder, it is at high pressure and temperature. The relatively high energy exhaust pulse enters the small volume turbine inlet. Passing through the diffuser and into the turbine housing, the exhaust gases expand and cool. The gas expansion pushes against the turbine blades, producing that work needed to drive the compressor wheel.
Turbines are incredibly efficient devices, but only when operated in a rather limited RPM range (turbine shaft RPM, not engine RPM). That is, a turbine/compressor is very efficient at a certain RPM/flow capacity, but once the shaft RPM deviates much, the efficiency drops off significantly. Spun too fast, the turbine blades cavitate and stall aerodynamically. Spun too slow, and the blades don't have enough air volume to compress. As a result, flow drops. This is why it is essential to match the size of the turbine and compressor wheels to the size of your engine. If your engine is too small, it will not produce enough exhaust gas energy to spin the turbine wheel fast enough to make boost. If your engine is too big, the turbo will be too small to produce the required air flow.
As one might expect, the compressed air at the outlet of the compressor housing of the turbo will have an increase in temperature (pv = Nrt). An intercooler is typically employed in the air stream past the compressor to cool the air before it is injested by the engine.
Turbochargers are positive feedback devices. As more boost is made, more exhaust is created, which in turn, makes more boost. Typically a device called a wastegate is employed, manipulated by a mechanical or electronic boost controller to bypass the exhaust gas around the turbine and directly into the exhaust pipe. This regulates the speed of the turbine so that a costant level of boost is produced.
As the turbine housing of a turbocharger is typically connected directly to the exhaust manifold of an engine, it operates at a very high temperature. The shaft housing of the turbo is usually connected to the vehicle oiling system as the shaft bearing rides on a thin film of oil. Engine coolant is also routed through the turbocharger housing to keep it cool.
Turbochargers are conceptually very simple, but in practice are extremely complicated devices. Typical turbine shaft speeds range between 80,000-100,000 RPM. The turbine and compressor blades, as well as the rest of the components and housings, must be precisely machined and balanced. Or the turbo will go BOOM.