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Hi Lofty, there is the standard turbo, and also the pressure wave supercharger (A hybrid device)
a normal turbo uses the expansion of exhaust gases to drive a turbine, the other end of the turbine is connected to another turbine that compresses the incoming air.. there is no connection between the exhaust gas and the incoming air.
The pressure wave device uses a common chamber and rotating vane, the pusles of exhaust gas compress a column of air for the inlet cycle, the chamber is designed so that even though the inlet air and exhaust gas share the chamber, the 2 exhaust gas is never re-inhaled. (or virtually never - some slight ingestion can actually be used to reburb any unburnt mixture to aid emissions (this is more commonly known as egr - or exhaust gas recirculation)
The turbo is basically a device that forces air into your engine by using its own exhausts gases. The unit is typically split into two chambers and has one linking axel. One side sits right in the flow of exhaust gases which pass over a propeller to make it spin. In turn this turns the other side of the axel where a impeller is attached, the effect of the spinning impeller is it sucks in air and force feeds it to the engine. The rest is a normal straight forward combustion process. Nice and simple.
as an aside, on a N/A engine.. (normally aspirated) it is theoretically possible to allow quite large quantities of exhaust gas into the induction system to overcome the low pressure effects that the piston has to overcome.
this can improve efficiency overall.
I cannot see this translated to turbo charged applications though.
Should also point out that turbos dont use the exhaust gas to increase their efficiency, they make the engine more efficient because they recycle the wasted energy in the exhaust gas. Thus the entire cycle is more efficient.