In non-turbocharged engines, air and fuel is sucked into the combustion chamber as the piston moves downward in the cylinder. In a turbocharged engine, the air that enters the engine is pressurized before it reaches the intake system; the air is forced rather than sucked into the engine. More air and fuel mixture is forced into turbocharged engines than is pulled into regular engines.
Turbochargers consist of three main components: a compressor wheel, a turbine wheel and a connecting shaft. The two wheels are basically fans. The compressor wheel is positioned before the intake manifold, and the turbine wheel is located in the exhaust stream that exits the combustion chamber. The exhaust gases spin the turbine wheel as they pass through its blades. The turbine wheel's motion is transferred to the compressor wheel through the shaft. The spinning of the compressor wheel causes it to compress the air that passes through it. As a greater amount of air is delivered to the cylinder, a greater amount of fuel is added to it.
Having a greater amount of air and fuel in the combustion chamber boosts engine power by creating a higher compression ratio. The ratio expresses the difference between a cylinder's maximum and minimum volumes as determined by piston position. This ratio describes how much the gases are compressed before ignition. The more compressed the gases are, the more powerful their combustion will be.
When a vehicle travels slowly, exhaust passes slowly over the turbine blades. This slow spinning does not lead to an increase in air pressure on the intake side. Turbo boost, or the increase in pressure of intake air by the turbocharger, is created when the engine works hard and the exhaust gases flow rapidly.
Most turbos contain protective devices to limit boost and compression pressures. A wastegate valve is positioned above the turbine wheel to divert exhaust gases away from it when doing so would prevent the compressor wheel from creating excessive turbo boost. Wastegate are controlled either by a diaphragm that responds to manifold pressure or a solenoid valve that pulses it to open and to close.
A blowoff valve can be used to prevent excess pressures on the compressor end when the throttle is closed. The high pressure created on the compressor side forces air past the throttle into the intake manifold. When the throttle is suddenly closed, the pressure is forced back down to the compressor wheel slowing and possibly stalling its movement. A blowoff valve positioned between the compressor and the throttle opens when its spring pressure is overcome by compressor pressure. The valve opens to expose a path that carries excess pressure to the atmosphere.
Turbochargers are mainly limited to performance and high-end vehicles due to their cost and their relatively poor fuel economy rate caused by greater fuel consumption and sometimes a heavy right foot.