How can engine exhaust help bring more air into the engine?

An engine’s exhaust still carries energy after leaving its cylinders. A turbocharger uses some of that energy to help supply the engine with more air. It has two rotating wheels joined by a shaft: a turbine in the exhaust path and a compressor in the incoming-air path.

Hot exhaust expands through the turbine and transfers energy to its wheel. The shaft carries mechanical work to the compressor wheel. The compressor raises the pressure of incoming air. The two gas streams remain separate; exhaust does not flow through the shaft into the intake. After compression, the fresh air reaches the engine’s intake.

Higher pressure alone does not tell us the air’s density, because compression also raises temperature. For an ideal gas of unchanged composition, density is proportional to absolute pressure divided by absolute temperature. If pressure doubles while temperature rises from 300 to 400 kelvins, density becomes 1.5 times its inlet value. If that air is cooled to 300 kelvins at the same doubled pressure, the ideal density ratio becomes two. A charge-air cooler can serve this cooling role.

More air can permit more fuel to burn under appropriate control. The extra output is not free energy: fuel supplies the engine’s energy, and the turbine also affects exhaust pressure. The simple diagram explains the connected energy path, not fuel economy or the performance of a particular vehicle.