A turbocharger is a forced induction device that increases the output of an engine by extracting energy from exhaust gases. You often see turbos in performance and diesel engines. Carmakers use them to help achieve better gas mileage by allowing carmakers to shrink the size of an engine and compensate for the lost horsepower. Turbos have a very attractive weight to power ratio, which makes them very popular. They compress the air flowing into an engine and increase the amount of air in each cylinder, allowing for the introduction of more fuel and a more powerful explosion.
The engine’s hot exhaust flow spins a turbine in the turbocharger, which then spins an air pump. Turbines spin at up to 150,000 rotations per minute. They boost the cylinder air pressure by six to eight pounds per square inch. That’s about a 50 percent boost over normal sea-level atmospheric pressure. This translates into 30 to 40 percent more power from your engine. Some power is lost when it is used to spin the turbine, and more power is lost by the exhaust air flow restriction caused by the presence of the turbine – known as back pressure.
Turbochargers attach to an engine’s exhaust manifold, where the exhaust gases pass through the turbine blades to spin the turbine. A shaft connects the turbine to an air compressor at the intake manifold to pressurize the air headed for the pistons. A compressor is actually a centrifugal pump: air travels into the center where the blades push it outwards.
As you might imagine, a turbocharger has to be carefully engineered to withstand blade rotation speeds of 150,000 rpm. The bearing is designed to be bathed in a thin layer of oil to facilitate lubrication and lower the operating temperature. Too much boost can cause knocking – pre-combustion due to high pressure and temperature. If the gas ignites before the spark plug fires, power will be lost and you will hear that familiar knocking sound. You may have to increase the octane level of your fuel to compensate. If knocking still occurs, you will have to lower the compression ratio of the engine.
Sometimes, it takes a second for the turbo to start delivering power, a phenomenon called turbo lag. To reduce the inertia of moving parts, the blades (often made from ceramics) and other pieces are made smaller and lighter. The downside is that the turbocharger may be overwhelmed at higher engine speeds and spin too quickly, though this can be controlled by a wastegate. The gate bleeds off excess pressure to avoid overspin.