How do turbochargers work?

Many car manufacturing companies offer turbo chargers on their vehicles.

A turbocharger or ‘turbo’ is one of the most effective ways to produce more power from an internal combustion engine and is commonly used in the automotive, marine or aviation industries. A simple addition of a turbo system to an engine can increase power by as much as 50% over the non-turbocharged variant of that engine. For this reason, several automotive manufacturers including Audi, Mitsubishi, Subaru, Mazda, Lotus, Mercedes Benz, Bentley and Volkswagen offer turbochargers on some of their models.

The internal combustion engine works by igniting a fixed mixture of air and fuel in a controlled environment to produce ‘power’ by causing a piston to turn the main crankshaft. The spent gases remaining after combustion are then expelled from the engine through the exhaust system and the combustion process repeats itself once more. So where do turbochargers fit into this process?

In simple terms, turbochargers work by forcing compressed air into an engine which increases power output and reduces both exhaust emissions and fuel consumption at the same time. But how is this compressed air generated and what prevents damage from being done to the engine due to the increased pressure? Let’s look at the typical turbo system in more detail:

If more air is drawn into the engine, then more power can be produced during the combustion phase. Turbochargers work by redirecting the spent exhaust gases which still contain un-burnt air and fuel through a series of pipes onto a turbine wheel. The turbine wheel is connected by a shaft to a compressor wheel on the other end of the housing. This housing, with the two wheels on either end, is commonly referred to as the turbocharger or ‘turbo’. As the exhaust gases reach the turbine wheel, they cause it to spin and as the compressor wheel is connected to the turbine wheel, it begins to spin simultaneously as well.

As the compressor wheel spins faster and faster, exhaust gases are drawn into the turbo housing and rapidly compressed. This compression happens very quickly as the compressor wheel can spin upwards of 150,000 revolutions per minute! The compressed air then exits the housing and is directed back into the intake system.

One by-product of compressing any gas very quickly is heat! As heat causes expansion, the hotter the gas the less dense the molecules will be. As a result of this, most modern turbocharger systems include a Heat Exchanger more commonly referred to as an ‘Intercooler’ which looks similar to a radiator, and works as a coolant system that cools the air leaving the turbo housing. As the temperature of the air leaving the intercooler is lower, the density of the air molecules increases. This cooler air is then directed back into the main intake system and into the engine where more power is produced during the combustion process. This entire process is then repeated over and over again with the exhaust gases that are expelled from the engine.

As this turbocharging process repeats itself, the pressure in the engine and entire system increases and if left unchecked, permanent damage will eventually be done to the engine. To prevent damage, all turbo systems have a wastegate in the intake system which opens up whenever the pressure exceeds a preset level e.g. 12 psi (pounds per square inch). The preset pressure in the system is commonly referred to as the boost level. The higher the turbocharger boost level, the greater the power that can be produced by an engine. In automotive applications, most turbocharged street cars run somewhere between 8-20 psi of boost, while racing cars with stronger engine internals can run upwards of 36psi.