Internal combustion engines produce power by burning a mixture of fuel and air. The more fuel and air the engine can burn, the more power it can produce, so the bigger the engine, the bigger the power output. It is possible however to increase the power of an engine without enlarging it by the use of ‘forced induction’ which increases the amount of air entering the cylinders thus allowing more fuel to be added.
The turbocharger is basically a pump that compresses the air entering the cylinders, which is connected to a turbine driven by the exhaust gas. A typical turbocharger will operate at a pressure of between 6 to 8 psi. Atmospheric pressure is 14.7 psi at sea level, so a theoretical power increase of up to 50% is feasible.
The two main technical hurdles facing the designers of turbochargers are heat generation and ‘turbo lag’. Heat is generated when air is compressed and red-hot exhaust gasses drive the turbine itself. Hot air is less dense, but the fitting of an intercooler to cool the air before it enters the engine now largely contains this problem.
Turbo lag refers to the delay between pressing the accelerator and the engine providing power. This was particularly noticeable on early turbocharged engines but modern engine management systems and the use of smaller (sometimes multiple) turbos with lighter rotating parts has greatly improved the throttle response of the latest models.
Supercharging (favoured by Mercedes for example) is another form of forced induction.
It works in a similar way to turbocharging except that a belt drives the pump directly from the engine instead of by an exhaust turbine. It has the advantage of an almost instantaneous power delivery but it is less efficient than a turbo because the belt drive is absorbing a proportion of the additional power.