How Turbochargers Work
A turbocharger is a great example of getting something from nothing. These devices use waste gases from engine combustion, normally expelled into the atmosphere through the exhaust pipe, to produce more engine power: they are essentially a ‘free’ performance boost and can be fitted to almost any engine.
They help make a smaller-capacity engine generate the power of a larger one, without suffering the fuel economy penalties associated with big-block engines. They also mean that smaller vehicles can have much higher outputs despite the restricted space that is afforded by their engine bays.
A turbocharger works by using the kinetic energy from the exhaust gas airflow to spin a turbine, which is connected to an air pump that compresses air via a compressor wheel. It is called forced induction, where the air is compressed in order to significantly increase its pressure, density and temperature. Alternatively, an engine without a turbocharger (an increasing rarity these days) is considered to be naturally aspirated.
Compressed, higher-density air contains more oxygen, which improves combustion and – when mixed with extra fuel – this ultimately produces more power. Each ‘bang’ of the engine’s combustion cycle is bigger and more potent; as a result, smaller, fewer cylinders can create the same cumulative power of more and larger naturally aspirated ones.
Because a turbocharger works from waste exhaust gases, the turbine gets very hot. This heats up the compressor that, in turn, further heats up the compressed air too. Because cool air is needed for maximum efficiency, most turbocharged engines have an intercooler, through which the hot air passes and is cooled before it reaches the engine.
Today, turbochargers are used by nearly every diesel engine on the market for one very good reason: they increase the power of an inherently less efficient type of engine without negating the impressive fuel economy that many choose diesel engines for. More and more petrol cars are using turbochargers too, as car manufacturers ‘downsize’ their engines – replacing bigger, less green motors with smaller, more efficient engines that offer the same power.
A turbocharger’s power is not quite completely free though – the additional backpressure in exhaust due to the turbine does cost a vehicle a little horsepower, but this is minimal compared to the mighty gains it brings.
TURBOCHARGING VS SUPERCHARGING
Turbochargers and superchargers boost engine power using similar principles – ie both devices force more air into the engine for combustion, increasing the pressure in the combustion chamber and thus improving its volumetric efficiency. Nevertheless, turbocharging and supercharging work in slightly different ways to get the job done.
A supercharger is not driven ‘indirectly’ by exhaust gases, but is ‘directly’ connected to the engine, via a belt or other rigid link. A supercharger is normally driven by the crankshaft of the engine, and this direct connection means it is effective at all engine speeds, not just when exhaust gases are flowing fast enough to spin the turbine sufficiently quickly.
The major downside of superchargers is the mechanical drag that they take from the engine – with the extra friction absorbing power. For example, if a supercharger boosts an engine by 75 kilowatts (100 horsepower), 15 kilowatts (20 horsepower) of this can be taken up by actually driving the supercharger, meaning the net power gain is 60 kilowatts (80 horsepower). For this reason, turbochargers are preferable and are more fuel-efficient, despite not being quite as fast-reacting.
A twincharger engine combines both turbocharger and supercharger elements – the turbo is used at higher engine speed and the supercharger at lower speeds, when it is at its most effective and its power consumption is minimal. This ‘best of both worlds’ solution is used on some new car models from Volkswagen, and while it’s more expensive than a regular turbo, it is both highly efficient and effective.