The full name of the turbo is turbo-supercharging and uses the same concepts – the main difference is the use of exhaust gases to turn a turbine rather than the crankshaft. The turbine is connected directly to the compressor impeller to provide the motion to build the boost. Clearly as the engine increases the rpm, the volume of gases increases which increases the speed of the impeller, which increases the boost.
But there is an optimal amount of boost for any engine and
if the boost goes over this point, the engine could be damaged by pre-ignition
where the fuel/air mix ignites before the spark plug does it’s job. This could
cause catastrophic damage, so a wastegate is added to direct some of the
exhaust away from the turbo unit.
One downside of early turbos was the concept of “lag”. This
happens when the throttle is closed – the turbo is still spinning so air is
suddenly blocked from exiting the induction system. This can damage the engine
but typically bounces back to the compressor causes a whistling sound and the
compressor to slow down, so when the throttle is opened again the compressor
needs to speed up again. The lag is caused by the time delay in getting back up
to speed. A blow-off valve is now used to dissipate the air after the throttle
is closed; this helps to keep the compressor spinning should the throttle be
opened again quickly.
Variable Geometry turbos were also developed to help the
compressors provide better boost across the rpm range by using different aspect
ratios at different rpms. This means that the turbine vanes change as the rpm
increases. Aspect ratios of turbos change for different speeds, so if you have
a turbo designed for low speed, it won’t work at higher speeds and vice versa.
The Porsche 911 uses this type of turbo from Borg Warner
The turbo was invented by Alfred Buchi, a Swiss engineer,
around 1905. They were fitted to large diesel engines first before General
Electric fitted one to a V12 aero engine in the 30s to help with the loss of
air pressure at altitude. During WW2, many turbocharged aircraft also used a supercharger as well.
It wasn’t until 1962 that the turbo made it to a production
car – it had been used in racing since the early 50s. General Motors fitted a turbo to the Oldsmobile Cutlass Jetfire V8 and the Chevrolet Corvair Monza Spyder flat 6. In
the 70s and 80s many European manufacturers like BMW, Porsche, SAAB and Renault all built turbo models and the Japanese manufacturers also developed turbo
sports cars.
In motor sport, Formula 1 and the World Rallying Championship both
have used turbos for extra power. From 1977 – 89 turbos were predominant in F1,
firstly using a 1.5 litre in place of a 3 litre normally aspirated engine. Renault were the first but Ferrari, BMW and Honda also produced turbo engines.