The full name of the turbo is “turbo-supercharging” and uses the same concepts as a supercharger – the main difference is the use of exhaust gases to turn a turbine rather than the crankshaft. The turbine is connected directly to a compressor impeller providing the motion to build the boost. Clearly as the engine increases the rpm, the volume of gases increases which in turn increases the speed of the impeller, which then 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 often added to direct some of the exhaust away from the turbo unit.
One downside of the early turbos was the concept of “lag”. This happens when the throttle is closed and 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 causing a whistling sound and the compressor to slow down. When the throttle is opened again the compressor needs to speed up again and 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 1930s 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 – although 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 1970s and 1980s 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 have both 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.
Turbos were typically seen on high performance sports cars or performance hatchbacks but today they are increasingly being used with diesel engines to provide a smoother delivery of power and for smaller displacement petrol engines to keep power up but use less fuel. I read a report recently that suggested a turbo 1.8 litre 4 cylinder motor could produce as much power as a normal 3 litre V6.
In 2010, the US Government updated its CAFE rules – the Corporate Average Fuel Economy ratings so that they rose over a period of time to be 35.5 mpg by 2016. This forced manufacturers to take turbocharging seriously for small cars as they could get higher fuel economy and cleaner emissions at the same time by having to meet the new rules.
As with superchargers, designers have put some thought into the efficiency of the turbo’s connected to the engine – balance is crucial as well as the intended use of the vehicle because a small turbo may not work and a big one could destroy the motor! So the designers have come up with interesting ways to balance the motor and keep costs down, one way is to use a twin scroll turbo where the cylinders are mapped to a separate scroll, making the turbo unit more efficient and balanced plus having less weight in the vehicle. This ensures that on a 4 cylinder motor, two cylinders each map to one scroll and the whole engine is more efficient in its burn cycle plus the exhaust gases are cooler which lengthens the life of the turbo impellers.
As turbo technology improves, I think it will spread to other vehicles like motorbikes and scooters so that we start to see a wider range of motors below 250cc again. However…. it will all depend on the battle between fossil and plant based fuels, electric and hydrogen powered engines.It may not be worth developing turbos any further if one of the other power sources takes a dominant market share.