A turbocharger is a device used in internal-combustion engines to increase the power output of the engine by increasing the mass of oxygen and fuel entering the engine. A key advantage of turbochargers is that they offer a considerable increase in engine power with only a slight increase in weight.
Principle of operation
A turbocharger achieves a similar result to a supercharger. Both have a gas compressor in the intake tract of the engine. A turbocharger also has a turbine, which powers the compressor using waste energy from the exhaust gases. Compressor and turbine have the same shaft, similar to a turbojet aircraft engine.
A turbocharger is like a supercharger in that both compress air for combustion, but a supercharger's compressor is driven directly by the engine's crankshaft.
The compressor increases the pressure of the fuel/air mixture entering the engine, so more fuel enters the engine in the same time interval. The increase in pressure is called "boost" and is measured in Bar or PSI. The energy from the extra fuel leads to more overall engine power. At 100% efficiency, a turbocharger providing 100 kPa (= 1 Bar or 14.7 PSI) of boost would effectively double the power of the engine. There are some parasitic losses due to heat and exhaust backpressure from the turbine, so turbochargers are generally only about 80% efficient. It still takes some work for the engine to push those gases through the turbocharger turbine (which is acting as a restriction in the exhaust). Normal boost pressure will not exceed 0.7 Bar, and a typical turbo charger will only start to deliver boost from 2500 RPM, while a supercharger will supply some boost at all engine speeds.
Design details
When air is compressed, its temperature rises. Using an 'intercooler' or a charge air cooler, basically an air/air heat exchanger, the compressed air from the turbo is cooled before it is fed into the cylinders.
To prevent detonation (also known as engine knock or "pre-ignition") the ECU will 'listen' for pinking and reduce the boost pressure by opening a waste gate, so the exhaust gasses will bypass the turbo charger, reducing the boost pressure to a safe level.
As the turbo spins very fast (10,000 to 100,000 rpm depending on size, weight and design), care must be taken in maintaining it. A turbo 'letting go' and shedding its blades requires an expensive replacement. The high speed also causes problems for standard ball bearings, which would explode in a turbo. All but the most expensive turbo-chargers use a fluid bearing. The fluid bearing of a turbo is a flowing layer of oil which suspends and cools the moving parts. More expensive turbochargers use incredibly precise ball bearings because they offer less friction than a fluid bearing. This lower friction in turn allows the turbo shaft to be built with lighter materials, which reduces 'turbo lag' or 'boost lag'. Some car makers use water cooled turbochargers.
As long as the oil supply is clean and the exhaust gas does not get too hot, a turbocharger is very reliable. Regular cleaning of both the turbine and the compressor sides of the turbo is recommended to remove any build-up of soot and dust.
Lag
Lag is sometimes felt by the driver of a turbocharged vehicle as a delay between pushing on the accelerator pedal and feeling the turbo 'kick-in'. As the exhaust is connected to the turbine side there will be delay before the compressor is up to speed to supply boost pressure. This, and inertial lag of the rotor, gives the characteristic turbine lag whereas a direct belt driven compressor as in a supercharger does not suffer this problem. Conversely on light loads or low RPM the turbocharger supplies less power and therefore the engine is more efficient than a supercharged engine.
Lag can be reduced by reducing the rotational inertia of the turbine. Using lighter parts is one way to allow the spin-up to happen more quickly, and in this way the lag is reduced. Another way to reduce lag is to change the aspect ratio of the turbine so that the diameter is reduced and the width is increased. Lag is also reduced by using a precision bearing rather than a fluid bearing, but this last one is to do with reducing friction rather than rotational inertia.
Applications
Turbocharging is very common on Diesel engines: in conventional automobiles, trucks, marine and heavy machinery applications. In fact, for current automotive applications non-turbocharged diesel engines are becoming increasingly rare. Diesels are particularly suitable for turbocharging for several reasons:
- Naturally-aspirated diesels have lower power-to-weight ratios compared to gasoline engines, turbocharging will improve this P/W ratio.
- Diesel engines require more robust construction because they already run a very high compression ratio, so they generally require little additional reinforcement to be able to cope with the addition of the turbocharger. Gasoline engines often require extensive modification for turbocharging.
- Diesel engines have a narrower band of engine speeds at which they operate, thus making the operating characteristics of the turbocharger over that "rev range" less of a compromise than on a gasoline-powered engine.
Turbocharging is most commonly used on gasoline engines in high-performance automobiles, particularly when there is no room to fit a larger-capacity (and physically larger) engine to a small car to increase its performance. Saab has been the leading car maker using turbo chargers, starting with the 1978 Saab 99. The Porsche 944 utilized a turbo unit in the 944S models, to great advantage, bringing its 0-100 speeds very close to its contemporary non-turbo big brother, the Porsche 928. Contemporary examples include the Subaru Impreza WRX and the Porsche 911 Turbo.