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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. A key disadvantage in gasoline engines is that the compression ratio must be lowered (so as not to exceed maximum compression pressure) which lowers engine efficiency when operating at low power. This disadvantage does not apply to specifically designed turbocharged diesel engines.
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 Pascal, 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 is 0.8 Bar, but it can achieve much more, and a typical turbo charger will only start to deliver boost from 2500 rpm (1800 in automotive turbo-diesel engines), while a supercharger will supply some boost at all engine speeds.
When a gas is compressed, its temperature rises. It's not uncommon for a turbocharger to be pushing out air that is 90 °C (200°F). Compressed air from a turbo may be (and most commonly is) cooled before it is fed into the cylinders, using an 'intercooler' or a charge air cooler (a heat exchange device).
A turbo spins very fast - 10,000 to 150,000 rpm depending on size (using low inertia turbos, 190,000 rpm), weight of the rotating parts, boost pressure used and design. Such high speeds would cause problems for standard ball bearings, which could explode in a turbo. Most turbo-chargers use a fluid bearing. This is a flowing layer of oil, which suspends and cools the moving parts. Some turbochargers use incredibly precise ball bearings that offer less friction than a fluid bearing. Lower friction means the turbo shaft can be made of lighter materials, reducing turbo lag or boost lag. Some car makers use water cooled turbochargers for added bearing life.
Turbochargers with foil bearings are in development (see picture above). This design eliminates the need for bearing cooling or oil delivery systems.
To reduce the possibility of damage to the turbocharger and to also reduce the amount of time required for the turbo to spool up after a shift on a car with a manual transmission, turbocharged engines are usually equipped with a blowoff valve.
As long as the oil supply is clean and the exhaust gas does not get too hot, a turbocharger can be very reliable, but careful maintenance is important. Replacing a turbo that 'lets go' and sheds its blades may be expensive. Regular cleaning of both the turbine and the compressor sides of the turbo is recommended to remove any build-up of soot and dust, to which a turbo may be sensitive because of its high rotational speed.
A 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'. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly driven compressor in a supercharger does not suffer this problem. Conversely on light loads or at low RPM a turbocharger supplies less boost, and the engine is more efficient than a supercharged engine.
Lag can be reduced by reducing the rotational inertia of the turbine, for example by using lighter parts to allow the spin-up to happen more quickly. Another way to reduce lag is to change the aspect ratio of the turbine to reduce the diameter and increase the width. Lag is also reduced by using a precision bearing rather than a fluid bearing, but this reduces friction rather than rotational inertia.
Some car makers combat lag by using two small turbos (like Toyota). A typical arrangement for this is to have one turbo active across the entire rev range of the engine, and one coming on line at higher RPM. Being such small units they do not suffer from excessive lag and having the second turbo operating at a higher RPM range allows it to get to full rotational speed before it is required.
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:
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 in production cars, 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, Lancer Evo VIII, and the Porsche 911 Turbo.
In Formula 1, in the so called "Turbo Era", engines with a capacity of 1500 cc could achieve 1500-1800 hp (Renault, Honda, BMW).
Small car turbos are increasingly being used as the basis for small jet engines used for flying model aircraft - though the conversion is a highly specialised job - one not without its dangers.
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