Turbocharger Principle Of Operation Design Details

A turbocharger is an exhaust driven supercharger. All superchargers have a gas compressor in the intake tract of the engine which compresses the intake air above atmospheric pressure, greatly increasing the volumetric efficiency beyond that of normally aspirated engines. 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.

The term "supercharger" is most often used when referring to a mechanically driven supercharger, which is most often driven the engine's shaft by means of a belt, whereas a "turbocharger" is exhaust driven.

The compressor increases the pressure of the fuel/ air mixture entering the engine, so a greater "charge" (of air and fuel) enters the engine in the same time interval. The increase in pressure is called "boost" and is measured in Pascal, Bar or PSIPounds-force per Square Inch PSI is a non- SI unit of pressure. For technical accuracy, psi must be expressed as psig (pounds-force per square inch gauge) or psia (pounds-force per square inch absolute; that is, gauge pressure plus sea level atmospheric p. The energy from the extra fuel leads to more overall engine powerMechanical power In physics, power (symbol: P is the amount of work W done per unit of time t''. This can be modeled as an energy flow, equivalent to the rate of change of the energy in a system, or the time rate of doing work, as defined by : where P is. 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.

2 Design details

When a gas is compressed, its temperature rises. It's not uncommon for a turbocharger to be pushing out air that is 90 ° CThe degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius ( 1701 1744), who first proposed it in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling po (200° FThis article is about the temperature scale; see also Fahrenheit graphics API. Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit ( 1686 1736), who proposed it in 1724. In this scale, the freezing point of water is 32 de). Compressed air from a turbo may be (and most commonly is) cooled before it is fed into the cylinderThe word cylinder has several meanings. For the geometric object, see Cylinder (geometry . For the engine component, see Cylinder (engine . In firearms the cylinder is the rotating device that contains the firing chambers of a revolver. The phonograph cyls, using an ' intercoolerAn intercooler is a device used on turbocharged and supercharged internal combustion engines to improve the volumetric efficiency and increase the amount of charge in the engine, thereby increasing power. The inter in the name refers to its location compa' 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 or a bypass valve.

3 Reliability

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.

4 Lag

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.

Lag is not to be confused with the turbo spooling up, however many publications still make this basic mistake. The spool up time of a turbo system describes the minimum RPM at which the turbo is physically able to supply the requested boost level. Newer turbocharger and engine developements have caused spool times to steadily decline to where day-to-day feels perfectly natural. Putting your foot down at 1200 rpm and having no boost until 2000 rpm is spool-up and not 'lag'.

5 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 at very high compression ratio, and then, at high temperatures, 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 in production cars, starting with the 1978 Saab 99. The Porsche 944 utilized a turbo unit in the 944S s, 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, Mazda RX-7, 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.

6 See also

engine technology

<Hide>

Home > Turbocharger

1 Principle of operation