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WHAT IS TURBOCHARGING

Turbocharging is a form of supercharging - the 'force feeding' of an internal combustion engine with air under pressure for improved performance.

Unlike a mechanical driven supercharger, a turbocharger is powered by an exhaust gasses. These drive a turbine wheel mounted on a single shaft with a second impeller or 'compressor wheel' drawing air into the turbo, compressing it.

A boost control system is fitted to ensure that exactly the right amount of air is delivered to the engine at exactly the right pressure. Simple enough in theory, perhaps: but the combination of high pressure, high temperature and high speed means that the quality of every component in a turbocharger is absolutely critical.

UNDERSTANDING TURBOCHARGERS

To give you a better insight on how a turbocharger works, it is worth looking at a non turbocharged (normally aspirated) engine breathing system and how the addition of a turbocharger can improve engine performance.

During the air intake stroke, air is brought into the engine via the air filter, airflow meter, inlet manifold and inlet valves to the cylinder. The motion of the piston moving from the top to the bottom of the cylinder, draws air into the cylinder. At an engine speed of 6,000 RPM the piston takes just a 2OOth of a second to travel down the bore, reaching maximum velocity at 90 degrees after crankshaft top dead centre. At this speed a relatively high partial vacuum exists in the cylinder. The potential difference in the pressure between the partial vacuum in the cylinder and the atmospheric pressure in the inlet manifold drives air into the cylinder. It is the basic difference in pressure levels that the turbocharger exploits.

The turbocharger itself harnesses energy from the engine's exhaust emissions. A combination of pressure waves from opening exhaust valves and inert gasses drive the turbine wheel. The turbine wheel is connected to the shaft by either inertia or electron welding processes. The shaft runs through the centre of the turbocharger and is held in position by a bearing assembly controlling the radial movement of the shaft. A compressor wheel is connected to the opposite end of the shaft and sits inside an aluminium cover (the compressor housing). The compressor area of the turbocharger draws air into the front of the compressor wheel (the inducers) which rotates at very high speeds up to 180,000 RPM and moves the air by centrifugal force towards the outer edge of the compressor housing (the scroll area). As the air travels from the throw of the compressor wheel towards the scroll area the decrease in air velocity creates a corresponding increase in air pressure. The motion of the compressor wheel creates a depression in air pressure at the compressor intake. The suction at the compressor intake combined with the positive exhaust pressure at the turbine wheel has the effect of drawing the rotor through the turbocharger. This force is often referred to as ''trust'' or ''axial'' load and is controlled by a trust bearing assembly located between the bearing and the compressor wheel.

Gas Flow Schematic

The greater the pressure at the engine intake, the greater the flow of air into the cylinder. An engine operating at sea level will be subjected to a greater air pressure than that of an engine at the top of a mountain, due to the change in atmospheric pressure and will consequently provide a higher power output. Taking the principal further, a turbocharger compresses atmospheric pressure to provide a substantial increase in air pressure to the engine air intake. The increase in pressure, often referred to as 'boost' pressure is usually quoted with respect to atmospheric pressure. This means that an engine with a boost pressure of 0.8 Bar will have an absolute or total pressure of 1.8 Bar. This is an addition of atmospheric pressure (approx. 1 bar) + boost (0.8 Bar).

A turbocharged application such as this will provide the engine with a far greater cylinder filling capacity than that of a non turbocharged engine.

A typical 2 litre turbocharged engine will flow similar air volumes as a non turbocharged 3.5 litre engine. The 2 litre engine will also have a lower reciprocating engine mass thus allowing the engine to rev faster and draw a fresh charge of air at a greater frequency then that of it's 3.5 litre counterpart.

The compressed air that is provided by the turbocharger is referred too as ''supercharged''. This is a term often associated with another type of compressor device; the supercharger. A supercharger shares the same principal of increasing air pressure to the engine intake as a turbocharger but differs greatly by how this is achieved. Superchargers use various methods of air compression, screw drive, reciprocating compression, offset diffuser etc. the defining aspect of a supercharger is that it is driven by a mechanical coupling to the engine. This is usually connected to the crankshaft via the drive belt. The benefit of using a turbocharger is its efficiency in using exhaust energy to power if. A supercharged engine requires a percentage of the energy the engine produces to drive the supercharger, therefore a turbocharged engine will produce more power at the engine flywheel than an equivalent supercharged engine.

The advantage of the direct drive of a supercharger is that it has a very good transient response to the changing engine loads and speeds. If the engine speeds doubles the supercharger speeds doubles, thus allowing ease of boost pressure control. Early turbocharger design with fixed geometry turbines operating from very slow to very high RPM, caused difficulty in regulating the rotor speed to that required by the engine and resulted in the dreaded ''turbo-lag'' associated with old turbocharger design - Modern turbochargers however with variable geometry technology and electronic positional actuator controls, have reduced the original benefits of the supercharger transient response to negligible amounts. The flexible broad operating range of the modern turbocharger combined with excellent power / efficiency and reduced emissions far outweigh any small benefits to be found in current supercharger use.