TURBO CHARGER

A turbocharger consists of a compressor and a turbine linked by a shared axle so if the turbine rotates, the compressor also rotates. The turbine inlet receives exhaust gases from the engine causing it to rotate. This rotation in turn drives the compressor, which compresses the ambient air and delivers it to the intake manifold of an engine at higher pressure, resulting in greater amount of air entering the cylinder.

Now to some basics before we go ahead. There are two ways of increasing the power of an engine. One of them would be to make the fuel-air mixture richer by adding more fuel. This will increase the power but at the cost of fuel efficiency and increase in pollution levels… prohibtive! The other would be to somehow increase the volume of air entering into the cylinder and increasing the fuel intake proportionately, increasing power and fuel efficiency without hurting the environment or efficiency. This is exactly what Turbochargers do, increasing the volumetric efficiency of an engine.

In a naturally aspirated engine, the downward stroke of the piston creates an area of low pressure in order to draw more air into the cylinder through the intake valves.  Now because of the pressure in the cylinder cannot go below 0 (zero) psi (vacuum) and relatively constant atmospheric pressure (about 15 psi) there will be a limit to the pressure difference across the intake valves and hence the amount of air entering the combustion chamber or the cylinder. The ability to fill the cylinder with air is its volumetric efficiency. Now if we can increase the pressure difference acorss the intake valves by someway we can make more air enter into the cylinder and hence increasing the volumetric efficiency of the engine. A turbocharger does exactly this, it increases the pressure at the point where air is entering the cylinder, thereby increasing the pressure difference across the intake valves and thus more air enters into the combustion chamber. The additional air makes it possible to add more fuel, increasing the power and torque output of the engine, particularly at higher engine speeds.

If the pressure in the cylinder goes too high it will cause the fuel to pre-ignite (remember more pressure ==  more temperature) in turn causing serious physical damage to the engine. To regulate pressure or boost, a wastegate is used. A wastegate controls the boost by routing some of the exhaust flow away from the exhaust side turbine. This controls the speed at which the axle rotates and in turn regulates the boost pressure by the compressor at the other end. The application of a compressor to increase pressure at the point of air intake is also commonly refferred to as forced induction.

The turbocharger application in an engine also introduce ‘Lag’ which is a symptomatic of the time taken by 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. That is why in turbocharged cars, you feel the turbo kicking in after ceratin rpm is reached, at which point the exhaust system overcomes the rotational inertia of the turbine and speed it up to supply boost pressure.

Advantages:

More power compared to the same size naturally aspirated engine.

Better thermal efficiency over natually aspirated engine and supercharged engine because the engine exhaust is being used to do the useful work which otherwise would have been wasted.

Better Fuel Economy by the way of more power and torque from the same sized engine.

Disdvantages:

Lack of response called the Turbo Lag. If the turbo is too big,  the boost will build up slowly because more exhaust pressure will be needed to overcome the rotational inertia on the larger turbine reducing throttle response but more peak power. If the turbo is too small the turbo lag wont be as big but the peak power would be lesser. So the turbocharger size is a very important consideration when deciding on it for a particular engine.

Non liner rise in power and torque.

Cost

Complexity: Turbocharger spins at very very high revolutions ( 1 lakh + per minute!!!) so proper cooling and lubrication is essential if it not to destroy the engine.