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To better understand the technique of turbocharging, it is useful to be familiar with the internal combustion engine's principles of operation. Today, most passenger car and commercial diesel engines are four-stroke piston engines controlled by intake and exhaust valves. One operating cycle consists of four strokes during two complete revolutions of the crankshaft.

1 Compressor Inlet

2 Compressor Discharge

3 Charge air cooler (CAC)

4 Intake Valve

5 Exhaust Valve

6 Turbine Inlet

7 Turbine Discharge

Suction (charge exchange stroke)

When the piston moves down, air (diesel engine or direct injection petrol engine) or a fuel/air mixture (petrol engine) is drawn through the intake valve.

Compression (power stroke)

The cylinder volume is compressed.

Expansion (power stroke)

In the petrol engine, the fuel/air mixture is ignited by a spark plug, whereas in the diesel engine fuel is injected under high pressure and the mixture ignites spontaneously.

Exhaust (charge exchange stroke)

The exhaust gas is expelled when the piston moves up. These simple operating principles provide various possibilities of increasing the engine's power output:

The components that make up a typical turbocharger system are:

The air filter (not shown) through which ambient air passes before entering the compressor (1)

The air is then compressed which raises the air’s density (mass / unit volume) (2)

Many turbocharged engines have a charge air cooler (aka intercooler) (3) that cools the compressed air to further increase its density and to increase resistance to detonation

Turbocharger Function

After passing through the intake manifold (4), the air enters the engine’s cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given size or displacement

After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke in to the exhaust manifold (5)

The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure

A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor

Turbocharging function

Swept volume enlargement

Enlargement of the swept volume allows for an increase in power output, as more air is available in a larger combustion chamber and thus more fuel can be burnt. This enlargement can be achieved by increasing either the number of cylinders or the volume of each individual cylinder. In general, this results in larger and heavier engines. As far as fuel consumption and emissions are concerned, no significant advantages can be expected.

Increase in engine rpm

Another possibility for increasing the engine's power output is to increase its speed. This is done by increasing the number of firing strokes per time unit. Because of mechanical stability limits, however, this kind of output improvement is limited. Furthermore, the increasing speed makes the frictional and pumping losses increase exponentially and the engine efficiency drops.

 

Turbocharging

In the above-described procedures, the engine operates as a naturally aspirated engine. The combustion air is drawn directly into the cylinder during the intake stroke. In turbocharged engines, the combustion air is already pre-compressed before being supplied to the engine. The engine aspirates the same volume of air, but due to the higher pressure, more air mass is supplied into the combustion chamber. Consequently, more fuel can be burnt, so that the engine's power output increases related to the same speed and swept volume.