Principles
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.
Schematic
of a fourstroke piston engine
Enlarge picture
- 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:
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.
Basically, one must distinguish between mechanically supercharged and exhaust
gas turbocharged engines.
Mechanical supercharging
With mechanical supercharging, the combustion air is compressed by a compressor
driven directly by the engine. However, the power output increase is partly
lost due to the parasitic losses from driving the compressor. The power to
drive a mechanical turbocharger is up to 15 % of the engine output. Therefore,
fuel consumption is higher when compared with a naturally aspirated engine with
the same power output.
Exhaust gas turbocharging
In exhaust gas turbocharging, some of the exhaust gas energy, which would
normally be wasted, is used to drive a turbine. Mounted on the same shaft as
the turbine is a compressor which draws in the combustion air, compresses it,
and then supplies it to the engine. There is no mechanical coupling to the
engine.