Power Plant Engineering

8.3 BASIC TYPES OF IC ENGINES
Although alike in main mechanical aspects, the oil engines differ from gas engines in fuels and
fuel handling i.e., when fuel and air are injected how much charge is compressed and how ignited. Many
variants exist.

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DIESEL POWER PLANT
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8.3.1 TWO-STROKE, SPARK IGNITION GAS ENGINES/PETROL ENGINES
The well-known automobile engine fueled with petrol (also called Gas) and Natural Gas Engine,
Bio-gas Engine is of this category. The low compression gas engine (petrol engine/natural gas engine)
mixes fuel and air, outside the cylinder, before compression. With the automobile engine, a carburator is
used for mixing the fuel and air and the mixture is injected in the cylinder. In a Natural Gas Engine, a
mixing valve is used for the same purpose instead of the carburator.
In the mixture, the gas fuel and air proportion is almost perfect to produce complete combustion
without excess air. This mixture flows into the cylinder and is then compressed. Near the end of the
compression stroke, an electric spark ignites the inflammable mixture, which burns rapidly. The pres-
sure in the cylinder rises rapidly and acts on the piston area and the piston is forced to move down on its
power stroke.
Since the compressed gas mixture rises in pressure during the compression stroke, the mixture
may get pre-ignited before the sparking resulting in loss of power. Hence compression pressure must be
limited in this type of engine. Compression Ratio is therefore an important parameter in establishing
combustion without pre-ignition.
The compression ratio is the ratio of cylinder volumes at the start and at the end of compression
stroke.
In general, higher the compression ratio, higher will be the maximum pressure reached during
combustion and higher is the efficiency of the engine.
Although it is desirable to have a high compression ratio, the nature of fuel imposes limits in
engines where a nearly perfect mixture is compressed.
With natural gas for example the compression ratio might be about 5:1 and compression pressure
of about 8 bar, pre-ignition being the limiting factor.