DIESEL POWER PLANT 2378.3.1 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.
8.3.2 Diesel Engines/Heavy Oil Engines
In contrast to the engines in which the fuel and air mixes before compression, in diesel engines:
air is compressed as the compression stroke begins and the fuel enters the cylinder at the end of compres-
sion stroke. Heat of compression is used for ignition of fuel.
In a typical diesel engine, air is compressed to about 30 bars, which increases the temperature
when finely atomised diesel fuel oil is sprayed into the heated air, it ignites and burns. High compression
ratio is therefore essential for reliable combustion and high efficiency. Compression ratios above those
needed to achieve ignition do not improve the efficiency.
The pressure ratio depends on engine speed, cylinder size and design factors. Typical compres-
sion pressures in diesel engines range from 30 bar to 42 bar. Small high-speed engines have higher com-
pression pressures.
8.3.3 Duel Fuel Engines
In a duel fuel engine, a small quantity of pilot oil is injected near the end of the compression
stroke. It is ignited by the compression and the mixture burns like standard diesel fuel. The pilot oil
burning provides enough heat to the mixture of gas/air. Precise control of pilot oil injection and a sepa-
rate set of fuel pumps and nozzles are added. Means are provided to reduce air quantity at partial loads.