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Answer Posted / rohit verma
1) Type of cycle used: In the case of SI engines, the Otto
cycle is used. In this cycle, addition of heat or fuel
combustion occurs at a constant volume. The basis of working
of CI engines is the Diesel cycle. In this cycle the
addition of heat or fuel combustion occurs at a constant
pressure.
2) Introduction of fuel in the engine: In the case of SI
engines, during the piston's suction stroke, a mixture of
air and fuel is injected from cylinder head portion of the
cylinder. The air-fuel mixture is injected via the
carburetor that controls the quantity and the quality of the
injected mixture. In the case of CI engines, fuel is
injected into the combustion chamber towards the end of the
compression stroke. The fuel starts burning instantly due to
the high pressure. To inject diesel in SI engines, a fuel
pump and injector are required. In CI engines, the quantity
of fuel to be injected is controlled but the quantity of air
to be injected is not controlled.
3) Ignition of fuel: By nature petrol is a highly volatile
liquid, but its self-ignition temperature is high. Hence for
the combustion of this fuel a spark is necessary to initiate
its burning process. To generate this spark in SI engines,
the spark plug is placed in the cylinder head of the engine.
The voltage is provided to the spark plug either from the
battery or from the magneto. With diesel, the self-ignition
temperature is comparatively lower. When diesel fuel is
compressed to high pressures, its temperature also increases
beyond the self-ignition temperature of the fuel. Hence in
the case of CI engines, the ignition of fuel occurs due to
compression of the air-fuel mixture and there is no need for
spark plugs.
4) Compression ratio for the fuel: In the case of SI
engines, the compression ratio of the fuel is in the range
of 6 to 10 depending on the size of the engine and the power
to be produced. In CI engines, the compression ratio for air
is 16 to 20. The high compression ratio of air creates high
temperatures, which ensures the diesel fuel can self-ignite.
5) Weight of the engines: In CI engines the compression
ratio is higher, which produces high pressures inside the
engine. Hence CI engines are heavier than SI engines.
6) Speed achieved by the engine: Petrol or SI engines are
lightweight, and the fuel is homogeneously burned, hence
achieving very high speeds. CI engines are heavier and the
fuel is burned heterogeneously, hence producing lower
speeds.
7) Thermal efficiency of the engine: In the case of CI
engines the value of compression ratio is higher; hence
these engines have the potential to achieve higher thermal
efficiency. In the case of SI engines the lower compression
ratio reduces their potential to achieve higher thermal
efficiency
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