Spark
Ignition Engines (S.I. Engine)
It works on
Otto cycle. In Otto cycle, the energy supply and rejection occur at constant
volume process and the compression and expansion occur isentropically. The
engines working on Otto cycle use petrol as the fuel and incorporate a
carburettor for the preparation of mixture of air fuel vapour in correct proportions
for rapid combustion and a spark plug for the ignition of the mixture at the
end of compression. The compression ratio is kept 5 to 10.5. Engine has
generally high speed as compared to C.I. engine. Low maintenance cost but high
running cost. These engines are also called spark ignition engines or simply
S.I. Engine.
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| SI engine P-V and T-S Diagram |
Compression
Ignition Engines (C.I. Engine)
It works on
diesel cycle. In diesel engines, the energy addition occurs at constant
pressure but energy rejection at constant volume. Here spark plug is replaced
by fuel injector. The compression ratio is from 12 to 25. Engine has generally
low speed as compared to S.I. engine. High maintenance cost but low running
cost. These are known as compression ignition engines, (C.I) as the ignition is
accomplished by heat of compression.
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| CI Engine P-V and T-S Diagram |
The upper
limit of compression ratio in S.I. Engine is fixed by anti knock quality of
fuel. While in C.I. Engine
upper limit of compression ratio is limited by thermal and mechanical stresses
of cylinder material. That’s way the compression ratio of S.I. engine has more
compression ratio as compared to S.I. Engine.
Dual
cycle is a combination of the above two cycles, where part or the energy is
given a constant volume and rest at constant pressure.
The
following are the main differences between SI and CI Engines:
Spark Ignition Engine (S.I Engine)
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Compression Ignition Engine(C.I Engine)
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Spark plug required
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No spark plug required
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The mixture of air and fuel is introduced into the cylinder from
carburettor.
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Only air is introduced into the cylinder.
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These type of engines compresses air and fuel together in the
cylinder
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In these engines air is only compressed in the cylinder.
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No fuel pump is used.
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Fuel pump is used to inject fuel.
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Fuel is mixed with air before compression starts.
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Fuel is mixed with air once compression is complete.
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Compression ratio is low.
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Compression ratio is high.
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This type of engine makes use of highly volatile liquid fuel.
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This type of engine makes use of less volatile liquid fuel.
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Less efficient.
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More efficient.
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Fuel used in this engine is expensive.
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Cheaper fuels are used in these engines.
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Higher fuel consumption in these engines for same power.
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These engines have lesser fuel consumption for same power.
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Engines are more compact and light.
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Heavier and strong engines due to higher pressure involved
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Initial cost is less
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Initial Cost is high.
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These engines have a smooth operation
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Roughness in engine operation encountered, especially when the engine
runs at high speed and low loads.
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