1. In SI engines, a properly mixed air-fuel mixture is provided by an electrical spark. Whereas in CI engines high temperature caused by compressing the air is sufficient for ignition.Hence they are called CI engines.
2.The SI engine works on Otto cycle. WhereasThe CI engine works on Diesel cycle.
3. the fuel used in SI engine is Petrol that burns if left in air at a little bit temperature. It is very volatile. The fuel used in CI engine is Diesel that has less self-ignition tendency and is less volatile.
4. In SI engines, aromatics are most preferred fuels whereas in CI engines alkanes are preferred as fuels.
5. A fuel must pass through four criteria before it is used : (a) volatility, (b) anti-knock quality, (c) gum deposits, and (d) sulphur content. Except volatility, other three can be managed.
More volatility means less ignition temperature, which in turn implies that a lower compression ratio can be obtained(as is the case with SI engines). In CI engines, diesel is used that is less volatile than petrol. So an appreciably larger compression ratio can be obtained that yields more efficiency. So volatility of fuels does play a crucial role in determining the efficiency of the Internal Combustion engines.
3. The air-fuel mixture is homogeneous throughout in SI while in CI only air enters and in the cylinder mixture is heterogeneous.
4. In SI engine, properly mixed air-fuel mixture in carburettor enters the cylinder. But in CI engine, only air enters the cylinder.
5.In CI engines the compression ratio is higher, which produces high pressures inside the engine. Hence CI engines are heavier than SI engines.
6. 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. In SI engine, the combustion chamber is smaller because there is no smoke. But in CI engine, due to smoke, combustion chamber has to be larger.
8. In SI engines, knocking is due to pre-ignition. It can result in detonation and can damage the engine altogether. But in CI engines, knocking is due to ignition delay, and hence it is least harmful.
9. The compression ratio is in the range of 6-10 in SI engines whereas in CI engines, it is about 12-20. Hence CI engines are considered efficient than the SI engines.
10.In the case of SI engines the lower compression ratio reduces their potential to achieve higher thermal efficiency. But in the case of CI engines the value of compression ratio is higher; hence these engines have the potential to achieve higher thermal efficiency.
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