The si or spark ignition engine uses petrol as a fuel and the ci or compression ignition engine uses diesel as a fuel. Both the fuels has different compression ratio.
In si engine the compression ratio is 8-12:1.
In ci engine the compression ratio is 16-22:1.
So incase of si engine, the compression ratio is not sufficient for fuel to burn so a spark plug is used, whereas in ci engine, the compression ratio is so high that due to its internal heat the fuel is combusted so there is no need for a spark plug.
so the technology used in si engine is different from ci engine.
Hope I have answered your question.
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By the virtue of ignition and chemical properties of the fuel used (diesel or petrol), same technology can't be used for both the engines.
Diesel has lower self-ignition temperature than petrol. Therefore flame-formation for diesel (in ci engines) is achieved by a temperature rise by compression only, where as petrol (in si engines) with comparatively higher self-ign temp needs an external spark for the same.
If the same technology of ci engine is to be employed for petrol, larger temp rise and thus a larger compression is needed, which in turn increases the surface area of piston, thus enormously increasing the size of engine.
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In SI engine the compression ratio is less so fuel can not reach it's self ignition state, for that spark plug is used i SI engine. in CI engine compression ratio is high so temperature goes high and fuel go ignite.
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The properties of both these fuels i.e., gasoline and diesel are different, Gasoline is less flamable than diesel.
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Self ignition temp of petrol in higher then diesel so it requires much pressure for self ignition as compared to diesel, due to this reason CI is not
favorable in case of petrol engine.
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