Difference between SI and CI engines
SI engine stands for spark ignition engine while CI engine stands for compression ignition engine.
SI engine works on the principle of working of the theoretical Otto cycle, while CI engine works on the principle of theoretical Diesel cycle.
In a SI engine, the fuel and air mixture is drawn into the cylinder during the suction stroke, while in a CI engine only pure air will be drawn during the suction stroke.
In a SI engine, the compression of the petrol and air mixture is less so that the heating of the mixture will be less than the ignition temperature of the fuel, where as in CI engine pure air will be compressed to very high pressure to attain a temperature greater the Ignition temperature of the diesel oil (higher compression ratio).
In a SI engine, the fuel-and-air mixture will be ignited by the spark plug, while in a CI engine the heat of the compressed air will auto ignite the diesel oil.