Compression Ignition and Spark Ignition

Spark burning engines (SI engines) use gasoline as fuel, while in compression ignition (CI) engines, the fuel used is usually diesel. Petrol engines have less weight and therefore they can achieve higher speed. Diesel engines are heavy and hence less efficient.

In the case of SI engines, the Otto cycle is used where addition of heat or fuel combustion occurs at a constant volume. In CI engines, in the Diesel cycle, the fuel combustion occurs at a constant pressure.
In the case of SI engines, a mixture of air and fuel is automatically injected from cylinder head portion. In CI engines, the quantity of air to be injected is not controlled.

To generate this spark in SI engines, the spark plug is placed in the cylinder head of the engine. in the case of CI engines, there is no need for spark plugs.

In the case of CI engines the value of compression ratio is higher; they have the potential to achieve higher thermal efficiency.