| Spark Ignition (SI) Engine | Compression Ignition (CI) Engine |
| Spark plug required | No spark plug required |
| The mixture of air and fuel is introduced into the cylinder from carburettor. | Only air is introduced into the cylinder. |
| These type of engines compresses air and fuel together in the cylinder | In these engines air is only compressed in the cylinder. |
| No fuel pump is used. | Fuel pump is used to inject fuel. |
| Fuel is mixed with air before compression starts. | Fuel is mixed with air once compression is complete. |
| Compression ratio is low. | Compression ratio is high. |
| This type of engine makes use of highly volatile liquid fuel. | This type of engine makes use of less volatile liquid fuel. |
| Less efficient. | More efficient. |
| Fuel used in this engine is expensive. | Cheaper fuels are used in these engines. |
| Higher fuel consumption in these engines for same power. | These engines have lesser fuel consumption for same power. |
| Engines are more compact and light. | Heavier and strong engines due to higher pressure involved |
| Initial cost is less | Initial Cost is high. |
| These engines have a smooth operation | Roughness in engine operation encountered, especially when the engine runs at high speed and low loads. |
