Four Stroke Cycle CI and SI Engine Comparison and Differences

Your four stroke cycle petrol engine is a great example of a SI (Spark Ignition) engine. Diesel engines are referred to as CI (Compression Ignition) Engines. How can you compare or differentiate SI and CI engines? To make things simpler to you, here is a list of some important facts:

- In the suction stroke, a SI engine draws air-fuel mixture into the cylinder. But, a CI engine draws only air in the suction stroke.

- You will find a carburetor in your petrol engine that mixes air and petrol. On the other hand, CI engines use atomizer or injector that injects the fuel at the end of the compression stroke.

 - Your petrol engine gets only 10 bar pressure at the end of the compression stroke. But, CI engines usually gets 35 bar pressure at the end of the compression stroke.

 - Spark plug provided with your petrol engine ignites the charge, but in case of a CI engine the fuel gets ignited by itself due to high pressure and temperature (Usually, it gets up to 600o C temperature).

- Practically, you will find your engine’s compression ratio in between 6-10 bars. But, the compression ratio of CI engines range from 15-35 bar.

- You will also find the speed of your petrol engine is higher than a CI engine (Diesel engine)

- Thermal efficiency of a petrol engine is a bit low than CI engines. Theoretically, thermal efficiencies of SI and CI engines are 26% and 42% accordingly.

- Your petrol engine is highly advantageous over CI engines due to low maintenance cost.