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Difference between SI (Spark ignition) and CI (Compression ignition) engines

SI and CI engines are the short form of saying spark ignition engine and compression ignition engine. this is also a way of saying petrol (gasoline) engine and diesel engine. these engines are four-stroke internal combustion engines but work in their various form. In other words, SI and CI engines are ways of classifying internal combustion engines, their difference is so broad.

difference between SI and CI engine

Today you’ll get to know the difference between SI engine and CI engine, functions, and working. You’ll get to know the advantages and disadvantages of the two engines.

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Difference between SI and CI engine

SI engine (spark ignition engine)

The spark ignition (SI) engine is a petrol engine that is often used on commercial vehicles such as scooters, motorcycles, cars, etc. it produces less noise and vibration, starting is easy, requires less maintenance, and has lightweight.

In their definition, we could say SI engines are internal combustion engines that work on the principle of spark ignition (that’s creating a spark with spark plugs). It uses petrol and fresh air to accomplish the otto cycle. The air/fuel mixture is performed in the carburetor before entering the cylinder, it also measures the amount of mixture to push in the cylinder.

Advantages and disadvantages of SI engine


  • It creates less noise
  • Engine is light
  • Less maintenance is required.
  • It starts easily
  • High speed and thermal efficiency.
  • Cheaper


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Watch video the learn how a SI engine works:

CI engine (compression-ignition engine)

The compression ignition (CI) engine is a diesel engine used in heavy-duty vehicles, machinery, railways, ships, trucks, and equipment. In the combustion process, only air enters into the cylinder during the suction stroke, unlike the SI engine that allows both air and fuel to enter. It ignites the diesel fuel with compressed hot air.

The CI engines are also internal combustion engines that use diesel fuel to operates on the diesel cycle. In the system, the spark plug is eliminated because the spark is not needed combustion. The combustion is accomplished by compressing pure air in the cylinder, which then compressed to a hot temperature so that ignition can occur when the fuel is injected into it.

This system has a high compression ratio due to the high ignition temperature of diesel fuel. More power is produced because of the high compression ratio. More hydrocarbon is produced because of the incomplete combustion, leading to air pollution from the exhaust. CI engines make more noise and vibration compare to SI engines. maintenance cost is also an issue in the CI engines.

Advantages and disadvantages of CI engine


  • Produces more power
  • High working efficiency
  • A huge amount of fuel can be burn
  • Low fuel cost
  • High energy density
  • Good lubricating properties
  • Low risk of catching fire because the flammable vapor is not produced.
  • Good exhaust-emission behavior.
  • No intake of air restriction apart from the air filters.
  • Fuel is injected directly into the combustion chamber.


  • Make more noise and vibration
  • Produces hydrocarbon, leading to environmental pollution.
  • High maintenance cost is required.
  • The engine is heavier.

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Watch the video to learn how a CI engine works:


So now let look at the difference between spark ignition (SI) engine and compression ignition (SI) engine in tabular form:

Difference between SI and CI engines


SI Engines

CI Engines

1. Spark is used to burn fuel Hot air is used to ignites fuel
2. Petrol is use as fuel Diesel is use as fuel
3. Operates on otto cycle Operates on diesel cycle
4. Low compression ratio High compression ratio
5. High thermal efficiency Low thermal efficiency
6. A spark plug is used to produce spark for ignition The heat of compressed air is used for ignition
7. High-speed engine Low-speed engine
8. Low pressure is generated after combustion High pressure is generated after combustion
9. Constant volume cycle Constant pressure cycle
10. Air and fuel Only air
11. It has less weight The engine is heavy
12. It makes less noise It makes more noise
13. Less hydrocarbon is produced More hydrocarbon is produced
14. Starting is very easy Starting can be difficult
15. Low maintenance cost High maintenance cost.
16. Less vibration problem Very high vibration issue
17. Engine cost is less Engine cost is high
18. Inject fuel with a carburetor Fuel is injected with an injector
19. It’s used on light commercial vehicles It’s used on heavy-duty vehicles.
20. Volume to power ratio is less The volume to power ratio is high.

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In conclusion, a spark-ignition engine and compression ignition engine are very useful in various suitable applications. Their difference is very clear and obvious making one serve better than others. We have stated and clarify the difference between the two engines; SI and CI their functions, applications, and working. Their advantages and disadvantages are also stated.

I hope you enjoyed the article, if so, kindly comment, and recommend this site to other technical students. Thanks!

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