Difference Between SI And CI Engine

As the name suggests, the basic difference between SI and CI engines is how fuel gets to ignite. In the case of the SI engine, a spark plug initiates the ignition, and in the case of the CI engine, the compression of air generates excessive heat to initiate the ignition.

Before going into detail about the difference between SI and CI engine, we should know a bit more about the SI engine and CI engine. Right? So, let’s get into it first.

What is a SI engine ( Spark Ignition Engine)

SI engine (Spark Ignition Engine) uses spark from a spark plug to initiate the ignition. SI engine works based on the OTTO cycle. The air-fuel mixture is done by the carburetor and injects the mixture into the engine cylinder for combustion.

A spark plug then generates a spark to start burning the air-fuel mixture. Since there is no such compression of air to initiate the burning of fuel-air mixtures, SI engines usually have a low compression ratio. And that is why SI engines are less powerful too.

During suction stroke, the carburetor supplies the air-fuel mixture to the engine unlike CI engines where only the air is sucked during a suction stroke

Difference Between SI And CI Engine

SI engines are also lightweight and simple in design. It produces less noise and vibration. The maintenance cost of the SI engine is low compared to CI engines. SI engines are mostly used in light commercial vehicles like passenger cars, scooters, etc.

What Is a CI Engine ( Compression Ignition Engine)

CI Engine ( Compression Ignition Engine ) is based on the diesel cycle. CI Engine does not use any carburetor for mixing air and fuel. Instead, the air is sucked during suction stroke, and it is pressurized to be heated. When the air is pressurized enough, fuel is injected through fuel injectors. The heat of the highly compressed air starts the ignition.

Since excessive pressure is required to compress the air, diesel engines have high compression ratios than any other SI engines. Also, the noise and vibration are more in CI engines.

Difference Between SI And CI Engine

There is no spark plug for timely initiation of a spark in the CI engine, and there is always a chance of incomplete combustion of fuel. Due to incomplete combustion, more hydrocarbons are generated, which is causing air pollution.

CI engines are complex in design and usually costly. Maintenance is also not easy. Usually, heavy vehicles like trucks, buses, ships, etc., use CI engines.

Difference Between SI and CI Engine

SI EngineCI Engine
SI engine uses a spark plug to initiate ignitionCI Engine does not have any spark plug
Spark is initiated by the spark plugSpark is initiated by the heat of compressed air
Petrol is the fuelDiesel is the fuel
Based on Otto CycleBased on diesel cycle
Constant volume heat additionConstant pressure heat addition
Carburetor supplies the fuelFuel injectors supplies the fuel
During suction stroke, the air-fuel mixture is suckedOnly Air is sucked during suction stroke
Compression ratio varies varies between 6-10Compression ration varies between 16-20
Less production of hydrocarbonHigh production of hydrocarbon
Simple design compared to CI engineComplex design
Low thermal efficiency due to the low compression ratioHigh thermal efficiency
Low pressure is generated after combusionHigh pressure is generated after combustion
Volume to power ration is lowVolume to power ration is high
Produce little noiseProduce too much noise
Maintenance is easy Maintenance is tough
Weight is lessWeight is more compared to SI engine
SI engines are high-speed enginesCI engines are low-speed engines
Very less vibration problemVibration is common in CI engines
The starting of the engine is easyStarting of the engine is difficult specially during winters
Used in small to medium sized vehicleUsed in heavy vehicles
Usually cheaper than CI engineCI engines are costly

Conclusion: Difference Between SI And CI Engine

That’s all we have in this article. I hope you all got an idea about the CI engine, what is SI Engine, and the key difference between SI and CI engine. If you still have any doubts, please write the queries in the comment section, and I will respond.

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A 2004 batch Mechanical Engineering graduate From NIT, Agartala. Close to 16 years of experience in the field of consumer electronics and appliances domain as a Sr. Design Engineer and Team Leader in India and the United States. A GD&T practitioner, with expertise in Lean Manufacturing and Six Sigma. Crazy about exploring CAD-CAM tools and in his free time love to go for a ride with his champ, Rian.