Comparison Between SI Engine And CI Engine

At first glance, it seems both SI engine and CI engine are completely similar, but they are working on an entirely different principle. A four stroke CI engine operating on diesel cycle (called diesel engine) and SI engine working on the Otto cycle (called petrol engine). They differ from engine design to its efficiency. Depending on working condition both have their own advantages and disadvantages. Here is the comparison of these two types of IC engines.

Weight/ design of cylinders

The diesel engine has a higher compression ratio than a petrol engine, means more pressure exerted on the internal element. In order to withstand high-pressure, its wall must be thicker than that of a petrol engine. That makes diesel engine heavier, stronger, and larger than a petrol engine. Generally, the diesel engine is more expensive than a petrol engine.

Air and fuel intake

In both engines, the actual working fluid is the air-fuel mixture. In SI engine, the carburetor is used to blend fuel-air mixture before they entered the combustion chamber. In CI engine fuel is injected into the cylinder by fuel injector at the end of compression stroke. They are working under different proportion of air fuel. A diesel engine requires more air than a petrol engine for combustion of fuel.

The power of CI engine under varying load controlled by regulating fuel flow into the cylinder, so it is not necessary to regulate air intake. CI engine always sucks same quantity of air while working, so they don’t need special valves (Throttle flap) as the SI engine needs it.

Compression ratio

Petrol engine cannot operate at high pressure because of the problem of self-ignition, meanwhile, diesel engine work at a high compression ratio. Diesel engine needs high pressure to achieve ignition temperature of the fuel. The compression ratio of a diesel engine can be up to 18:1 and for a petrol engine, it is about 10:1.


The main difference between SI and CI engine is how they start combustion process. In SI engine, the fuel is ignited by a spark created by the ignition system (spark plug). The diesel engine has no spark plug, the fuel ignited by other means known as compression ignition. As pressure increases, the temperature of the air-fuel mixture also increase. At high pressure when the temperature reaches ignition temperature, the fuel gets self-ignited.

Temperature and thermal efficiency

The CI engine is more fuel efficient than SI engine because high compression ratio. The SI engine had a thermal efficiency of 20%-30%, the thermal efficiency of petrol engine is between 30%-45%. The efficiency varying due to change in compression ratio and other design specification.

Speed and power

CI give more torque, but deliver at lower speed range. That is the reason CI engine used in almost all heavy vehicles. Petrol engine operates at high speed. They are suitable for light vehicles such as cars and bikes. CI engine takes the time to build up some speed.

Vibration, noise and pollution

Because of the high compression ratio in the cylinder, diesel engine produces more vibration and noise than a petrol engine. The combustion process in petrol engine quieter and smoother. Diesel engine causes more pollution than a petrol engine.

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