As the name suggests, the basic difference between SI and CI engine is the way 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
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 is more in CI engines.
As there is no spark plug for timely initiation of a spark in the CI engine, there is always a chance of incomplete combustion of fuel. Due to the 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 Engine||CI Engine|
|SI engine uses a spark plug to initiate ignition||CI Engine does not have any spark plug|
|Spark is initiated by the spark plug||Spark is initiated by the heat of compressed air|
|Petrol is the fuel||Diesel is the fuel|
|Based on Otto Cycle||Based on diesel cycle|
|Constant volume heat addition||Constant pressure heat addition|
|Carburetor supplies the fuel||Fuel injectors supplies the fuel|
|During suction stroke, the air-fuel mixture is sucked||Only Air is sucked during suction stroke|
|Compression ratio varies varies between 6-10||Compression ration varies between 16-20|
|Less production of hydrocarbon||High production of hydrocarbon|
|Simple design compared to CI engine||Complex design|
|Low thermal efficiency due to the low compression ratio||High thermal efficiency|
|Low pressure is generated after combusion||High pressure is generated after combustion|
|Volume to power ration is low||Volume to power ration is high|
|Produce little noise||Produce too much noise|
|Maintenance is easy||Maintenance is tough|
|Weight is less||Weight is more compared to SI engine|
|SI engines are high-speed engines||CI engines are low-speed engines|
|Very less vibration problem||Vibration is common in CI engines|
|The starting of the engine is easy||Starting of the engine is difficult specially during winters|
|Used in small to medium sized vehicle||Used in heavy vehicles|
|Usually cheaper than CI engine||CI engines are costly|
Conclusion: Difference Between SI And CI Engine
That’s all we have in this article. I hope that you all got an idea about what is CI engine, what is SI Engine, and what is the key difference between SI and CI engine. If you still have any doubts, then please write the queries in the comment section and I will be happy to respond.