Difference between Spark Ignition and Compression Ignition Engines

By | March 22, 2019

Abbreviations used

SI : Spark Ignition, CI: Compression Ignition

Many people generally find it difficult to differentiate between SI and CI engines. To solve this problem we came with an exhaustive comparison between SI and CI engines (spark ignition vs compression ignition).

This comparison has been drawn in the form of a table to make it easy to understand.

Description SI Engine CI Engine
Basic Cycle Otto cycle Diesel Cycle
Fuel Gasoline Diesel
Introduction of Fuel Fuel and air mixture is supplied to the engine with the help of carburetor or gasoline injection Fuel directly injected into combustion camber, with the help of a fuel pump and injector
Load Control throttle valve controls the supply of fuel air mixture Quantity of fuel is regulated, not air
Ignition Uses spark plug for ignition Self ignition due to high compression
Compression ratio* 6 to 10 16 to 20
Speed used for high speed used for comparatively low speed
Thermal efficiency* lower compared to CI engines Higher compared to SI engines
Weight Lighter due to low peak pressure Higher due to high peak pressure

Compression ratio

The ratio of the maximum to minimum volume in the cylinder of an internal combustion engine.

Thermal efficiency

The thermal efficiency of a heat engine is the amount of useful work an engine can do based on the amount of heat input.

Spark Ignition

spark-ignition engine is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug.


A compression-ignition engines, typically diesel engines, where the heat generated from compression together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.

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