SI Engines & CI Engines- Spark Ignition & Compression Ignition Engine

SI Engines and CI Engines along with differences are explained along with a lot of details. Engines are among one the marvellous inventions by humankind. It helped humankind to be equipped with a machine that has made commuting easier. Let’s explore SI Engines & CI Engines!

What are SI Engines & CI Engines & Differences?

Let’s try to understand the basics of SI Engines & CI Engines before going to the differences. Engines act as the heart for any automobile, making it quite essential to get comprehensive information about the same.

  • The genre of the engine is extensive, and here we are providing some of the information that can help you grab a fair share of knowledge about the same.
  • This excerpt is all about two of the main types of IC engines, SI and CI engine.

Apart from the core introduction, we will provide all the relevant information regarding the application, characteristics, advantages, and disadvantages of both. In addition to that, the difference between both is also elaborately discussed. Without further ado, let’s delve into the details.

SI Engine & CI Engine

Specifically, there are two types of engines that are based upon the internal combustion ignition technique. The SI engine stands for the spark-ignition engine, while CI is for compression-ignition engines. Let’s start with the core definition of both.

SI Engine or Spark Ignition Engine

A spark-ignition engine (SI engine), generally known as a petrol engine, is an internal combustion engine.

  • Here the combustion process occurs due to the ignition provided by a spark from a spark plug to the air-fuel mixture.
  • There are several misconceptions regarding the petrol engine to be the SI engine, but the petrol engine is just a type of SI engine.
  • There are various other engine types of SI engines such as hydrogen engines, LPG engines, gas engines, etc.
  • In a nutshell, we can define an SI engine as the engine in which fuel is ignited with a spark plug or a specific ignition source.

SI engines specifically depend upon the sparking mechanism that causes combustion due to the compressed fuel-air mixture’s ignition. The SI engine is equipped with a spark plug mounted on each cylinder to provide the ignition to the air-fuel mixture.

Characteristics and Working of SI Engines

The working of SI engines may be withering four strokes or two strokes. Those SI engines which work as a four-stroke are an Otto cycle engine.

  • It consists of four strokes, suction or intake stroke, compression stroke, expansion or power stroke, exhaust stroke.
  • The whole cycle of four strokes is completed through 720 degrees of crank rotation as all the four strokes consist of 180 degrees of crankshaft rotation.
  • In SI engines, the essential thing is the correct amount of fuel-air mixture in the cylinder.
  • The fuel-air mixture gets compressed, and the ignition takes place with the help of a spark plug.

Application of SI engine

As we know, SI engines come in both types of strokes, such as two strokes and four strokes, and both are used for different purposes. Some of the application of SI engines are as follows:

  • SI engines, which come with two strokes, are widely used in scooters and mopeds.
  • Small equipment such as lawnmowers also uses this kind of engine.
  • These engines are also used in pumping sets and small electricity generating sets.
  • The usage of four-stroke SI engines is abundant in automobile industries.
  • Four-stroke SI engines that run on CNG are used in buses.
  • Small aircraft also use radial SI engines.

Advantages of SI Engine

The advantages of SI Engine are as follows,

  • Comparatively lower cost
  • These are capable of providing high rpm at low torque.
  • It causes comparatively less air pollution.
  • These engines require less space and are also lightweight.

Disadvantages of SI Engine

The disadvantages of SI Engines, are as follows

  • Comparatively low efficiency
  • Not economical and preferable for high load capacity
  • The consumption of fuel is on the higher side.
  • The problem of knocking persists in this type of engine.

CI Engine or Compression Engines

A compression ignition engine is an engine in which the ignition of fuel charge is due to compression heat.

  • In this kind of engine, the process of combustion is different from the SI engine.
  • In CI engines, air pressure comes into play, which gets compressed in the combustion chamber.

Characteristics & Working of CI Engines

CI engines’ characteristics and working can be understood with the help of strokes, which defines the whole compression and ignition process of the same. Let’s delve into the different strokes through which the entire process takes place in CI engines.

Suction stroke

This stroke occurs when the piston is due for moving down towards the bottom dead center or BDC. Here the inlet valve opens with the help of the inlet mechanism. In this stroke, that exhaust valve remains closed, and as the piston travels downwards, suction comes into the play, and fresh air gets sucked into the cylinder.

Compression Stroke

When the suction stroke gets completed, and the piston is at BDC, then the inlet valve gets closed, and the exhaust valve also remains in the closed position.

  • When the piston moves upwards, then the compression of air takes place.
  • Here generally, the compression ratio lies between 16 to 22.

Power stroke

After the end of the compression stroke, the process of atomization occurs, specifically the injection of fuel in the form of fine spray. The fuel gets ignited due to the excessive compression.

  • The fuel supply continues in the expansion stroke also, which helps the whole combustion process take place at a constant pressure.
  • After this, the piston moves to BDC due to the high pressure.
  • The power stroke is responsible for providing the much-needed power to the engine.

Exhaust Stroke

In this stroke, the inlet valve remains closed, and the exhaust valve opens, due to which a part of burnt gases escapes out. The cylinder pressure also falls to atmospheric pressure, and the piston again starts the cycle by moving towards the top dead center.

Applications of CI engine

CI engines are useful and provide maximum efficiency. These are widely used in different types of automobiles as well as pieces of machinery. Some of the core applications of the same are as follows:

  • High power CI engines are used in ships.
  • Small CI engines or diesel engines are used in compression machinery, drilling rigs, pump sets, etc.
  • CI engines are widely used in different types of automobiles, such as jeep, trucks, cars, etc.
  • The supercharged CI engines are used in earthmovers such as excavator, paver, loader, etc.
  • These are also used in high power electricity generators.

Advantages of Compression Ignition Engines

CI engines have different advantages as this kind of engine is used for numerous applications that provide support to various industries. Some of the core advantages are provided below:

  • CI engines’ mechanical efficiency is on the higher side as it prevents pumping losses at part load.
  • The availability of heterogeneous mixtures doesn’t need the intake of air.
  • The control of the load is dependent upon the quantity of injected fuel in the cylinder.
  • These engines are also capable of providing maximum thermodynamic efficiency.
  • In every expansion stroke, it does more work.
  • CI engines can work with the help of a minimal proportion of air to fuel ratio.
  • The specific heat ratio is also higher, which ultimately results in higher conversion efficiency. Due to these, it has lower emissions of CO2 and also lowers fuel consumption.
  • When it comes to volumetric efficiency, the CI engines can provide it on the higher side. This is because these engines are more significant and also capable of attaining minimal heat losses. It doesn’t have that efficiency loss due to the heating up of the manifold.

Disadvantages of CI Engine

  • Generally, CI engines are heavyweight, which increases the overall kerb weight of the vehicle.
  • CI engines produce comparatively more vibration than SI engines.
  • At the time of exhaustion, it provides an unpleasant odor.
  • This kind of engine comes at a higher cost.

Comparison Between SI and CI Engines

It is essential to compare the SI and CI engine as it provides general ideas about the best one, though both have similarities; on some points, these provide different characteristics. Here we are providing some of the differences between both.

  • The grounds on which we are differentiating are the attributes that provide maximum efficiency to any automobile.
  • Depending on different working conditions, both types of engines have their benefits and limitations.

Weight and design of cylinders

When it comes to the design of the engine, various processes define the same. Due to these processes, the design and weight of the engine vary.

  • In CI engines, the higher compression ratio stands for massive pressure exertion, which needs thicker walls.
  • This makes the diesel engines have thicker walls than that of petrol engines.
  • It also makes the CI engines heavier, larger, and stronger than petrol engines.

Compression ratio

SI engines cannot work at high pressure due to the self-ignition process, whereas CI engines work at a high compression ratio.

  • The main thing which is needed by CI engines is high pressure, which helps in achieving the ignition temperature.
  • The compression ratio for SI engines generally lies at 10:1, whereas CI engines have 18:1.


One of the core differences that acts as the most effective one is the ignition or the combustion process.

  • Specifically, in the SI engine, the combustion starts due to the spark plug, whereas the CI engine doesn’t have any spark plug, and the fuel ignites due to the excessive compression.
  • In CI, engine pressure affects the temperature of the air-fuel mixture.
  • The temperature of the air-fuel mixture increases with the increase in temperature.

Thermal efficiency and temperature

When it comes to fuel efficiency, the CI engines provide a nudge over SI engines due to CI engines’ high compression ratio.

  • CI engines’ thermal efficiency is between 30-45%, whereas the SI engine only provides 20-30% of the same.
  • The efficiency of both differs due to their design specifications as well as the compression ratio.

Power and speed

CI engines can provide high torque at a lower speed range, which is the sole reason for their usage in heavy vehicles.

Whereas SI engines generally work at high speed, it is preferable for lightweight automobiles such as bikes and cars. CI engines take time to get some great speed.

Pollution, vibration, and noise

There is a certain emissions level regarding the operation of both types of engines.

  • CI engines are equipped with a high compression ratio that produces more noise and vibration than SI engines.
  • The whole combustion process in SI engines is smoother and quieter.
  • In a nutshell, we can deduce that CI engines cause more pollution than SI engines.

Differences Table SI Engine and CI Engine

We can tabulate the key differences between SI Engine and CI Engine, as follows,

SI Engine or Spark Ignition EngineCI Engine or Combustion Engine
Light weight, main application is in Small vehiclesHeavy weight, main application is in heavy vehicles.
Main thermodynamic working cycle is OTTO cycle.The main thermodynamic working cycle is DIESEL Cycle.
Here, heat addition happens at constant volume.Here, heat addition happens at constant pressure.
In SI Engine, petrol is used as working fluid and it is highly volatile.In CI Engine, diesel is used as working fluid and it is less volatile comparative to petrol.
Carburetor is used for the supply of fuel.Fuel injector is used for the supply of fuel.
Self-Ignition temperature of working fluid (petrol) is high.Self-Ignition temperature of working fluid (diesel) is low.
The compression ratio is generally considered in the range of 7 to 10.The compression ratio is generally considered in the range of 14 to 20.
Operating speed is high approx. (3000 RPM to 6000 RPM)Operating speed is low approx. (400 RPM to 3500 RPM)
Thermal efficiency is low due to low compression ratio (7 to 10).Thermal efficiency is high due to high compression ratio (14 to 20).
The upper limit of compression ratio is finalized by fuel’s antiknock quality.The same is done by engine’s weight increase methodology.
Homogeneous combustion.Heterogeneous combustion.
Cranking effort is less due to low volume, & starting is easyCranking effort is more due to high volume, & starting is comparatively difficult.
Low pressure is produced.High pressure is produced.
Air and fuel both are introduced in the suction stroke.Only air is introduced in the suction stroke.
Low cost comparatively CI engine.High cost comparatively SI engine.
Low maintenance costHigh maintenance cost
Pollution is less as it produces less hydrocarbon.Pollution is more as it produces high hydrocarbon.
Less noise produces.It produces high noise.
Difference between SI Engine and CI Engine

Differentiation Summary

As per the above differentiating points, it is quite clear that SI engines are type of IC engine in which the fuel and air mixture ignites with the help of a spark plug, whereas in CI engine, the combustion takes place with the help of air compression, which is being heated at high temperature.

When it comes to preference, both have their justification for different kinds of applications. SI engines are also known as petrol engines, which are comparatively preferable for fuel economy and minimal emissions, whereas CI engines or diesel engines are mainly preferable for those automobiles that are exceptionally performance-oriented. With these points, we can help ourselves better understand both types of IC engines.


Here we have learned about the two most popular types of IC engines and their all-around attributes. However, numerous points can be covered to get further explanation. This excerpt provides holistic and all-round information about IC engines. Our Videos Refer YouTubeOpens in a new tab..


Hello Everyone! We, Rituparna Guha & Firan Mondal, are delighted to present our Mechanical Engineering articles. We will capture all types of articles and try to explain in the simplest way with a lot of diagrams! Happy Reading....

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