IC Engines : Combustion in SI & CI Engine

By Vijay Pratap Singh|Updated : September 13th, 2021


In an Internal combustion engine,  the combustion process uses the chemical energy released by the fuel in the form of heat which further used for producing mechanical work.
The engines are made to work for cycles which include intake, compression, expansion, or power and exhaust cycles.
The fuel-air mixture formation is the main governing parameter for the combustion process to occur and hence the combustion in SI and CI engine depends upon the method of air-fuel mixture formation.
The first basic difference is the fact that in an SI engine, spark initiates the combustion while in a CI engine initiation of combustion is solely dependent on the self-ignition of the fuel being used. 



  • In Internal combustion engine,  the combustion process uses the chemical energy released by the fuel in the form of heat which further used for producing mechanical work.
  • The engines are made to work for cycles which include intake, compression, expansion, or power and exhaust cycles.
  • The fuel-air mixture formation is the main governing parameter for the combustion process to occur and hence the combustion in SI and CI engine depends upon the method of air-fuel mixture formation.
  • The first basic difference is the fact that in an SI engine, spark initiates the combustion while in a CI engine initiation of combustion is solely dependent on the self-ignition of the fuel being used. 

Air fuel mixtures: An engine is generally operated at different loads and speeds. For this, a proper air-fuel mixture should be supplied to the engine cylinder. Fuel and air are mixed to form three different types of mixtures.

(i). Chemically correct mixture: 

  • A chemically correct or stoichiometric mixture is one in which there is just enough air for complete combustion of the fuel.
    For example, to burn one kg of octane (C8H18) completely 15.12 kg of air is required. Hence chemically correct A/F ratio for C8H18 is 15.12:1 i.e. usually approximated to 15:1.
  • Complete combustion means all carbon in the fuel is converted to CO2 and all hydrogen to H2O.

(ii). Rich mixture: A mixture that contains less air than the stoichiometric requirement is called a rich mixture (example, A/F ratio of 12:1, 10:1, etc.).

(iii). Lean mixture: A mixture that contains more air than the stoichiometric requirement is called a lean mixture (example, A/F ratio of 17:1, 20:1, etc.)


Combustion in Spark Ignition (SI) Engines: 

  • In spark-ignition engines, a nearly homogeneous mixture of air and fuel is formed in the Carburetor. In a homogeneous gas mixture, the fuel and oxygen molecules are more or less, uniformly distributed.
  • Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet manifold. Fuel droplets that remain in suspension continue to evaporate and mix with air even during suction and compression processes.
  • Stages of combustion in SI Engines: A typical theoretic pressure-crank angle diagram, during the process of compression (a → b), combustion (bc), and expansion (c → d) in an ideal four-stroke spark-ignition engine is shown in Fig.
  • Pressure variation:
    The first stage (A → B) is referred to as the ignition lag or preparation phase in which the growth and development of a self-propagating nucleus of flame take place.
    The second stage (B →C) is a physical one and it is concerned with the spread of the flame throughout the combustion chamber.


  • The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to the engine cylinder is called carburetion and the device which does this job is called a Carburetor.
  • The process of mixture preparation is extremely important for spark-ignition engines. The time purpose of carburetion is to provide a combustible mixture of fuel and air in the required quantity and quality for efficient operation of time engine under all conditions.

Factors affecting Carburetion: Of the various factors, the process of carburetion is influenced by

  • The engine speeds
  • The vaporization characteristics of the fuel
  • The temperature of the incoming air
  • The design of the Carburetor

Abnormal Combustion in SI Engine:

Under certain operating conditions the combustion deviates front its normal course leading to loss of performance and possible damage to the engine. This type of combustion may be termed abnormal combustion or knocking combustion. The consequences of this abnormal combustion process are the loss of power, recurring preignition and mechanical damage to the engine.

The phenomenon of Knock in SI Engines: 

  • A definite flame front that separates the fresh mixture from the products of combustion travels from the spark plug to the other end of the combustion chamber. Heat-release due to combustion increases the temperature and consequently the pressure, of the burned part of the mixture above those of the unburned mixture.
  • In order to effect pressure equalization the burned part of the mixture will expand, and compress the unburned mixture adiabatically thereby increasing its pressure and temperature. This process continues as the flame front advances through the mixture and the temperature and pressure of the unburned mixture are increased further.
  • If the temperature of the unburnt mixture exceeds the self-ignition temperature of the fuel and remains at or above this temperature during the period of flame reactions (ignition lag), spontaneous ignition or autoignition occurs at various pin-point locations. This phenomenon is called knocking. The process of autoignition leads towards engine knock.

Increase in variable

Major effect on unburned reduce charge

Action to be taken to knocking

Can operators usually control?

Compression ratio

Increases temperature & pressure



Mass of charge inducted

Increases pressure



Inlet temperature

Increases temperature


In sine case

Chamber wall temperature

Increases temperature


Not ordinarily

Spark advance

Increases temperature & pressure


In some cases

A/F ratio

Increases temperate & pressure

Make very rich

In some cases


Decreases time factor


Somewhat (through engine speed)

Engine speed

Decreases time factor



Distance of flame travel

Increases time factor



Combustion in compression ignition (CI) Engines: 

  • In the CI engine, only air is compressed through a high compression ratio (16:1 to 20:1) raising its temperature and pressure to a high value.
  • Fuel is injected through one or more jets into this highly compressed air in the combustion chamber. Here, the fuel jet disintegrates into a core of fuel surrounded by a spray envelope of air and fuel particles.
  • This spray envelope is created both by the atomization and vaporization of the fuel. The turbulence of the air in the combustion chamber passing across the jet tears the fuel particles from the core. A mixture of air and fuel forms at some location in the spray envelope and oxidation starts.
  • The liquid fuel lets evaporate by absorbing the latent heat of vaporization from the surrounding air which reduces the temperature of a thin layer of air surrounding the droplet and sometimes elapses before this temperature can be raised again by absorbing heat from the bulk of air.
  • As soon as this vapor and the air reach the level of the autoignition temperature and if the local A/F ratio is within the combustible range, ignition takes place.
  • Combustion in CI Engines:
    Ignition delay period:  A definite period of inactivity between the time when the first droplet of fuel hits the hot air in the combustion chamber and the time it starts through the actual burning phase. This period is known as the ignition delay period.
    Physical delay: The physical delay is the time between the beginning of injection and the attainment of chemical reaction conditions. During this period, the fuel is atomized, vaporized, mixed with air and raised to its self-ignition temperature.
    Chemical delay: During the chemical delay, reactions start slowly and then accelerate until inflammation or ignition takes place. Generally, the chemical delay is larger than the physical delay.

The phenomenon of knock in SI engines: 

  • In CI engines the injection process takes place over a definite internal of time. Consequently, as the first few droplets to be injected are passing through the ignition delay period additional droplets are being injected into the chamber.
  • If the ignition delay of the fuel being injected is short, the first few droplets will commence the actual burning phase in a relatively short time after injection and a relatively small amount of fuel will be accumulated in the chamber when actual burning commences. As a result, the mass rate of mixture burned will be such as to produce a rate of pressure rise that will exert a smooth force on the piston.
  • If the ignition delay is longer, the actual burning of the first few droplets delayed and a greater quantity of fuel droplets gets accumulated in the chamber. When the actual burning commences, the additional fuel can cause too rapid a rate of pressure rise, resulting in jamming of forces against the piston and rough engine operation. If the ignition delay is quite long, so much fuel can accumulate that the rate of pressure rise is almost instantaneous. Such a situation produces extreme pressure as differentials and violent gas vibrations known as knocking and is evidenced by an audible knock.

The factors that tend to increase autoignition reaction time and prevent knock in SI engines promote knock in CI engines. Also, good fuel for a spark-ignition engine is a poor fuel for the compression-ignition engine. The spark-ignition fuel has a high octane rating of 80 to 100 and a low cetane rating of about 20, whereas diesel fuels have a high cetane rating of about 45 to 65 and a low octane rating of about 30.



SI Engine

CI Engine


Ignition temperature of the fuel




Ignition delay




Compression ratio




Inlet temperature




Inlet pressure




Combustion wall temperature




Speed, rpm




Cylinder size



Lubrication of Engine Components:

The method of reducing friction by introducing the substance called lubricant between the mating parts is called lubrication.


  • Reduce friction thus increase efficiency.
  • Reduce wear and tear of moving parts.
  • Carry away heat.
  • Provides sealing action between the cylinder and piston rings, thereby it reduces blow-by.
  • Provide protection against corrosion.
  • Lubrication film acts as a cushion and reduces vibration.
  • Carrying away the grit & other deposits and provide cleaning.
  • Reduce noise.

The main component of the IC engine to be lubricated.

  • Piston & cylinder
  • Main crankshaft bearings
  • Small and big end bearings of the connection rod
  • Cam, camshaft and its bearing
  • Valve and valve operating mechanism
  • Timing gears

Lubricating Oil

Types of lubricants

  • Solid (e.g. Graphite molybdenum, Mica)
  • Semi-Solid (e.g. Heavy greases)
  • Liquid (e.g. Mineral oils, Vegetable Oils, Animal Oils)

Properties of lubricants.

Viscosity: It is a measure of resistance to the flow of oil. It is measured in Saybolt Universal Seconds (SUS).

Viscosity Index:

  • The variation of viscosity of oil with change in temperature is measured by viscosity index.
  • Smaller the variation of viscosity, the higher the VI.
  • VI of Paraffin oil is 100 (small change) and VI of Naphthenic oil VI is 0 (large change).

Cloud Point: The temperature at which the oil starts solidifying is called cloud point.

Pour Point:

  • It is the temperature just below which the oil sample will not flow under certain prescribed conditions.
  • The sample is cooled until no movement of the oil occurs for 5 sec after the tube is tilted from the vertical to the horizontal.

Flash Points

  • The flashpoint is defined as the lowest temperature at which an oil will vaporize sufficiently to form a combustible mixture of oil vapour and the air above the surface of the oil.
  • It is found by heating a quantity of the oil in a special container while passing a flame above the liquid to ignite the vapour. A distinct flash of flame occurs when the flashpoint temperature is reached.

Fire Points

  • The fire point is obtained if the oil is heated further after the flashpoint. The fire point is the temperature at which the oil, once lit with flame, will be burnt steadily at least for 5 seconds.
  • Fire point temperature is usually 10°C higher than flash point temperature.


  • The property of an oil to cling to the metal surface by molecular action and then to provide a very thin layer of lubricant under boundary lubrication condition is called the oiliness or lubricity or film strength.

Carbon residue

  • It is the quantity of carbon residue which remains after evaporation of a simple oil under specified conditions.


  • To prevent the formation of deposits, the engine oil has the property of detergency to clean the deposits.
  • It has also the ability to disperse the particles, preventing them from clotting and keeping them in a finely divided state.


  • Any violent agitation in the crankcase engine oil to foam. It is because of the presence of air bubbles in the oil. This action accelerates oxidation and reduces the mass flow of oil to the bearing and other moving parts causing insufficient lubrication.


These are the compound added to lubricant oils to promote and improve their desired properties.

Major classes of engine oil additives and their primary function

  • Detergent Dispersant (Metallic Salts, Organic acids)
  • Anti-wear
  • Anti-rust ( Metal Sulphonates, Fatty acids, Amines)
  • VI improver (Butylene Polymers, Polymerized Olefins, Iso-olefins)
  • Pour point depressant (Phenols, Easters, Alkylated Naphthalene)
  • Anti-foam (Silicone Polymers)
  • Antioxidant (Zinc Dithiophosphate, Sulphur and Phosphorous compounds, Amine & Phenol Derivatives)

Comparison Between Air and Water Cooling

S. No.

Air cooling

Water cooling


No danger of water freezing at low temperature.

There is a danger of water freezing at low temperatures.


Maintenance is low and the design is simple.

Maintenance is high due to the radiator and the design of the engine is more complicated.


The engine warms up quickly.

Engine warm-up takes comparatively more time.


Lower specific fuel consumption.

Higher specific fuel consumption.


The weight to power ratio is low.

The weight to power ratio is high.


Suitable for low-capacity engines.

Suitable for high-capacity engines.


Regulation of cylinder temperature is not possible.

Regulation of cylinder temperatures is possible.


Volumetric efficiency is low.

Volumetric efficiency is high.


Cooling is not even.

More even cooling is achieved.   


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