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Hope everyone is safe and doing great😊 

Good to see you again…We’re aware about your concern for information related to combustion in SI engine and we’re pretty much confident that all your queries and concepts would be clarified in this article. 

Let’s deep dive on how combustion in SI engine takes place and improve our knowledge related to this topic. 


The stages of combustion in SI engine can be divided into three major areas:  

(1) Ignition and flame development. 

(2) Flame propagation. 

(3) Flame termination.  

The flame outbreak is generally considered to be the consumption of the first 5% of the air-fuel mixture (some sources use the first 10%).  

At the flame development stage, the process of ignition and combustion in SI engine takes place, but only a slight increase in pressure is noticeable and little or no useful work is performed.  

Almost all useful work that occurs in an engine cycle is the result of the flame execution time of the combustion process.  

Cylinder pressure in the combustion chamber of an SI engine as a function of crank angle. The increase in pressure rise is very slow after ignition during the flame development period. This results in a slow pressure force increase on the piston and a smooth engine cycle. Maximum pressure occurs 5 ° to 10 ° aTDC.

This is the period during which most of the fuel and air mass burns (that is around 80-90%).  

During this time, the pressure in the cylinder rises significantly, which provides the force to work in the expansion stroke.  

The last 5% of the mass of air-fuel ratio burned (some sources use 10%) is classified as flame extinguishing. During this time, the pressure drops rapidly and combustion stops.  

In a petrol engine, combustion ideally consists of a heat-generating subsonic flame that continues through a premixed homogeneous air-fuel mixture.  

The spread of the flame surface is significantly increased by the induced turbulence, swirl and squish within the cylinder.  

With the right combination of fuel and operating characteristics, knocking can be prevented or almost eliminated.  


Combustion in SI engine is initiated by an electric discharge through the electrodes of the spark plug.  

This occurs between 10° and 30° in front of the TDC (Top dead centre), depending on the shape of the combustion chamber and the direct operating conditions of the engine.   

This high temperature plasma discharge between the electrodes ignites the air-fuel mixture in the immediate vicinity, from which the combustion reaction spreads outward.  

Combustion in SI engine begins very slowly because of the high heat loss due to the relatively cold spark plug and gas mixture.  

The flame can usually be detected at about 6° of crank rotation after the spark plug has ignited. 

The applied potential is typically 25,000 – 40,000 volts, with a maximum current on the order of 200 amps lasting about 10 nsec. This provides a peak temperature on the order of 60,000 K.  

The total spark discharge takes about 0.001 seconds at an average temperature of about 6000 K. A stoichiometric mixture of hydrocarbon fuels requires about 0.2 mJ (0.2 x 10-3 J) of energy to ignite self-sustaining combustion.  

This fluctuates up to 3 mJ for non-stoichiometric mixtures.  

Discharging a spark plug provides 30-50 mJ of energy, most of which is lost by heat transfer.  

Various methods are used to generate the high voltage potential required to cause a discharge between the electrodes of the spark plug.  

A common system is a combination of battery coils.  

Most cars use a 12-volt electrical system that includes a 12-volt battery.  

This low voltage is multiplied by the coil that supplies the very high potential supplied to the spark plug.  

Some systems use capacitors to discharge from the spark plug electrodes at the right time.  

Most small engines and some large engines use a magneto driven by the crankshaft of the engine to generate the required spark plug voltage.  

Some engines have a separate high voltage generation system for each spark plug, while others have a single system with a distributor that switches from one cylinder to the next.   

The electrode gap of the latest spark plugs is about 0.7-1.7 mm. 

Shorter distances are acceptable if the air-fuel mixture is abundant or the pressure is high (e.g., high inlet pressure or high compression ratio with a turbocharger).  

The normal quasi-steady state temperature of the spark plug electrode between ignitions should be approximately 650° to 700 °C. 

Temperatures above 950 °C endanger the possibility for surface ignition, and temperatures below 350 °C tend to promote surface contamination over long periods of time.  

For older engines where the piston rings wear and burn too much oil, a hotter spark plug is recommended to prevent contamination. 

The temperature of the spark plug is controlled by the heat loss path created within the plug. Hot plugs have higher thermal resistance than cold plugs.  

Modern spark plugs are made with better, more expensive materials, and have a much greater life span than those of a decade ago. 

Some high-quality spark plugs with platinum tips last over 160,000 km (100,000 miles). One of the reasons this is desirable is that it is difficult to replace the plug on some modern engines. Due to the increased equipment of the engine and the smaller engine compartment, it is very difficult to replace the spark plug.  

In extreme cases, modern cars require the engine to be partially removed in order to change the connector.  

The voltage, current, electrode material, and gap size should be compatible with the long-life plug (for example, if the current is too high, the spark plug electrode will wear).  

When the spark plug ignites, a plasma discharge ignites the air-fuel mixture between and near the electrodes.  

This creates a spherical flame surface that extends outward toward the combustion chamber. Due to its small original size, the flame surface moves very slowly at first.  

Diffusion is very slow because it does not generate enough energy to heat the surrounding gas quickly.  

Again, this does not cause a sharp rise in cylinder pressure and almost no compression heating occurs. 

Only after the first 5-10% of the air-fuel ratio has burned will the flame velocity reach a higher value, and the corresponding pressure will rise sharply, creating a flame diffusion region.  

It is desirable to have a rich air-fuel mixture around the electrodes of the spark plug when ignited.  

The rich mixture ignites more easily, has a faster flame rate and allows a better start of the entire combustion in SI engine.  

Spark plugs are usually placed near the intake valve to ensure a richer air-fuel mixture, especially when starting a cold engine. 

Spark plugs with several electrodes and two or more simultaneous sparks are now available. These give a more consistent ignition and quicker flame development. 

One modern experimental system gives a continuing arc after the initial discharge.  

It is reasoned that this additional spark will speed combustion in SI engine and give a more complete combustion as the air fuel mixture is swirled through the combustion chamber.  

This system is quite similar to methods tried over a hundred years ago.  

Development work has been carried out to manufacture spark plugs with variable electrode gap sizes. This provides flexibility in ignition under different operating conditions.  


When the first 5-10% of the air-fuel ratio burns, the combustion process is well established and the flame surface moves very quickly through the combustion chamber.  

Due to induced turbulence, swirl, and squish, the flame propagation velocity is about 10 times faster than in the case of a laminar flame front moving through a stationary gas mixture.  

In addition, the flame surface, which expands spherically from the spark plug in static air, is strongly distorted and spread by these movements.  

When the gas mixture burns, the temperature, and therefore the pressure, rises to a high value.  

The combustion gas behind the front of the flame is hotter than the unburned gas on the front, and the pressures of all the gases are about the same.  

This reduces the density of the combustion gases and causes them to expand and occupy a larger percentage of the total volume of the combustor.  

If only 30% of the gas mass burns, the burned gas already occupies almost 60% of the total volume and compresses 70% of the unburned mixture to 40% of the total volume. 

Due to the compression of unburned gas, the temperature rises due to pressure heating.  

Mass percent burned vs volume percent burned in the combustion chamber of a typical SI engine

In addition, the radiant heat on the order of 3000 K emitted from the flame reaction zone further heats the unburned and combustion gases in the combustion chamber. 

The increase in temperature due to radiation further increases the pressure.  

Due to the very short real-time of each cycle, heat transfer by conduction and convection is lower than that of radiation. 

As the flame passes through the combustion chamber, it passes through an environment where temperature and pressure gradually rise.  

This reduces the chemical reaction time and increases the flame tip velocity.  

With radiation, the temperature of the unburned gas behind the flame surface rises further, reaching a maximum at the end of the combustion process.  

The temperature of the combustion gas is not uniform throughout the combustion chamber, but it is high near the spark plug where combustion started.  

This is because the gas experiences more radiant energy due to the later flame reaction. 

Ideally, about two-thirds of the air-fuel mixture should be burned at TDC and almost completely at about 15 ° TDC.  

This causes the maximum temperature and pressure of the cycle to occur somewhere between 5 ° and 10 ° aTDC (after top dead centre).  

This is mostly ideal for 4-stroke cycle SI engines.  

Therefore, combustion in a real 4-stroke petrol engine is a process of near constant volume, but not accurate, as approximated by an ideal petrol cycle with air standards.  

The closer the combustion process is to a certain volume, the higher the thermal efficiency.  


At about 15° to 20° aTDC, 90-95% of the air fuel mass has been combusted and the flame front has reached the extreme corners of the combustion chamber.  

The last 5% or 10% of the mass has been compressed into a few percent of the combustion chamber volume by the expanding burned gases behind the flame front.  

Although at this point the piston has already moved away from TDC, the combustion chamber volume has only increased on the order of 10-20% from the very small clearance volume.  

This means that the last mass of air and fuel will react in a very small volume in the corners of the combustion chamber and along the chamber walls.  

Due to the closeness of the combustion chamber walls, the last end gas that reacts does so at a very reduced rate.   

Near the wall, the turbulence and mass motion of the gas mixture are damped and the boundary layer stands.  

A large block of metal wall also acts as a heat sink, dissipating most of the energy released by the reactive flame.  

Both mechanisms slow down the reaction rate and flame rate, and the combustion ends with a slow extinction.  

Due to its slow reaction rate, the piston does little extra work during this fire extinguishing time, but it is still a desirable event.  

During fire extinguishing, the cylinder pressure tends to rise slowly to zero, so the force transmitted to the piston also slowly decreases and the engine runs smoothly.  

During the end of the flame, self-ignition and engine knocking may occur in the tail gas in front of the front of the flame. 

The temperature of the unburned gas in front of the flame surface continues to rise during the combustion process, reaching its maximum at the last end gas.  

The maximum temperature is often higher than the self-ignition temperature. At this point, the flame surface moves slowly, so gas is not consumed during the ignition delay and self-ignition often occurs.  

The resulting knocks are usually unpleasant and unobtrusive.  

This is because there is very little unburned air-fuel ratio left at this point, so auto-ignition produces very small pressure pulses.  

The engine achieves maximum performance when operating with almost no automatic ignition and knock at the end of the combustion process.  

This occurs when the combustion chamber is at maximum pressure and temperature and the pressure rises slightly due to knocking at the end of combustion. 


  • These engines have lower cost. 
  • They generate high rpm at low torque. 
  • It creates less pollution compare to CI engines. 
  • These engines are light in weight and required less space. 


  • SI engines have low efficiency compare to CI engine. 
  • These engines are not economical at high load.  
  • Higher fuel consumption.  
  • Knocking problem 


So that’s it from us for now!!! Hope you enjoyed going through the content. For any questions or suggestions please feel free to use the comments section. We would be more than happy to be in touch with you all👍 

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