COMBUSTION IN IC ENGINES.



Hey guys in this post we will talk about the fuel behaviour and the combustion process in the IC (internal combustion) engines and we will also look at the various important terms that each and every automotive enthusiast should know about the combustion process in IC engines.

What is Combustion?

It is the rapid and high temperature oxidation of fuel with liberation of fuel energy.
The requirements for initiating the combustion process are:
·       Combustible air-fuel mixture
·       Means for starting the combustion and formation, stabilisation and propagation of flame inside the combustion chamber of the engine.
Some of the very important terms:

  • Ignition Lag or Delay period- It is the first time interval between the instant of spark and the instant when there is noticeable rise in the pressure due to combustion. 
  • Self-ignition temperature- The temperature at which fuel will ignite itself without flame or in absence of flame i.e we don't have to give any spark or use any external matchstick or any other source of flame but the fuel will start to burn itself.
  • Auto-ignition- It is simply the self-ignition of fuel in the absence of flame , this term may confuse you in between the self-ignition and auto-ignition as both are very similar.
these terms are used very frequently when we talk about combustion process in the IC engines and when we talk about the process such as Detonation and Knocking, we will discuss about the Detonation and Knocking in detail in our next blog.
 Now lets move further and continue with our topic.

Stages of Combustion in S.I (spark ignition) Engines:



 Figure shows the stages of combustion graphically.
There are 3 main stages of combustion in SI engines :
  1. Preparation phase or period of ignition lag
  2. Flame propagation phase
  3. After burning or flame termination phase.

  • Preparation phase or period of ignition lag: This phase is shown in the figure between point a & b, ignition is given at point a & combustion starts at point b, so the time interval between point a & b is called Ignition lag and it is expressed in terms of crank angle.
"Period of ignition lag is very small approximately (0.0015 seconds-0.002 seconds). It is also known as ignition delay period."
  • Flame propagation phase: This starts at point 'b' when the self sustaining travels actually appears and this flame travels in combustion chamber outwards and burn the fuel in layers.Flame propagates across the combustion chamber at very high speed (15-35 m/sec).It is very difficult to differentiate between these 2 phases of combustion but the start of 2nd phase is considered when appreciable pressure rise is seen on the Pressure-Crank Angle diagram. Point b to c represent the rate of pressure rise.
  • Flame Termination Phase: Actual combustion is not completed at point 'c' but this point 'c' represents the completion of flame travel. It is because burning continues due to left over fuel. This combustion beyond point 'c' is called after burning which continues along  with expansion stroke upto point 'd'. This phase occurs when rich mixture is supplied to the engine and flame velocity is decreased during this phase.
                 

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