Why cant we use petrol as a fuel in CI engines?

Replies

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  ashish_mittal

  just tell me if first air is compressed and then fuel is sprayed then y not petrol will ignite as compression raised temp of air to 700 C
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  Velshankar MJ

  Knocking will occur if you to auto ignite the petrol at 700 C. Also petrol may vapourise at such higher temperature. NOx emission is also possible due to this high temperature combustion...
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  ashish_mittal

  you are right but if i reduce the compression stroke of ci engine which will raise temperature of air upto auto ignition temperature of petrol,will petrol ignite or it will cease the engine ?
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  Velshankar MJ

  It will ignite but due to vaporization of fuel there wont be complete combustion and engine will produce very high noise and increase in load increase knocking and engine will tend to break down
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  ashish_mittal

  ok thanks velshankar...😀
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  Shashank Moghe

  Velashankar M, please stop posting wrong stuff.
  
  1. Vaporization is the essence of combustion. Its not vaporization that causes incomplete combustion.
  
  2. The real answer lies in the methods of combustion of both fuels: Petrol and Diesel.
  
  - Petrol combustion is dependent on flame kernel development and subsequent flame front propagation. This (the kernel) is accomplished with the help of a spark in the SI engine.
  
  - Diesel can combust upon compression. But petrol needs a spark to initiate combustion.There is no flame kernel and hence no flame front in Diesel combustion. Combustion occurs at all locations upon achievement of the necessary temperature.
  
  This is an excellent question, although a "diesel in petrol engine" question would be easier to answer. Any rightful corrections are welcome.
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  Velshankar MJ

  You can ignite petrol as he told by using high compression temperature of about 700 C without a spark plug (The problem is that if u compress that much petrol droplets will become gases)......... Flame propagation is a concept of burning of fuel droplets. In diesel fuel flame progates not to initiate the next droplet.. Every droplet will burn on its own.
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  Shashank Moghe

  - The droplets are created (atomized) with the intention of helping them evaporate! Who told you that evaporation is undesirable? Since you seem too attached to that idea, let me quote Wikipedia for you:
  
  "Liquid gasoline itself is not actually burned, but its fumes ignite, causing the remaining liquid to Evaporate and then burn".
  
  Some may qualify Wikipedia as an unreliable source, but if you want you can refer most other books and will find that the evaporation is what leads to combustion.
  
  - What do you mean by "Flame propagation is a concept of burning of fuel droplets"? That does not make much sense.
  
  Flame kernel is created with a heavy electrical discharge through a small gap between the electrodes of the spark plug (the air fuel mixture is between the gap). This kernel expands (you can think of an expanding sphere) and the combustion chemical reactions take place of the flame front (the surface of the sphere, which is also the boundary between unburned air-fuel mix on the outside and burned air-fuel mix on the inside of the sphere).
  
  - In diesel there is simultaneous combustion in all parts of the combustion chamber, hence I said there is no flame front propagation.
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  Shashank Moghe

  Also, I would like to retract my statement claiming Petrol cannot burn in a diesel engine. Velshankar M mentioned this somewhere in this conversation.
  
  Although petrol has a higher Auto-Ignition temperature than diesel, at the diesel engine compression ratios, enough high temperatures are reached to cause petrol to auto-ignite. The only problem is that since the petrol is more volatile than diesel, it burns very spontaneously as compared to diesel (when allowed to auto-ignite). And also much hotter.
  
  Now, this spontaneous combustion is obviously uncontrollable ("knocking") and causes severe structural damage to the engine. Besides, the NOx emissions are increased since it has a higher maximum combustion temperature.
  
  Controlling combustion over several crank angle degrees is the prime motive in extracting power out of an engine. Hence knocking is undesirable (besides the obvious reason that knocking damages the engine).