Idea discussion: Petrol pre-injection PLUS Direct post-injection

Replies

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  msuresh444

  Shashank Moghe

  Let us scientifically discuss an idea: The idea is to have petrol to be direct injected in a very small quantity, using the stratified charge methodology, making the air-fuel mix appropriately rich near the spark plug tip. The spark plug ignites the charge thereby increasing the pressure (and hence the temperature) beyond the auto-ignition point of Diesel. In the mean time (which is a very very short time), as the pressure (and temperature) is rising, we have diesel Direct injected through a different nozzle into the chamber. After the pressure and temperature hits the desired auto-ignition temperature, it combusts.
  
  Of course, this idea must have some pros and cons, here are some I envision:
  
  Pros:
  1) No high compression ratios needed for Diesel combustion- which serve only in increasing engine size and weight.
  2) A smaller + lighter engine, capable of providing the efficiency of a diesel.
  3) Possible fuel savings - since the engine has smaller Frictional losses (due to a smaller stroke length)
  4) Greater peak temperatures and pressures (greater torque) due to the already high starting point for the diesel combustion thermodynamic cycle (thanks to the petrol pre-injection).
  
  Cons:
  1) Emissions of a diesel engine.
  2) Combustion timing to maximize the torque - the diesel injection has to be timed perfectly so that it starts combusting after TDC. To make this happen, the petrol pre-injection+spark must be timed well before TDC to let the combustion peak pressure (hence temperature) just occur before TDC.
  3) Better material requirement for engine components to withstand such high thermal+structural loads.
  4) Complex instrumentation.
  
  I am discounting the instrumentation part, since this is just an idea under consideration. All people, just chip in.
  
  Thanks to #-Link-Snipped-# for this discussion on this post:
  #-Link-Snipped-#

  This is a good idea.
  The volume of combustion chamber will increase, to accommodate both petrol & diesel components. Engine size will be big, even though the pressure inside the chamber increases due to petrol ignition, it would only momentarily last. As the diesel is pumped in, with duration of time, the pressure would fade away.
  But, the electronics and interlocks will prove very costly for the project feasibility.
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  Shashank Moghe

  msuresh444

  Engine size will be big, even though the pressure inside the chamber increases due to petrol ignition, it would only momentarily last. As the diesel is pumped in, with duration of time, the pressure would fade away.

  The engine size will be smaller than a comparable Diesel with the same BMEP. Thats because we are reducing the stroke to that of a SI, which is considerably huge.
  
  I understand you are saying the pressure would only momentarily last because you are considering the latent heat utilization by diesel spray to vaporize, but you are not considering the fact that there is a combustion reaction going on, releasing the LHV of the injected petrol (which is substantially too much to be used up completely by the injected diesel). Hence, the pressure would be enough to provide the requisite activation energy for diesel auto-ignition. Besides, the petrol injection amount will be metered by the ECU to exactly provide for this activation energy once the vaporization of diesel is complete.