Thermal efficency of IC engine

Replies

• 

  din_Power Engg

  First basics,​

  
  

  The thermal Efficirncy can be given as,
  
  
  η th = Energy Out / Energy in​

  
  in this case (internal Combustion Engines) it becames, η th = Work Output / Heat Supplied
  
  Work Output is the difference between Heat Supplied and Heat rejected
  
  i.e., Work output comes as = Heat supplied – Heat rejected
  
  Therefore, the thermal Efficiency becomes = (Heat Supplied – Heat Rejected)/ Heat Supplied
  = 1- (Heat Rejected / Heat Supplied)………Eqn 1
  
  The inside engine temperature, in which combustion takes place is kept as Tin and the outside atmosphere to which the heat is rejected is kept as Tout . So ideally the Eqn. 1 can be modified as,
  
  η th = 1- (Tout / Tin)……Eqn 2
  
  The Eqn. 2 is an ideal one. But in real cases other factors like friction, etc.. also play a part. But for designing purposes only ideal cases are taken into account.
  
  Now, coming to your question,
  
  From the Eqn. 2 it is clear that the thermal Efficiency is dependent on two factors, that is outside (atmospheric) temperature and inside temperature (Engine)….So, while designing an IC engine these two factors are to be taken in to account….By optimizing any one of the factors the efficiency can be maintained throughout or improved further….as in your question, and in general the outside temperature will not be same everywhere (not Rajasthan and Kashmir, it may not exactly be the same between even 100 m distance for even 1 minute)…So, the IC engines is not designed considering the outside temperature, but designed keeping the inside combustion temperature (Tin) and other factors like heat added by friction, load etc.. in consideration.
  
  Even in ideal case, generally the inside temperature will be much higher, somewhere between 800 – 1000 degree Celsius than the ambient temperature, some where between - 10 to 60 degree (though the average figure will be some where between 15 – 35 degree Celsius)… so there wont be much difference in the ratios of the temperature, resulting insignificant difference in the efficiency values……
  
  But in colder conditions, there may some other problems like starting trouble, choking, etc….which are due to completely different reasons…..
• 

  ShrinkDWorld

  good explanatin din
• 

  jhbalaji

  Draw up a heat balance sheet mate...