Module 3: Influence of Engine Design and Operating Parameters on Emissions
  Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 


CI Engine Design and Operating Variables and Emissions

Important diesel engine variables that influence emissions are:

Engine Design Variables

  • Compression ratio
  • Combustion chamber type
  • Combustion chamber design
  • Injection system: injection pressure and timing, nozzle holes, nozzle sac volume

 

Operating Variables:

  • EGR
  • Engine speed
  • Engine load
  • Fuel quality

Compression Ratio

  •   In the diesel engines the minimum compression ratio that can be used is governed by the ease of engine cold starting ability. For the high speed direct injection engines CR of around 16 to 17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold starting requirements prevents further reduction in the compression ratio.

  •   Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher combustion temperatures obtained at a higher compression ratios tend to increase oxidation of the unburned HC.

  •   At a low compression ratio, a longer delay increases the fraction of fuel burned during the premixed phase resulting in higher peak pressures and temperatures which cause an increase in NO x formation. On the other hand, increase in compression ratio due to higher combustion temperatures would tend to increase formation of NOx. If the ignition delay is too long the combustion may begin in the expansion stroke reducing combustion pressure and temperature. Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.

  •   Use of a low compression ratio results in too long a delay during engine warm up under cold conditions, and it causes high emissions of unburned fuel which due to its appearance is called ‘white smoke'.

  • A high compression ratio leading to high combustion temperatures would increase soot formation while on the other hand it increases soot oxidation.
  •   For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio is to be used.