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Friday June 23rd 2017

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SI (Spark Ignition) IC Engine Exhaust Emissions

Please read the previous post on IC Engine Emissions

The exhaust emissions from SI engines vary greatly with the operating conditions and mode of operation.

  • Cold start. HC and CO emissions produced within the combustion chamber will be high due to the fuel enrichment required to compensate for the poor vaporisation.
  • Idle. High HC emissions tend to occur due to the low load. High CO emissions are experienced at idle to the fuel enrichment required for reducing idle roughness.
  • Deceleration. HC during deceleration has been greatly reduced with fuel injection systems because they can cut off injection.
  • Acceleration. NOx emissions will be highest.
  • Full load. High CO emissions due to the fuel enrichment required for max power output.

 

The effect of fuel/air equivalence ratio on the emissions of and Sl engine is shown in the following figure. NOx peaks at about 10% weak of stoichiometric where the charge temperature and oxygen availability are still relatively high. CO is low at weaker than about 10% lean of stoichiometric but increases rapidly for rich mixtures. HC reduces rapidly as the mixture is weakened but rises sharply at excessively lean conditions due to partial burns.

Variation of HC, CO and NO Concentration with Fuel/Air Equivalence ration in an SI Engine (Heywood 1988)


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