SI Engine Variables and Emissions

Any engine variable that affects oxygen availability during combustion would influence CO emissions. The factors which influence flame quenching, quench layer thickness and post flame oxidation control engine out HC emissions. The burned gas temperature-time history and oxygen concentration control NO formation and emission. Hence the engine variables that influence burned gas temperature and oxygen concentration would affect the NO emissions.
Principal design and operating variables affecting engine emissions are:

Design Variables :

• Compression Ratio
• Combustion chamber surface to volume ratio
• Ignition timing
• Valve timings and valve overlap
• Air motion, swirl tumble etc
• Charge stratification

Operating Variables :

• Air-fuel Ratio
• Charge dilution and exhaust gas recirculation (EGR)
• Speed
• Load
• Coolant temperature
• Transient engine operation: acceleration, deceleration etc.

The effect of some variables discussed below is typical in nature and variations in the trends with specific engine design change are observed.