Coolant Temperature

As the coolant temperature is increased, the contribution of piston ring zone crevice becomes lower due to decrease in gas density within this crevice. Secondly, the top piston-land side clearance is also reduced due to higher thermal expansion of the piston. A thinner oil film and reduced fuel vapour solubility would result in reduced absorption of fuel vapours in engine oil. Increased postflame oxidation at high temperatures also contributes to reduction in HC emissions. Increase in coolant temperatures has been observed to reduce HC emissions by about 0.4 to 1.0 % per K increase in temperature.  An increase in the coolant temperature from 20 to 90º C, roughly results in 25% lower HC emissions and hence, the need of a rapid engine warm up is obvious.
For reduction of the cold start and warm up HC emissions, an important area of development is to improve the fuel injection and delivery to the cylinder with minimum wall wetting. Over-fuelling during cold start and warm-up is to be kept at a minimum, while still forming the combustible charge.

Summary

The effects of some of the important  engine design and operating variables on emissions from SI engines are summarized  below  Table 3.1.

Variable Increased	HC	CO	NOX
Fuel-air equivalence ratio
Compression ratio	↑	-	↑
Surface/volume ratio	↑	-	↓
Bore/stroke ratio	↑	-	↓
Ignition timing advance	↑	-	↓
Port fuel injection	↑	↓	↓
Engine speed	↑	-	↑↓
Engine load	↑	-	↑
Coolant temperatures	↓	-	↑
Combustion chamber deposits	↓	-	↑
EGR	↓	-	↓
Intake swirl and turbulence	↓	-	↑