Combustion chamber design can affect the combustion process also, and therefore the level of emissions.
Designs that increase gas flow rate, and promote vaporization, distribute the fuel more evenly in the combustion chamber.
Quenching of the combustion flame can occur in zones in the combustion chamber where surface temperatures are low. The flame temperature drops so low in these areas that the flame goes out, or is quenched. Fuel left unburned in these zones is then exhausted, as hydrocarbon and carbon monoxide emissions.
If the spark plug is positioned so that the flame front travels evenly through the combustion chamber, combustion is more complete.
Gas flow rate, and volumetric efficiency, can be improved by using 2 intake valves in each cylinder. The effective port opening is increased, and the gas flow rate increases.
Changing valve timing also alters the combustion process. Reducing valve overlap reduces the scavenging effect. It also reduces hydrocarbon emission.
Source: CDX Global
