Engine Speed

Volumetric efficiency of the engine changes with speed, it being highest in the mid-speed range. At high engine speeds the volumetric efficiency generally decreases resulting in high residual gas dilution. Although heat transfer rates increase with increase in engine speed as a result of higher turbulence, but total amount of heat transfer is lower due to shorter cycle time. This gives higher gas temperatures at higher speeds.  However, at high speeds a shorter time is available for NO formation kinetics. The net result is a moderate effect of speed on NO  although this is specific to the engine design and operating conditions. Increase in exhaust gas temperatures at higher speeds enhances post flame oxidation of unburned hydrocarbons. A reduction of 20 to 50 percent in HC emissions has been observed with increase in speed from 1000 to 2000 rpm.

Cold Start and Warm-up Phase

Engine cold start and warm-up phase contribute significantly to unburned hydrocarbons. One of the main sources of HC emissions during cold start and engine warm-up period is very rich fuel-air ratio needed for ignition and combustion for several seconds after engine start. During cold start, the engine has to be over-fuelled 5 to10 times the stoichiometric amount of gasoline. To obtain robust ignition on the first cycle on cold start, a fuel vapour- air equivalence ratio above lean threshold limit (f = 0.7-0.9) is required. This threshold is independent of the engine coolant temperature. The fuel-air equivalence ratio supplied to the engine during cold start is in the range, f = 4 to 7.
For the first few engine cycles, a large fraction of inducted fuel is stored as liquid film in the intake port and cylinder as only the most volatile fractions evaporate when the engine is cold. The liquid fuel films do not participate in combustion and is emitted as unburned fuel emissions.