Under-mixing of Fuel

HC emissions also result from under-mixing of fuel with air. This can happen for the fuel injected later in the cycle or because of over-fuelling of the engine. The fuel left in the injector sac volume and nozzle holes   at the end of injection on heating during combustion gets fully   or   partially vaporized. The vaporized fuel from the nozzle sac and holes enter the engine cylinder at low velocity later in the cycle during expansion stroke and has little time to mix with air when the gas temperatures are still high. This portion of fuel therefore remains mostly unburned and is emitted in the exhaust. Nozzle sac volume has been observed to be the main contributor to HC emissions in a DI engines
through the process of under-mixing of fuel.  Effect of nozzle sac volume on HC is shown on Fig. 2.19.  Two types of nozzle designs one with low sac volume and another valve covered orifice (VCO) design are also shown. In VCO nozzles   the nozzle sac is eliminated.  The VCO nozzles however, are not preferred as their durability is low due to overheating of the nozzle tip as in the other nozzles fuel in the sac absorbs latent heat of vaporization and the nozzle temperatures are lower.

Fuel contained in the nozzle holes also contributes to the HC emissions as seen when the curve is extrapolated to zero sac volume.
For the DI engines, at full load a minimum of about 40 percent excess air (f< 0.7) is usually supplied to limit smoke emissions. Over-fuelling may occur during acceleration especially in turbocharged engines, as the response of turbocharger to increase airflow rate is slower than the increase in fuel injection rate. With increase in engine load (increase in fuel-air equivalence ratio), engine and cylinder gas temperatures increase and therefore, HC emissions generally decrease until a critical fuel-air ratio is reached. When excess air is reduced to around 10 percent (f = 0.9) or below HC emissions increase sharply.
At low ambient temperatures during engine warm up or with retarded injection timing, some engine cycles may misfire resulting in bulk quenching and high HC
emissions. Under these conditions, liquid fuel droplets may appear in the exhaust giving the exhaust a white coloured appearance, known as ‘white smoke’