An Investigation of the Effect of Fuel-Air Mixedness on the Emissions from an HCCI Engine

This research work has focused on measuring the effect of fuel/air mixing on performance and emissions for a homogeneous charge compression ignition engine running on propane. A laser instrument with a high-velocity extractive probe was used to obtain time-resolved measurements of the fuel concentration both at the intake manifold and from the cylinder for different levels of fuel-air mixing. Cylinder pressure and emissions measurements have been performed at these mixing levels. From the cylinder pressure measurements, the IMEP and peak cylinder pressure were found. The fuel-air mixing level was changed by adding the fuel into the intake system at different distances from the intake valve (40 cm and 120 cm away). It was found that at the intake manifold, the fuel and air were better mixed for the 120 cm fuel addition location than for the 40 cm location. Measurements of the fuel concentration in the cylinder showed an increase in unmixedness of 50% when the fuel was added at the 40 cm location vs. the 120 cm location. The better mixed operating point showed a decrease in oxides of nitrogen of approximately 30%, and decreases in levels of unburned hydrocarbons and carbon monoxide of about 20% each. The IMEP was essentially unchanged for the given change in fuel-air mixing. The cycle-to-cycle variations in peak cylinder pressure were decreased when the fuel and air were better mixed.

Additionally, a multi-zone combustion model has been run to determine the qualitative effects of fuel-air mixture non-uniformities on NO_X emissions. This model has shown that when standard deviation of fuel concentration (normalized by mean fuel concentration) increases from 0 (i.e., uniform) to 20%, NO_X levels increase by a factor of four. Additionally, the effect of fuel-air non-uniformity on combustion duration and ignition timing was found to be minimal.