Emission formation study of HCCI combustion with gasoline surrogate fuels

HCCI combustion can be enabled by many types of liquid and gaseous fuels. When considering what fuels will be most suitable, the emissions also have to be taken into account. This study focuses on the emissions formation originating from different fuel components. A systematic study of over 40 different gasoline surrogate fuels was made. All fuels were studied in a CFR engine running in HCCI operation. Many of the fuels were blended to achieve similar RON's and MON's as gasoline fuels, and the components (n-heptane, iso-octane, toluene, and ethanol) were chosen to represent the most important in gasoline; nparaffins, iso-paraffins, aromatics and oxygenates. The inlet air temperature was varied from 50°C to 150°C to study the effects on the emissions. The compression ratio was adjusted for each operating point to achieve combustion 3 degrees after TDC. The engine was run at an engine speed of 600 rpm, with ambient intake air pressure and with an equivalence ratio of 0.33. NO _x emissions were low for all operating points, and ethanol and toluene addition was found to decrease NO_x emissions for higher octane fuels. CO emissions were related to in-cylinder temperature by formation from HC and oxidation to CO₂. HC emissions were found to be mainly dependent on the compression ratio used in each case. Ethanol addition was shown to reduce HC emissions for combustion at a given compression ratio. Toluene addition was found to increase HC emissions at a given CR when high concentrations (40-60 vol.%) were added. Low concentrations of toluene showed a small decrease of or no effect on HC emissions.