The NO_x and N₂O Emission Characteristics of an HCCI Engine Operated With n-Heptane

This paper presents the oxides of nitrogen (NO_x) and nitrous oxide (N₂O) emission characteristics of a Cooperative Fuel Research (CFR) engine modified to operate in homogeneous charge compression ignition (HCCI) combustion mode. N-heptane was used as the fuel in this research. Several parameters were varied, including intake air temperature and pressure, air/fuel ratio (AFR), compression ratio (CR), and exhaust gas recirculation (EGR) rate, to alter the HCCI combustion phasing from an overly advanced condition where knocking occurred to an overly retarded condition where incomplete combustion occurred with excessive emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO). NO_x emissions below 5 ppm were obtained over a fairly wide range of operating conditions, except when knocking or incomplete combustion occurred. The NO_x emissions were relatively constant when the combustion phasing was within the acceptable range. NO_x emissions increased substantially when the HCCI combustion phasing was retarded beyond the optimal phasing even though lower combustion temperatures were expected. The increased N₂O and UHC emissions observed with retarded combustion phasing may contribute to this unexpected increase in NO_x emissions. N₂O emissions were generally less than 0.5 ppm; however, they increased substantially with excessively retarded and incomplete combustion. The highest measured N₂O emissions were 1.7 ppm, which occurred when the combustion efficiency was approximately 70%.