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

This paper presents the NO_x and N₂O emission characteristics of a Cooperative Fuel Research (CFR) engine modified to operate in Homogeneous Charge Compression Ignition (HCCI) combustion mode using an air-assist port fuel injector. The single-cylinder engine was fuelled with n-heptane for these experiments. The parameters examined include intake air temperature and pressure, air/fuel ratio, compression ratio, and exhaust gas recirculation (EGR) rate. The parameters were varied in order to change the combustion phasing from advanced (knocking) to retarded (incomplete combustion) conditions. NO_x emissions were less than 5 ppm for a fairly wide range of combustion phases, except when knocking or incomplete combustion occurred, and were largely unaffected by the parameter varied when the combustion phase was within the acceptable range. It was also found that NO_x emissions increased significantly when retarded and incomplete combustion was observed even though lower combustion temperatures were expected. The increased N₂O and unburned hydrocarbon (THC) emissions usually observed with retarded combustion phasing, as well as the deteriorated combustion efficiency, may contribute to this unexpected increase in NO_x emissions. It was also shown that N₂O emissions were extremely low (less than 0.5 ppm) except when incomplete combustion was observed.