Cetane Number and Engine Speed Effects on Diesel HCCI Performance and Emissions

The effects of cetane number (CN) on homogeneous charge compression ignition (HCCI) performance and emissions were investigated in a single cylinder engine using intake air temperature for control. Blends of the diesel secondary reference fuels for cetane rating were used to obtain a CN range from 19 to 76. Sweeps of intake air temperature at a constant fueling were performed. Low CN fuels needed to be operated at higher intake temperatures than high CN fuels to achieve ignition. As the intake air temperature was reduced for a given fuel, the combustion phasing was retarded, and each fuel passed through a phasing point of maximum indicated mean effective pressure (IMEP). Early combustion phasing was required for the high CN fuels to prevent misfire, whereas the maximum IMEP for the lowest CN fuel occurred at a phasing 10 crank angle degrees (CAD) later. The high CN fuels exhibited a strong low temperature heat release (LTHR) event, accounting for more than 15% of the total heat release in some instances, while no LTHR was detected for fuels with CN ≤ 34. All of the fuels had comparable NOx emissions and pressure rise rates at their respective maximum IMEP timing, with NOx emissions below 6 ppm at 3.5 bar IMEP. At advanced combustion phasing, low CN fuels had significantly higher pressure rise rates and higher NOx emissions than the high CN fuels. At retarded phasing, the CO emissions for the high CN fuels were excessive, with a CO:UHC ratio of up to 8, while these remained <1 for low CN fuels. These results suggest that the products of LTHR, which are high in CO, are more sensitive to the quenching effects of cylinder expansion. Thus high CN fuels, which exhibit significant LTHR, require early combustion phasing, whereas low CN fuels can be retarded to later combustion phasing. Increasing engine speed had the effect of reducing the total LTHR. Further investigation showed that the LTHR rate is constant on a millisecond basis, so the effect of higher engine speed is to reduce the time allowed for the reaction without changing the rate of reaction.