To gain a better understanding of how the onset of incomplete bulk-gas reactions changes with engine speed and fuel-type, a parametric study of HCCI combustion and emissions has been conducted. The experimental part of the study was performed at naturally aspirated conditions and included fueling sweeps at four engine speeds (600, 1200, 1800 and 2400 rpm) for research grade gasoline, pure iso-octane and two mixtures of the primary reference fuels (i.e. n-heptane and iso-octane) with octane numbers of 80 and 60. Additionally, single-zone CHEMKIN computations with a detailed mechanism for iso-octane were conducted.
The results show that there is a strong coupling between the ignition quality of the fuel and the required intake temperature to phase the combustion at TDC. There is also a direct influence of intake temperature on the completeness of combustion. This is the case because the CO-to-CO2 reactions are highly sensitive to the peak combustion temperatures.
For fuels with very little cool-flame activity (i.e. gasoline and pure iso-octane), the fuel/air-equivalence ratio for onset of incomplete bulk-gas reactions is independent of engine speed. This occurs because the increased compression temperatures required to maintain ignition as the engine speed is increased also increase the rate of CO-to-CO2 conversion, therefore balancing the shorter time available for the combustion event. The minimum peak combustion temperature for complete combustion is only weakly dependent on engine speed and ranges roughly from 1470 K at 600 rpm to 1550 K at 2400 rpm.
However, for fuels with a significant fraction of n-heptane, the onset of incomplete bulk-gas reactions is dependent on engine speed with a shift towards higher fuel/air-equivalence ratios for lower engine speeds. This is caused by the strong speed dependence of the cool-flame chemistry, which necessitates lower intake temperatures at lower engine speeds to maintain combustion phasing at TDC. This leads to lower peak combustion temperatures and a commensurate rise of the CO-emissions unless the fueling is increased.