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Authors
J.P. Szybist and
B.G. Bunting
Publication
SAE Paper No. 2005-01-3723, 2005 SAE Powertrain & Fluid Systems
Conference and Exhibition.
Abstract
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 heir 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. |
