Fuel Effects on Homogeneous Charge Compression Ignition Combustion
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Ghandhi, Jaal B.
- Jul 08, 2009
- Homogeneous charge compression ignition (HCCI) is one combustion strategy that has shown the possibility of both lower emissions and lower fuel consumption than spark ignition combustion. However, HCCI combustion can be sensitive to changes in fuel composition.
The effects of fuel composition on HCCI combustion were experimentally investigated in a single cylinder 4-stroke SI engine adapted to run HCCI combustion with the Negative Valve Overlap (NVO) valve timing strategy. This strategy traps large amounts of hot residual gases in the cylinder to raise the mixture temperature and promote auto-ignition. Direct fuel injection was used, with one injection occurring during the NVO period to increase the residual temperature and for fuel reforming, and the other injection occurring just after intake valve opening. The timing and duration of the NVO injection were used to control the combustion phasing; an advance in timing or increase in delivered fuel mass caused an advance in combustion. The testing was performed at three operating conditions: 2100RPM 3bar IMEP, 2100RPM 2bar IMEP, and 3500RPM 2bar IMEP.
Three gasoline-like fuels and an 87 octane pump gasoline were tested to examine the effects on fuel consumption and HC, CO, and NOx emissions. The trends for the fuels were consistent over all conditions and correlated well with combustion phasing. Most differences were small with a few exceptions. One of the test fuels, which was the only oxygenated fuel tested, showed significantly higher fuel consumption, which was mainly due to its lower energy density. This fuel also had lower HC emissions than the other test fuels; however, this may have been caused by the HC analyzer?s lower response to oxygenated hydrocarbons. The pump
gasoline had significantly higher NOx emissions, but the reason for this difference is less clear and may have been caused by chemical effects.
The combustion phasing of the four fuels was also compared. Heptane and Iso-Octane were also tested as they provided limits of low and high octane rating respectively. The phasing of the three test fuels, which ranged in RON from 66 to 88 and MON from 69 to 98, were all advanced compared to the pump gasoline by 2� at 2100RPM and 1� at 3500RPM. The phasing of Heptane was advanced by 6� at 2100RPM and 3� at 3500RPM, while the Iso-Octane was retarded 6� at 2100RPM and 3� at 3500RPM. Correlations of RON and MON with the change in phasing were poor, but Octane Index correlations were good.
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