Effect of high-octane oxygenated fuels on n-heptane-fueled HCCI combustion

The effect of the addition of high-octane oxygenated fuel including methyl tertiary butyl ether ( MTBE), ethanol, and methanol on combustion phasing and combustion rate of homogeneous charge compression ignition (HCCI) combustion fueled with n-heptane as base fuel was comparatively investigated. In this paper, n-heptane was blended with MTBE, ethanol, and methanol from 10 vol % to 60 vol % at 10 vol % increments, respectively. Three kinds of blended fuels were injected into intake air, and their premixed HCCI combustions were studied comparatively. The results show that the HCCI combustion fueled with high-octane oxygenated fuel/n-heptane blended fuels is delayed in the combustion phasing and suppressed in combustion rate in contrast to that with pure n-heptane. Given the higher fuel/air equivalence ratio, the combustion phasing of blended fuels is progressively delayed with the increasing proportion of MTBE, ethanol, or methanol, so that the high-temperature heat release (HTHR) can occur near top-dead center (TDC) or later; at the same time, the HCCI combustion can be suppressed to partial burn or even misfire when the proportion of MTBE, ethanol, and methanol is increased to some extent, respectively. On the other hand, MTBE, ethanol, and methanol have some differences in how they affect the n-heptane- fueled HCCI combustion. In view of the capability of delaying combustion phasing, methanol is exhibited as the most powerful of the three fuels and ethanol is stronger than MTBE. In view of suppressing the combustion rate of HCCI combustion, methanol is the most effective and MTBE is the least powerful among the three fuels. When total fueling rate is kept at 18.7 mg/cycle, the misfire of methanol/n-heptane blended fuels occurs near methanol 40 vol % (M40), and that of methanol/n- heptane blended fuels occurs near ethanol 50 vol % ( E50); however, MTBE/n-heptane blended fuels do not misfire till MTBE 60 vol % (MTBE60). The differences among the three fuels fundamentally result from their performance of difficult autoignition. At the same time, the oxygen content in the high-octane oxygenated fuels may improve the HTHR of HCCI combustion that is still suppressed by the addition of these fuels. On the basis of the difference of three high-octane oxygenated fuels in the control of combustion phasing and combustion rate, it is noted that MTBE has more advantages over methanol and ethanol in the potential of extending the operating range of n-heptane- fueled HCCI combustion by adjusting the addition of MTBE, ethanol, and methanol, respectively, at the same total fueling rate.