Correlation of Low Temperature Heat Release With Fuel Composition and HCCI Engine Combustion 2005-01-0138
Low temperature heat release (LTHR) in HCCI combustion changes according to fuel chemical composition and engine test conditions. In this study 11 pure hydrocarbon components were blended into 12 different model fuels to evaluate the effects of fuel composition on LTHR heating value, LTHR CA50 (crank angle at 50% completion of LTHR), high temperature heat release (HTHR), and engine performance. From the heat release analysis of the test data from a supercharged 4-cylinder engine, it was determined that the HTHR CA50 (crank angle at 50% completion of HTHR) was strongly indicative of combustion stability and maximum rate of pressure rise. Moreover, the functional dependence of HTHR CA50 on LTHR heating value and LTHR CA50 was quantified.
Test fuels denoted MD05, Base, MC05 and MX05 were prepared by adding 5.2vol%, 9.3vol%, 15.0vol%, and 18.2vol% of n-hexane, respectively, to a blend of 10 pure hydrocarbons. These four fuel blends were applied to HCCI combustion tests at the same engine speed and IMEP to evaluate how the LTHR heating value and LTHR CA50 change according to the amount of n-hexane, a fuel that exhibits large LTHR. The test results showed that the heating value and volume percent of n-hexane correlated linearly over the range of n-hexane tested. However, the relation between LTHR CA50 and volume percent of n-hexane was linear only at low volume percents, and became less so as the n-hexane content increased.
From the rate of heat release calculations it was apparent that n-paraffins contribute the most towards a large LTHR heating value followed by iso-paraffins. In direct correspondence, the propensity for early initiation of HTHR was clearly distinguishable by fuel group as follows:
Furthermore, it was found that the aromatics, except benzene, and some of the naphthenes and olefins, have a function to reduce the LTHR that would be expected of the remaining mixture counterparts. The discovery of this “LTHR inhibitor effect” is a key finding of this study.
From these results, it was obvious that the amalgam of each chemical component LTHR, including inhibitor effects, determines the HTHR CA50 timing, which in turn determines engine performance. Thus, the LTHR plays a key role in HCCI combustion and the chemical component dependencies of LTHR were examined.
The significance of octane number is discussed. In the case of regular gasoline (research octane number (RON) 90.5) and PRF 90.5, though the research octane numbers are identical, the heat release patterns differ significantly. This can be explained by the difference in fuel composition and the LTHR inhibitor effect of certain components of regular gasoline.