Realization of Dual Phase High Temperature Heat Release Combustion of Base Gasoline Blends from Oil Refineries and a Study of HCCI Combustion Processes

It was reported that n-heptane and toluene blended fuels (NTL series fuels) showed the dual phase high temperature heat release (DP-HTHR) combustion in a previous SAE paper [1]. DP-HTHR has the potential to enlarge the engine operational range to high load conditions and lower the engine combustion noise. Further research has been reported in this paper. Initial interests were in the combustion characteristics of a second “bump” in the high temperature heat release (2^nd HTHR) in DP-HTHR, since this kind of two-stage combustion appears, when CO oxidation radically occurs over the 1450K temperature range. To prove this 2^nd HTHR of DP-HTHR was not caused by a rapid CO oxidation process, and indeed by the decomposition and oxidation of the benzene ring in toluene (benzyl radicals and aromatics), exhaust emissions (THC, NOx, CO, CO₂) and in-cylinder pressure data were measured at the same HTHR CA50 (the crank angle of 50% burn of high temperature heat release) and IMEP condition with the DP-HTHR fuel (NTL75) and non-DP-HTHR fuels (PRF90, NDB90 and NMP85). The CO emissions were evaluated by vol% scale and emitted quantity to correct the differences of fuel density, the H/C ratio and injected fuel quantity between the fuels. These data analyses supported that the decomposition and oxidation of the benzene ring at hot flame temperature was the starting trigger of the 2^nd HTHR in DP-HTHR combustion. The second interest was the possibility of DP-HTHR combustion with the blends of the refinery-based gasoline. NTL fuels are the surrogate fuels consisting of n-heptane and toluene.

To apply this DP-HTHR technique to the refinery blended fuels, three base gasoline blends of Nippon Oil refinery were chosen. Three different gasoline prototypes (GAS2, GAS3 and GAS4) were made, and the combustion of each fuel was evaluated by a 4 cylinder HCCI engine. The engine test results showed that DP-HTHR combustion could be achieved with GAS4 fuel. Furthermore, a uniform low temperature heat release (LTHR) was also reported, which is a special characteristic of DP-HTHR. In DP-HTHR combustion, the magnitude of LTHR and LTHR CA50 do not change even if the load changes. This is a unique combustion characteristic of DP-HTHR and gives a better idea of HCCI combustion processes, since DP-HTHR is a simple combustion model. The fundamentals of HCCI combustion processes are further investigated and discussed in this paper.