Dual Phase High Temperature Heat Release Combustion

To allow the HCCI vehicles to enter the market in the future, it is important to investigate the combustion deviations and operational range differences between the same research octane number fuels. In this paper, eighteen kinds of two hydrocarbon blended fuels, which were composed of n-heptane and another hydrocarbon, such as iso-octane, diisobutylene, 4-methyl-1-pentene, toluene or cyclopentane, were evaluated. Those fuels were blended to have the same research octane numbers of 75, 80, 85 and 90 by changing the blending volume ratio of n-heptane and counterpart hydrocarbon. Intake air was supercharged to 155 kPa abs and its temperature was kept at 58 °C. The HCCI engine was operated at 1000 rpm. Neither hot EGR, nor any other combustion stratification system was utilized in order to investigate the purely hydrocarbon effects on HCCI combustion. In-cylinder pressure data were taken at 7 different maximum pressure rise rate conditions, which were 900, 800, 700, 600, 500, 400 and 300 kPa/deg of 400 cycles averaged in-cylinder pressure traces, by changing the fuel injection quantity. The test results showed that the engine operational range of n-heptane and iso-octane blended fuels (PRF) were the same as those of n-heptane and diisobutylene blended fuels (NDB), and n-heptane and 4-methyl-1-pentene blended fuels (NMP). The operational ranges of n-heptane and toluene blended fuels (NTL) were at high IMEP conditions, but those of n-heptane and cyclopentane blended fuels (NCP) were only at low IMEP conditions, compared with the cases of PRF series fuels, NDB series fuels and NMP series fuels. To investigate those operational range differences, heat release data were analyzed at the same HTHR CA50 (the crank angle of 50 % burn of high temperature heat release) and IMEP conditions. As NTL fuels show the largest low temperature heat release (LTHR), high temperature heat release (HTHR) starts earlier than the other fuels. Furthermore, the HTHR combustion period of the NTL fuels was longer than the other fuels since the NTL fuels displayed dual phase high temperature heat release (DP-HTHR) combustion. The first part of the HTHR is caused by n-heptane, and second is caused by benzyl radicals and aromatics produced from toluene. Therefore the maximum pressure rise rate can be reduced, and NTL fuel is effective in achieving low combustion noise and high IMEP engine operation.