The Influence of PRF and Commercial Fuels with High Octane Number on the Auto-ignition Timing of an Engine Operated in HCCI Combustion Mode with Negative Valve Overlap
Journal article, 2004
A single-cylinder engine was operated in HCCI combustion mode with different kinds of commercial fuels. The HCCI combustion was generated by creating a negative valve overlap (early exhaust valve closing combined with late intake valve opening) thus trapping a large amount of residuals (~ 55%). Fifteen different fuels with high octane numbers were tested six of which were primary reference fuels (PRF's) and nine were commercial fuels or reference fuels. The engine was operated at constant operational parameters (speed/load, valve timing and equivalence ratio, intake air temperature, compression ratio, etc.) changing only the fuel type while the engine was running.
Changing the fuel affected the auto-ignition timing, represented by the 50% mass fraction burned location (CA50). However these changes were not consistent with the classical RON and MON numbers, which are measures of the knock resistance of the fuel. Indeed, no correlation was found between CA50 and the RON or MON numbers. However, when only the PRF's were considered, a correlation was found between the auto-ignition timing and the RON number. Although a substantial difference in auto-ignition timing between PRF 70 ON and PRF 98 ON was expected, the difference was only 1.5 CAD. Furthermore, the differences in auto-ignition timing between all fuels spanned only 4.5 degrees.
It was found that the reason for the different behavior of the fuels during these measurements was the method of generating the HCCI combustion mode. Retaining residuals with a negative valve overlap makes the combustion mode less sensitive to different fuel qualities. This is because chemical reactions during the negative valve overlap affect the auto-ignition timing proportionally to the amount of unburned hydrocarbons remaining from the main combustion.