Auto-Ignition Characteristics of Hydrocarbons and Development of HCCI Fuel Index

It is known that the regular gasoline and primary reference fuel (PRF), that have the same research octane number, show the different HCCI engine performance, because of the different phasing and heating value of low temperature heat release. This means that the research octane number is not an “all-round” auto-ignition index, and another index must be developed to evaluate the HCCI combustion characteristics. In this paper, eleven pure hydrocarbon components were blended into twenty three different kinds of model fuels (surrogate fuels), labeled BASE, MC01-MC11 and K01-K11, and the HCCI engine tests were performed under five different intake air temperature conditions to change the auto-ignition characteristic of each hydrocarbon component. As HCCI combustion can be described as a lean and slow gasoline knocking phenomenon, an analysis of HCCI combustion data gives us much more important knowledge of gasoline knocking phenomenon. The octane number calculation method from HCCI heat release data was developed, and the octane numbers of eleven pure hydrocarbons, that composed the model fuels (surrogate fuels), were calculated. The ignitability trends of hydrocarbons were different depending on the hydrocarbon types. The physical and chemical differences of research octane number (RON) and motor octane number (MON) were also investigated. From these research data, it was obvious that the fuel parameters that related to the HCCI engine performance were LTHR, LTHR inhibitor effect and fuel base potential, and new HCCI fuel auto-ignition indices that could be widely applicable were developed.