A novel reactivity index for SI engine fuels by separated weak flames in a micro flow reactor with a controlled temperature profile

The reactivities of n-heptane, n-pentane, PRF50, PRF80, iso-pentane and iso-octane were investigated by separated weak flames in a micro flow reactor with a controlled temperature profile at a pressure of 500 kPa. At this elevated pressure condition, low-temperature reactions in the cool flame and blue flame were enhanced. High temperature reactions in the hot flame became weaker and independent of the base fuel as they mainly consist of CO and hydrogen–oxygen reactions. By conducting simulations, it was found that the ratio of heat that is released in the blue flame to the total released heat is a measure that well describes the reactivity of a given fuel. This ratio was named the Blue flame Heat Contribution Index (Blue flame-HCI). It is primarily governed by the molecular structure that controls the low-temperature isomerization reaction RO₂ ⇌ QOOH. This step is important, as the QOOH, in turn, reacts with O₂ to form OOQOOH, which decomposes to keto–hydroperoxide and OH, where the keto–hydroperoxide immediately dissociates and releases another OH. This formation of two additional OH radicals controls the chain branching at low temperatures. As straight-chain alkanes have more pathways for the isomerization reaction to form QOOH, n-heptane and n-pentane showed higher reactivity than the branched iso-pentane and iso-octane. In the experiments, formaldehyde (CH₂O) concentrations were measured in the cool flame to evaluate the fuel reactivity using the newly introduced FormAldehyde Index (FAI). The FAI and the Blue flame-HCI were in good agreement with each other and described well the fuel reactivity. Furthermore, the FAI showed a linear correlation with the critical compression ratio as the reactivity of a given fuel is governed by low-temperature chain branching that is initiated by the isomerization reaction.