Autoignition Chemistry Studies on Primary Reference Fuels in a Motored Engine

Autoignition chemistry of n-heptane, iso-octane and an 87 octane blend, 87 PRF, was studied in a single-cylinder modified Wisconsin model AENL engine under motored conditions. Use of a fast-acting sampling valve and gas chromatographic analysis allowed measurement of in-cylinder gas composition during the ignition process. Crank angle resolved species evolution profiles were generated for all three fuels at a fixed inlet temperature of 376 K. For n-heptane, the measurements were made during a cyclically repeatable two stage ignition process up to the point of hot ignition (the second stage ignition). These n-heptane experiments were run at ø = 0.3 to avoid excessive pressure rise at hot ignition which might damage our engine. iso-Octane and 87 PRF were run at stoichiometric equivalence ratio which did not have a second stage ignition, and species were measured only during the first stage of ignition. In addition, a series of in-cylinder gas composition measurements were made for iso-octane at inlet temperatures ranging from 350 to 500 CAD at a fixed residence time (20 CAD ATDC) where the first stage of ignition ceased.

An oxidation mechanism has been assembled to describe the chemical processes responsible for the measured species and ignition behavior. Some reactions have been examined and discussed in detail based on the experimental data. The results of 87 PRF oxidation show that the oxidation paths for each fuel component are essentially the same as in the case of neat fuel, with the interaction between the two components primarily via the reactive radical pool.