The objective of this work was to develop reference data for validation of kinetics mechanisms at engine-relevant conditions where limited data exist. Fuel substitution sweeps were run for six homogeneous-charge compression-ignition conditions. The sweeps were performed with constant combustion phasing and boundary conditions. Baseline mixtures of both methane/hydrogen and methane/propane were used and the required ratio change of the two baseline fuels to meet the operation constraint as a test fuel was added was used to define the test fuel’s reactivity.
Three in-cylinder temperature measurement methods were performed: a trapped mass state equation calculation, water absorption spectroscopy, and fast-response thermocouples. At intake valve close (IVC), the 95% confidence intervals for the three methods overlapped for most test conditions. Uncertainty in the final IVC measured temperatures ranged from 5-10 K.
The required simulation IVC temperatures in a two-zone model for five mechanisms were compared to the experimental IVC temperatures. Four of the five mechanisms were accurate within experimental uncertainty for the methane/hydrogen cases, but only one of those four mechanisms was also accurate for the methane/propane cases.
Two simulation methods, using the three most accurate mechanisms from the previous simulation, were used to evaluate mechanism performance regarding the reactivities of nine test fuels relative to the two sets of baseline fuels: 1) fixed fueling, variable IVC temperature and 2) fixed IVC temperature, variable fueling. Consistent results were seen with the three mechanisms’ ability to predict test fuel reactivities relative to methane/hydrogen; most test fuels’ reactivities were under-predicted. Varying results were seen with the three mechanisms’ ability to predict test fuel reactivities relative to methane/propane. One mechanism showed consistent results to the methane/hydrogen cases, but the remaining two mechanisms tended to over-predict test fuel reactivities relative to methane/propane, which was different from the under-predictions seen for the methane/hydrogen cases.
A simplified method to characterize relative fuel reactivities using ignition delay calculations was developed to generate maps of the relative fuel reactivities over a wide range of thermodynamic conditions. At high temperatures, all hydrocarbon test fuels had similar reactivities to propane, but at low temperatures there was a large variation in relative reactivities.