This study investigates the octane requirements of a
hybrid flame
propagation and controlled autoignition mode referred to as mixed-mode
combustion (MMC), which allows for strong control over combustion
parameters via a spark-initiated deflagration phase. Due to the throughput
limitations associated with both experiments and 3-D computational
fluid dynamics calculations, a hybrid 0-D and 1-D modeling methodology
was developed, supported by experimental validation data. This modeling
approach relied on 1-D, two-zone engine simulations to predict bulk
in-cylinder thermodynamic conditions over a range of engine speeds,
compression ratios, intake pressures, trapped residual levels, fueling
rates, and spark timings. Those predictions were then transferred
to a 0-D chemical kinetic model, which was used to evaluate the autoignition
behavior of fuels when subjected to temperature–pressure trajectories
of interest. Finally, the predicted autoignition phasings were screened
relative to the progress of the modeled deflagration-based combustion
in order to determine if an operating condition was feasible or infeasible
due to knock or stability limits. The combined modeling and experimental
results reveal that MMC has an octane requirement similar to modern
stoichiometric spark-ignition engines in that fuels with high research
octane number (RON) and high octane sensitivity (S) enable higher loads. Experimental trends with varying RON and S were well predicted by the model for 1000 and 1400 rpm,
confirming its utility in identifying the compatibility of a fuel’s
autoignition behavior with an engine configuration and operating strategy.
However, the model was not effective in predicting (nor designed to
predict) operability limits due to cycle-to-cycle variations, which
experimentally inhibited operation of some fuels at 2000 rpm. Putting
the operable limits and efficiency from MMC in the context of a state-of-the-art
engine, the MMC showed superior efficiencies over the range investigated,
demonstrating the potential to further improve fuel economy.