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High octane fuel (e.g., E85) effectively suppresses knock, but the octane ratings of such fuels are much above what is required under normal driving conditions. It is important, therefore, to understand the octane requirement of the engine itself over its full range of operation and apply that knowledge to practical driving cycles to understand fuel octane utilization, especially of a turbocharged engine. By carefully defining knock limits, the octane requirement of a 2-liter turbocharged spark ignition engine was experimentally quantified over a wide range of loads and speeds using PRF blends and gasoline-ethanol blends. Utilizing this knowledge and engine-in-vehicle simulations, the octane requirements of various driving cycles were calculated for a passenger car and a medium duty truck model.

A computer simulation of the four-stroke spark-ignition engine cycle has been developed for studies of the effects of variations in engine design and operating parameters on engine performance, efficiency and NO emissions. The simulation computes the flows into and out of the engine, calculates the changes in thermodynamic properties and composition of the unburned and burned gas mixtures within the cylinder through the engine cycle due to work, heat and mass transfers, and follows the kinetics of NO formation and decomposition in the burned gas. The combustion process is specified as an input to the program through use of a normalized rate of mass burning profile. From this information, the simulation computes engine power, fuel consumption and NO emissions. Predictions made with the simulation have been compared with data from a single-cylinder CFR engine over a range of equivalence ratios, spark-timings and compression ratios.