A set of experiments was performed to investigate the effects of relative air-fuel ratio, inlet boost pressure, and compression ratio on engine knock behavior. Selected operating conditions were also examined with simulated hydrogen rich fuel reformate added to the gasoline-air intake mixture. For each operating condition knock limited spark advance was found for a range of octane numbers (ON) for two fuel types: primary reference fuels (PRFs), and toluene reference fuels (TRFs). A smaller set of experiments was also performed with unleaded test gasolines. A combustion phasing parameter based on the timing of 50% mass fraction burned, termed “combustion retard”, was used as it correlates well to engine performance.
The combustion retard required to just avoid knock increases with relative air-fuel ratio for PRFs and decreases with air-fuel ratio for TRFs. PRFs, which require about 5° CA of combustion retard per bar of net indicated mean effective pressure (NIMEP), need about three times as much combustion retard as TRFs when boosted to achieve the same NIMEP. Both fuel types require an average of about 3° CA of combustion retard per unit of increased compression ratio. The trends for gasoline are about halfway between PRF and TRF trends. An end-gas model that employs detailed chemical kinetics and experimental cylinder pressure data successfully approximated the response of PRFs and TRFs to compression ratio, air-fuel ratio and boost.
Adding gasoline reformate (a mixture of H2, CO, and N2) decreases the combustion retard required to avoid knock by about 2° CA per 3% fuel reformed fraction for PRFs. For TRFs with low alkane content reformate addition is less effective. Reforming up to 30% of the fuel entering an engine allows increased compression ratio or increased turbocharging without increasing combustion retard. A simplified analysis suggests that increasing compression ratio and downsizing the engine to maintain constant maximum brake torque would increase brake fuel efficiency by about 9%. Turbocharging and downsizing would increase efficiency by about 16%.