Knock Behavior of a Lean-Burn, H2 and CO Enhanced, SI Gasoline Engine Concept 2004-01-0975
Experiments were performed to identify the knock trends of lean hydrocarbon-air mixtures, and such mixtures enhanced with hydrogen (H2) and carbon monoxide (CO). These enhanced mixtures simulated 15% and 30% of the engine's gasoline being reformed in a plasmatron fuel reformer . Knock trends were determined by measuring the octane number (ON) of the primary reference fuel (mixture of isooctane and n-heptane) supplied to the engine that just produced audible knock.
Experimental results show that leaner operation does not decrease the knock tendency of an engine under conditions where a fixed output torque is maintained; rather it slightly increases the octane requirement. The knock tendency does decrease with lean operation when the intake pressure is held constant, but engine torque is then reduced. When H2 and CO are added to the mixture, the knock susceptibility is reduced, as illustrated by a decrease in the measured octane number of the primary reference fuel resulting in knock. Experiments conducted with the addition of H2 and CO separately show similar trends, but to a lesser degree; therefore, both H2 and CO act as octane enhancers when added to a hydrocarbon-air mixture. The extent to which H2 and CO improve the knock resistance of a mixture can be estimated by finding the bond-weighted octane numbers for these non-traditional blends of fuels.
To understand these results better, a reduced chemical kinetic model was also used to predict autoignition of the end-gas for various conditions and fuel-air mixtures. Predicted model trends of knock onset of primary reference fuels agree with experimental observations. A comprehensive isooctane chemistry mechanism was used to demonstrate that H2 and CO are effective in lengthening the ignition delay, thereby reducing knock tendency.