Knock Behavior of a Lean-Burn, H2 and CO Enhanced, SI Gasoline Engine Concept

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 [1]. 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.