Knock behavior of a lean-burn hydrogen-enhanced engine concept

Experiments to identify the knock trends of lean gasoline-air mixtures, and such mixtures enhanced with hydrogen (H 2) and carbon monoxide (CO), were performed on a single-cylinder research engine with boosting capability. The experimental method used to investigate knock trends consisted of determining the octane number (ON) of the primary reference fuel (mixture of isooctane and n-heptane) supplied to the engine that just produced audible knock. All tests were completed at 1500 rpm, MBT spark timing, with coolant temperature at fully warmed-up conditions and intake air temperature at 200 C. Various relative air-fuel ratio (lambda) sweeps were performed, while holding different parameters constant. First, testing with primary reference fuels investigated knock limits of lean operation; selected tests were then repeated with H 2 and CO-enhancement. These mixtures simulated 15% and 30% of the engine's gasoline being reformed in a plasmatron fuel reformer. 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 onset of knock does decrease with lean operation when the intake pressure is held constant, but engine torque is then reduced. When H 2 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 H 2-show similar trends, but to a lesser degree. Therefore, both H 2 and CO act as octane enhancers when added to a hydrocarbon-air mixture. The extent to which H 2 and CO improve the knock resistance of a mixture can be estimated by finding the bond-weighted octane number for the mixture of fuels. To substantiate these results, a chemical kinetic ignition model was used to predict autoigntion of the end-gas for various conditions and fuel-air mixtures. Predicted model trends of knock onset partially agree with experimental observations. A comprehensive isooctane chemistry mechanism was used to demonstrate that H 2 and CO-enhancement are effective in lengthening the ignition delay, and thereby reduce knock tendency.