Chemical Kinetic Analysis with Two-Zone Model on Spark Knock Suppression Effects with Hydrogen Addition at Low and High Engine Speeds

Spark knock suppression with hydrogen addition was investigated at two engine speeds (2000 rpm and 4800 rpm). The experimental results showed that the spark knock is strongly suppressed with increasing hydrogen fraction at 2000 rpm while the effect is much smaller at 4800 rpm. To explain these results, chemical kinetic analyses with a two-zone combustion model were performed. The calculated results showed that the heat release in the end gas zone rises in two stages with a remarkable appearance of low temperature oxidation (LTO) at 2000 rpm, while a single stage heat release without apparent LTO process is presented at 4800 rpm due to the shorter residence time in the low temperature region. The mechanism of the spark knock suppression with hydrogen addition can be explained by inhibition of the LTO reactions and H₂O₂ loop reactions by the OH radical consuming reaction with hydrogen, leading to a reduction in the heat release from hydrocarbon fuel at the initial stage of the ignition process. However, the OH radical is simultaneously produced from H radical with hydrogen addition at the later stage of the ignition process. The reduction in the heat release with hydrogen addition can only be obtained at the initial stage of the ignition process, and the differences in spark knock suppression with hydrogen addition under low and high engine speeds are due to absence or appearance of LTO at the initial stage of the ignition process.