The effect of ignition intensity and in-cylinder pressure on the knock intensity and detonation formation in internal combustion engines

Engine downsizing with a higher compression ratio and a higher boosting pressure improves the thermal efficiency of engines. However, with further downsizing of engines, knock intensity is aggravated until super knock occurs which is accompanied by a detonation wave that potentially can destroy engines rapidly. Therefore, it is essential to reveal the parameters that influence the knock intensity and the formation of a super knock. Selfdesigned detonation bomb experiments and corresponding numerical simulations were conducted to explore the end-gas combustion mode as well as the knock intensity problem. The experiment was conducted at three typical in-cylinder pressures for representing different mixture energy densities, which classify the knock phenomenon into subcritical (lower pressure), critical (medium pressure) and supercritical (higher pressure) conditions. Spark ignitions with low and high intensity realized by adjusting the spark ignition energy were given in each condition, respectively. The in-cylinder combustion process and pressure oscillation process were monitored by the synchronous acquisition of three pressure sensors that were installed in different positions of the chamber. It is found that irrespective of the low or high ignition intensity, the super knock as well as the detonation would not occur at the lower pressure, and only mild knock occurs. At the medium pressure, the high ignition intensity would result in a detonation wave, while the low ignition intensity would not. At the higher pressure, despite the intensity of ignition, a detonation wave as well as the super knock would always occur. Therefore, the ignition intensity and the in-cylinder pressure are two essential factors that affect the knock intensity of engines.