Numerical Investigations on Strong Knocking Combustion under Advanced Compression Ignition Conditions

Homogeneous charge compression ignition (HCCI) combined with high compression ratio is an effective way to improve engines’ thermal efficiency. However, the severe thermodynamic conditions at high load may induce knocking combustion thus damage the engine body. In this study, advanced compression ignition knocking characteristics were parametrically investigated through RCM experiments and simulation analysis. First, the knocking characteristics were optically investigated. The experimental results show that there even exists detonation when the knock occurs thus the combustion chamber is damaged. Considering both safety and costs, the effects of different initial conditions were numerically investigated and the results show that knocking characteristics is more related to initial pressure other than initial temperature. The initial pressure has a great influence on peak pressure and knock intensity while the initial temperature on knock onset. Further analysis shows that knock intensity is mainly related to the energy density of the in-cylinder mixture and energy density is higher under higher pressure conditions. Then the effects of different cylinder wall temperature on the local autoignition thus knocking characteristics were further discussed. The results show that the increase of wall temperature can reduce the peak pressure and knock intensity, which is ascribed to that autoignition of the end gas is inhibited under high wall temperature conditions. Thus, the type of engine knock is translated from an SI-type knock to an HCCI-type knock. And HCCI-type knock is milder because the overpressure is over the whole combustion chamber. Finally, knocking combustion under different turbulence intensity were qualitatively evaluated by the function of MAPPING. It shows the autoignition is a little delayed under high turbulence intensity due to the enhanced mass and heat transfer. However, the knocking characteristics are nearly the same under the three turbulence conditions in terms of combustion phase and knock intensity, which proved the conclusion that knock intensity mainly depends on energy density.