Study on Knocking Intensity under In-Cylinder Flow Field in SI Engines Using a Rapid Compression Machine

Knocking intensity under the in-cylinder flow field was investigated by using a rapid compression machine (RCM). The nitrogen diluted and non-diluted fuel-air mixtures were employed for the examination of the combustion characteristics under the in-cylinder flow field. The behaviors of flame propagation and the spontaneous ignition in end gas were observed. The analyses of the in-cylinder flow field and the dependency of the knocking intensity with considering the volume fraction for flame propagation and the heat release rate of the spontaneous ignition in end gas were carried out. As a result, the flame propagation velocity increased with heightening the turbulent intensity. The change of the flame propagation velocity provided the change of the volume fraction for flame propagation. The knocking intensity depended on the volume fraction for flame propagation and it reached a peak at about 0.6 in the volume fraction, when the heat release rate due to the spontaneous ignition was high enough. This agreed with the numerical prediction qualitatively. The combustion due to the spontaneous ignition in end gas was prolonged by the broader temperature variation by turbulence when the spontaneous ignition in end gas was delayed or turbulence was strong. In these cases, the knocking intensity was lowered with the prolongation of the combustion in end gas. It was to be expected that the dependency on the volume fraction for flame propagation remained the same even when the heat release rate due to the spontaneous ignition was lowered, by the numerical simulation.