Understanding engine knock
The problem of engine knock has been around since the inception of internal combustion engines. The premature ignition of the air-fuel mixture causes unnecessary engine wear, reduces power output and increases emissions of harmful air pollutants. The MINKNOCK project sought to address the lack of knowledge surrounding engine knock. It is a complex issue involving engine geometry, fuel characteristics, flame propagation to name but a few factors. Mechanical engineers with the Instituto Superior Tecnico in Portugal constructed an experimental set-up complete with Particle Image Velocimetry (PIV) equipment to closely monitor the conditions inside the combustion chamber. Data collection and analysis focused on the flame-wall interaction for three types of flames: hemispherical, symmetrical and non-symmetrical. New post processing software enabled the estimation of flame velocity (Sd) and the flame stretch factor (K). The MINKNOCK experiments showed that, contrary to popular belief, stagnation points do exist in the flow field inside the combustion chamber. The positions of the stagnation points, where regions of unburned fuel exist, were determined for each type of flame geometry. In addition, a high correlation between flame velocity and stretch factor was observed. Finally, and most importantly, the research revealed that flames are remarkably robust, able to survive large positive and negative stretching without being extinguished. The MINKNOCK consortium will use this new knowledge to improve engine design with the aim of reducing engine knock, helping Europe's legendary automobile industry remain globally competitive.
