Pressure Oscillations Generated by Knocking Combustion of C2H2 in a Constant Volume Vessel

Abstract

Abstract. Onset of super-knock in highly intensified spark ignition engines poses a great threat to the reliability. In this context, An approaching attempt was exerted on the initial condition of C 2 H 2 -O 2 -N 2 mixtures ignited within a constant volume vessel, in order to determine the critical condition of knock and response to oxygen ratio, acetylene ratio and mixture density. Results show that with a low acetylene content, sole elevation of oxygen doesn't necessarily lead to knock onset. On the other hand, with oxygen content fixed to a sufficient value, elevation of acetylene content results in louder combustion noise and when it approaches the stoichiometry ratio, auto-ignition happens in the unburnt far end and strong oscillation appears on the combustion pressure curve. Surprisingly, elevation of mixture density has no significant effect on knock propensity. IntroductionDownsizing is an effective strategy for gasoline engines to reduce fuel consumption and CO 2 emission. In the approach to downsizing, turbocharge and direct injection are commonly reckoned to be the most important techniques. However, application of them exerts a greater knock risk on the engines and moreover a new abnormal combustion phenomenon, namely super knock, which is even more devastating, stochastically occurs. Super knock usually happens before the spark ignition, which drives researchers to relate it to pre-ignition. Therefore, endeavors were made to dig the cause of pre-ignition [1][2][3][4]. On the other hand, although preignition is reckoned to be the indicator, it actually doesn't necessarily result in super knock. Regarding this, the transformation of deflagration to detonation is speculated by many researchers to be the key point.In this context, acetylene was ignited within a constant volume vessel for knock research due to its high knock tendency and gaseous nature convenient for mixture preparation. An approaching attempt was exerted on the initial condition of C 2 H 2 -O 2 -N 2 mixtures in order to determine the critical condition of knock and response to oxygen ratio, acetylene ratio and mixture density. During the experiment, pressure profile and combustion images were simultaneously recorded to observe pressure waves generated in the vessel.