A Fundamental Study on the Potential LSPI Promoting Mechanisms of Ca and Mg Based Oil Detergents

Abstract Abnormal combustion, in its various forms, has presented a challenge to the design and operation of spark-ignited internal combustion engines for well over a century. During the last decade the relative prevalence of low speed pre-ignition (LSPI) in highly boosted and downsized engines has led to significant research efforts directed at understanding its origins. Two key mechanisms; oil droplet release from the piston top land crevice region and the accumulation and release of in-cylinder deposits, have both been shown to trigger LSPI. With respect to the oil release mechanism, multiple authors have reported a link between LSPI propensity and the calcium treat rate as detergent in the oil additive pack. It is, similarly, widely reported that magnesium based detergent packs do not promote LSPI in the same way. However, no fundamental explanation as to why calcium is ‘bad’ and why magnesium is ‘good’ has yet been presented in the literature. This paper reports the preliminary results from a series of fundamental shock tube experiments exploring the reactions between calcium and magnesium oxides, and a range of gases in relation to their potential as a trigger for LSPI. Visible light emissions are shown from shock heated calcium oxide particles in a 100% carbon dioxide environment at high temperature, T > 1173 K. Unexpectedly, light emitting reactions are also shown for calcium oxide in a 100% nitrogen environment at temperatures T > 773 K, suggesting that there may be a previously unidentified LSPI trigger mechanism in which nitrogen plays a key role. No visible emissions were seen from magnesium oxide powders.