Improvements in Early Flame Development in a Spark Ignition Engine Brought About by “Spark-Aider“ Fuel Additives

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Abstract
The effects of certain fuel additives—alkali and alkaline earth metal compounds collectively known as “spark-aiders”—on early flame development in a spark ignition engine have been investigated using a laser based technique for flame speed measurement. Iso-octane and unleaded gasoline were the fuels used and two different engines were considered. The results for a calcium and two potassium additives are presented and demonstrate that “spark-aiders” increase early flame development rates and reduce cyclic variation. The leaner the mixture, the more marked is this improvement. As the additive concentration is increased, the maximum improvement in combustion characteristics brought about by the additive increases up to a limiting value and the rate at which this improvement occurs also increases. There is a minimum additive concentration below which no effect can be observed. The “spark-aider” effect, once fully established, lingers on, though at a progressively decreasing level, when the engine is switched back to additive-free fuel. Upon coating the electrodes directly with different potassium and calcium compounds, the average spark duration increases but there is no significant change in the peak current. The early flame speed also increases but the extent and the pattern of this increase are variable, unrepeatable and not well correlated with increases in spark duration. There is also no direct correlation between improvements in early flame development and the presence of potassium in the gas phase at that time, as indicated by its atomic emission. It appears that “spark-aiders” act by causing deposits of low work function materials to be formed on the spark electrodes. These in turn reduce the early flame development time and its variability most probably by increasing the proportion of the spark energy transferred to the gas. Thus “spark-aiders” reduce cyclic variations in combustion and hence engine operation, at source, without requiring any changes to the ignition system hardware.

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