Increasing the signal-to-noise ratio of sparkplug ion sensors through the addition of a potassium acetate fuel additive

Sensing ion production is one way of detecting the start of combustion in homogeneous charge compression ignition (HCCI) engines. However at low equivalence ratios, the ion concentration becomes too low to detect. This paper discusses experimental research that explores the potential of enhancing ion signals at low equivalence ratios by adding small amounts of potassium acetate (KOAc) to ethanol fuel (EtOH). We use KOAc as a surrogate for a large number of metal acetates that could be added to fuel. For each concentration, the research examines the additive's effects on the ion signal and any side effects from this additive. Pure ethanol fuel is used as a starting point, and different amounts of KOAc are added. Concentrations of 0, 180, 360, and 720 mg/L of KOAc-in-EtOH are used. The experimental results reveal significant increases in ion signals with addition of just 180 mg/L of KOAc at low equivalence ratios, however the benefits of the additive level off with higher concentrations. The research also reveals a side effect, where increasing KOAc concentrations cause a reduction in the heat release rates and therefore a lower engine power output. This side effect of lower heat release rates with KOAc addition may be beneficial as the equivalence ratio (and thus the power output) in HCCI is often limited by excessive rates of heat release.