Increasing signal-to-noise ratio of spark-plug ion sensors through addition of salt-based fuel additives

Sensing ion production during the combustion of hydrocarbons is one way of detecting combustion time in homogeneous charge compression ignition (HCCI) engines. This process, however, is particularly difficult when low fuel concentrations typical of HCCI are used. For low equivalence ratios, distinguishing ion signal peaks from ambient noise is difficult, making ion sensors an unreliable means of performing combustion timing control. One strategy of increasing the ion signal magnitude is by including fuel additives, such as salt acetates, which produce greater numbers of ions during combustion. This paper discusses experimental research that quantifies the effects of adding different concentrations of salt acetates in fuels. Specifically for each concentration, the research examines the salt additive's effects on the ion signal, and the changes in intake conditions required to maintain fixed combustion times. Pure ethanol (EtOH) fuel is used as a starting point, and different amounts of potassium acetate (KOAc) are added. The goal is identification of the minimum KOAc concentration required under low equivalence ratios to produce ion signal magnitudes that are clearly distinguishable from background noise.