Experimental and numerical investigation of effect of fuel on ion sensor signal to determine combustion timing in homogeneous charge compression ignition engines

Homogeneous charge compression ignition (HCCI) engines offer promise owing to low emissions and high efficiency. However, the control of the combustion process in HCCI engines, specifically the control of the start of combustion (SOC) or ignition timing, remains a challenge. Piezoelectric pressure transducers are used in research engines for determination of the start of combustion; however, these pressure transducers are too expensive and fragile for applications in commercial engines. Recent work by the authors as well as other investigators has shown the potential of inexpensive ion sensors in HCCI engines fuelled with propane or gasoline. However, the working range of ion sensors is limited in HCCI engines owing, in large part, to the fact that the peak cycle temperature in HCCI combustion is quite low (∼ 1700-1900 K). With the guidance of detailed chemical kinetic modelling it is shown that fuels or additives producing a higher concentration of CH radicals will probably produce higher ion concentrations. Acetylene (HC≡CH) is known to produce large concentrations of CH radicals. Hence, various mixtures of propane and acetylene are numerically and experimentally studied. The ion concentration substantially increases with the addition of a small amount of acetylene. This research is an effort to understand the ion generation mechanism in HCCI engines with a view towards improving the ion signal.