Characterization of ion production using gasoline, ethanol, and N-heptane in a homogeneous charge compression ignition (HCCI) engine

Determining when the combustion event occurs in a Homogeneous Charge Compression Ignition (HCCI) engine can prove challenging given that the combustion event is governed by temperature sensitive chemical-kinetics which results in auto-ignition of the fuel/air mixture. Auto-ignition variability can affect emissions and efficiency. Thus, measuring and controlling the combustion event is of the utmost importance. Ion sensors can be used reasonably for detecting the combustion event for various fuels used in HCCI engines over a wide range of operating conditions. Experiments are conducted to measure ion signals produced from the combustion of gasoline, ethanol, and n-heptane in a 4-cylinder HCCI engine with varying equivalence ratio, intake pressure, intake temperature, bias voltage, and dimensions of the ion sensor. It is found that the ion signal is reduced under several situations: with an increase in intake pressure, reducing equivalence ratio (under lean conditions), reducing intake temperatures, decreasing the bias voltage source, and decreasing the surface area of the positive electrode (positively bias voltage source). Results from a Well-Mixed-Reactor (WMR) numerical model suggest that gasoline and ethanol produces more ions than n-heptane during combustion under the similar operating conditions (intake temperature is the only variation due to the different octane ratings for each fuel); these predictions agree with the experiments. Ion signals from ethanol and gasoline are comparable and have the potential for use in gasoline-ethanol fuel blends (E85). The ability to measure ions in n-heptane suggests ions may be measurable in diesel fueled HCCI as well.