There is a steadily increasing concern that the products of combustion of fossil fuels is eroding our environment; causing problems with the earth's protective ozone layer and overall human health. These concerns are augmented by the fact that fuel consumption is steadily increasing. For these reasons, any improvement in fuel efficiency, while reducing pollutant emissions, is highly desirable. Homogeneous Charge Compression Ignition (HCCI) is an ideal engine for addressing these problems. HCCI engines have high thermal efficiencies, due to a high compression ratio (comparable to Diesel engines), low NOx and low particulate emissions due to a homogeneous mixture with a lean equivalence ratio (and thus low combustion temperature). Determining when combustion occurs in a HCCI engine can prove challenging given that the combustion process 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. In research, auto-ignition (Start of Combustion) is measured by pressure sensors that are too expensive to use on production engines. Ion sensors can be used accurately to predict the Start of Combustion for various fuels used in HCCI engines. Spark plugs converted to ion sensors have the potential of replacing the more costly and less robust pressure transducers. Currently our focus is in modeling chemical kinetics for various fuels and experimentally measuring ions to identify combustion events in a HCCI engine.