Chemistry of Autoignition
Combustion chemistry is driven by chain reactions of reactive radicals and the progress of combustion depends on the balance of chain-branching and chain-terminating reactions. Autoignition, the spontaneous ignition of a fuel-air mixture, happens when initially slow thermal reactions have a large enough chain-branching component to sustain and accelerate oxidation. The increasing radical concentration and the increase in reaction rate build onthemselves and eventually lead to a rapid explosive rise in radical concentration, oxidation rate, and temperature – ignition! These reactions typically release heat, increasing the temperature of the system, and at the same time their rate is also strongly dependent on temperature and pressure. These characteristics result in a complicated interplay of positive and negative feedback loops that determine when the ignition event will occur. The accurate prediction of autoignition times and their dependence on pressure,temperature and composition, is essential for advanced engine technologies,such as HCCI, where the ignition event is timed by chemical kinetics. In fact autoignition is very sensitive to details of chain branching and chain terminating in the initial reactions, and hence depends sensitively on the chemical structure of the fuel. Although our understanding of alkane autoignition is relatively good (but still incomplete!), oxygenates, typical components of biofuels, can exhibit very different and often unexpected behavior. Moreover, one critical class of intermediates, the hydroperoxyalkyl radicals (called QOOH radicals), which control low-temperature chain branching and hence play the most important role in autoignition, have never been observed directly by any means in any experiment. The CRF maintains a strong experimental and theoretical program aimed at analyzing and understanding fundamental autoignition reactions in rigorous detail.