Relative Roles of Premixed and Diffusion Burning in Diesel Combustion

Relative roles of premixed and diffusion burning in diesel combustion have been examined in terms of characteristic times for ignition delay (τ_ig), combustion (τ_c) and fuel-air mixing (τ_m). Results indicate that the majority of the diesel combustion process is diffusion controlled, as in gas turbine combustion, since τ_c << τ_m over the entire range of operating conditions. During the ignition phase, some premixed burning can occur in the fringe of the fuel spray where τ_m < τ_ig; however, most of the fuel injected prior to ignition also burns in the diffusion mode, since τ_m > τ_ig in the majority of the fuel spray. The fraction of premixed burning which occurs during the ignition phase is increased by longer τ_ig, high rates of fuel injection, high air swirl and the use of multi-hole injectors, which increase the surface area of the spray.