Relative Roles of Premixed and Diffusion Burning in Diesel Combustion

Abstract

<div class="htmlview paragraph">Relative roles of premixed and diffusion burning in diesel combustion have been examined in terms of characteristic times for ignition delay (τ<sub>ig</sub>), combustion (τ<sub>c</sub>) and fuel-air mixing (τ<sub>m</sub>). Results indicate that the majority of the diesel combustion process is diffusion controlled, as in gas turbine combustion, since τ<sub>c</sub> << τ<sub>m</sub> 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<sub>m</sub> < τ<sub>ig</sub>; however, most of the fuel injected prior to ignition also burns in the diffusion mode, since τ<sub>m</sub> > τ<sub>ig</sub> in the majority of the fuel spray. The fraction of premixed burning which occurs during the ignition phase is increased by longer τ<sub>ig</sub>, high rates of fuel injection, high air swirl and the use of multi-hole injectors, which increase the surface area of the spray.</div>