Effect of Secondary Mixing on Starting Characteristics of a Cold Diesel Engine

Abstract

Results of experiments performed in a constant-volume bomb are presented. The conditions in the bomb were typical of starting regimes of diesel engines. Cooled (down to 286 K) or noncooled inserts were placed in the bomb volume on the way of the fuel–air jet. These inserts were plates simulating a sector of the combustor wall. Leaf deflectors of the jet were mounted on the plates to obtain the effect of secondary mixing. In the case of an elevated pressure in the medium (2.5 MPa and higher), the presence of the inserts, including those equipped by deflectors, independent of their thermal state, affects ignition only at extremely low temperatures of the medium, at which the primary ignition of fuel vapors and products of its thermal decomposition occurs in an immediate vicinity of the insert. If the initial pressure in the medium is rather low (0.9 MPa), the influence of the insert is significant: a noncooled surface reduces both the induction period (by a factor of 1.5 as compared to injection into a free volume) and the limiting temperature of ignition. If the insert surface is cooled, however, the ignition delay increases by 20%, and the limiting temperature of ignition increases by 100 K. Under these conditions, a leaf deflector of the jet mounted on the wall-simulator surface significantly improves the ignition characteristics of the mixture: the ignition delay and the limiting temperature decrease down to values that do not exceed those upon injection into a free volume. It is assumed that the secondary injection of the mixture from the edge of the leaf deflector into the free space of the working volume leads to the formation of a closed circular vortex containing both the liquid phase (atomized fuel drops) and the gaseous phase (fuel vapors, products of its partial thermal decomposition, and air). Key words: diesel engine, mixing.