Simulation of electrosprays in model direct-injection spark-ignition engine in-cylinder flows

Abstract

The calculated effects on injected charged sprays of dielectric and conductive in-cylinder wall materials are presented for a half-cycle of an axisymmetric flow model direct-injection spark-ignition (DISI) engine. A plain orifice electrostatic atomizer, previously used in experiments for application in fuel burners, is embodied into the EPISO code and this is assumed to pump a charged spray while working at moderate pressures of 5 MPa and fuel deliveries of 5 cm³/s, the maximum rates currently reported in electrostatic atomization of hydrocarbons. The transition mode operation of DISI engines is selected for the study and this consists of multiple pulses of 5 mg each, occurring at 80, 150, and 300° crank angle with the engine running at 3000 r/min. In the case of the third pulse, which impinges on the piston surface, the wall impaction model of Park and Watkins is used particularly when an electric potential of 1 kV is applied on this surface for it helps to reduce excessive fuel film build-up. Particles impinging on the cylinder roof and liner are treated with the stick impaction model of Naber and Reitz. A simple axisymmetric engine geometry of flat piston and cylinder heads is considered and computations of an uncharged spray are taken as a reference case. It is found in the study, firstly, that charge improves mixture preparation when dielectric in-cylinder surfaces are used, secondly, that the need for charge drainage in metallic surfaces produces poor spray characteristics in comparison with an ideal charged spray with boundary electric fields (this is shown by the large impingement of drop parcels relative to an ordinary spray as well as to a charged spray with electric boundary fields) and, thirdly, that charge reduces the fuel film thickness on the piston surface during late injection.