High pressure injection of natural gas for diesel engine fueling

The high velocity unsteady methane jet emerging from a suddenly-opened conical poppet type nozzle is investigated, with the objective of characterising the penetration rate and the jet distribution under different injection conditions. The results are to be utilised in the development and optimization of a prototype injector for late-cycle injection of natural gas in a diesel engine. Transient underexpanded turbulent jets of methane were visualized utilising schlieren and shadowgraph photography. The methane injections were principally performed in ambient air, with lateral visualization of the jet. Axial visualization of the methane injected into a pressurized cylinder was also executed. Pressure ratios between 1.5 and 8 were utilised to generate the jets. The Reynolds number of the jets covered a range between 7x103 to 5.6x104. The steady-state turbulent conical sheet jet originating from the conical poppet nozzle is characterised using an integral approach in conjunction with published empirical results. A model of the transient conical sheet jet is developed, in which the transient jet is described as a quasi-steady jet feeding a moving vortex structure. The model is found to predict similar penetration rates as the ones observed experimentally for different conditions, except in the early moments of the injection where the proposed modelling does not describe adequately the initial jet behaviour. The penetration of the methane jet is found to be proportional to the square root of the time and initial velocity, and to the 1/4 power of the methane to air density ratio (taken at ambient conditions), the upstream to ambient pressure and the product of the poppet lift by the seat radius. The jet penetration is also dependent on the ratio of methane to air densities at standard temperature and pressure, the poppet angle and the injection duration. The conical sheet penetration is shown to be approximately half of that of round holes, given the same flow area. Observations of the jet revealed that the conical sheet jet has a distinct curvature towards he inside of the sheet, resulting in the jet collapsing under the nozzle for poppet angles greater than approximately 20o. When the injector is positioned near a top wall, the jet exhibits a bi stable behaviour, either collapsing under the poppet or clinging to the top wall. The immediate effect of the jet collapsing and clinging to the top wall is a reduction in mixing between the gas and the air and a slower penetration rate. Both these conditions are undesirable for an optimum combustion of the methane in a diesel environment. It is deduced that with the current conical poppet nozzle design the distribution of the gas within the chamber is inadequate and the penetration rate of the jet is insufficient. Conical sheet interruptions at the nozzle is investigated as a potential solution for the clinging and collapsing problems. It is found that interruptions can successfully prevent both phenomena. In addition, proper interruption arrangement generates jets of a different type that are propagating significantly faster than the conical sheet jet.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat