An experimental and numerical study of natural gas jets for direct injection internal combustion engines

Natural gas predominantly contains methane which is the cleanest burning hydrocarbon. It can be considered as the most promising alternative fuel that has the potential to replace gasoline and diesel. This is mainly due to wider availability, lower fuel price, favourable fuel properties and cleaner burning characteristics. Currently in all spark ignited CNG engines fuel is port injected that results in lower peak torque due to reduced volumetric efficiency. Direct injection can easily overcome this drawback and can match the performance levels of modern gasoline direct injection engines while emitting far less harmful pollutants. Gaseous direct injection produces highly diffusive under-expanded turbulent gaseous jets. In order to optimise mixture preparation and maximise combustion performance, the flow characteristics of gaseous jets need to be further understood and modelled. In this work, the fundamentals of gaseous flow from an outward opening direct injector is studied. Schlieren imaging is used at high spatial and temporal resolution to characterise the far-field jet growth. The nozzle in a typical outward opening direct injector has been found to produce a hollow conical jet at the beginning of injection which later collapses depending on the nozzle geometry and pressure ratio. The experimental data are used to validate a CFD model. A numerical investigation is carried out to predict the jet growth for different nozzle geometries and their effect on the jet characteristics such as mass flow rate, penetration and mixing is discussed.