Experimental investigation of near-nozzle characteristics of gasoline sprays from pressure-swirl atomisers
This thesis presents an experimental investigation of the near-nozzle region of the fuel spray issuing from a pressure-swirl injector commonly found in a direct injection spark ignition engine. Specifically, the effects of the back pressure and impinging air flows were investigated. In addition to this the effect of fuel pressure was investigated. The literature survey reveals only limited published work on the near-nozzle characteristics of fuel sprays, and the requirement for more detailed knowledge of the area to aid in the general understanding of the topic and the development of mathematical models. Two separate experimental test rigs were designed and built to carry out the testing, firstly a static pressure chamber to identify the effects of back pressure on the fuel spray, and secondly a steady state flow rig, to investigate the effects of an impinging air flow. The steady state air flow rig was designed to simulate the air flow from the inlet valves of a direct injection spark ignition engine cylinder head. A novel void fraction technique was utilised to quantify the fuel spray break up process. It was found that there was a correlation between the void fraction and the fuel spray break-up length. The results from this analysis were compared to mathematical models currently available in the literature. It was found that the models do not compare well with the experimental results. A modified mathematical model was therefore proposed by the author, which can be easily integrated into existing computational codes. It was shown that impinging air flows do not affect the primary fuel spray break-up process, however they do affect the secondary break-up of the fuel spray from ligaments into droplets. Impinging air flows also affect the general fuel spray shape, and will determine the location of the fuel spray within the cylinder far downstream of the injector.