Fundamental Characteristics of an Air-Assisted Direct Injection Combustion System as Applied to 4-Stroke Automotive Gasoline Engines

Gasoline direct injection technology is rapidly evolving. At present, there does not exist a dominant combustion system, however many believe that the ultimate system will be spray or jet guided. These combustion systems rely on an ignitable mixture being promoted directly from the injection source to the spark plug gap location. Very small droplet sizes, coupled with low penetration rates and diffuse sprays, characteristic of air-assisted direct injection systems, are ideally suited to these spray guided combustion systems. The ability of the air assisted system to function with the spark plug directly in the injected spray without plug fouling problems is testament to the quality of fuel preparation enabled by the dual fluid injection process.

This paper presents the findings of some fundamental characterisation of Orbital's low pressure air-assisted, spray guided combustion system, including effects of injection pressure, injected gas composition, and in-cylinder charge motion. The results presented demonstrate the ability of the system to operate in highly stratified conditions at very lean A/F ratios with an injection pressure of only 6.5bar. As well, the direct injected air is identified as playing an important role at part load conditions by increasing combustion stability when using high EGR levels, enabling the simultaneous achievement of low fuel consumption and low raw engine emissions. The injected air is also instrumental in providing “soft” stratification gradients which result in excellent stability and high levels of robustness, both of which are important in ensuring easy transition from engine testcell to vehicle platforms.