Ultra-High Pressure Spray for Gasoline Direct Injection Engines

Doktorsavhandling, 2022

There is an increasingly urgent need for vehicle manufacturers to reduce emissions from passenger vehicles to protect the environment and comply with increasingly stringent emissions standards such as Euro 6d, which came into force in January 2021, and Euro 7, which will probably come into force around 2025. Current emissions standards limit passenger vehicles' particulate number emissions to at most 6*10^{11} [#/km], making it necessary to find ways of reducing the engine-out particulate emissions of gasoline direct injection engines to meet this requirement. A promising way of doing this is to use fuel injection pressures above 200 bar, which was the most commonly used injection pressure until recently. This thesis presents a comprehensive experimental evaluation of the potential for reducing particulate number emissions from GDI engines by using high fuel injection pressures.

Basic spray characteristics such as spray tip penetration, droplet size, and spray-induced air motion were investigated in a constant-volume spray chamber. Spray tip penetration was determined by post-processing images captured with a high speed video camera, while droplet size was measured by Phase Doppler Interferometry and the motion of air around the spray was captured by means of Particle Image Velocimetry using tracer particles and a CCD camera. In addition, the behavior of combusting sprays inside a cylinder was studied using a single cylinder optical engine with a high speed video camera, and the effects of the fuel injection pressure on combustion characteristics and emissions were investigated using a single cylinder metal engine.

Increasing the injection pressure increased the spray velocity and therefore increased spray tip penetration. However, raising the injection pressure from 200 bar to 1500 bar also reduced the droplet size by over 50% because high injection pressures enhance droplet atomization. The high spray velocities and small droplet diameters seen with an injection pressure of 1500 bar increased air entrainment into the spray: the mass of air entrained at 1500 bar injection pressure was 40% higher than at 200 bar. In addition, air entrainment occurred in a shorter time frame at higher injection pressures, which improved fuel-air mixing in the cylinder. Engine tests revealed that the use of high injection pressures reduced PN emissions by up to 50% at standard SOI timings, and by a factor of 1000 when using advanced SOI timings under hot-steady conditions. Interestingly, the most significant effects on spray characteristics were seen when raising the injection pressure from 200 bar to 600 bar, and only relatively minor changes in spray behavior were seen when raising the injection pressure further. However, beneficial effects on emissions continued to be observed even when increasing the injection pressure to above 1000 bar.