Spray Angle and Rail Pressure Study for Low NOx Diesel Combustion 2007-01-0122
The effects of varying nozzle spray angle and rail pressure on emissions and thermal efficiency each were explored using a 103-mm bore direct-injection single-cylinder diesel engine. Spray angles from 120° to 158° significantly changed the spray targeting within the 16:1 compression ratio reentrant-shaped piston bowl. At one part load operating condition injection timing was varied over a range of 15° to 30° btc to investigate pre-mixed compression ignition (PCI) combustion with 800 bar rail pressure while varying EGR to maintain a constant low NOx emission index of 0.4 g/kg. The observed trends are explained by the combined effects of spray angle and injection timing and, in particular, the calculated amount of liquid spray that misses the piston bowl is directly linked to the measured increases in HC, CO, and smoke emissions and a reduction in thermal efficiency. Rail pressure was varied from 800 to 1600 bar with a good performing narrow angle of 105° and the standard wide angle of 158°. The range of injection timing explored was expanded (0° to 30° btc) to include the timings more typically used for conventional diesel combustion with higher smoke and then retarded timings that can lead to low smoke while again maintaining a constant low NOx of 0.4 g/kg for all cases. The narrower angle spray (105°) flows within the piston bowl in a direction opposite to that of the wider angle tip and none of the spray ever misses the bowl for the range of injection timings explored. Increasing rail pressure improves atomization and vaporization of the spray as evidenced by significant reductions in smoke for either spray angle, particularly in the more conventional diesel injection timing range. However, for conditions where the spray misses the bowl with the wider spray angle (158°), increased rail pressure exacerbates the deterioration in emissions and thermal efficiency due to increased spray penetration outside of the bowl.