Optimization of the piston geometry of a diesel engine using a two-spray-angle nozzle

Abstract

The objective of this research was to optimize a light-duty diesel engine in order to minimize the soot, nitrogen oxide (NO _x ), carbon monoxide (CO), and unburned hydrocarbon emissions, and the peak pressure rise rate while minimizing the gross indicated specific fuel consumption (GISFC) in a low-oxygen low-temperature environment. The variables considered were the injection timings, the fractional amount of fuel per injection, the swirl, and the piston geometry. The KIVA-CHEMKIN code (a multi-dimensional computational fluid dynamics program with detailed chemistry) was used and was coupled to a multi-objective genetic algorithm, together with an automated grid generator.

A unique two-spray-angle nozzle (2SAN) was explored. This nozzle allows both the lower and the upper portions of the bowl to be targeted simultaneously. With the 2SAN, stepped-bowl piston, and split-injection strategy, the NO _x and soot emissions and the GISFC were further reduced in comparison with well-performing cases that used a single-hole nozzle (SHN) with two injections. The soot and NO _x emissions were reduced by 67 per cent and 63 per cent respectively while the fuel consumption was improved by nearly 4 per cent. The second-injection timing was also varied with the selected case and compared with the well-performing SHN cases. This showed that the GISFC did not depend on the second-SOI timing as much as in the SHN cases. This leads to greater flexibility as to when the second SOI can occur. The 2SAN case performed particularly well with a highly diluted intake charge with an early second injection. Soot was further reduced while maintaining the GISFC. The last part of the study explored the benefits of the 2SAN at midload conditions (10.5 bar) at both 2000 r/min and 4000 r/min. The 2SAN reduced CO emissions by 20 per cent and 50 per cent for the low-speed and high-speed cases respectively. This research shows the importance of the bowl geometry, spray targeting, injection timing, split fuel amounts, and swirl on the emissions and fuel efficiency in direct-injection diesel engines.