Theoretical Study on Spray Design for Small-Bore Diesel Engine (Second Report)

Generally, soot emissions increase in diesel engines with smaller bore sizes due to larger spray impingement on the cavity wall at a constant specific output power. The objective of this study is to clarify the constraints for engine/nozzle specifications and injection conditions to achieve the same combustion characteristics (such as heat release rate and emissions) in diesel engines with different bore sizes. The first report applied the geometrical similarity concept to two engines with different bore sizes and similar piston cavity shapes. The smaller engine emitted more smoke because air entrainment decreases due to the narrower spray angle. A new spray design method called spray characteristics similarity was proposed to suppress soot emissions. However, a smaller nozzle diameter and a larger number of nozzle holes are required to maintain the same spray characteristics (such as specific air-entrainment and penetration) when the bore size decreases. This second report proposes another approach that maintains the same nozzle diameter and number of nozzle holes. When the similarity ratio is defined as the ratio of the bore diameter between a targeted engine and the base engine, this new method (called the "geometric similarity of fuel spray" concept) can be expressed as functions of the spray penetration, spray angle, and excess air ratio. This method requires the fuel spray angle to be the same regardless of the engine size. To achieve this condition, a newly designed step shaped nozzle was adopted to increase the fuel spray angle compared to a conventional straight hole nozzle. In the case of the geometric similarity of fuel spray concept, soot emissions are the same as those generated by a longer nozzle, while maintaining the same spray angle as the conventional straight nozzle. Consequently, it was confirmed experimentally that theoretical solutions based on the geometric similarity of fuel spray concept can obtain almost constant soot emissions in different sized engines.