Search Results

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.

1 Recently, demand for small-bore compact vehicle engines has been increasing from the standpoint of further reducing CO2 emissions. The generalization and formulation of combustion processes, including those related to emissions formation, based on a certain similarity of physical phenomena regardless of engine size, would be extremely beneficial for the unification of development processes for various sizes of engines. The objective of this study is to clarify what constraints are necessary 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.

A three-dimensional numerical model has been developed to predict spray formation process of swirl or slit type injectors which are currently used in direct-injection gasoline engines. The Discrete Droplet Model (DDM) is totally enhanced: a new droplet deformation model is developed, which is theoretically introduced with a spheroidal shape assumption. The droplet drag model and droplet break-up model via Kelvin-Helmholtz and Rayleigh-Taylor instabilities are modified taken into account with the deformation. The break-up model parameters are dynamically changed according to a droplet Weber number. The model functions are developed using single droplet breakup measurement data. A liquid sheet injection and breakup models are incorporated into the DDM. A new parcel radius model is also introduced to get rid of the grid dependence of the droplet collision-coalescence model.