Effect of cold temperatures on the in-nozzle cavitation of fuel injections

Abstract

The term “cold-start” refers to the transient engine startup phase, in which a low-temperature fuel is injected into a combustion chamber. During such early cranking loads, a number of discrepancies are introduced to the spray structure and the fuel-air mixture formation. These are driven primarily by variations to the phenomenology of phase change, which exhibits a behaviour that is markedly different compared to the fully warmed-up operation. This study examines the in-nozzle cavitation and near-nozzle jet formation at temperatures as low as −10°C, corresponding to the regulatory limit of cold-start. Large Eddy Simulations have been performed with an enlarged prototype geometry and a configuration with engine-realistic dimensions and features. The former investigation assesses different fuel compounds, including mid-distillate and high-volatility hydrocarbons, along with bio-derived isomers. The latter elaborates on geometric effects that are relevant for the modern generation of injectors. Both cases explore conditions corresponding to the operation of Direct-Injection Spark-Ignition engines, as reflected by characteristic flow and phase change parameters. The reduction of the injected fuels’ temperature has been noted for the alterations it introduces to the size and the intensity of the in-nozzle vapor formations. These discrepancies are accompanied by a shift to the transient entrainment of air from the discharge chamber, changes to the driving mechanism that causes a detachment of cloud structures and variations to the fuel jet’s angle and symmetry. The observations have been linked to the temperature- dependent fuel thermodynamic properties, which appear to impede the in-nozzle phase change at cold conditions. This is evident in the test cases with the simplified geometry, which span across different cavitation regimes: from non-vaporizing to incipient and supercavitation. The investigation with the engine-realistic geometry reaffirms those trends. A non-flashing early injection and two cold conditions were examined using iso-octane as the working fluid. This configuration operates on a hydraulic flip with vapor forming in regions where the flow encounters an adverse pressure gradient. An increased entrainment of gas from the downstream chamber has been observed, which intensifies at lower temperatures and detaches the plumes from the orifice walls. Hole-to-hole disparities are also noticeable, arising due to asymmetries in the injector geometry as well as due to the influence exerted by the upstream flow domain.