The Influence of Variable Fuel Properties in High-Pressure Diesel Injectors

High pressurization of Diesel fuel in modern common-rail injectors, in addition to its effect on spray atomization, can result to increase of fuel density and viscosity in comparison to atmospheric conditions; moreover, due to the sharp de-pressurization experienced by the fuel at the inlet of the injection holes significant gradients of the above properties are established. Consequently, the characteristics of cavitation taking place at the entrance to the injection holes are affected. The present study quantifies the role of these effects in automotive Diesel injectors operating at pressures in excess of 1500 bar through use of a cavitation CFD model. The flow solver is accordingly modified to account for such effects during the solution of the conservation equations. Two different injector designs have been considered, both based on the same sac-type nozzle body; one with sharp-inlet cylindrical holes and one with tapered holes with inlet rounding. Pressurization of the fuel affects mainly liquid viscosity, which can increase up to approximately 10 times in comparison to atmospheric conditions, while density differences do not exceed 10%. The results indicate that for the nozzle with sharp-inlet cylindrical holes, in which the flow is highly cavitating and for this reason can be referred to as ‘low efficiency’, the consideration of variable fuel properties effects due to pressurization is overshadowed by cavitation effects; moreover, the predicted discharge coefficient considering variable density and viscosity is comparable with the one obtained using constant fuel properties, having a 2% difference. In contrast to this, these effects become relatively more important for the ‘high efficiency’ nozzle equipped with tapered holes, which cavitates much less in comparison to the cylindrical one. For the tapered hole nozzle it has been found that when variable density and viscosity effects are considered the predicted average flowrate is approximately 4% lower than when constant properties are used.