Étude de l'effet d'injection double sur le comportement d'un système d'injection directe de diesel à rampe commune

Abstract

In order to answer the climate change problem, different technical solutions aimed at reducing the carbon emission of vehicles are studied such as electric, hybrid vehicles and improved combustion engines running on cleaner fuels. However, each of these technologies is subject to technical difficulties. For compression ignition engines an improvement in pollutant emission performance is needed, particularly for NOx and soot emissions. As pollutant formation mechanisms in compression ignition engines are sensitive to the fuel-air mixing process in the combustion chamber, complex fuel induction systems such as the common rail direct injection systems (CRDI) allowing better control on the mixture formation process were developed. This additional control on the fuel injection process comes at the expense of a complex fuel injection jet dynamics, an already misunderstood process, by introducing potential jet-to-jet interactions between successive injections. The research work covered in this dissertation aims at studying these interactions and the associated physical mechanisms. The studied potential interactions mechanisms are: Interaction via the emission of shock waves at the spray tip, interaction via needle dynamics and interactions via induced air velocity reducing the aerodynamic drag seen by following injections. Interaction via shock waves is studied in a two-step process involving both an experimental and a numerical approach. Experimental results with a multihole injector have shown the presence of shock waves significantly weaker than what was reported in the literature and obtained with single hole injectors. Shocks are followed with decompression waves restoring the ambient pressure after the shock passage. The numerical approach is based on a 1D model simulating a shock wave going through a droplet cloud at rest. The numerical results allowed to conclude that the experimentally observed shocks were not strong enough to act as a coupling mechanism between successive injections. However, shocks of strengths reported in the literature might allow an emitted shock wave to push forward upstream droplets from a previous injection. The study of the hydraulic coupling between injections shown that the injector closing delay and injected mass grows while the injector opening delay drop as the dwell time between injections is reduced. Adimensionalisation of the experimental results expressed in discharge coefficients and adimensional time based on the injector opening delay have shown self-similarity of the injection flowrate growth and reduction rate independent of the injection duration and pressure. A simple empiric model is developed to isolate the impact of the injection effective duration on the total injected mass and the observed coupling behavior. The modeling results allowed to conclude that the observed additional injected mass is associated with this increased injection duration and shown an adequate approximation of the resulting injected mass over the injections strategies covered in this dissertation. High speed visualisations using Mie scattering are used in order to study aerodynamic interactions between successive fuel jets. A significant difference is observed on fuel spray penetration as a function of the dwell time between injection commands. The injection strategies showing an increased jet penetration relative to an electrically equivalent single injection were observed to be corresponding to the strategies showing an increased injected mass. A fuel spray penetration model proposed by (Desantes et al.,2006) is modified to short and multiple injections. This model while neglecting the entrained gas velocity allowed an adequate modeling of the fuel spray penetration with the studied injector under short and multiple injections.

Date	11 Sept 2019
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Patrice Seers (Supervisor)