Experimental and Numerical Analysis of the Hydraulic Circuit of a High Pressure Common Rail Diesel Fuel Injection System

Abstract

This thesis deals with the analysis of fluid transients in the hydraulic circuit of a high pressure common rail diesel fuel injection system for light vehicle applications. The injection process triggers flow and pressure pulsations that have an adverse effect on the predictability of subsequent fuel injections. To achieve maximum fuel efficiency during the combustion process it is mandatory to meter the amount of injected fuel precisely.

Firstly, a brief review of modelling techniques for analysing fluid transient problems is provided. The basic governing equations are presented as a second step. The individual terms are discussed and evaluated in respect of their importance for modelling the present system.

Experimental measurements are conducted on a hydraulic test bench. The input parameters are defined by injection specific parameters and the geometry of the connection line between rail and injector. The main output parameters are based on the pressure histories at various locations, the oscillation frequency of the main perturbation, and the amount of injected fuel per injection.

To extend the range of analysed parameters, a one-dimensional simulation model is used to describe the complete system characteristics. Additionally, local flow phenomena are modelled by means of three-dimensional simulations. The agreement between measurements and simulations is good. The results show that the fluid transients in the system are strongly dependent on the analysed input parameters. In this work, the individual correlations are pointed out and evaluated. Finally, the simulation models are utilised to analyse parameters that are not accessible by measurements and to study theoretically different measures to minimise unwanted flow and pressure oscillations as much as possible.