High pressure pipes of the diesel injection system seem to represent a weak point in terms of vibration and acoustic radiation of the whole injection system. Investigations have highlighted this phenomenon. The injectors induce acoustic waves which propagate in the viscous diesel contained in the injection pipes. A strong coupling can occur sometimes between these acoustic waves and the duct structural modes leading to intensive mechanical vibration and acoustic radiation; and sometimes to a possible failure of the pipe.
Numerical simulations offer a good platform to predict such vibration and can be used in order to prevent any structural component failure and to decrease the resulting acoustic radiation. This paper presents a vibro-acoustic study performed with the finite element code ACTRAN to estimate which parameters play a role in this process and to provide some guidelines for avoiding problems.
During this study the acoustic excitation due to the injector is directly applied in the hydraulic circuit, which is simulated by using visco-thermal acoustic elements. These elements are used for acoustic propagation in narrow section ducts, based on a reduced wave equation taking into account both viscous and thermal losses. The hydraulic system is coupled with the duct which radiates in free field conditions.
The structural behavior of the pipes depends both on the definition of the hydraulic system and on the definition of its own boundary conditions (connection to the engine block). Using a Nastran “superelement” at the extremities of the pipes, it is possible to take more precisely into account the mechanical impedance of the engine block without having to include it explicitly in the duct model. This superelement technique is based on a dynamic reduction procedure (Craig-Bampton, [6]) of the entire engine (without the duct) which is then applied in the ACTRAN analysis.
In this paper, the simulation method is first briefly described. The different inner fluid - structure interactions are then analyzed on different duct models. Finally, several structural results are retrieved globally and locally in order to guide the choice of the high pressure pipe to select.