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INFLUENCE OF BELLOWS TYPE FLEXIBLE JOINT ON EXHAUST DYNAMICS

By RAKESH .K.R

SYNOPSIS

Most of the modern cars have bellows-type flexible joint between the manifold and the catalytic converter to allow for thermal expansion and to decouple large engine movements and vibrations from the rest of the exhaust system. To obtain better understanding of the influence of this joint, the dynamic response of a typical exhaust system is studied when exited via different joint configurations.

Measurements show the great order of reduction in vibration transmission to the exhaust system that a bellow joint, with and without an inside liner, gives in comparison with a stiff joint. For the combined bellows and liner joint, vibration transmission is, however, higher than for the bellows alone.

Anyhow, the combined bellows and the liner joint makes the exhaust system behaviour significantly non-linear, whereas the system behaviour proves to be essentially liner when the bellows has no liner, which imply that the liner needs to be included .

INTRODUCTION

Increased customer awareness, demands on shortened time to market and the greater complexity of todays cars has made the traditional cut-and-try product design strategy obsolete. Together with the strong development of computer capacity this has made dynamics analyses of car components considerably more comprehensive over the years. The exhaust system is of particular interest since it is one of the main sources of structure-borne noise and needs to fulfill high demands on safety and durability. Front-wheel-driven cars have become common on the market. These cars have the engine mounted in the transverse direction and have a flexible joint included between the manifold and the rest of the exhaust system to allow for thermal expansion and to decouple larger engine movements and vibrations from the rest of the exhaust system. The commonly used bellows type joint has, however, caused car and component manufactures severe problems in some case and it is generally recognized that there is need for better understanding of the influence of this type joint on the system dynamics.

The Exhaust System Design

3 TYPES OF JOINT CONFIGURATIONS HAVE TO BE STUDIED

PIPE JOINT BELLOWS JOINT BELLOWS JOINT WITH LINER

Exhaust System With The Pipe

Modified Exhaust System With The Flexible Joint

Basic Design of Flexible Joint

EXPERIMENTAL INVESTIGATION

The Experiment Setup

TWO TYPES OF EXCITATIONS

BURST RANDOM EXCITAION FORCE SINUSOIDAL EXCITATION FORCE

Sinusoidal excitation is well suited to analysis of non-linear structures as the input signal can be accurately controlled. Furthermore the signal-tonoise ratio is good since all energy is concentrated at one frequency at a time. The main drawback of the method is that it is very time consuming. This function makes it possible to control the force amplitude so that the system can be excited with a constant force over the whole frequency interval of interest

Fig7: Linearity Check For Exhaust System including the Bellows Joint

Typical FRFs at the Middle of the system Including the combined Bellows and Liner joint

Transmissibilities For the Pipe and Bellows Joint Respectively

Transmissibilities For the Bellows and the combined Bellows and Liner joint respectively

DISCUSSION AND CONCLUSIONThe transmissibility for the combined bellows and the liner joint is, however, generally higher than for the bellows without the liner. Thus, from the view point of reduction in transmission of engine vibrations through the joint a bellows without a liner, or with a liner, with as low a friction limit as possible, should preferred. The liner may, however, be advantageous by reducing the response through friction-based damping. In addition to the above it should be considered that the liner reduces the bellows temperature and improves flow conditions. Friction in the liner should, however, not be necessary to fulfill these objectives, but a liner with no (or very low) friction may cause additional undesired noise.