Vehicle Powertrain Systems

Chapter 6

Driveline Dynamics

6.1 Introduction

The vehicle driveline is a dynamic system consisting of inertia and elastic components and when it is subjected to excitations, mechanical resonances occur. These span a large range of frequencies and are generally referred to as noise, vibration and harshness (NVH). Vehicle drivelines with manual transmissions are lightly damped and tend to be more prone to vibration. Driveline torsional frequencies are in general excited by changes in torque which arise from engine or driver inputs.

There are many sources of driveline excitation and in general the disturbing inputs can be categorized into two types: sudden/discrete and persistent. The first category includes throttle inputs, tip-in/tip-out and sudden changes in the engine torque. The second type may arise from engine torsional/torque fluctuations or from worn or misaligned components in the driveline such as Hooke's joints. The driveline vibrations are sometimes categorized as engine speed, vehicle speed or acceleration related. Free play between the components or gear lash will often exacerbate driveline torsional oscillations.

The lowest frequency driveline oscillation (longitudinal vehicle oscillation) is called vehicle shunt. This phenomenon arises from high rate of pedal application to change the driveline torque instantly. The existence of a large gear backlash in the driveline will amplify this type of oscillation starting with an initial jerk. The driver can, therefore, distinguish ...