Vehicle Powertrain Systems

Chapter 3

Vehicle Longitudinal Dynamics

3.1 Introduction

The longitudinal motions of a vehicle include accelerating, cruising, uphill and downhill motions. The performance and the driveability of a vehicle including accelerations at various loadings and driving conditions are the main topics of this chapter. Vehicle engineers have long realized the importance of driveability in the commercial success of vehicles. Vehicle longitudinal dynamics involves the study of several issues including engine behaviour, tyre tractive force generation, resistive forces acting on the vehicle and the drivers' gearshifting habits.

In this chapter first, simple vehicle models are developed which are useful for the initial estimation of vehicle longitudinal performance. For more detailed analyses and reliable design, however, more elaborate models including the engine characteristics, tyre slips, shifting delays, rotating masses and driveline losses have to be used. Such models with increasing details are being developed and many examples using typical vehicle data are included throughout and MATLAB® codes for vehicle performance calculations are provided in this chapter.

3.2 Torque Generators

The vehicle is accelerated by a tractive force at the driving wheels which in turn depends on the torque applied at these wheels. In the vehicle longitudinal performance analysis, therefore, the power source which delivers the torque – usually an internal combustion engine (ICE) – plays an important role. Different ...