2 Vehicle Mechanics The fundamentals of a vehicle design are embedded in the basic mechanics of physics, particularly in Newton’s second law of motion relating force and acceleration. Newton’s second law states that the acceleration of an object is proportional to the net force exerted on it . The object accelerates when the net force is non-zero, where the term net force refers to the resultant of the forces acting on the object. In the vehicle system, several forces act on it with the resultant or net force dictating the motion according to Newton’s second law.
A vehicle propels forward with the aid of the force delivered by the propulsion unit overcoming the resisting forces due to gravity, air and tire resis- tance. The acceleration and speed of the vehicle depend on the power available from the traction unit and the existing road and aerodynamic conditions. The acceleration also depends on the com- posite mass of the vehicle including the propulsion unit, all mechanical and electrical components and the batteries. A vehicle is designed based on certain given specifications and requirements. Furthermore, the electric and hybrid vehicle system is large and complex involving multidisciplinary knowledge.
The key to designing such a large system is to divide and conquer. The system-level perspective helps in mastering the design skills for a complex system where the broad requirements are first determined and then system components are designed with a more focused guidelines. For example, first the power and energy requirement from the propulsion unit is determined from a given set of vehicle cruising and acceleration specifications. The component-level design begins in the second stage where the propulsion unit, the energy source and other auxiliary units are specified and designed.
In this stage, the electrical and mechanical engineers start designing the electric motor for electric vehicles (EVs) or the combination of electric motor and internal combustion engine for hybrid EVs. The power electronics engineers design the power conversion unit which links the energy source with the electric motor. The controls engineer works in conjunction with the power electronics engineer to develop the propulsion control system. The chemists and the chemical engineers have the primary responsibility of designing the energy source based on the energy requirement and guidelines of the vehicle manufacturer.
Many of the component designs proceed in an iterative man- ner, where various designers have to interact with each other to insure that the design goals are met. The design is an iterative process, which starts with some known factors and other educated guesses or assumptions to be followed by scientific analysis in order to verify that the requirements are met. In this chapter, we will develop the tools for scientific analysis of vehicle mechanics based on Newton’s second law of motion. After defining and describing a roadway, the vehicle kinetics issues will be addressed. The roadway and kinetics will be linked to establish the equation for the force required from the propulsion unit.
The force from the propulsion unit, which can be an electric motor or an internal combustion engine or a combination of the two, is known as the tractive force . Once force requirement is defined, one would proceed to calculate the power and energy required for a vehicle under consideration. The emphasis of study in this book is on the broad design goals, such as finding the power and energy requirement and predicting the range for a given energy source, thereby maintaining a top-level perspective. The design details of a subsystem are beyond the scope of this book, and the readers are referred to the literature of the respective areas for further details.
2.1 ROADWAY FUNDAMENTALS A vehicle moves on a level road and also up and down the slope of a roadway. We can simplify our description of the roadway by considering a straight roadway, since the horizontal maneuvering has minimal impact on the force and power requirement from the propulsion unit. Furthermore, we will 23