8 Electric and Hybrid Vehicles where k m is mass factor related to the translational equivalent of all rotating inertias, N p is the number of persons in the vehicle and m p is the average mass of the persons. k m is a dimensionless mass factor that accounts for the inertia of all the rotating components such as wheels, driveline components, engine with ancillaries and hybrid electric machines. The mass factor is given by [3] 4 J 2 2 2 2 k = + 1 w J eng e ξ ξ ng FD J em ξ ξ em FD m 2 + + m r v wh m r 2 v wh m r 2 cv wh where ξ eng e / m and ξ FD are the engine/electric machine transmission and final drive gear ratios.
The steering performance of a vehicle is measured in terms of the steering angles, which is the average of the left and right wheel angles [4]. Right to left vehicle mass balance is also needed for good steering performance. Packaging the hybrid components above the vehicle center of gravity will result in a good steering performance and handling of the vehicle. 1.5 ELECTRIC MOTOR AND ENGINE RATINGS The strengths of electric motors and IC engines are typically described with kilowatt (kW) or horse- power (HP) ratings, although a comparison between electric motors and IC engines in terms of power units only is not fair.
The power that an electric motor can continuously deliver without over- heating is its rated power, which is typically a derated figure. For short periods of time, the motor can deliver two to three times the rated power. Therefore, higher torque and power is available from an electric motor for acceleration, and the motor torque can be the maximum under stall conditions, i.e., at zero speed. The motor type determines whether maximum torque is available at zero speed or not. On the contrary, an IC engine is rated at a specific r/min level for maximum torque and maximum power. The IC engine maximum torque and power ratings are typically derived under idealized laboratory conditions.
In practical situations, it is impossible to achieve the rated power; the maximum power available from an IC engine is always smaller than the rated power. The torque characteristics of motors are shown in Figure 1.6 along with torque characteristics of IC engines. The characteristics of specific motors and IC engines will differ somewhat from these generalized curves. For electric motors, a high torque is available at starting, which is the rated torque of the motor. The peak or rated power from a motor is obtained at base speed ( ω b ) when the motor characteristics enter the constant power region from constant torque region once the voltage limit of the power supply is reached.
The motor rated speed ( ω rated ) is at the end of the constant power region. The IC engine peak power and torque occur at the same speed. At this stage, it will be helpful to review the power and torque relation, which is as follows: Torque Maximum Motor power Rated Engine torque Maximum Engine torque IC Engine Constant Power region Electric rated b Speed (r/min) FIGURE 1.6 Electric motor and IC engine torque characteristics.