71 Vehicle Architectures and Design Vehicle Velocity (m/s) FIGURE 3.16 Velocity profile for the three types of initial acceleration. TABLE 3.2 Final Numbers Following Initial Sizing F TR,peak P TR,peak Time and Velocity of Mode Transition Time for 0–60 mph Time for 50–70 mph 7,500 N 120 kW 4.6s/16 m/s 9 s 5 s of Equation 3.6 requires siginifantly large force or torque initially, while the constant accelera- tion profile requires high power at the end of the initial acceleration period. The power and force required with constant acceleration at t = 9 s are 6,899 N and 184 kW.
The practical approach will be to accelerate faster than the constant acceleration profile, but less than the smooth acceleration profile, and transition to constant power acceleration approximately midway through the initial acceleration. The constant force for initial acceleration up to about 4 s obviously needs to be greater than 6,000 N. The total power required to meet the acceleration requirements prior to accounting for the drive- train losses is calculated through design iterations using computer simulation.
The numbers given in Table 3.2 are arrived at after a few design iterations with different peak tractive force, peak trac- tive power and the time for transition of the control algorithm from constant force mode to constant power mode. The tractive force of 7,500 N is supplied by the combination of the electric motor and the IC engine. Furthermore, there will be a gear between the electric motor and the drivetrain, which will bring the torque requirements of electric motor but increase its speed requirement. The F TR versus velocity profile of Figure 3.17 matches well with the torque speed characteristics achiev- able from electric motors.
The velocity ratio of the force characteristics between the peak vehicle velocity (60 m/s) to the onset of the constant power mode (16 m/s) is approximately 4:1, which can be obtained from well-designed electric motor drives. Therefore, assuming the drivetrain efficiency to be about 90%, the minimum combined power required is 132 kW. 3.5.3.2 IC Engine Power: Cruising Speed Once the initial acceleration power requirements are calculated, the sizes of the IC engine and the electric traction motor can be determined. The IC engine size can be determined by the vehicle cruising power requirement at its rated velocity independent of the electric motor power capacity.