66 Electric and Hybrid Vehicles ρ P m TR = + gv sin β mgC A + 1 F D C v 3 ,max max m + ax mgv C max 0 (3.3) 2 The dominant resistance force at high speeds is the aerodynamic drag force with the power require- ment to overcome it increasing at a cubic rate. For vehicles designed with fast acceleration char- acteristics, P m is likely to be greater than P TR ,max . If P P TR ,max > m derived earlier to meet the initial acceleration requirement, then P TR ,max will define the electric motor power rating. The natural mode region of electric motors can be used to meet very high maximum vehicle velocity requirements to minimize the motor size.
3.5.1.4 Maximum Gradability The maximum gradability of a vehicle for a given motor and gear ratio can be derived from 100 F Max.% grade = TR . ( ) mg 2 2 − F TR The maximum traction force F TR available from the preliminary motor design can be plugged into the above equation to check whether the vehicle maximum gradability conditions are met or not. If the maximum electric motor power derived for acceleration or maximum vehicle velocity is not enough to meet the maximum gradability requirement of the vehicle, then either the motor power rating or the gear ratio has to be increased. Care must be taken not to violate the maximum vehicle velocity requirement when increasing the gear ratio.
The gear ratio and motor power is decided in a coordinated manner to meet both the requirements, while maintaining a reasonable size for both the electric motor and the gear. 3.5.2 heV p owerTrain s izing The HEV powertrain architecture and control technique depends on the desired requirements includ- ing, but not limited to, performance, range and emission. The performance requirements of initial acceleration, cruising velocity, maximum velocity and gradability dictate the power requirements of the IC engine and motor. The power requirements can also be specified in terms of multiple driving schedules that have the worst-case demands embedded in them.
The energy required by the drive- train to meet the range specification dictates the design of the energy storage system, which can be a battery-pack or a combination of battery and ultracapacitors. Meeting the emission standard depends solely on the IC engine emission characteristics, since the electric motor has zero emission. The power required in a hybrid vehicle comes from a combination of the electric motor and IC engine outputs. The mission of the vehicle plays a big role in apportioning the power require- ment between the electric motor and heat engine.
A hybrid vehicle designed for urban commute- type transportation will have a different combination of drivetrain subsystems than a family sedan designed for both urban and highway travel. The power requirement of the electric motor and the IC engine for an urban commute vehicle will definitely be lower than that required for cars designed for highway travel. For urban vehicles that are used solely for daily commutes of less than 100 miles, the designer must consider the BEV instead of a PHEV. The design engineer evaluates the design trade-offs based on the mission and specifications and selects one of the series or parallel configura- tions discussed earlier.
The subsystems of the drivetrain have their individual control units and the components are coordinated through a supervisory control unit. The mission of the vehicle also dic- tates the type of control to be employed for the vehicle. For example, the highway vehicles require a control strategy designed to maximize the fuel economy. The designed system must be capable of handling all real-world situations within the limits of the design requirements. Appropriate safety measures must also be incorporated in the controller to handle situations when certain subsystems fail or underperform.