69 Vehicle Architectures and Design 3.5.2.4 Maximum Gradability The maximum gradability condition must be checked once the sizing of the IC engine and the elec- tric motor is done from the previous three requirements. The maximum gradability of a vehicle is given by = 100 F TR Max.% grade ( ) mg 2 2 − F TR If the condition is not met, then the size of either the engine or the motor or both must be increased or the gear ratio must be changed to meet the gradability requirements.
Although the design philosophy outlined above states that the IC engine sizing primary comes from the rated cruising velocity and the electric motor sizing comes from the initial acceleration, the practical design involves extensive computer simulation using various drive cycles, parameters of the vehicle and characteristics of chosen battery, motor, generator and IC engine. As in all systems cases, the sizing and design of the components of electric and hybrid vehicles is an iterative process that ends when all the design requirements are met. The discussions presented provide the theoreti- cal basis for initial estimates avoiding unnecessary oversizing of components.
An HEV powertrain sizing example is presented below for a series–parallel 2 Ч 2 hybrid vehicle. The vehicle was re-engineered from a 2005 Chevrolet Equinox for the national collegiate student vehicle design competition called Challenge X. This vehicle is termed as Akron hybrid vehicle in this book. The vehicle parameters and performance requirements for the design are given in Table 3.1. The overall design goal for the vehicle is increased fuel economy and reduced emissions without sacrificing customer amenities.
The gains in fuel economy with vehicle hybridization can be achieved on the basis of downsizing the engine, reduction in engine idling operation, recovery of braking energy losses and flexibility in choosing engine operating point to enhance energy effi- ciency. These considerations form the basis of component sizing and selection and control strategy development for the Akron hybrid vehicle. 3.5.3 heV p owerTrain s izing e xample The design considers initial acceleration without the trailer but includes the mass of the driver. The vehicle cruising speed is 75 mph on a 3% grade and against a 20 mph headwind at high-engine fuel efficiency as well as high-electric drive efficiency.
The Akron hybrid vehicle architecture is the same as that shown in Figure 3.5. The vehicle components have been sized to meet the initial acceleration demand through a combination of power delivered by the electric motor and the IC TABLE 3.1 Akron Hybrid Vehicle Parameters and Requirements Description Requirements Vehicle mass 4,400 lbs./1,995 kg Driver/one passenger 176 lbs./80 kg Trailering capacity 2,500 lbs./1,133 kg Rolling resistance coefficient, C 0 0.009 Wheel radius, r wh 0.3305 m Aerodynamic drag coefficient, C AD 0.45 Frontal area, A F 2.686 m 2 0–60 mph 9.0 s 50–70 mph 6.8 s