Post-transmission parallel configurations

The second option for locating the ac drive in a hybrid vehicle is to insert the M/G at the transmission output shaft, but ahead of the final drive. In this post-transmission configuration the M/G does not have the benefit of gear ratio changes; therefore, it must operate over the very broad vehicle speed range. This demands a high torque ac drive that can function over wide CPSR.

The disadvantages of post-transmission hybrids are the high torque levels, impact of continuous engagement spin losses on fuel consumption, and package difficulty.

Higher torque M/Gs are always physically larger since more rotor surface area is needed to develop surface traction. Larger moment arms to this surface traction are more restricted because the package diameters are usually constrained to fit within transmission bell housing diameters (200 to 350 mm OD).

An example of a post-transmission hybrid would be an in-wheel motor or hub motor hybrid. The GM Autonomy, for example, could be classified as a post-transmission hybrid because the hub motor is separated from the wheel by a non-shifting epicyclic gear.

Figure 2.24 Autonomy with in-hub M/G

The Autonomy (Figure 2.24) is a concept automotive chassis designed for wide ranging body style flexibility and cross-segment application. All propulsion, energy storage, chassis functions and wiring for power distribution and communications are packaged within the skateboard like chassis. Communication is via a controller area network, CAN. Power for propulsion is at high voltage, 300 V typical or higher when fuel cells are used. Chassis and passenger amenities are powered by 42 V or 12 V for lighting.

A concern with hub motors is their higher unsprung mass, a tendency for torque steering, and durability. Because of lower speeds and high torques, a hub motor will be inherently heavier than its higher speed axle or pre-transmission equivalent. Torque steer is a phenomenon due to steering and suspension geometry design. Durability is a persistent issue with hub motors because of simultaneous vibration, temperature, water/salt spray ingress, and sand, dust and gravel impingement.

Torque steer can be understood by recognizing that vehicle steering geometry will generally have non-zero scrub radius. When the suspension king pin axis intercepts the tyre-road patch inside the plane of the wheel the distance from the wheel plane to the king pin axis is referred to as the tyre scrub radius. If the intercept point is inside the wheel plane the scrub is positive and if outside it will be negative. A negative scrub radius puts the wheel turning axis outside the wheel plane on which the corner mass of the vehicle sits. The wheel torque develops a longitudinal component of tractive effort at the wheel plane that is in board of the steering axis. This off axis steering moment due to tractive effort tends to re-align the wheel so that the axis of applied wheel torque and the king pin axis align.

2.4.1 Post-transmission hybrid

There has been work on electric M/Gs connected to the vehicle propeller shaft ahead of the final drive, but these programmes were discontinued in the case of electric

propulsion due to the high demands on machine power density, speed range and physical size. Figure 2.25 illustrates the concept of a post-transmission hybrid in which an electric machine is interfaced to the driveline via a gear reduction.

The speed range concern with a post-transmission hybrid has to do with operating deep into field weakening of the electric machine and not incurring electrical and mechanical spin losses when the M/G is un-energized. If spin losses become a major fuel economy issue, the post-transmission M/G would require an additional clutch to remove it from the driveline during coasting periods.

Wide CPSR is more problematic. With a post-transmission M/G there is no option – it must possess CPSRs >6 : 1 and preferably 10 : 1 in order to deliver both high torque at low speeds for tractive effort plus constant power at higher speeds for

Battery pack

ICE XM

3

E-steer M/G

FD

Figure 2.25 Post-transmission hybrid architecture

T, Nm

300

Speed, krpm 0 1 2 3 4 5 6

Figure 2.26 Post-transmission hybrid capability curves

optimum propulsion. Figure 2.26 illustrates the motor capability curves required from a post-transmission hybrid. A high torque, in the vicinity of 300 Nm, is necessary to deliver low speed tractive effort and wide CPSR is necessary to hold shaft power at high vehicle speeds. Efficiency contours are estimated for such a post-transmission electric M/G to illustrate the placement of peak plateaus. An even more advantageous efficiency contour map would have high efficiency islands extending toward zero on the chart so that best operation would be available at low demands regardless of speed as well as at higher demands.

The capability curve mapped in Figure 2.26 is also needed for in-wheel motors.

Such hub motors have no option for gear shifting and generally are direct drive units.

2.4.2 Wheel motors

There have been many projects over the years to adapt hub motors as post-transmission wheel motors. DOE has funded some of these activities and others have been privately funded. Ontario Hydro developed an in-wheel motor for hybrid propulsion.

Volvo Car Company examined hub motors to determine the package benefits of fully packaged wheel assemblies that contained propulsion, steering, suspension and braking all integrated. The recent GM Autonomy is a similar concept.

The University of Sheffield in the UK developed a demonstration wheel hub motor for application in their Bluebird EV formula 3000 vehicle. The hub motor is a direct drive,φ310 mm by L220 mm capable of delivering 382 Nm of continuous torque.

Toyota Motor Co. has unveiled a wheel motor fuel cell hybrid called the FINE-S (Fuel Cell Innovative Emotion – Sport). Toyota has already leased four FINE-S vehicles to city officials in Japan for operational use. The design goal of FINE-S is to focus on modularity of components and subsystems. The fuel cell components and wheel motors permit versatile packaging freedom not available in conventional cars.

Individual wheel motors in the FINE-S FCHV enable low centre of gravity, high performance handling and smooth ride qualities. Fine tuning the wheel motor torque levels provides high dynamic response traction control and longitudinal stability. The four seat FINE-S concept vehicle is shown in Figure 2.27.

A very recent illustration of in-hub motors can be found in Reference 15 in a concept demonstration motorcycle having the complete power plant housed inside the rear wheel hub. Developed by Franco Sbarro, and unveiled at the 2003 Geneva Motor Show, the semi-encapsulated motorcycle has a 160 hp (119 kW) Yamaha engine plus 5-speed gearbox, including radiator, exhaust, brake, battery, fuel tank and suspension all packaged within a single 22 inch wheel. The system is cited as being an autonomous motor unit, or independent wheel-drive.