84 Transmission Efficiency

All vehicles need a transmission that connects the output of the motor to the wheels. In the case of an internal combustion engine vehicle the engine is connected to a clutch which in turn connects to a gear box, a prop shaft, a differential (for equalising the torque on the driving wheels) and an axle.

All of these have inefficiencies that cause a loss of power and energy. The transmission of electric vehicles is inherently simpler than that of IC engine vehicles. To start with no clutch is needed as the motor can provide torque from zero speed upwards. Similarly, a conventional gear box is not needed, as a single ratio gear is normally all that is needed. The three basic variations of electric vehicle transmission are illustrated in Figure 8.6.

The most conventional arrangement is to drive a pair of wheels through a differential. This has many advantages, the differential being a well tested, reliable, quantity-produced piece of engineering. The disadvantage is that some power is lost through the differential, and differentials are relatively heavy. It can also take up space in areas where the space can be usefully utilised. An example of a motor and differential fitted to an experimental battery powered vehicle is shown in Figure 8.7. In this system the motor is transversal, but otherwise it is similar to Figure 8.6. This arrangement can also be seen in the diagram of the electric bus in Figure 11.7.

The differential can be eliminated by connecting a motor to each wheel via a single ratio gearbox or even a toothed belt drive. The torque needs to be equalised to each wheel by the electronic controller. This system has the advantage of clearing space within the vehicle, and the disadvantage of needing a more complicated electronic controller. Also, in terms of cost per kilowatt, two small motors are considerably more expensive than one larger one. An example of a small motor connected via a simple gearbox to an axle, which would be suitable for this sort of application, is shown in Figure 8.8.

The third method is to directly connect the motor to the wheels via a shaft, or to actually design the motor as part of the hub assembly. This system has huge potential advantages, including a 100% transmission efficiency. The trouble with this system is that most electric motors typically run at 2 to 4 times faster than the vehicle wheels, and designing a

Figure 8.7 Example of type (a) of Figure 8.6 on an experimental electric vehicle by MES-DEA of Switzerland. The mounting of the motor is transverse, so there is no drive shaft
Figure 8.8 Commercial motor and single speed gearbox to axle connection. This type of motor is designed for use in systems like that of Figure 8.6(b), or on vehicles with a single drive wheel, or on vehicles like go-karts which have no differential

motor to work slowly results in a large heavy motor. However, this arrangement has and can be used. It is particularly popular in electric scooters and bicycles. An example is shown in Figure 8.9. The General Motors Hy-wire of Figure 8.16 uses this approach, and it can also be seen in the electric bicycle of Figure 11.1. Normally a vehicle's handling is improved if the unsprung mass is kept to a minimum. Placing the motor in the hub has

Figure 8.9 The rear wheel of an EVT electric scooter. Here there is no transmission, the wheel and motor are one. This is an example of Figure 8.6(c)

advantages for space saving in vehicle layout, but will adversely affect handling. Also, the motor is certain to be considerably more expensive in terms of cost per kilowatt.

Of course, if you were designing a three-wheeler and driving the single wheel you would not need any differential, mechanical or electronic! You may still need to gear the motor to the wheel. A tricycle arrangement with one driven wheel at the back could also help in the production of a near teardrop shape with its associated low aerodynamic drag. Such an arrangement has been used in some experimental vehicles. A power unit that could be the basis for such a vehicle is shown in Figure 8.11.

Whatever the arrangement for the transmission, the transmission efficiency is important. A 10% increase in transmission efficiency will allow a similar reduction in battery mass and battery cost or alternatively a 10% increase in the vehicle range.

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