EV transmissions

Wednesday, July 09, 2014

Tags :  transmission

It is a popular myth that electric and hybrid vehicles, thanks to their use of electric motors capable of delivering maximum torque from zero rpm, do not need a transmission. But while it is perfectly feasible to run an electric motor (or motors) directly driven to the wheels, it is increasingly common to use a transmission, particularly among EV racers. Why is this? The answer is that, by using different ratios, the efficiency of a motor can be increased by ensuring that it is operating in its optimum speed range. Electric motors have a wider power range than an internal combustion engine; their torque curve is flatter, but their efficiency curve is far from flat.

For example, one electric drag car I have studied features a pair of astoundingly powerful electric motors, pushing out in the region of 700 bhp, which would have more than enough power to propel it down the strip at great speed. However, it also has a planetary geared transmission, which helps make the most efficient use of the car’s motors.

As the owner and builder of the car explains, “The power band of our motors gives us peak horsepower between 2400 and 3400 rpm, so it is a narrow band. What we are able to do with the use of a transmission is go through that peak four times in a run. Our goal, after studying the data we gained from our first car, was to keep the car in that power band for as much of the run as possible.”

The result is blistering acceleration all the way down the track. By using an under-driven ratio for first gear, the car achieves very low 60 ft times, leveraging the instantaneous torque delivery of the motors to the maximum, while the additional three ratios help it reach a speed of more than 150 mph by the end of a run.

Another interesting EV transmission design draws its technology directly from that used in the current generation of MotoGP ‘seamless-shift’ transmissions. It eliminates the dog rings found in a regular sequential transmission, and instead uses ‘bullet rings’ to engage the gears. A gear is selected when one ring is moved until its bullets hook onto drive teeth on the side of that gear. A second bullet ring moves in the same direction, with its bullets filling the gap between the teeth, eliminating any slack between the gears.

Eliminating this slack is what creates seamless upshifts and downshifts. By integrating the dampers inside the gear hubs, the manufacturer claims that the need for a clutch is eliminated – and this, combined with a seamless change of ratio, means the motor’s torque delivery can remain constant.

On a simpler level (although in terms of overall system complexity, far higher), hybrid vehicles that use electric drive motors on one or both axles can also benefit from a transmission. Taking the latest generation of hypercar road-going hybrids and both Porsche’s and Audi’s LMP cars as an example, the use of clutch systems and reduction gears helps increase motor efficiency. For instance, Porsche’s 918 roadcar uses a system of clutches on the front drive motors to disengage them above 146 mph, with the clutches being electronically controlled to provide the function of an active differential.

Meanwhile, other cars use reduction boxes to maximise the already high torque output of their electric motors in order to assist acceleration. This is a logical approach when thinking of an optimised racecar package. The boost provided by an electric motor should be far more beneficial to lap times if it can increase corner exit speed, rather than being used at high speed where the extra power has far less impact.

The age of electric and hybrid racers is still young, but rest assured that the rise of electric motors does not mean the end of multi-speed transmissions.

Written by Lawrence Butcher

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