Electric Vehicle Frequently Asked Questions

TRANSMISSION & DIFFERENTIAL

Can I build an EV with an automatic transmission?

Automatic transmissions have been used in EV conversions, but the modifications required are likely beyond the scope of most enthusiasts. The torque curve of an EV motor/controller combination is not the same as the ICE it replaces, therefore the shift points (what speed/RPM the transmission shifts gears) will have to be changed. In older transmissions this can be accomplished by modifying the valve body within the transmission (the assembly that routes the fluid under pressure, activating the bands), or in the case of more modern transmissions, modifying the computer code that controls the electronic servos inside the transmission. It may also be possible to simply shift into "L2" or "L1" instead of "D". Since the benefits are rather minor, the work required complex, and there is a decrease in overall efficiency compared to a standard transmission, you don't see very many automatic transmission EV's.

An external fluid pump should be used to provide the constant fluid circulation that an automatic requires, and usually derives from the idling ICE.

As in many other aspects of converting a vehicle, it comes down to what you want out of the vehicle. For many, the convenience is worth the slight decrease in efficiency. For others, premium efficiency and control is a requirement, and this requires a manual transmission.

Why do I need a transmission?

Though it can be omitted through careful design, I see a transmission as being necessary in an EV for both performance and practical reasons. To prevent overloading the motor and controller at lower RPM's, it is helpful to use a lower gear to get the motor RPM's up to improve efficiency, torque and motor cooling. At higher speeds it is desirable to use a higher gear to keep the motor from over-revving, and possibly to increase torque as well. An EV motors' torque band is quite a bit wider than an ICE's, so shifting gears doesn't occur as often as with an ICE. Actual gears used will depend on the motor's torque curve, the output of the controller, the transmission's gear ratios and the final drive ratio. In a practical sense, the transmission serves to provide a convenient interface between the motor's output shaft and the vehicle driveline. Since it usually comes with the car and is likely still functioning (usually the ICE dies first), to discard it and custom fabricate this part of the EV driveline would be complex and expensive. EV suppliers can provide adapter plate kits that bolt to the transmission's bell housing and allow any number of electric motors to connect to it. By using the vehicle's original transmission in this manner solves a number of engineering issues in conversion EV's.

Should I keep the clutch?

Discussing keeping the clutch in an EV conversion first requires a clarification of the terms "necessary" and "desirable". Numerous enthusiasts, and even a few prominent suppliers will say that keeping the clutch isn't necessary. It's true, it is possible to operate an EV without the clutch. However, it may be a very desirable thing to keep and really doesn't add much, if at all, to the cost or complexity of the conversion. If the donor vehicle originally had a clutch, all of the necessary hardware is already there, so there's not much reason not to incorporate it. Inside a manual transmission, synchronizers are used to match the speeds between the gears during a shift. When a clutch is present, those synchronizers only have to speed up or slow down the mass of a couple of gears and a shaft or two during a shift. Without the clutch, and an electric motor permanently coupled to the input, those synchronizers also have to speed up or slow down the entire mass of the rotating motor armature and coupler, which can be as much as 80lbs or more. This increases shift time to a number of seconds, which can get kind of exciting when you need to shift while merging on the freeway or trying to climb a steep grade. Under this severe duty of having to speed up and slow down this 80lb armature mass, it's inevitable that the synchronizers will eventually fail. Keeping the clutch not only ensures the synchronizers never see a load greater than they were designed for, but offer other advantages over going clutchless. One is the ability to slip the clutch to allow precise parking and backing maneuvers. Another is a safety feature. If the motor controller ever fails full on and your emergency disconnect doesn't work (or isn't there!), you can push in the clutch and disconnect the runaway motor from the driveline. The motor will be destroyed but that's better than careening out of control down the street. During normal EV driving you don't use the clutch quite the same as you would in an ICE, for instance sitting at a stoplight in gear doesn't require you to hold the clutch in, since an electric motor doesn't idle. Starting from a stop doesn't require slipping the clutch, as the motor can accelerate from 0 RPM on up. You also tend to shift much less often, due to the broad torque band most EV motors have. As such, the clutch in an EV will see very light service and should last the life of the EV, in addition to preserving the synchronizers. Add to that the additional motor-disconnect safety feature and this makes keeping the clutch not exactly necessary, but very desirable.

What are the effects of using one separate motor for each drive wheel? Can I remove the differential to save weight or control the motors to emulate a differential?

It depends on the motors.

With series DC motors, the problem is trivial. Wire the two motors in series, and they behave exactly like a normal differential; same torque at both wheels regardless of speed. With one wheel in the air, you have no torque at the other wheel either.

Wire the two motors in parallel, and it behaves like a limited-slip differential. Each motor operates independently, adjusting its speed according to the torque. Same speed, same torque to both wheels. When you turn, the inside wheel slows down a bit, so it delivers a bit more torque.

AC induction motors and PM DC motors are "stiffer" (more change in torque as their speed changes), but basically work the same as the series motors in parallel. When you turn, the inside motor delivers more torque than the outer motor (and more than series motors would have under the same conditions), but not enough to cause problems. It still works as long as you don't try it with extremely tight turning radius (like a skid-steered vehicle).

AC synchronous and brushless DC motors are the only ones where you need a separate controller for each wheel. These motors "fight hard" to run at precisely the speed commanded. With them, you would either use one motor with a differential, or separate controllers for each motor with some control scheme so each runs at the right speed.

Editor's note: Though direct motor drive has been accomplished, It takes careful system design. Usually some sort of reduction is necessary. For example, if one were to direct drive a 23" diameter wheel (tire size 205/50 R15) with a typical EV motor such as an ADC 9" series wound motor, the vehicle would be going about 68 MPh at only 1000 motor RPM. Since these particular motors like to be run at around 5000 RPM for best efficiency, the motor would draw excessive current (requiring a more expensive controller) and generate a lot of excess heat. To perform well would require a lot of torque (and thus current) at low speeds. This drives up the cost as well.

The examples above allow one to eliminate a differential, but a reduction would most likely still be needed. One can be used on each motor, or the two motors can be ganged together on one differential, as on the Solectria EV S-10 trucks.