Here’s a conversation that piqued my interest today: “Are electric vehicle transmissions necessary?”, or “Do electric cars have gears?”. Really, could multispeed gearing do anything for electric vehicle performance, acceleration, speed, efficiency, or otherwise? (For the purposes of this discussion, we’re leaving out heavy-duty applications, such as delivery trucks or tractor-trailers.)
After decades of driving conventional vehicles, one gets used to the idea of shifting gears, multispeed transmissions an absolute necessity in combination with an internal combustion engine. In an electric vehicle, perhaps one comes to the conclusion that a multispeed transmission would be a given, but practically none of them do. Are electric vehicle transmissions necessary to improve performance? First, let’s take a look at why we have multispeed transmissions in the first place.
Actually, the very first production automobile, the 1886 Benz Patent Motorwagen, had a single-speed transmission and clutch. Then again, its engine generated less than 1 hp and it had a top speed of about 10 mph. Today, with engines pushing 1,000 hp and tops speeds over 200 mph, five- and six-speed transmissions aren’t at all uncommon, automakers are toying with transmissions with up to ten gear ratios, and CVT (continuously-variable transmissions) have infinite gear ratios, but why?
The problem is that the ICE (internal combustion engine) only generates usable torque and power in a narrow band of engine speeds. To accelerate the vehicle, multispeed transmissions step that down, in varying gear ratios, to keep the engine in its power band. Keeping the engine in its power band also proves to be the most efficient and durable. An engine, in first gear, can easily accelerate a car to 30 mph, but would also shake itself to bits attempting to accelerate to highway speed. Likewise, that same engine would hardly be able to accelerate from a stop in 6th gear.
Interestingly, electric motor-generators (MG) generate 100% of their torque at very low speeds, DC MGs near stall (zero rpm), and AC MGs around 1,000 rpm, as a general rule. As rpms increase, torque falls off at a fairly linear rate, at the same time that power is increasing. According to an ORNL (Oak Ridge National Laboratory) Evaluation of the 2004 Toyota Prius (PDF), for example, the MG generates up to 300 N•m of torque around 1,500 rpm, trailing off to about 50 N•m at 6,000 rpm. At its most-efficient, 93%, the MG is pushing only 100 N•m at 2,250 rpm, perfect for cruising. In any case, a multispeed electric vehicle transmission is unnecessary because even 100 N•m is plenty of torque at cruising speed.
Of course, this doesn’t mean that electric cars couldn’t have gears, but they aren’t necessary. First, given MG torque / power / rpm characteristics, they’re perfect for transportation. Torque is necessary for acceleration, the most of which is generated near stall. Power is necessary for cruising, the most of which is developed at high rpm. Second, a multispeed electric vehicle transmission would add weight, complexity, friction, and inefficiency to an otherwise simple system, actually robbing the powertrain of torque, power, and efficiency. The Tesla Model S, for example, is equipped with a single-speed 9.73:1 step-down transmission, not even a two-speed transmission, for two reasons. First, there are very few transmissions capable of handling 600 N•m torque. Second, you can’t spin the wheels at 10,000 rpm. Well, you could, but it would be totally unnecessary.
Where do we see multispeed electric vehicle transmissions? Not in production vehicles, but in conversion vehicles. The easiest way to convert a conventional vehicle to electric is simply to swap out the ICE with an AC or DC MG and throw some batteries in the trunk. Granted, that’s an oversimplification, but why replace the whole drivetrain, multispeed transmission included, if you only want to achieve a relatively convenient electric vehicle powertrain?