Home Transportation Electric Vehicles

Do Electric Cars Have Gears? No. Here’s Why

11639
18
Single-speed electric vehicle transmission by Borg Warner for the original Tesla Roadster.

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.

AC MG Torque / RPM / Efficiency
AC MG Torque / RPM / Efficiency

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?

(Visited 41,208 times, 18 visits today)

18 COMMENTS

  1. 10,000 rpm in an ICE is far different than and electric motor, which has no reciprocating mass, spinning at 10,000 rpm. The electric motor is actually harder to punish with aggressive speeds as long as it doesn’t grenade from poor manufacture. ICEs only last a very short time when pushed to those same speeds and must be torn down and rebuilt regularly.

  2. If the different gearing woun’t change the range of Tesla, the what about this: We take Tesla Model-S, with a gear transmition of 9.73/1. We take these gears-off, and we put a new one, with 9730/1. That’s a thousand times higher. Now, the engine will work for 6-7 hours, and will dry-off the battery. But for this time, the car will be traveled only 0.5km, instead of 500km. So, as you see, the whole battery charge will be spend for only 0.5km. And if this reduces the range, why not the smaller gear rations, like 5/1, to increase it?

  3. In concept, the variable ratio transmission is very appealing. If it were possible, I’d be happy to have a sort of overdrive or alternate drive ratio to extend range for longer trips. Mine goes as quick and fast as I could want. Range improvement would be one of the best improvements. (Of course, I have no technical knowledge.)

  4. Some quick calculations indicate that with a fixed final drive ratio of 9.73:1 and 19″ wheels, with a maximum RPM of 9500, the same Tesla that smoked your Ferrari off the line is going to have trouble keeping up with your Volkswagen once your speed starts exceeding 100 mph.

    The Tesla would in fact benefit greatly from a variable ratio transmission, in terms of overall performance if not 0-60 times. With the right gearing and proper materials, I feel like the acceleration characteristics of the vehicle could remain largely untouched, while freeing the vehicle to explore the upper limits of the speedometer range.

  5. So, you might not need a trans for normal driving. Now lets say I’m building an electric car for drag racing. I’m familiar with the times of the Model S, but wouldn’t I actually accelerate faster with a two speed gear box in order to maintain lower RPM? Just curious…

  6. No wonder the major car makers have been slow to embrace EV’s. Half the time when something goes on a car, it’s the transmission. There goes a huge chunk of revenue.

  7. “once you get to freeway speeds (high motor speeds as well with a single speed trans), the torque drops dramatically”… have you ever driven a P85D, or you’re just theorizing? I suggest you go try one and while at freeway speeds floor it. I’m telling you – it goes.

    • Well it’s different for DC and AC motors. For DC it drops, but has a whole lot or torque in the beginning. AC, used in Teslas, are mor constant with their torque. It isn’t as high as DC but is still very high

    • Yes, it goes. It’s about relative torque though. A PxxxD beats about anything but a top fuel dragster from 0-60. However, after about 80, the high end gasoline muscle cars have a significant advantage. 80ish, IMO, is close enough to freeway speed to be valid for this concept.

      Theorizing with all the detail in the article? It’s you who seem to be relying on feelings instead of data.

  8. I realize this article is a half a year old, but someone has to read it every once in a while still. I mean, I did (: The automatic transmission has evolved as much, if not more than internal combustion engines have over the last few decades. We have smart transmissions with dry and wet clutches to engage and disengage drive trains when idling and/or decelerating (obviously counter intuitive for regeneration on an EV), which dramatically increases efficiency. Obviously, inductive motors can do 10000 RPM without too much worrying, but how much torque is produced at that motor speed? Sure, a Tesla P90D can do 0-60 mph in 2.8 seconds. Amazing for a 5000 lb sedan. But, once you get to freeway speeds (high motor speeds as well with a single speed trans), the torque drops dramatically. Keeping motor RPM in the lower end of that range would allow the electric motor to still produce monster torque at higher vehicle speeds, would it not? Real driving speeds, Tesla has done a great job. But everyone knows that their fastest car will be overtaken by high performance petrol fed cars in the same segment because of horsepower, and multi-range transmissions. Figure out a way to minimize the parasitic effect this drive train could have on an EV battery (increase range capabilities), and you can have an EV that’s track ready, and the consumers will come. An EV that performs like a 911 around the track would have a lot of buyers…

    • Question: when you’re already at freeway speeds, why would you NEED higher torque? We’re not building race cars here.

      • Then why does any car need gears?!!! Just put a third gear ratio in a final drive and by your logic that’s enough lol Think it through. EV’s have a huge torque drop off at higher motor speeds, ICE’s GAIN torque through the powerband. This is why the Model S gets over taken through the quarter mile traps. It doesn’t have to be for race cars, but having the motor in the correct RPM range at highway speeds would be beneficial regardless of how or where the car is driven.

    • I lack any expertise in this area but your response confirmed my thinking that a single gear means that the electric motor will have to spin very fast to keep the vehicle moving at higher speeds – I am not dumb after all 🙂 I am amazed that those motors don’t produce a lot of noise at those RPMs or so I guess since people don’t complain. But I still don’t get it – having to spin continuously at some crazy 10000 or so RPM just so you can cruise is more efficient and effective than having a transmission? What about engine wear? Explain to me please someone 🙂

      • A little late, but consider that since there are no gears, the electrical motor is running at the same speed as the wheels. so wear and tear is the same as that within the wheel of a conventual vehicle.

        Also consider your engine isn’t necessarily going to have to spin much faster if you have no gears, knowing that most of the smaller gears (1st, 2nd and 3rd) in normal cars, gear DOWN, meaning the engine spins faster at lower wheel spin speed.

        So having no gears at all likely means your engine will spin slower on average (considering you don’t average over a 100 mph or something.

        Summarized, the main function of gears is essentially to make an engine spin faster to reach its power band even at low vehicle speeds, so without gears the engine will run slower most of the time.

      • More to the point, it will not be doing 10000rpm at cruising speed – more like at 250kph. at cruising speed it will be doing something a bit more sensible.

LEAVE A REPLY

Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.