What's the problem with EVs and geared transmissions?

[swbluto]

It seems that gasoline vehicle use gears all the time, and it's often more like 5+ gears. I've heard that Tesla was planning on using a 2-speed transmission(for getting the greatest acceleration and speed), but they devolved it to a fixed gear after high failure rates. Also, I don't seem to notice gears on vehicles where it would seem common place for a gasoline equivalent(Many electric motorcycles don't seem to have a "crank case" or where the gears are, I think.). What's with EVs and the relative lack of multiple speed gears?

I would think that the technology would be readily adaptable from gasoline technology that's had tons of years to work out the kinks and produce reliable configurations, but despite how much it seems it could benefit EVs, it doesn't seem like they're particularly common. Why not? Is there a sudden technical barrier created by an EV system?

 

[Lapwing]

Torque. It destroys gears. Strong enough implies heavy and ineffcient.

Probably the bottom line is EV's don't need even 2 gears, given recent developments in poly-phase controllers and motors.

 

[Toshi]

It seems that gasoline vehicle use gears all the time, and it's often more like 5+ gears. I've heard that Tesla was planning on using a 2-speed transmission(for getting the greatest acceleration and speed), but they devolved it to a fixed gear after high failure rates. Also, I don't seem to notice gears on vehicles where it would seem common place for a gasoline equivalent(Many electric motorcycles don't seem to have a "crank case" or where the gears are, I think.). What's with EVs and the relative lack of multiple speed gears?

I would think that the technology would be readily adaptable from gasoline technology that's had tons of years to work out the kinks and produce reliable configurations, but despite how much it seems it could benefit EVs, it doesn't seem like they're particularly common. Why not? Is there a sudden technical barrier created by an EV system?

all time spent shifting when the clutch is engaged is time during which energy is lost. this is why automobiles are moving to DSGs and other fanciness in order to minimize this time. if the RPM range and power curve of electric motors is such that a drivable highway speed vehicle is possible with 1 or 2 gears then i'm all for minimizing the number of gears.

 

[Roy Von Rogers]

Now I have never driven a converted to electric ICE vehicle, but when driving an ICE manual shift, there is a certain area where one should shift to get economy. One doesnt want to shift too early to keep from lugging the engine, same with shifting late and over revving the engine.

I would assume such is also the case with an electric to manual transmission. Its just the power/economy band area is different.

Roy

 

[Malcolm]

Car conversions often use the existing gearbox with or without a clutch. Drivers don't need to use all the gears though, as the ratios are usually too closely spaced for an electric motor. Most only use two gears, with roughly a 2:1 difference between ratios. The heavier the vehicle, the more important it becomes to use a multispeed transmission.

If you go for a single ratio drive on a car you generally need a bigger motor to provide sufficient torque for pulling away on hills. Most motorcycle conversions eliminate the gearbox to save weight and more importantly make space for batteries. It does compromise performance though, so it would be really nice to have a compact, two-speed, wide ratio box that's specifically designed for EV use. Likewise for car conversions.

 

[swbluto]

Yes, I did notice a rather significant difference between a gas engine's and electric drive's torque curve, so it seems reasonable that a gasoline engine would need variable gearing to get any reasonable translational force/"Thrust" at a given speed, especially when starting, so gassers basically need it for creating reasonable acceleration over the speed range whereas electric drives have great torque off the line so they don't quite need it for starting acceleration but it seems it could improve acceleration at the higher end of speeds as the torque curve declines. Might not be a problem, though, if you have the right motor and gearing system to produce reasonable acceleration over the typical driving speed(0-70mph) and gearing does add cost and weight.

Anyways, I've found these curves that depict the torque curve shape:

Gas Engine


View attachment DCmotorTorqueCurve.jpg

 

[Toorbough ULL-Zeveigh]

ur on the right track.
now superimpose both those on the same scale then the answer will be made clear.
electric has a wider bandwidth whereas ICE is a sliver, more peaky by comparison.
throw in electronic 'gearing' like the aforementioned polyphase controller & the band is even flatter & wider.
short answer, electric can get away without one, or at worst needs fewer.

 

[johnrobholmes]

I think two speeds would be perfect, spaced 2:1 apart. One gear for climbing hills with efficiency, and one for going on flats and down hills with speed. If the vehicle is heavier or needs faster acceleration or a highway cruising speed, maybe 3 gears.

 

[dogman dan]

If you are familiar with an electric potters wheel, you will see the kind of gearing we need, if any. A potters wheel with an ac motor is a bit awkward to controll with the degree of accuracy a potter needs. So they use a cone drive type setup to get an infinitely variable gearing. On a car though, we don't need that much accuracy, so a two speed automatic may appear in a few years to improve performance in the first 20 mph. If you look at car conversion simulators, you see very little is gained by using a high gear, even to go 60 mph. So a two speed would be like first gear and thrid on a 5 speed. You'd spend like 5 seconds in first, and then go to third.

 

[spinningmagnets]

IMHO, adding gears can reduce the size of the motor (cost/weight) while maintaining performance, and the added efficiency of using a smaller motor can extend the range of an existing battery pack.

Whether brushed or brushless, indexing the input can add power and efficiency to an electric motor if it is used in only one rotational direction. By using a gearbox you can use a small uni-directional motor, while still having great hill-climbing ability without overheating, long battery range, and a reverse. (it is assumed that bicycle motors dont need a reverse, and are all indexed for uni-directional best power).

To get the same power and top speed from a non-gearbox motor it will have to be larger, and if you require a reversible motor, you sacrifice some efficiency.

Because the Cyclone system uses the bikes gearset, it can climb steep hills well without overheating, and it can get great range and top speed while only using a 500/650W motor using just 36/48 volts.

A direct-drive hubmotor is always going to be the least expensive entry-level option, but you sacrifice some efficiency and range because it spins only 200-300 RPMs, and some Watts are wasted as heat. Instead of up-shifting for higher top-speed or down-shifting for better hill-climbing, you must "up" the pack voltage, leading to battery packaging and cost considerations.

For a motorcycle or car EV around town (49 MPH max) I would consider converting with a reversible non-geared motor, much easier and possibly cheaper. But, for freeway driving I would want to adapt onto an existing transmission. I'm guessing that using reverse, 3rd, and 5th on most 5-sp manuals would be close to ideal. Gasoline and electric motors all seem to run well near 3,000 RPM's.

You can get a transaxle (VW?) or differential at the salvage yards cheap for conversions, but building a new transaxle for an electric motor adds cost, weight, and complexity to a new EV purchase price.

My 18-wheeler only has 500-HP and can still haul 80,000 lbs (imagine a 100-HP car hauling 15,000 lbs, they usually weigh 3,000), but my work-truck has 14 close-ratio gears...

 

[donob08]

I agree a two speed box w/ reverse would be all that's needed. I think it would add A LOT to battery life. Yes the DC motor has lots of torque to start from a standstill, but that torque costs a lot in battery energy (current). That's why when I'm trying to get the best range out of my ebike, I always pedal from a standstill to 5 mph or so, in order to save my battery charge.