Why Tesla doesn't use an "overdrive" for better range?

[MikeRTX]

Seems like a higher gear that operates like the old overdrive on ICE cars would greatly extend range by causing the drive motor(s) to spin more slowly while maintaining the same road speed. Battery use increases with speed, so that would "fool" the motors into thinking they're going at a slower speed. The overdrive would automatically engage as the car reached a certain highway speed, then disengage as the car slowed to a lower speed.

I see no EV maker uses an overdrive, so there must be some reason why it's not practical. Anyone know?

 

[zoomer0056]

Good question. That would be adding a transmission to the system of multiple motors. That seems complicated and heavy.

 

[804son]

Battery use increases with speed, as a function of the wind resistance of the vehicle (motor load), not so much as a function of the motor rpm.

 

[LoudMusic]

Electric motors, certainly the ones in electric cars, deliver more consistent torque throughout the RPM range than an internal combustion engine. That's why selectable transmissions aren't being used anymore. Some previous EVs did have multiple gear transmissions but it's been determined that isn't necessary.

Transmissions are an inherent power loss. The engine or motor would have to turn the parts of the transmission in addition to wheels, with more weight and friction in motion. Even if you're gaining a few percent at interstate speeds, you're losing so much more in the transmission that it isn't worth the end result.

This is a similar conversation to, "Why don't EVs have solar panels?". The benefit is so small and the deficit for carrying the equipment is so large, you'd be better off spending the money and weight in a bigger battery.

There may be some specific use cases where large gearing would be truly beneficial to an EV, such as a long haul truck, but likely not enough in private vehicles to warrant the research, development, and manufacturing, for a company that has no trouble making sales as it is.

 

[bruce4000]

That shows you how awesome / efficient motors are.

 

[smogne41]

The Taycan has a 2 speed transmission on the rear motor, but it is for performance reasons not efficiency. As others said, the gearing is not a major source of efficiency losses in modern EV motors, usually one ratio covers everything reasonably well. Not to say it is nothing, but usually not worth the effort/complication. The Tesla Semi does actually do a form of this though, it has a 'cursing axle' and a 'torque axle', which can be fully disengaged for efficient highway travel. I believe the Hyundai/KIA eGMP platform cars have clutched front drive units for a similar reason.

 

[zroger73]

And, aren't the drive units in AWD Teslas optimized for power in the rear and economy in the front, so the rear wheels get more power while accelerating then the front wheels take over while cruising?

 

[MP3Mike]

And, aren't the drive units in AWD Teslas optimized for power in the rear and economy in the front, so the rear wheels get more power while accelerating then the front wheels take over while cruising?

The manual for the Model Y says that the gear ratio is the same front/rear. Most Teslas use a PM motor on one end and an induction motor on the other. (Not true of the current Model S/X that use PM on both ends.) So they can use just the PM motor for cruising, but they don't disconnect the induction motor.

 

[gearchruncher]

Battery use increases with speed, so that would "fool" the motors into thinking they're going at a slower speed.

Physics hates this one weird trick!

 

[Darmie]

Coming from the RC racing world, when you gear up, things can heat up and will pull more amperage ie, deplete the battery quicker.
So, gearing up through a transmission will not have the benefit that you are thinking of with an electric motor.

I'm so amazed how Tesla has figured out how to make the Model S Plaid the quickest production car and can reach speeds over 200 MPH all without a transmission.

 

[richrootes]

I think the cars already do this with ‘torque sleep’ - introduced seven or eight years ago

 

[mongo]

Seems like a higher gear that operates like the old overdrive on ICE cars would greatly extend range by causing the drive motor(s) to spin more slowly while maintaining the same road speed. Battery use increases with speed, so that would "fool" the motors into thinking they're going at a slower speed. The overdrive would automatically engage as the car reached a certain highway speed, then disengage as the car slowed to a lower speed.

I see no EV maker uses an overdrive, so there must be some reason why it's not practical. Anyone know?

Lower motor speed at the same power level (which depends on vehicle speed) means higher motor current which is likely less efficient due to resistive losses which are proportional to the square of current.

 

[bruce4000]

The Taycan has a 2 speed transmission on the rear motor, but it is for performance reasons not efficiency. As others said, the gearing is not a major source of efficiency losses in modern EV motors, usually one ratio covers everything reasonably well. Not to say it is nothing, but usually not worth the effort/complication. The Tesla Semi does actually do a form of this though, it has a 'cursing axle' and a 'torque axle', which can be fully disengaged for efficient highway travel. I believe the Hyundai/KIA eGMP platform cars have clutched front drive units for a similar reason.

For Tesla plaid, I think they went with the carbon fiber “motor rap” that allows it to spin at higher rpm .

 

[gearchruncher]

For Tesla plaid, I think they went with the carbon fiber “motor rap” that allows it to spin at higher rpm .

Common misconception that even Elon has.
The Model 3 and Base Model S motors spin faster at the 162 MPH top speed than the Plaid motor does at 200 MPH because the Plaid runs 7.56 gear ratio vs 9.04.

The carbon wrapping is to maintain a smaller rotor-stator gap and support a very unique laminate core structure, not improve overall RPM handling.

 

[CKwik240]

Seems like a higher gear that operates like the old overdrive on ICE cars would greatly extend range by causing the drive motor(s) to spin more slowly while maintaining the same road speed. Battery use increases with speed, so that would "fool" the motors into thinking they're going at a slower speed. The overdrive would automatically engage as the car reached a certain highway speed, then disengage as the car slowed to a lower speed.

I see no EV maker uses an overdrive, so there must be some reason why it's not practical. Anyone know?

An electric motor that turns slower to produce the same torque at the wheels will have to produce more torque at the motor. If you ignored losses, it would be the exact same power output as electrical current draw has to increase. There are some differences in efficiency, but it's likely quite small so the benefits would be rather small.

Engines also require more torque at lower engine speeds to make the same wheel torque, but the efficiency differences at different speeds are more drastic due to windage inside the crankcase at higher engine speeds, and a slower turning engine at the same power output will require the throttle to be open more to increase the engine torque output which reduces pumping losses as it does not have to fight as much vacuum to suck air into the cylinder.

 

[holmgang]

Battery use increases with speed, so that would "fool" the motors into thinking they're going at a slower speed.

The software engineers simply need to divide the speed readout by 10 to decrease battery usage.

Elon, PM me for a CV

 

[Sophias_dad]

Let us not forget to look at the ICE side of the equation as well. A slower engine speed benefits an ICE, at least in part because every time a piston(or pretty much anything) moves in an ICE, there's a TON of friction. If nothing else, you are sliding a bunch(12,18, or 24 usually) of tight fitting piston rings back and forth in their cylinders, and if you can get away with doing that less there's less wasted energy(heat from friction). Most of the big bearings in an ICE engine are also not ball bearings, which adds even more friction.

Obviously there's a limit where other factors make slowing the engine down further impractical, whether it causes inefficient combustion or unpleasant vibration, or something else.

 

[zroger73]

A slower engine speed benefits an ICE, at least in part because every time a piston(or pretty much anything) moves in an ICE, there's a TON of friction.

"Ton" is subjective. You can turn a crankshaft on an ICE by hand if the valves are open. Most of the resistance to turning is due to the compression of the air in the cylinder and the valve springs and most of the that energy is returned on the down stroke. There is layer of oil between the piston rings and cylinder walls and between the crankshaft and journal bearings to prevent metal-to-metal contact. Friction certainly is a factor in ICE efficiency, but it's a very small piece of the puzzle.

 

[mongo]

"Ton" is subjective. You can turn a crankshaft on an ICE by hand if the valves are open. Most of the resistance to turning is due to the compression of the air in the cylinder and the valve springs and most of the that energy is returned on the down stroke. There is layer of oil between the piston rings and cylinder walls and between the crankshaft and journal bearings to prevent metal-to-metal contact. Friction certainly is a factor in ICE efficiency, but it's a very small piece of the puzzle.

Oil and water pumps waste energy and most energy losses increase with speed, so ability to rotate by hand is not very representative of operational losses.

While some compressive energy is returned, consider that standard ICE engine braking (not diesel Jacob's style compression dump) is highly effective at slowing a vehicle.

 

[zroger73]

Oil and water pumps waste energy and most energy losses increase with speed, so ability to rotate by hand is not very representative of operational losses.

Our Teslas have oil and water pumps, too - one oil pump in each drive unit and either 2 (3/Y) or 3 (S/X) water pumps. A typical ICE-powered vehicle has only one oil pump and one water pump.

While some compressive energy is returned, consider that standard ICE engine braking (not diesel Jacob's style compression dump) is highly effective at slowing a vehicle.

Now, open the throttle valve and try that again.