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

[mongo]

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.

And those pumps are demand speed controlled and sized appropriately, versus ICE water pump that needs to cool engine in gridlock or the oil pump that provides pressure at idle but increases flow with RPM.

Now, open the throttle valve and try that again.

So you are saying engine off, power brakes and steering disabled, going downhill with throttle open?
(Note, really don't do this with a carburated manual, you'll regret it)
Throttle losses are losses anyway.

 

[mpgxsvcd]

A vehicle like the Plaid that has a nearly flat HP curve(after the peak) does not need a multi gear transmission. If you have a flat HP curve throughout your entire rev range then having multiple gears would be meaningless for acceleration.

 

[holmgang]

A vehicle like the Plaid that has a nearly flat HP curve(after the peak) does not need a multi gear transmission. If you have a flat HP curve throughout your entire rev range then having multiple gears would be meaningless for acceleration.

depends how long the rev range is...

 

[mpgxsvcd]

depends how long the rev range is...

Constant Horsepower means gearing is irrelevant.

 

[gearchruncher]

Constant Horsepower means gearing is irrelevant.

Their point was that no system can rev to infinity, so you have some upper rev range. Plus, you can easily make a car with "constant power" if you limit the range. I mean, the M3P is constant power to 50MPH. Constant power also doesn't mean infinite torque, so you can't just slap any gear you want on the system and make it go to 200 MPH. So gearing isn't completely irrelevant.

If your upper range for "constant power" is 4000 RPM and that's 50 MPH, then a gear might help.

Clearly the Plaid method is way better though and it's awesome to see that performance EV's aren't going to need multiple gears.

 

[Sendit1]

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.

Is the "cursing axle" used when bad drivers cut the big rig off?? heh

 

[mpgxsvcd]

I mean, the M3P is constant power to 50MPH.

This is 100% false. Torque in the Model 3 Performance is nearly constant to about ~40 mph. Horsepower is not. HP rises until 55 mph. Then It starts to drop off after that.

That is the difference between the Model 3 Performance and the Plaid vehicles. Plaid vehicles have rising Horsepower until their peak but then they hold nearly that Horsepower throughout most of the rest of the rev range.

The Model 3 has very little of its rev range that could be considered nearly constant.

 

[gearchruncher]

@mpgxsvcd you are correct that I was remembering flat torque for the M3P, not flat power. Not my best moment,

Of course, a Plaid does the same thing. It's flat torque to 60 MPH, which means it's not constant HP. This is just the reality of a torque limited motor, and was my exact point that you can't just throw any gear you want at the car and get flat power. You ARE limited at low RPM where you're against the torque limit, and at some point you will also be limited by your RPM.

 

[mpgxsvcd]

@mpgxsvcd you are correct that I was remembering flat torque for the M3P, not flat power. Not my best moment,

Of course, a Plaid does the same thing. It's flat torque to 60 MPH, which means it's not constant HP. This is just the reality of a torque limited motor, and was my exact point that you can't just throw any gear you want at the car and get flat power. You ARE limited at low RPM where you're against the torque limit, and at some point you will also be limited by your RPM.

No problems. Happens to the best of us. The real distinction is that once the Plaid HP goes essentially flat then shifting gears can’t change wheel torque at all. For a given speed once HP is flat shifting gears is meaningless for acceleration.

 

[holmgang]

Constant Horsepower means gearing is irrelevant.

Horsepower is a measurement requiring rate.

I can get tons of torque driving the wheels, by employing gears for mechanical leverage.... except I would run out of revs.

 

[Darmie]

This is 100% false. Torque in the Model 3 Performance is nearly constant to about ~40 mph. Horsepower is not. HP rises until 55 mph. Then It starts to drop off after that.

That is the difference between the Model 3 Performance and the Plaid vehicles. Plaid vehicles have rising Horsepower until their peak but then they hold nearly that Horsepower throughout most of the rest of the rev range.

The Model 3 has very little of its rev range that could be considered nearly constant.

View attachment 982812

I've been looking at this chart for a few days. So, could the Model 3 Performance benefit then from a second gear? Would that push out the constant torque further and offer more HP further down the MPH? That would really sweeten up 1/4 mile racing for the hobbyist.

 

[mpgxsvcd]

Horsepower is a measurement requiring rate.

I can get tons of torque driving the wheels, by employing gears for mechanical leverage.... except I would run out of revs.

If you have constant Horsepower throughout the REV range then no matter what gear ratio you use you will have equal torque at the wheels for a given vehicle speed. This is one of the most misunderstood things in physics.

 

[mpgxsvcd]

I've been looking at this chart for a few days. So, could the Model 3 Performance benefit then from a second gear? Would that push out the constant torque further and offer more HP further down the MPH? That would really sweeten up 1/4 mile racing for the hobbyist.

Yes, for acceleration above 55 mph the Model 3 could benefit from a second gear. I went through the calculation for optimal 1/4 mile time before and the ideal second gear would require a shift at about 80 mph and that would cut motor rpm in half. It would be solidly in the 10s with weight reductions then.

The issue is that it would almost never get fully utilized. The vast majority of Model 3 owners would never exceed 90 mph. A lot of people cruise at 80 mph too. The car would be constantly shifting up and down as you went from 79-81 mph and back. It would add significant cost, complexity, and reduce efficiency for a very rarely used scenario which is 1/4 mile drag racing and roll racing. Makes no sense to actually do it.

 

[zroger73]

Horsepower = Torque x RPM / 5,252

 

[mpgxsvcd]

Horsepower = Torque x RPM / 5,252

Certainly. Calculate wheel torque at a given vehicle speed for constant HP as you change gear ratio. You will see that torque at the wheels is unchanged if HP is constant and vehicle speed is constant regardless of the gear ratio.

 

[mongo]

If you have constant Horsepower throughout the REV range then no matter what gear ratio you use you will have equal torque at the wheels for a given vehicle speed. This is one of the most misunderstood things in physics.

I think we all agree that, when operating within the flat powerband of the motor, gearing is moot (other than motor and geartrain efficiency effects).

The point they are raising is that the motor can't spin at infinite RPM, so gearing does matter.
Similarly, at the other end, power is torque limited (both at wheel and motor) so gearing matters there too.

 

[Unclepaul]

"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.

You cannot turn an ICE by hand to 5,000 rpm

 

[mariusciclistu]

@mpgxsvcd you are correct that I was remembering flat torque for the M3P, not flat power. Not my best moment,

Of course, a Plaid does the same thing. It's flat torque to 60 MPH, which means it's not constant HP. This is just the reality of a torque limited motor, and was my exact point that you can't just throw any gear you want at the car and get flat power. You ARE limited at low RPM where you're against the torque limit, and at some point you will also be limited by your RPM.

Hi. If power is constant while rpm rises then it is not ideal because the torque drops. You can view the simulation above. Look at torque if you want to see clearly. Power is just energy rate at which energy is transformed into force, but without mentioning the rotational speed to convert it to torque, it (energy rate) is useless.

 

[gearchruncher]

If power is constant while rpm rises then it is not ideal because the torque drops.

Ideal is a motor with infinite torque and RPM, 100% efficiency, and no weight. So any discussion of "ideal" is kind of pointless.

Yes, if torque stayed constant, the power would increase at higher RPM. That would be nice, as it would keep acceleration constant and the tires at their limit of traction all the way to infinite speed. We're actually getting there as many of these EV's are traction limited to 60 MPH and beyond, so effectively we do have an ideal motor in that it can overpower the tires at all times and the motor is no longer a limit on the car's acceleration.

The reality is that the battery has a power limit. If you can do 1MW, then by definition your torque is going to go down as your RPM goes up.
This is the same as an ICE engine. It can't create more than X horsepower, but it does it over a narrow RPM band. So we have to put a transmission there, which just trades torque and RPM (reducing torque and increasing RPM as you shift up gears). The power stays the same in all gears, but the torque drops.

Power is just energy rate at which energy is transformed into force, but without mentioning the rotational speed to convert it to torque, it (energy rate) is useless.

No it's not. If you tell me you're putting 1MW into a 5,000 lb car, I can tell you how quickly it will accelerate. I cannot do that with only torque or RPM, and if you give me torque and RPM, I just multiply those to get power.

There's a reason you can get a general sense of a car's speed by just knowing the horsepower (which is the same as kW), even if you don't know the RPM and torque. You assume the gearbox is converting that to a usable wheel speed and torque.

 

[mariusciclistu]

Ideal is a motor with infinite torque and RPM. So any discussion of "ideal" is kind of pointless.
Yes, if torque stayed constant, the power would increase at higher RPM. That would be nice, as it would keep acceleration constant and the tires at their limit of traction all the way to infinite speed.

I agree.

The reality is that the battery has a power limit. If you can do 1MW, then by definition your torque is going to go down as your RPM goes up.

That limit can be seen in the engine's torque curve as a result, so I also agree.

No it's not. If you tell me you're putting 1MW into a 5,000 lb car, I can tell you how quickly it will accelerate. I cannot do that with only torque or RPM, and if you give me torque and RPM, I just multiply those to get power.

There's a reason you can get a general sense of a car's speed by just knowing the horsepower (which is the same as kW), even if you don't know the RPM and torque. You assume the gearbox is converting that to a usable wheel speed and torque.

With this I don't agree. I would give you the detailed explanation but I was warned by the admin to stop posting links here, so I will not. The explanation is not short... It took me several years to understand it (rallying years + simulations on computer). Recently I found that in 1770 James Watt thought of power as Force * Velocity and also that back then power could mean "ability to act". Only in 1840 joule introduced the energy concept.

PS. Battery Power transforms into force +heat. The calculated power from measured force (torque) is only the energy rate at which the not wasted energy is transformed into force.