High speed acceleration - revisited

[MikeyC]

Thanks for admitting you were wrong in your previous blanket statement. Real-life torque curves are often not flat at all, especially high in the rev range.

A transmission is just a mechanical device to help ICE engine to stay in the power band, and a crude one at that. Imagine an efficient CVT and make your argument with that assumption. If it doesn't work, you're doing something wrong.

All you need are the first two letters: CV. Continuously variable. A CVT is nothing more than a gearbox with effectively infinite gears. It continuously changes the gear ratio. So rather than being some form of magic, even if you have a CVT that can continuously hold the peak torque RPM in the power band, your acceleration will continuously decrease as your CVT changes ratios. The only thing that changes is that acceleration will drop smoothly with speed rather than stepped dropoffs with each gear change. Bottom line: the faster you go, the more acceleration you lose.

The bottom line is the same. Both ICE vehicles and EV's will lose acceleration with speed, even if you ignore air friction. They are both "fighting physics" but in two different ways: trying to apply the same force over a greater distance.

Mike

 

[Mash]

You are confusing torque which is a unit of force and power which is work (force and distance).

This is exactly where you got confused. Power is not work. Work is power over time.

 

[Mash]

And by your logic, you can get the same amount of acceleration by starting in second gear. Or third gear. Or any gear. The fact is, even if you had an ICE engine that had a perfectly flat power curve from 0 to redline, as soon as you change gears, your acceleration will drop by the same ratio as the gear change.

Mike

The reason you start with 1st gear is that your powerband is not flat on ice. That's exactly the reason why Tesla pretty much starts on 5th gear and stays on it the whole time.

 

[MasterC17]

I am very glad Tesla was able to solve the back EMF issue with the Plaid. MUCH better solution than running a second gear like the Taycan (weight, complexity, reliability, cost). I also assumed the third motor would be geared differently; impressive what has been accomplished and bodes well for the future.

Sadly, on the other hand, a 100kWh pack is not nearly adequate enough for any sort of sustained road course use at these horsepower levels. I am guessing you will be consuming 4kWh/mi, considering the Performance 3 is over 2kWh/mi. Given you can really only use 80% of the pack, that's about 20 miles of driving before you need to find a Supercharger, as AC charging would be way too slow. That's a letdown, and the reason why I had an order in for the Plaid+ which likely would have had a 125-135kWh battery pack.

Therefore, I suppose we can only hope that they bring the motor technology to the Model 3. Even if the peak power did not change, sustained power at higher speeds would be an incredible upgrade and with the bigger 82kWh battery pack we really only need improved thermal capacity to make the 3 into a great track day car. I'll dream.

 

[Sam1]

Thanks for admitting you were wrong in your previous blanket statement. Real-life torque curves are often not flat at all, especially high in the rev range.

A transmission is just a mechanical device to help ICE engine to stay in the power band, and a crude one at that. Imagine an efficient CVT and make your argument with that assumption. If it doesn't work, you're doing something wrong.

Not worth debating it with him. It's the usual Tesla Stan stuff...try to rationalize out an anti-Tesla situation by using a technical observation, but not understanding that the application of that observation they're using, is incorrect.

 

[TIppy]

Power has nothing to do with acceleration. Accelerating from a standstill(zero rpm), the power is zero, but the torque(and therefor acceleration) in an ev can be the max available. That's why the power curve ramps up from zero at start until max power is achieved; power is torque times rpm. Higher available power allows you to maintain torque at higher rpms. Likewise, you can achieve very high torque through low gear ratios and achieve high acceleration for a brief time(as rpms will climb quickly) with low hp.

Even though plaid's hp is constant to 200 mph, the torque available is dropping with speed as the motor rpm increases. Torque is hp / rpm, so if the rpms double the torque is one half. T2 / T1 = RPM1 / RPM2. And it is torque that accelerates the car. Torque to the wheel divided by the radius of the tire is the longitudinal force applied to the car, assuming the tire doesn't break free.

Now if you want to go fast you need hp as force(drag) times velocity is power.

 

[MikeyC]

The reason you start with 1st gear is that your powerband is not flat on ice. That's exactly the reason why Tesla pretty much starts on 5th gear and stays on it the whole time.

Wrong again. You start in 1st gear because you have more wheel torque in first gear and that gets you to your powerband faster. In an ICE car, you always want to use the lowest gear possible that keeps you within your powerband. A Model 3 has a final drive ratio of 9:1 which is roughly equivalent to 2nd gear in an ICE car. You don't seem to understand that wheel torque is directly correlated to acceleration and that each time you upshift, you lose wheel torque (and therefore acceleration). If you don't understand that simple concept, there is no point continuing by trying to "teach" you.

Here's a dyno graph of a 2018 Mustang GT:

https://www.speednik.com/files/2017/10/lmr-gives-its-2018-mustang-gt-the-lie-detector-test-0001.png

Third gear in the automatic is 2.15 and fourth gear is 1.77. If you short shift at 5500 RPM in third, you'll end up at 4500 RPM in fourth: right at the engine's torque peak. According to your faulty logic, you should do that: do that and see how many races you win! In fact, 5th gear is 1.52 so according to your logic where gear ratio has no effect on acceleration, if you take third all the way to 6500 RPM and skip shift from third to 5th, you'll end up at 4600 RPM: again, right at the torque peak in the powerband. And again, I emplore you: do that in a race and see what happens.

The reason you don't do that is that is that at 6500 RPM you still have about 93% of your torque available but the moment you switch to fourth gear, you only have 83% of the torque to the wheels that you had in third just by virtue of the gear change.

Mike

 

[Mash]

I am very glad Tesla was able to solve the back EMF issue with the Plaid. MUCH better solution than running a second gear like the Taycan (weight, complexity, reliability, cost). I also assumed the third motor would be geared differently; impressive what has been accomplished and bodes well for the future.

Sadly, on the other hand, a 100kWh pack is not nearly adequate enough for any sort of sustained road course use at these horsepower levels. I am guessing you will be consuming 4kWh/mi, considering the Performance 3 is over 2kWh/mi. Given you can really only use 80% of the pack, that's about 20 miles of driving before you need to find a Supercharger, as AC charging would be way too slow. That's a letdown, and the reason why I had an order in for the Plaid+ which likely would have had a 125-135kWh battery pack.

Therefore, I suppose we can only hope that they bring the motor technology to the Model 3. Even if the peak power did not change, sustained power at higher speeds would be an incredible upgrade and with the bigger 82kWh battery pack we really only need improved thermal capacity to make the 3 into a great track day car. I'll dream.

I hear that Randy commented no power loss at 35%, so I assume something was done not only to evacuate heat, but cells are higher rating. There is no way that model 3 cells (let alone model s) would give like 12C discharge that you need with such low voltage to maintain power.

Also, a lot depends if they can do much higher power regen. If you have much better cooling and higher rating cells - why not to return 350kW of braking power.

My understanding that there is no track mode and I don't know the source of 2 week rumor for it. Without track mode and some kind of hybrid braking you can't use high regen realistically.

But, yeah, I hope they understand that amateur sport cars needs to be smaller and technology of roadster and plaid will trickle down to Model 3 RS.

P.S. It's not like they don't have back EMF, but clearly 4% loss at 160mph (and 7% at 200mph) is much better than 30% loss of Model S 2019 and 60% loss of Model 3 Performance.

 

[Mash]

Wrong again. You start in 1st gear because you have more wheel torque in first gear and that gets you to your powerband faster. In an ICE car, you always want to use the lowest gear possible that keeps you within your powerband. A Model 3 has a final drive ratio of 9:1 which is roughly equivalent to 2nd gear in an ICE car. You don't seem to understand that wheel torque is directly correlated to acceleration and that each time you upshift, you lose wheel torque (and therefore acceleration). If you don't understand that simple concept, there is no point continuing by trying to "teach" you.

Here's a dyno graph of a 2018 Mustang GT:

https://www.speednik.com/files/2017/10/lmr-gives-its-2018-mustang-gt-the-lie-detector-test-0001.png

Third gear in the automatic is 2.15 and fourth gear is 1.77. If you short shift at 5500 RPM in third, you'll end up at 4500 RPM in fourth: right at the engine's torque peak. According to your faulty logic, you should do that: do that and see how many races you win! In fact, 5th gear is 1.52 so according to your logic where gear ratio has no effect on acceleration, if you take third all the way to 6500 RPM and skip shift from third to 5th, you'll end up at 4600 RPM: again, right at the torque peak in the powerband. And again, I emplore you: do that in a race and see what happens.

The reason you don't do that is that is that at 6500 RPM you still have about 93% of your torque available but the moment you switch to fourth gear, you only have 83% of the torque to the wheels that you had in third just by virtue of the gear change.

Mike

Mike, I don't want to debate with you. I already told you that work is not power and you should check your other logic from that. Changes of gears have nothing to do with acceleration.

If you have specific power at specific speed - you have exactly same acceleration doesn't matter your gear ratio.

If you have specific wheel torque and disregarding air resistance - you have the same acceleration at any speed.

 

[MikeyC]

Mike, I don't want to debate with you. I already told you that work is not power and you should check your other logic from that. Changes of gears have nothing to do with acceleration.

If you have specific power at specific speed - you have exactly same acceleration doesn't matter your gear ratio.

If you have specific wheel torque and disregarding air resistance - you have the same acceleration at any speed.

This is the basis of your error. What do you think wheel torque is? It is engine torque multiplied by the gear ratio then multiplied by the axle ratio. As soon as you upshift, you lower the gear ratio which lowers wheel torque.

Effective torque - Wikipedia

en.wikipedia.org

You can't ignore N.

Mike

 

[TIppy]

Changes of gears have nothing to do with acceleration.

Are you saying that changing gears has no effect on acceleration? Optimum shift point is selected to keep torque to the rear wheels constant across gear changes, so it's directly related to acceleration. The higher gear ratio reduces the torque to the wheels, but the lower rpm moves to higher torque output from the engine. It's true the acceleration doesn't change at the shift point(unless you hit red line), but it drops off more slowly than if you stayed in the lower gear.

This is the basis of your error. What do you think wheel torque is? It is engine torque multiplied by the gear ratio then multiplied by the axle ratio. As soon as you upshift, you lower the gear ratio which lowers wheel torque.

What you say is true, but misses the fact that at the lower rpm the engine can generate more torque.

 

[thors_hammer]

I hear that Randy commented no power loss at 35%, so I assume something was done not only to evacuate heat, but cells are higher rating. There is no way that model 3 cells (let alone model s) would give like 12C discharge that you need with such low voltage to maintain power.

Also, a lot depends if they can do much higher power regen. If you have much better cooling and higher rating cells - why not to return 350kW of braking power.

My understanding that there is no track mode and I don't know the source of 2 week rumor for it. Without track mode and some kind of hybrid braking you can't use high regen realistically.

But, yeah, I hope they understand that amateur sport cars needs to be smaller and technology of roadster and plaid will trickle down to Model 3 RS.

P.S. It's not like they don't have back EMF, but clearly 7% loss at 200mph is much better than 30% loss at 160mph.
View attachment 672957

The article where he said it is here Unplugged Performance Reveals Its Model S Plaid Pikes Peak Racer

I'm not expecting power levels this high in the 3 obviously, but it's exciting to think about the possibilities of the other improvements. Torque vectoring, improved thermals, no/less power drop off with lower SOC, and nearly flat power band through the rpm range.

 

[Mash]

This is the basis of your error. What do you think wheel torque is? It is engine torque multiplied by the gear ratio then multiplied by the axle ratio. As soon as you upshift, you lower the gear ratio which lowers wheel torque.

Effective torque - Wikipedia

en.wikipedia.org

You can't ignore N.

Mike

If you upshift to lower power RPM - you lose torque. If you upshift to higher power RPM - you gain torque.

 

[Mash]

Are you saying that changing gears has no effect on acceleration? Optimum shift point is selected to keep torque to the rear wheels constant across gear changes, so it's directly related to acceleration. The higher gear ratio reduces the torque to the wheels, but the lower rpm moves to higher torque output from the engine. It's true the acceleration doesn't change at the shift point(unless you hit red line), but it drops off more slowly than if you stayed in the lower gear.


What you say is true, but misses the fact that at the lower rpm the engine can generate more torque.

I'm saying that the loss of acceleration at speed on gearboxes is not due to gear ratios.

Any motor is torque limited first and power limited next. Staying at maximum motor power all the time gives you the maximum wheel torque you can make.

ICE cars have a narrow powerband, so you have to use multiple gears.
Electric motor has a much wider powerband, so it's possible to make the car lighter and lose less power on a clutch.

 

[Mash]

The article where he said it is here Unplugged Performance Reveals Its Model S Plaid Pikes Peak Racer

I'm not expecting power levels this high in the 3 obviously, but it's exciting to think about the possibilities of the other improvements. Torque vectoring, improved thermals, no/less power drop off with lower SOC, and nearly flat power band through the rpm range.

I mean people comment that Tesla will make Track Mode for Model S Plaid available in 2 weeks.
But I don't see a source for that including this article.

 

[TIppy]

I'm saying that the loss of acceleration at speed on gearboxes is not due to gear ratios.

Generally, this is true. It depends on the shape of the torque curve. It may not be possible to maintain torque to the wheels across a gear change. But if you can, then, yes, the loss of acceleration at constant hp is due to the increased rpm and thus less torque.

 

[Mash]

Generally, this is true. It depends on the shape of the torque curve. It may not be possible to maintain torque to the wheels across a gear change. But if you can, then, yes, the loss of acceleration at constant hp is due to the increased rpm and thus less torque. The gear ratio does determine how fast the engine rpms increase with speed, though.

If you switch to another gear and at that point happens to be the same motor power - you will have exactly the same wheel torque and acceleration.
Gear ratio has nothing to do with acceleration - the same power at the same speed = same wheel torque = same acceleration.

 

[TIppy]

If you switch to another gear and at that point happens to be the same motor power - you will have exactly the same wheel torque and acceleration.
Gear ratio has nothing to do with acceleration - the same power at the same speed = same wheel torque = same acceleration.

Yes, if you can make the same hp which depends on the torque curve. I said, in general, I agree with what you are saying. Same hp to the rear wheels at a given speed same acceleration. The motor has to adjust its output torque to keep the hp at the wheels constant across gear changes for fixed wheel speed.

 

[play150]

Isn't it like:

Lower gears = more torque (i.e more force driving wheel rotation), but you need to upshift because you reach the engine's redline i.e RPMs can't rise further to increase the # of times the wheel is spinning per second.

So then you upshift into a higher gear where 1 spin of the engine = more spins of the wheel than previously, at the cost of there being less torque/energy/oomph per wheel rotation since now 1 engine rotation is dedicated to more wheel rotations. The decreased torque behind each wheel rotation translates to less acceleration.

e.g in Gear 1 6000 RPM = 25 mph -> Gear 2 6000 RPM = 60 mph, each engine rotation is dedicated to more wheel rotations in order to achieve the # of rotations that gives the car 60 mph.

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I always relate it to a bicycle where in the lower gear you can really accelerate (or go uphill), but you have to upshift because your legs can't spin fast enough once the speed goes up. But then try moving from a stop in the higher gear, it's more difficult and you accelerate slower (because each rotation of the pedals is being divided across several rotations of the bike wheel).

 

[TIppy]

Isn't it like:

Lower gears = more torque (i.e more force driving wheel rotation), but you need to upshift because you reach the engine's redline i.e RPMs can't rise further to increase the # of times the wheel is spinning per second.

So then you upshift into a higher gear where 1 spin of the engine = more spins of the wheel than previously, at the cost of there being less torque/energy/oomph per wheel rotation since now 1 engine rotation is dedicated to more wheel rotations. The decreased torque behind each wheel rotation translates to less acceleration.

e.g in Gear 1 6000 RPM = 25 mph -> Gear 2 6000 RPM = 60 mph, each engine rotation is dedicated to more wheel rotations in order to achieve the # of rotations that gives the car 60 mph.

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I always relate it to a bicycle where in the lower gear you can really accelerate (or go uphill), but you have to upshift because your legs can't spin fast enough once the speed goes up. But then try moving from a stop in the higher gear, it's more difficult and you accelerate slower (because each rotation of the pedals is being divided across several rotations of the bike wheel).

This would only be true if the torque from the motor was constant, but you don't shift until the torque to the wheels is the same in the new gear. This is possible because the torque ultimately falls off with higher rpm, so changing to a higher gear allows the engine to move back to a lower rpm where is makes more torque.