High speed acceleration - revisited

[play150]

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 allow the engine move back to a lower rpm where is makes more torque.

True :c After posting that I went to read about why engine torque decreases at higher RPMs and apparently it's because air can't flow in/out of the cylinders efficiently enough. So replace "being able to achieve higher RPMs" with "inability to generate torque at higher RPMs"!

I suppose the electric motor (not relying on air flowing in/out the cylinders) is one reason why the Plaid can go up to 20k+ RPMs then

 

[Mash]

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).

Lower gear means MORE torque if power is the same, but power is not the same since RPM is different.
Again, if you have the same power at the same speed it would be the same acceleration doesn't matter which gear you are in.

 

[MikeyC]

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.

You are mixing terms and making false statements. You can't have the same "motor power" after upshifting, because you'll be at a lower RPM. If you are talking torque, your statement that gear ratio has nothing to do with acceleration or wheel torque is just false. See the wiki article I posted. Wheel torque (and therefore acceleration) is a direct factor of gearing.

Look at the 2018 Mustang GT dyno graph I posted. The car has the same torque at 3200 and 6200 RPM: 350 lb/ft. At 6200 RPM the engine is outputting 350 lb/ft of torque, then you skip shift a couple gears to a gear that lands you at 3200 RPM after the shift. The engine is still outputting the same 350 lb/ft of torque but your wheel torque and acceleration drop by almost half after the shift! It's simple math.

Mike

 

[Mash]

True :c After posting that I went to read about why engine torque decreases at higher RPMs and apparently it's because air can't flow in/out of the cylinders efficiently enough. So replace "being able to achieve higher RPMs" with "inability to generate torque at higher RPMs"!

I suppose the electric motor (not relying on air flowing in/out the cylinders) is one reason why the Plaid can go up to 20k+ RPMs then

In turbo cars, you can push any amount of air-fuel mixture, so it can keep flat torque almost to the end, but it can't provide you that torque at low RPM.
Any static geometry fan (including turbine fan in the car) can only provide similar mass airflow at roughly 2x difference of air resistance.
ICE is MUCH MUCH lower torque at the crank compared to the same weight electric motor and stalls at low speed.
Most ICE is ONLY torque limited, so your power would be climbing with RPM to the redline.
So you end up using a lot of gears and your power curve over speed keeps dropping with every gear change.

As I've said before - every motor is limited first by torque (cylinder pressure or maximum electrical current or maximum gear load) and then by power (maximum RPM of turbo or maximum flow of injector or maximum temperature of coils or maximum battery cell power).

 

[Mash]

You are mixing terms and making false statements. You can't have the same "motor power" after upshifting, because you'll be at a lower RPM. If you are talking torque, your statement that gear ratio has nothing to do with acceleration or wheel torque is just false. See the wiki article I posted. Wheel torque (and therefore acceleration) is a direct factor of gearing.

Look at the 2018 Mustang GT dyno graph I posted. The car has the same torque at 3200 and 6200 RPM: 350 lb/ft. At 6200 RPM the engine is outputting 350 lb/ft of torque, then you skip shift a couple gears to a gear that lands you at 3200 RPM after the shift. The engine is still outputting the same 350 lb/ft of torque but your wheel torque and acceleration drop by almost half after the shift! It's simple math.

Mike

If you going to upshift from 6750 RPM to 5250RPM you're going to have MORE power and MORE acceleration.

You made multiple contradicting statements and I've corrected you:
1. Work is NOT power
2. ICE is not dropping power due to gearing, but due to a shift to RPM where motor power is lower.

 

[MikeyC]

Not sure where you got that very peaky graph with a torque scale that is different than the HP scale... but let's work with it. At 6750 RPM it looks like you have about 210 lb ft of torque and at 5750 RPM you have about 270 lb/ft of torque. So you get 22% more torque from the engine after shifting. The problem is, you've shifted to a gear that is 22% lower ratio and that drops wheel torque (and therefore acceleration) by 22%. So in fact, you'll have the same acceleration after shifting and then, only for an instant as at 5250 RPM, the curve has a pretty big negative slope. So just a fraction of a second after shifting, you'll have less acceleration than you did in the previous gear at 6750 RPM.

Mike

 

[TIppy]

Not sure where you got that very peaky graph with a torque scale that is different than the HP scale... but let's work with it. At 6750 RPM it looks like you have about 210 lb ft of torque and at 5750 RPM you have about 270 lb/ft of torque. So you get 22% more torque from the engine after shifting. The problem is, you've shifted to a gear that is 22% lower ratio and that drops wheel torque (and therefore acceleration) by 22%. So in fact, you'll have the same acceleration after shifting and then, only for an instant as at 5250 RPM, the curve has a pretty big negative slope. So just a fraction of a second after shifting, you'll have less acceleration than you did in the previous gear at 6750 RPM.

Mike

The large negative slope applies to both gears. If you stayed in the lower gear the torque would also decrease as your speed increased, but at 6750 rpm the torque is about to fall off a cliff or that's red line. The higher gear is higher up on the torque curve so it can continue to 6750 rpm again. Both gears provide about the same torque to the rear wheels until the lower gear hits the curve where it takes a sharp dive. This is where the higher gear begins to prevail. Since the hp is a little higher at 5250, the acceleration would be better in the higher gear.

It would be better to have the shift points straddle peak horsepower. That way you have the most torque to the rear wheels at any given speed.

 

[Mash]

I'm going to repost the updated chart I made.
I don't know why nobody makes acceleration vs speed chart - that would be an actual representation of perceived performance.

Model 3 Performance is limited by torque way below tires limit and at speed is limited by rolling and air drag.
Model S Plaid is limited by torque around tire limit and at maximum speed rolling and air resistance only eat 35% of power, so it's limited by motor RPM.

Model 3 Performance is NOT very fast at high speed - many cars with the same price can beat it.
Plaid solves all those highway GT bragging needs. Doesn't matter what is the speed - it's just accelerating faster than anything short of a few hypercars.
If it had a gearbox it theoretically could have reached ~260mph, but even 200mph is quite stupid on Autobahn.

To be tire limited up to 200mph it would need to be ~4`500hp (~10`000 hp with street usable aero).
That would be a quarter-mile below 7 seconds at ~300 mph. Below 6 with cold thrusters.
Theoretically, 4680 cells with 40kg supercapacitors buffer can be that powerful.
Motors would need 10x cooling power - big pumps, compressor, radiators, but it can be done with today's technologies.
But there is no point in doing that now, so Roadster is going to be probably 2MW 200kWH tire limited to 100mph.
Still fast enough to have no chance during the crash.

I believe M3P is still a better track car (even if slower) than Plaid though, but this is a thread about the highway bragging.

 

[Mash]

Not sure where you got that very peaky graph with a torque scale that is different than the HP scale... but let's work with it. At 6750 RPM it looks like you have about 210 lb ft of torque and at 5750 RPM you have about 270 lb/ft of torque. So you get 22% more torque from the engine after shifting. The problem is, you've shifted to a gear that is 22% lower ratio and that drops wheel torque (and therefore acceleration) by 22%. So in fact, you'll have the same acceleration after shifting and then, only for an instant as at 5250 RPM, the curve has a pretty big negative slope. So just a fraction of a second after shifting, you'll have less acceleration than you did in the previous gear at 6750 RPM.

Mike

I'm telling you - math is simple. Gears don't matter - what matters is motor power at vehicle speed always defines your acceleration.
In that particular example, you would immediately have 15% faster acceleration with upshifting 6750->5250.

 

[TIppy]

I don't know why nobody makes acceleration vs speed chart - that would be an actual representation of perceived performance.

There are plots of speed vs time. If you plot the slope of those curves against speed, that would give you acceleration vs speed. At a chosen speed on the plot, find the time and do a delta speed over delta time around that time for small delta time. Then plot that calculated acceleration vs the chosen speed.

 

[Mash]

There are plots of speed vs time. If you plot the slope of those curves against speed, that would give you acceleration vs speed. At a chosen speed on the plot, find the time and do a delta speed over delta time around that time for small delta time. Then plot that calculated acceleration vs the chosen speed.

Thanks, but I know how to calculate it.
I'm merely saying that such chart instead of dyno/0-60/quarter-miles would give much more realistic information to consumers.
A lot of cars don't really have properly measured speed vs time charts either.

 

[MikeyC]

I'm telling you - math is simple. Gears don't matter - what matters is motor power at vehicle speed always defines your acceleration.
In that particular example, you would immediately have 15% faster acceleration with upshifting 6750->5250.

Your choice if you want to continue believing a false statement (that gears don't matter WRT acceleration). Wheel torque is cut by the change in gear ratio each time you shift. At that point, you factor in how much wheel torque you lost to the gear change versus how much engine torque you gained by being more in the powerband. You have to consider both. There are thousands of these graphs online. This is just one:

http://teamghettoracing.com/wp-content/uploads/2015/03/Figure_04-1024x768.png

If you want to know where to shift, the optimal shift is dictated by the moment at which the ratio gain in engine torque exceeds the amount of wheel torque you lost to the change in gear ratio. In this graph, it's where the lines intersect:

http://teamghettoracing.com/wp-content/uploads/2015/03/Figure_09-1024x768.png

Mike

 

[Mash]

Your choice if you want to continue believing a false statement (that gears don't matter WRT acceleration). Wheel torque is cut by the change in gear ratio each time you shift. At that point, you factor in how much wheel torque you lost to the gear change versus how much engine torque you gained by being more in the powerband. You have to consider both. There are thousands of these graphs online. This is just one:

http://teamghettoracing.com/wp-content/uploads/2015/03/Figure_04-1024x768.png

If you want to know where to shift, the optimal shift is dictated by the moment at which the ratio gain in engine torque exceeds the amount of wheel torque you lost to the change in gear ratio. In this graph, it's where the lines intersect:

http://teamghettoracing.com/wp-content/uploads/2015/03/Figure_09-1024x768.png

Mike

Most of what you say is correct and I never debated that, but...
You absolutely not reading what I'm telling you and flooding the thread with repeating messages.
You ridiculously changing my statements to different meanings.

It's a mix of confirmation bias and a strong desire to never be wrong.
We have a very similar guy who has an eternal need to prove that only tires relevant for braking.
I'm not interested in such a debate, because it's not.

 

[MikeyC]

Most of what you say is correct and I never debated that, but...
You absolutely not reading what I'm telling you and flooding the thread with repeating messages.
You ridiculously changing my statements to different meanings.

It's a mix of confirmation bias and a strong desire to never be wrong.
We have a very similar guy who has an eternal need to prove that only tires relevant for braking.
I'm not interested in such a debate, because it's not.

OK, I agree the back and forth is a bit of flooding comms. So I'll ask you one last question to clarify because maybe we're talking about two different things. I made the statement that ICE cars lose wheel torque to gearing while EVs lose wheel torque to back EMF. You seemed to object to that and kept making a statement that "gearing has nothing to do with acceleration" and making a statement that if your engine has the same power at the same speed, you'll have the same acceleration: doesn't matter what gear you are in. So I have to know...

Let's say we have an ICE engine that has 400 lb/ft of torque and 400 HP at 5250 RPM. We don't need the whole curve because we are just going to measure acceleration at 5250 RPM. In the first case, we are full throttle in first gear and we measure the acceleration when the engine hits 5250 RPM. In the second case, we go full throttle in second gear and measure the acceleration at the same 5250 RPM. The engine has the same torque/power in both cases. Are you saying the acceleration will be the same in both first and second at the same 5250 RPM? The engine is outputting the same power. The only thing that has changed is the gear. So you're saying the acceleration at 5250 RPM (or any RPM for that matter) would be the same in first and second? We can ignore air resistance. It probably won't contribute a lot in first/second at lower speeds anyway.

If you answer yes, I'll label myself done and head for the door!

Mike

 

[play150]

 

[TIppy]

OK, I agree the back and forth is a bit of flooding comms. So I'll ask you one last question to clarify because maybe we're talking about two different things. I made the statement that ICE cars lose wheel torque to gearing while EVs lose wheel torque to back EMF. You seemed to object to that and kept making a statement that "gearing has nothing to do with acceleration" and making a statement that if your engine has the same power at the same speed, you'll have the same acceleration: doesn't matter what gear you are in. So I have to know...

Let's say we have an ICE engine that has 400 lb/ft of torque and 400 HP at 5250 RPM. We don't need the whole curve because we are just going to measure acceleration at 5250 RPM. In the first case, we are full throttle in first gear and we measure the acceleration when the engine hits 5250 RPM. In the second case, we go full throttle in second gear and measure the acceleration at the same 5250 RPM. The engine has the same torque/power in both cases. Are you saying the acceleration will be the same in both first and second at the same 5250 RPM? The engine is outputting the same power. The only thing that has changed is the gear. So you're saying the acceleration at 5250 RPM (or any RPM for that matter) would be the same in first and second? We can ignore air resistance. It probably won't contribute a lot in first/second at lower speeds anyway.

If you answer yes, I'll label myself done and head for the door!

Mike

This is different, because you are now talking about the same hp at different speeds, not the same hp at the same speed. At 5250 rpm in first gear you are going slower than 5250 rpm in second gear. For a given hp you can accelerate more quickly the slower you are going. The same hp is going to the wheels, but in the lower gear the rear wheels are spinning more slowly, so the torque and acceleration are greater. In the earlier discussions, the engine rpm changed so that the wheel rpms stayed the same. Yet because of the shape of the torque curve, you can sometimes actually accelerate more quickly in higher gears when you are going the same speed.

At a given speed (any speed) you want to pick the gear that gives you the highest hp from the engine for max acceleration. A higher gear can move the rpms back into the power band.

 

[MikeyC]

Yet because of the shape of the torque curve, you can sometimes actually accelerate more quickly in higher gears when you are going the same speed.

This is actually very rare. You need a very peaky torque curve to end up with higher acceleration in a higher gear. Most modern engines have a relatively flat torque curve between 3000 and 6000 RPM, varying by maybe 10-15% (350-400 lb/ft for example) throughout that range. And transmissions are designed to keep you in that powerband once you reach it. I'm not saying it's flat or that you shouldn't account for the curve, but you need a pretty big drop in torque near redline to make changing to the next gear produce more acceleration: why it doesn't make sense to short shift most cars. And even if you do briefly get more acceleration, you're still following the same curve (only at lower wheel torque) so as you increase speed, you'll quickly end up with less acceleration than the previous gear.

A good example is the Mustang GT graph I posted. There is at most a 12.5% drop in engine torque between 3000 and 6000 RPM but the gear changes produce a larger drop in wheel torque:

1-2: 36%
2-3: 28%
3-4: 18%
4-5: 14%
5-6: 16%
6-7: 22%
7-8: 15%
8-9: 19%
9-10: 7%

So on this car (like the vast majority of performance engines), you lose some acceleration each time you shift because you ran out of usable RPM.

Mike

 

[TIppy]

This is actually very rare. You need a very peaky torque curve to end up with higher acceleration in a higher gear. Most modern engines have a relatively flat torque curve between 3000 and 6000 RPM, varying by maybe 10-15% (350-400 lb/ft for example) throughout that range. And transmissions are designed to keep you in that powerband once you reach it. I'm not saying it's flat or that you shouldn't account for the curve, but you need a pretty big drop in torque near redline to make changing to the next gear produce more acceleration: why it doesn't make sense to short shift most cars. And even if you do briefly get more acceleration, you're still following the same curve (only at lower wheel torque) so as you increase speed, you'll quickly end up with less acceleration than the previous gear.

A good example is the Mustang GT graph I posted. There is at most a 12.5% drop in engine torque between 3000 and 6000 RPM but the gear changes produce a larger drop in wheel torque:

1-2: 36%
2-3: 28%
3-4: 18%
4-5: 14%
5-6: 16%
6-7: 22%
7-8: 15%
8-9: 19%
9-10: 7%

So on this car (like the vast majority of performance engines), you lose some acceleration each time you shift because you ran out of usable RPM.

Mike

If you are on the back side of the power curve, that is, the power is decreasing as rpms increase, any gear shift that lowers the rpms will increase hp at your current speed. So you will accelerate faster in the higher gear. This is true unless it lowers the rpms so much you are on the other side of peak hp and the power is less than what it was in the lower gear.

If the power is higher in the higher gear, the final torque to the wheels will be higher, taking into account both the increased torque from the motor and the fact it is reduced due to the taller gear ratio. And even though the torque will decrease as the speed increases, it will still be higher than if the lower gear had been selected at these higher speeds.

 

[MikeyC]

If you are on the back side of the power curve, that is, the power is decreasing as rpms increase, any gear shift that lowers the rpms will increase hp at your current speed. So you will accelerate faster in the higher gear.

You lost me on that one... or I'm misunderstanding. Any gear shift that lowers RPM will lower engine HP by definition: HP is torque multiplied by (a factor of) RPM.

Mike

 

[TIppy]

You lost me on that one... or I'm misunderstanding. Any gear shift that lowers RPM will lower engine HP by definition: HP is torque multiplied by (a factor of) RPM.

Mike

I think this is where the disconnect is. Lowering the rpm can increase power if you are past the peak hp rpm. Hp increases as rpm increase until it reaches a peak. then any increase in rpm lowers the hp the engine can generate. This is a consequence of the torque falling off faster with rpms than rpms are increasing.