High speed acceleration - revisited

[MikeyC]

Sure, but the brief hit you're taking is because the hp is less, not because of any specific gear ratio.

I agree with what you said. I think this is tomato vs tomahto at this point. Your logic is fine in a theoretical example. The problem here is that in reality, you can rarely design your gear ratios so the gear change doesn't cause a loss of power. In order for the gear change itself to not cause a loss of power, your RPM after the shift cannot cause you to land at an RPM on the dyno graph that has less power than where you left off in the previous gear. Manufacturers can rarely do this because it would be counterproductive to do things like have 8 shifts in a 1/4 mile. So what ends up happening is something like this:

In order to not have a loss of overall power delivery due to a gear change, you'd have to run each gear from only 6500 to 7500 RPM. In reality and depending on which shift we are talking about, you get more like the "Actual" range in the above. The part between 5400 and 6500 RPM is lower power than where you shifted from so you have less power because of the gear change... or more accurately, because you can't always pick optimal gears.

In addition, you have more frictional losses with a transmission (compared to an EV) so you lose a minimum of 5% power there plus some time lost with each gear change itself. So my point is still that: both ICE with transmissions and EVs both lose some efficiency up top. ICE loses some power over the run due to not being able to run the optimal RPM range in each gear while an EV may lose power due to decreasing efficiency of the motor at higher RPM... although that doesn't seem to be the case with the Plaid S. It just depends on how good the EV motor is to determine whether or not one is better than the other.

Mike

 

[TIppy]

View attachment 673731

These guys have ruined this thread with their uniformed opinions that contain no information. So nobody wants to read them.

 

[MikeyC]

View attachment 673736
These guys have ruined this thread with their uniformed opinions that contain no information. So nobody wants to read them.

I like the funny posts because we're all just pondering and being civil. I only wonder: which guy am I? Wait. Don't answer that.

Mike

 

[TIppy]

I agree with what you said. I think this is tomato vs tomahto at this point. Your logic is fine in a theoretical example. The problem here is that in reality, you can rarely design your gear ratios so the gear change doesn't cause a loss of power. In order for the gear change itself to not cause a loss of power, your RPM after the shift cannot cause you to land at an RPM on the dyno graph that has less power than where you left off in the previous gear. Manufacturers can rarely do this because it would be counterproductive to do things like have 8 shifts in a 1/4 mile. So what ends up happening is something like this:

View attachment 673721
In order to not have a loss of overall power delivery due to a gear change, you'd have to run each gear from only 6500 to 7500 RPM. In reality and depending on which shift we are talking about, you get more like the "Actual" range in the above. The part between 5400 and 6500 RPM is lower power than where you shifted from so you have less power because of the gear change... or more accurately, because you can't always pick optimal gears.

In addition, you have more frictional losses with a transmission (compared to an EV) so you lose a minimum of 5% power there plus some time lost with each gear change itself. So my point is still that: both ICE with transmissions and EVs both lose some efficiency up top. ICE loses some power over the run due to not being able to run the optimal RPM range in each gear while an EV may lose power due to decreasing efficiency of the motor at higher RPM... although that doesn't seem to be the case with the Plaid S. It just depends on how good the EV motor is to determine whether or not one is better than the other.

Mike

@Dolemite, don't read this.

It doesn't matter that there is power loss across gear changes. It's the same loss for each gear change. So the average power over the gear shift rpm range will be constant, and dividing that average power by rpm will give you the average torque at a given speed.

Apparently, lots of cars have 8-speed transmission. Toyota even make a 10-speed now.

Why So Many Carmakers Use ZF's Eight-Speed Automatic

It's on-par with dual-clutch in terms of performance, but smoother, cheaper, and easier to package. It's no wonder so many cars use it.

www.roadandtrack.com

 

[TIppy]

I like the funny posts because we're all just pondering and being civil. I only wonder: which guy am I? Wait. Don't answer that.

Mike

This was not directed at you!

 

[MikeyC]

@Dolemite, don't read this.

It doesn't matter that there is power loss across gear changes. It's the same loss for each gear change. So the average power over the gear shift rpm range will be constant, and dividing that average power will give you the average torque at a given speed.

Apparently, lots of cars have 8-speed transmission. Toyota even make a 10-speed now.

Why So Many Carmakers Use ZF's Eight-Speed Automatic

It's on-par with dual-clutch in terms of performance, but smoother, cheaper, and easier to package. It's no wonder so many cars use it.

www.roadandtrack.com

No argument! But not being able to keep the power close to peak is a compromise caused by gearing. And in an EV, not being able to keep power optimal is a compromise caused by electric motors being inefficient at the very high RPM they have to run. The good thing for ICE is, like you said, assuming each gear runs a similar RPM, you don't continue losing more and more power the faster you go. So at some point, the EV will have lost more efficiency to back EMF than the ICE lost due to gearing not being able to keep peak power. At that point, the ICE comes past the EV. Unless of course the EV is a Plaid S.

Mike

 

[Mash]

HP is a consequence, not a cause! HP increases linearly until about 5500 RPM on the Plaid S and then stays relatively flat until 20,000 RPM. All that means is that from about 60 MPH to 200 MPH, your acceleration is decreasing because torque to the wheels is decreasing. The cause of this is back EMF in electric motors. In an ICE, the cause is volumetric efficiency. So what's your point?

Now, in the case of the Plaid S, you could add another gear but it would make zero difference to acceleration. Why? Because the HP curve is flat. All that means is that when you change to 2nd gear, the increase in wheel torque due to lower RPM is the same (ratio wise) as the loss of wheel torque due to the ratio change of the gears. You could use any gear (ratio) you want and you'd get the same acceleration (within the flat part) because the the increase in wheel torque that you'd get at the lower RPM is canceled out by the decrease in wheel torque by running a lower gear ratio. Unfortunately ICE doesn't have flat HP curves so gear ratio DOES matter!

Mike

The reason why Plaid motors losing torque is not back emf, but simply the power limit defined around maximum cooling and what's safe for the battery. ICE is limited in HP by max air mass flow and safe EGT limits.

 

[MikeyC]

This is just a recent video that is a point of reference. I see this same scenario repeated over and over:

If you watch the position of the RS7, it starts to gain on the Model 3 when the Model 3 crosses the first set of double cones (1/4 mile). At that point, the model 3 is going 190 KPH (118 MPH). Ignore the position of the cars because the Model 3 driver knows to leave on the last yellow and got the jump. I'm just talking about the speed at which the RS7 starts to out-accelerate (gain on) the Model 3 which starts happening right at those first two cones. And the RS7 has 590 HP. So however you want to cut it or explain it, the Model 3 becomes less efficient as a package than a relatively similar performance ICE car above 110-120 MPH. Below that it is as efficient or more efficient at generating acceleration.

Mike

 

[gearchruncher]

So however you want to cut it or explain it, the Model 3 becomes less efficient as a package than a relatively similar performance ICE car above 110-120 MPH. Below that it is as efficient or more efficient at generating acceleration.

Your overall observation is correct, but "efficent" isn't the right term. The Model 3 is not wasting power as it goes faster. It's just unable to create as much power, and the energy being drawn from the battery is going down.

No different than an ICE engine that is at high rpm and reducing in torque. The fuel flow is also decreasing. It's making less power, but that doesn't mean it's less efficent at converting fuel to power.

Also, high HP cars always do better at high speeds, where aero drag starts to dominate. Yep, a 590 HP car can go faster than a 400 HP car, even if it weighs way more and takes longer to get to 100 mph.

 

[MikeyC]

Yeah, maybe efficiency is the wrong term. I think of it as efficiency because when a powerplant can't produce its peak power in a certain range, seems like it is less efficient at delivering power. Even if you have to pull power from the battery, there must be a reason: I take that as a sort of energy delivery efficiency deficit.

Mike

 

[gearchruncher]

think of it as efficiency because when a powerplant can't produce its peak power in a certain range, seems like it is less efficient at delivering power.

You mean kinda like how an average ICE engine can't produce much torque at 2,000 RPM or 10,000 RPM, and is massively inefficient in converting fuel to shaft power, wasting at least 60% of it?

All engines have a torque curve. ICE goes up, then down. We HAVE to have transmissions, or they are pretty useless because the RPM range is very narrow.
EV's start at peak, and then maintain that until they decrease. This range is so wide, that a transmission is often not needed to get amazing real world performance. But you aren't calling an ICE engine inefficient even though it has a narrower powerband than an EV, and we all know about dyno charts for an ICE engine are way more important than a single peak HP number.

So yes, in one sense, a single speed gearbox EV as a whole is less efficient at using the capabilities of the motor to maximize top speed. In just about everything else (0-100 acceleration, total powerplant weight, parts count, cost, total energy used) it is phenomenally efficient.

Big strides are being made in this area- the Plaid motors in the Model S can now maintain torque up to about 20,000 RPM, which is 200 MPH as geared. The Porsche Taycan uses a 2 speed transmission. This is not a fundamental limitation of EV's, just an indication of being about 20 years into EV development vs 140 years for ICE, yet the EV's are already dominant in many environments.

 

[MikeyC]

Big strides are being made in this area- the Plaid motors in the Model S can now maintain torque up to about 20,000 RPM, which is 200 MPH as geared.

Minor correction: the Plaid motors maintain HP, not torque. Torque steadily falls from about 6000 to 20,000 RPM. If torque didn't decrease over that range, the HP curve wouldn't be nearly flat.

Mike

 

[Mash]

Minor correction: the Plaid motors maintain HP, not torque. Torque steadily falls from about 6000 to 20,000 RPM. If torque didn't decrease over that range, the HP curve wouldn't be nearly flat.

Mike

Yep, HP would be climbing up after 60mph. And would need 4000hp to keep it going. But thats not due to back emf. It's due to the cooling, battery and invertors power limits.

 

[dsgerbc]

it starts to gain on the Model 3 when the Model 3 crosses the first set of double cones (1/4 mile).

Visible gaining comes way later than the point where acceleration is the same. A car behind by a constant time interval will appear to be visibly falling back due to increasing speed. The RS7 starts catching up (as in cutting the time delta) much sooner than at 118mph.

 

[MikeyC]

Visible gaining comes way later than the point where acceleration is the same. A car behind by a constant time interval will appear to be visibly falling back due to increasing speed. The RS7 starts catching up (as in cutting the time delta) much sooner than at 118mph.

I was referring to the point in the video where the car behind stayed at the same distance for some time, and you can tell when it started to get closer to the lead car. We're talking about the distance between car A and car B, not about being behind by the same time interval. At the moment car B behind car A starts to close the distance gap, it is out-accelerating it.

Mike

 

[dsgerbc]

I was referring to the point in the video where the car behind stayed at the same distance for some time, and you can tell when it started to get closer to the lead car. We're talking about the distance between car A and car B, not about being behind by the same time interval. At the moment car B behind car A starts to close the distance gap, it is out-accelerating it.

Mike

If two cars are accelerating exactly the same but one car is 1 second ahead, the gap is growing in line with speed. If the gap is growing at a rate slower than that at which cars are gaining speed, the second car is out-accelerating the first.

When the distance starts shrinking, the second car has been out-accelerating the first for quite a while already. It's just math, try it.

 

[MikeyC]

If two cars are accelerating exactly the same but one car is 1 second ahead, the gap is growing in line with speed. If the gap is growing at a rate slower than that at which cars are gaining speed, the second car is out-accelerating the first.

When the distance starts shrinking, the second car has been out-accelerating the first for quite a while already. It's just math, try it.

Not sure what magic you think occurs at higher speed that would create a larger gap at higher speed if two cars are accelerating at the same rate. We are talking about one car's speed relative to the other. If two cars are lined up side by side and they both take off at 1G acceleration, both cars will stay side by side regardless of speed. Now move one car 20 feet back and start them at the exact same time and accelerate at 1G. The lead car will always stay 20 feet ahead of the trailing car. The gap will not change. But as soon as the car in the back starts accelerating at 1.01G, it will start closing the gap: 19 feet, 18 feet, until eventually it'll pass the lead car. Sure, the faster you go, the more distance they'll cover in the same time... but that is irrelevant. It's all relative.

Mike

 

[Mash]

Not sure what magic you think occurs at higher speed that would create a larger gap at higher speed if two cars are accelerating at the same rate. We are talking about one car's speed relative to the other. If two cars are lined up side by side and they both take off at 1G acceleration, both cars will stay side by side regardless of speed. Now move one car 20 feet back and start them at the exact same time and accelerate at 1G. The lead car will always stay 20 feet ahead of the trailing car. The gap will not change. But as soon as the car in the back starts accelerating at 1.01G, it will start closing the gap: 19 feet, 18 feet, until eventually it'll pass the lead car. Sure, the faster you go, the more distance they'll cover in the same time... but that is irrelevant. It's all relative.

Mike

Eh. Why are you insisting on wrong math again? Cars would be decreasing gap based on speed. If one was accelerating slower and they flip, obviously ICE would need to get up to EV speed first by having faster acceleration.

 

[MikeyC]

Eh. Why are you insisting on wrong math again? Cars would be decreasing gap based on speed. If one was accelerating slower and they flip, obviously ICE would need to get up to EV speed first by having faster acceleration.

Not sure what "math" you are using unless you are talking about the lead car starting at a higher speed. I'm talking about the video I posted: two cars in a race where the lead car gets the initial jump and at higher speeds, we see the trailing car stay the same distance behind (~20 feet) for about 3 seconds, matching the speed of the lead car. At this point (during that 3 seconds where the two cars are "locked" relative to one another) they have the same acceleration. Sure, prior to that point, the trailing car must have momentarily had slightly higher acceleration at some point (probably during optimal power in one gear) in order to catch the speed of the first car. But if we are looking at higher speeds and we look at those 3 seconds where the cars have the same acceleration and we want to know when the trailing car starts consistently out-accelerating the lead car, it is only when the trailing car has more acceleration.

So looking at that part of the race, I'll say it again. If two accelerating cars have a known gap and that gap isn't changing, they are going the same speed. So at any given time they will maintain the same gap as long as acceleration is the same... regardless of the speed involved.

Mike

 

[Let's Drive]

If two cars are accelerating exactly the same but one car is 1 second ahead, the gap is growing in line with speed.

Bolding mine. How is the gap growing, if the cars are accelerating at exactly the same rate?

The one second difference exists because they started at different times (reaction) and/or the lead car was initially accelerating faster.