High speed acceleration - revisited

[MikeyC]

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.

I think the disconnect here is how we are defining the initial setup. If we think about an extreme case, consider two cars running the half mile. One car takes off a few seconds early and the second car doesn't launch until the first car reaches the 1/8 mile mark. At that point, the second car leaving late can out-accelerate the first car for a while with the gap between the two cars still increasing: because the first car is already up to speed (say 90 MPH) and is gaining ground on the second car. At the point when the late car catches the speed of the first car and finally starts to decrease the gap, yes, it has been out-accelerating the lead car for quite a while.

My setup was different, as in the video. One car dropped behind but they were close enough that there were about 3 seconds where the trailing car neither gained nor lost ground. During that 3 seconds, both cars had the same speed and acceleration: during that 3 second window (looks like about 80-115 MPH), that has been established since the only way to change the gap after that 3 second dead heat is for one car to out-accelerate the other. We were talking about EVs losing rolling races at higher speed so in this case, in an 80-115 MPH rolling race, both cars would be dead even. After about 115 MPH, the ICE would pull away. And keep in mind, in the video example I posted, the ICE is rated for more than 100 HP more than the EV.

Mike

 

[dsgerbc]

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.

I simplified things and assumed both cars are going at the same speed. In that case, the distance is V*t. So, if acceleration is the same, t stays constant, and distance is growing with V.

In reality, the car that accelerates faster at higher speeds (but slower initially) will be going at a slower speed at the point where acceleration rates will cross over and the second car will begin out-accelerating the first one. So, in this case the faster-accelerating car is still visually falling behind faster than the rate of speed of the car ahead. So, this more realistic case is even worse for judging when an ICE car begins to out-accelerate an EV by when the distance starts shrinking.

 

[MikeyC]

That's true. If we look at the whole race, by the time the car that is behind matches the speed of the car in front, it is a given that at some point before that, it had to have been accelerating more. For example, if the car in the back matches the front car's speed at 115 MPH, they both reached 115 MPH in the same time which means their average acceleration at that point is the same. And since the car in the back accelerated slower than the front car in the first part of the race, it had to have out-accelerated it some in the last part leading to 115 MPH, else it would have never matched it's speed.

In reality, even two ICE cars will out-accelerate one another at different points in the race due to gearing matched with torque curve and other factors. They may swap back and forth several times (and often do) as to which one is accelerating more at any given moment. So as you said, it's hard to judge when one is truly out-accelerating the other consistently. Fortunately other than the initial burst off the line, most races between similarly powered cars don't vary much by time vs speed. Maybe 1-2 MPH which is a noticeable "walk" on the losing car. So while we can't prove when the back car started consistently out-accelerating the front car, when we see a ~3 second dead heat where both cars are going the same speed (with a gap), we can predict what a rolling race would look like at the beginning of that 3 seconds at high speed. It'd be a dead heat until they hit about 115 MPH and then the ICE pulls ahead.

If you ran the race just prior to when the initial race matched speed between both cars, there would have to be a speed range where the ICE would accelerate faster as well. But like I pointed out, acceleration is not constant in any race. You can always pick a sweet spot/speed/gear for a particular car and get an advantage. The only thing we know for sure (given that the two cars reached 115 MPH in the same time) is that up to 115 MPH, both cars have the same average acceleration, and that above 115 MPH, the ICE accelerates faster.

Mike

 

[MasterC17]

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.
View attachment 672990

No (perceivable) power loss at 35% would be amazing. I didn't mean to insinuate that the back EMF is gone, rather it is so minimal now that it's almost irrelevant. I think your graph highlights that very well. That being said, the horsepower of the Model 3 on the graph seems much lower than actual (closer to 570hp peak at the motors).

I guess the only other question now is does the battery/motors overheat on the track and if so how long does it take?

 

[gearchruncher]

I guess the only other question now is does the battery/motors overheat on the track and if so how long does it take?

I'm guessing the drivetrain overheats about 3 laps after the brake system melts.
This kind of power being dumped into a heavy, slippery aero car is a whole new challenge for brake systems. Would be super cool if the 3 motors/inverters enabled a much higher regen rate. We know the batteries can do 250kW (~330 hp).

 

[Dolemite]

No (perceivable) power loss at 35% would be amazing. I didn't mean to insinuate that the back EMF is gone, rather it is so minimal now that it's almost irrelevant. I think your graph highlights that very well. That being said, the horsepower of the Model 3 on the graph seems much lower than actual (closer to 570hp peak at the motors).

I guess the only other question now is does the battery/motors overheat on the track and if so how long does it take?

Sounds promising - Mate Rimac coincidentally also mentions the 35% threshold in this video:

Let's hope this continues... it's one of my main gripes about my M3P.

And yeah the 'real' tests I've seen are quoting the M3P much closer to 570 - 580.

 

[zipzap1]

Sorry to dig up an old post but my m3p 2021 is really slow from 100-200 kmh. For a “performance” car it really dies off after 120kmh. I have tasted this several times and it takes around 11-12 seconds 100-200kmh. My tuned BMW 335d did it in 9-10 sec range but it only had 340 hp.

 

[Sam1]

Sorry to dig up an old post but my m3p 2021 is really slow from 100-200 kmh. For a “performance” car it really dies off after 120kmh. I have tasted this several times and it takes around 11-12 seconds 100-200kmh. My tuned BMW 335d did it in 9-10 sec range but it only had 340 hp.

Welcome to the world of the model 3. My 2019 and 2021 M3P were also in the 11 second range (60-130mph), and my 135i could regularly do it in 6 as long as it had traction.

Hopefully Tesla addresses this with a similar motor/controller/battery configuration as they did with the plaid, on the 3.

 

[terranx]

It makes no difference how long it takes your car to shift. When you shift, you change the gear ratio and you lose torque to the wheels. It's simple math. For example, a Challenger with an A8 transmission has a 1st gear ratio of 4.71 and 2nd gear is 3.14. When the transmission shifts from 1st to 2nd, you lose 33% of the torque to the wheels and your acceleration will drop accordingly.

Mike

You’re losing torque in electric cars too, just in a more gradual fashion rather than a stepwise drop. P = τ ω. If P is constant, as ω increases, τ decreases. In our case the power actually decreases at higher speeds, so the effect is larger.

 

[MikeyC]

Sorry to dig up an old post but my m3p 2021 is really slow from 100-200 kmh. For a “performance” car it really dies off after 120kmh. I have tasted this several times and it takes around 11-12 seconds 100-200kmh. My tuned BMW 335d did it in 9-10 sec range but it only had 340 hp.

A 60-130 MPH run is going to be tough for any performance electric car other than a Plaid. The motors are just geared to give you the best output from maybe 0-80 MPH. Much above that, ICE cars will start to have an advantage. With that said, the Model 3 (performance and LR boost) trap 115-117 MPH in the 1/4 mile so they seem to do quite well up to maybe 105 to 110 MPH. My 2015 Challenger SRT had 485 HP and went 12.1 and trapped 114 in the 1/4 mile and both the M3P and M3 LR with boost beat it on both ET and MPH even though we are talking about a race that ends at about 115 MPH.

When you start ending your "race" at 130 MPH, you're approaching the top end of a Model 3 so of course it's going to taper off at those high speeds. Put it another way: the top speed of the Challenger is 182 MPH. So if you did a 60-170 race, the time of the Model 3 is infinite. Lesson learned: race under 100 MPH.

Mike

 

[gearchruncher]

A 60-130 MPH run is going to be tough for any performance electric car other than a Plaid. The motors are just geared to give you the best output from maybe 0-80 MPH. Much above that, ICE cars will start to have an advantage.

You mean an EV without a transmission.
An EV can work exactly like an ICE engine, where torque reduces at higher RPM. Put a gear in, and you can put the motor back in the power band. This is exactly what the Porsche Taycan does.

 

[MikeyC]

You mean an EV without a transmission.
An EV can work exactly like an ICE engine, where torque reduces at higher RPM. Put a gear in, and you can put the motor back in the power band. This is exactly what the Porsche Taycan does.

Right. That's what I meant: all EVs to date except the Taycan (and I think its Audi brethren) which, for whatever reason, doesn't seem to get a lot of benefit from the extra gear (still gets overcome by ICE vehicles on the top end). I don't profess to know much about the Taycan but I think I remember only one of the motors has the second gear so... maybe that's not enough.

I prefer to have one gear and have the usable power from 0-100 MPH. I don't spend any time above 100 MPH except at the dragstrip. Others may differ in how they drive. In any case, there are now obviously two ways to do high speed acceleration: transmission as you said... or the Plaid way with one gear and the right motor tech.

Mike