1. EV has torque available from 0 RPM. But my question is why doesn't the gas car have peak torque available at 0 RPM?
2. EV (P85D/L) does not have to shift gear. But my question is why does gas car have to shift gear?
I thought these are obvious question, but after having some "debate" in forums, it seems people aren't versed in basic Physics and manual transmission to be able to understand what I'm saying. Therefore, if you haven't had basic Physics or you don't understand how manual transmission and clutch works, you should understand them before continuing with this post. It also helps if you know how to pull a wheelie on a motorcycle.
I'm going to be using Corvette Z06 as an example gas car for this post, but it could be any car with comparable performance. While Dodge Hellcat has more power, I can't stand Sergio (their CEO) for making stupid comments about EV and not improving on Fiat500e with DCFC or making better EV, so I won't be using it in this post. Sorry Hellcat; I like you, but not your CEO.
Making an EV out of gas car
Gas cars produce peak torque at specific engine RPM, and fully open throttle. Outside of these conditions, torque is far less. One only has to look at power profile curve to see this. Below is 2015 Corvette Z06 power profile.
If there's a way to keep the engine running at 3600 RPM with FULLY open throttle at 0 MPH, full torque 650 ft-lb from the engine would be available from the very start. Is there a way? Of course there is; simply FULLY open the throttle with clutch disengaged, and the engine will happily rev far past 3600 RPM. Hopefully, it has rev limiter so that the engine doesn't fall apart!
So if there's a way to make the engine speed to peak torque RPM from 0 MPH, how do we control the engine speed with FULLY open throttle so that it stays at peak torque of 3600 RPM instead of going to rev limiter? Simple: clutch slip. If the clutch is allowed to engage partially and allowed to slip, the torque at either ends of the clutch shaft must be the same to keep the engine RPM constant. Essentially, the torque at 0 MPH now becomes peak torque of the engine, not the torque at idle.
Some suggested that clutch slip results in loss of torque; if the engine RPM is kept constant, there is no loss of torque. Some suggested power is lost on clutch; yes, but power to wheels is related to wheel RPM, and clutch friction loss does not enter into the picture. Even for Tesla, instantaneous power at 0 MPH would be 0 horsepower (or watts or whatever unit of power you want to call it). This is why I asked for basic Physics as prerequisite for reading this post.
Doing this requires careful control of the clutch. Some have suggested that such fine control over clutch slip is very difficult or not possible. Or is it?
Enter the Motorcycle
Let me back up here, and give an example of this type of activity being done on daily basis by rank amateurs: motorcycle wheelie. Most motorcycles don't have enough power at low RPM to be able to pull a wheelie. The problem is worse for high peak power sports motorcycles as they have very little power in low RPM. Knowing that even 90cc motorcycles can pull a wheelie that sometimes require 2G of acceleration, something is being done to allow that from even tiny engines.
Don't try this at home, kids. The way they pull a wheelie is to rev the engine at sufficiently high RPM, and let the clutch slip to accelerate much quicker than their low RPM engine torque would allow. On high power sports bikes on the freeway speeds, they "blip" the clutch (don't do it!). Once the front wheel rises and the center of mass has shifted sufficiently, less acceleration is needed, and the clutch is allowed to fully engage. At that point, it's a matter of balancing, and very little power is needed.
It's easier to wheelie with high power sports bikes since they have higher center of mass. Doing this on 90cc motorcycle requires careful dance on the clutch lever for much longer time to allow sufficient wheel rise to take place. Indeed, I tried this many times long ago when I had one cylinder, 250cc 300 lb motorcycle that got 15HP peak power and 80 MPG (only in private parking lots with owner's permission; please don't arrest me!). Even something as wimpy as 20 lb/HP vehicle can pull 2G of acceleration, surely something with 650HP engine at 5.4 lb/HP should be able to pull 1.1G for 2.5 seconds, right?
Another problem often cited by EV folks is that gas car must change gears. That's not true. If the gas car keeps it in one gear that'll allow it to run at 60 MPH, there is no need to change gears. Corvette would run under 6000RPM at 60 MPH even in first gear. But clutch slip would be the mechanism to match engine's single RPM (peak torque RPM) to variable wheel RPM, not gear change. Looking at Corvette Z06 gearing, it seems 60 MPH would result in 4161 RPM in second gear and 3128 RPM in third gear. The gearing data is from Chevy web site.
The RPM at 60 MPH calculation is from
Let's use third gear and clutch slip to allow peak torque to take place. Second gear means more torque to the wheels, but we have to remember to fully engage the clutch and allow the engine to rev beyond 3600 RPM within the run. For driver sanity, using the third gear would let him (me!) not worry about RPM other than 3600.
There is one potential problem with this approach, and that is traction. Given that Corvette is RWD while P90DL is AWD with intelligent traction control, putting down all that power may simply spin the rear wheels. Again with basic Physics, static coefficient of friction is far higher than kinetic coefficient of friction.
For example, if Corvette did the run with third gear (1.21 * 3.42 final drive = 4.14), and 26.7 inch diameter wheel, driving force at the contact patch of the tires would be
650 ft-lb * 4.14 / (26.7 in / 12in/ft / 2) = 2418 lb (each rear tire would be 1209 lb)
Corvette weighs 3524 lb. Knowing that Corvette can pull more than 1G, the minimum power needed to break traction would be 3524 lb. Third gear is not enough to break traction, but that's also not enough to push it quicker than P90DL mode. 2418 lb to push 3524 lb car would result in acceleration of only 0.69G, about that of P85D sports mode. Using peak torque RPM will not allow the Corvette to out accelerate P90DL. Scotty, we need more power!
May the force be with Vette
What we need is at least 3524 lb of driving force. Using first gear, 60 MPH would be 5919 RPM; let's say it's 6000 RPM. At this RPM, there would be about
550 ft-lb * 7.83 / (26.7 in / 12in/ft / 2) = 3871 lb
3871 / 3524 = 1.1G of acceleration! We are there! If we keep the engine running at 6000 RPM with fully open throttle and modulate the clutch to keep it there while accelerating, Corvette would pull 1.1G from stand-still all the way to 60 MPH. It's 10% quicker than falling.
This assumes traction holds. This is not "popping the clutch" where the torque at the wheel could exceed the traction limit via flywheel momentum change. This is controlled modulation of the clutch, and the torque at the wheels is not allowed to go beyond accelerating at 1.1G. Whether the stock tire would allow this is unknown, but with rear wheel contact patch pushed down from body torque due to acceleration, I'm pretty sure it will hold, especially considering that skid pad (maximum lateral acceleration) is rated at 1.2G.
At 60 MPH, it is 88 ft/sec. 1.1G is 35.42 ft/sec2. This would result in 0-60 MPH time of (drum roll...)
Corvette just beat Tesla P90DL. HOLY SHEEEEEEE! Oh well, it's only in theory in some obscure blog about a "compliance" EV.
Help me, Obi-won
If this was actually carried out, the price we pay to win may make the victory hollow.
First is the question of traction. We have to get the RPM just at 6000, and no less (more would be ok). If the RPM dips any less, there will be more torque, and the rear tires could make plenty of smoke. The advantage of using the peak torque gearing (third gear) was that no matter which way you go, it's not likely to break traction due to less torque, but that wouldn't be quick enough. One can argue that using higher RPM where less torque is available while keeping the car at 1.0G (2.73sec 0-60 mph) may make it more manageable. Maybe, maybe not. That will take experiments to find out.
And those experiments will cost you dearly. For each run, you're essentially dumping lots of power into the clutch, with standstill dumping almost 600 HP to poor clutch. Assuming that clutch response with all that power (aka, heat) is manageable, it will surely reduce its life. It's not clear how long it'll last. One run? Two runs? I suppose Chevy engineers would know, but would you burn up the clutch on each 0-60 MPH run with ~$100K car? Each clutch job may cost more than 3.25 years of SparkEV lease! It's not likely that anyone would do such a thing, and such run would not be valid, would it?
Leia, I am your experiment
Each time a manual transmission car starts from stand-still, it slips clutch. Each gear change involves minute amount of clutch slip. Basically, clutch slip is normal operating mode of manual transmission cars. If that's the case, extreme clutch slip is not out of the operating mode, although it's not something you see everyday (other than motorcycle wheelies; no officer, I did not pull a wheelie!). Without quantifying what amount of clutch slip is legitimate, slipping the clutch all the way to 60MPH would be valid.
Question is, is anyone nutty enough to do it? Well, there are plenty of people nutty enough to drive a 4 door sedan with electric drive train that can take them from 0-60MPH in 2.8 seconds. Worse, there are those who dare drive a "compliance EV" and damn proud of it! I don't see why there wouldn't be some who would burn up their Corvette clutch for bragging rights.
Now where are my girlfriends, one with Chemical engineering degree who own bunch of Corvettes and another with Electrical engineering degree who own bunch of Tesla P90DL? Are you reading this? ;-)
Edit Nov 4, 2015
After sleeping over this (don't we all dream about cars and/or motorcycles in our sleep?), I think sanity check is in order. We know Corvette's 0-60mph is 2.95 sec. If we assume that clutch is fully engaged as soon as possible off the line and first gear is maintained, we can calculate how long it'll take for 0-60. Then we can compare to actual run time to see how close the calculations are.
Unfortunately, the torque data isn't available, only the graph. We can "eye-ball" some values from the graph and make rough calculations. How far off can we be? As far off as the eye ball and my biases allow!
The torque number we are after is average torque from lowest to 6000 RPM. It's pretty hard to tell, especially only 2 numbers are available: 650 ft-lb at 3600 RPM and 533 ft-lb at 6400 RPM converted from peak horsepower number. I don't know, lets just guess 450 ft-lb. While it could be higher, torque above 600 ft-lb would result in greater than 1.2G skid pad limit in first gear and resulting tire spin, so one would have to try to stay below it. Assuming 550 ft-lb is used as peak torque, using 450 ft-lb as average would not be unreasonable.
450 ft-lb * 7.83 / (26.7 in / 12in/ft / 2) = 3167 lb
3167 / 3524 = 0.9G
88 ft/sec / (0.9G * 32.2 ft/sec2) = 3.03 seconds
We are in the ballpark to actual experiment data with our calculations, so I feel more confident that clutch slip method could yield quicker 0-60 mph than P90DL.
Edit Nov. 5, 2015
I just can't seem to leave it alone, can I. I've been accused of being ignorant of Physics and not knowing how manual transmission and clutch works. Of course, both are wrong. It seems people who don't understand basic Physics or who haven't driven manual transmission in many years forget things. Here's a very simple way to picture the concept I present: riding the clutch.
When you were a beginner manual transmission driver, the fear you had while stopped on a hill is rolling back and hitting the car behind you. What you did (or what I did) was to partially engage the clutch while revving the engine so that the car doesn't roll back. Obviously, this wears out the clutch very quickly. What is significant is that you are accelerating via fighting gravity through clutch slip and higher power of the engine than idle can provide.
For steeper hills, you needed higher RPM. 45 degree hill would be about the maximum on most cars due to traction. Then the power needed would be? Physics homework for you! If the hill is 90 degrees and there is traction (fly paper tires?), you'd need 1G of acceleration to avoid rolling back.
The proposal I make in this post is simply "riding the clutch" taken to extreme levels. We know that it can be done at lower power levels; beginners do it all the time. The question of whether it can be done at extreme level requires (expensive) experiments. At least the calculations I present show that it's possible. Now I need to go find girlfriends with ChemE and bunch of Corvettes. BRB.
There's no question that Corvette has enough power to out accelerate Tesla P90DL with proper abuse of the clutch. But the question was if the tires can hold traction. There's an excellent video by Engineering Explained that stock tires probably won't hold traction, and limit it to about 0.8g in RWD cars like Corvette. That is probably why Corvette is rated only for 2.95 sec 0-60 MPH, and even that's very optimistic.
Video does an excellent explanation of normal force and shifting center of mass. With shorter wheelbase, peak acceleration would be much higher. That's why lateral acceleration (as in cornering) on Corvette is 1.2g and probably far larger than linear acceleration since width of the car (left to right) is much shorter than length (front to rear).
So the conclusion is that Stock Corvette will not out accelerate Tesla P90DL only due to lack of traction. But given flypaper tires of infinite traction, (if there is such a thing), appropriate clutch abuse (and destruction) would allow Corvette to out accelerate even P100DL that's rated for 0-60 MPH in 2.5 seconds.
Many arguments given by EV folks that gas cars lack "instantaneous torque" or that gear shifting make them slow are not true. Rather, gas cars are slow for most parts due to drivers' inability or unwillingness abuse the car to the extreme. Even with capable driver (aka, abuser), traction limit would not allow stock gas cars' tires on RWD configuration to be quicker than intelligent AWD Tesla.
Which begs the question, is there a tire combination that will allow properly abused Corvette to be quicker than P90DL or even P100DL? I really need to make more money if I'm to satisfy my curiosity. I should setup a "go fund me" page asking for tens of billion dollars so I could take over GM to be able to run these experiments. Anyone care to donate GM to me?