# Has this been done?



## Jimdear2 (Oct 12, 2008)

Woke up with one of those weird dreams (No not that kind)

How about a very small motor controller package tied to a variable speed transmission, possibly speed sensors on the CVT in the mix? In other words keep the motor close to the spot with lowest possible power consumption and use the CVT to take the variable loads. It works for ICE tech, allows small engines to move big loads with some versitility.

I'm new to EV tech so I don't know if this is original thinking or not.

Hope it's something new and somebody can run with it.


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## stormcrow (May 28, 2008)

I was actually wondering the same thing. It seems like an ideal solution. I will definately be following this thread.


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## TX_Dj (Jul 25, 2008)

The problem of course, in my mind, is that the torque of an electric is inversely proportional to its RPM. You make the most torque near 0, and the most efficiency near max RPM. In situations where you need torque, you want to have the motor as close to stall speed (actually on its torque peak) just as you would want an "undersized ICE" to be running somewhere near its torque peak.

For example, the Crazyhorse Pinto makes 1260 ft-lbs ~750 RPM, but at the max motor speed it makes far less than that (though still respectable). That's an extreme example, but it's true- you'd want to make use of the torque peak during times when you need torque, and make use of max efficiency during times when you need that as well.


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## Jimdear2 (Oct 12, 2008)

That torque thing is just what I was talking about. The CVT trades rpm for torque through reduction, the motor stays in its more effcient range. Using the CVT allows you to find the most efficient trade off. Could even be set up to overdrive. In other words trade torque for speed while still keeping the motor in it's efficient range. As I say it works one way for ICE's It should work the other way for electric motors.

I know CVTs are torque limited due to belt slippage so you could never dump big electric motor torque into them but it may be a way to make lighter vehicles get away with smaller high rpm motors to allow a bigger battery pack. It seems that carring 200 lbs of motor when 75 lbs of motor with a 25 lb belt type CVT would give the same results is a bit wasteful. 

I guess i should if something so obvious would work, it would be working.

If it's a bum idea I'll bow to the experts, I just thought it sounded interesting. I'm very new to EV tech.

Jim


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## 1clue (Jul 21, 2008)

Keep in mind here that I'm NOT an expert, I've been learning about motors on another forum and through modeling, but have not yet built a car or even seriously started yet.

I'm not trying to rain on your parade, and what I have to say amounts to hearsay anyway. I've been here for a while and compiling information on these electric vehicles so I don't do something stupid when I finally do start. My ideal design has dramatically changed since I started here.

Not sure about how efficient an automotive CVT is, but back when I was researching that sort of thing the general rule of thumb was that the CVT lost 30% of the energy that went in. At the time there were a half dozen varieties of CVT that I looked at, and they all hovered around that for design calculations. This is WAY worse than a conventional manual transmission.

The only exception I saw back then was what amounts to a modified conveyor belt. You have two pulleys which are cones, and a very wide flat belt and a very long distance between centers. You want the driven axle to turn slow, then move the belt to the side with a big driven and small driver, and vice versa. In that case, the efficiency of the CVT was up in the 90's. The only thing is, for a car you would wind up with a transmission which is several times as big as the car.

Cars use a metal belt idea, which evidently works OK but seems kinda dicey to me. However, it's using one of those models that would be around 70% efficient if it were a normal belt. It's basically a modified snowmobile transmission.


Another thing to consider is that motors and engines have several characteristics which don't match up. A heavily loaded electric motor is less efficient than a lightly loaded one. One reason I know about for that is that the iron core gets saturated and all the electric field does is heat it up after that, wasting electricity and no benefit.

Another mismatched characteristic is the torque curve. On a DC motor, the torque near zero is very high and it drops off sharply after that. On an AC motor, the torque can be pretty much constant until the wires would start to melt, then it drops off for a while, and then you reach max RPM. On an engine, you have very low torque near zero and then a peaky curve that we're all familiar with. Basically, though, you have electric motors which have torque curves much better suited to driving than gas engines have.

The thing is, the guys who actually drive EV's right now insist that there is very little shifting while you drive an EV. You don't often shift on the fly, you just leave it in second until you reach redline, which is generally never in a city setting. Then when you get on the highway you pick 3 or 4. Many EV builders leave the clutch off because it's not really needed.

Again, I'm not an expert. I've never even gotten a ride in an EV so I can't say how the end product will be. I'm convinced that EV's are how it must go, but I'm not there yet.


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## enganear (Jun 16, 2008)

The Prius uses a CVT and can operate fin full electric mode up to 42mph, so yeah, it has kinda been done....


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## major (Apr 4, 2008)

enganear said:


> The Prius uses a CVT and can operate fin full electric mode up to 42mph, so yeah, it has kinda been done....


Hi enganear,

The Prius uses a clever system called a power split device. It is not a CVT. A CVT implies various ratios of mechanical advantage. Like 1st, 2nd and 3rd gear in a 3 speed without the gaps inbetween. The Prius power split system adds (or subtracts) the torque applied to the 3 ports. It does not multiply torque by a ratio of speed reduction which would be characteristic of a varible ratio transmission. A common misconception.

Regards,

major


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## ElectriCar (Jun 15, 2008)

I thought the torque on an electric motor was basically flat until several thousand rmp where it steadily declines. 

Regardless, is you're looking for efficiency the question comes to mind "Would it save more than the energy needed to haul it around?" If so then is it worth the added complexity and maintenance expense/breakdown repairs etc? My answer is I sincerely doubt both of the above but doubts don't mean very much. It's only my guessing...


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## 1clue (Jul 21, 2008)

ElectriCar,

Various motors have different torque curves. A series-wound DC motor starts with extremely high torque at zero and then falls off sharply. This is what most of the conversions being discussed on this site seem to be.

An AC induction motor has a curve like you're describing. IMO, this is a strong reason to go AC, and nobody seems to pay any attention to it. Regenerative braking is a neat idea, and that seems to be what everyone focuses on when you say "ac," but I personally think it's a minor feature for a whole lot of money to do it right. 

As to your cost analysis, I agree completely only I would say that you need to add the efficiency of the device into the equation. Maybe you said that or intended to.


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## enganear (Jun 16, 2008)

major said:


> Hi enganear,
> 
> The Prius uses a clever system called a power split device. It is not a CVT. A CVT implies various ratios of mechanical advantage. Like 1st, 2nd and 3rd gear in a 3 speed without the gaps inbetween. The Prius power split system adds (or subtracts) the torque applied to the 3 ports. It does not multiply torque by a ratio of speed reduction which would be characteristic of a varible ratio transmission. A common misconception.
> 
> ...


Hi Major,
Amazing! I am in my 2nd week of renting one of these technical marvels and I was completely fooled. You are correct, of course. I have rented two other cars in my travels with CVTs (Ford Freestyle, Mercedes A Class) and the sensation was exactly the same. I did some research and found an excellent tutorial with pictures and diagrams at http://www.cleangreencar.co.nz/page/prius-transmission

A video showing the changing engine speed vs road speed can be found at http://video.google.com/videoplay?docid=3834454834833982503 . The rpm is the 2nd value from the top on his OBDII scanner.

Thanks for setting me straight and prompting the research. I view engineering as a search for the truth.
-enganear


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## TX_Dj (Jul 25, 2008)

*TORQUE IS WHAT DOES THE WORK*

Use the torque!

If you've never ridden or driven an EV before, get ahold of your nearest EV club (see National EAA's Chapter List). Someone from the club will be very happy to let you experience their car, from either seat.

I would not be willing to trade instant torque (from stall) for anything.

Yes, it is advisable to configure an ICE to run extremely low RPMs or an EV to run near max RPMs for **cruise efficiency**, but it is also advisable to run an ICE on its peak torque (few thousand RPM, depending on engine) or an EV on its peak torque (near 0 RPM, depending on motor) when you need to make something move.


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## stormcrow (May 28, 2008)

It would seem to me like the best option (although I dont know if it is possible) would be to have two different settings in some sort of a transmission. You could have one setting that would just hold it in a "starting out" gear, thereby giving you your maximum torque and then when you got the car up to X miles per hour, you could shift into the second setting. This setting would vary the gear output rather than the RPMs on the motor, thus keeping it at peak efficiency.
So you would have a starting gear to give you oomf and a cruising "gear" once you got going.
Now I have no idea how you would go about creating such a masterpiece, but it seems like it would be ideal.


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## TX_Dj (Jul 25, 2008)

stormcrow said:


> It would seem to me like the best option (although I dont know if it is possible) would be to have two different settings in some sort of a transmission. You could have one setting that would just hold it in a "starting out" gear, thereby giving you your maximum torque and then when you got the car up to X miles per hour, you could shift into the second setting.


Sounds like a manual transmission to me.  Most folks use 2nd gear to start out (because 1st gear tends to multiply the torque and has a very short RPM range), and shift to 3rd for their next higher range. Some even shift as high as 4th or 5th gear, depending on their setup and their locale.




> This setting would vary the gear output rather than the RPMs on the motor, thus keeping it at peak efficiency.
> So you would have a starting gear to give you oomf and a cruising "gear" once you got going.
> Now I have no idea how you would go about creating such a masterpiece, but it seems like it would be ideal.


The "oomph" is torque. Torque on a typical electric is available at low RPMs (low voltage) and high torque (high amps). You need to be close to zero to get "oomph".

At max RPM there is very little torque available. In order to feel the "oomph" one would have to drag the motor down to put a load on it so that it will draw some amps.

Volts = RPM
Amps = Torque

Let me pull some numbers out of thin air- not real world by any means, just to serve an example.

Lets say you're running 72v and on your system 72v gives you 4000 motor RPM. Lets also say that at 4000 RPM under load you're seeing 25 amps at the motor. 72 * 25 = 1800W, or about 2.4 HP.

Now lets say you're running at 666.66 RPM (I took 4000/72*12) at 12v, but you're seeing 500A... remember, amps make torque, and HP is a function of torque and RPM (or a conversion from watts). 12 * 500 = 6000W, or about 8 HP.

Would you rather have the punch of 8 HP or the lag of 2.4 HP?


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## Jimdear2 (Oct 12, 2008)

enganear said:


> Hi Major,
> Amazing! I am in my 2nd week of renting one of these technical marvels and I was completely fooled. You are correct, of course. I have rented two other cars in my travels with CVTs (Ford Freestyle, Mercedes A Class) and the sensation was exactly the same. I did some research and found an excellent tutorial with pictures and diagrams at http://www.cleangreencar.co.nz/page/prius-transmission
> 
> A video showing the changing engine speed vs road speed can be found at http://video.google.com/videoplay?docid=3834454834833982503 . The rpm is the 2nd value from the top on his OBDII scanner.
> ...


Thank you both (major and enganear) I learned something and re-learned something. 

1.) That Prius trans is a wonder, elegant I wonder who thought of it It marries a bunch of different disiciplines. Simple in principle when you see how it's done but darned hard to do the first time.

2.) NEVER assume (been relearning this one all my life) check things out firsthand.

Jim


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## Jimdear2 (Oct 12, 2008)

TX_Dj said:


> Let me pull some numbers out of thin air- not real world by any means, just to serve an example.
> 
> Lets say you're running 72v and on your system 72v gives you 4000 motor RPM. Lets also say that at 4000 RPM under load you're seeing 25 amps at the motor. 72 * 25 = 1800W, or about 2.4 HP.
> 
> ...


Just an aside to clear up something for me above quote (I took 4000/72*12) did you mean 4000/72*25?

I may be wrong, but it seem you left something out of the figuring above that I implied in my original post.

What is the result in torque when you reduce that 2.4 HP at 4000 rpm to 666.66 rpm (torque multiplcation through gear reduction). Torque turns the wheel and yes, I know there is a loss in the reduction.

Think about the efficiency of a small ICE turning 20,000 rpm. You get Big HP, but little torque, but they can still smoke em off the line, thats torque multiplcation through reduction.

It's hard to make an ICE turn 20,000 RPM and live, not so hard to make an electric motor do it.

I was asking about the trade off in weight, smaller motor with much higher more efficient rpm, with a snowmobile type trans (CVT) for reduction, just sufficient to move the vehicle comfortably. Traded against battery weight (capacity = distance). I was just looking for a way to pass that 40-50 mile range limit that seems to be part of so much lead acid tech. I'm 80 miles round trip from work. So an EV for me is difficult until lithium arrives at an affordable up front price for 95% of us.

I dont yet have the math to figure this *YET,* (yes I've been visiting the WIKI). I'm old with a thick head, but it's coming. Hopefully, you that do have the math can put up some numbers and I can use those as a target to learn how you got there. I't going to be a long cold winter and I need something to do.

Jim


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