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Old 8 Oct 2002, 17:42 (Ref:398535)   #1
engstudent
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CVT -- engine RPM

When running a CVT, for max acceleration do you want to run at your engines maximum torque or your engines maximum power. Torque ultimately gives you acceleration right? but it is driving me crazy why wouldn't you run at max power (torque * w).
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Old 9 Oct 2002, 01:08 (Ref:398849)   #2
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The way it was explained to me, is that the engine's torque peak is where you get the max acceleration in any given *gear*...... but, providing there is a suitable gear ratio available, the engine's power peak is where you get the max aceleration for a give road speed.

Therefore I think you should run the engine at it's power peak. Anyone got any different theories?
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Old 9 Oct 2002, 03:25 (Ref:398893)   #3
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Here is a very simple example on how to differenciate between torque and power.

'Think of slowing a free-spinning tire with your hand. Feel the tug on your palm and the tension in your arm? That's a measure of torque, the torque the tire experiences as a result of your palm slowing it down. Feel the heat build up from friction? That's a measure of power.'

The Continuously Variable Transmission (CVT) has the ability to maintain the rpm at any given driving situation (example - from a steady pace to an uphill pace) eventhough it has reached it's maximum torque. Traditional automatic transmission would need to shift it's gears in order to go through those road conditions.

I would say for CVT, you would want to run at max torque as there's less friction to use max power.
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Old 9 Oct 2002, 03:39 (Ref:398895)   #4
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would like to edit the last sentence.....

I would say for CVT, you would want to run at max torque as you won't need to accelerate further to maintain your speed, wherelse with the traditional auto transmission...you would need to accelarate more after achieving the max torgue of each gear in order for it to shift to the next and rpm would definitely decreace after each gear shift, so max power would be more appropriate for the none CVT.
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Old 9 Oct 2002, 04:08 (Ref:398900)   #5
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The following link may help explain better than I can:
http://www.stanford.edu/~voloshin/lhowwhy.html

Second to last paragraph says: "the simulation data supports our claim that HP is the important factor in car acceleration."

You are probably right about less friction at lower RPM, but the question was about acceleration, not fuel efficiency.

I don't think a CVT makes any difference - let's say I have a 5 speed box and for arguments sake, lets say it has a special shift/clutch system that gives me totally lossless gear changes (full power applied all the time during the change)..... now lets say it's a 6 speed, or 7 speed or 20 speed? In practice, when I have enough gears, with close enough ratios, it's that same as a CVT.
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Old 9 Oct 2002, 04:45 (Ref:398907)   #6
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I've extracted this form the link you've pasted..and the research that they have made was not concentrated on CVT.

On the other side, we have CVTs - continuously variable transmissions. Using novel technologies, these can vary the gear ratio such that the engine always runs at the same speed regardless of the speed of the car. In that case, design of the cam will be very simple. Since power can be freely adjusted, you don’t need a wide power band.

There are huge difference between CVT and the conventional auto transmission.

CVT is based on a coned shaped pulleys and a metal belt that runs between them that slides between the narrow and wide end of each pulley. When the belt slides, it allows continous variable gear ratios without any gear being changed.

The normal auto transmission consists of gears, friction plates, hydraulic fluid and a torque converter.

Anyway, you can't make a comparison between the manual gear and CVT....simply because CVT is an alternative for the traditional automatic transmission system. Clearly there would be power loss when gears are being changed as opposed to CVT even you were to close the ratios of each gears and CVT is not an option for performance based cars.
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Old 9 Oct 2002, 05:07 (Ref:398910)   #7
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So Jukebox, do you agree with the studies presended in the link - at least for a conventional gearbox with fixed ratios?

http://www.stanford.edu/~voloshin/lhowwhy.html

Would you then agree that the power peak would also be more important than torque in my example with my hypothetical lossless 20 speed gearbox, even though the revs change very, very little between one gear ratio and the next?

If you answered yes to the questions above, then why do you think it is torque that matters more than power, for a CVT with cone and pulley or whatever?

All gearboxes have powerloss (CVT too I'm sure), but I can't see how that matters to the maths of power and torque. Hopefully if there is something I'm missing here, you'll let me know, but the way I see it, at any given instant, the CVT has a fixed ratio too and the normal laws of physics apply to the CVT example.
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Old 9 Oct 2002, 05:30 (Ref:398923)   #8
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alfasud...i did metion earlier in my first reply that 'so max power would be more appropriate for the none CVT.' so it's a yes but CVT based would rely on torgue

Try to read in this link http://www.canadiandriver.com/articles/jk/020828.htm of a Honda Civic Hybrid powertrain which uses CVT
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Old 9 Oct 2002, 06:14 (Ref:398941)   #9
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Quote:
Originally posted by Jukebox
alfasud...i did metion earlier in my first reply that 'so max power would be more appropriate for the none CVT.' so it's a yes but CVT based would rely on torgue
O.k. thanks Jukebox, I wanted to make sure I fully understood you, but you're right, you did already say max power for the conventional gearbox.
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Try to read in this link http://www.canadiandriver.com/articles/jk/020828.htm of a Honda Civic Hybrid powertrain which uses CVT
Thanks for the link and it's an interesting article, but if you don't mind me saying so, the article seems to support the idea that the revs of max power may also be important for the CVT case too.

To quote the article "93 HP at 5700 rpm and 105 lb.ft. torque at 3000 rpm". So if it's the revs that produce max torque that are the most important for acceleration, then there wouldn't be any good reason to rev past 3000 rpm except for a top speed run, should there?

But later in the article it says "Step harder on the gas and the tachometer goes up to between 4000 and 5000 rpm and stays there while you accelerate". Now 5000 rpm is getting rather close to that max power peak at 5700 rpm isn't it?

There doesn't seem to be any good reason for the car to be reving past 3000rpm, unless the car accelerates better at those higher revs.... which leads me to conclude that the peak power is most important for acceleration in both the conventional multi-speed gearbox and CVT case.

Last edited by alfasud; 9 Oct 2002 at 06:17.
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Old 9 Oct 2002, 07:58 (Ref:399008)   #10
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alfasud...after giving much thought, i could conclude that for CVT, both torgue and power works in a unison way theoritically like both sides of a coin...means both works together to reach maximum engine revs.

When accelerating, CVT immediately increases rpm to a steady point and then increases speed and torque by continuously varying it's gear ratios....i'm getting myself confuse but i'm pretty sure torgue have everything to do with it cause you can achieve a steady rpm even if you drives through different type of road conditions.

In a manual or auto transmission mode...each gear shift will ultimately reduce the rpm (even in a very tight ratios), so you'll need power to reach the max torgue for the next gear shift.

make sense dosen't it?
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Old 9 Oct 2002, 13:23 (Ref:399281)   #11
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Simply assuming that the transmission fulfills its function decently, ain't the question simply: what (CVT-)car accelerates faster: the one on max. torque or max. power? To accelerate a mass, i.e. car, over a certain distance, a certain amount of energy is required. No matter if it takes 20 s or 10 s that amount remains the same. To do it faster requires more power though and that's the crux. Max power is everything.

Torque is most visible when the slipcoefficient of the tires gets very small (spinning on ice) or very high (fixed wheels, axles break).
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Old 10 Oct 2002, 01:29 (Ref:399859)   #12
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Dino...i suggest you read the link i paste here http://vettenet.org/torquehp.html In there you'll find a very clear explanation on how torque and power is calculated and how they are measured when rpm are concerned.
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Old 10 Oct 2002, 10:38 (Ref:400101)   #13
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Isn't hp just proportional to torque x revs?

Anyway great discussion, I've come in a bit late, but do you think Patrick Head and Frank Williams had this discussion when planning their CVT?

Shame it was banned before it got going.
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Old 11 Oct 2002, 19:09 (Ref:401365)   #14
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Can I try to explain the difference between torque and power and which is suitable for CVT?

When an engine burns fuel in a piston engine the combustion creates a pressure to push the piston down. Some engineers call it brake mean effective pressure. The more pressure on the piston the greater these values called torque and power FOR THOSE engine revs.

A camshaft is always a compromise in when it should time the opening and closing of a valve. A racing engine around a town is a pig because the cam won't allow it to pull at low revs, but it is a dream at high revs. Conversley a diesel cannot run fast because its big fuel molicule burns slower than petrol. So its camshaft is designed for low revs; the camshaft is designed to ensure maximum filling of the cylinder with air and fuel - you can design a petrol to pull at low revs.

So the more air and fuel in EACH piston stroke meas more torque. If you like the amount of power from each piston stroke. But what if we double the revs from the maximum torque figure of 2500 revs to 5000 revs. Each piston has half the time to get the air into the cylinder. The piston is very sorry but say it only gets 75% or 3/4 of the air in at 5000 revs. It will generate 75% of the torque for that piston stroke. But at 5000rpm it it is doing two of these 75% strokes. So the engine produces 2 times 75% power or 150% of the power generated at maximum torque.

Does that explain how torque is related to a specific speed but power is also dependant on the number of revs and the torque figure?

The beauty of a CVT is that at maximum power it will give maximum acceleration because power is what is needed to accelerate the car. A CVT can be confusing because at part throttle and a lot of revs can equal full throttle at less revs - like travelling in 4th or 3rd 30 mph.
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Old 11 Oct 2002, 23:06 (Ref:401525)   #15
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Quote:
Originally posted by Jukebox
Dino...i suggest you read the link i paste here http://vettenet.org/torquehp.html In there you'll find a very clear explanation on how torque and power is calculated and how they are measured when rpm are concerned.
Why, Jukebox? Did I say something silly?
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Old 14 Oct 2002, 02:40 (Ref:403376)   #16
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Ok fellas, looks like the basics have been well and truly covered, but I'm going to throw my 2 bob's worth in. SO please bear with me.
The site proposed by jukbox is good but is imperial, so I'll talk metric (just to confuse the issue)
Power (watts) = Pi (3.1415..etc) x N (rpm) x T (torque Nm) all divided by 60. So everyone who said power is dependent on torque and revs, well done.
But still the question exists, what do you want for a engine driving a CVT, power or torque ?. And the answer is, another question, does it really matter ?. All engines have a specific rpm range at which they are most efficent, due to factors such as cam timing, air flow thru the head, piston filling blah blah blah. Thus it would make sense (for reasons of fuel economy and reduced emissions) to design the CVT around that rpm.
However other factors are at play. Say the engine needs to do 15,000rpm (theoretical, lets not get too concerned about practicality just yet) to produce a torque of 50 Nm. Ok not a fantastic engine but it would mean that all the elements in the CVT (gears, shafts, bearings) need only to cope with 50 Nm of torque. Bugger all really, except for the 15,000rpm. This then means that the CVT itself would be very small and very light. Of course to make it all work you would then need a ridiculous final drive ratio to move a 1500kg car, but you have less parasitic drag from the gearbox and the other driveline components.

CVT's are probably not the thing to use for automotive engines though. They work very well on motors with fixed rpm such as electric motors for conveyor drives and the such. And they were used on harvesters, before everyone started going to hydrostatic transmissons, for controlling ground speeds. However here again, the harvester runs at max. engine revs all the time so it has enough power to drive all the screens, augers and cutters.

Anyhow thats all I wanted to say, hope it's helped.
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Old 14 Oct 2002, 07:15 (Ref:403484)   #17
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CVT has it's advantages (in theory at least)....

1. Having a narrow power band isn't a problem.... you don't have to worry about those gaps between gears.... there are no gaps. And think of all the effort that people go to for a wide power band, compromise induction and exhaust, variable valve timing, variable length induction. You could get rid of all that. See the link to the Mazda 4-rotor engine in the other message topic - here it is again:
http://www.mymazdarotary.com/mazda_r...paper_html.htm

2. and then there's those harpins that should really be 1st gear, but you don't want to be changing down to 1st right at the turn in point and then pulling 2nd on the exit.... so you take it in 2nd and accept that the revs are lower than optimum.... just as well you've got a wide power band (see point 1 above).... but with CVT it's not a problem.

3. And what about those decreasing radius bends, that needs 3rd gear on entry, but really need 2nd gear on exit.... are you going to unbalance the car by changing down to 2nd gear, midway through the corner? See 1 and 2 above.

With CVT, you could have a touring car engine that has peak power at 8250rpm (assume 8500 limit) and peak tourque at 8000rpm and still have the same acceleration out of those harpins and decreasing radius corners as someone with a much wider power band.

Last edited by alfasud; 14 Oct 2002 at 07:17.
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Old 15 Oct 2002, 01:52 (Ref:404392)   #18
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Why, Jukebox? Did I say something silly?
Not at all..Dino, i really appreciate a healthy debate and discussion. I just don't agree with what you've said that's all.
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Old 22 Oct 2002, 08:31 (Ref:410001)   #19
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Interesting info on agricultural machines, boyracer. There must be some fascinating angles to that.

That's no problem at all, Jukebox, I only don't understand with what and why you disagree, that's all.
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Old 24 Oct 2002, 01:32 (Ref:411805)   #20
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hey engstudent if you are setting up the system u propose it would probably benefit us all to just test it at both points. I would personally like to know and can't say for sure that i do. It shouldn't be harder to try than a simple final drive or computer code change. The way i understood it is that its sorta both with torque needed at slow speeds and hp at hi but this thread has my headspinning, my only beef is that i can't really see the comparison between a civic econobox built for fuel economy and a race car especially in transmission setup
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Old 29 Oct 2002, 03:38 (Ref:416070)   #21
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Absolutly,positively run it at maximum torque.

I think you should learn about what torque and horsepower are.

Don't feel too bad though.Only a couple of people i know understand it properly and i know a hell of a lot of mechanics and such...
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Old 29 Oct 2002, 22:09 (Ref:416771)   #22
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RWC

I am sorry, I disagree.
Units measuring torque are in energy.
Units measuring horsepower (or kw) are energy per second or we simply use the word power.
To accelerate you need as much energy per second as possible. In other words absolutely, positively run it at maximum power.

Has anyone got access to an MGF fitted with a CVT?
Granted it uses "stepped" changes.
Get at just below maximum torque, time acceleration for say next ten mph. Timed run finishes with engine running just above maximum torque, so its roughly averaged the maximum torque figure.
Repeat excercise over same speed differential with engine just below revs for maximum power.
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Old 30 Oct 2002, 07:27 (Ref:417111)   #23
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A tractor that pulls a plow has power roughly somewhere around 20 bhp. A formula 1 car car that pulls a plow will not achieve the same acceleration () as the tractor even if it has 800+ bhp. Yes, basically you need as much energy per second as you can get to accelerate, but that energy should be the result of a higher torque. Not higher rpm.
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Old 30 Oct 2002, 11:21 (Ref:417290)   #24
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Yes, basically you need as much energy per second as you can get to accelerate, but that energy should be the result of a higher torque. Not higher rpm.
Yes you need more torque..... but it is rear axle torque that matters.... not engine torque, that's what the gearbox is for. Rather than explain it myself, it might pay to read the following link (already mentioned in this message topic):

http://vettenet.org/torquehp.html

If you have engine A with 100Nm of torque at 2000rpm and engine B with 50Nm of torque at 8000rpm (or maybe it's the same engine), then what do you do?

You gear down the output of the engine to 1/4 of crankshaft speed, then run the engine at 8000rpm, the output shaft will run at 8000/4=2000rpm and the torque of the output shaft will be 50x4=200Nm.

Power wins because more power can be converted to more torque at the rear axle, providing you have a suitable gear ratio.

There are occasions where you don't have a suitable gear ratio. For example, a standing start (or maybe pulling a plow with a F1 car), but once you are on the move and you have a suitable gear ratio (CVT or overwise), then the answer to the question is "power".

Last edited by alfasud; 30 Oct 2002 at 11:27.
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Old 30 Oct 2002, 11:41 (Ref:417299)   #25
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Quote:
Originally posted by alfasud
Yes you need more torque..... but it is rear axle torque that matters.... not engine torque
Hmmm..... I guess in the Honda Civic example it's the front axle torque that matters

Last edited by alfasud; 30 Oct 2002 at 11:44.
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