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Old 8 Oct 2002, 18:42 (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, 02:08 (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, 04:25 (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, 04:39 (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, 05:08 (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, 05:45 (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 donít 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, 06:07 (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, 06:30 (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, 07:14 (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 07:17.
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Old 9 Oct 2002, 08:58 (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, 14:23 (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, 02:29 (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, 11:38 (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, 20:09 (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 12 Oct 2002, 00:06 (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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