Continuously Variable Transmissions

[atrowe]

This isn't really a computer question, but it is technical in nature. I've been looking at buying a new low to midrange SUV and I noticed that the Saturn VUE is advertised as having a continuously variable transmission. I understand that this type of transmission does not have any gears and allows for an infinite range of gear ratios within the range of the transmission, but I'm not sure I grasp the concept behind this technology. How is it possible for a wheel on a pully to dynamically adjust its diameter? Is this type of transmission as reliable as existing manual or automatic transaxles?

 

[CTho9305]

well, we have yet to see about reliability, but here is one way it can be done: pic (sorry, crude drawing ). the middle piece rotates. the way it is drawn right now, if the input is at, say, 1rpm, the output would be much higher. if you rotated it counter-clockwise, you would decrease the ratio. hope that makes sense

 

[Mark R]

Torotrak are designers and builders of one type of IVT, which is used in the latest Ford SUVs.

The key component is a rotary variator, but the whole transmission is an exceptionally elegant system.

Here's a detailed discription of how the whole system works, with links to how the individual components work. Link

 

[Mingon]

I think perhaps a simple explanation is in order, standard CVT transmissions use a belt or pulley connected between between a shaft and a cone which are aligned in parallel. As the belt moves up the shaft the opposite end attached to the cone moves with it, this cone allows the change in ratio's associated with normal gearing. Most of the people in this forum are american and as such wont have seen many CVT gearboxes as they were restricted to the amount of torque the belt could withstand (100lbft) but newer cvt's such as that used in the audi's can handle upto 250lbft. The newer Torotrak gearboxes as mentioned earlier are totally different and very difficult to explain.

(simple drawing)

I - /\

Where:

I = shaft
- = belt
/\ = cone

 

[kylef]

I remember back in 1990 when Subaru introduced the first car in the U.S. market with a CVT... the $8000 Subaru Justy! It never caught on...

 

[Oda]

I looked at the Torotrak link. Tell me if I misunderstood the whole concept, but wouldn't the traction fluid get heated up because it is used as friction inside the variator?

 

[Mingon]

Most CVT engines sound terrible, the newer one have programmable steps to make it feel like gears. but the whine from the car is just nasty.

 

[RayH]

I think one of the first ones in the US was on the Subaru Justy. Audi is offering one on their new A4.

 

[FrustratedUser]

We used to have this kind of transmission in Sweden back in the 70's. They were called DAF 66 and Volvo 343. They were belt driven.
Funny. They were just as quick in reverse as forward.
LOL

 

[puffpio]

<< Most CVT engines sound terrible, the newer one have programmable steps to make it feel like gears. but the whine from the car is just nasty. >>



such a waste. if you make a cvt behave like it has gears it defeats the purpose. the great thing since you have an infinite ratio selection, you can have the TCU make the CVT choose a ratio depending on engine optimal engine load.

For example. Your engine produces the best torque/horsepower output at 5000 rpm. You stomp on the gas, the RPM's shoot to the optimal point (5000) and the CVT modulates it's ratio so the engine STAYS at 5000 rpm. So you are optimally accelerating, gear ratio constantly changing, while engine RPM remains constant.

You can also apply the same thing to a engine output that gives a good gas mileage/performance tradeoff.

 

[Nefrodite]

hm i thought cvt civic HX's have been sold in the US for years, i swear i've seen some. and yes, its a kewl idea, sweet efficiency

 

[earthman]

These transmissions are NOT as durable as either a conventional automatic or a conventional manual transmission. Justys never caught on because they were very small, very slow, and went very few miles before they wore out. As some of the other posters pointed out, this has been tried before with poor results, thats why it never caught on, because on paper it is a superior design. Think about it, do you really want your car powered by rubber belts? I don't know about the new Audi design, that may be more successful.

 

[jamarno]

Only the early CVTs used a rubber belt; all the current belts are metal links, most operated so the belt actually pushes rather than pulls.

 

[Soccerman]

there is no CVT standard.. why? well becuase first of all CVT's haven't caught on due to one common problem: ability to transmit torque. most CVT's would slip under heavy load (and by heavy, I mean the power of a typical mid sized car or greater).

so until there is a transmission able to transmit torque in larger quantities without slipping or whatever might happen, CVTs probably won't catch on. They're a great idea though, it's just no-one has thought of a CVT design capable of transmitting lots of torque!

I think Mercedes also released a car recently with a 'fairly' powerful motor and new CVT design. I think it produced something like 160 HP or so..

 

[ElFenix]

peak fuel efficiency is at peak torque. peak acceleration should be there too (F=MA). top speed would still be at peak horsepower, though.

 

[dkozloski]

The best examples of IVT's ( hydro-static drive) are snow machines which use rubber belts. These are pretty impressive but are very intense and subject to instant self destruction when abused. Automobiles using laminated metal belts in IVT's are limited to stinky little cars with weak engines as any slippage at all means lots of metal shavings in the oil. Severe weather conditions such as extreme heat or cold speed up the process of failure. Torotrak is essentially a pie in the sky stock promotion with constantly slipping schedules and down graded performance predictions. The key sticking point appears that it requires a magic fluid that has not been discovered yet.

 

[Soccerman]

top speed is where peak horsepower is??

I somehow doubt that.. can u explain why this is so? Personally I think the top speed depends on a great many things.. first of all, the gear ratios! how much force can you put to the ground at the RPM that your car reaches it's top speed? (requires both the torque curve and the gear ratios) it has to equal the force of friction, so u also need to know the coefficient of friction..

anywho, Audi has their CVT mated with a 220hp 3 litre V6... according to Audi, the CVT is 13% quicker (I guess that's acceleration wise), and 8.5% more economic (cruising at low RPM for example) than a 5 speed automatic. they also say it beats the 5 speed manual, though the gains are much less.

 

[Scootin159]

<< top speed is where peak horsepower is??

I somehow doubt that.. can u explain why this is so? Personally I think the top speed depends on a great many things.. first of all, the gear ratios! how much force can you put to the ground at the RPM that your car reaches it's top speed? (requires both the torque curve and the gear ratios) it has to equal the force of friction, so u also need to know the coefficient of friction..
>>



He's saying that using a CVT to keep the engine at it's peak horsepower RPM would allow you to attain the best top speed.

 

[jamarno]

Peak fuel efficiency for a gasoline engine usually comes at about 65-75% of the RPM where torque is maximized.

 

[dkozloski]

A good rule of thumb is to gear your machine so that you turn the engine slightly beyond the RPM at which peak horsepower is achieved, otherwise you approach top speed asymptodically and never get there. Winston Cup racers give up a small amount of optimum gearing in order to have some slight margin left over for acceleration in the draft. The choice is critical and can mean the difference between winng or losing a restrictor plate race by only tenths of a mile per hour.

 

[KIAman]

Another big reason Audi uses the new CVT is because it is lighter and has less moving parts than an automatic. Nowadays, automatics can pretty much keep up with CVT in the fluidity of the car's motion, look at any acceleration graph of any new car with automatic (such as the MB S500) and it is perfectly smooth. Of course a broad torque curve on the engine helps.

Currently the only thing CVT has going for it is the lighter wieght, and lower gas mileage, acceleration is still on par with manuals.
Bringing in automatic manual transmissions (ex alfa romero or ferrari f360 paddle) offer the best of manual effiecency without the hassle of clutch engagement, but slightly less fluid in the acceleration graph.

So the disadvanatages of CVT include the low threshold of torque, the "infinite amount of gearing" is not a complete truth because of the fact that infinite ratios are impossible, and CVTs are rather given a RANGE of ratios.

What would be interesting is using some a controllable fluid viscocity transmission. Basically the shaft from the engine is already geared to spin very fast, maybe 10x the rpm of the engine or more, and the output shaft has no contact with the input. They are both encased in a fluid that the viscocity can be controlled by electrical current, thus creating way more options for gear ratios, in this case the car would be geared down and gradually geared down less. The more thick the fluid, the more the output shaft rpm equals the input shaft rpm. Of course i am making all this up and there is no application for this since i dont think there is a fluid out there that can change its viscocity according to electricity (only chemical composition and heat that i know of can change visc). So my bogus controllable fluid viscocity transmission would completely depend on the fluid that has all the forementioned properties. But it would be cool.

 

[dkozloski]

Many years ago(there is nothing new under the sun), a transmission was designed and built that used the electrically variable viscosity fluid described in the previous post. It was a mixture of oil and iron filings operating in a variable electro-magnetic field. As with all systems that depend on slippage it fell afoul of the laws of physics and succumbed to the heat of friction from the churning of the fluid and magnetic hysterisis. Like all these contraptions, starting with the 1910 Metz, as soon as you put some tire ripping power to them you realize just how well the existing systems have been developed and what a difficult task the engineers have to make the next leap to the "next new" technology.

 

[KIAman]

Kewl, i learn something new every day.