Does anybody understand how a CVT works?!?

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Just checking out some of my regular car sites this morning and came across a Murano review at www.canadiandriver.com. (http://www.canadiandriver.com.)

It's becoming more and more apparent that many automotive journalists simply do not have the mental faculties to grasp how a CVT works. I've seen a number of acticles which have explanations of how Xtronic works, and only a few actually seem to understand how a CVT works.

Here's this fellow's description:

"Briefly, for those who don't know, a CVT transmission utilizes an expandable belt for motivation, rather than gears. There are no shift points, and the belt stretches or contracts, according to engine rpms and load. Bruce Crick, Nissan's manager for product information training, claims that the advantages include no transmission "hunting" as the vehicle climbs inclines, and better engine braking during descents. In this case, the belt is made out of segmented steel and is mated to an electronically controlled all-wheel-drive system that directs engine power to the appropriate wheels when it's needed. The Murano is not a down-and-dirty off-road rig, in other words. "

What the heck does a expanding and contracting belt have to do with how a CVT works? The only expanding and contracting belt that I know of is the one I wear to keep my pants up. http://forums.freshalloy.com/ubbthreads/images/icons/smile.gif

Even if you could somehow control how "long" the belt was, why would that have any impact on the effective gear ratio? You just have a bunch of slack in your belt, which would cause it to slip over the pulleys.

This guy should do himself a favor and pop-open a skidoo engine...

[jberger]

Okay, here are some good websites for CVT reading.
Enjoy!

Overview of Transmission Technologies with all of the Nissan technologies explained: http://autozine.kyul.net/technical_school/gearbox/tech_gear.htm#CVT

Overview of CVT by MIT: http://web.mit.edu/klang/www/cvt.pdf

SAE Review of CVT V-Belt Related Losses: http://www.bath.ac.uk/%7Eenscjb/cvtlosses.pdf

Excellent "How it works: Audi CVT" by SAE: http://www.sae.org/automag/techbriefs_01-00/03.htm

Powerpoint of Toroidal CVT from Jatco http://www.jatco.co.jp/E_JATCO/SHOHIN/CVT_E.HTM

[MuranoFANATIC]

Belts in CVT do not "expand" the clutch sheeves move but the belt ceratinly cannot expand and contract, that statement is flat out wrong!

[Zed]

Taken from NissanNews on CVT:

2003 Nissan Murano Features Advanced Xtronic CVT, Offering Quiet, Fuel-Efficient, Shift Shock-Free Operation

Nissan, in the latest example in setting industry standards of automotive technology, offers the first North American availability of its advanced Xtronic CVT™ (Continuously Variable Transmission) beginning December 2002 with the introduction of the all-new 2003 Nissan Murano crossover sport utility vehicle. The Xtronic CVT, which comes standard in Murano, is mated to a 245-horsepower 3.5-liter V6 producing 246 lb-ft torque - a new record for CVT torque capacity.

The Xtronic CVT transforms conventional transmission design, operating as essentially “one gear” - resulting in a smoother, more efficient operation. Arguably the most technologically advanced transmission in automotive history, the Xtronic CVT operates more seamlessly when compared to even the very best automatic transmissions.

“Nissan has a strong heritage of leadership and innovation in technology, a reputation that is upheld with the Xtronic CVT,” said Bill Kirrane, vice president and general manager, Nissan Division. “The Xtronic CVT is a superior automotive transmission that provides outstanding efficiency and smooth, powerful driving performance.”

How it Works

While automatic or manual transmissions use multiple gear sets with an integral clutch and bands to achieve separate gear ratios, the Xtronic CVT replaces those discrete gear ratios with an infinitely adjustable ratio design.

Instead of using fixed gears, Xtronic CVT transmits power through two variable diameter pulleys and a high-strength steel belt. The Xtronic CVT “changes” ratios smoothly and seamlessly with no perceptible pauses, jerks or lurches – virtually eliminating traditional shift shock.

During acceleration, the pulleys move in opposite directions and the steel belt rides on a smaller diameter, closer to the axis; when the pulleys are close, the steel belt rides on a larger diameter, near the outer rim. The pulleys are controlled to expand and contract in opposite directions so the ratio of the drive diameters between the two pulleys can be varied over a wide range, resulting in an infinitely variable drive ratio. The Xtronic CVT offers a spread of “gear” ratios between its lowest and highest range (2.371:1 to 0.439:1).

At the heart of the Xtronic CVT is a high-strength steel belt, which is as strong as steel, but as flexible as a belt. The strength enables it to handle a high amount of torque input, fitting for Murano’s V6 engine.

Xtronic CVT Benefits

Xtronic CVT brings a number of benefits that are immediately recognizable. The most obvious benefit is the quick response and smooth ratio changes, versus the “clunk” of a shifting transmission that is familiar to all drivers.

In contrast, the Xtronic CVT is perfectly smooth – it naturally changes “gears” discreetly and minutely such that the driver or passenger feels only steady acceleration.

Another advantage of the Xtronic CVT is its powerful driving performance. CVT offers the convenience of an automatic with the performance similar to a manual. Due to its pulley and chain design, Xtronic CVT can maintain the ideal ratio to keep the engine in its optimal power range.

This power is especially felt in mountainous driving ranges, as it can smoothly tailor the gearing to suit the hills, rather than constantly downshifting, searching for the appropriate gear.

The Xtronic CVT also allows for improved fuel economy. By varying the gear ratio continuously, the Xtronic CVT suffers no power loss during shifting allowing the engine to operate under conditions of optimum efficiency. Thanks to these remarkable features, a CVT can improve fuel economy by approximately 10% over a conventional automatic transmission. As a result, the Murano has outstanding fuel economy estimates of 20/25 mpg City/Highway for 2WD models and 20/24 mpg City/Highway for Murano AWD models.

A History in CVT Engineering

Nissan Motor Co., Ltd. has been a leader in CVT research since the 1970s and currently offers CVTs in Asia and Europe.

In 1992, Nissan implemented the N-CVT on the March in Japan and subsequently developed the HYPER CVT in 1997 as the world’s first unit designed for application to front-wheel-drive cars fitted with 2.0-liter class engines. Today, Nissan sells about 180,000 vehicles per year equipped with CVT technology in Asia alone.

“We believe the new Nissan Murano, with its breakthrough styling, utility and technology, will have a huge impact on the crossover SUV class,” said Kirrane. “The Xtronic CVT gives Murano a competitive edge in this very competitive segment.”

The 2003 Nissan Murano goes on sale in December 2002 at Nissan dealers nationwide.

[MikeSable]

An easy way to think about it is to compare it to the sprokets on a bicycle. As you "change speeds," one end uses a smaller while the other uses a larger.

However, in CVTs, there are no sprockets- rather expandable pulleys, and essentially a rubberband (yes, not technically, but it's the best way to illustrate) between them.

If the drive side is smaller than the wheel side, you have a higher (numerically) ratio suited for acceleration. If the drive side is larger than the wheel side, you have a lower ratio sutiable for high-speeds.

In a conventional transmission, "gears" are placed equally throughout the range so that 1st is the best for accel, and 5th (or 6th, etc) is best to keep the engine working easy at higher speeds.

In a CVT, it eliminates the steps, and varys constantly (hence the name) while keeping the engine at a peak power output point during accel and at a peak fuel effiency point during cruising. This is why any suggestes to have a "sport shift" mode or something to that effect on a CVT would be counterproductive.

I personally thumb my nose at it, since it is one more thing interfeering in the connection between the driver and the road. But, I can see it's advanatages, and it has it's places: family sedans, economy cars, and trucks (imagine the releif of big rig drivers no longer having to deal with 18 speed transmissions!).

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Theoretically, a CVT can extract better performance out of a vehicle because it can keep the engine at the HP peak (assuming the belt isn't slipping).

The Audi CVT certainly bears this out, with the CVT equipped cars being faster (0-60, 1/4 mile) than the 6-spd.

I wouldn't mind a CVT in my 2k2 Max http://forums.freshalloy.com/ubbthreads/images/icons/smile.gif