福特公司CVT自动变速箱介绍说明书

O ver the years there have been a number of automotive tech-nologies promoted as next year’s new thing. But some-how, they never quite arrived.Surely, the Continuously Vari-able Transmission fits into that category. Since it was first put into tiny little Dutch-built DAF cars 40 years ago, the benefits of CVTs have been explained to countless classes of automo-tive technicians. But as a mainstream trans-mission, CVTs have been stuck in low gear.This is about to change in a big way. Ford will introduce a CVT transmission in three new vehi-cles this summer—the Five Hundred, the Mer-cury Montego and the Freestyle. And these will not be small vehicles. This transmission will be in high-volume, domestically produced vehicles with an engine capable of 250 ft.-lbs. of torque. While other makers have been in production with CVTs,this is a mass-market break-through for this technology.Ford is not alone in mov-ing CVTs into higher pow-ered cars. The only transmis-sion option of-fered by Nis-san in its V6-powered Mu-rano is a CVT.Honda contin-ues to make the Multitron-ic transmis-sion. The Au-di A4 has a CVT option,as does the Mini Cooper.In addition, both the Toyota Prius and Honda Insight are equipped with CVT transmissions.While mass-market CVTs strapped to higher output engines is a story in itself, there are other new transmissions on the market and on the way that are also significant. Mercedes has a new seven-speed automatic in production, Audi an electronically clutched and shifted manual.Ford and GM are working on a common six-speed automatic that will be installed in both brands of vehicles. While Ford may have called this the year of the car, it might have been more correct to call it the Year of the Transmission.A major part of what’s finally making these long-desired changes possible is the electronic content of those trannies. The on-vehicle con-trol electronics, sensor technology and software make what was a struggling technology ready for mainstream application.The basic benefits of improved-technology transmissions have been around for quite some time. The truth is that internal combustion en-gines are really efficient only over a fairly nar-row range of rpm. The tach in your car might suggest that the engine can run from idle to red-line and you’d never have to worry about it.While mechanically this is true, it’s not true in terms of fuel economy, emissions and reliability performance. The window for best emissions and economy may be less than 1000 rpm wide.This new generation of CVTs has about half the parts of a comparable automatic. In a world where simpler means less expensive and more reliable, this is good. One of the major subsys-tems that will disappear is the torque converter;it’s not needed with a CVT. When it comes to size, a CVT can be about the same physical size and weight as a standard five-speed manual transmission. Cutting the weight of the trans-mission in half has a direct impact on vehicle fuel economy.With a conventional transmission, three or four gear ratios are used to adapt engine speed to vehicle speed. This means that variations in engine rpm have to cover the range between As the need for improved fuel economy and component
simplicity grows in importance, several automakers have
refocused their energies on CVTs to achieve both goals.
Mike Dale
continued on page 18
This CVT, made by ZF Transmissions in Batavia,Ohio, is used in Europe on the Ford Focus. Ford has recently taken full control of that joint venture,signaling the ramp-up in CVT production and usage by Ford and other world automakers.P h o t o c o u r t e s y Z F T r a n s m i s s i o n s
each gear. If there were more available gear ratios, the actual engine rpm op-erating range could be narrowed, keeping the engine in its most efficient operating range longer. The claims from automakers vary, but the general thrust of things is that with these im-proved-technology transmissions (not just CVTs), fuel economy can be im-proved from 6% to 10%. There are further claims of lower NO X emissions because the engine can be kept in a range where temperatures are low enough to prevent NO X emissions from forming.
The value of six- and seven-speed transmissions is that they can handle the torque load of higher powered en-gines, such as the typical American V8. The range of gear ratios is wider with the new improved-technology trans-missions than it was with the old. The ratio span of the typical four-speed au-tomatic is between 3.5:1 and 4.0:1; Ford’s new CVT has a ratio span of 6.0:1.
You can see how this works in the
new Mercedes seven-speed. Most
Overdrive ratios are in the range of
15% to 20%. This Mercedes transmis-
sion has a 50% Overdrive. The output
of the transmission will be two full ro-
tations for every engine rotation. This
should be good for high-speed, light-
load, flat-land cruising. With European
gasoline prices over five bucks a gallon,
you can understand the concern they
have over fuel economy.
When you put it all together—better
fuel economy, lower emissions, fewer
parts, lower cost (if not today, eventual-
ly)—CVTs seem to make so much sense
that you wonder why we haven’t had
them for a long time. The answer is reli-
ability and torque limitations. Van
Doorne’s original CVT used an upgrad-
ed drive belt made of much the same
materials and construction as a V-belt.
These belts originally had a life of about
20,000 miles, when expected vehicle
life was maybe 80,000 to 100,000 miles.
Even at a time when 50,000 miles was
acceptable for the life a VW Beetle en-
gine, this was not good enough.
More importantly, CVTs were not
scalable. You couldn’t simply build a
bigger belt and have it work on a 6- or
8-cylinder engine. The CVTs that have
been out there have always been on
small cars with relatively small dis-
placements and torque-handling re-
quirements. GM, for example, offers
them on 1.5- and 2.0L engines, and on
the 2.2L Ecotech engine used in Sat-
urn’s VUE and ION.
The road to an improved CVT has at
least two lanes. One of them is im-
proved mechanical integrity, which has
been brought about by the use of met-
al link belts and improved lubricants.
In today’s belt systems, the belts oper-
ate in compression, not in tension. This
means that the belt actually pushes the
drive pulley instead of pulling it.
The development of these new belts
has been the result of extensive com-
puter modeling. An outfit called
Abaqus Inc. has recently been talking
Eye On Electronics
about how its software was used by Honda to model the ef-fect the stiffness of the pulleys has on the life of the belts. Previously, it was assumed that the pulleys were perfectly stiff. When they actually programmed in the true material properties of the pulleys, they found that they did have some flexural stress, and that this could lead to misalignment of the belt. Computer simulations showed that stress on the belt could be lowered by 7% by making sure the pulley really was stiff enough to prevent misalignment.
The second lane on the road to improved CVT durability and torque-handling capability is something called integrat-ed drivetrain torque control. While there are steady-state loads on any transmission, it’s the transient overloads, or spikes, that could possibly do damage.
In GM’s VTi electronic CVT, torque management coordi-nation between the engine and transmission algorithms pro-vide overall transmission input torque protection. A recent SAE paper authored by GM engineers says that this soft-ware protects the variator system (their term for the pulleys and belt) from the shock loads during rock cycling and rolling garage shifts. It does this by controlling the combina-tion of input speed and torque limits needed to maintain the necessary belt clamp. They say their uniquely applied torque management strategy eliminates potential damage to the belt caused by inertia torque from sudden increases in wheel speed due to a loss of traction or harsh road inputs.
Further electronic content is in the software used to con-trol the CVT. In GM’s case, fuzzy logic is used to determine driver intent, between acceleration and steady-state opera-tion. This allows the CVT to operate effectively during accel-eration, providing a linear acceleration linked to increasing engine speed. This prevents one of the “dislikes” about earli-er CVTs called motorboating. In those versions, the engine could accelerate up and down in an odd way while the pul-leys moved in and out. Integration of throttle control and transmission ratio control provides superior cruise control without inducing high engine speeds or cycling, even in rolling terrain. A grade braking logic provides speed control and enhances cruise control operation, allowing more accu-rate cruise set speeds.
There have been some teething problems with the new-generation CVTs. Ford had hoped to have its CVTs in produc-tion a year ago. In January of this year, GM temporarily halted production of its CVT after making nearly 100,000 of them. Nearly 40% of Saturn VUEs and IONs were being equipped with CVTs at the time. There also have been reports of prob-lems with Honda’s Multitronic, especially in cases where power-increasing mods had been made to the engine.
Some drivers have found the completely smooth shifting of the CVT to be a bit disconcerting. Companies like Nissan, Audi and BMW have programmed in “bumps,” so drivers would sense that the CVT was correctly “shifting” gears. From a service standpoint, there are a couple of things for you to know. Typically, the fluid fills in CVT transmissions are special for each particular transmission. Also, the recommend-ed service intervals are shorter for CVTs than for conventional automatics. Your customers need to know that these service intervals must be followed if they’re going to enjoy the long service life these transmissions can provide.。