Continuous Variation Transmissions
History of the CVT
Would you believe Leonardo DaVinci sketched the
first CVT in 1490. A Dutch automaker first started
using CVTs in their cars in the late 1950s, but technology
limitations made CVTs unsuitable for engines with more
than a 100 horsepower. Improved CVTs capable of
handling more powerful engines were developed in
the late 90s, and now can be found in cars from
Nissan, Audi, Honda, Ford, GM, and other automakers.
How the CVT works
Traditional transmissions use sets of clutches and
gears to provide a speed ratio for each portion of
the increase to full throttle. The transmission shifts
thru the gears to provide the appropriate gear/torque
level for the given situation that the vehicle is experiencing.
There are several types of CVTs, most cars use a pair of variable-diameter pulleys with a pair of opposing cones. A belt of chain is connected between the cones. The input shaft is attached to one cone, while the output shaft is connected to the other cone. Each of the pulleys are moveable; as the pulley halves come closer together the belt or chain is forced to ride higher on the pulley, this makes the pulley's diameter larger, and as the car accelerates, the pulleys vary their diameter and find the best output speed vs. engine speed. Think of it as the pulley system at an old grist mill. If you’ve ever seen the operator move the big wide belt to the next pulley he was effectively changing the rpm of the belt without changing the orginal speed of the pulley. In a sense this is exactly what a CVT unit does.
Driving a car with a CVT
The controls for a CVT are the same as an automatic: P – R – N – D – L, But while an automatic transmission has a set number of gears the CVT changes output speed smoothly from a dead stop to full speed with no gear change or rpm drop from shifting gears. When driving a car with a CVT, you never hear or feel the transmission shift -- it simply raises and lowers the engine speed as needed.
Many people find the CVT hard to get used to, because of the way the engine revs. When you step on the accelerator, the engine races as if it’s going slip out of gear similar to a faulty automatic transmission you would find on most every other vehicle. But the increase in engine rpm as the car takes off is a normal part of a CVT unit.
Advantages of the CVT
An engine doesn’t deliver a perfect torque range at all speeds or have good fuel economy at high rpm levels. This is where the CVT excels. The CVT can provide a quicker acceleration than a conventional transmission while keeping a higher fuel economy curve over a wider rpm range.
Disadvantages of the CVT
Automakers have gone to great lengths to make the CVT feel more like a conventional transmission. Most CVTs are set up to creep forward when the driver takes their foot off the brake. This provides a similar feeling to an automatic transmission, and helps an unfamiliar operator to feel more comfortable operating a CVT. In other words the big disadvantage is the consumer who will need to get used to how they work compared to what they have used in the past.
Still don’t understand the principles behind CVT?
If you’ve ever changed the speeds on a drill press you’ve operated a CVT system. In fact, a lot of small yard tractors, mowers, mini-bikes with torque converters all use a form of a CVT type transmission. Since the mid 50’s a lot of aircraft generators have used a CVT system. You’ll also find them on different types of heavy equipment and even a few military tanks. It’s a simple concept with great potential. It just takes a little different understanding when it comes to diagnosing them.
Different variations of a CVT
VDP CVT (Variable diameter pulley)
In this most common CVT system there are two pulleys with a V-belt running between them. The gear ratio is changed by moving the two halves of one pulley closer together and the two halves of the other pulley farther apart. This causes the belt to ride higher on one pulley and lower on the other. Doing this, changes the effective diameters of the pulleys, which in turn changes the overall gear ratio. The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) in order to maintain the proper amount of tension on the belt.
Toroidal CVTs are made up of discs and rollers that transmit power between the discs.. One disc is the input, and the other is the output (they do not quite touch). Power is transferred from one side to the other by the rollers. The roller can be moved along the axis, this will cause the roller to contact the near-conical parts at varying diameters which gives a smooth transition from a dead stop to full speed. A full toroidal system is the most efficient design while partial toroidals may still require a torque converter, possibly lose some fuel economy.
The magnetic CVT has two rotating transmission disks, each with magnets attached, synchronously revolving. A change in the radius of the magnets on each of the disks causes a change in the transmission ratio.
Infinite Variable Transmission
The IVT dates back to the 1930s; the original design converts rotary motion to oscillating motion then back to rotary motion using roller clutches. This original design is still manufactured today.
Most IVTs uses a form of planetary gear systems the output shaft rotation speed is equal to the difference between two other speeds within the IVT. This IVT configuration uses a continuously variable regulator (CVR). The maximum output/input ratio can be chosen from infinite practical possibilities through the selection of additional input or output gear, pulley or sprocket sizes.
The ratcheting CVT is a transmission that relies on static friction and is based on a set of elements that successively become engaged and then disengaged between the driving system and the driven system.. The transmission is adjusted by changing linkage geometry within the spinning components. Power is transferred from input to output only when the clutch or ratchet is engaged.
These CVTs can transfer substantial torque, because their static friction actually increases relative to torque. The drawback to ratcheting CVTs is vibration caused by the successive transition in speed required to accelerate the element, which makes it harder to develop a smooth ride in passenger cars.
Diagnosing the CVT
The latest CVT vehicle I had in the shop was a 2005 Ford Freestyle, 155,000.0 miles. This was one of those "sweet deals" at a bank repossesion sale that the guy couldn't pass up. The customers complaint: Will not go any faster than 50 mph. No harsh engagement, no delay or slip. No service codes. As a tech I was not prepared for what I first saw under the hood. All around the intake and valve cover gaskets were mouse droppings and signs of nesting material. I was already thinking... wiring problems. But I need to look further, due to the fact not a single code was set. The likelyhood of wires disconnected and not setting a code is rare to say the least. Using an IDS I ran the main pressure test while driving. (Engine rpm should be above 2100 rpms during this test.) I find this as one of the best tests to perform on these transmissions. Putting the car under driving conditions and watching the scanner information is the best way I know of to determine a problem like this. Comparing the difference between the PCA_MES and the PCC_MES pids (Main pressure sensor and pulley 2 pressure (secondary sensor) confirmed the customers complaint. The difference between the pids should be less than 44 psi. It was way off the charts. This led to the possibility of internal leakage or damaged “O” rings in the transmission. Now the mice thing was making a little more sense. My guess was the original owner probably had a problem with the transmission and never had it repaired. Probably tried adding fluid (the wrong type) but soon gave up on the whole thing. Then they left the car sit along side their house and the mice started to make a home in the car since it was sitting in the same spot for quite some time. Later on, the owner stopped paying the monthly statement and the bank repo'd it. It's only a guess as to how it all played out, but, I'll bet I'm not too far off.
Ford recommends using only the Motorcraft Continuously Variable Chain Type Transmission Fluid XT-7-QCFT. CVT fluid is bluish-green when it’s new but as it ages it will turn a darker green to a light brown. If there is an internal concern with the mechanical components, there will be some metallic residue in the pan and possibly you will see metal particles on the dipstick. One other special note; with other tranny fluids the odor is a good indicator of heat damage (burnt fluid) but not with the CVT fluid.
The fluid on the dipstick wasn’t bluish-green or even light brown, it was red. I think I’ve found the problem without going much further. Chances it was Mercon or Dexron fluid in the transmission. (On every page of the diagnostics there is a warning about using the correct fluid with these transmissions.) It clearly states; “The use of any other fluid or cleaning agents will cause internal transaxle damage.” Checking the dipstick for metallic debris in this case didn’t show much at all.
After pulling the pan down the internal damage was very noticeable. Flushing wasn’t going to be an option. The best solution was to replace the transmission and flush the remaining cooler lines.
What’s in the future for CVT’s?
There’s no telling if these will be the main stream transmissions of the future. They are a lot different than working on an automatic transmission that most of us are use too. But they’re out there, and it won’t be long until you see one in your shop. There’s a good chance the engineers will be working on improving these for many years. With that in mind, I wouldn’t doubt we’ll see a lot more of them in the future.