CVT (Continuously Variable Transmission)


  • General
  • Operation of the CVT
  • Stepless acceleration
  • Planetary Gear System
  • Accelerate in different stages
  • push belt
  • Video of the instrument panel when accelerating
  • Graphic

The CVT principle is old. In the year 1490 Leonardo da Vinci already designed the first concept of a continuously variable transmission.
In the 50s, these gearboxes were used in the popular DAF passenger cars (under the name Variomatic). These cars could drive forward as fast as they could go backwards.
Today, the CVT principle is still widely used. Certain car brands; Audi and Seat with the multitronic, Nissan, Toyota, Ford, Fiat, Suzuki, Subaru, Rover, Mini (BMW group), Doge, Jeep, Mitsubishi, Saturn (and perhaps another manufacturer) use or used a CVT in certain types gear box.
CVT is not only used in cars, but also in karts, snowmobiles, scooters and certain ATVs.

Operation of the CVT:
CVT stands for Continuously Variable Transmission. That means the gear ratios are constantly changing. In the manual, automatic and DSG gearboxes, each gear has its own gear ratio. This gear ratio is determined by the number of teeth of the gears on the shafts (with the manual gearbox) and the gear number of the planetary gear system (with the automatic gearbox).
With a CVT gearbox there are no fixed transmission ratios. It is not the gears that provide the transmission, but a rubber or metal push belt that moves over discs that vary in thickness. Changing the diameter of the discs results in a different transmission ratio. Because the discs become continuously larger and smaller in a smooth movement, there are no shifting shocks as is the case with a conventional gearbox.

The image shows how it works. However, the checkers vary in 1 movement from neutral, to small, to large. In reality, this happens evenly.

Stepless acceleration:
The primary conical disc is driven by the engine (crankshaft) and the secondary conical disc is connected to the differential and drive shafts of the car. At low speed, the primary drive is wide and the secondary is thin (see image below). Now you can accelerate at a high rpm and a low speed. In this situation a lot of engine torque can be transferred to the wheels for acceleration. As the speed increases, the primary drive changes from wide to narrow and the secondary drive from narrow to wide. During this change, the engine speed remains the same, but the vehicle speed continues to increase.

Accelerate in different stages:
Often with a CVT transmission you can also choose to switch in different stages. This can be done, for example, by moving the gear lever to position S (Sport) or to Manual.
With the Manual selection, as with the conventional Automatic transmission, the programs 1 (high speed at low speed) to 3 (low speed at high speed) can be selected. Or you can manually switch between 1st and 6th gear.
When manually switching on, for example, the 2nd gear (2nd stage), the gearbox behaves the same as a normal automatic gearbox. This is not because there is a construction in the box that makes this possible, but because the primary and secondary discs adopt a certain position. The electronics of the gearbox have a certain width of the discs in the memory for each step. Because the width between the two discs does not vary, the engine hangs at eg 3000 rpm at a speed of 60 km/h.

This function can be used when a caravan is being driven, or before the driver wants to overtake. Selecting a particular stage that causes the motor to rotate around the torque range makes it easier to start acceleration. In D (drive) mode, the speed automatically drops back to around 1500 rpm. When depressing the accelerator pedal (the kickdown), the discs must first assume a different position, which takes time. This problem is prevented by the Sport and Manual mode.

Planetary Gear System:
A single planetary gear system is also coupled to the primary pulley, which is also part of a conventional automatic transmission. This planetary gear system makes it possible to engage reverse gear.
More information about this can be found on the page automatic transmission. (After all, it works the same, and is not really important on this page).

push belt:
The push belt in passenger cars is designed as shown in the image below. The steel band is basically a ring, with metal discs clamped around it. The guide pins allow the metal discs to pivot relative to each other, so that a variation in the diameter is possible by making the moving discs wider or narrower.
Because the metal discs all abut each other, large pushing forces can be transferred. The angle at which metal discs are placed may be small when the disc assumes a small diameter.

Variable Disks:
The regulation of the diameters of the discs is determined by the ECU (the engine management). The engine management receives information from sensors, on which the position will be determined and the transmission ratio will be changed. The following information is important for the ECU:

  • Engine speed
  • Driving speed
  • Throttle valve position
  • Selector lever position (lever)
  • Position of planetary system
  • Driving resistances

Video of the instrument panel under acceleration:
Below is a movie of an Audi A4 with a Multitronic gearbox. This also works according to the CVT principle. This video gives a good impression of how this gearbox works and how this is experienced by the driver. This Audi driver kicks the gas completely. With calmer acceleration, the speed also remains the same with increasing speed, only then, for example, at 2000 rpm.

As can be seen in the video above, the speed remains constant when accelerating. This can also be seen in the graph below. A conventional automatic transmission shifts up a number of times, so that peaks in the driving speed are always visible. The CVT has a linear line without peaks. The advantage of this is that the motor is optimally loaded in the most favorable torque range.

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