Cvt transmission – how the continuously variable automatic transmission works

CVT transmission

A CVT transmission makes it possible. Continuous acceleration from a standstill without shift pauses and without interruption of tractive power. How a CVT transmission works, what advantages and disadvantages it has and much more can be found out in this guidebook.

What is a CVT transmission?

A CVT transmission is a continuously variable automatic transmission used in motor vehicles, agricultural machinery and machine tools. The abbreviation CVT stands for "continuously variable transmission". A CVT transmission operates without gearshift stages and therefore without interruption of tractive force.

How does a CVT transmission work??

In principle, a CVT transmission consists of two shafts, on each of which two conical disks, the so-called variators, are mounted so that they can be shifted. The tips of the variators are opposite each other, resulting in a V-shaped cross-section between the variators. The primary shaft is connected to the engine via a clutch. The second shaft leads to the differential gearbox. Power is transmitted between the two shafts by a wide, multi-row chain. The chain lies between the variators on both shafts without touching the shafts. Changing the distance between the variators of one shaft changes the transmission ratio.

When starting, the variators and on the first shaft are shifted as far apart as possible. The chain is close to the shaft. The variators of the second shaft, on the other hand, are as close to each other as possible. The chain is far from the shaft. This results in a reduction ratio similar to that of a small to a large gear wheel. For acceleration, the distance between the variators on the first shaft is reduced and increased on the second shaft. The reduction becomes a gear ratio like a big gear on a small one.

Due to the continuous change of the transmission ratio, the second shaft rotates faster without the first shaft having to rotate faster. This allows the engine to always work in the optimal power range. Only when the maximum ratio of the CVT transmission is reached does the engine speed increase to accelerate further. Power is transmitted from the variators of the first shaft to the chain and from the chain to the second pair of variators by force transmission. In modern CVT transmissions, the variators are adjusted hydraulically.

What are the advantages?

Probably the most important advantage of a CVT transmission is the stepless change of the gear ratio. This allows the engine to operate over a wide speed range with optimum power and minimum fuel consumption. Another advantage is that acceleration takes place without pauses between shifts or interruptions in tractive power.

Further advantages of a CVT transmission over manual transmissions and double-clutch transmissions are its smaller size and lower weight. Since a CVT transmission does not require any gear pairs for each gear stage, it can be built smaller and is also significantly lighter. Because of these advantages, mercedes has developed CVT transmissions specifically for the W169 A-Class and the W245 B-Class. Both series did not offer enough space for a conventional torque converter automatic transmission.

CVT transmissions

What are the disadvantages?

CVT transmissions are subject to higher wear than other transmissions. Due to the high contact pressure between the variators and the chain, both components are subjected to high loads and wear more quickly. Likewise, despite the high contact pressure, there is always some slippage between the chain and variators. According to transmission manufacturer luk, a special chain design has increased the efficiency to 98%, but 2% of the power is still lost.

In addition, CVT transmissions require more maintenance than other transmissions. They usually require more frequent oil changes according to the manufacturer’s instructions. Another disadvantage is that the maximum transmittable torque is limited. These transmissions are not suitable for large diesel engines and high-torque gasoline engines.

The biggest disadvantage for many drivers, however, is the unfamiliar noise when accelerating. When a vehicle with a CVT transmission accelerates, the engine revs up and then remains at that speed while the vehicle accelerates. This is not the same as the noise produced by a manual or torque converter automatic transmission.

Since when are there CVT transmissions for cars?

The first vehicle to be fitted with a CVT as standard was a car from the dutch vehicle manufacturer DAF. In 1958, the DAF 600 was first introduced with a CVT transmission called variomatic. This is a small car with a 600 cm3 and 22 hp engine. In the variomatic transmissions, a V-belt took over power transmission in the gearbox. In the successor models of the DAF 600, 2 CVT transmissions were often installed in the small cars. Each driven wheel had its own gearbox. This made it possible to dispense with a differential gearbox. The slippage of the belts on the variators is sufficient to prevent the inside wheel from spinning on the curve.

In addition, the variators in the variomatic are not adjusted hydraulically, but with the aid of vacuum cans similar to those used for brake boosters. Theoretically, it was possible to drive a DAF backwards as fast as forwards. By 1975, more than 800 DAF passenger cars with variomatic transmissions had been built.000 examples built. this year volvo took over the dutch manufacturer. After that, other vehicles with variomatic CVT transmissions were sold under the volvo brand name. The last vehicle produced in the netherlands for volvo with this automatic transmission came off the line in 1991.

The first CVT transmission, known as multitronic, was installed by audi in 1999 in the A6 models and later also in the A4 models. Since these transmissions have repeatedly caused major mechanical and electronic problems, they have now been replaced by the modern S-tronic dual-clutch transmissions. In europe today there are only a few vehicles that are equipped with a CVT transmission. Most of these are small cars or lower mid-range vehicles. In contrast, one in three vehicles registered in Japan is equipped with such an automatic transmission. In Japan, these transmissions are still very popular because they offer a high level of driving comfort in dense urban traffic.

CVT transmissions

How can I tell if my car has a CVT transmission??

You can easily find out if your car is equipped with a CVT transmission or with another automatic transmission. If, when accelerating, the engine speed does not change over a wide speed range, but the car nevertheless speeds up, then your car most certainly has a CVT gearbox. With a torque converter automatic transmission, on the other hand, smooth gear changes are noticeable and also audible due to the changing engine speed. A modern dual-clutch transmission accelerates with virtually no interruption in traction, but a change in engine speed is still clearly audible with every gear change.

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Christina Cherry
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