More and more mass models of small and medium-sized engines now use CVT transmissions and its variants such as D-CVT, Direct Shift CVT or iCVT.

Many mass car manufacturers in Vietnam are using at the maximum CVT gearboxes for their models, such as Honda (City, Civic, CR-V), Toyota (Vios , Altis, Corolla Cross, Raize), or Mitsubishi (Outlander)… CVT is the solution to the problem of fuel economy and smooth operation on mass cars today.

In terms of terminology, CVT stands for Continuous Variable Transmission. The variable here is the gear ratio. Because it is continuously varying, the car will not be limited to a finite number of gear ratios, i.e., having no gear levels.

To put it simply, CVT is an automatic transmission, but there is no specific gear number hierarchy (6 speed, 7 speed…). Instead of using gears to engage with each other at each speed change, the CVT uses belts that connect the pulleys. When the position of the pulleys change, the gear ratio also changes.

The CVT gearbox structure will include active pulley (input), belt, and passive pulley (output). In order for the car to go backwards, on the input will install a sun gear system, a planetary gear like on a traditional AT.

The CVT box has no gears, but to meet the needs of traveling on many road conditions, manufacturers have created simulated levels through the controller. This controller will fix the position between the pulleys corresponding to each gear, which fixes the gear ratio, creating a virtual gear like the AT gearbox. Usually, brands will create 6-7 levels of virtual levels, or will only have a single number to support downhill (symbolled L or B).

CVT gearboxes are increasingly popular because of many advantages such as simpler structure, lighter, more compact, more efficient, smoother acceleration and deceleration, and quieter movement, especially on small and urban cars. However, because the gearbox is driven by metal belts, the downside is the noise when stepping on the accelerator, or the belt slipping if the torque is large. In the certain areas, the CVT’s simulated gearbox will not be as optimal as the manual transmission.

To overcome these weaknesses, manufacturers try to create variations of the traditional CVT gearbox by interfering with a certain stage in the operation mechanism of this type of gearbox.

Daihatsu, a subsidiary of Toyota, has Dual CVT (D-CVT) integrated on the Rocky, or on the Toyota Raize in Vietnam. The basic principle of the D-CVT is that there is an additional gear system connecting the two input and output shafts of the gearbox in addition to the belt.

This gear system is responsible for reducing the load on the belt and increasing power when the vehicle needs more traction or speed. When the car starts or runs at low speed, the gearbox still works according to the principle of a normal CVT. But at high speed, the gear system will participate to reduce the load on the belt, thereby providing more efficient power transmission, reducing waste, and increasing efficiency.

While the changes of Daihatsu D-CVT affect the high revving area, its parent company Toyota has the opposite method, affecting low speeds. This type of Toyota transmission is called Direct Shift CVT. It has the same structure, also adds a gear system that Toyota calls the Launch Gear, which will operate when the car has just started moving to reduce belt slip, helping the car accelerate better, and reduce noise.

The D-CVT will have a start-up process like a regular CVT model, and the Direct Shift CVT will start like a car with a geared transmission.

This difference comes from the purpose of use, when the D-CVT is produced by Daihatsu, provided for small cars such as Raize, Avanza or Veloz. Because small cars with weak powers will often be maxed out when running at high speeds, auxiliary gears will assist high-speed running, improve performance when accelerating.

Meanwhile, Direct Shift CVT, applied to large models like Camry, will reduce the delay when igniting, avoiding belt slip due to large engines, heavy weights. These factors may impact belts beyond their capacity.

CVT still has other disadvantages such as maintenance costs, expensive production, and that it is especially difficult to apply on vehicles with large capacity and does not create an exciting driving feeling.

Recently, Hyundai has created another variant of the CVT, which Hyundai calls iVT, for use on the new Creta or Kia Sonet model. A special feature of this gearbox is that it can be used on even high-power machines such as 1.6 turbo, due to the new material with high load capacity.

Basically, Hyundai’s iVT uses metal straps with a chain link structure instead of a conical eye like a traditional CVT belt. The Korean automaker says that the chain belt has a longer life, requires less maintenance and is important to use the tension of the belt to adjust the diameter of the pulley instead of having to act from the outside.

In addition, many other manufacturers also have ways to overcome the disadvantages of CVT, to make this type of gearbox a standard on small cars.

Source: vnexpress.net