CVT GEARBOXES: COMFORT AND FUEL SAVINGS
Save fuel, improve comfort and go into medium or small cars. Discover in our insights on CVT (Continuously Variable Transmission) boxes
In 2017 CVT gearboxes will be present in nearly one in ten vehicles produced. The CVT boxes are used primarily in small and medium cars and offer good fuel economy, especially in the start / stop function. At first they were comparable to automatic three-, four- or five-speed gearboxes, but since the introduction of version six, seven and eight-speed , the CVT have been optimized to match their fuel consumption.
This type of CVT boxes are very popular in Asia, especially in Japan and China. Leading manufacturers of these continuously variable automatic gearboxes are Robert Bosch, JATCO, Aisin Warner, Honda, Punch, Mercedes and LuK.
As they are very light boxes , their low weight compared to other help to save fuel. Moreover, they are simple to manufacture. Last, they improve drivers’ comfort as they allow smooth gear changes and less vibration.
The CVT boxes, inside
Let us now examine the inside of these boxes. The CVT have two pulleys connected by a chain or belt. A pump exerts a hydraulic pressure to vary the ratio between the pulleys. To change the distance (radius) between the two sides of each pulley varies also the speed at which the belt or the chain moves. Thus, a pulley radius increases while the other decreases.
As a result, it changes the pair , but the belt tension is maintained. These changes in the spokes create a continuously variable ratios, hence the name of these boxes CVT.
Which type of oil for CVT gearboxes?
As for the oils needed for these boxes , their characteristics depend greatly on the design. In fact, the friction of the oil needed depend on the material used to manufacture the pulley (metal), the chain (metal or rubber) or belt.
The improvements that have been incorporated into these boxes involve changes in the variator and weight loss , the addition of an auxiliary transmission with planetary gears, the redesign of the oil pump , belts and metallurgy.
All this means that each manufacturer requires specific products .
At first automatic transmission oils were commonly used, but as CVT system specifications have changed, now different products are used
The main difference with ATF lubricant is a need for a single coefficient of friction for the system formed by the belt or chain and the pulleys. In fact, it should be raised to reduce the compression forces and improve efficiency. It must also be suitable for a wet clutch of the torque converter.
Thus, a coefficient of friction high and stable implies a low possibility that the chain or belt slides suffer under certain load conditions. It also helps to diminish the wear .
But the demands on lubricants for continuously variable transmission boxes do not stop there: they must have adequate viscosity , protect materials against wear and fatigue, be consistent with such materials, remove foam and provide stability to oxidation , between many others.
Moreover, these lubricants must have characteristics especially severe in viscosity stability and wear protection . In fact, the viscosity tends to be between 5.5 and 6 cSt to increase fuel economy. That's so important to have a really suitable oil to each gearbox, which really has the ability to maintain its operation in perfect condition.
Did you know? TOTAL developed a specific product for CVT gearboxes, TOTAL FLUIDMATIC CVT MV
http://catalog.lubricants.total.com/automotive/cars/fluidmatic-cvt-mv