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CVT Evolution Part 1: Seamless Driving and Improved Fuel Efficiency!

What is CVT? A subtle usage of pulleys and belt
When starting off or on a slope, you switch down to a low gear and then it will be easier to pedal. But when your speed increases...

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2013/05/27

Three types of automatic transmission
Imagine you are riding a bicycle with gears.

When starting off or on a slope, you switch down to a low gear and then it will be easier to pedal. But when your speed increases or when you’re going down a hill, you will get the feeling that you are pedaling more than necessary if you stay in the low gear.

So you go up a gear or two, and then you zip along.

Well, an automobile transmission is fundamentally the same idea. In the same way that the strength of a human body has limits, so too is it hard to greatly change the torque in an engine. And that’s where gears come in. By using the high gears and low gears at different times, you can move forward smoothly without needing to change the amount of effort you put into pedaling – or, in case of a car, the amount of power from the engine.

With a car, a driver selects a low gear (i.e. first or second) when starting off from a traffic signal or going up a hill. And then, as the car picks up speed, the drive will shift up to fourth or fifth gear.

In Japan, over 90% of vehicles have automatic transmissions (often called just “automatic”) that can change gears by itself. We can classify automatic transmission systems into three main types. The first is the conventional automatic transmission (AT). Then there is the dual clutch transmission (DCT), and last but not least, continuously variable transmission (CVT).

Of the three, CVT is unique
Transmission, including the formerly mainstream manual transmission, selects the combination of gears from many types and then shifts gear. However, CVT sets the transmission gear ratio freely with the pulley and the belt. This is why CVT is known as “stepless” transmission.
Transmission Mechanism for CVT
CVT_002.jpg
CVT is the mechanism that can deliver the power of the engine efficiently to the tires through pulleys, one for the engine (left) and one for the tires (right). The pulleys are set up between two plates. As the plates get closer or further away from each other, the diameter of the front and back pulley parts in contact with the belt changes. CVT shifts gears (transmission) through this change in diameter, in other words, by the transmission gear ratio changing.
How does it work?
Okay, let’s go back to your bicycle. Think about how you switch gears on a bike? Changing gear changes the size correlation (i.e. transmission gear ratio) of the front and back wheels. Well, CVT uses a pulley and belt. As in the diagram below, by changing the width of two pulleys it becomes possible to change transmission gear ratio.

The advantage of CVT is that it can choose the optimum transmission gear ratio. For example, there are times with AT and DCT when the gear that is optimum for the vehicle’s speed is actually between fourth or fifth gear. The “gear” that would really give the best driving results would be “4.25” or “2.16,” but the number of gears is limited, so the system has to compromise.

CVT doesn’t have to compromise. It’s a “stepless” transmission where you can set the transmission gear ratio freely, meaning the car can always drive in the gear ratio that is most efficient. It can be used when driving at a set speed, as well as for continuous optimal transmission gear ratio during acceleration.

Due to this, fuel efficiency also improves. Another benefit is a smooth ride without the jerk you can get from shifting gears.

CVT is transmission with great potential, achieving a powerful driving experience that is smooth and with superb fuel efficiency.



Read more CVT with the next article in this series,
CVT Evolution Part 2: Transmission ‘Rivals’ Learning From Each Other.