Why a Continuously Variable Transmission?

The continuously variable transmission has gained popularity in recent years for its ability to improve fuel efficiency and its simplicity. A decade or so ago, it was difficult to find a vehicle manufactured by a big name company that used a CVT. Since the start of the governments improved fuel efficiency requirements, these transmissions have become more and more common.

Many auto makers now offer an automatic transmission that is a continuously variable type or “CVT” type. This is an automatic transmission that doesn’t have traditional gears. Instead, it uses a drive section and a driven section that are usually connected through a metal belt. The belt that transfers the power from the engine to the wheels runs on two pulleys. There is a pulley connected to the engine side of things, and a pulley connected to the drive line side of things. These pulleys can vary their size so that the “gear ratio” changes.

A traditional automatic transmission is basically a manual transmission that shifts on its own. It has anywhere from 2 to 8 actual physical gears with fixed ratios. It jumps from one gear ratio to another, but it is always engaging the transmission in a fixed gear with a fixed ratio and then varying the engine RPM to meet the situation it is presented with.

A CVT type transmission is always in gear and the ratio between the RPM of the engine and the output to the wheels is constantly changed to meet demand. There is no set of gear ratios that the trans jumps to like in a normal automatic. The computer that controls a CVT transmission can set the trans to any ratio by adjusting the diameter of the two pulleys inside of it. In this way, a CVT can vary the gear ratio to achieve the optimum fuel economy, optimum acceleration, or anywhere in between depending on its programming.

continuously variable transmission

     Why this transmission is called continuously variable

This type of transmission is called “continuously variable” because it doesn’t have set gear ratios that it jumps to like in a traditional automatic transmission. This type of transmission never shifts gears from one ratio to another, it is always in gear, and the ratio is changed continuously based on the needs of the vehicle at any given time.

One of the big differences you will notice when driving an automobile equipped with a CVT is the lack of shifting. When you encounter a big hill in a vehicle with a traditional automatic transmission, you can expect that when the RPMs drop to a pre-determined speed, the transmission will downshift and the RPMs will go up so that you can maintain your speed up the hill. A CVT does not do that. When you encounter a hill with a variable transmission, there is no noticeable downshift or big jump in RPM. Instead what you get is a very gradual, almost undetectable change in the gear ratio of the trans, and a correspondingly slow and gradual increase in RPM.

It’s an odd feeling when the transmission doesn’t downshift. I’ve logged about 1000 miles behind a Chrysler CVT II transaxle. I still expect it to downshift when I notice the initial engine RPM dip while going up a hill. It never does though. It just slowly and evenly changes the pulley sizes in the transmission and increases the RPM slightly.

CVT

What’s that feel like?

The “seat of the pants” feel to this is not so great. It makes you feel like the vehicle is slower than a normal car, but it’s not. When you look at the other cars on the road you can see that you’re basically going the same speed as them. It doesn’t seem like you should be though. I guess it’s just the sensory feel of downshift, and sound of the RPM jumping that makes a traditional automatic transmission seem to have more power, or feel “faster”.

variable trans

Why are these transmissions becoming more popular?

The main reason that this type of transmission is becoming popular with automotive manufacturers is due to its ability to return better fuel economy numbers over a traditional automatic.

In the United States, the federal government has passed legislation that requires auto manufacturers to improve the average fuel efficiency of their consumer vehicle offerings. These regulations require that the car companies continuously improve the fuel economy ratings of their vehicles and set milestone MPG ratings that every manufacturer must meet. The car companies are looking for every way possible to keep up with these ever more stringent legal requirements while still offering a vehicle that consumers actually want to buy and drive.

One of the aspects that have been looked at carefully while trying to reduce fuel consumption is the transmission. Chrysler for example claims that the CVT2 used in their Jeep products is capable of achieving 6%-8% higher MPG than a standard 4 speed automatic. This is just the type of change that automotive companies are looking for to reduce their fuel consumption and keep within federal regulations.

The secondary advantage to companies who offer this type of transmission is the ability to advertise and market a vehicle that will almost certainly have better fuel economy than others in its class. Taking the Jeep example again, it wasn’t very long ago that there were no Jeep vehicles available that got better than about 20 MPG. Now there are Jeeps offered that can get 30+ Miles per gallon! A big part of that is due to the adoption of the continuously variable transmission.

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