The Continuously Variable Transmission (CVT)

At the moment there are basically two types of gear changing transmissions common in domestic cars. These are:

  • manual transmissions, or what, for some reason, Americans refer to as ‘stick shifts’, and
  • automatic transmissions.

Manual transmissions require the driver to manually clutch the engine in order for gears to be changed. This is a manoeuvre that requires a reasonable degree of training and practice to get right. According to a recent survey by the British Royal Automobile Club about 40 percent of drivers who have been driving manual vehicles regularly for more then ten years are still not performing ‘good’ manual gear changes.

Automatic transmissions perform the clutching of the engine automatically. Well technically they don’t really actually clutch at all. They have a non-rigid connection to the engine, known as the fluid-drive torque converter, that allows them to change gears without technically clutching the engine.

The problem with automatic transmissions is this fluid-drive torque converter. Even though most modern automatics (almost all automatics made in the last seven or so years) have final-drive-lockup—whereby in the final gear selection, which is normally overdrive, is a rigid connection is made between the engine and the transmission—a small amount of engine power is lost in the torque converter when final-drive-lockup is not engaged. This loss of engine power impacts fuel economy and vehicle performance.

Admittedly over the last eight years or so automatic gearboxes and torque converters have become very good. The economy and performance loss in new automatic transmissions is now well below 5 percent compared to the same model vehicle with a manual transmission. In fact, due to final drive gearing tricks, many automatics now achieve the same fuel economies as the same model car in its manual version.

Even so, there is still the unavoidable power loss from the torque converter, and when automatics change gears the engine is basically running at normal revolutions while the transmission is slipping as the next gear change is made. This wastes fuel and power.

Introducing the CVT

So, along comes the Continuously Variable Transmission or CVT. The CVT is not new. The idea of a CVT style transmission has been around for 30+ years and has been used in military vehicles for at least the last 15 (in one version or another). But due to a couple of factors the CVT has not made it to mainstream domestic vehicles—until about now.

The two main issues with the CVT for use in domestic vehicle has been:

  • The size and weight of the transmission (which is not a concern for military vehicles).
  • The noise the transmission makes (again, generally not a concern for military vehicles).

But with the development of new metals and materials, new manufacturing methods, and with the aid of computer design, both of these issues have been largely overcome. Modern CVTs are small enough and light enough to use in domestic vehicles.

They still do make a bit of noise, a whining whirring sort of noise, especially as you accelerate, but this is also becoming less of an issue.

Why are CVTs exciting?

Basically a CVT has no torque converter. You get all the advantages of an automatic transmission without the downside of the torque converter.

Like a manual transmission it has a rigid connection to the engine. So it provides all the power of the engine through the transmission, just like a manual. And there are no gear changes! With the CVT, as the name might suggest (continuously variable transmission), the gear ratios are continuously variable from the lowest possible gear ratio through to the highest ratio. Basically there are NO gear changes—ever.

So, as you might expect, not only do CVTs provide better fuel economy than an automatic transmission (which loses power through the torque converter), they provide better fuel economy than a manual transmission (which wastes fuel, power, and momentum during every manual gear change).

If you imagine a car going up a hill a CVT equipped car is never going to have to ‘stop’ and change gears. It will simply slide through the required ‘gears’ continuously—either moving down or up ratios as required— with no gaps or loss of power at any point. However a manual driver may have to clutch and change down during which time the car loses forward power and hence loses speed, which then has to be made up when the next gear is finally engaged. Then this will happen again when the driver changes back up a gear; and so on.

In the same model vehicle a CVT should provide about 3 to 5 percent better fuel economy over a well driven manual vehicle in city driving.

So there are no actual gears to change with a CVT?

Technically, no. With a CVT the ratios start at one end and go through the the other end with no ‘gear’ stops in between.

However, because a lot of people really like to change gears, what they do is put in some artificial stop points along the range so that you can actually change ‘gears’—should the desire overtake you. Obviously you are not changing any gear selections at all in reality, but you simply slide the CVT slip-chain one way or the other along the cones to a pre-determined ‘gear’ point. This allows those drivers that feel the need to ‘change down’ and thereby use the breaking energy of the engine to assist in slowing the car down.

Who uses a CVT?

VX

I am sure there are probably a number of car manufacturers that have a CVT vehicle but the main one of interest to me is Subaru. I am a huge fan a Subaru vehicles. Subaru currently have a CVT version of the Outback, Liberty (called the Legacy in the USA), and Impreza. And in 2012 in Australia they will be releasing the new Subaru XV, pictured above, which will be available either with a 6-speed standard manual transmission or a 6-speed paddle-controlled CVT.

Note that the combined (urban+highway) fuel consumption of the standard 6-speed manual version is 7.3 litres/100 klm whereas the consumption of the 6-speed CVT version is 7.0 litres/100 klm. On the open highway the fuel usage of the CVT is a respectable 5.9 litres/100 klm (remembering the XV is an all-wheel-drive car).

So, with its 60 litre fuel tank, on the open highway the Subaru XV CVT will cover over 1,000 kilometres (600 miles) on a full tank of fuel or about 850 until the yellow warning light comes on.

Now … how long before they put a CVT into the Forester?

I should add that I have never driven a car with a CVT so I have no idea what they feel like to drive.

BarryMark