Nissan's CVT technology

By Jim Kerr

Continuously Variable Transmissions (CVT) have been used by several manufacturers over the course of the life of the automobile. DAF, a Netherlands car company was the first to build a CVT in any volume. It used a simple rubber band and cone system that proved to be jerky and only suitable for low horsepower engines. Other manufacturers such as Subaru, Audi, Saturn, Honda and Ford have used or are currently using CVT's on vehicles, but the trend is to use multi-speed conventional automatic transmissions instead of CVT's. Nissan is bucking this trend, starting with their use of the CVT on the Murano SUV and now on the new 2007 Versa sub-compact car and Maxima sports-luxury sedan. They will add more CVT-equipped models in the near future until their entire sedan line-up is available with this transmission.

What does a modern CVT look like? Externally, it looks similar to a compact conventional automatic transmission. Inside, it is much different. The most common CVT today uses a steel push belt running between two variable width pulleys, similar to that snowmobile. Instead of pulling on the belt, the belt is pushed to drive the pulley. Imagine trying to push a rope. Every time you pull it, it turns into a stick, but push on it and it becomes a rope again. Sounds like magic but by using a special belt made of a series of small plates held in position by a cable, the belt can flex when no torque is applied to it, such as when going around a pulley but can lock solid when straight line force is applied to it.

A special oil is used to help the steel belt grip the pulleys. As pressure is applied to the oil, the molecules compress, turning into a crystalline form that locks the belt and pulley together. Relax the pressure and the oil returns to its original state.

There are not many parts inside a CVT. Two variable width pulleys are connected by the steel drive belt. A small planetary gear set and clutch is used to provide reverse gear operation. The final drive gears and differential are some of the biggest parts. An oil pump provides hydraulic pressure and lubrication. Finally, a computer-controlled valve body varies the width of the pulleys hydraulically so the belt can ride at different heights on the pulleys. This provides the varying gear ratio.

Nissan has been using CVTs for many years in Japan and has been able to adapt the technology for use with high torque, high horsepower engines. When Nissan originally introduced the Murano, they contemplated programming in seven specific shift points so that the CVT would feel more like a conventional automatic. Instead, they decided to go strictly with continuously variable operation, but use a torque converter in conjunction with the CVT to enable smooth quick starts. Once under way, the torque converter locks up at 18 kph so the CVT belt and pulleys provide all the gear advantage.

Since the appearance of the Murano over four years ago, Nissan has experienced virtually no problems with their CVT. For 2007, the internal mechanicals have remained much the same, but the computer controls have been improved for even better operation. Nissan refers to it as an Xtronic CVT to differentiate it from the earlier models. Downshift response has been improved by 30%, so when you step on the gas pedal, a lower gear ratio is selected faster. The transmission selects gear ratios more accurately than before, instead of "overshooting" the desired ratio and then compensating. Drivers can feel the difference as smoother operation and quicker performance. While they were at it, Nissan increased the torque capacity of the CVT too to ensure reliability.

Globally, Nissan will build about one million CVT-equipped vehicles in 2007. This is significant, when you consider the CO2 emissions reductions offered by the CVT's ability to optimise engine power. Five CVT-equipped vehicles reduce CO2 as much as one hybrid vehicle without the complexity of the hybrid technology.

Steel belt CVT's can approach 97% efficiency, similar to manual transmissions. Conventional automatic transmissions typically have efficiency in the 80% range although this improves as more gears are added. The increased efficiency and always-correct gear ratio enables CVT equipped vehicles to achieve up to 10% better fuel economy, lower engine emissions, and faster acceleration compared to conventional automatics. It will be interesting to watch how other manufacturers react to Nissan's increased implementation of CVT technology.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).