Supply Side – Proving CVT Technology

CFT23 is ZF’s first CVT that uses a torque converter.

When continuously variable transmission (CVT) technology was first introduced in the mid 1970s, it was not without its problems. The pulley system was questionable. Simple maintenance adjustments of belts and clutches involved climbing underneath the car, meaning most owners rarely bothered, resulting in short belt and clutch life and the perception that CVTinstalled cars were unreliable and expensive to repair. The transmission package itself was bulky and needed its own space under the rear of the car making it available only on specially designed cars. The CVT tended to burn up the lubricant with some frequency. The list goes on.

But as ZF has demonstrated with its VT1F continuously variable transmission, that’s all changed.

Installed in the wildly popular Mini Cooper, the ZF VT1F CVT offers improved fuel economy and better acceleration figures than a traditional 4- or 5-speed automatic transmission.

The heart of ZF’s VT1F CVT is the variator. The variator consists of a high-performance steel thrust belt, looped around two pairs of double belt pulleys which transmit the drive output from the engine, essentially offering infinitely variable transmission ratios.

The electronic control unit of the VT1F allows the transmission to simulate gear ratios. According to ZF, drivers can use a tip shift system to select “gears” or allow the transmission to operate as a normal automatic.

Through substantial improvements to detail, CVTs have evolved considerably.

But despite the improved performance and reliability of CVTs, as Herbert Mozer, chief engineer at ZF’s Batavia, Ohio, facility points out, public opinion of CVT is still negative.

“The biggest obstacle for our first generation CVT was customer acceptance,” says Mozer. “The basics of CVT have not changed. Most of our refinements are in terms of software and how it will react in certain driving conditions. We now have ABS braking and traction control. We made refinements to have the right line pressure to the belt at the right time, improving fuel economy.

“The first CVT had no electronics, only hydraulic controls,” Mozer adds. “The famous rubber band feel is still on everyone’s minds.” Mozer admits that addressing public perception was a priority even in the early stages of development for the ZF VT1F.

“We hosted a number of customer clinics,” says Mozer. “We did different setups with different calibrations and showed it to the end users and got their feedback. We took their comments and used it to help us develop how the CVT had to be calibrated.

“When we started development with CVT, we realized that the CVT duty cycle has to be different than a step automatic,” says Mozer. “Ten years ago, we started to develop our own CVT duty cycle. This was developed out of experience, out of in-vehicle measurements and out of special driver condition measurements. We believe that we now have a CVT duty cycle developed that really represents 150,000 mile customer usage. And we can demonstrate this on the test stand.” The CVT test stands are capable of testing input and output power and are able to provide full load testing. Transmission test stands for its CVT are located at ZF facilities in Germany and North America.

Mozer sees the popularity of the Mini as a step in the right direction for CVTs, but he is also quick to point out that ZF isn’t letting the success of the VT1F affect its next CVT launch. The ZF CFT23 will be introduced later this year and when it is, it will have already performed more than 1 million miles of real world testing. “Knowing that in order to introduce new technology you have to first convince the customer, for the CFT23, we did something that we normally have not done before, fleet testing.” According to Mozer, ZF ran a fleet of cars in Europe more than 310,000 miles to prove the CFT23 in real world testing and is currently operating a fleet of cars in North America with a target of 1 million miles.

“We are about 50 percent done with our North American fleet testing,” says Mozer.

“The target is that every car will perform 150,000 miles. So far, there have been no problems. The fleet began operation in summer 2002 and will complete testing this June.”

ZF’s CFT23 is a continuously variable transmission for front-wheel and all wheeldrive applications. It is an integrated transmission control unit with adaptive shift programs and a chain driven variator. Torque capacity is 250 Nm (185 lbs.-ft.). Continuous variation of the rotational radii yields an infinite number of ratios.

A major difference between the VT1F and the CFT23 is that the CFT23 employs a torque converter.

“Especially in North America, we now believe that CVT has to have a torque converter,” says Mozer. “The North American customer really wants to have the performance feel of a torque converter and we are 100 percent sure that our concept with the torque converter is the right one.

“There will always be ongoing improvements with CVT,” says Mozer. “The key is to be prepared for ongoing changes and to have highly sophisticated software and electronics that are able to react in every driving condition. And we have had that installed since the very beginning.”