CVTs Part 3: CVT in the Passenger Car Market
DLN Feature Series: Continuously Variable Transmissions
Part 2: CVTs for Farm Tractors (links to Commercial Lubricants)
Part 3: CVTs in the passenger vehicle market
As discussed in parts 1 and 2 of this series, continuously variable transmissions (CVTs) have become more and more common in both on- and off-highway applications in recent years. Advances in technology have been the motivating force behind this increase in popularity and many analysts predict CVT’s upward trend in the passenger car market will continue.
There are many reasons for manufacturers to pursue alternatives to manual and traditional automatic gearboxes. These include: the potential to improve fuel efficiency; advances in electrification and hybridization of the powertrain; and the importance of reducing overall vehicle mass. Automakers are clearly buying in to these advantages, and market authorities such as the IHS Automotive Supplier Business Transmissions Report are predicting global Continuously Variable Transmission. An automatic transmission that replaces conventional gear ratios with a chain or belt and a system that adjusts diameters on either side of two pulleys to generate seamless transitions of speed and torque. production to reach 11 million units a year by 2017, up from 7 million in 2012. Japan and Korea are expected to lead the way here, closely followed by North America, China and South Asia.
Global presence of CVTs
Around the world, CVTs are more popular in certain countries than others, partly due to consumer preference, but also because of how local legislation influences vehicle size and engine capacity. With this in mind, Japan – with its Kei class (mini-vehicles limited to 660 cc and a torque capacity of 65 Nm) – is a major market for the technology. The fact that CVT is simple and well suited to engines with limited torque capacity encourages their use here, as well as in other parts of Asia such as China. Consumers have also embraced CVT technology on larger vehicles in Asia, favoring its excellent smoothness and drivability in regions with high levels of traffic congestion.
In North America, CVTs have had a mixed history, especially because early transmissions suffered from poor performance and high unit costs. A low point in the industry came in the mid-2000s, when GM and Ford decided to end in-house CVT production, but since late last year there have been signs that those two manufacturers may be willing to re-visit the CVT principle. The incentive, once again, has been technical improvements in the transmissions themselves – especially their perceived potential to help boost fuel efficiency figures.
In Europe, CVT’s biggest hurdle has been public perception, especially the notorious “rubber band” effect, where the engine revolutions remain constant as the vehicle accelerates. To counteract this negative view, CVT engineers developed what amounted to manual “steps” to override the basic control programming of the transmission. It was believed that this improvement would break down consumer barriers by re-introducing the perceptible gear ratio changes that drivers had come to expect from other transmissions.
A familiar solution, as seen on the likes of the Toyota Prius as well as other hybrid passenger cars, is a sophisticated system which uses a planetary gearset to blend torque from the electric motors and the internal combustion engine (ICE) to achieve a CVT effect. In these applications, power can be split along multiple paths to allow both series and parallel operation and so achieve the optimum balance between torque at the road wheels and power for the electrical generator. This power split provides many of the same benefits as a conventional CVT, but achieves it using a combination of two power sources.
Automakers and their CVTs
One of the most notable European brands to embrace CVTs has been premium automaker Audi – yet recently, the company announced that its Multitronic CVT is to be discontinued. The German automaker says it has made the decision for “efficiency reasons,” and because its own S tronic stepped DCT. An automatic transmission with two concentric shafts and two separate clutches for odd and even gears. The two-shaft design allows another gear ratio to be pre-selected to provide faster and more efficient gear changes. can now achieve the same performance and economy levels delivered by the Multitronic.
Audi’s stance does not appear to have deterred other manufacturers and suppliers, who are working on new CVTs for all regions. Despite problems with its early CVTs, Nissan has been a powerful advocate for the technology and has adopted, among others, a high torque CVT from Jatco (Japanese Automatic Transmission Company) which offers high ratio spreads and more efficiency. On the 2013 Nissan Altima, fuel economy has been improved by up to 10 percent on the Japanese test cycle, helped by the latest Jatco Xtronic CVT8. This transmission has a ratio spread of 7.0 – compared to the 6.0 of the preceding unit – making it the widest span in its segment, according to Jatco.
Available in two variants – a belt-type model for four-cylinder applications and a chain-type model for diesels and V6 engines of up to 380 Nm – the CVT is able to simulate stepped gear shifts to provide consumers with a more traditional automatic transmission experience.
New technology applications
A more niche and unconventional approach to CVTs has been made by Torotrak. The company initially developed gearless traction drive transmissions for bus and track applications, but is now looking to adopt the technology on passenger cars. Torotrak’s full-toroidal traction-drive infinitely variable transmission is a variation on the CVT concept. While the company says the technology is suitable for passenger cars, any commercial tie-up with an automaker is yet to be seen.
A further important innovation in CVT technology is the smaller Jatco CVT7. Aimed at mini and mid-range vehicles, this transmission incorporates an auxiliary two-speed planetary gear set to broaden the effective ratio span of the CVT’s variator belt. The benefits are lower weight and smaller package space, a much wider ratio span and significantly improved fuel economy.
Elsewhere, CVT technology has been deployed on all-wheel-drive vehicles, with Fuji Heavy Industries developing a chain-type CVT capable of handling torque values up to 400 Nm for Subaru in 2009. The result was a reduction in fuel consumption of 5 percent compared to a belt-type CVT of similar torque capacity. Other large vehicles have benefited from Jatco’s CVT8HT, which offers a ratio spread of 6.3, up 17 percent from the company’s previous 350 Nm unit.
Market predictions for CVTs
Bosch remains positive about the North American market and has been investigating the geometry of its variator pulleys for CVTs. Previously angled at 11°, they now have a curved profile – ranging from 7° in the center to 11° at the outer edges. The bigger flank angle at the large running radius helps minimize deformation losses, while at the narrower angle there is less oil displaced by the pump, enabling a smaller pump to be specified.
The other factor that works in favor of the CVT is the ever-decreasing costs involved. Experts including IHS Automotive believe that, in terms of price, CVTs now fall between a four-speed automatic transmission and a dry Dual Clutch Transmission. An automatic transmission with two concentric shafts and two separate clutches for odd and even gears. The two-shaft design allows another gear ratio to be pre-selected to provide faster and more efficient gear changes.. The most expensive six-speed automatics and DCTs cost more, and CVTs are improving their price position.
The IHS report also adds that new-generation CVTs under development, such as those at Bosch, promise to further increase performance and reduce mass – and, therefore, reduce material costs. There are many who insist that despite their theoretical advantages, CVTs will never be able to match other forms of transmission for pure efficiency. The blame here goes to the energy losses in clamping the belt or chain, but such is the rate of improvement in lubricant and belt technologies that it would be unwise to bet against the CVT just yet.