Tech Briefs - Tokyo Motor Show supplier technology
February 2002
More 1
2
3
4
5
6
7
NSK Powertoros Unit CVT
 The half-toroidal CVT from NSK uses a pair of tilting power rollers to transfer power from one spinning cone to another. The angle of the power roller determines where it contacts the cones on the input and output sides of the transmission, varying the effective gear ratio. |
NSK Ltd. revealed details of the design and development of its Powertoros Unit half-toroidal CVT, as used since 1999 in the Japanese-market Nissan Cedric and Gloria models. The toroidal version differs from the belt-driven one in that it uses a spinning power roller to transfer force between disks. The reduction ratio depends on the power roller angle, which determines where it contacts the input disk powered by the engine and the drive disk on the output side of the transmission.
The difference between the half- and full-toroidal CVT is that the half-toroidal design uses a more efficient hemispherical geometry rather than a fully spherical geometry between the input and output disks. The hemispherical geometry reduces the power roller angle, trimming the spinning losses compared to a full-toroidal design.
CVTs have been slow to realize their potential because of practical challenges to a simple idea. Charles Hunt invented the toroidal CVT in 1877, and the so-called "friction-drive" transmission was employed in early cars through the 1920s. But poor durability and limited power capacity killed off the CVT until better materials and manufacturing processes could be developed.
The huge force between the power roller and the disks limited the durability of toroidal CVTs. This force caused metal-to-metal contact that accelerated wear, so a suitable lubricant was needed to transmit power between the disks and the roller without allowing direct contact.
NSK employs a specialized oil that becomes a glass-like solid under high pressure at the contact points. Force is transmitted through an elasto-hydrodynamic lubrication-oil-film layer that is 0.001 mm (0.0003 in) thick, rather than through the metal, so wear from metal-to-metal contact is avoided. Also, the large forces involved cause great strain on the parts, so an improved pure steel alloy was needed to withstand the strain. NSK responded by developing "extremely purified steel technology," which reduces non-metallic inclusions in the material.
The remaining challenge was to find a way to let the metal surfaces survive the inevitable contamination by hard particles that are present in transmissions and differentials as the result of wear to gears and other parts.
Squeezing debris between two metal surfaces causes indentations in the metal that lead to flaking of the hard surface and rapid wear that can cut service life to 1/5th the expected duration. Bearing specialist NSK developed a metallurgy for bearings that face similar challenges, and applied that to the CVT. The solution was to increase retained austenite, a soft metal structure, in the alloy to provide some resilience. A carbon nitriding surface treatment preserved the needed hardness.
The resulting steel-alloy components are heat-treated, then polished and post-processed to a mirror finish accurate to within a few microns.
Visteon CO2 air-conditioning system
 The only unusual component in Visteon's CO2 air-conditioner system is the heat exchanger, which uses coolant from the evaporator to chill the high-pressure circuit between the condenser and the evaporator. Click to enlarge  |
Concern about the ozone-depleting properties of CFC air-conditioner refrigerants is leading component suppliers to develop automotive air-conditioning systems that employ CO2 as a refrigerant instead. Using CO2 not only eliminates a widespread application of an ozone-depleting chemical, it also creates a more efficient system.
The enhanced efficiency of the system halves the increase in fuel consumption caused by the use of an air conditioner in hot locations such as Phoenix, AZ, according to Visteon Corp. In that hot climate, a conventional air conditioner causes a 16% increase in fuel consumption, but a CO2 air conditioner increases consumption only 8.3%. Cooler climates, such as western Europe, showed a similar reduction, with the increase falling from 9.1 to 4.6%.
The first CO2 air-conditioner system from Visteon is likely to appear in a European luxury car in 2005 or 2006, according to Joern Froehling, Visteon Supervisor, Advanced Technology, Europe. "But it is too early to say what brand," he said.
While automotive and residential air-conditioning systems have traditionally relied on CFC refrigerants, use of CO2 is not entirely new. "The idea is pretty old," said Froehling. "It has been used in ship refrigeration for a long time."
The obstacle to its adoption in automotive systems is the extremely high pressure required to contain the CO2. Otherwise, the layout of the system is similar to that of a conventional design. "It uses the same components as today, but the system operates at a much higher pressure level," said Froehling. "The process is super-critical, about 14,000 kPa (2030 psi)."
Froehling also explained the trick to containing CO2 under such high pressure. "We use special material for the hoses because of the high pressure. CO2 is a small molecule, so it would just escape."
While the components employed will be familiar, their design will be tweaked from current practice to better suit the new refrigerant. "It requires the same components, but a redesign of those components," he said. The new components either will be the same size or smaller than their current equivalents. "It is a question of dimensioning parts correctly and selecting the right materials."
While accommodating the high pressure in an automotive CO2 system is challenging, similar pressures are handled routinely by hoses in the power steering system in many cars, according to Froehling.
One additional component required for the system is a heat exchanger that uses the coolant to reduce the temperature of the high-pressure part of the system. "The heat exchanger can go in an air-conditioning line or (be) integrated into some other components," he said. "You connect the cold line to the hot line. The cold line going into the compressor cools (the) hot line coming out of gas cooler."
Good news, especially for customers who live in hot climates such as Phoenix, is that this refrigerant is actually more effective than the chemical it replaces, so they can look forward to cars that cool off even more quickly. "CO2 systems can be cooled down even more than conventional systems," said Froehling. "The CO2 system shows better efficiency."
The increased efficiency also makes CO2 air conditioners useful as heat pumps to provide supplemental heat for vehicles such as direct-injected diesels, which generate little waste heat that can be used for cabin heating.
Service should also be simplified because CO2 need not be recovered when the system is repaired. "It is different, but from a service station (standpoint), it should be easier," said Froehling. "You don't have to recover the stuff; you can release it into the atmosphere." The cost of CO2 refrigerant is very low, which is also attractive. "The supply base is huge and the cost is very low."
The cost of CO2 air conditioners initially will be higher than for conventional systems. However, nothing is inherently more expensive about CO2 designs, so as volume increases, the cost should fall to similar levels as CFC systems, according to Froehling.
