Saturday, 22 November 2014
Question: Is RVT the new CVT?
OEMs, seemingly, are taking an interest in the reversible variable transmission (RVT) but is it a better transmission than the toroidal continuously variable transmission (CVT)?
One man thinks so, but then he would, being the inventor. And inventors, by their nature, consider their own inventions far superior to those of other people
Filip de Mazière is a director of Mazaro, a business based in Belgium. Filip de Mazière is the designer of the RVT. He has been designing transmissions and clutches for over 25 years for major manufacturers of high-end sports cars, passenger cars, off-highway vehicles and material handling equipment.
De Mazière has several patents for automatic transmissions, DCTs and off-highway power shifts, some of which have been in production since the 1990s.
In 2009, de Mazière co-founded Mazaro, an engineering company in Destelbergen, Belgium, concentrating on making the RVT production-ready for specific applications. The work consists of design and calculation of mechanical aspects, physical models and controls software. Mazaro is also engaged in research and testing to evaluate components, transmissions and subsystems.
The de Mazière RVT is described as one that offers up to 23 per cent less fuel economy and emissions while at the same time being a compact design without a clutch yet giving exciting dynamics.
It is said to be suitable for a range of vehicles, including passenger cars, city buses, coaches, trucks, off-highway machinery, all-terrain vehicles as well as “vehicles running on any kind of energy source: gasoline, diesel, hydrogen….including hybrids.”
One unit, designed for a projected input torque of 560Nm, has a maximum outer diameter of only 280 mm as the transmission does not many of the components usually to be found in transmissions. The complete RVT (including housing, but excluding hydraulics and electronics) comprises only 37 different parts, many times less parts than all other transmissions.
Such a build-up is surely enough to make any OEM sit up and take notice. Especially the “come on” of a 23 per cent fuel economy gain.
But that 23 per cent improvement in fuel economy? Is that really possible? Is it achievable? And if it is, how do manufacturing costs stack up against a 10-speed double-clutch transmission, or even a toroidal CVT?
Many attempts have been made to produce a stepless continuously variable transmission since Milton Reeves’ design in 1879 and attempts by Daimler and Benz in 1896. Forbes Perry, creator of the Perbury transmission (precursor of the Torotrak design), continued to invent CVTs until his death late last year. CVTs have long been a happy hunting ground of inventors seeking a transmission with 'dynamic' characteristics and fuel-savng capabilities. Ford Motor Company has adopted CVTs in the past and dropped the idea - even after building a plant to make them.
How does the RTV work? Six tilting planet wheels which rotate around their own axis and around the main shaft, transmit power by traction without drill-slip. The ratio and the driving sense are controlled by changing the axial position of the main shaft: the position of the main shaft defines the tilting angles of the planet wheels.
A forced zero output speed is obtained by keeping the planet wheels horizontal. The highest speed ratio of one design is 2.4 (corresponding torque ratio: 0.417), but different ratio ranges in forward and reverse can be designed according to the requirements of the application.
Not a toroidal CVT
According to de Mazière, the rolling surfaces of a toroidal system roll over each other as do cones of which the tips do not coincide. When the surface speed of both cones is equal at one point, slip occurs at all other contact points.
The rolling surfaces of an RVT system roll over each other as do cones of which the tips coincide. When the surface speed of both cones is equal at one point, the surface speeds are equal at every point on the contact line.
“Mathematically, pure rolling is obtained, avoiding energy-consuming ‘drill-slip’”, claimed de Mazière.
“The rolling surfaces of all CVTs suffer drill-slip, causing power losses,” explained de Mazière. “But the RVT’s rolling surfaces are specially designed to avoid this effect; they simply roll over each other as do cones with coinciding tips. This mathematically pure rolling minimizes power losses and oil heating.”
At present, four designs have been conceived: the RVT125 for passenger cars of up to 150bhp output and 507Nm torque; the RVT220 for vans, trucks and buses of up to 3.500Nm torque; the RVT230 for leisure vehicles and snowmobile of 100bhp and 556Nm torque, and finally, the SV210 for auxiliary drives.
No weight figures are given for the RVT125 transmission, but the RVT220 weighs 250kg, the RVT230 54.6kg and the SV210 some 16.5kg.
The RVT125 design is a first generation machine leaving the RVT220 and RVT230 as second generation transmissions.
Development appears to centre on a 50Nm design. The big question is whether this modest arrangement can be scaled up ten-fold to produce a transmission suitable for passenger car applications.
“We plan to build a second generation prototype of a single stage variator of 50Nm output to drive an auxiliary around spring 2015,” de Mazière told this newsletter. “Next year we also plan to start the integration of an RVT into a demonstration bus, a van or a refuse vehicle.”
When is the 540Nm RVT expected to run?
“At the moment, this unit is only a calculated model. We have no real prototype,” said de Mazière. “We have produced data of a design according to the second generation with an output torque of 556Nm. This version will be smaller and lighter than the first generation design. At what point in time this unit will run is too early to say, as OEMs have just started to look into this new technology.”
“Currently, we have some detailed designs for other applications following requests from customers though,” he added
As to the expected transmission efficiencies, according to de Mazière the new RVT of 556Nm output torque will have a maximum efficiency of 92.2 per cent, whereas an RVT with 3,500Nm output torque is designed with a max efficiency of 94.2 per cent.
“The single stage variator is calculated with a maximum efficiency of 96 per cent,” declared de Mazière.
But what efficiencies have been achieved in practice?
“On the current prototype, a peak efficiency of 90.6 per cent has been measured,” claimed de Mazière. “This prototype is still a first generation machine and the 90.6% value perfectly matches the calculated expectations. In the next (second generation) prototype, minor component optimizations should effect an extra 2-3 per cent efficiency gain.
Asked to estimate the manufacturing cost of a 540Nm unit for a production volume of 50,000 units a year, de Mazière said it was “difficult to say at the moment”.
“To define the cost, the complete project has to be finalized,” said de Mazière. “But knowing that in an RVT there are only about 35 ‘different make’ parts, and that the parts are ‘easy to manufacture and assemble’, we estimate that the serial cost should be attractive. Naturally, this also depends on the volumes we talk about.”
It seems that so far the transmission has been used for various development tests, with the result that no serious endurance tests have been conducted on this unit.
“Probably it has been running for several 100 hours,” suggested de Mazière, adding “Endurance tests have been performed in a special test set-up for the traction wheels. Here 116 hours at full load and maximum speed (22 million revolutions) have been obtained without a trace of wear.”
According to de Mazière the RVT is a “highly innovative transmission enjoying the benefits of a continuous system without the limitations of current CVTs.”
Rocky and undulating road
Notwithstanding these comments, toroidal CVTs (or IVTs as it prefers to call them) of the type developed by Torotrak plc in the UK over very many years have proved that the road from design through development is long and tortuous.
Torotrak Group generates revenue through licensees which include: Allison Transmission, Carraro, Iseki, MTD Holdings Inc., TATA, Univance and Xtrac.
Torotrak’s work with Allison Transmissions Inc. in the US is well known, as is its involvement with the unknown (to journalists) so-called European Truck and Bus Manufacturer. But both of these, not to mention earlier skirmishes with Ford Motor Company and others, point to the fact that the road to bring the toroidal CVT into volume production for road vehicle use is a rocky one, and one that is still incomplete. Yes, ride-on mowers and outdoor power equipment have brought revenues, but the ‘big one’ still eludes.
To hedge its bets, Torotrak has evolved its strategy in the form of a three-legged stool – gearless traction drive transmissions, variable drive superchargers and mechanical kinetic energy recovery systems (KERS) – and there may be more ‘legs’ to come. All are linked to CO2 and emissions reduction.
But for Mazaro there is a lesson to be learned. The road is hard and what looks simple on paper often turns out problematic to implement without a great deal of time and effort, even aside from the engineering, materials, fluid hydrodynamics and control issues.
The Torotrak design can be traced back to the 1960s. That’s a long time in anyone’s language for a digestion period. Mazaro should take note.