Friday, 17 July 2015
Are single-regime CVTs the way forward?
Step-less automatic transmissions continue to fascinate the automotive industry.
News of the 11-speed transmission should not be that surprising. It is only one ratio short of the 10-speed automatic that will be made available for the 2017 F-150 Raptor. The 10-speed transmission could then trickle down through the remainder of the F-150 model lineup and maybe beyond. (NOTE: the Fiat 500X has a nine-speed gearbox and 100bhp/litre gasoline engine.)
But could there be automatic transmissions with higher ratios than 11- and 12- speed gearboxes? Or will the CVT make a comeback? Ford has dabbled in CVTs before and eventually opted out. It even dabbled with UK innovator, Torotrak. But is Ford still interested? In Germany, both Bosch and Getrag have taken out patents for CVTs - the notion of taking out patents appeals to many researchers in transmission businesses.
And what about off-highway uses for CVTs? JCB, for example, has Fastrac models with 24 ratios.
A recent spike of interest in the last few days regarding an item published here in January 2014 suggests the interest in CVTs is far from waning. An item in Auto industry Newsletter – the obituary to Forbes Perry in January 2014 – has attracted attention and suggests that someone, somewhere has more than a passing interest in toroidal continuously variable transmissions.
It is known that before his death late in 2013, Forbes Perry – the inventor of the Perbury transmission, which formed the initial basis of Torotrak's foray into CVTs (and before that the British Technology Group (BTG) and before that the National Research Development Corporation (NRDC)) – was working on single-regime toroidal CVTs. Up to the time of his death, Perry probably knew more about toroidal CVTs than any living person. He certainly had been associated with them far longer than anyone.
Ever secretive when journalists were present, but nevertheless extremely clever and with contacts the world over, Perry produced a multiplicity of designs for single-regime (and other) CVTs which could have paved the way to a low-cost transmission.
This type of CVT can have either a roller epicyclic (like a variator but with a fixed ratio) which returns the direction of rotation to input as well as balancing the end load, or it can take the form of a conventional (even shorter) epicyclic which deals with the direction. However, this design requires a thrust bearing for the end load with the result that the design is not very efficient.
Single-regime CVTs are slightly less efficient, however the fact they are compact makes them ideal for small, low-powered units – such as lawn mowers, quad bikes and small city cars. The offset to their lower efficiency arrives in the form of small size, low weight and lower cost. However, efficiency becomes increasingly important as the torque numbers go up. It is this that rules them out for higher powered vehicles.
Nevertheless single-regime CVTs, with a single rotor, do offer advantages for vehicles in certain applications where fuel economy might not be so critical. For example, transmission efficiency alone may not be a deciding factor compared to being able to run a diesel engine close to its optimum during much more of the operating cycle.
This may be one reason why specialist equipment makers, such as JCB, have shown more than a passing interest in CVTs. Single-regime CVTs might offer certain advantages as a replacement for a hydrostatic transmission.
JCB discloses little about its work in the area of CVT, just as years ago it denied all rumours that it was planning to make its own range of diesel engines, when in fact the company’s founder was working on the concept and today JCB makes its own diesel engines.
Nevertheless, in the clandestine world of CVT technology it is known that JCB is knowledgeable in the area of toroidal film drive transmissions. There is just the possibility that this type of transmission could cover all or part of JCB’s vehicle range allowing its engineers to be rid of hydrostatics. And these transmissions could be made in-house.