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Application of Torotrak IVT to Optare Bus Delivers 19% Improvement in Fuel Economy
2 October 2007
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| Components of the Torotrak variator. Click to enlarge. |
Torotrak, a developer of full-toroidal traction drive technology (earlier post), and Optare UK, one of Europe’s largest bus manufacturers, have recently achieved a 19% improvement in fuel economy in an Optare Solo Bus by replacing the standard fit 5-speed automatic transmission with a prototype Infinitely Variable Transmission (IVT).
Computer simulation of an optimized “production intent” IVT design demonstrates further efficiencies that increase the improvement in fuel economy to 23%.
An Optare Solo European Midi-Bus was selected as the IVT test vehicle. The 11,300 kg gross design weight Solo accommodates up to 60 passengers and is fitted with a Euro-3 compliant four-cylinder diesel engine and a 5-speed automatic transmission.
The standard fit 5 speed automatic transmission was replaced with a prototype IVT, originally developed for a Sports Utility Vehicle (SUV), with gearing modified to suit the Optare application. Testing was independently performed on the “Millbrook London Transport Bus” (MLTB) Cycle, at Millbrook Proving Ground, UK. The MLTB is an intensive stop-start cycle replicating a demanding urban bus route.
The core of the IVT is Torotrak’s ‘variator’, which comprises four identically shaped discs, being two input discs (shaded green in figure above) and two opposing output discs (shaded yellow), that form two symmetrical cavities. Each disc is formed so that the space created between the opposing pair of input and output discs is doughnut shaped; that is, the toroidal surfaces on each disc form the toroidal cavity.
Each cavity contains three rollers, positioned so that the outer edge of each roller is in contact with the toroidal surfaces of the input and the output discs. When power is supplied to the input disc, the disc rotates and the power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc.
The full-toroidal traction drive variator is torque controlled, meaning that the required system torque is set by hydraulic pressure and the variator follows the ratio automatically. Force is applied to the rollers via hydro-mechanical actuation which determines the output torque, and the variator is clamped together via a simple hydraulic end-load arrangement.
The transfer of power through the contacting surfaces of the discs and rollers takes place via a microscopic film of traction fluid. This fluid separates the rolling surfaces of the discs and rollers at their contact points and prevents metal-to-metal contact, increasing the durability and life of the variator components. The hydraulic forces and clamping pressures act at the contact points between the rollers and discs to make the traction fluid highly viscous, creating an efficient traction drive mechanism for transferring power between the rotating discs and rollers, both smoothly and quietly.
The variator alone cannot provide neutral and reverse drive, nor can it provide the ratio spread to achieve high overdrive. However, the torque control capability of Torotrak’s IVT allows the variator to be incorporated within a “two regime mechanical shunt” transmission arrangement to provide forward and reverse operation, generation of high output torques and extraordinary overdrive capabilities (approximately 60 mph / 1000 rpm). In addition, the IVT employs a geared neutral function which provides a zero output speed with the engine rotating so eliminating the need for an inefficient starting device such as a slipping clutch or torque converter.
Due to the ratio range and the torque controlled nature of the transmission, the IVT is able to decouple engine speed from vehicle wheel speed while accurately defining the load demand placed upon the engine. These features enable the engine to be operated at its optimum engine speed and load condition, regardless of the vehicle speed or power demand, so maximizing fuel efficiency and reducing emissions.
The result of this driveline control strategy is a vehicle which is simple to drive and that offers a smooth, gear shift free ride for the passengers, while delivering excellent fuel economy with a steadier and lower engine speed.
In addition to the power train efficiency benefits, the IVT also provides additional functionality including:
An automatic anti-rollback feature, without making use of the foundation brakes, where sufficient torque is applied to the wheels to ensure a stationary vehicle. A safety feature that can only be applied with a ‘geared neutral’ system.
An enhanced engine braking feature mimicking the electric retarder fitted on the base vehicle—the IVT increases the engine speed to fully exploit the overrun torque absorbing capability of the engine whilst maintaining the desired vehicle speed. Hence the retarder can be deleted.
The IVT decouples the engine speed from the vehicle speed enabling engine design to exploit new combustion regimes, such as HCCI, that are not easily accessed when using fixed ratio transmissions.
By operating the engine at the most efficient point then, depending upon application, the IVT may enable downsizing of the engine leading to further efficiency, fuel economy and carbon improvements.
For ultimate fuel economy, the IVT can itself be incorporated into a hybrid driveline. With the torque controlled nature of the transmission and the ability to manage power flow in either direction, the IVT is able to transfer power to and from any energy storage system—not only an electrical hybrid arrangement but also enabling mechanical hybrid systems.
Torotrak and Optare are now in exploratory discussions with partners regarding series production.
October 2, 2007 in Fuel Efficiency, Transmissions | Permalink | Comments (17) | TrackBack (0)
Comments
Posted by: gavin walsh | October 02, 2007 at 10:01 AM
gavin,
I don't know if Nissan uses it in any current cars (maybe in Japan?) but they developed a toroidal IVT similar to this one several years ago.
Posted by: Patrick | October 02, 2007 at 10:06 AM
@ gavin walsh -
toroidal CVTs are expensive because you need tight manufacturing tolerances to distribute engine torque equally among the rollers. You also need advanced traction oils, preferably formulations that briefly change into the solid phase in the small area of very high Hertzian (contact) pressures. This avoids the shear force losses of laminar flow but also exposes the roller elements to the risk of dry friction and associated wear-and-tear. Traction oils therefore require significant materials engineering effort.
For low torque applications (e.g. subcompact and compact cars), the simpler and cheaper cone ring transmission also offers mechanical CVT functionality without a belt. The principle is the same as for toroidals, but you're limited to one point of torque transfer.
http://www.gif-ac.com/en/developments/
Note: this animation is a 140MB download but well worth the wait.
http://www.ecurie-aix.rwth-aachen.de/downloads/krganimation.mpg
Posted by: Rafael Seidl | October 02, 2007 at 10:35 AM
There are other validated and working variators that could be used with power split to establish an IVT (by itself, HSD is essentially IVT with electrical variator). And ultimately, one can use a smaller IVT as a variator to control larger power split (staged IVT?), thus enabling extremely high ratio ranges and additional engineering degree of freedom for optimizations.
Posted by: jaros | October 02, 2007 at 01:55 PM
thanks Rafael, you're a goldmine of knowledge. :-)
Posted by: gavin walsh | October 02, 2007 at 04:31 PM
Why isn't this technology being used in F1 where the engines develop max power in a very narrow range near peak rpm? Back in the late 1960's DAF raced a F3 with their CVT tranny. This allowed them to tune the Ford Cosworth engine for more peak hp.
http://www.ritzsite.demon.nl/DAF/DAF_cars_p17.htm
Posted by: gary | October 02, 2007 at 05:19 PM
There may be something in the technical regulations preventing CVT usage.
For what it's worth, F1 engines already run most courses in a very narrow range between about 16000 and 19000 rpm (upper end limited by rules). The gearboxes are very robust, very light, very compact, and this season are changing gears in about 10 milliseconds.
Posted by: rob | October 02, 2007 at 08:22 PM
"There may be something in the technical regulations preventing CVT usage."
There is. McLaren et al were lobbying for its introducion years ago but the FIA ruled against it.
Posted by: clett | October 03, 2007 at 02:27 AM
The reasons why up till now there were few takers for CVT technology were:
1) CVT's couldn't handle higher-powered engines.
2) The acceleration feel of CVT's turned off many drivers.
If you've driven a 2007 Nissan Altima or Maxima with the JATCO CVT, note that CVT's can handle over 250 bhp of engine power and JATCO programmed the CVT so you minimizes the slipping clutch feel that drivers often dislike. In fact, it's very likely the 2009 Honda Fit when it arrives in the USA in summer 2008 will switch to a CVT instead of using a 5AT transmission like it does with the current Fit models sold in North America.
Posted by: Raymond | October 03, 2007 at 06:53 AM
The toroidal CVTs are also expensive because the toroids and the rollers have to be made with a very pure steel in order to avoid failure due to stress concentrations. Nissan had to develop a new process to produce ultra-pure steel.
Posted by: Jorge | October 03, 2007 at 10:26 AM
Sorry, but they were racing DAF's CVT transmission back in the 1960's in European F3 racing. There has never been a technical regulation in F1 that prevented CVT transmissions. And there is absolutley no cost factor when it comes to developing technology in F1.
The question was why hasn't this this supposedly (New)superior (IVT) transmission tecnology been used in F1 racing not CVT's
Posted by: gary | October 07, 2007 at 09:25 AM
FIA regulations concerning this area are quite clear:
9.3 Gear ratios:
9.3.1 The minimum number of forward gear ratios is 4 and the
maximum is 7.
9.3.2 Continuously variable transmission systems are not
permitted.
Taken from: http://argent.fia.com/web/fia-public.nsf/71EDB18A1DD3F7C3C12573290033806D/$FILE/06F1_TECHNICAL_REGULATIONS.pdf?Openelement
Posted by: Robert | October 08, 2007 at 03:37 AM
The FIA F1 rules, as mentioned by Robert, were a quick respons of the FIA following testdrives in the early 90's of a Williams Renault with a Van Doorne's Transmissions belt type CVT.
The Nissan Toroidal CVT comprises of a toroidal variator and a conventional torq converter. The Torotrak variant is 'smarter'.
Posted by: Ramshoek | October 09, 2007 at 06:01 AM
The Nissan Toroidal CVT is a half toroidal where as the Torotrak IVT is Full Toroidal.
Torotrak IVT "projected costs" are falling all the time and they currently have a low cost IVT in production and on marketing trials in ride on lawn mowers. If you are really interested in the Torotrak IVT have a read at www.torotrak.com
The Torotrak IVT is an inherent part of a new KERS device which is in advanced development in preparation for the 2009 F1 season under new rules published early this year.
Posted by: Jack | October 12, 2007 at 12:11 AM
Of more importance, certainly financially, is the recent licence agreement with Tata: http://www.torotrak.com/Investor/Information/RNS/01.10.07+New+Licence+Agreement+with+Tata+Motors+Limited.htm
Gaining the approval of India's Foreign Investment and Promotion Board should be a formality, and then the size of licence payment, likely volumes, and projected start dates will hopefully be revealed.
Posted by: Robert | October 12, 2007 at 05:29 AM
If necessity is the mother of invention, then $84 per barrel oil causes a lot of necessity.
Posted by: sjc | October 15, 2007 at 10:09 AM
I would imagine that the IVT would, over time, take over from the current 4 to 8 speed auto boxes, on the basis that it does what is said about it. It appears to be the 'next step' product, but I would imagine that it will take a considerable number of years to have a meaningfull share of the market. New technology in engineering products which take substantial investments to develope usually do, unlike electronic technology, which seems much cheaper to get into ptoduction. I can't see why one would chose to have stepped, and sometimes jerky, gearboxes when an alternative is becoming available. Just ride a bus up a hill and it often jerks about as it tries to find the best ratio. This, I expect, would be eliminated. It just takes time!
Posted by: Lewis Blght | April 23, 2008 at 01:43 PM
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I've been hearing good things about Torotrak's IVT for quite some time now; can anyone enlighten me as to why it has not found widespread application? presumably the big car co.'s don't want to pay licencing fees so they develop their own versions?