Torotrak Infinitely Variable Transmission Finds Business in Outdoor Power, Targets Hybrids for the Future

15 December 2005

Cross-section of the IVT.

Torotrak, the UK developer of a fuel-efficient toroidal “infinitely variable transmission” (IVT), has formed a 50–50 joint venture with MTD Holdings Inc to apply technology to the Outdoor Power Equipment (OPE) market.

The new venture, named Infinitrak, will boost Torotrak’s revenue while the company continues to work on placing its IVT not just in conventional vehicles, but in new parallel-hybrid systems as well. An IVT-based parallel hybrid can, according to the company, provide a greater than 35% increase in fuel efficiency over conventional vehicles, thereby matching the fuel efficiency gains of more complex and expensive series-parallel hybrid drives.

The outdoor power deal. Under the terms of the joint venture, Torotrak will vest in Infinitrak exclusive global rights to IVT technology in the 0-25kw power range, and exclusive North American rights in the 25-45kW power range for certain products.

Infinitrak expects to commence manufacture of IVT units in 2006. Production in the first full year of manufacture is expected to exceed 100,000 units. The OPE Market includes annual sales of more than 2 million powered ride-on products in the United States alone.

The automotive market. Automotive, however, remains the key target market for Torotrak, which has been working with Equos and Koyo for a number of years to develop production prototypes of the IVT, although the company has yet to ink a volume distribution deal with an auto major. (Hence the importance of the outdoor power market as a revenue stream.)

Equos is the company within the Aisin AW group that undertakes new product development. Aisin is the world’s largest tier 1 supplier of automatic transmissions, and a key early partner with Toyota in the development of the Hybrid Synergy Drive.

According to Torotrak CEO Dick Elsy, the company is making progress with its automotive work.

The current three-way project to develop a transmission with Equos and Koyo is now focused on the calibration of the compact IVT in the unnamed target customer’s vehicle.

The calibration process is crucial to getting the best out of the transmission in the eyes of their customer, one of the world’s foremost car companies. This involves a comprehensive optimization of engine and transmission, to achieve the optimum combination of fuel economy, performance and driving feel. This is a normal but critical development process for any new transmission.

The continued extensive evaluation of IVT by another global car company has led to a program of work to understand the performance of IVT in a specific vehicle which they manufacture. This vehicle, which features a large diesel engine, already enjoys good fuel economy but we have been able to demonstrate that a further improvement of 9.5% is possible with the IVT design considered for this application.

The competitive bidding process started last year by a further car company has reached the point where that company has informed the bidders that it wishes to consider working with another car company using the same transmission manufacturer to build IVTs. Torotrak is assisting in this process to bring together interested parties to realize this goal.

—Dick Elsy, CEO

A decision by Toyota Motor in 2000 to terminate a licence agreement with Torotrak sent the company’s shares plummeting more than 60%. Toyota gave as its reason “unacceptable driveability” in the form of vibration in the Toyota IVT-equipped prototype.

At the time, Torotrak’s potential had pushed the company’s valuation to nearly £400 million (US$705 million). A similar decision by GM early in 2002 had a similar effect, precipitously knocking down the stock price even further.

Generic simplified (and expanded) layout of the IVT.

The IVT. Torotrak was created in 1998 out of the BTG Group, although work on what would become the Torotrak IVT began in 1988. This type of approach to a transmission has a long history, reaching back to an 1877 patent by Charles Hunt.

Key elements of the IVT are:

• The input gearset. The input gearset transmits the power from the engine via the low regime clutch to the planet gear in the epicyclic gear train.

• The variator. The variator is the means by which the IVT can deliver an infinite range of ratios. It effects the speed of rotation of the sun gear in the epicyclic and is responsible for the smooth variation of ratios which the Torotrak transmission produces.

  Compact variator assembly with roller detail.

  Inside the variator are two pairs of discs. The space between each pair of discs forms a hollow doughnut shape or toroid. Within each toroidal space there are three rollers. These rollers transmit drive from the outer, engine driven, discs to the output discs located in the centre. The rollers transfer power between the input discs and the output discs via a traction fluid.

  Each roller is attached to a hydraulic piston. The pressure in the pistons can be increased or decreased to create a range of reaction torque within the variator.

  When the rotational velocities of the input and/or output discs change, the rollers automatically alter their inclination in order to adjust to the new operating conditions.

  By varying the angles of the variators, the drive ratio can be changed infinitely, and by using a system of lockable clutches and the epicyclic gearset, the drive transmitted to the road wheels is totally seamless, unlike that of a conventional automatic, in which fixed ratios are engaged as required.

• Traction fluid. The traction fluid—a special oil—separates the rollers from the discs. Long chain molecules used in the traction fluid interlock with each other when the fluid is compressed, becoming highly viscous (glassy) under pressure. As pressure is exerted at the contact points between the rollers and the discs the oil resists the tendency to slide and transmits the power effectively.

  Thus, power transmission in the IVT is achieved by traction, (by shearing the thin, elasto-hydrodynamic fluid film) and not through metal-to-metal friction.

• Epicyclic (planetary) gearset. The central gear (sun gear) is driven by the output of the variator. The planet gears are driven directly by the engine. The outer annulus gear is connected to the wheels. The change of ratio in the variator (the speed it transmits) alters the relative speeds of the sun and planet gears.

• Fixed ratio chain. This takes the drive from the output discs and transmits it to the sun gear of the epicyclic gearset and the input of the high regime clutch.

• High regime clutch. The Torotrak IVT operates in two regimes activated by the engagement and disengagement of clutches which act as on/off switches. In low regime the epicyclic gearset is included in the power path from the engine. In high regime the output from the variator is directly connected to the road wheels.

The basic IVT offers four main benefits:

1. Torque-Based Control. The IVT is torque controlled, which essentially means that the required system torque is set by hydraulic pressure and the variator follows the ratio automatically. This control approach eliminates the need for a starting device such as a torque converter or friction clutch (also adding to the increased fuel efficiency).

2. High Overdrive. Very high levels of overdrive have been demonstrated in vehicles equipped with Torotrak’s IVT—the Expedition test vehicles have achieved 74mph @ 1,000 rpm.

3. Optimized Engine Running. The ratio range of the IVT provides an almost total decoupling of engine and vehicle speed. Thus a control line can be set which, for any given power demand, will operate the engine at a pre-determined torque and speed, largely independent of vehicle speed. This control line ensures that the engine always operates at minimum specific fuel consumption, i.e. at maximum efficiency. Typically, this means that the engine operates at low speed (reduced friction losses) and high torque (reduced throttling losses).

   Fixed ratio transmissions, with a smaller ratio range, must operate at higher speed and lower torque when delivering the same power to the wheels—hence their inferior fuel economy. In practice, the control line is the result of a multi-variable optimization, to ensure emissions and NVH are also controlled to provide the optimum trade-off. The result is that the IVT delivers a validated fuel economy benefit of 20% over a conventional four-speed automatic transmission, and more than 10% over a six-speed automatic transmission with emissions well within requirements.

4. Low-Speed Control. The IVT is able to provide sensitive, safe control of vehicle speed under extreme conditions, without the need for a low range gearset.

Torotrak has recently developed a lower-cost variator control mechanism—Epicycloidal Roller Control (ERC)—that promises further significant cost reductions for the transmission.

The IVT in Hybrids. On the hybrid front, Torotrak is collaborating with Cranfield University in developing a parallel-hybrid IVT applied in the Ford Expedition.

We are able to offer a 20% fuel economy improvement from the fitment of IVT to a conventional vehicle growing to more than 35% improvement with the parallel hybrid IVT derivative.

Our results also show that the hybrid IVT is capable of producing very significant fuel economy gains whilst the vehicle is cruising at speed—an area where many current generation hybrid vehicles perform relatively poorly. These results bode well for “real-world” fuel economy improvement.

—Dick Elsy

Achieving reductions in fuel consumption at highway speeds is one of the design rationales behind the GM-Chrysler-BMW work on a two-mode hybrid system. (Earlier post.)

Parallel hybrid layout featuring IVT

Basically, Torotrak’s contention is that a parallel-hybrid drive (a “mild” hybrid where the electric motor provides additive traction but does not drive the vehicle on its own) built with an IVT could match the increased fuel efficiency benefit of a more complex, series-parallel (“power split”) hybrid architecture (e.g., the Prius), but at lower cost and less complexity—primarily through the efficiencies gained from the optimum control of the engine.

In other words, Torotrak is suggesting that although its IVT hybrid wouldn’t have an all-electric mode, its increased base engine efficiency combined with the boost from the electric motor would match the fuel efficiency of series-parallel hybrids on the market (and be less expensive).

To test this out, Torotrak is working with Cranfield University and Newage (an electric motor company) with support from UK Government program, Foresight Vehicle. The purpose of the project—Electrically Assisted Infinitely Variable Transmission—is to quantify the combined benefits in combining a mild parallel hybrid system with IVT, first through modelling and then in a physical vehicle.

Resources:

• Torotrak interim results (with management discussion)

• IVT Technology: Building on the Fuel Economy Gains Delivered by Hybrids

• Increasing Power Density in a Full Toroidal Variator

• Full Toroidal IVT Stability