Torotrak, world leaders in the development of full-toroidal traction drive technology, and
Optare UK, one of Europe’s top bus manufacturers, have recently achieved an outstanding 19% fuel economy improvement 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 optimised “production intent” IVT design demonstrates further efficiencies increasing the fuel economy improvement to 23%.
With the growing concerns regarding “greenhouse gas” emissions and their impact on the environment, local authorities, bus operators and vehicle manufacturers are keen to embrace low carbon technologies. Comparable in price, weight and package to existing transmission technologies, the Torotrak IVT offers a simple stand-alone “low carbon” solution providing real world fuel economy benefits approaching those delivered by electric hybrid drivelines but at a fraction of the cost. 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.
Optare Solo with a Torotrak IVT
An Optare Solo European Midi-Bus was selected as the IVT test vehicle. The 11,300 kg gross design weight Solo, a very popular class of vehicle in Europe, accommodates up to 60 passengers and is fitted with a Euro 3 emissions 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.
How full-toroidal traction drive Infinitely Variable Transmission Technology works
At the heart of the IVT is Torotrak’s ‘variator’, which comprises four identically shaped discs, being two input discs and two opposing output discs, 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 unique 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 exceptional 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 maximising 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.
Additional features and benefits of the IVT
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
Delivering a measured 19% fuel economy improvement in prototype form (23% predicted with an application specific design) in an Optare Midi-Bus, the Torotrak Infinitely Variable Transmission demonstrates that significant strides towards a low carbon bus infrastructure can be achieved with simple driveline modifications.
Furthermore, the adaptability of the technology for use with either electrical or mechanical hybrid designs provides a platform for further carbon reduction.
Torotrak’s IVT offers a simple, stand alone solution to the increasing demands for ‘low carbon’ technologies from local authorities, bus operators, manufacturers and customers. Torotrak and Optare are now in exploratory discussions with partners regarding series production.