Gear change

UK gearbox designer Torotrak claims its transmission technology could boost the environmental performance of public transport after it achieved 19 per cent improved fuel economy in a bus.

The company said its Infinitely Variable Transmission (IVT) delivered the fuel savings when test-fitted to an Optare 60-seater bus, replacing the standard five-speed automatic gear system with a prototype IVT first developed for an SUV.

The Lancashire company claimed that a production-optimised design could squeeze a further four per cent economy out of the vehicle while remaining comparable in price, weight and performance to existing transmissions.

Public transport is the latest automotive sector targeted by Torotrak, which has spent most of this decade developing IVT and this year signed agreements with partners in areas as diverse as F1 and agricultural vehicles, including a licensing deal with India's Tata Motors.

At the heart of IVT is a simple hydro-mechanic variator, which comprises four identically shaped discs — two input discs and two output discs — that form two symmetrical cavities. The space between the opposing pair of discs is 'doughnut' shaped so that the surfaces on each form a toroidal cavity.

Each cavity contains three rollers, and the outer edge of each contacts the toroidal surfaces of the discs. When power is supplied to the input disc, it rotates and power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc.

'The variator technology is completely different to anything you can buy,' claimed Steve Murray, engineering director at Torotrak. With currently available continuously variable transmission technology, transmission relies on belts and pulleys to change ratios.

Torque for the Torotrak's variator is set by hydraulic pressure and the variator adjusts the ratio accordingly. Output torque is determined by hydraulic-mechanical force to the rollers. The variator is clamped together by a basic hydraulic end-load arrangement.

Power is transferred through the contacting surfaces of the discs and rollers by a microscopic film of traction fluid. This separates the rolling surfaces of the discs and rollers at their contact points and prevents metal-to-metal contact, which increases the durability and life of the variator components.

The key selling point will be the transmission's 'enormous ratio range,' which leads to better fuel economy said Murray.

'In the demonstration SUV, the overdrive ratio can go as high as 74mph with 1,000 rpm which means it is twice as high as any automatic transmission,' said Murray. 'On motorways you can go 70mph with less than 1,000 rpm on the engine.'

Another feature that leads to better fuel economy is the transmission's 'geared neutral' function that provides a zero output speed with the engine rotating. This eliminates the need for starting devices like slipping clutches or torque converters, which Murray said are inefficient.

While achieving 19 per cent fuel economy improvement was a good start, he said there is still room for improvement. Specifically, Torotrak will try to optimise the end-load clamping system and work on reducing hydraulic losses. Murray said computer simulations show that with an optimal production design fuel economy could improve to 23 per cent.