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14/11/2008
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Some tantalising technologies are being employed in British hybrid drive buses for the London Olympics. Tom Shelley reports
Oil-cooled induction motors are crucial to a new design of hybrid bus – hundreds of which may well be carrying visitors to the 2012 London Olympic Games.
Designed by a company (now owned by BAe Systems) in Johnson City, New York, the motors are being incorporated into British designed and built buses through a facility set up in Rochester, Kent. Here, a suitably equipped red double decker bus has been seen doing repeated stops and starts as it simulates a London bus route round the car park.
So far, hybrid drives with the motors – using the name, ‘HybriDrive’ – have been installed in more than 1,200 buses delivered to New York, San Francisco, Houston and Toronto, with another 1,500 buses on order. According to Rob Lindsay of BAe Systems at Rochester, these vehicles have been driven more than 70 million miles, saving five million US gallons of diesel. The order the company is most keen to secure right now is the 500 hybrid buses that Transport for London says it wants to purchase to take visitors to the 2012 Olympic Games - and then use locally thereafter. Initially, TfL is planning to have a 60-bus trial, for which Alexander Dennis and BAe Systems are contracted to supply 17 buses.
Using smaller diesel engines does not save the fuel. In fact, Lindsay explains that the engines are comparable to those used in conventional buses. However, by using regenerative braking to take the energy released during stopping, it is stored in the batteries (initially lead acid, but now nano technology Chinese-made lithium ion). This energy is then released to assist acceleration. “We look after the batteries very carefully,” says Lindsay. “If we were to use a small engine, this would significantly reduce battery life.” The original lead acid batteries weighed 1.8 tonnes, but the new ones weigh only 270kg to 360kg. The resulting improvement in fuel economy is regarded as well worth the extra cost, leading to a programme to retrofit the new batteries in place of the old ones on existing buses.
For a double decker bus, the motors are rated at 120kW continuous, or 175kW peak, and weigh 280kg. Torque is 425Nm continuous, 650Nm for four minutes and 900Nm peak. The 185 HP, 4.5 litre diesel engine drives a 145 kW, 135kg permanent magnet generator, which also functions as the starter motor.
Development of what is termed the ‘Propulsion Control System’ is still continuing. The company also intends to develop what it terms ‘Intelligent Energy Management’. This means that, if the control system knows which route a bus is on, it can draw power out of the battery going uphill, knowing that, even if nearly discharged at the top, it will become charged again when it goes downhill. It would also be possible to ensure the battery is fully charged when the bus enters a zero emissions area in a town centre that requires the engine should be switched off.
Pointers
* Hybrid buses are being driven by oil-cooled induction motors
* Engine sizes are similar to conventional buses. Fuel savings are achieved with regenerative braking
* Further savings are expected by using electrically-driven air conditioning, brake reservoir pumps and steering, and by improving the intelligence of the propulsion control system
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Author
Tom Shelley
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