 As the green revolution takes root, Harrogate-based SR Drives is applying its switched reluctance technology to hybrid traction drives for environmentally friendly urban buses and local authority vehicles.
Urban vehicles are least efficient, and most polluting, on repeated stop/start duties and, with European cities facing the prospect of meeting ever increasing carbon reduction targets, public transport and municipal vehicles are coming under the environmental microscope.
Working closely with Green Propulsion, a Belgian company that specialises in developing cleaner prototype vehicles, SR Drives has developed two switched reluctance motor-generators for a hybrid power-train project designed to cut carbon emissions in vehicles such as buses and waste collection vehicles
In Green Propulsion’s ‘Series and Parallel’ hybrid transmission scheme, a 50kW and a160kW SR Drive® motor-generator both work in conjunction with a 2.5 litre diesel engine (downsized from 7.5 litres in a conventional bus). This combination is powered from lightweight low-loss Lithium Polymer batteries and enables the vehicle to seamlessly switch between pure electric running or hybrid power - whichever offers optimum efficiency. The technology also eliminates the need for a main gearbox, enabling direct coupling to the wheel drive, resulting in improved efficiency at all operating speeds.
SR Drive® technology was chosen because of its ability to maintain high efficiency over a wide operating range and for the exceptionally wide speed range over which it can produce rated power - essential in a system that does not employ a gearbox. Also, the absence of magnets in the switched reluctance motor-generator eliminates uncontrolled generation.
In tests, Green Propulsion’s combined hybrid system achieved reductions in CO2 emissions in excess of 40% and fuel savings of over 30% compared to conventional diesel engined vehicles. These savings are partly due to the increased fuel efficiency of the hybrid power-train and partly due to the vehicle’s ability to take energy from a conventional electrical supply during a charging cycle. This enables the vehicle’s energy to be obtained efficiently from conventional low-emission sources of electrical power. Another key feature of the Green Propulsion scheme is the use of low-loss Lithium Polymer batteries that provide significant weight savings and improved electrical efficiency compared with lead-acid batteries.
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