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A flywheel-based brake energy recovery system concept has a projected fuel saving of around 10% when retrofitted to a diesel multiple unit (DMU) rolling stock, a research and development project has shown.
Although it has previously been used exclusively on electric rolling stock, regenerative braking represents a particularly attractive fuel-saving proposition for DMUs. The launch phase efficiency of conventional diesel rail vehicle transmissions, which typically use a torque converter on starting from rest, can be as low as 30%. By capturing and storing energy otherwise dissipated during braking, a significant improvement in fuel consumption is possible if this energy is subsequently reused to augment acceleration from rest during the period of poor transmission efficiency.
The DDFlyTrain project, conducted by Ricardo, Artemis Intelligent Power and Bombardier Transportation, set out to demonstrate, through simulation and rig testing, the practical feasibility, operational fuel and energy savings, and economic investment case for the use of high-speed flywheel energy-storage technology on DMU trains.
Ricardo’s TorqStor high-speed flywheel energy-storage technology is connected to the DMU driveline via Artemis Digital Displacement hydraulic pump-motors in which computer-controlled solenoid valves coordinate the responses of individual pistons to the overall power and torque requirements. Compared with conventional hydraulics, Digital Displacement pump-motors have inherently high part-load efficiencies and controllability, making it economically feasible to use hydraulics in energy-sensitive applications such as DMU regenerative braking.
TorqStor differs from other flywheel designs in that it uses an innovative permanent vacuum. This is made possible by its advanced magnetic gear system that enables the transmission of torque across a vacuum without the limitation of rotating seals or necessity for vacuum pumps.
In the DDFlyTrain project, the optimal configuration for a DMU was found to be two 4.5MJ capacity TorqStor units with a maximum speed of 45,000rpm. The proof of concept demonstration test rig was a scaled-down version of the DMU architecture, based on a single 220kJ TorqStor.
David Rollafson, Ricardo vice-president of innovation, said: “The fuel savings and short commercial payback demonstrated for this technology make it attractive both as a retrofit solution for existing fleets and for new-build rolling stock.”