Ricardo, Artemis Intelligent Power and Bombardier Transportation have conducted a project called DDFlyTrain, which showed that a flywheel-based energy recovery system could reduce fuel consumption on diesel multiple units (DMUs) by up to 10%.

The high-speed, flywheel-based, brake energy recovery system concept was developed for DMU rolling stock.

This hybrid technology is based on Ricardo’s TorqStor high-speed flywheel energy storage system and Artemis’ Digital Displacement hydraulic pump-motor transmission.

The optimal configuration for a DMU was found to be two 4.5MJ capacity TorqStor units with a maximum speed of 45,000rpm.

Ricardo Innovation vice-president David Rollafson said: "Ricardo’s TorqStor high-speed flywheel technology linked to the Artemis Digital Displacement high-efficiency transmission provides a technically feasible and commercially attractive means of enabling regenerative braking on DMUs, so providing a pathway to reduce the carbon footprint of this form of rail travel.

"The double-digit percentage fuel savings and short commercial payback demonstrated for this technology make it attractive both as a retrofit solution for existing fleets."

"The double-digit percentage fuel savings and short commercial payback demonstrated for this technology make it attractive both as a retrofit solution for existing fleets, as well as for application on new-build rolling stock."

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The project is set out to show the feasibility, operational fuel and energy savings, and economic investment case for the use of high-speed flywheel energy storage on DMUs.

Led by Artemis, the project has also produced a proof of concept test rig used to show the technology to rail industry stakeholders and a concept for integration onto a Bombardier Turbostar DMU.

Started in 2013, the research project was co-funded by Innovate UK and the Rail Safety and Standards Board.

The project included extensive simulation work based on field service data, which was used in the optimal sizing and design of a practical installation high-speed flywheel brake energy system for rail-based application, and the construction and commissioning of a test rig for demonstration purposes.

The company said that 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%.

Apart from its promising application on DMU rolling stock, the high-speed flywheel technology demonstrated in the DDFlyTrain project is also attractive for electric multiple units operating on DC conductor rail networks such as the London DC electrified lines region.

Artemis managing director Dr Niall Caldwell said: "Digital Displacement technology is ideally suited to railway driveline applications requiring highly efficient fluid power, as demonstrated in the DDFlyTrain project where we have used this high-efficiency variable transmission as the powertrain interface for the TorqStor high speed flywheel.

"With the DDFlyTrain test rig commissioned in our Edinburgh lab, we are now in a position to demonstrate this very promising application to rail industry customers."

Image: TorqStor unit used on DDFlyTrain demonstrator rig. Photo: courtesy of Ricardo.