Artemis Digital Displacement technology represents a new approach to hydraulic pump-motors that combines robust mechanics, ultra-fast valves and digital controls. Digital Displacement replaces the port plates and swash plates in conventional hydraulic machines with computer-controlled high-speed solenoid valves. The technology offers the following key benefits:

• Very high efficiency at full and part-load;
• Multiple independent variable displacement services from one compact package;
• Fine output control and very fast response; and
• Low noise emission.

Digital Displacement machines are typically multi-displacement cylinder pumps and/or motors. Each cylinder has two poppet valves, one connecting it to low pressure fluid (the LP valve) and one to a high pressure output (the HP valve). A microcontroller reads a shaft position sensor and controls at least the LP valve.

The microcontroller is able to choose to operate or not to operate one or more valves each revolution to determine how much flow is generated. Hydraulic motors are achieved with the addition of a controlled high pressure (HP) valve. Motors are also pump/motors—operation as a pump is inherent.

Digital Displacement pumps and motors completely isolate idling cylinders from the load, eliminating losses due to leakage and shear and delivering efficiency across the whole operating range. For example, Digital Displacement machines have idling losses as low as 1% of the rated output power, according to Artemis.

Basic concepts of using Artemis Digital Displacement technology in a hybrid application. Click to enlarge.

At the Low Carbon Vehicle Event 2011 in the UK, Dr. Win Rampen, Artemis Intelligent Power managing director, outlined a conceptual Digital Displacement KERS (kinetic energy recovery system) for rail:

• Existing engine and mechanical transmission is retained.
• Digital Displacement Pump/Motor added to driveline.
• Gas accumulator storage unit installed under floor.

We believe that the Artemis Digital Displacement technology is ideally suited both to railway driveline applications requiring highly efficient fluid power, and to use with an advanced mechanical energy storage system such as Ricardo’s Kinergy.

—Win Rampen

To demonstrate a complete rail driveline incorporating this combined energy storage technology, the system will be coupled to a wheel-set supplied by Bombardier and will be tested on a dynamometer rig at Artemis’ facility in Midlothian, Scotland. It is anticipated that a follow-up project will progress to installing and testing the system on an operating train.

The system can be tailored to suit various operating philosophies, including alteration of the engine demand to enable it to operate closer to its optimum brake specific fuel consumption, hence saving fuel and reducing carbon dioxide emissions. The system may also be configured to use stored energy to augment the peak acceleration of the vehicle and thus increase the operational flexibility of older rolling stock.

Faster acceleration rates—as typified on more modern vehicles—allow for increased network capacity, hence enabling more rapid recovery from delays and minimizing consequential impacts across the network.

While we are already evaluating the Kinergy in a commercial bus application, this project will be the first to deploy this very promising, cost-effective and efficient mechanical energy storage technology in a rail application. Combined with the Artemis high efficiency hydraulic transmission technology and Bombardier’s established position as a leader in rail vehicle design and construction, I believe that this project has the potential to demonstrate a highly compelling fuel saving and performance enhancing solution, equally applicable to retro-fit installation or incorporation in new rolling stock.

—Ricardo head of rail vehicle technology, Jim Buchanan

The “Digital Displacement Rail Transmission with Flywheel Energy Storage” project is a partnership between Artemis Intelligent Power Ltd, Bombardier Transportation UK Ltd, and Ricardo UK Ltd, with co-funding from the UK government-backed Technology Strategy Board. Artemis is the lead company.