Digital pumps finally get wheels

A digital hydraulic pump originally revealed in Eureka’s October 1990 cover story has finally made it into a saloon car – to demonstrate how it is used in a hydraulic transmission system. It is also being developed for off-road use. It still uses poppet valves to turn individual cylinders in a radial piston pump on and off. The technique has now been extended to a parallel family of motors. The technology was originally invented for Professor Stephen Salter’s ‘Nodding Duck’ wave power machines, and is “working towards” use in wind turbines. The advantage of the Artemis digital displacement technology transmission system is its efficiency at part load. One of the pumps with two conventional hydraulic motors is fitted to a 1.8 litre Ford Focus. This project was part-funded by the Department of Transport, their licensee (formerly Dana, now Bosch-Rexroth) and managed by the Energy Saving Trust. The pump was originally developed at the University of Edinburgh but was spun out as Artemis Intelligent Power in 1994. Managing director Win Rampen says: “It is frustrating that it has taken so long. The trouble with conventional hydraulic transmissions is their poor efficiencies at part loads, which puts them out of contention for most applications. Ours is around 85% efficient at 20% load and most of the map is above 90%.” On the C-class car, the pump transfers 75kW of shaft power. “And it can do it all day and is three times more power dense than the best electric motors,” says Dr Rampen. The first car transmission used in the original EST programme includes two separate motors, one Linde and one Bosch Rexroth. One motor is used for cruise while the other is only powered up during acceleration. The pump has 18 cylinders, each of 2cc capacity. Peak efficiency is 97%. The poppet valves operate in 2ms. On the Ford Focus, the pump supplies an oil manifold equipped with valves to engage either or both motors. The new digital motor, which will be used in the current EST programme, will do away with the need for the manifold and switching valves while reducing overall weight at the same time as further improving system efficiency. Artemis is also working with licensee Sauer-Danfoss in the mobile hydraulics area. The company has also been working with a Tier 1 automotive supplier to build an active suspension system that works directly from the pump without any external valves. Zero to full output is typically achieved in 15ms. Bandwidth is typically 20Hz, direct from the pump without external valves. The system has been demonstrated in a Mercedes SUV. The transmission also being developed for wind turbines is rated at several MW, where it offers significant weight reduction, improved grid-loss ride-through, robustness and controllability.. The Carbon Trust has recently awarded a grant to part-fund a ground based transmission demonstrator. Artemis Intelligent Power Pointers * Digital switching on and off of individual cylinders means that there is no energy wasted in partially opened valves * Energy density is about three times as great as in an electric transmission system * Efficiency at 20% load is 85%, and more than 90% over most of the operating map. Peak efficiency is 90%