EFFICIENCY OF A DIGITAL DISPLACEMENT PUMP OPERATING WITH PARTIAL STROKES

Digital Displacement (R) pumps use solenoid operated on/off valves to control the pressurization of each individual cylinder on a stroke-by-stroke basis. The overall pump displacement is commonly varied by changing the proportion of cylinders which are not pressurized to those delivering their maximum volume. This strategy leads to high efficiency but can increase pulsation in the downstream hydraulic system. As an alternative, the timing of the valve actuation can be adjusted so that cylinders are only pressurized for a portion of their stroke. This paper presents measurements of overall power loss and efficiency of a 96cc/rev pump at a variety of speeds and pressures with variation of the cylinder stroke size from 10-100%. The efficiency of the pump decreases as cylinder stroke is reduced, but the change is relatively small, with efficiencies over 86% measured in all conditions above 100 bar and 50% stroke. The measured data is used to develop and validate a loss model for the Digital Displacement pump when operating using partial cylinder strokes. By combining this loss model with operating data from a 16-tonne excavator, the impact of different cylinder operating strategies on energy savings in an off-road system is evaluated. For the excavator performing digging, grading and travel it is predicted that changing from a full-stroke to a partial-stroke operating strategy has the effect of reducing the average pump efficiency by less than 2% in all duty cycles.