Noise, Vibration & Harshness of Digital Displacement Pumps

Abstract

Digital Displacement® pumps are radial piston hydraulic pumps with independently controlled, solenoid driven inlet valves and check-type outlet valves. The displacement of each cylinder is determined by timing the low pressure valve closure. If the low pressure valve is closed after piston bottom dead centre partial displacement is achieved. However pump Noise, Vibration & Harshness (NVH) performance is significantly worse than when completing full strokes, where the low pressure valve closes at piston bottom dead centre. The primary research goal is to make a significant reduction to part stroke noise by means of hardware design of the pump.

This thesis details the root causes of why pump NVH degrades with partial strokes. A 96cc/rev Digital Displacement pump was studied on a bespoke test stand designed and commissioned for this research. During test stand commissioning, ISO 10767 1:2015 was found to be incompatible with pumps whose outlet is a check-type valve. Characterisation results indicate two prominent fluid-borne noise phenomena. The highly transient nature of the outlet check valve opening excites a large modal response of the cylinder volume as it couples to the high pressure galleries within the pump. Simultaneously, the initial high flow rate through the galleries generates a significant amount of fluid-borne noise at frequencies up to 1.5 kHz. This is the first time that the causal mechanisms of noise generation with a Digital Displacement pump have been understood.

A pre-existing lumped parameter SIMULINK model was given a major update, with a focus on analytically modelling poppet dynamics. This included the derivations for a number of stiction models which proved to be critical for understanding HPV dynamics. The resulting model correlated across a wide range of shaft speeds, stroke sizes, pressures and temperatures, becoming an invaluable design tool for investigating hardware modifications.

The updated SIMULINK model indicated large stiction forces acting on the High Pressure Valve (HPV). Modifications to HPV geometry were designed and modelled to reduce this stiction, with two designs manufactured and tested. Results showed modest improvements of up to 2 dB(A), which became a secondary result to evidence that HPV dynamics excite a modal response of the entire high pressure side of the circuit, and not just a modal response contained within the pump.

A novel piston concept is introduced wherein a ‘false’ base displaces as a function of cylinder pressure, dynamically modifying the piston-cylinder volume and consequently changing cylinder pressurisation characteristics to reduce the energy of dynamic forcing during HPV opening. A design is presented that demonstrates a significant reduction in radiated noise levels, typically greater than 3 dB(A) improvement at a range of pressures and displacements at 1800 rpm, while high frequency fluid-borne noise content was reduced by an order of magnitude. The concept demonstrated only minor losses of tens of Watts per piston.

Date of Award	25 Mar 2026
Original language	English
Awarding Institution	University of Bath
Sponsors	Danfoss Scotland Ltd
Supervisor	Nigel Johnston (Supervisor) & Nathan Sell (Supervisor)