Combinations of electric and hydraulic machines, also known as e-pumps or electro-hydraulic energy converters, are essential for the electrification of mobile working machinery. Currently, these machines are typically combined by axial stacking, and the electric machine directly drives the hydraulic machine. Alternatively, the hydraulic machine can be radially integrated within the core of the electric machine, or a gearbox in combination with a downsized electric machine can be used. However, to the authors’ knowledge, no systematic comparison of these different concepts has been published. This paper uses analytical methods to determine the dimensions of the active parts of hydraulic machines, electric machines, and gearboxes in order to compare different design concepts based on volume, aspect ratio, total mass, copper mass, magnet mass, electromagnetic efficiency, and inertia. Axially stacked concepts can yield the highest compactness. However, they achieve this compactness at low aspect ratios, with their lengths being several times greater than their outer diameters. For balanced aspect ratios, where the outer diameter and total length of the machine are similar, the radially integrated, direct-driven concept is most compact.

In mobile working machines, there is a trend towards replacing combustion engines with electric machines to reduce their carbon footprint. This provides several advantages and challenges for their hydraulic systems. The low efficiency of conventional hydraulic systems is no longer acceptable because of the volume and cost of batteries. Fortunately, the advantages offered by electrification can be exploited for increased system efficiency.

Electrified pump drives enable variable speed control, energy recuperation, power-on-demand, and new system architectures with flexible control. However, legacy hydraulic machines (pump/motors) were not optimised to match the electric machines’ capabilities. This thesis has two main focus areas: the development of hydraulic machines that better match the abilities of electric drives, and the integration of a hydraulic machine with an electric machine.

The electric drive places new demands on the hydraulic machine to enable more sustainable hydraulic systems: higher efficiency, a broader speed range (lower and higher speeds), multi-quadrant capability, and reduced noise. Lubrication interfaces need to be redesigned for enhanced speed capabilities, and commutation requires attention due to its influence on the noise behaviour. Variable displacement can help downsize the electric machine, but control losses should be reduced compared with conventional displacement control. This thesis aims to support developments on the above-mentioned aspects.

Commercial electro-hydraulic energy converters usually combine hydraulic and electric units by axial stacking. Alternatively, research projects consider the radial integration of the hydraulic machine within the core of the electric machine or downsizing the electric machine using a gearbox. This thesis summarises analytical sizing methods for the active parts of each concept and examines trade-offs between volumes, aspect ratios, masses, and efficiencies.

Finally, noise is a challenge for electrified hydraulic systems. This thesis provides a starting point to investigate the combined noise of hydraulic and electric machines by describing the noise contributors and harmonic frequencies of both machines. Furthermore, the hydraulic noise from different electrically driven pump setups (e.g., speed control, displacement control, multi-pump) is assessed using audio files created from simulated flow pulsations.

Bland mobila arbetsmaskiner ersätts förbränningsmotorer av elmaskiner i allt större utsträckning. Syftet är i regel att minska maskinernas koldioxidavtryck. Elektrifieringen medför flera möjligheter och utmaningar för hydraulsystemet. Den låga verkningsgraden hos konventionella hydraulsystem är inte längre acceptabel på grund av storleken och kostnaden för batterierna som skulle krävas. Genom att nyttja elektriskt drivna pumpar (elektrohydrauliska energiomvandlare) möjliggörs varvtalsstyrning, energiåtervinning och nya systemarkitekturer med mer flexibel styrning. I dagsläget realiseras elektrohydrauliska energiomvandlare oftast genom att koppla samman konventionella hydraul- och elmaskiner som finns på marknaden. Konventionella hydraulmaskiner är dock inte optimerade för att kombineras med elmaskiner, och därför finns utrymme för förbättringar. Denna avhandling angriper två förbättringsområden: utveckling av hydraulmaskiner som bättre matchar egenskaperna hos elmaskiner, och integration av hydraulmaskiner med elmaskiner.

Elektrisk drivning ställer nya krav på hydraulmaskiner: högre verkningsgrad, ökat varvtalsområde (både till lägre och högre varvtal), minskade ljudemissioner, samt multikvadrantkapacitet för energiåtervinning och ökad system-flexibilitet. Variabelt deplacement kan också vara önskvärt då det kan minska storleksbehovet för elmaskinen, men förluster relaterade till deplacementstyrning bör minskas. Denna avhandling syftar till att stödja utvecklingen inom ovan nämnda punkter.

De elektrohydrauliska energiomvandlare som idag finns på marknaden kombinerar hydraul- och elmaskiner genom axiell stapling. Ett alternativ till detta är radiell integration av hydraulmaskinen i kärnan av elmaskinen. Ett annat alternativ är att nyttja en växellåda mellan hydraul- och elmaskinen, vilket möjliggör neddimensionering av elmaskinen. Denna avhandling sammanfattar analytiska dimensioneringsmetoder för de aktiva delarna i varje koncept och syftar till att ge insikter om avvägningarna mellan volymer, längd–diameterförhållande, massor och verkningsgrader.

Ytterligare en aspekt som behandlas i denna avhandling är det kombinerade ljudet från hydraul- och elmaskiner, ett område som ännu inte är väl undersökt. Denna avhandling syftar till att vara en utgångspunkt till detta genom att ge bakgrundsinformation om ljudkällor och harmoniska frekvenser för båda maskinerna. Vidare presenteras resultat från en enkät där det hydrauliska ljudet från olika elektriskt drivna pumpkonfigurationer (t.ex. hastighetsreglering, deplacementsreglering, multipump) har undersökts.

Noise is a critical parameter for electrified mobile machinery. Both electric and hydraulic machines generate periodic forces that excite the surrounding structures and generate noise. This paper discusses harmonic orders and their origin in hydraulic pumps and permanent magnet synchronous motors (PMSMs). By combining the pump and motor, the harmonics of both machines interact, and the noise experience changes. This must be taken into account when designing electrically driven pumps. The effect of the combination of the design parameters; number of pistons, number of slots, and number of poles on the system harmonics is discussed. Traditionally, hydraulic pumps use odd piston numbers to reduce noise. However, if identical numbers of poles and pistons are chosen, the pump and motor harmonics coincide and can interact constructively or destructively. This paper shows, using torque ripple as an example, that selecting odd piston numbers is not straightforward when combining pumps with a PMSM.

There is a trend of electrification for off-road construction vehicles. This trend includes installing electric prime movers for the hydraulic motion systems. Drive cycles are highly dynamic and differ strongly between applications, thus making it challenging to guarantee that the electric machine does not overheat under any operating condition without over-dimensioning or limitation of the functionality. Variable pump displacement can reduce the electric machine’s torque load and thus protect it from overheating when needed. However, conventional continuously variable displacement controllers introduce losses to the system. For speed-controlled systems, a small number of discrete displacement settings can reduce the torque while requiring less power than continuously variable controllers.

Mirrored axial piston pumps with two identical pump halves have some inherent benefits, amongst others the balancing of internal axial forces and bending moments. Therefore, they typically only require small bearings. Furthermore, they can provide a reduced displacement setting by separating one pump half from the consumer and circulating its flow instead. However, in this case, the axial force and bending moments from the other pump half are no longer balanced.

This paper suggests to increase the pressure level of the circulating flow to balance the axial forces of the active pump half. Lumped parameter simulations for passive and active control are presented, and parameter sweeps show the influence of valve opening areas, volume of circulated flow, and pump leakage on the robustness of axial force compensation to varying delivery pressures.

Piston-type positive displacement machines are used across diverse applications and operating conditions, making it a critical design challenge to balance the minimization of solid-body contact while maintaining efficiency. This study investigates the potential of hydrostatic pockets between the cylinder block and valve plate to provide dynamically and passively controlled pressure forces, mitigating contact issues at low speeds without excessive losses at high speeds. Simulations of a baseline pump design revealed persistent solid-body contact under low-speed and high-pressure conditions, indicating the need for enhanced lubrication strategies. Retaining the baseline design, the study examined multiple hydrostatic pocket configurations through simulation, varying their location and quantity. Although the primary focus is on low-speed high-pressure and high-speed high-pressure scenarios, additional operating points at high-speed low-pressure and medium-speed medium-pressure were also considered. The effectiveness of each design was evaluated on the basis of film thickness, contact pressure, and viscous losses under key operating conditions. The experimental findings from previous studies were used to validate or challenge the conclusions from the simulation results. This paper seeks to deliver a better understanding of the hydrostatic pockets, offering design guidance for optimizing the lubrication management for future piston-type positive displacement machines and informing strategies for improved efficiency and longevity in demanding applications.

Noise is one of the major challenges for the electrification of hydraulic systems for mobile working machinery. Without the masking sounds of a combustion engine, the distinct noise of the hydraulic pump becomes more audible and thus needs to be reduced. Compressible pump flow pulsations caused by imperfect commutation are one of the main noise sources. Electric motor control offers the possibility to inject torque pulses at the pump's first harmonic frequency in order to reduce common noise. This paper uses lumped parameter simulation to drive a hydraulic pump with different shapes of drive torques, and in different operating conditions. The potential of injecting harmonics into the drive torque for the reduction of flow pulsations is explored. At the right phase and amplitudes, these pulses can basically eliminate flow pulsations at their frequency. Scaling laws for comparisons of different piston numbers are summarised, and the behaviour of a 9 and a 10 piston pump are compared, demonstrating that lower torque amplitudes are required when using smaller piston numbers. Required torque amplitudes are quantified, showing that torque amplitudes in the region multiples of the pump's ideal drive torque are required at medium or high speeds. Furthermore, these torque amplitudes significantly increase losses and increase the requirements for the inverter, making harmonic injection impractical at medium and high speeds. At low speeds, the first pump harmonic can be eliminated, but this is of limited benefit due to the low sensitivity of human hearing at these frequencies.

This paper explores the transition towards the electrification of hydraulic systems in material moving machines, highlighting both environmental benefits and the noise challenges posed by such technological change. With a focus on noise, the study examines the perceptual impact of replacing diesel engines with electric motors, which alters the noise profile of these machines. A survey assesses human responses to different hydraulic pump configurations in electrified setups, revealing significant variations in noise perception based on the specific design and the familiarity of the respondents with hydraulic sounds. The findings underscore the need for a nuanced approach to noise management in electric hydraulic systems, advocating for designs that consider both decibel levels and sound quality to enhance operator comfort and public acceptance.

Conventionally, variable hydraulic axial piston machines vary displacementby adjusting the length of the piston stroke. Another method to achieve variabledisplacement is to rotate the valve plate and thus adjust the effectiveuse of the piston stroke. This paper provides an analytical methodology tocalculate control torques for valve plate rotation. This methodology considerscompensation ratios in the contact between the valve plate and thepiston plate, and compensation ratios in the contact between the valve platein the housing. This paper adds the consideration of a spring force, pressuredependentviscosity, and a surrogate model for the compensation force fromthe area between high-pressure and low-pressure to the traditional calculationof the compensation ratio. The influence of the cylinder barrel’s rotation angleand the valve plate rotation angle is taken into account. The calculation resultsfor an exemplary pump of floating piston type reveal that the main shareof the required control torque originates from the contact between the valveplate and the housing. A hydrostatic compensation force in that interface canreduce this torque, but it is illustrated that a full compensation is not possible.For large valve plate rotation angles, the risk of valve plate tipping caused bythe axial ports in the housing is shown, which shows that valve plate rotationis not suitable for displacement control to small displacement levels whenusing axial ports.

In search of more efficient hydraulic systems, new system architectures are explored. These system architectures are often electrically driven and include energy recuperation. This requires hydraulic machines to function both as pumps, converting mechanical power into hydraulic power, and as motors, converting hydraulic power back into mechanical power. However, the availability of machines that can operate in all desired modes is limited. This indicates that operation in multiple modes comes with performance penalties. This paper highlights the challenges for multi-quadrant operation of hydraulic piston pump/motors, with a particular focus on commutation, i.e., the transition between high- and low-pressure level for each chamber. Various commutation strategies for piston machines are examined. Furthermore, other important aspects for pump/motor operation such as hydrostatic compensation ratios, design of inlet channels, low-speed capability, and flow control through speed or displacement control are discussed. The article shows that the design of multi-quadrant machines is challenging, and this has to be considered when choosing the system architecture.

In mobile working machines, there is a trend towards replacing combustion engines by electric machines to reduce their carbon footprint. This provides several advantages and challenges for the hydraulic system. The low efficiency of conventional hydraulic systems is no longer acceptable due to the volume and cost of batteries. Luckily, the advantages offered by electrification can be exploited for increased system efficiency. 

Electrified pump drives (electro-hydraulic energy converters) enable variable speed control, energy recuperation, power-on-demand, and new system architectures with more flexible control. Currently, electro-hydraulic energy converters are typically made by stacking off-the-shelf components. However, off-the-shelf hydraulic machines are not optimized to be combined with electric machines, and thus there is room for improvement. One of these potentials is the volume at the core of electric machines which does not contribute to torque creation. This volume can be used to tightly integrate a hydraulic machine. This tight integration leads to increased power density and the elimination of some parts (e.g., a pair of bearings). This thesis investigates and discusses the design of electro-hydraulic energy converters. 

Furthermore, this thesis discusses valve plate rotation for a double pump of floating piston type with two valve plates for the following reasons: Firstly, without the noise of the combustion engine, the noise of the hydraulic machine becomes more audible. Valve plate rotation provides variable pre- and de-compression, and is therefore investigated to reduce fluid-borne noise. Secondly, electric machines can be overloaded for some time. In order to protect them from overheating when maximum pressure is demanded continuously, the torque load can be reduced by reducing the hydraulic machine’s displacement. Conventional swash-plate tilting needs significant leakage to be stable, which reduces the efficiency. Valve plate rotation requires low control power and could therefore increase efficiency, and is thus investigated in this thesis. However, valve plate rotation remains challenging for the following reasons: For low displacement setting ratios, the axial speed of the pistons at commutation is increased, increasing the throttling effect. Also, the hydrostatic forces acting on the valve plate change when rotating the valve plate.