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DISPLACEMENT UNITS AND DEVELOPMENT POTENTIAL

Hubertus Murrenhoff*, Katharina Schrank* and Dirk Schulze Schencking*

* Institute for Fluid Power Drives and Controls (IFAS),

RWTH Aachen University

Steinbachstr. 53, 52074 Aachen, Germany

(E-mail: [email protected])

ABSTRACT

In fluid power systems, displacement units are the key component for power transformation from mechanical to

hydraulic power and vice versa. Depending on the requirements of the application different designs have been

developed so far. Each of which has design dependent advantages but also restrictions in operation. The applicability

and efficiency of the components highly depend on its working principle as it determines the location and type of

energy losses as well as the operation range.

In this paper, the main operating principles of constant and variable hydraulic displacement units are shown and the

differences in design and operation are systematically pointed out. Starting from there, the possibilities for optimization

of displacement machines with regard to new simulation tools are presented. To be able to expand the use of hydraulics

in the future, novel displacement unit designs have been developed at different research facilities and are characterized

in this paper.

KEY WORDS

Displacement Unit, Simulation of Displacement Machines, New Pump Designs

NOMENCLATURE

F Force [N]

M Torque [Nm]

n Rotation speed [rpm]

P Power [W]

p Pressure [bar]

Q Flow rate [l/min]

V Displacement volume [cm³]

ε Volumetric entrained air content [-]

ηhm Hydro-mechanical efficiency [-]

ηvol Volumetric efficiency [-]

INTRODUCTION

Hydraulic drives are widely used to transform energy

especially in applications where a high power density is

required or linear movements have to be realized or

used. Fluid power systems have the advantage of a good

controllability due to their control parameters flow rate

and pressure. These control parameters also lead to an

easy and reliable system overload protection based on

pressure control, pressure relief and flow control valves.

Basis of oil hydraulic systems is always the pressure

fluid that provides the power transmission and also

leads to a good lubrication of the engaged components

Copyright © 2014 JFPS. ISBN 4-931070-10-8

Proceedings of the 9th JFPS International Symposiumon Fluid Power, Matsue, 2014

Oct. 28 - 31, 2014

1

S-1

as well as the removal of locally emerging heat. The

achievable dynamic behavior is excellent due to low

inertia. In contrast to the advantages, hydraulic systems

have some downsides that have to be optimized in order

to be able to compete with other forms of energy

transfer. Based on its working principle the energy

consumption is problematic because of losses through

friction and leakage. An additional parameter for

optimization arises from the effects on the environment

caused by noise emission and external leakage.

Furthermore, highly optimized systems are sensitive to

particle contamination and therefore demand a strict

fluid maintenance.

The basis of fluid power systems is the provision of

hydraulic power. This is always based on mechanical

energy that can have different origins, starting from a

rotating shaft from an electric motor or a combustion

engine or from a linear movement of e.g. a wave energy

converter or a free piston engine. A pump has to convert

this mechanical energy into a flow rate to transfer the

energy hydraulically. In order to use this power, a motor

transforms the hydraulic energy back into a mechanical

torque or a force.

REQUIREMENTS FOR DISPLACMENT UNITS

Depending on the drive application, different

requirements for the hydraulic system arise. On the one

hand these requirements can be based on the design of

the system. On the other hand they depend on the user

and the environment. When regarding a conductive

controlled or secondary controlled hydraulic circuit a

flow supplied system incorporating a fixed

displacement pump can be used. In contrast, a primary

controlled system always needs a variable or

speed-controlled displacement pump. [1]

For the use of variable displacement units the

adjustment time is an important factor in designing the

system. Also, the hydraulic circuit layout has an

important influence on the demands towards the unit.

When an open loop system is considered the low

pressure level cannot be individually adjusted. This

leads to a restriction of the pump’s rotational speed as

with high speeds and therewith high flow rates the

pressure in the suction line decreases. When reaching

pressures below atmospheric pressure dissolved air is

released from the oil and below the vapor pressure the

oil starts to vaporize. In regions of high pressure inside

the pump these cavitation bubbles implode and heat up

rapidly. Effects of cavitation are damage to the

components and an increased oil aging. Therefore the

maximal flow rate of pumps is restricted by the design

of the suction channels, cavitation phenomena and also

on the load on the bearings.

Noise emission is another important criterion in the use

of hydraulic systems especially for the user and the

environment where it is operated. Due to the power

transmission via liquid, sound can easily be transmitted

throughout the whole system too. The generated noise

can have different origins, see Figure 1. It can be

distinguished between structure borne, fluid borne or air

borne noise.

Figure 1 Overview of location of noise production [2]

The main noise source is the pump with its frequent

pressure changes by the commutation process in a finite

number of displacement volumes. In addition, the

dynamic forces acting on the housing lead to sound

excitation. The produced level of sound depends on the

displacement principle. In general, machines with a

small number of displacement volumes such as axial

piston machines result in larger noise levels than

machines with a larger number such as gear machines.

In addition, the finite amount of displacement volumes

also leads to pulsation, which affects the system

behavior. The transportation of fluid from the low

pressure level up to a higher pressure level requires

additional fluid as a result of the compressibility of the

fluid. To reduce pulsation a pre-compression or

non-delivery angle can be used in positive displacement

units in order to compensate the oil compressibility by

reducing the chamber’s volume during commutation.

The pre-compression angle depends on the dead volume

of the displacement chamber, the compressibility of the

fluid and the operation point. Therewith pulsation

increases when the fluid is polluted with entrained air

due to an increased compressibility, see Figure 2.

Figure 2 Measurements of pressure build-up depending

on different entrained air amounts [3]

Copyright © 2014 JFPS. ISBN 4-931070-10-8 2

In addition to pulsation effects, the compressibility of

the fluid also results in a reduced volumetric efficiency

ηvol. It is defined as the ratio of the effective flow rate at

high pressure behind the pump and the theoretical flow

rate because of the geometry of the machine, see

equation (1). In a pump the theoretical flow rate is

additionally reduced caused by internal and external

leakage. In analogy the hydro-mechanical efficiency ηhm

of a pump is defined to be the quotient of theoretical

torque and the reduced effective torque.

th

effvol

Q

Q

eff

thhm

M

M (1)

The total efficiency of a unit is the product of

volumetric and hydro-mechanic efficiency. On the one

hand the efficiencies depend on the pressure difference,

see Figure 3. With increasing pressure the volumetric

efficiency decreases due to higher leakage and

compression losses whereas the hydro-mechanical

efficiency increases due to lower friction losses.

Figure 3 Pressure dependent efficiencies

On the other hand the total efficiency depends on the

unit’s rotational speed, the designated temperature of

the fluid and the gap heights inside the machine. Higher

rotational speeds lead to an increase of the overall

efficiency in most operating points. With an increase in

temperature and hence in a reduction of viscosity, the

volumetric efficiency decreases and the

hydro-mechanical efficiency increases. The product

leads to a higher overall efficiency for low pressure

levels and a lower overall efficiency for higher

pressures. A low sealing gap height inside a unit has a

positive effect on leakage losses but also leads to higher

friction and reduced hydro-mechanical efficiency.

Therefore a proper balance of the gap height has to be

provided.

A high efficiency is a major requirement for the users of

hydraulic systems as well as a long life span and low

acquisition costs.

DISPLACEMENT PRINCIPLES

Due to the different requirements for hydraulic

displacement units a variety of displacement principles

exists to transform mechanical power into hydraulic

power and back [4]. An overview of the major machine

designs is given in Figure 4.

53 of 46Displacement Units

Murrenhoff

Englisch

Overview – Displacement principles

Vane machine

External piston support

Internal piston support

In-line piston machine

External gear machine

Internal gear machine

Orbit motor

Swash plate machine

Wobble plate machine

Bent axis machine

Rotary screw pump

Variable

displacement

volume

Constant

displacement

volume

Axial piston machine

Radial piston machine

Gear machine

Screw machine

Piston

Gear

Vane Figure 4 Overview of different displacement units

Displacement machines can be categorized by the

geometry of the displacement body into piston machines,

gear machines and vane machines. Piston units can be

distinguished into axial piston machines or radial piston

machines. Swash plate pumps are the most commonly

used axial piston pumps. Their basic design principle is

displayed in Figure 5. The pistons and the piston drum

rotate and the pistons are supported by the fixed swash

plate. Thanks to the swash plate the displacement

volume and therefore the flow rate is easily adjustable

by pivoting the swash plate. In addition a reversal of the

flow direction is possible with constant direction of

rotation. Disadvantageous in this design are the lateral

forces acting on the piston, as the slipper – swash plate

slide contact can only transmit forces normal to the

contact area. The pressure force Fpr and the piston force

Fpi act in direction of the pistons axis. Due to the angle

of the swash plate, the supporting force of the slipper

and swash plate only compensate part of these forces,

which leaves a resulting force Fr. This is compensated

by the forces FA and FB on the piston.

Fpr

Fsl

Fpi

Fr

FA

FB

Figure 5 Schematic design of a swash plate machine

(Parker)


In contrast to swash plate machines, bent axis machines

have no lateral forces acting on the piston due to the use

of ball joints between piston and flange, see Figure 6.

The pressure force Fpr acting on the piston is directly

induced into the flange where it is split into its axial and

radial component FA and FB. The pistons are not aligned

parallel to the rotating axis and the driving flange

rotates with the same speed as the slanted cylinder block.

To adjust the displacement volume the entire cylinder

block has to be swiveled against the driving axis. This

large moving mass results in longer adjustment times

and larger dynamic actuation forces.

Fpi

FA

FB

Fpr

Figure 6 Schematic design of a bent axis machine

(Parker)

In comparison with swash plate machines, bent axis

machines have larger swivel angels up to 45°

(16° … 22° for swash plate machines) and better

volumetric efficiencies due to fewer locations of

hydrostatic balancing. Bent axis machines demand

bigger bearings due to higher axial forces and therefore

have a higher manufacturing effort.

The pistons in radial piston units are moving

perpendicular to the rotating axis. These machines can

be subdivided into machines with external piston

support, machines with internal piston support and

in-line piston machines. In radial piston machines with

external piston support, the pistons are supported on the

external stroke ring via slippers or rollers. The pressure

is applied from inside. Commonly, a cylindrical control

journal that provides commutation is arranged in the

cylinder star and is not rotating. Another distinction can

be made between single-stroke and multiple-stroke

designs. With a multiple-stroke machine, several strokes

are performed within one rotation by use of a curved

stroke ring. Therewith, units with large displacement

volumes of up to 38,000 cm³ and very high torques can

be designed. Multiple-stroke piston pumps are not

adjustable and operate at low rotation speeds in contrast

to single-stroke units. Figure 7 shows the schematic

design of a fast running, adjustable single-stroke piston

machine with external piston support.

Figure 7 Schematic design of a radial piston machine

with external piston support (Moog)

The pistons of internally supported radial piston

machines are arranged in a star-shaped fashion in the

housing and do not rotate. The pistons are supported on

an eccentrically rotating shaft. The commutation is

realized in analogy to axial piston units with a valve

plate aligned to the driving shaft. The schematic design

of a radial piston machine with internal piston support

can be found in Figure 8. As can be seen, the

adjustability of units with internal piston support is

more complex than in the previously shown units. This

type of piston unit is used for low speed applications

because of the large displacement volumes.

Figure 8 Schematic design of a radial piston machine

with internal piston support (Parker)


In contrast to piston machines that consist of a huge

number of parts, gear and vane machines have a simpler

design. In Figure 9 the schematic design of a gear

machine is given. The low pressure fluid and the high

pressure fluid are separated by meshing gears.

Figure 9 Schematic design of an internal gear machine

Due to the small number of parts, gear machines are

commonly used when a fixed displacement pump at

moderate pressure levels is required. Gear machines can

be divided into external gear machines and internal gear

machines. In external gear machines the fluid is

transported inside the volume between teeth and

housing. In internal gear machines it is transported

between the teeth volume and a sickle or between two

teeth volumes. The bearings of external gear machines

are liable to high loads due to the gradual pressure build

up and have to be designed robustly. The resulting

pressure force presses the gears towards the suction port

in the housing which can lead to material contact there

and hence wear. Therefore non-compensated external

gear machines can only be used at low pressures. The

combination of internal and external gear in internal

gear machines leads to longer meshing sections that

separate low and high pressure. Thereby a better sealing

effect is reached and larger suction and pressure angles

can be obtained. In contrast to external gear machines

internal gear machines have more favorable dimensions

due to the centric arrangement of the driving shaft.

Additionally, they can easily be combined as multiple

pumps but have the disadvantage of higher

manufacturing costs.

The displacement volume of gear machines cannot be

varied easily in contrast to vane pumps. The use of

vanes as displacement principle combines a simple

design and a variable displacement with the

disadvantage of a lower volumetric efficiency due to a

high number of tribological contacts. In vane machines

the vanes move in radial slots located in the rotor or in

special designs in the housing and are pressed against

the housing/rotor. The fluid is transported in the volume

between two vanes, the housing, and the rotor. Like

radial piston machines, vane machines can have a

single-stroke or multiple-stroke design but only

single-stroke units allow adjustability of the

displacement volume. In Figure 10 an adjustable

single-stroke vane machine is shown.

Figure 10 Schematic design of a vane machine (BR)

Vane pumps have the advantage of a lower noise

emission than piston units. As a result of comparatively

high volumetric losses and high vane loads they are

commonly only used for pressures up to 150 bars.

All displacement principles have unique and principle

based advantages and disadvantages. An overview of

these is given in Table 1. Here, the displacement

principles are rated in terms of number of tribological

contacts, number of parts, adjustability, scalability, the

capability for motor operation and the noise emission

grade.

Number of tribo contacts

Number of parts

Adjustability

Scalability

Motor operation

Sw

ash

pla

te m

ach

ine

Bent axi

s m

ach

ine

Radia

l pis

ton m

ach

ine

Gear m

ach

ine

Vane m

ach

ine

J

J

Noise emission

JK L L

K K K L

JJK L L

K J L J

L L J JK

J

L L K J K

Table 1 Evaluation of displacement principles

The number of critical tribological contacts directly

corresponds to internal leakage respectively effort to

reach a good volumetric efficiency. Especially vane

machines have to deal with a high number of

tribological contacts and therefore with a low

volumetric efficiency. The number of parts in general

has a great influence on manufacturing costs. Gear and

vane machines consist of a manageable number of


simple parts and are therefore the cheapest machines in

contrast to piston units.

Swash plate machines, radial piston machines and vane

machines are practical for the use as variable

displacement units due to small actuating forces and

easily adaptable variation of the displacement volume.

The scalability is an important factor when the flow rate

or the torque should be duplicated. Gear and vane

machines are not suitable for larger power based on

their principle. Radial piston pumps as well as swash

plate machines can be designed to fit high power

demands. The possibility to work in motor operation is

important for the design of hydraulic systems especially

when secondary control is demanded. Mostly bent axis

machines or radial piston units are used for motor

operation. The noise emission is an important factor for

the user of a system as well as the environment of the

application. In general, axial piston pumps are louder

than gear and vane machines due to their smaller

number of displacement volumes.

This overview and rating shows that the optimal unit is

predefined by the application and that there is no prime

unit in general. Which unit is to be chosen rather

depends on the application and the requirements of the

user.

POTENTIAL FOR OPTIMIZATION

From Table 1 it can be taken that no displacement unit

is optimal in all evaluated properties. Therefore several

companies and research facilities are working on

optimizing individual displacement units or parts of

them. Commonly this is done by using simulation tools

to understand the physical effects inside the machine

which are not visible during operation.

For these simulations different forms and accuracy

grades can be used, see Figure 11. Most physical

phenomena can be described by mathematical formulae

via equations of conservation, for example conservation

of mass, conservation of momentum etc. These

equations form a set of partial differential equations that

have to be solved in the simulation approach or can be

neglected because of simplification. A lumped

parameter approach as in 0 dimension formulations is

very fast but leads to less accurate results compared to

the complex physical phenomena. The application of

0-D and 1-D simulation approaches is very useful in

optimization algorithms or at the beginning of the

design process. It is commonly used to optimize the

reversing process or to analyze the adjustment time of

the displacement unit. More complex simulations are

2-D simulations where the state variable is dependent

on two positions and in dynamic simulations on time.

With it, the complex behavior of seal deformation can

be investigated as well as friction forces and leakage

across seals. Most complex, but also most comparable

to the real physical behavior, are 3-D simulations that

are used to investigate the fluid flow performance

through a unit, the fluid-structure interaction, thermal

effects as well as sound excitation and transmission.

Figure 11 Different simulation approaches for

optimization

Most accurate results can be obtained by 3-D

simulations of the entire machine. But these types of

simulations demand high effort regarding model design

up and computation as the whole structure first has to be

transformed into a three dimensional mesh. Next, all

physical conservation equations have to be solved for

each node respectively volume. For the optimization of

the geometry of a machine a new mesh has to be created

for each geometry iteration. The solving process of the

conservation equations is an iterative optimization

process. The robustness of the solution depends on the

initial conditions of the mathematical solving algorithm

as well as on the mesh structure and quality. Therefore a

good mesh and initial conditions are essential for

accurate results. An exemplary mesh for the 3-D

simulation of the fluid flow through a pump can be

found in Figure 12.

Figure 12 Set-up of a 3-D simulation of a pump [5]

Theoretically all physical phenomena can be

synchronously simulated with 3-D simulations but in

regard of necessary effort it is only reasonable to focus

on one specific detail of the machine. In computational


fluid dynamics (CFD) simulations the focus lies on the

behavior of the fluid flow. With that, for example the

suction channels can be investigated and locations of

high pressure losses can be found. In the next step new

and optimized geometries can be found to allow larger

flow rates through the channels with reduced pressure

losses. [6]

By including two-phase and phase change models into

the CFD simulation, the location of cavitation

occurrence can be predicted, see Figure13. By these

simulations, the optimization of the geometry of the

valve plate is possible.

Figure 13 Results of a CFD simulation – Possible

locations of cavitation [7]

In contrast to CFD simulations where the fluid flow is

in the focus of the investigation, the aim of finite

element method (FEM) is the solid body structure.

These simulations help to calculate the stability of a

solid structure and to optimize the size of the design as

well as the amount of material used. By including

damping models, the acoustic emission of a structure

can be simulated. In Figure 14 sample results of a modal

analysis of a hydraulic motor are shown [8].

Figure 14 Results of a modal analysis – Location of

structure borne sound [8]

The location and direction of oscillation can be seen at a

previously defined frequency. This oscillation can lead

to structure borne noise. With the help of modal analysis

the sound emission of hydraulic displacement units can

be reduced by optimizing for example the geometry of

the housing.

By including the temperature behavior of fluid and

structure, see Figure 15, physically more accurate

results can be obtained and the individual regions can be

optimized.

Figure 15 Modeling approach used to predict the lubri-

cating interface performance in axial piston pumps [9]

All of the previously presented 3-D simulation

approaches have in common that optimization

calculations require a high effort as the mesh of the

whole machine has to be recalculated in every step. To

minimize the computational effort, simpler 3-D

simulations can be performed which do not focus on the

entire machine but on one part. This can e.g. be the

tribological contact between piston and cylinder block

or between cylinder block and valve plate. Optimization

goals are i.e. minimizsation of mechanical contact and

leakage losses. A set-up of a 3D simulation of the piston

– cylinder block contact is shown in Figure 16. The

mesh for the calculation of the Reynolds equation only

consists of the lubrication area between piston and bore

of the cylinder block.

Figure 16 Set up of a 3-D tribological simulation [10]

Therefore, the effort to generate new meshes is low.

Using this simulation tool it is possible to optimize the

contour of the piston and of the bushing as well as the

contour length. Due to the small mesh element number,

numerous variations are calculated within an acceptable

time frame.


Figure 17 depicts such a simulation result. The total

losses in this tribological contact are seen with varying

radius and length of the piston contour.

Length of contour [mm]

Overa

ll lo

sses [

%]

Figure 17 Results of the tribological simulation piston

and cylinder block [10]

But not only axial piston pumps are in the focus of

optimization efforts. By combination of fluid dynamic

models and fluid structure thermal modelling the

geometry of the gears of gear machines was optimized

by [13]. With these simulations pressure ripples,

pressure peaks, and cavitation could be drastically

reduced.

In addition to the piston-cylinder block contact other

details of tribological interest can be simulated.

Figure 18 shows the simulation results of the contact

between cylinder block and valve plate of a swash plate

axial piston pump. On the left side the resulting fluid

film height is depicted and on the right side the

corresponding pressure field is seen.

Figure 18 Results of the tribological simulation cylinder

block and valve plate – Fluid film height and pressure

field [11]

Besides to optimizing the shape of the piston and the

contour of the bushing, the texture of the components

surface may be a detail that leads to higher efficiencies

as well as the use of new materials and coatings [12].

Depending on the aim of the simulation it can be

sufficient to use a two dimensional simulation. One

example can be found in [14] where a 2-D simulation

tool is used to optimize the geometry of the valve plate

of an oscillating slide machine. The reversing process is

also an important and often optimized part in axial

piston pumps. The fastest simulation approach is the

one dimensional modelling, see Figure 19. Here, every

displacement volume is displayed by a cylinder and the

reversing is conducted via orifices. This simulation

approach is based on serially connected 1-D

components such as cylinders and valves.

Figure 19 1-D Modeling of the reversing process of a

piston machine

The main advantages of this simple 1-D simulation are

that a huge number of variations can be investigated and

the influence of the changed parameter on a large

system can also be analyzed. Often the pump or motor

cannot be investigated and optimized individually but

the complex fluid power system and the specific

application have to be considered.

INNOVATIVE DISPLACEMENT PRINCIPLES

In the last decades, the existing displacement principles

have been optimized with increasing effort. To ensure

the use of hydraulic systems in the future new

components have to be developed.

A new design of an axial piston displacement unit is the

Floating Cup machine by INNAS, see Figure 21. The

basis of the design is an axial piston machine. In

contrary to ordinary designs where the pistons move in

a cylinder block, the pistons in the Floating Cup move

in individual cylinders, called cups, that are supported

by two barrel plates. Two pistons face each other

back-to-back to compensate forces and to allow a low

pressure pulsation due to a high number of pistons.


Fpr

Fr

FpiFr

Figure 21 Schematic design of the Floating Cup [15]

The Floating Cup machine is designed to combine high

efficiencies, a low noise level as well as low pressure

and torque ripples. Measurements have shown overall

efficiencies up to 96% [16]. Therefore it can be used for

a large number of applications. First units are available

to be tested by third parties.

Another innovative design of a piston machine is the

new radial piston unit with axial cone valve plates

(RAC). It combines the advantages of axial and radial

piston machines. The basis is a radial piston machine

with external support, but axial cone valve plates similar

to axial piston machines are used for commutation. The

schematic design of the RAC is depicted in Figure 20.

Fpr

Fpi

Fr

Figure 20 Schematic design of the RAC [17]

The pistons are held by the cam ring as in radial piston

units (e.g. see Figure 7). By adjusting its eccentricity the

displacement volume can be varied. The pistons tilt

inside the cylinder bore which leads to a cylindrical

wedge in the pressure chamber, leading in a sickle

shaped pressure field which results in direct torque

generation on the rotor. Due to a contoured and flexible

piston sealing, the sealing plane is always orthogonal to

the piston axis. The contact between pistons and cam

ring is fully hydrostatically compensated. Thus no

solid-state friction or loading on the pistons occur.

The main advantages of the RAC are the decreased

friction losses thanks to a direct torque generation on

the rotor, the large amount of fully hydrostatically

compensated contacts, and its simple design. In

conclusion, low production costs combined with a good

efficiency are aspired.

A fundamentally new approach is the design of the free

piston engine. Here, the hydraulic power is directly

transformed out of combustion energy. The schematic

design is displayed in Figure 22. In the left part the

combustion chamber can be found where fuel is

combusted like in an internal combustion engine. The

thermic energy then moves the combustion piston to the

right, serving as a hydraulic plunger, which is directly

part of the pump. With this movement fluid is pressed

from the low pressure grid into the high pressure part.

Therewith losses due to multiple transformations of

energy and gear boxes are avoided.

Figure 22 Schematic design of the free piston engine

[18]

The layout of the free piston engine points out a basic

problem. In general, displacement units and also

systems in general are rated based on the overall

efficiency of the fluid power component. However, in

order to find the best energy efficient solution for an

application the power generation has to be included into

the efficiency calculation by a holistic approach as well,

see also [19].

SUMMARY AND OUTLOOK

The need for a conserving use of energy leads to new

challenges in hydraulic applications. The improvement

of fluid power systems efficiency is one of the major

challenges which have to be dealt with by the fluid

power community. Hydraulic displacement units are the

key component in all fluid power systems and

applications. Because of their large influence on the

overall efficiency of fluid power systems, the units are

the focus of improvement. Furthermore, additional

requirements arise from their use in optimized systems


that do not negatively influence the environment.

In this paper the requirements for displacement units

were described and optimization methods to improve

the properties of existing as well as future designs were

illustrated. In addition, the basic principles of generally

used displacement units were explained as well as

innovative principles for prospective units. Especially

the free-piston engine shows the great potential of

combining different specializations into one holistic

drive and circuit approach in order to design the most

energy efficient solution not only on the component

level but for the entire system.

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