Hydraulic Energy Efficiency of Concepts on an Articulated Hauler : Design and evaluation of different hydraulic concepts with focus on energy efficiency

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Hydraulic Energy Efficiency

of

Concepts on an Articulated Hauler

Design and evaluation of different hydraulic concepts with focus on energy efficiency

Written by

Andr´

e Arana Escobedo

Oskar Gunnarsson

Link¨oping University Link¨oping

Master Thesis LIU-IEI-TEK-A--15/02236–SE

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Hydraulic Energy Efficiency

of

Concepts on an Articulated Hauler

Design and evaluation of different hydraulic concepts with focus on energy efficiency

Written by

Andr´

e Arana Escobedo

Oskar Gunnarsson

Link¨oping University Link¨oping

Examiner at LiU: Liselott Ericson Supervisor at LiU: L. Viktor Larsson Supervisor at Volvo CE: Robert Morelius Supervisor at Volvo CE: Vilhelm Fredriksson

Master Thesis LIU-IEI-TEK-A--15/02236–SE

Department of Management and Engineering Fluid and Mechatronic systems

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Abstract

This master’s thesis has evaluated different system designs for the hydraulic system on an articulated hauler at Volvo Construction Equipment (CE). The current system suffers from great losses when running on low pump displacement settings. This is due to large installed displacements as a result of regulations and market demands. New system concepts have been generated and simulations in Matlab and Amesim show that some of the concepts can be implemented in order to increase energy efficiency up to 65%. However, increasing efficiency does in most cases also increase cost, making some of the concepts unrealistic to implement. The suggested solution for Volvo CE is to remove one pump and allow for the fan pumps to supply oil to the steering and dumping, as described in the Displacement reduction concept. They should also examine the possibility to implement clutches further.

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Acknowledgements

When our time as master students at Volvo CE in Eskilstuna is coming to an end we feel grateful that we have had this opportunity to do our thesis at a company with such a great atmosphere between co-workers. In this thesis we have put a lot of the knowledge achieved during our studies at Link¨oping University to use and we are proud of the result. We hope that Volvo CE feels the same and that our collaboration has been worthwhile.

We would like to thank the hydraulics department at Volvo CE for making us feel welcome and for taking the time to answer our questions. Specifically we would like to express our gratitude towards our supervisors, Robert Morelius and Vilhelm Fredriksson, who supported us throughout our work. Another person who has our gratitude is Erik Norlin who has been a great support when programming in Matlab. We would also like to thank our supervisor at Link¨oping University, L. Viktor Larsson. We would like to thank our friends and family and Oskar would like to thank his girlfriend for her support and for putting up with him being away all the time.

May 2015

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List of Figures

1.1 Figure showing when the ”Over Center” function is needed. . . 3

1.2 A simplified schematic of the hydraulics on the Volvo A40. . . 4

3.1 Principle sketch of a constant flow and constant pressure system. . . 10

3.2 Principle of a load sensing system . . . 11

3.3 Principal figures of different clutch types . . . 12

3.4 Pump pressure margin as a function of the flow . . . 13

3.5 Schematic view of a flow sharing pressure compensator valve. . . 13

3.6 Principle for the force and pressure levels of a digital piston. . . 14

3.7 The principle of a multi camber cylinder. . . 15

3.8 Artemis Digital Displacement® Pump. . . 16

3.9 Mathers Fuel Saving Vane, FSV, Pump . . . 16

3.10 Different operational modes when using individual metering. . . 18

3.11 Different type of hydraulic accumulators . . . 19

3.12 A conventional hydraulic transformer . . . 20

3.13 Simplified schematic of a pump controlled actuator . . . 21

3.14 Principle sketch of an electro hydraulic actuator . . . 22

4.1 A typical Volvo test track . . . 24

4.2 Flow Chart showing the calculation process. . . 24

4.3 Geometry for the hydraulic functions on the hauler. . . 25

5.1 Clutch placement in the system . . . 35

5.2 Alternative steering configuration and accumulator size . . . 38

6.1 Distribution of inserted energy with the Current system. . . 42

6.2 Displacement settings with the Current system. . . 43

6.3 Distribution of inserted energy with a Constant flow concept . . . 45

6.4 Engaged displacement for a Constant flow concept with clutches. . . 46

6.5 Distribution of inserted energy with a Constant pressure concept . . . 48

6.6 Distribution of inserted energy with the Displacement reduction concept . 48 6.7 Displacement settings for the Displacement reduction concept . . . 49

6.8 Distribution of inserted energy during a drive cycle using clutches. . . 50

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6.12 Results with a Mathers Fuel Saving Vane Pump . . . 52 6.13 Piston rod diameter vs. Pressure and Pump Displacement . . . 53 A.1 AMESim model of the steering circuit . . . 66 A.2 Simulink model to calculate the valve opening to achieve the correct flow. 67

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List of Tables

6.1 Sensitivity analysis of the constant flow system. . . 47

6.2 Steering system with accumulators . . . 54

6.3 The current system with different sizes of the auxiliary steering pump. . . 54

6.4 Power savings for different configurations of the fan circuits. . . 55

6.5 Cost for different concepts . . . 57

B.1 Different pump displacement setting in a constant flow system . . . 70

B.1 Different pump displacement setting in a constant flow system . . . 71

B.2 Different pump displacement setting in a constant pressure system . . . . 72

B.2 Different pump displacement setting in a constant pressure system . . . . 73

B.3 Displacement reduction w. clutch and various displacement combinations – auxiliary steering pump always engaged . . . 75

B.4 Displacement reduction w. clutch and various displacement combinations – auxiliary steering pump engaged when needed . . . 76

C.1 Component price . . . 77

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Nomenclature

α Steering angle rad

γ Angle between right cylinder attachments rad

κ Turn speed deg/s

µ Angle between left cylinder attachment rad

ω Angular velocity rad/s

θ Dumping angle rad

$ Price SEK

A Length between front cylinder attachments and steering joint mm

Api Cylinder area on piston side mm2

Apr Cylinder area on piston rod side mm2

B Length between rear cylinder attachments and steering joint mm

D Displacement cc

E Energy kWh

L Cylinder length mm

NoP Number of pumps –

NoT Number of Turns –

P Power kW

R Ratio –

T Torque Nm

V Volume l

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l Perpendicular length between steer cylinder attachment and steering joint mm

lcw Width between lower dump cylinder attachments mm

n Number of revolutions per minute rpm

p Pressure bar

q Flow cm3_/

s

t Time s

ucw Width between upper dump cylinder attachments mm

v Velocity km/h x Distance in x-direction mm z Distance in z-direction mm Subscripts + Positive direction – Negative direction E Combustion engine G Auxiliary steering pump Ideal Ideal calculations PTO Power Take Out b Trailer unit cur Current system ext External

f Pull unit

h Right

hm Hydromechanical m Hydraulic Motor max Highest value min Lowest value

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otc Length between dump cylinder attachment p Hydraulic Pump pi Cylinder pr Piston rod req Required st Steering

tl Length between lower cylinder attachment and body rotational joint

v Left

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Acc Accumulator

CE Construction Equipment DDP Digital Displacement® Pump EHA Electro Hydraulic Actuator

FS Full Suspension system, an optional feature on the Volvo A35/A40 FSV Fuel Saving Vane Pump

LS Load Sensing NA Not Applicable PTO Power Take Out cc Cubic Centimetres rpm Revolutions per minute

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Chapter 1

Introduction

As a result of increased climate awareness, energy efficiency in vehicles has become more and more important in recent years. Construction vehicles are expected to carry greater loads and drive faster while consuming less fuel. Studies in the academia have shown flaws in the hydraulics commonly used in the industry and that there are other viable options available that can improve both hydraulic systems and components with regard to fuel efficiency. This thesis focuses on mobile hydraulic systems, exploring and evaluating different hydraulic concepts on an articulated hauler, namely the Volvo A40.

1.1 Background

Volvo Construction Equipment, Volvo CE, manufactures a large variety of construction machines. The product range includes wheel loaders, excavators, pipe layers and artic-ulated haulers. This paper focuses on the haulers which are mainly developed in Bra˚as and Eskilstuna, Sweden. An articulated hauler is an all wheel drive off-road vehicle. The assignment on a construction site is mostly to transport goods such as soil and gravel in rough terrain from a working site to a dump site. The body that the soil and gravel is loaded into, usually by wheel loaders or excavators, is connected to the frame through a rotation linkage, usually at the rear end of the vehicle. Two hydraulic cylinders are used to lift the front of the body when dumping the load, meaning unloading of the soil and gravel. The hydraulic system also controls steering cylinders that make it possible to turn with the hauler. It can also provide flow to fan motors, cooling systems and the brake system. Controlling many functions with hydraulics gives rise to challenges, such as how to dimension the hydraulic pumps to achieve an energy efficient system. Functions that are not used often but require much power during a small period of time, like dumping, can make the system over-dimensioned most of the time. The articulated hauler used as reference in this project is the Volvo A40. The present Volvo haulers have load sensing, LS, hydraulic systems installed. The LS system adapts the pump pressure to the highest active load pressure. This will cause energy losses when simultaneously using functions that requires widely different pressures, although this rarely occurs on an articulated hauler. When no load is active there is a minimum pressure equal to

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the system pressure margin. The losses existing in the system have previously been identified and located in a master’s thesis carried out at Volvo CE [10].

1.2 Aims

The primary aim of this thesis is to generate new concepts for the hydraulic system on an articulated hauler in order to increase energy efficiency. All concepts have to keep the same functionality as in the existing system. The generated concepts also have to be evaluated with respect to cost and safety.

1.3 Problem Description

Losses in the existing system derives from the fans, brakes, steering and dumping. The main source of energy losses in the hydraulic system is the steering system which has high drag losses when driving the hauler [10]. This function will therefore be the most influential when exploring new concepts. The new system will have to fulfil certain requirements. They are:

Dumping time – The body has to be able to rise in less than 12 seconds in order to be competitive on the market.

Steering time – A full turn has to be achieved in less than 3.5 seconds at 1200 rpm on the diesel engine.

Power take-out at engine idle speed – The installed hydraulic power is higher than what the diesel engine delivers when idling. Therefore stalling has to be prevented in some way.

Dump modes – If the current dumping valve is removed all existing modes have to be implemented in a new system. These includes the over center function, a safety function when dumping downwards a slope. It prevents free fall of the body when its center of mass move over the rotational point. The principle can be seen in figure 1.1.

System cost – The cost of the system has to be equal or less than today’s system. If the cost increases, the payback time has to be one year maximum.

Comfort – The hauler should be comfortable to use. Safety – All safety legislations have to be fulfilled.

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Chapter 1. Introduction

Figure 1.1: Figure showing when the ”Over Center” function is needed.

1.4 Delimitations

Due to limited knowledge and the complexity of the system a complete Amesim1 model will not be produced in this work. Only subsystems will be modelled in order to evaluate concepts. The full suspension system, an extra feature available on the Volvo A35/A40, will in no aspect be analysed in this thesis. The differences in dynamic characteristics, such as stability or damping, between hydraulic concepts will not be analysed. Complete control strategies for the systems will not be evaluated or produced. Further on the brake system will not be analysed other than to decide which pump it should be supplied by. There are no prototype machines built to verify the models of the generated concepts.

1.5 System Description

A simplified schematic of the hydraulics on the Volvo A40 can be seen in figure 1.2. It consists of six variable displacement pumps. Five of which are connected to the diesel engine, in the figure represented by the Power Take-Off, PTO. The sixth pump, denoted 8, is an auxiliary steering pump connected to the dropbox. This pump will always rotate 1_{LMS Imagine.Lab Amesim - Integrated simulation platform for multi-domain mechatronic systems}

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when the vehicle is moving. The dropbox is a transmission component whose purpose is to distribute engine torque to the front and rear axles in all wheel drive vehicles. This pump makes steering possible even if the engine malfunctions and disables the other pumps. P T O PWM 1. 3. PWM PWM 2. 6. 5. 8. 10. LS-block MTRV1 MTRV2 Steering cylinders Dumping cylinders Dumping valv e Steering valv e 9. Brake Coolant System

Figure 1.2: A simplified schematic of the hydraulics on the Volvo A40.

A 63 cc pump, labelled pump 1, with both load sensing and electrical displacement control exists in the system. The electrical displacement control has been implemented to avoid stall of the engine. Stalling could otherwise be a problem at low engine speeds since the hydraulic power demand can be greater than what the engine can deliver. This pump and two 60 cc load sensing pumps, pump 2 and 3, provide the hydraulic power for the steering and dumping. The steering 6/3-valve is of open-centre type enabling the steering to always be functional, even during full dumping. The open-center valve also allows for a quick steering response. The steering is given priority over the dumping by connecting the flow through the open-centre steering valve to the inlet of the dumping valve. The steering valve is mechanically controlled by the operator through the steering wheel. It has a self compensating hydro-mechanical feedback meaning that a certain movement in the steering wheel always corresponds to the same steer angle. The dumping 6/4-valve can except for normal operations also be used to achieve float mode, meaning that the body is lowered by its own weight without the use of the pumps. The motors in the system are of an axial piston design with a fixed displacement. Pump 5 of size 28 cc provides an 11 cc motor with flow to power an intercooler fan. An intercooler is used to achieve higher engine efficiency by supplying cold air, which has a higher concentration of oxygen. A radiator fan is powered by a 14 cc motor, receiving flow from pump 6, 45 cc. Pump 6 also supplies the brake cooling system as well as the brake circuit. Pump 5 and 6 are electronically controlled variable displacement pumps.

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Chapter 1. Introduction

1.6 Contribution

This thesis will bring clearance to the energy efficiency and potential of new hydraulic systems, as well as their advantages and disadvantages. Raising the knowledge of what potential upcoming hydraulic systems have, will be of importance for future work in the field of energy efficiency. There will also be an introduction of alternative ways to manoeuvre the hydraulic functions. The results and analyses will show what possible directions Volvo CE can take to further develop and improve their articulated haulers.

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Chapter 2

Method

The purpose of this work is to generate hydraulic concepts that have higher energy efficiency than the existing system, while in the same economic range. The existing system will therefore be a point of reference from which all other concepts are compared. A proper way of comparing one concept to another is crucial to the work. In order to successfully carry out this project a structural method is needed.

The functionality of the existing system has to be preserved in any new system. In order to fulfil this demand when generating new concepts it is first important to understand the functionality of the existing LS-system and its hydraulic components. This is achieved by studying hydraulic schematics and other material regarding the functionality and configuration of a hauler, provided by Volvo CE, but also previous master’s theses work done at Volvo CE [10]. The specifications in those documents are the basis when creating a mathematical model of the existing system, made in Matlab1. This model will serve as the reference point from which the models for all other concepts originate. The ongoing process after initial modelling is performed in the manner further described below.

Concept generation To learn about new concepts, theses and publications from academia are studied. When initially generating concepts no aspects on the func-tionality is to be taken. This strategy is chosen to encourage new ideas to come through without criticism stopping or limiting them.

Initial Evaluation All ideas are evaluated to ensure that there is a possibility to fulfil all requirements. Simple calculations in Matlab and partial models in Amesim are made to examine the plausibility of all ideas. Ideas found to be inadequate are eliminated from further studies.

Concept Modelling All the remaining concepts are now to be modelled. The mathe-matical model for the existing system will be modified according to the principal differences of the concept, sections in the script are removed for some concepts 1

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while other sections are added on other concepts. This process will enable the con-cepts to be accurately comparable. All models are verified individually to ensure that they are mathematically correct, meaning that a change in parameter value should give a predictable model behaviour. All models are verified with equations found in [6]. Normally a validation of the model would be made. The point of validating a model is to examine if the model is an accurate representation of a specific concept. The non existing data of the generated concepts prevents valida-tion of those models towards collected data. Therefore a validavalida-tion is only possible of the initial model. This is done by comparing it against other models previously made at Volvo CE.

Result Evaluation The result from simulating the models are evaluated to see if they fulfil the requirements and that the output parameters such as pumps displacement setting and inserted power are reasonable. The final step in the evaluation is to include the economical aspect of the concepts.

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Chapter 3

Hydraulic systems

This section introduces new ways of controlling and design hydraulic systems that has been researched in the academia. A deeper understanding of load sensing, individual metering, flow control and valveless systems is to be achieved, as well as the digital hydraulics. The knowledge of these theories are to form a base when generating new concepts for the hydraulic system on an articulated hauler.

Removing losses such as the pressure margin, that is needed in a LS-system, and idle losses are the main objectives when exploring other options. Idle losses, also known as drag losses, occur when a pump’s displacement setting is turned to zero. The pump pressure as well as pump speed and size affect how great the power loss is. If the speed of the pump can be set to zero the idle losses can be fully removed. This since there can be no losses without an energy input to the system.

3.1 Conventional Systems

The most commonly used mobile hydraulic systems are constant flow and constant pres-sure systems. These basic systems are considered to be simple but ineffective from a energy efficiency point of view. The efficiency is reduced further if the hydraulic functions are rarely used. More detailed description on both systems follows in this section.

3.1.1 Constant Flow

A constant flow system, also called open center system, consists of a fixed displacement pump delivering a continuous flow. Flow not required by any function is drained, through an open center in the control valve, to the tank. A simple schematic of this can be seen in figure 3.1a. The pressure in such system is set to whatever is required by the active functions including the pressure drop from pump to load [1]. The losses in an open center system is big at partial flow requirements due to valve losses. The losses can be described by the equation

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3.1.2 Constant Pressure

Holding a constant pressure in the system is most commonly done using a variable displacement pump where the pump displacement is changed to control the system pressure. In this system the flow delivered is what the active functions require. The constant pressure causes great losses in the system if the functions require low pressure, causing a great pressure drop over the valve. The system principle can be seen in figure 3.1b. As above, the losses can be calculated according to

PLoss= q · (ppump− pload) (3.2)

qpump, ppump q_load,

pload

(a) Constant flow

q, ppump q,

pload

(b) Constant pressure

Figure 3.1: Principle sketch of a constant flow and constant pressure system.

3.2 Load Sensing System

Combining the constant flow and the constant pressure system will result in a load sensing system. This system has a higher efficiency than any of the two conventional systems. A load sensing system consists of a variable displacement pump where the highest load pressure sets the displacement, resulting in a flow and pressure output. The principle for such a system can be seen in figure 3.2. This results in a system delivering the flow required at a pressure just over the pressure required by the highest load. One downside is that the efficiency is highly reduced if the load difference is great. Another disadvantage, described in [1], with this concept is that a pressure margin must be added to compensate for losses in the system, mainly from hoses, valves and compensators. The margin is set to a constant high value to cover for all operating points, possibly decreasing efficiency. The idle losses are increased with increased pressure. Therefore the constant pressure margin will have negative impact on the energy efficiency. LS-systems can in some implementations have poor hydraulic damping, causing oscillations in the systems [4]. Load interference might occur on the lighter loads in this system if load pressures vary [1]. Load interferences is a phenomena that can occur when several functions are active simultaneously. The pump pressure affects the flow to the loads. Changing the

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Chapter 3. Hydraulic systems

valve opening on one function will affect the speed of the other [13]. Therefore a load sensing system has high losses if active load pressures differs widely. A load sensing system is what can be found in most of the articulated haulers on the market today.

Figure 3.2: Principle of a load sensing system

3.3 Clutch

A clutch is a type of mechanical connector that allows for two axles to be connected or disconnected to each other while rotating. Enabling this possibility in a system allows for pumps to be disconnected when the flow is not required and thereby reduce losses. There are several types of clutches available on the market such as friction clutches, centrifugal clutches and non-slip clutches. Some clutches are internally controlled as for example the centrifugal clutch. Others are externally controlled and can be controlled using hydraulic, mechanic, pneumatic or electronic control. These clutches can be disc clutches or non-slip clutches. The clutches can be seen in figure 3.3.

Friction Clutch A friction clutch, also called disc clutch, is basically two or more discs that are forced against each other in order to transfer torque. This type of clutch is allowed to slip and therefore a smooth engagement of the clutch is possible. A friction clutch can either be wet or dry. A wet clutch can transfer more energy that an equally sized dry. This is since the oil is forced in between the friction surfaces enabling torque to be transferred via viscous shear forces [8]. Other advantages of the wet clutch is that the oil reduces mechanical wear of the friction plates and cools the clutch, allowing more energy to be absorbed [8].

Centrifugal Clutch Centrifugal clutches transmits higher torque while the speed of the input axle increases. This forces the flyweights, held together by springs in the clutch, to move outwards transferring torque to the output axle. This type of clutch has no external engagement mechanism and is therefore hard to control when the output speed should not depend on the input speed.

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Non-slip Clutch A non-slip clutch is also known as a dog clutch in the automotive industry. An example of the non-slip clutch is the claw coupling where the claws are inserted between each other and thereby transferring torque. As the name implies, this clutch can not slip and therefore a smooth engagement under load is hard to achieve.

(a) Multi disc friction clutch (b) Centrifugal clutch (c) Non-slip jaw clutch

Figure 3.3: Principal figures of different clutch types

3.4 Flow Control

Flow control of hydraulic actuators is commonly used in applications where control of velocity is important. Often in these cases the pump is still pressure controlled and the flow control is achieved by using pressure compensating valves. In a system with flow controlled valves where the demand from the operator is an electronic signal, e.g. from a joystick, it would according to Axin make more sense to also control the pump according to flow demand. This type of control has several benefits but requires a higher complexity in the controller, [1].

With a flow control system the losses can be reduced at low flow and pressure com-binations. This since the pressure margin depends on the resistance in the system in contrast to a LS-system where the pressure margin is constant and has to be set to cover the worst case. A simplified model for the pressure margin can be expressed with the square of the flow [2]. The principle for this can be seen in figure 3.4.

Flow control can easily be implemented in an existing LS-system. It requires an electronically controlled pump and a new controller. All other components can often be the same [4]. This means that the cost increase when implementing flow control will be relatively low. Load interference can occur in a flow controlled system in similar ways as in a LS-system, but it is easily handled using pressure compensators. There are two types of compensators that can be used, traditional pressure compensators or flow sharing pressure compensators. Flow sharing pressure compensators, seen in figure 3.5,

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Chapter 3. Hydraulic systems Hose Losses Valve Losses pmargin Flow Compensa tor Losses Hose Losses Energy Saved Valve Losses pmargin Flow Compensa tor Losses

Load Sensing

Flow Control

Unnecessary Losses

Figure 3.4: Pump pressure margin as a function of the flow for a LS system (Left) and a Flow control system (Right). The dashed red line indicates the margin.

are preferable since they do not require the same accuracy in system components and control. The reason being that in a system containing more than one function, the flow from the pump should be the sum of the demand from all functions. The speed of the functions will be affected if there are deficiencies or excess flow. By using flow sharing pressure compensators these deficiencies or excess will be split between all functions affecting them equally. This since the pump pressure and the highest load pressure also affects the valve position. The use of pressure compensators are however not desired, since they reduce system efficiency due to added valve losses.

PWM max load_pressure

Figure 3.5: Schematic view of a flow sharing pressure compensator valve.

3.5 Digital Hydraulics

In recent years new technology in the area of digital hydraulics have evolved. The idea of digital hydraulics is that the output of the system is discrete. This can be realized

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for example by the use of on/off-valves with only discrete inputs. Digital solutions have been presented for linear actuators as well as pumps and motors.

3.5.1 Linear actuators

In [3] a system for linear actuators on an excavator arm containing a four chamber cylinder and three pressure levels is presented. The idea is that different pressure levels in the four chambers create different forces on the piston. The force also depends on the area ratio between the chambers. A piston can be seen in figure 3.7 and an example of the force steps can be seen in figure 3.6.

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A

D

B

C

Figure 3.7: The principle of a multi camber cylinder.

3.5.2 Angular actuators

This section will introduce two types of digital pumps presented by Artemis Intelligent Power Ltd and Mathers Hydraulics Pty Ltd. Their technology show great potential and it could be of interest for future implementation.

Artemis Digital Displacement Unit

Artemis Intelligent Power Ltd presents a radial piston machine where the traditional mechanical valves have been replaced by high speed solenoid valves. Using high speed valves allows for active selection of when to engage a piston, thereby changing the dis-placement in a digital way [12]. The pump can be seen in figure 3.8. When the pump is idling the poppet valve in the end of a cylinder, the cyan valve in figure 3.8, stays open and the pump suck oil from the low pressure volume and then return it to the same volume, preforming no output work. This way of idling is according to Artemis very energy efficient [12]. When a piston is to be used the low pressure poppet valve is closed and pressure builds up in the cylinder. As the pressure rises the high pressure valve opens letting the oil into the system. In [16] Wadsley presents data with a total efficiency over 90% when running the Digital Displacement® pump on partial loads and low speeds in a hydraulic transmission. This technology might prove useful and efficient in a system that has to be dimensioned for a large flow demand that rarely is required. Mathers Fuel Saving Vane Pump

Mathers Hydraulics Pty Ltd invented a vane pump where the vanes can be retracted into the rotor thus forming no pumping chambers. This makes it possible to turn the pump on/off. The vanes are locked in its retracted position using a pilot pressure pushing a ball into a dent in the vane [5]. When the pump is turned on the pressure is removed and the vane is released, starting to preform useful work. Retracting the vanes to just the right point the rotor will be cylindrical with no protruding object, making the idle losses low. Another advantage is that the vane pump is quiet [13].

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Figure 3.8: The Artemis Digital Displacement® Pump. ©Artemis Intelligent Power Ltd. Used with permission of the copyright holder.

Figure 3.9: Mathers Fuel Saving Vane, FSV, pump. In the left figure the vanes are retracted and the pump is turned off. In the right figure it is active, working as a regular vane pump.

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3.6 Individual Metering

The use of spool valves in conventional systems has a restriction in that the meter-in and meter-out orifices are mechanically connected, thereby decreasing the degree of flexibility. Higher flexibility is achieved by controlling the orifices independently. This is called individual metering. The advantages, presented in [4], of this concept is the possibility to acquire a higher efficiency by changing the direction of flow during operations and to reduce valve losses. Functions like float, recuperation and regeneration can be used to potentially lower the energy consumption. There are different types of individual metering systems depending on what type of valves are chosen and in which way they are connected, see [4] for more configurations than what is presented in this thesis. The four most common modes are normal, recuperation, float and regeneration mode. These are explained below and can be seen in figure 3.10.

Normal mode

The pump and tank are connected to the meter-in valve and the meter-out valve, respectively. This is the most common sort of operation since it is how a conven-tional system operates [1].

Regenerative mode

Losses can be reduced by directing the output chamber to the pump-line, thus increasing the flow to the actuator without increasing the pump flow. This function is most useful when manoeuvring light and heavy loads simultaneously.

Floating mode

Also called Energy Neutral Operation is profitable when lowering a load. No pump flow is delivered while in this mode, the oil to the actuators is moved from one chamber to the other. The excessive oil is provided by the tank.

Recuperative mode

The cylinder is used as a pump in negative strokes, pressurizing the return oil and directing it back to the pressure system, thereby producing energy that can be used on other functions, or stored in an accumulator.

Introducing this concept in to the system will make the controller more complex, because there must be an input signal for each valve. An additional input signal is possible to add if an electronically controlled pump is implemented, resulting in more flexibility. The valves and pump can be controlled either electronically or directly by the operator. Usually the operator generates one input, for example desired cylinder speed, pressure or flow. The ability to switch mode, i.e. change the flow direction, during operations presents further challenges in the controller. Transition between modes must be managed in a smooth manner. Also, a strategy for when to switch mode must be made. The ability to divide the flow into several directions with individual metering can reduce the number of pumps in the system.

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PWM PWM

(a) Normal mode

PWM PWM PWM PWM (b) Regenerative mode PWM PWM PWM PWM (c) Floating mode PWM PWM PWM PWM (d) Recuperative mode

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3.7 Hydraulic Accumulators

A hydraulic accumulator is typically used to store or recover energy in a hydraulic system. Pump flow or excess flow from a operation can be used to charge the accumulator. The flow can for example be efficiently used when a cycle consists of high flow demand during short periods of time. An accumulator can also work as a safety component, providing the system with flow if other flow sources are disabled. Rapid valve movement or acceleration of masses can produce pulsations in the system. These can be dampened by the spring characteristics of an accumulator. An accumulator consists of two different chambers divided by a flexible ”wall”. One of the chambers working as a spring is filled with gas, often nitrogen N2, to a certain pre-charge pressure. The other chamber is connected to

the hydraulic system. When the hydraulic pressure increases the nitrogen is compressed and the oil flows into the accumulator. When the hydraulic system pressure decreases the oil flows out from the accumulator as the gas strives to reach the pre-charge pressure, [13]. There are three different types of accumulators available on the market today. They are described below and can be seen in figure 3.11.

Figure 3.11: Different types of accumulators. From the left a Bladder accumulator followed by the Diaphragm accumulator in the middle and a Piston accumulator to the right.

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Bladder accumulators, where the gas is stored inside a balloon, bladder. The bladder is compressed when oil enters the accumulator. This type typically has a fast output flow and is often used for medium size volumes [14].

Diaphragm accumulators has a membrane that can move back and forth depending on the pressure on each side. This type of accumulator is usually used when the accumulator size is small and is often more robust than bladder accumulators. Piston accumulators is the type of accumulator that can handle the largest volume.

It consists of a light weight piston that separates the gas from the liquid, and moves inside a well defined cylinder depending on the pressures. The pressure ratio limit between maximum pressure and pre-charge pressure can be very high [14] [9].

3.8 Valveless Systems

When eliminating the valves in a system, the metering losses caused by the valve is eliminated as well. This section will present different systems that can be used to control hydraulic actuators in a system without the use of valves.

3.8.1 Transformers

A hydraulic transformer consists of two hydraulic machines mechanically connected to each other, see figure 3.12. This allows for a linear actuator to be manoeuvred using secondary control. A secondary controlled system can according to Heybroek [7] be described as the hydraulic equivalent of an electric grid. Additional functions and com-ponents are simple to attach to such a system. The transformer converts an input flow at a certain pressure to a different output flow at the cost of a pressure change. Compare with an electric circuit where the current is increased if the voltage is decreased and vice versa. In order to implement this at least one of the machines must have variable dis-placement, and therefore may experience low efficiency at certain operating conditions. The disadvantage for this system is the number of machines used, requiring space and increasing the system cost. The possibility to recuperate energy and the high efficiency when operating several functions simultaneously is the advantage. In order to operate in all four quadrants, that is positive and negative piston velocity and external force, additional valves are needed causing additional losses.

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3.8.2 Pump Controlled Actuators

Introducing one bidirectional variable pump for each actuator would eliminate the need for directional valves and give the possibility to control each actuator by changing the displacement setting on the pump. In a system as this, see figure 3.13, the system efficiency highly depends on the pump efficiency at low displacement settings. One pump for each actuator means that it has to be dimensioned to handle the highest flow to and from the actuator and for this reason it gets a low displacement setting most of the time. The installed displacement in a system tends to be unnecessarily high if the highest flow from each function is not used at the same time but still have to be dimensioned for. The main advantage for this concept is the efficiency when operating more than one actuator simultaneously [1].

Figure 3.13: Simplified schematic of a pump controlled actuator

3.8.3 Electro Hydraulic Actuators

Electro hydraulic actuator system, often refereed to as EHA, primarily consists of a bidirectional fixed displacement pump and an electric motor. It is basically a special configuration of the Pump Controlled Actuator described in 3.8.2, where each pump have its own electric motor. By varying the speed of the electric motor the flow and therefore also the motion of the actuator is controlled. If the actuator is symmetric there is no need for a tank to the oil. This is because of the fact that the pressurized area is the same for both positive and negative strokes in such a cylinder, resulting in a constant system volume regardless of the piston position. In mobile machines however the actuators are often asymmetric so some adjustments to the traditional concept would be needed [15]. The overall system efficiency in an EHA system will be high since it runs only when needed. One other advantage is that it does not require a variable pump, thereby reducing the pump cost. The main disadvantages are the need of one electric motor and pump for each actuator, as well an additional energy conversion.

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Chapter 4

Mathematical model

The characteristics of a system and its efficiency must be examined in a manner that provides accurate and reliable results. This is achieved by simulations of all concepts, based on mathematical models. Provided data and calculations are presented in this chapter.

4.1 Simulation Data

The data is collected during tests executed on tracks available for Volvo in Bra˚as. A typical track can be seen in figure 4.1. The tests were made with different vehicle conditions, such as speed and load. A test cycle typically consists of two laps on the same track. One lap is loaded and the other is unloaded and run in the opposite direction on the track. Data collected when dumping was not available for the same size of the hauler but have been processed to be usable in these evaluations. The data is stored with a fix frequency. The time between two data points are refereed to as a time step. The logged parameters that are used as input in the mathematical models are:

Engine speed rev_/

min

Vehicle speed km/h

Radiator fan speed rev/min

Intercooler fan speed rev/min

Pressure in steering cylinders bar

Pressure in dumping cylinders bar

Pressure at radiator fan bar

Pressure at intercooler fan bar

Steering cylinder length mm

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Figure 4.1: A typical Volvo test track

4.2 Calculations

In this section the calculations for all concepts found in chapter 5 are presented. First calculations for the current system are presented and later on the changes that were made in the initial code to make it valid for the new concepts.

The calculation process can be described in a flow chart, as in figure 4.2, where the the mathematical model is based on the hydraulic concepts. How the calculations are made and what changes that are done between different concepts is described in this chapter. The model is constructed in such a way that the measured data functions as input and the result is the energy consumption. The result for different concepts can then be further analysed and compared.

Hydraulic

Concepts Mathematical Model Results Comparison / Evaluation

Measurement Data

Figure 4.2: Flow Chart showing the calculation process.

4.2.1 Geometry

The hauler geometry affects the model by influencing the flow demand caused by steering and dumping during a test cycle. The specific purpose for the geometry calculations will be to compute the length difference between every time step on both steering cylinders and dumping cylinder. The geometries can be seen in figure 4.3.

Positive and negative stroke areas on the cylinders will affect the flow demand during both steering and dumping. The pressurized area can be calculated as shown in equation (4.1) and (4.2).

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Chapter 4. Mathematical model Load unit Pull unit bb bf lf Lh Lv β A B lb γ µ α

(a) Gemoetry for the steering

xtl a b Lc rtipp xotc zotc ztl β α α1 α2

(b) Geometry for the dumping

Figure 4.3: Geometry for the hydraulic functions on the hauler.

Api= π d2_pi 4 (4.1) Apr = π d2_pi− d2 pr 4 (4.2) Steering

When calculating the steer angle the values of A and B are needed. They are calculated according to A = s bf 2 2 + l2_f (4.3) B = s bb 2 2 + l2_b (4.4)

There is a slight non linearity between the lengths on the steering cylinders, meaning that a certain change in length on one cylinder will not correspond to the same length change on the other cylinder. This give rise to compute the right cylinder length from measured data on the left cylinder length. Begins by calculating the steering angle, α, which depends on the angle γ which in turn depends on parameters calculated in (4.3) and (4.4). γ = arccos(A 2_{+ B}2_{− L}2 v) 2AB (4.5) α = π − γ − arctan bf 2lf − arctan bb 2lb (4.6)

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The expression for the right steering cylinder length depends on the angle µ that in turn depends on the previous calculated steering angle, α, (4.3) and (4.4).

µ = π + α − arctan bf 2lf − arctan bb 2lb (4.7) Lh= p A2_{+ B}2_{− 2AB cos(µ)} _(4.8)

Knowing both the right and left cylinder lengths throughout the test cycle, the flow to or from the cylinders can be calculated. The flow calculations are further described in section 4.2.2.

Dumping

The dumping cylinder length expression depends on the angle β, that is the angle be-tween the upper and lower cylinder attachments measured at the body rotational point. β is the sum of the tilt angle θ and β0. Two other parameters used to calculate the

cylinder length is the distances a and b according to β0= arctan ztl xtl + arctan zotc− ztl xtl− xotc (4.9) β = θ + β0 (4.10) a = q x2 tl+ ztl2 (4.11) b =p(xtl− xotc)2+ (zotc− ztl)2 (4.12) Lc= s

a2_{+ b}2_{− 2ab cos(β) +} ucw − lcw

2

(4.13)

4.2.2 Current System

The Matlab models used to simulate the current system does also act as a base when generating models for some of the new concepts. Starting with the measurement data, calculations are made as described below resulting in an engine input power. These calculations are made for all data points. Combining these results and knowing the time step the energy consumed during a simulation can be found.

System pressure is generated by three LS pumps and an auxiliary steering pump, pump 8 in figure 1.2. The pump pressure is a sum of the pressure in the steering cylinders, called steering pressure, and the pressure margin which is constant on a LS-system.

pp = pst+ pmargin (4.14)

The flow demand from steering and dumping depends on the difference in volume between each time step, see equation (4.15). It is computed from the cylinder positions and areas. A leakage flow is included in the model. If the calculated flow demand is

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Chapter 4. Mathematical model

below the leakage flow value, the flow demand is set to the leakage flow. qst =

dV

dt (4.15)

qst(qst0 qleak) = qleak (4.16)

The displacement setting, ε, calculations are made under the assumption that all LS pumps and the auxiliary pump share the same setting. Based on what is known about the system configuration this assumptions seems valid. The highest ideal flow from each pump is calculated using (4.17) and setting ε = 1 and ηvol,p = 1. Knowing these flows

the displacement setting can be calculated using (4.18) qp = εpDp

np

60ηvol,p (4.17)

ε = qst

qIdeal,E · ηvol,E+ qIdeal,G· ηvol,G

(4.18) where qIdeal,E and qIdeal,G are the ideal flows from the engine mounted pumps and

the auxiliary steering pump respectively. Pump efficiency data provided by the pump manufacturer serves as the base for interpolating an efficiency, η, for a specific running condition. The efficiency depend on several parameters and can be described as a η = η(p, ε, n). Since the displacement setting and the efficiency is a function of each other the actual displacement setting is iterated to a precision of ∆ε = 1e−4. If the calculated displacement setting is greater than 1 the concept can not deliver the flow required. Torque required by the pumps are calculated using equation (4.19). Knowing the torque, the engine and vehicle speed the power consumption for each pump can be calculated using equation (4.20).

Tin= εpDp 2π ∆p 1 ηhm,p · R (4.19) P = T · ω ⇔ P = T · n 602π (4.20)

Fan circuit calculations require the flow demand from the fan motor to be calculated. Since the motors have a fixed displacement this is done using equation (4.21).

qm = εmDm nm 60 1 ηvol,m (4.21) The flow calculations, resulting in the power consumption, are the same as described above using equation (4.17)–(4.19).

Useful power is calculated by equation (4.22) with the stored data of the steering pressure, pst, and the flow demand qst from equations (4.15) and (4.16).

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The powers from equations (4.22) and (4.20) are thereafter used to calculate the energy consumption in one test cycle. This makes comparison between simulations with different time steps possible.

E =X

Z

P (t)dt (4.23)

4.2.3 Conventional Systems

Constant flow

The displacement settings, ε, on all pumps are set to 1 in the constant flow concept, consequently excluding equation (4.18). Since there is no data available showing when the steering wheel is turned, i.e. the driver wants to change the driving direction, the available data had to be analysed in an attempt to determine this. It is done using equation (4.24) where a pressure level, G, is decided based on the data. If the pressure drops below that value the system is assumed to be idling and the system pressure is set to the idling pressure, F.

pst(pst < G) = F (4.24)

Constant Pressure

The current system is easily changed to a constant pressure system by modifying equa-tion (4.14). The pump pressure, pp, is set to the constant pressure desired in the system.

4.2.4 Displacement Reduction

When implementing displacement reduction the displacement of a pump is simply set to zero in order to eliminate it from the calculations.

4.2.5 Clutches

When implementing clutches into the system some changes to the initial code have to be made. Now calculations are made to decide what each pump can deliver at all time. The flow demand from the steering is then compared with these values in a predefined order until the full required flow can be delivered. It is then known how many pumps that are required. The check is made using an if-statement like the one presented below. All other calculations are made in the same way as for the initial system.

for n = 1:data length

if q st(n) _{> (q8 Ideal(n)*eta vol 8(n) +} ...

(q1 Ideal(n) + q2 Ideal(n))*eta vol(n)) NoP(n) = 4; % Auxiliary steering pump, pump 1, 2 and 3

elseif q st(n) _{> (q8 Ideal(n)*eta vol 8(n) +} ...

(q1 Ideal(n))*Pump eta vol(n))

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elseif q st(n) _{> (q8 Ideal(n)*eta vol 8(n))}

NoP(n) = 2; % Auxiliary steering pump and pump 1

else

NoP(n) = 1; % Only the Auxiliary steering pump

end end

4.2.6 Flow control

The reduction of the pressure margin was described in section 3.4, stating that the margin can be expressed with the square of the flow. A mathematical model according to equation (4.25) is made where the coefficient k is calculated by a Matlab fit function. The function requires maximum flow, (4.17), and pressure margin boundaries as inputs. The pressure margin now depending on the flow is thereafter used in equation 4.14 to implement flow control.

pmargin(qst) = k · qst2 (4.25)

4.2.7 Digital pump

Since only the total efficiency is available for the digital pump, changes were made in the model so that the total efficiency is added after calculating the displacement and torque of the pump. Since no data for idle losses is available those sections of the code is removed and the result for this concept is presented without idle losses. Although the idle losses stands for most of the losses, these calculations determine whether the concept is reasonable at all.

4.2.8 Individual Metering – Regenerative mode

The advantage of individual metering would be to obtain the regenerative mode. When the regenerative mode of the individual metering concept is active, both sides of the piston are connected to the pump line. The external force, Fext, has to be equal to the

force in the current system. The force of the current system can be calculated using the same initial equations, (4.26) – (4.28), where the pressure is different on both sides. The force equation for the piston would in this case be

0 = F+− F−− Fext (4.26)

F = p · A (4.27)

The cylinder area, A, is calculated using equation (4.1) and (4.2). Combining (4.26) and (4.27) and replacing A with the corresponding area equation, the expression becomes

p+· π d2_pi 4 − p−· π d2_pi− d2 pr 4 = Fext (4.28)

Since the external force has to be the same in the regenerative system as the current system the force equation, (4.28), for both systems can be set equal to each other. For

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the current system p is replaced with the known pressures. Assuming that the low pressure ,p−, is zero. Breaking out the pressure in the regenerative system, preg, results

in preg· π d2_pi 4 − preg· π d2_pi− d2 pr 4 = pcur· π d2_pi 4 − 0 · π d2_pi− d2 pr 4 ⇔ preg· d2pi− d2pi− d2pr = pcur· d2pi ⇔ preg= pcur· d2_pi d2 pr (4.29) 4.2.9 Accumulators

All systems that includes accumulators were made in co-simulation between Amesim and Simulink. In these simulations the rest of the system was simplified so that the steering was modeled as an ON/OFF-valve and only one pump component is used. Despite these simplifications the model was found accurate. This is verified by comparing flow demand to the flow in the model. The Amesim and Simulink models can be seen in appendix A. When looking at replacing the auxiliary steering pump with accumulators as de-scribed in section 5.7 the required accumulator volume is based on calculations dede-scribed in this section. The oil volume needed is calculated from the steering cylinders using (4.1) and the total stroke length. This volume is then multiplied by the number of turns that have to be made according to safety standards, resulting in

Vst = N oT · (Lst,max− Lst,min) · (Api+ Apr) (4.30)

There is also a defined turning speed, κ, in degrees per second and vehicle speed, v, used to calculate the time it takes to preform a full turn and the angular velocity of the pump. With these numbers the volume from the pump can be calculated according to

Vp = qp

90

κN oT (4.31)

Subtracting the flow delivered by the pump from the required flow yields the working volume needed in the accumulator. Knowing the working volume and assuming the highest and lowest working pressure needed, the accumulator volume is calculated ac-cording to equation (4.33). In these calculation both the emptying and filling processes are assumed to be adiabatic.

∆V = Vst− Vp (4.32) V0= ∆V p1 p0 1 − (p1 p2) 1 n (4.33)

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4.2.10 Cost

Cost is an important factor when constructing a system today. Since some systems are cheap but do not improve the energy efficiency that much while others are more expensive but improves the efficiency all the more. In order to get a fair comparison between the concepts a payback time was calculated for each concept. The payback time is based on the cost of the components and the energy consumption. Some constants such as fuel price and diesel engine efficiency are also included in the calculation. The amount of hours accepted as payback time during one year is 1400 hours. This is given if there are 250 working days with eight hour shifts during one year and the hauler is driven 70% of that time.

P ayback = _∆E ∆Cost

Concept

EDiesel·ηEngine ·

1

tSimulation · $Diesel

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Chapter 5

Concepts

During this project different concepts have been evaluated with respect to functionality, cost and safety. The results of evaluating these concepts will be presented here. Con-cepts which do not fulfil the requirements will be excluded from further evaluation and modelling of system. The first concepts to be presented are the base concepts, some of whom will thereafter be combined into different complete system concepts further down in this section.

5.1 Conventional Systems

A constant flow concept is expected to have great losses since all the flow supplied by the pumps will often not be required. Most of the flow will then be drained through the open center during a driving cycle.

The pressure demand when performing a dump is much higher than the pressure needed for the other functions, this will cause a constant pressure system to have great losses.

Both conventional systems should be examined to find out how efficient they need to be in order to compete with the current system. The goal with these calculations is to find which values for the losses and leakage in equation (4.24) that generates an energy consumption equal to the current system. As these systems are expected to consume more energy than the current, a request from Volvo CE was to evaluate if the increased energy consumption could be motivated by the reduced system cost. A constant flow concept can for example be upheld with cheaper fixed displacement pumps.

5.2 Displacement Reduction

This concept is intended to optimize the usage of the pumps in the current LS-system. If installing valves so that the flow from the fan pumps can be directed to the main control valve and thereby supply the steering and dumping functions there would be an excess of available flow. Dimensioning the system to fulfil the demands of today would allow

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elimination of one pump, thereby reducing the losses in two ways. First the idle loss for the removed pump would be eliminated, secondly the reduced displacement during normal driving would increase the displacement setting on the remaining pumps hence increasing their efficiency. This concept would work under the condition that the fan pumps are over dimensioned so that the cooling can be shut off for some time, when they are used for steering or dumping. The fact that the pump is over dimensioned allows for a higher pump flow when turning the fans on again, resulting in a higher fan speed and cooling effect. Looking at the measurement data this seems to be the case, see figure 6.2a. This concept would reduce the cost since one pump less is required, however some extra valves are used to direct the flow from pump 5 and 6 to the main control valve. The main downside of this concept is that if cooling is required at the same time as those pumps are wanted for dumping the driver might feel that the function acts slowly. In this concept the 63 cc pump is removed and the 45 cc fan pump is reduced to 28 cc. This reduction can be done since the brake cooling is assumed to be removed from the circuit.

5.3 Clutch

5.3.1 Current system with Clutch

The idle losses have been shown to be the greatest losses in an average driving cycle. One way to eliminate these would be to add clutches between the PTO and the pumps, as shown in figure 5.1. Since the clutches are implemented as an interface between the mechanical and the hydraulic system, and therefore does not require any changes in the hydraulic circuit, they can be introduced in different kind of systems such as constant flow, constant pressure, LS or flow control. This would allow disconnection of pumps during normal driving which would reduce the idle losses. The displacement setting on the connected pumps would be higher and thus increasing efficiency. Centrifugal clutches are obviously not applicable in this type of system since the timing of when to connect a pump is of great importance. The controllability of friction and non-slip clutches are more suited. Depending on the type of clutch, wet or dry, this would reduce or completely eliminate the losses when the pumps are not used. As mentioned above the clutch does not interfere in the hydraulic system. This allows for further system development and improvements in other operation points. The clutches are assumed to be ideal in all calculations. The clutches concept is only evaluated on the steering circuit since it is the part where it will be most useful. They might also be implemented on pumps in the fan circuits but since the fans are expected to run most of the time they will not reduce the energy that much.

5.3.2 Displacement Reduction with Clutch

This concept is a combination of Displacement Reduction, section 5.2, and Clutch, section 5.3. The inserted energy is expected to be less than for the current system with clutches since the idle losses on the downsized radiator pump will be reduced. The first connected

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Chapter 5. Concepts P T O PWM 1. 3. PWM PWM 2. 6. 5.

Figure 5.1: Clutch placement in the system

pump, pump 2, being 60 cc instead of 63 cc should only have a minor effect on the improvement.

5.4 Flow Control

5.4.1 Current system with Flow Control

Since the hydraulic system constantly runs at low pressure and flow to cover the pressure margin, this causes a constant loss. In order to reduce this loss Flow Control could be implemented in the system. With the existing steering valve some problems regarding a control strategy will exist. Those would however be eliminated if introducing electrically controlled steering. The losses should with this concept be reduced according to section 3.4.

5.4.2 Displacement Reduction with Flow Control

This concept is a combination of Displacement Reduction, section 5.2, and Flow Control, section 5.4. The losses are expected to be reduced as much as they are when implement-ing Flow Control in the current system, improvimplement-ing the system further.

5.4.3 Clutch and Flow Control

This concept is a combination of Clutch, section 5.3, and Flow Control, section 5.4. Flow control is believed to be the most efficient way of controlling a system that includes clutches. This since the flow demand and how much flow each pump can deliver at each moment is known. Then the clutches can be manoeuvred, engaging the appropriate pump configuration at all time. This concept will be implemented on both the current system and the Displacement Reduction concept.

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5.5 Digital hydraulics

5.5.1 Linear actuators

To use a digital linear actuator on the dumping piston will never be desired. The dumping is to be performed as fast as possible. This is achieved if directing all flow to one side while keeping the pressure on the other side of the piston as low as possible. Neither is applying them on the steering an option, because of safety legislations. A malfunction on any of the numerous valves on a digital linear actuator would potentially be dangerous. No further evaluation of this concept will be conducted.

5.5.2 Angular actuators

A digital hydraulic pump is a future concept that might prove useful. Such a pump could replace both a clutch and pump. The pumps evaluated here are the Artemis Digital Displacement® radial piston pump and Mathers vane pump. They are said to have low idle losses, however no data was provided for the Artemis pump so all calculations assume no idle loss and the efficiency is interpolated from graphs found in [16]. For the Mathers pump an estimated idle loss was provided by the inventor for one pump size, 1 kW for a 90 cc pump. The digital pumps are implemented on the LS-system in the evaluated concept. The fan circuits still have the current configurations.

5.6 Individual Metering

Big volumes are in play during dumping and much can be won by taking advantage of the energy stored in those volumes, which in the existing system goes to waste. The use of individual metering simplifies the use of regeneration mode. This is believed to potentially be a good concept to implement on the dumping since it will reduce the flow needed from the pump and therefore also allow for downsizing of the pumps. Individual metering raises the flexibility of the hydraulic system, providing more control of the flow through meter-in and meter-out orifices, this enables less valve losses.

5.7 Accumulator

Accumulators can be used to recover energy. Doing so on a hauler has previously been evaluated in [10]. This has been further investigated at Volvo and internal reports [11] shows energy savings no higher than 4%. The cost and weight of these systems are expected to be too high for implementation.

5.7.1 Accumulator in constant pressure system

Introducing accumulators in the system would result in an extra flow source to assist during high flow demand. This would allow for downsizing or disconnection of pumps. The system evaluated here is intended for the steering, allowing for the pumps to be

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Chapter 5. Concepts

disconnected in intervals. In this concept the accumulators are installed in a system with pump/clutch units so that the pumps are disconnected when the maximum sys-tem pressure is reached and connected again when the pressure drops below a pre-set level. Different pressure levels and sizes of the pumps and accumulators were evalu-ated. Another advantage when implementing accumulators is its natural characteristic of damping the hydraulic system, the degree of damping will however not be evaluated in this thesis.

5.7.2 Accumulator to reduce auxiliary steering pump

The auxiliary steering pump is the largest pump in the whole system, 71 cc, running at low displacement settings most of the time. If the size of this pump could be reduced and the additional flow be provided by an accumulator, the losses should be reduced during normal operations. In figure 5.2b it can be seen what accumulator size that is required for each pump size. This volume will also depend on what pressures that are allowed in the accumulator or system. Accumulators are preferably used in a constant pressure system. Implementing the accumulator instead of the auxiliary steering pump in the current system thus requiring additional valves in order keep the accumulator pressure high, even when the system pressure is low in normal driving conditions. In this system configuration the accumulator is to be charged when the hauler is started and the oil in the accumulator is to be used only in extreme situations such as engine failure.

In order to reduce the maximum flow demand from the pump and consequently the accumulator size an alternative configuration is possible, placing a valve between the steering pistons so that only the positive stroke area creates the torque and requires flow. The principle for this can be seen in figure 5.2a. During normal operation the system works equally as today, but in case of an engine failure or loss of power to the valve the valve switches to reduce the flow demand in the system. The downside of this configuration is the reduced steering torque and the need for a small accumulator, compensating for the non linearity in the steering geometry.

5.7.3 Dump using accumulators

One question raised at Volvo CE is if it would be possible to perform the dumping using only accumulators. These accumulators are then to be charged by the engine mounted pumps while driving, hence increasing the efficiency and allow for pump downsizing. The volumes needed to achieve a full dump are expected to be large, requiring big expensive accumulators. This volume will probably be hard to install on the hauler and add weight, reducing maximum payload.

5.8 Valveless Systems

These concepts require more machines than the other systems evaluated. All valveless systems will therefore add to the vehicle weight. This disadvantage and the highly

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LS-block

Steering

cyli

nders

Steering valve

(a) Alternative steering configuration. During normal driving the system works as today but if the power to the valve is lost the flow demand is reduced.

(b) Accumulator size requirement for a certain displacement on the auxiliary steering pump. p1= 150 bar, p2= 250 bar and polytropic exponent is 1.4

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Chapter 5. Concepts

increased cost, that is likely to follow, makes these systems low prioritized and no further evaluation are made in this thesis.

5.9 Fan Circuits

The fan circuits in the current system are quite luxurious using one pump for each fan motor. Alternatives for this system in order to reduce cost could be to connect the fans in series or parallel. Enabling the motors to run independently in a series connection is possible if implementing bypass valves. A series connection would require only one of the pumps used today, but increase the pressure delivered by the pump and also the pump efficiency. Connecting the motors in parallel requires a pump that can deliver the sum of the flow demands from the motors. To split the flow correctly a variable flow divider is required.

Hydraulic Energy Efficiency of Concepts on an Articulated Hauler : Design and evaluation of different hydraulic concepts with focus on energy efficiency

Hydraulic Energy Efficiency

of

Concepts on an Articulated Hauler

Andr´

e Arana Escobedo

Oskar Gunnarsson

Hydraulic Energy Efficiency

of

Concepts on an Articulated Hauler

Andr´

e Arana Escobedo

Oskar Gunnarsson

Abstract

Acknowledgements

Contents

List of Figures

List of Tables

Nomenclature

Chapter 1

Introduction

1.1

Background

1.2

Aims

1.3

Problem Description

1.4

Delimitations

1.5

System Description

1.6

Contribution

Chapter 2

Method

Chapter 3

Hydraulic systems

3.1

Conventional Systems

3.2

Load Sensing System

3.3

Clutch

3.4

Flow Control

Load Sensing

Flow Control

3.5

Digital Hydraulics

A

D

B

C

3.6

Individual Metering

3.7

Hydraulic Accumulators

3.8

Valveless Systems

Chapter 4

Mathematical model

4.1

Simulation Data

4.2

Calculations

Chapter 5

Concepts

5.1

Conventional Systems

5.2

Displacement Reduction

5.3

Clutch

5.4

Flow Control

5.5

Digital hydraulics

5.6

Individual Metering

5.7