Sổ tay bôi trơn tribology

1-11 Characteristics of Engine Oil and Functions of Its Additives • Viscosity Effects • Engine Oil Quality and Oil Degradation During Vehicle Use • Fluid Film Lubrication • Future Concer

Taylor & Francis Group

6000 Broken Sound Parkway NW, Suite 300

Boca Raton, FL 33487-2742

© 2006 by Taylor & Francis Group, LLC

CRC Press is an imprint of Taylor & Francis Group

No claim to original U.S Government works

Printed in the United States of America on acid-free paper

10 9 8 7 6 5 4 3 2 1

International Standard Book Number-10: 0-8493-2095-X (Hardcover)

International Standard Book Number-13: 978-0-8493-2095-8 (Hardcover)

This book contains information obtained from authentic and highly regarded sources Reprinted material is quoted with permission, and sources are indicated A wide variety of references are listed Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials

or for the consequences of their use.

No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers

For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc (CCC) 222 Rosewood Drive, Danvers, MA

01923, 978-750-8400 CCC is a not-for-profit organization that provides licenses and registration for a variety of users For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.

Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for

identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data

Catalog record is available from the Library of Congress

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Taylor & Francis Group

is the Academic Division of Informa plc.

her perseverance that literally defines the character of our family.

Handbook of Lubrication and Tribology: Volume I Application and Maintenance, Second Edition is sponsored

and copublished by the Society of Tribologists and Lubrication Engineers (STLE) This book is one of athree volume series covering: in Volume II, Theory and Practice; in Volume III, Monitoring, Materials,Synthetic Lubricants, and Applications; and in this volume, Applications and Maintenance The goal ofthis book is to provide an update to the first edition and to provide the latest information regardingapplication and maintenance in the broad field of Lubrication Engineering

The first edition was written over twenty years ago In the intervening period, the science of tribologyand the development of engineering best practices have evolved markedly As a result, each of the chapters

of the first edition has been rewritten and updated A number of new chapters have been added to capturenew information: In the section on Applications, the Hydraulics chapter was split into two chapters onpumps and fluids, and all new chapters on Tribology of Data Storage Devices, and Biotribology wereadded In the section on Industrial Practices, a new chapter was added on Tribology of Metal FormingProcesses In the section on Maintenance, three new chapters were added on Lubricant Cleanliness,Environmental Implications of Lubricants, and Centralized Lubrication Systems — Theory and Practice.This volume was written by a peer recognized team of expert contributors from a wide variety ofindustry segments Each chapter was written by an expert both knowledgeable and active in the subjectarea Thanks go to these individuals; without their expertise and hard work this work would not havebeen possible Thanks must also go to their employers for their support of this effort and contribution toour industry

Because of its emphasis on the practice of Lubrication Engineering, this book is an excellent referencefor those preparing for STLE’s Certified Lubrication Specialist® Certification examination As such, ithas been recommended in the Body of Knowledge by STLE’s Certified Lubrication Specialist Certifica-tion Committee This volume, like its predecessor, belongs in the reference library of all professionals

The first edition of the Handbook of Lubrication: Theory and Practice of Tribology — Volume I: Application

and Maintenance was edited by E.R Booser and was sponsored by the Society of Tribologists and

Lubrication Engineers (STLE) to provide the latest information in the field Volume I of the Handbook

of Lubrication: Theory and Practice of Tribology covers Applications and Maintenance Volume II covers

theory and design and was published in 1984 Volume III which covers Monitoring, Materials, SyntheticLubricants, and Applications was published in 1994 to extend the topical areas covered by Volume I andVolume II since their initial publication

Over 20 years have elapsed since the First Edition was published in 1983, and enormous changescontinue to occur in the lubrication and tribology engineering sciences Although Volume III did extendthe areas covered, all of the areas initially covered in Volume I needed to be significantly updated In view

of these changes and the time that has elapsed since the appearance of the First Edition, STLE initiatedthe Second Edition of this invaluable text

The Second Edition of the Handbook of Lubrication and Tribology: Volume I Application and

Mainten-ance, has been reorganized slightly to aid the reader in identifying chapters and topics of interest All of

the chapters from the First Edition, with the exception of the chapter on Marine Equipment, have beenrevised or completely rewritten In addition, a number of new chapters have been added including: Bio-tribology, Tribology of Data Storage Devices, Tribology of Metal Forming Processes, and EnvironmentalImplications of Lubricants The chapter on Compressors and Vacuum Pumps was significantly expandedand the original chapter on Hydraulic Systems and Fluids was divided and expanded into two separatechapters: Hydraulic Pumps and Hydraulic Fluids Altogether there are a total of 37 chapters much ofwhich is either a totally new treatment of the subject or completely new information

This handbook provides the reader with an extensive reference to the most important and commonlyencountered lubrication systems and fluids in industry This text is of value to the practicing tribologistand lubrication engineer, mechanical or materials engineer, and failure analysis personnel

I am indebted to all of the contributing authors of the book for their tremendous effort and patience.Without their dedication and support, the successful completion of this text would not have been possible

George E Totten

Seattle, WA

ix

I wish to thank the Society of Tribologists and Lubrication Engineers (STLE) for their continued supportthroughout this project Very special thanks to Robert Gresham and Barbara Rapacz, without whosesupport the successful completion of this project would not have been possible.

I am especially indebted to Theresa Delforn and Shelley Kronzek of CRC Press, Inc for their continuedguidance and expert assistance throughout this process, from the beginning to the end They have made

a potentially difficult task into an absolute delight

Finally, and most importantly, I am especially indebted to my family, especially my wife Ah Kum forallowing me to be so totally involved in this project for such a long time Their continued patience with

my sometimes bad behavior is most especially appreciated

xi

George E Totten is President of G.E Totten & Associates, LLC in Seattle, Washington, and a

visit-ing professor of materials science at Portland State University Dr Totten is coeditor of a number of

books including Steel Heat Treatment Handbook, Handbook of Aluminum, Handbook of Hydraulic Fluid

Technology, Mechanical Tribology, and Surface Modification and Mechanisms (all titles of CRC Press), as well

as the author or coauthor of over 400 technical papers, patents, and books on lubrication, hydraulics, andthermal processing Dr Totten is a Fellow of ASM International, SAE International and the InternationalFederation for Heat Treatment and Surface Engineering (IFHTSE) and a member of other professionalorganizations including ACS, ASME, and ASTM Dr Totten formerly served as president of IFHTSE

He received bachelor’s and master’s degrees from Fairleigh Dickinson University in Teaneck, New Jerseyand a Ph.D degree from New York University, New York

xiii

James R Anglin

Aluminum Company of America

Alcoa Technical Center

School of Mechanical Engineering

The University of Leeds

University of LeedsLeeds, UK

James C Fitch

Noria CorporationTulsa, OK

Malcolm F Fox

De Montfort UniversityLeicester, UK

G.S Fox-Rabinovich

Department of MechanicalEngineering

McMaster UniversityHamilton, Ontario, Canada

O.-C Göhler

Institute of Fluidpower Drives andControls (IFAS)

RWTH Aachen UniversityAachen, Germany

Hooshang Heshmat

Mohawk Innovative Technology,Inc

Albany, New York

Emile van der Heide

TNO Industrial TechnologyEindhoven, The Netherlands

xv

Douglas M Jahn

Delphi Saginaw Steering Systems

Saginaw, Michigan

Mark J Jansen

NASA Glenn Research Center

Tribology and Surface Science

NASA Glenn Research Center

Tribology and Surface Science

Michael L McMillan

General Motors R&D Center,Chemical and EnvironmentalScience LaboratoryWarren, MI

T Meindorf

Argo-Hytos GmbHKraichtal, Germany

Hans M Melief

The Rexroth CorporationIndustrial Hydraulics DivisionBethlehem, PA

Paul W Michael

Milwaukee School ofEngineeringFluid Power Institute,Milwaukee, Wisconsin

H Murrenhoff

Institute of Fluidpower Drives andControls (IFAS)

RWTH Aachen UniversityAachen, Germany

Barbara J Parry

Mohawk LubricantsNorth Vancouver, Canada

Tribology GroupEnschede, The Netherlands

Rick Schrama

Dofasco Inc.,General Maintenance ShopsHamilton, Ontario, Canada

Shirley E Schwartz

General Motors (retired)

Will Scott

School of Mechanical,Manufacturing and MedicalEngineering

Queensland University ofTechnology

Brisbane, Australia

Paul D Seemuth

Tribology Consulting,International LLCHixson, TN

L.S Shuster

Department of MechanicalEngineering

Ufa Aviation InstituteUfa, Russia

Jacek Stecki

Subsea Engineering ResearchGroup

Department of MechanicalEngineering

Monash UniversityMelbourne, Australia

Portland State University

Department of Mechanical and

Materials Engineering

Portland, OR

Drew D Troyer

Noria CorporationTulsa, OK

Simon C Tung

General Motors R&D CenterChemical and EnvironmentalScience LaboratoryWarren, MI

S.C Veldhuis

McMaster ManufacturingResearch Institute (MMRI)Department of MechanicalEngineering (JHE-316)McMaster UniversityHamilton, Ontario, Canada

R.E Yungk

Air BP LubricantsMelbourne, Australia

SECTION I Applications

Simon C Tung, Michael L McMillan, Edward P Becker, and

Shirley E Schwartz

C.D Tipton

Arup Gangopadhyay and Farrukh Qureshi

Douglas M Jahn and Simon C Tung

D.J.W Barrell and M Priest

Andy Hall

Hooshang Heshmat and James F Walton II

B.C Pettinato

T Kazama and G.E Totten

Richard K Tessmann, Hans M Melief, and Roland J Bishop

xix

11 Hydraulic Fluids 11-1

H Murrenhoff, O.-C Göhler, and T Meindorf

S.C Veldhuis, G.S Fox-Rabinovich, and L.S Shuster

13 Lubricating Industrial Electric Motors 13-1

16 Tribology of Hard Disk Drives — Magnetic

Data Storage Technology 16-1

José Castillo and Bharat Bhushan

17 Biotribology:Material Design, Lubrication, and Wear in Artificial

Z.M Jin, J Fisher, and E Ingham

SECTION II Industrial Lubrication Practices

R Lal Kushwaha and Jude Liu

Roger Lewis and Rob Dwyer-Joyce

23 Lubrication in the Timber and Paper Industries 23-1

Paul W Michael

Paul D Seemuth

25 Food-Grade Lubricants and the Food Processing Industry 25-1

James C Fitch, Sabrin Gebarin, and Martin Williamson

26 Aviation Industry 26-1

H.A Poitz and R.E Yungk

27 Lubrication for Space Applications 27-1

William R Jones and Mark J Jansen

28 Friction and Wear in Lubricated Sheet Metal Forming Processes 28-1

E van der Heide and Dirk Jan Schipper

SECTION III Maintenance

29 The Degradation of Lubricants in Service Use 29-3

Malcolm F Fox

30 Lubricant Properties and Test Methods 30-1

Larry A Toms and Allison M Toms

31 Contamination Control and Failure Analysis 31-1

35 Conservation of Lubricants and Energy 35-1

Robert L Johnson and James C Fitch

36 Centralized Lubrication Systems — Theory and Practice 36-1

Paul Conley and Ayzik Grach

37 Used Oil Recycling and Environmental Considerations 37-1

Dennis W Brinkman and Barbara J Parry

Applications

Automotive Engine Oil

Simon C Tung and

Michael L McMillan

General Motors R&D Center,

Chemical and Environmental

1.2 Issues Related to Energy Consumption in an Engine:Service Effects 1-8

1.3 Engine Oil 1-11

Characteristics of Engine Oil and Functions of Its Additives • Viscosity Effects • Engine Oil Quality and Oil Degradation During Vehicle Use • Fluid Film Lubrication • Future Concerns

1.4 Gasoline Engine Oil Performance Categories andAssociated Test Methods 1-16

Introduction • ILSAC GF-4 and API SM Standard Tests1.5 Diesel Engine Oil Performance Categories andAssociated Test Methods 1-19

Service Effects of Diesel Engines Related to Engine Oil Degradation • Examples of Test Methods for Diesel Engine Oils • A Model for the Rate of Engine Oil Degradation in Diesel Engines

1.6 Future Directions 1-21

Concerns Related to Conservation of Fuel • Effects at the Molecular Level • Insights Gained from Tests with an Alternative Fuel • Prolonging the Working Life of Engine Oil • Minimizing Emissions and Pollutants and Ensuring Backward Compatibility • Future InvestigationsAcknowledgments 1-25

References 1-25

This chapter describes the functions of typical engines (gasoline and diesel), engine oil characteristics,and test methods Included are descriptions of the tribological concerns associated with various enginecomponents, service effects on engine oil, standard tests for engine oil and the types of service theyrepresent, and an overview of the issues that need to be addressed in the future

1-3

Rockers Valve springs Piston rings Oil filter

Journal bearings

Oil pump Oil

Camshaft Valve Piston

Cylinder block Con rod

Crankshaft Oil consumption Sump

FIGURE 1.1 The main components in an internal combustion engine.

1.1 Automotive Engines

1.1.1 Engine Operation

An internal combustion engine, such as illustrated in Figure 1.1, is the predominant power source formost types of cars and trucks [1] Various conditions influence engine development, such as the desirefor high power output, legislative requirements for reduced emissions, increased fuel economy, andminimal generation of hazardous substances Many technical and environmental challenges await thosewho attempt to address these concerns The following discussion contain examples of current conditions(and in some cases the evolution of current conditions) to provide insight into the types of issues thatmust be understood and actions that are desirable to meet future concerns successfully Directions inwhich future developments may evolve are included

Passenger car engines in North America typically use a “four stroke” cycle, which represents the number

of times a piston changes direction before the events in the process of powering the engine are repeated.Some diesel and spark-ignited engines use a “two stroke” cycle, but this is not common for passenger carapplications because two stroke engines may provide higher emissions of unburned fuel Some engineslocate the camshaft and valves above the engine and others locate the valves within the engine block.Engines also differ with regard to the number of cylinders and the orientation of those cylinders, such as

The working mechanism of a spark ignition engine is as follows:

1 Intake: One of the valves (the intake) in the cylinder head opens when the piston is near the top

of the cylinder, and as the piston moves downward, air and fuel are injected and move downwardwith the cylinder

2 Compression: When the piston begins to move upward again, both valves are closed, and the contents

of the cylinder (vaporized fuel and air) are compressed

3 Power: As the piston nears the top of the compression stroke, a spark plug fires and combustion of

the fuel takes place The burning fuel creates carbon dioxide, water vapor, and other compounds

As a consequence of this gas formation, pressure rises rapidly within the cylinder The force of thecombustion gases pushes the piston down again

4 Exhaust: As the piston reaches the bottom of the power stroke, energy from the expanding gases

has been transferred from the piston to the crankshaft via the connecting rod At this point, theexhaust valve opens and the piston then rises and sweeps most of the combustion products out of

FIGURE 1.2 Cross-section view of a V-6 engine.

the cylinder When the piston is near the top of the exhaust stroke, the exhaust valve closes and theintake valve opens The sequence of events then repeats

Any given engine can have various numbers of cylinders and arrangements of those cylinders, butcommon arrangements are V-6, V-8, and inline 4 The “cam-in-block” engine type is becoming lessprevalent than an overhead cam, since a cam-in-block engine requires stiffer springs and a higher load onthe springs Lighter loads on engine components tend to reduce both wear and energy consumption.Many diverse surfaces interact and have the potential to experience wear when converting the chemicalenergy of the fuel into the mechanical energy of the crankshaft These interactions are described below

1.1.2 Crankshaft to Crankshaft Bearing

As the crankshaft turns, sliding occurs between the crankshaft and the engine block structure as well asbetween the crankshaft and the connecting rod The load is transferred through journal bearings, whichare designed to run primarily under hydrodynamic conditions In this bearing interface, the engine oilacts mainly as a viscous fluid, and the friction in the bearings is directly related to the viscosity of theengine oil Since the proper clearance between the shaft and the bearing is important for good engineperformance, both the shaft and the bearings should be manufactured using strong and stiff materials, tominimize deformation

Vehicle engines, however, are often shut down for long periods of time A shaft will then settle intocontact with its bearing until the engine is started again Also, solid particles (such as residues frommanufacturing, contamination, wear, etc.) can be entrained in the engine oil, and these particles have

the potential to damage the shaft or bearing surface if the particles are larger than the minimum ance between the shaft and bearing Soft, compliant bearing surface materials minimize the sticking

clear-of the shaft to the bearing during shutdown, and such materials also can capture some small debrisparticles and remove them from circulation This property of a bearing material to capture debris is calledembedability [2]

To meet these contradictory requirements, crankshafts are usually made from a hard, stiff material such

as cast iron or steel The bearing is made using a steel backing (for strength and dimensional stability)and coated with a soft alloy (for embedability) For many years, lead-based alloys were used in crankshaftbearing applications However, legislation now forbids the use of lead in many applications, and the lowstrength of the lead alloys limits the output of engines Modern engine bearing coatings are usually madefrom aluminum-tin alloys, which are stronger but also have poorer embedability, so that engine and oilcleanliness become critical for long-term engine durability [2]

1.1.3 Piston Pin to Piston

The piston pin transfers force from the piston to the connecting rod The interface between the pin andthe piston is also a type of journal bearing, but the motion in this case is not full rotation In the fixedpin design, the pin is press-fit into the connecting rod, and the motion between the pin and the piston

is fully reversed partial rotation In the floating pin design, the pin is free to rotate within both the rodand the piston, and the motion is indeterminate The floating pin has been shown to reduce the operatingtemperature of the piston pin boss and is therefore the preferred design [3] In either case, the velocity

of the pin is not sufficient to generate a full fluid film between the surfaces, and a condition of boundarylubrication results

The tribological properties in the pin to pin–bore interface are primarily controlled by the materialproperties of these parts Automotive pistons are usually made from aluminum–silicon alloys The pistonpins are usually made from low or medium carbon steel, which is formed into a hollow cylinder and

is then carburized The carburization process results in very high hardness of the pin surface and helpsminimize adhesion between the pin and piston It has been demonstrated that increasing the oil supply

to this interface reduces the tendency for scuffing [3]

1.1.4 Piston Skirt to Cylinder Block

The piston skirt to cylinder block interface is one of the primary contributors to total engine friction [4].The design challenge in this case is to maintain a small clearance between the piston and the block in order

to avoid seizure, while minimizing noise and vibration [5] The aluminum–silicon alloys used for mostautomotive pistons are lighter than the cast iron pistons of the past, which therefore reduces engine massand vibration Also, the higher thermal conductivity of aluminum helps prevent overheating of the top

of the piston Sometimes, however, the piston requires additional cooling, which is usually provided byadding devices to direct a jet of oil onto the underside of the piston In this case, the engine oil is acting

as a coolant

The cylinder bore is usually made from gray cast iron, which has a lower coefficient of thermal sion than aluminum This creates a design challenge, since a piston with adequate clearance at runningtemperature may be too loose (and hence noisy) at low temperature To reduce friction and preventscuffing of the piston, oil must be supplied to the cylinder bore walls Nevertheless, the clearances are sotight that special coatings are applied to most pistons, such as nickel ceramic composites or molybdenumdisulfide [6,7] These coatings also reduce the friction in the interface of the piston rings and the pistonskirt with the cylinder walls

expan-1.1.5 Piston Rings to Cylinder Block

The piston rings function as a set of sliding seals that try to separate the combustion gases above thepiston from the crankcase environment below The most common arrangement is a set of three rings,

the upper compression ring, lower compression ring, and the oil control ring, as can be seen in Figure 1.1.The ring-block sliding interface has been estimated to account for 20% of the total engine mechanicalfriction [8].

Oil usually reaches the cylinder bore surface by being thrown from the crankshaft after flowing throughthe bearings Some oil is necessary for the compression rings to function properly, but the oil that escapespast the compression rings is lost The oil control ring ensures that only the necessary amount of oilreaches the compression rings

The upper compression ring experiences the highest loads and oil temperatures, and it must provide agood seal to the cylinder surface with very little engine oil To provide acceptable durability, this ring isusually made either from nitrided stainless steel or from steel coated with molybdenum

1.1.6 Camshaft to Cam Follower and Valve Train

As the camshaft rotates, it presses against a flat or roller surface, which reciprocates to open and close thevalves The interface between the camshaft and follower is unidirectional sliding between nonconformalsurfaces Although engines are designed to provide oil to this interface, it is likely that oil will be scarce attimes For example, when starting a cold engine, the cams will begin turning before pressure is sufficient

to pump oil to the top of the engine Only a few material combinations are used successfully in thisapplication, and even those wear sufficiently during the life of an engine to require periodic adjustment orthe use of self-adjusting hydraulic elements The severity of these various surface interactions is reduced

by the presence of engine oil

is an important means for conserving energy and preserving nonrenewable fuel sources, techniques forreducing friction will continue to become increasingly important

1.1.7 Oil Pump

To assure an adequate distribution of lubricant through the engine and sufficient flow to maintain dynamic conditions in engine bearings, automotive engines use a pressurized lubrication system The oilflows in a circuit beginning with the sump, from which the oil is drawn into a pump The pump thendelivers pressurized oil through a filter, then to passages in the block and head, to the crankshaft andcamshaft bearings, as well as to the hydraulic valve lifters in engines equipped thus The oil is then thrownfrom the rotating components onto the cylinder walls, valve lifters, and other components As the oil runsoff these surfaces, gravity directs it back to the sump through passages in the head and block

older design and have the advantage of relatively quiet operation However, the gerotor has the advantage

of greater efficiency and can be made to take up less space in the engine compartment, so that most recentdesigns use the gerotor The pump incorporates a relief valve for pressure regulation

1.1.8 Oil Filter

The oil filter is intended to remove potentially harmful particles from circulation The filter element isusually either a pleated paper or metal mesh Oil filters are rated by various tests, including industrystandard methods (e.g., SAE 1858) and proprietary tests The most commonly reported performancefigures are for particle removal and flow restriction It is desirable to have the highest level of particleremoval with the lowest flow restriction Since flow decreases as trapped material increases, filter systemsgenerally have a bypass circuit included which opens when the pressure ahead of the filter reaches apredetermined level This allows the oil to continue circulating if the filter becomes plugged, althoughcontaminants are then allowed to circulate through the engine

Rocker arm Valve keys

Valve spring retainer

Spur gear Gerotor

Suction Suction Output

FIGURE 1.4 The two main types of pumps in internal combustion engines.

FIGURE 1.5 Engine bearings and seals.

operating temperatures, service history and age of the vehicle, and extent of component wear [9,10]

transmission, and axles represents approximately 11% of the energy consumed by a light-duty vehicle such

as a gasoline-fueled passenger car Within this 11% portion of energy usage, the piston skirt and pistonrings contribute significantly to energy loss Cooling and exhaust also represent a significant fraction ofthe energy loss

The severity of surface interactions between the moving components in an engine is reduced, to a greater

or lesser extent, by the presence of engine oil The oil provides different functions in different regions

of the engine Lubrication conditions are often subdivided into boundary, mixed, and hydrodynamic

various engine components usually operate

Figure 1.7 indicates the relationship between the coefficient of friction (vertical axis) and a termconsisting of the oil’s viscosity at a given operating temperature, multiplied by the relative difference inspeed between the two surfaces, divided by the load that one surface exerts on the other The range overwhich various engine components operate is indicated by the horizontal arrows It should be noted thatthe vertical axis is drawn on a logarithmic scale, and the differences in friction would be greater if drawn

on a linear scale The low point on Figure 1.7 indicates the condition under which friction is a minimum

Research and development center

Piston skirt friction 25%

Bearings 22.5%

Valvetrain 6%

Axle & transmission friction 3%

Cooling

29%

Wheels 12%

Engine friction 7.5%

Air pumping 6%

Braking & coasting 7.5%

Accessories 4%

Exhaust

33%

Piston rings 19%

Crankshaft 5%

Axle & transmission 22.5%

Distribution of energy losses in a typical light-duty vehicle

FIGURE 1.6 Typical values for energy loss in a light-duty vehicle.

.001 01 0.1 1.

Engine bearings

Piston rings Piston skirt

FIGURE 1.7 Stribeck diagram, including the operating regions of several engine components.

(and thus fuel consumption will be minimized for a given vehicle) Engines do not operate at a constanttemperature, vehicles sometimes drive on rough roads, and various additional conditions influence vehicleoperation, so that Figure 1.7 represents a highly idealized assessment of friction effects

In the hydrodynamic region, the sliding surfaces are completely separated by an oil film, and friction

is essentially due to shearing of the fluid As the sliding speed and viscosity of the engine oil decrease andloads increase, the two opposing solid surfaces begin to interact Moving to the left on the Stribeck curve,the coefficient of friction rises sharply as the load is shared between the fluid and the solid surfaces in theregion identified as mixed lubrication At some sufficiently low value of viscosity and component speedand at a sufficiently high load, the contact zone moves into the domain of boundary lubrication [1,2]

1.3 Engine Oil

1.3.1 Characteristics of Engine Oil and Functions of Its Additives

The major component of engine oil is its base stock (i.e., the oil itself) Various additives are added

to the engine oil, each of which provides a highly specific mode of action to protect the engine andreduce the rate at which the engine oil degrades Zinc dialkyl-dithiophosphate (known as ZDP) is anessential additive in engine oil, and it has two functions: inhibition of oil oxidation and protection againstwear To protect the oil against oxidation, ZDP tends to react faster with oxygen than the rate of attack

by oxygen on the oil base stock In this way, the oil base stock and its other additives are less likely to beoxidized In addition, ZDP reacts with iron on an engine’s surface (particularly in a heavily loaded contact)

by laying down a phosphorus and sulfur coating that is resistant to wear The phosphorous in ZDP canpoison catalytic converters, which has contributed to a trend in recent years to reduce ZDP concentrations

in engine oil Thus, additional compounds that provide supplemental oxidation protection are generallyalso incorporated into an engine oil formulation

A detergent in the engine oil behaves somewhat like a soap, in that it reduces the tendency of partiallyoxidized oil to form tar-like deposits on a hot surface A dispersant helps keep degraded oil from coagu-lating, so that the coagulated oil will not be able to block narrow lubricant passageways A pour-pointdepressant allows the oil to flow at low temperature

1.3.2 Viscosity Effects

Appropriate engine oil viscosity is essential for satisfactory engine performance, but maintaining suitable

additive in the engine oil (a viscosity index improver, typically called a “VI” improver) that helps tominimize the adverse consequences of large temperature fluctuations A VI improver is a long-chainpolymer that is less soluble in cold oil but more soluble in warm oil When cold, the VI improver folds

in upon itself and offers less resistance to oil flow Thus, the VI improver facilitates cold starting of anengine When the oil is hot, the VI improver expands into a loose coil, so that the viscosity of the engineoil increases over what it would otherwise be at an elevated temperature This expansion and contractioneffect may diminish as the VI improver ages and is broken down by high-shear conditions, which arelikely to be experienced whenever engine oil passes through narrow, hot contact points such as in a heavilyloaded bearing or underneath the piston rings

The oil container will display a term such as SAE 5W-30, in which the “5W” signifies the oil viscositywhen the oil is cold; the “30” indicates the viscosity at normal operating temperatures Viscosity require-ments under various shear conditions for the different viscosity grades are established by the Society ofAutomotive Engineers (SAE) and are included in SAE J300, “Engine Oil Viscosity Classification.” The

Oil containers usually also display something relating to the performance capabilities of the oil The twomost common symbols indicating that engine oil satisfies a particular performance standard for gasolineengines in the United States are the API Certification Mark (starburst) and the API Service Symbol (donut)

1.3.3 Engine Oil Quality and Oil Degradation During Vehicle Use

A container of engine oil, such as one would buy in a store or at a service station, should have a symbolthat indicates whether the oil meets current standards (as indicated above) Unfortunately, some storesalso carry engine oils that do not have a current designation, and an uninformed purchaser is at risk ofbuying an inappropriate grade of engine oil In addition, overly degraded engine oil puts an engine at risk

TABLE 1.1 SAE Viscosity Grades for Engine Oilsa

Low-shear-rate kinematic viscosityd( −mm 2 /sec

at 100 ◦C, min)

Low-shear-rate kinematic viscosityd(mm2/sec

at 100 ◦C, max)

High-shear-rate kinematic viscositye(mPa sec

e ASTM D 4683, CEC L-36-A-90 (ASTM D 4741), or ASTM D 5481.

paper can be found in the Annual Book of ASTM Standards, available from ASTM International, WestConshohocken, PA.) Loss of antioxidant/antiwear protection as a consequence of exposure of engine oil

to the exceedingly high heat and pressure of combustion processes can be measured using ASTM D 5483.Remaining oil alkalinity (including remaining corrosion protection) can be measured using ASTM D 2896

or D 4739 (Note: the authors have observed that ASTM D 2896 is particularly useful when attempting tocompare the rates of engine oil degradation in a broad range of service types.) Accumulation of acids inthe engine oil (due to incomplete combustion of the fuel or oil oxidation in hot spots) can be measuredusing ASTM D 664 The extent to which wear debris or corrosion residues have entered the engine oil(e.g., measurements of iron, copper, lead, aluminum) can be determined via analysis of the chemicalelements in the oil This type of analysis is particularly beneficial when one uses a nontraditional fuel or

a new material in an engine application That is, whenever engines are modified to meet new conditions,

it becomes important to determine whether the engine oil degrades differently than was the case beforethe modification If there are different mechanisms of oil degradation, tactics will have to be developed tounderstand those mechanisms and to find techniques to reduce any adverse consequences

Engine operating conditions can be particularly harsh on the engine oil compared with conditionsexperienced by most other types of automotive lubricants An operating engine produces carbon dioxide,water vapor, nitrogen compounds, and partially burned oil or fuel, which then become pollutants Theeffects of pollutants can be minimized by exhaust after-treatment

In reality, service effects can be more complex than the above description The complexities arise fromdifferences between one type of engine and another, the type of service that the engine is experiencing at agiven instant, the nature of the fuel, the severity of the terrain, the weather conditions during use, and theextent to which the driver uses rapid accelerations Service effects can be roughly categorized under fourheadings: easy freeway, high-temperature high-load service, taxi service, and extreme short-trip service atlow outside temperatures [9,10] Even though one or another of these service conditions may predominatefor a given driver, in reality, most vehicles, at some point, are driven under each of these conditions

The sequence of events that engine oil experiences in freeway service is approximately as follows.

An adequate supply of oil is pulled up from the oil pan, filtered through an oil filter, and distributed tothose engine components that require lubrication Sufficient oil pressure needs to be available to permitbearings to ride on a fluid film Engine oil is moved along an engine cylinder bore by the motion of thepiston rings The conditions within the cylinder are extremely harsh, since the fuel typically explodesseveral times a second, creating extreme heat and high pressures, and the fuel sometimes creates corrosivechemicals whenever combustion is not complete The heat of combustion would be strong enough topartially oxidize the engine oil and form organic acids (i.e., oxidized hydrocarbons), except that theoil’s antioxidant (as long as it has not degraded) blocks oxidation and acid formation [11–14] Once

an antioxidant has degraded, the second line of defense against acid attack is the detergent, which is analkaline agent that neutralizes acids that form during exposure of the engine oil to heat in the presence

of oxygen Thus, the detergent reduces the rate at which polar reaction products accumulate on enginesurfaces, and therefore the detergent helps keep the engine’s surfaces clean When the detergent has becomeinactivated during long-term vehicle use without an oil change and without oil refreshing via addition ofmake-up oil, engine corrosion (from acid attack) becomes more likely, and excessive deposits may form

on engine surfaces Since the oil’s antioxidant is also its antiwear agent, once the antioxidant is largelydegraded, the wear rate in an engine can then accelerate These harmful effects are highly unlikely infreeway service, except under conditions in which the engine oil has not been changed for far greaterdistances than recommended in owner’s manuals or when a driver has installed an engine oil of inferiorquality

In many ways high-load service is similar to freeway service, except that oil temperatures and enginespeeds are higher, and thus the rate of engine oil aging will be faster In addition, oil viscosity is lower at hightemperature, which means weaker oil films are present in high-temperature contact points An increase inengine speed also means that a flame front will impinge on the cylinder bore more frequently than duringservice at slower engine speeds Thus, higher engine speeds promote higher oil temperatures, faster oildegradation, and increased stress on an oil film [15] These effects accelerate the wear rate, though use of

an oil cooler can reduce some of the adverse consequences of high-temperature service If the engine oilhas not been changed soon enough in high-load service, so that the oil’s protective additives have becomeoverly degraded and ineffective, various ill effects are likely to be observed For example, the lighter ends

of the engine oil will boil away faster than in freeway service, so that the engine oil’s viscosity increases.The antioxidant/antiwear agent in the oil will experience faster thermal degradation, which means that anincrease in the engine wear rate may occur sooner once the oil’s antiwear agent is no longer effective Oilthickening due to chemical interactions within the oil will increase If high-load service continues longenough without an oil change or if an inappropriate engine oil has been used (e.g., an engine oil thathas not passed the standard tests, but which is easily available to the public), the oil can become viscousenough that engine failure becomes a concern [15]

Chemical and physical effects of city driving (such as taxi service where the oil is completely warm butsevere accelerations may occur) differ from those found during freeway or high-load service For example,

if a taxi is attempting to move through a large, congested city during rush hour, the taxi’s engine will beidling when the vehicle is at a red light When the light turns green, the taxi will accelerate, but will moveslowly whenever traffic is again blocked A sudden acceleration tends to produce incomplete combustion

of the fuel and formation of both organic acids and additional harsh chemicals Periods at idle or at slowvehicle speed (so that the engine receives very little cooling from the wind) also promote formation ofaggressive chemicals that can condense in or be formed in the engine oil

The effects resulting from extreme short-trip service during cold weather (e.g., all trips lasting only

5 or 10 min with outside temperatures below freezing) can be particularly harsh [15] On start-up, theoil and the engine are cold Once the engine has started, fuel condenses in the engine oil Even if theweather warms or a longer trip is taken, so that the lighter ends of the fuel evaporate, the heavier ends ofthe fuel are likely to remain in the oil and cause the oil to have a lower-than-normal viscosity Partiallyburned fuel (including organic acids and other reactive chemicals) also condenses in the oil These fuel-derived agents can degrade protective oil additives, attack the oil’s base stock, and attack some engine

materials [11] For example, organic acids derived from the fuel begin to neutralize the detergent andcause the antioxidant to become less effective Acids can also modify an engine’s surface properties, so thatengine materials are more easily removed by mechanical action (i.e., rubbing) than would normally bethe case Thus, these corrosive wear conditions accelerate removal of metal from rubbing surfaces duringshort-trip winter driving Water-in-oil emulsions form a “white sludge” that also contains fuel, partiallyburned fuel, and oil additives If an engine has not been turned on for several weeks in winter (after havingbeen used in extended short-trip service), a layer of “white sludge” (containing measurable amounts ofpolar oil additives and water) may form and drop to the bottom of an oil pan.

This condition of water in oil occurred during a test in which vehicles that had been driven exclusively

on 3-km trips for 2 years (without having changed the engine oil) were left parked for 2 weeks in winter [12,13] By the end of the 2 weeks, water and polar oil additives had settled to the bottom of the oilpan and had frozen The oil uptake in the oil pan was totally blocked by the frozen water Thus, the engineoil did not flow when the engine was started The driver immediately turned off the engine when he noted

mid-a lmid-ack of oil pressure Wmid-armth mid-applied to the bottom of the oil pmid-an resolved the problem However, such

a scenario poses a risk of severe engine problems if a vehicle is driven very far under conditions in whichthe engine oil cannot flow up from the oil pan to lubricate the engine

1.3.4 Fluid Film Lubrication

The thickness of a fluid film, in many cases, plays a crucial role in the durability of an engine component.The film thickness and the oil film’s capacity to protect against wear, corrosion, and excessive friction arerelated to the viscosity of the bulk of the oil at the operating temperature, the oil temperature increase in

a heavily loaded contact, the roughness of each of the surfaces in the contact zone, the speed at which onesurface is moving relative to the adjacent surface, the presence or absence of any debris (such as honingburrs on the cylinder of a newly manufactured engine or dust in the engine oil), and the extent to whichthe lubricant has degraded The “fluid film ratio” is a useful measurement determined by comparing theoil-film thickness to a term representing the roughness of each of the mating surfaces If the oil-filmthickness is of the order of magnitude of the surface roughness, undesirable wear may result To avoid thispossibility, a designer can modify the surface roughness, increase the viscosity of the lubricant, changeengine design to supply more oil to the contact zone, reduce load, or upgrade the oil’s additive package(all of which may be difficult to modify if at the same time acceptable lubrication is still to be provided toother regions in the complex engine structure)

This assessment assumes that the lubricant wets the surface of interest If a surface material and itslubricant are not mutually attractive (i.e., the fluid beads up on a surface rather than spreads spontaneouslyover it), a mating surface has the potential to wipe away the lubricant rather than use the lubricant toform a lubricating film An accelerated wear rate then occurs The following simple test can determine thewetting characteristics of a surface and any fluid that may come into contact with that surface A drop oflubricant is placed on a level portion of the surface If the drop beads up, especially if the edge of the drop

or nearly so

An even more surprising effect related to a nonwetting surface can sometimes be identified A drop oflubricant is smeared to form a thin film over a clean sample of the surface of interest Next, the point of apin is lightly dragged a distance of a centimeter or more over the middle of the thin layer of fluid on thesurface If the fluid is not attracted to the surface, the fluid can recoil from the path over which the tip

of the pin has moved, so that a hole forms in the fluid film This nonwetting condition is not desirable

in a lubricated contact Such concerns must be addressed whenever one considers utilizing fuels otherthan hydrocarbons Simple compatibility tests can often be conducted in advance to determine whetheralternative fuels may influence the durability of engine components (seals in particular) or diminish theability of the engine oil to provide suitable lubricating films

These examples illustrate that it is not always easy to replace one automotive material or fluid with analternative substance Unexpected consequences may result

1.3.5 Future Concerns

Ongoing engine-related concerns include such issues as reduction in exhaust emissions, expense andeffort required in the development of standard tests for engine oils, and creativity needed to envision zero-

emission standards that designate the allowed values for nitrogen oxide emissions up to the year 2007.Limits on particulate emissions are also shown in Figure 1.8 The metric used in the vertical scale ofFigure 1.8 is grams per brake horsepower-hour (g-bhp/hr) Figure 1.9 describes lower nitrogen oxideemissions and better volumetric efficiency can be achieved by using a cooled exhaust gas recirculation

0 1 2 3 4 5 6 7

more heat

to coolant

Potential bearing wear

15%

cooled exhaust gas

(a)

(b)

Exhaust manifold Intake manifold

EGR recirculation duct

FIGURE 1.9 Lower nitrogen oxide emissions and better volumetric efficiency when using a cooled exhaust gas recirculation system: (a) exhaust gas recirculation system; (b) exhaust gas recirculation system wear control challenges.

5 5

8 12

15

0 4 8 12 16

CD CE CF-4 CG-4 CH-4 CI-4 API:

Number of tests to qualify products

Increasing oil quality

FIGURE 1.10 Improved engine protection when using exhaust gas recirculation in engines with CI-4 lubricants.

gas recirculation Figure 1.10 shows the increase in the number of required standard tests for certifyingperformance of engine oils in diesel engines Each additional test represents increased expense to thelubricant manufacturer

In addition to efforts to make engine oils more environmentally friendly, many automobile turers have now incorporated devices into their vehicles that indicate the point at which engine oil should

manufac-be changed In some cases, sensors of various types are used (e.g., acid, base, or temperature sensors, alongwith a computer assessment of the point at which a given reading has become excessive) In other cases, amicro processor determines the rate at which the engine oil has degraded, signal is given to the driver to

“change oil soon,” and at a slightly later date the driver will be provided with a “change oil now” warning.Such systems can both reduce the chance of harm to the engine and, for a driver who spends most of thetime on the freeway, greatly extend the point at which the engine oil needs to be changed

Examples presented in this section indicate the complexity of simultaneously attempting to reduceenergy consumption, reduce friction, prolong the life of the engine oil, minimize wear and corrosion in anengine, identify environmentally friendly power sources, reduce the amount of polluting substances thatenter the environment after lubricant disposal, but at the same time still retain personal mobility Insight,continuing effort, and creativity are required in these domains One can assume that efforts will continue

to be aimed at enhancing fuel economy, reducing pollution, and exploring various alternatives to gasolineand diesel fuel

1.4 Gasoline Engine Oil Performance Categories and Associated Test Methods

1.4.1 Introduction

In the early years of automobile use, engine oil had to be added or changed after an exceedingly briefinterval In addition, oils were not standardized Individual vehicle operators could be at the mercy oftheir intuition with regard to the purchase of an appropriate automotive lubricant for their cars Withtime, people began to realize that something had to be done to avoid the possibility of serious adverseconsequences if a driver had used the wrong kind of oil for an engine Thus, there was strong motivation

to look for chemical agents that both provided protection to an engine and promoted long life of theengine oil This trend (toward improving engine and oil durability and using additives in the oil toprovide specific beneficial attributes) is ongoing Promoting environmental acceptability has also become

an essential ingredient of responsible engine oil formulation Throughout these developments, it has beenessential to create and conduct appropriate engine oil test methods that ensure oils available to the publicproduce appropriate engine protection

Various classes of standard tests are available to confirm that current automotive engine oils provide thedesired protection, including long oil life, corrosion and wear protection, resistance to the formation of

sludge and deposits, ability to remain within an appropriate viscosity range, etc ILSAC (the InternationalLubricant Standardization and Approval Committee) and API (the American Petroleum Institute) aretwo organizations that play a major role in overseeing the availability of standard engine-oil-related testmethods The American Society for Testing and Materials (ASTM) is typical of the organizations thatpublish standard procedures to be used when conducting an automotive test Such standard tests areprepared in painstaking detail, so that there will essentially be no chance of conducting a standard testincorrectly if one has followed the written directions Automotive companies tend to be the developers ofsuch tests In general, different tests are used for gasoline-fueled engines than are used for diesel engines.Tests typically need to be updated periodically, for various reasons In some cases, test components, such

as a specific type of engine, may no longer be available Changes in the chemical nature of the fuel, such

as the transition from leaded to unleaded fuel, may mean that a former test is no longer pertinent tocurrent engine wear, corrosion, and sludge characteristics Future engine designs that differ from currenttest engines mean that standard tests will have to be created using the newer types of engines, since theolder engines may not be predictive of current performance If it is possible to legitimately substitute abench test for an engine test (such that the fundamental mechanisms of oil and engine damage correlatestrongly with the results from the bench test), the bench test becomes far less labor intensive and expensive

A brief overview of the evolution of standard engine oil test methods and the status of current automotiveengine oil test method development is provided in the following paragraphs

Early test methods for engine oils were far less sophisticated and less specialized than the tests of today

It can be anticipated that the tests of the future will be even more specialized Wherever possible, benchtests will be substituted for engine dynamometer tests, such as was the case in the development of the BallRust Test, a bench test that replaced the Sequence IID (i.e., Sequence 2D) engine test, which measured theability of an engine oil to protect against the kind of corrosive damage that can occur during extendedshort-trip winter service in which water and corrosive chemicals (derived from the partial combustion ofthe fuel) enter and remain in the engine oil for extended periods and cause engine corrosion

At the fundamental level, oil analyses can determine whether a given engine oil has all the requiredadditives in its formulation (and thus is not deficient, such as an “SA” quality oil would be) Such inform-ation can be pertinent to engine durability field problems, since most vehicle warranties are invalidated ifthe wrong grade of engine oil has been used in an engine

As of early 2004, the only two designations widely used to describe light-duty, gasoline engine oilperformance were API SL and ILSAC GF-3 Later in 2004, API SM and ILSAC GF-4 oils became available

in the marketplace The engine test and bench test performance requirements for API SL are similar tothose for ILSAC GF-3, but, in addition, ILSAC GF-3 oils must also meet energy conserving requirements.Similarly, API SM requirements as well as energy conserving requirements must be passed before an engineoil can be designated as ILSAC GF-4

The test methods for engine oils must be in accordance with the requirements outlined in the AmericanChemistry Council (ACC) Product Approval Code of Practice These requirements include registration

of all tests, use of only calibrated equipment and facilities, and guidelines for acceptable modificationsduring program development These requirements were implemented when the API SH and ILSAC GF-1designations for engine oil were adopted in 1993, and the requirements have been continued as newperformance categories have evolved

1.4.2 ILSAC GF-4 and API SM Standard Tests

In January 2004, ILSAC issued its latest Minimum Performance Standard for Engine Oils, ILSAC GF-4.Compared with GF-3 (the previous engine oil category), oils meeting GF-4 requirements provide improvedoxidation resistance, improved high-temperature deposit control, better cam and lifter wear discrimina-tion, improved low-temperature wear protection, and improved low-temperature used-oil pumpability.ILSAC GF-4 oils also have reduced phosphorus and sulfur contents to provide enhanced emissions systemprotection and to help vehicles meet the stringent Tier 2 Bin 5 emissions standards, which require,among other things, that vehicles emit no more than 0.07 g/miles (0.045 g/km) of nitrogen oxides over

120,000 miles (190,000 km) of driving GF-4 oils also provide improved fuel efficiency for both newand used oils, compared with GF-3 oils GF-4 oils began to be marketed during the second half of 2004,and all oils licensed to display the API Certification Mark (starburst) must meet GF-4 requirements

Although the performance limits in many of the engine and bench tests in ILSAC GF-4, as well as thechemical compositional requirements, were modified to achieve the benefits described previously, therewas only one new engine performance test developed for GF-4 (the Sequence IIIG Test, which replaced theSequence IIIF Test, ASTM D 6984) The IIIG Test utilizes the same General Motors 3800 Series II engineused in the IIIF Test, but the IIIG Test has different operating conditions and uses retrofitted valve trainmetallurgy The measured parameters in the IIIG Test include average cam plus lifter wear, end-of-testkinematic viscosity increase, and a composite assessment of piston deposits An end-of-test oil samplefrom the IIIG Test is also evaluated for its low-temperature engine oil pumpability characteristics (ASTM

D 4684) In addition, the test length was increased to 100 h (from 80 h in the IIIF Test), engine load wasincreased from 200 to 250 Nm, and sampling and additions of make-up oil were minimized to increase

(a decrease in test severity), because of concerns over abnormal depletion (degradation) of the engine

same alloy-cast-iron lifters used in the IIIF Test, but in the Sequence IIIG Test, the camshaft is phosphated(with a manganese phosphate coating) to minimize scuffing during break-in of the test engines Thus, theSequence IIIG Test addresses the issues that were of concern at the time of its inception As conditions andissues evolve, it can be anticipated that this test (and other test methods) will evolve to meet future needs.The Sequence IVA (i.e., 4A) Test mimics city service and determines whether the engine oil providessufficient wear protection to an overhead cam and slider followers The Sequence VIII (i.e., 8) Test measuresthe extent of shear of the viscosity index improver In addition, the Sequence VIII Test determines whetherthe engine oil provides sufficient protection to copper-lead bearings when using unleaded fuel Thepreviously available test (L-38) used leaded fuel, and thus the L-38 test is no longer appropriate forvehicles using the current unleaded fuels The Sequence VG Test (i.e., 5G) addresses some of the issuesrelated to partial replacement for the Sequence VE Test (ASTM D 5302) Sequence VG measures the sludgeand deposit control tendency of engine oils under engine conditions that simulate stop-and-go city service

to about 4000 to 5000 miles (6400 to 8000 km) of vehicle operation Different levels of fuel efficiencyimprovement are required, depending upon the SAE viscosity grade of the engine oil (the same groupings

of viscosity grade as were defined in the ILSAC GF-2 requirements for the Sequence VIA Test) SequenceVIB fuel efficiency requirements apply only to ILSAC GF-3 and GF-4 oils, not to API SL or SM oils.The Ball Rust Test is a bench test that mimics the effects of extreme short-trip winter driving It replaced

a previously used engine dynamometer test, and thereby saves considerable expense, time, and effort inthe testing process In the test, an engine component (ball) is immersed in a fluid that contains engine oil

to which has been added the kinds of corrosive chemicals that are generated from incomplete combustion

of the fuel when the oil and the engine are very cold (e.g., organic acids and other oxidized compounds)

At the end of the test the extent of rust formation is evaluated electronically

As can be seen, test methods for engine oils can be complex, time consuming to develop, and expensive

to run, and they need to be revised whenever engine designs have changed (e.g., as a consequence

of environmental issues, including modifications to fuels, lubricants, or engine materials) Thus, theupgrading of standard tests is an essential and ongoing effort to ensure that new materials, engine design,fuels, and government mandates related to vehicle operation are adequately addressed

1.5 Diesel Engine Oil Performance Categories and Associated Test Methods

1.5.1 Service Effects of Diesel Engines Related to Engine Oil Degradation

An overview of service effects on engine oils used in diesel engines includes the following Under freewaydriving conditions, in the absence of hilly terrain, a modern diesel engine operating under a light load willmove along the freeway with no (or very little) visible evidence of generation of soot The rate of engine oiloxidation is influenced primarily by the engine oil volume, oil temperature, engine characteristics, enginespeed, and load Thus, under light-duty freeway conditions, engine oil life will be a maximum for a givenengine design, oil sump volume, and engine oil temperature, as was the case for gasoline-fueled vehicles

In contrast, if the same diesel vehicle is heavily loaded and driving up a long, steep incline (such as can

be found in mountainous regions) or experiencing stop-and-go city driving with frequent stops followed

by heavy accelerations, the rate and extent of engine oil degradation will increase, and soot may formduring an acceleration The harder and hotter an engine works, the faster the engine oil’s antioxidantdegrades, the engine oil alkalinity decreases, and the engine oil acidity increases City driving produceslower vehicle speeds with frequent stops and starts, which generate the potential for both soot formationand an increased rate of engine oil degradation Thus, two major differences between service with gasolineand service with diesel fuel are the generation of soot in diesel engines and the fact that diesel engines inNorth America do not normally experience short-trip cold-start driving [16,17]

Mandates for the reduction of sulfur in diesel fuel, starting in 2006, should help promote longer engineoil life, since fewer acidic reaction products should be generated from the fuel

1.5.2 Examples of Test Methods for Diesel Engine Oils

Both the physical and chemical properties of diesel engine oil change during use Standard tests for engineoils used in diesel service focus on those conditions that may produce damage to the engine or the oil, ormay cause oil-related engine failure, as was also the case with gasoline fuel

The various active diesel engine oil performance categories include at least two (and typically morethan two) engine performance tests that must be conducted to demonstrate compliance with categoryrequirements Various bench tests are also included A description of the standard tests for diesel engineoils can be obtained from such organizations as the ASTM or the API

Examples of test methods that determine the physical and chemical properties of engine oils used indiesel applications include the following The volatility of the engine oil should not exceed 15% in theNoack Volatility Test (ASTM D 5800) Shear stability of the engine oil is measured using ASTM D 6278,and the limiting values for shear stability depend on the initial viscosity designation of the engine oil(e.g., a 15W-30 engine oil will be compared with 15W-30 a standard oil, and a 15W-40 engine oil will

be compared with a 15W-40 standard oil) The low-temperature pumpability of used diesel engine oil is

High-temperature, high-shear characteristics of the engine oil can be determined using ASTM D 4683.The ability of the engine oil to resist forming foam is determined by ASTM D 892 The oil’s capability tocontrol aeration (i.e., ability to allow bubbles in the oil to escape at a sufficiently fast rate) is confirmedwith the EOAT (Engine Oil Aeration Test) The extent of engine oil thickening due to the accumulation ofsoot in the engine oil can be measured by the Mack T-8E Test, ASTM D 5967 The engine oil’s capability

to inhibit corrosion of bearings is measured by ASTM D 6594, in which metals of interest include copper,tin, and lead.

Bench tests that are much simpler than standard engine tests can provide useful insights into the nature

of interactions between contaminated engine oil and wear of engine surfaces For example, bench weartests have documented the role that diesel soot can play in increasing the wear rate of engine materials [16].Since vehicle service modifies the properties of engine oil in a variety of ways, bench tests with fresh oil

do not necessarily provide useful information about the characteristics of used engine oil Measurements

of the changes in physical and chemical characteristics of diesel engine oil tend to be a reflection of boththe nature of the service as well as the characteristics of the engine Engine tests related to performance

of engine oils in diesel service look for stress or failure of the engine or its components, such as extent

of protection against wear in both sliding and rolling contacts, the extent to which corrosive wear occurs,and unacceptable changes in oil properties during severe service such as soot loading, acid formation,and viscosity increase Several brief descriptions of standard tests that have been used in currently activeAPI performance categories for diesel engine oils (i.e., API CG-4, CH-4, CI-4, CF, and CF-2) are asfollows

The Detroit Diesel 6V92TA Test lasts 100 h and measures oil volatility, wear, and protection to the engine.Various 8-h segments are interspersed with 3-h shutdowns The engine speed and load are specified forthe different segments of the test Scuffing of cylinder liners is noted, and pistons and rings are inspectedfor wear or other forms of damage Limiting values for various measurements are provided, to determinewhether the engine oil has exceeded its ability to adequately lubricate and protect the engine For example,the maximum allowed port plugging in a given cylinder is 5%, and the overall average port pluggingshould be no more than 2% Required engine oil analyses include a measurement of wear metals as well

as additive metals If the concentration of additive metals has increased, this provides a direct measure ofthe amount of oil volatility that has taken place That is, if a metallic element such as calcium, which ispart of the engine oil formulation, has increased in concentration by 5%, that means approximately 5%

of the oil’s base stock has evaporated

The Caterpillar 1M-PC Test is conducted using a Caterpillar 1Y73 single cylinder indirect injectionengine and measures scuffing of rings, pistons, and cylinder liner, piston deposit formation, and pistonring sticking The test is part of the requirements for API CF and API CF-2

The Cummins M11 EGR Test is part of the API CI-4 category and measures ring and overhead wear,extent of filter plugging, and sludge formation, as related to exhaust gas recirculation (i.e., EGR) Fifty-hour segments of the test are conducted overfueled (i.e., using more fuel than would be required duringthe service conditions of the test) at an engine speed of 1600 rpm and alternated with 50-h segments at

1800 rpm overfueled with retarded timing The total test duration is 300 h Engine measurements includering and overhead wear, filter plugging, sludge formation, weight loss of engine crossheads, and sludgeformation on the engine valve covers and in the oil pan Oil analysis measurements include viscosity,base number and acid number, concentration of additive elements in the engine oil, and accumulation

of wear metals in the engine oil For example, under high-temperature operation zinc concentration inthe engine oil (from ZDP) will increase when the more volatile engine oil hydrocarbons have evaporated.Under severe service conditions, once the engine oil’s additives are no longer effective, concentration ofwear metals such as iron will also increase

The Engine Oil Aeration Test uses a 1994 7.3 L V-8 engine The test is conducted at 215 brake horsepower

at 3000 rpm The amount of air in the engine oil is determined at 1, 5, and 20 h during the test Wearmetals are determined at the start of the test and at 20 h No more than 10% air is allowed for API CG-4oils Additional standard tests measure characteristics such as roller follower wear and the influence ofsoot in the engine oil on engine wear

The Roller Follower Wear Test (for categories API CG-4, CH-4, and CI-4) uses a General Motors 6.5 L,indirect injected diesel engine The engine speed during the test is 1000 rpm at near-maximum load.The test duration is 50 h, and make-up oil is added at 25 h Oil gallery and coolant-out temperatures

the test, roller followers are removed and the extent of wear is measured Oil analyses are conducted at

40 and 100◦C The alkalinity (total base number) of the engine oil is determined, as are wear metals and

7.6 µm for CH-4 and CI-4 engine oils Various additional tests for engine oils used in diesel service are

also available

The above discussion documents most of the test methods currently deemed necessary for ensuringacceptable engine oil performance in diesel engines Proposed future regulations regarding emissions

of nitrogen oxides and particulates (such as soot) for future diesel engines mean that current engines

modifications may include catalysts and particulate traps A very low sulfur concentration in the fuel will

the temperature of an engine and its engine oil Higher engine and engine oil temperatures mean greatersusceptibility to increased wear and a faster rate of engine oil degradation Faster oil degradation has thepotential to accelerate the reduction of the corrosion–inhibition capabilities of the engine oil In addition,engine oils that provide benefits for 2007 model engines may not necessarily provide the desired benefitsfor previous engines Thus, it becomes worthwhile to explore any areas in which uncertainties exist

1.5.3 A Model for the Rate of Engine Oil Degradation in Diesel Engines

To investigate the fundamental mechanisms of engine oil degradation in diesel engines, a number of testswere conducted with a vehicle having a medium-duty diesel engine, covering a full range of service con-ditions An extensive number of computerized measurements were taken, and oil samples were collectedand analyzed, including soot generation, remaining antioxidant in the engine oil, and oil acidity andalkalinity Then, when attempting to generate a mathematical model for the rate of engine oil degradation

in diesel engines, it became necessary to include a term for both soot generation and a term for the rate ofdegradation of the engine oil’s antioxidant, because these two measurements interacted with each other,according to the results of the testing that had been conducted Thus, to provide the desired statisticalcorrelation, the two terms had to be multiplied together, which strongly confirmed an interaction betweenthe effects of soot generation and engine oil degradation The results of this testing are described in U.S.Patent 6327900 [17]

1.6 Future Directions

1.6.1 Concerns Related to Conservation of Fuel

In several countries, Government regulations mandate improvements in fuel economy Vehicle weightreduction is one way to address this mandate If aluminum components are used in an engine or inany automotive wearing surface, it becomes important to identify a design such that aluminum oxide(which can form on an aluminum surface) is not rubbed off and allowed to enter the lubricant, becausealuminum oxide is abrasive and can act as a severe polishing agent, which will increase the wear rate.Lubricant additives that are optimal for use with an iron surface do not necessarily provide the same wearand scuffing protection for an aluminum surface Aluminum engine blocks with cast iron cylinder boresmust be designed such that differences in the rate and magnitude of thermal expansion of these metals

do not cause unacceptable gaps to open between contacting surfaces

1.6.2 Effects at the Molecular Level

When considering interactions between one material and another, or between materials and their ricants, conventional lubrication wisdom does not necessarily provide a complete understanding ofinteractions at the molecular level Investigators believe that it may be feasible to create a number of bene-ficial effects if nano-materials can be optimized for specific applications Desired applications include

lub-materials optimized for such attributes as friction reduction, optimum hardness, scuffing resistance,enhanced strength, and thermal stability.

1.6.3 Insights Gained from Tests with an Alternative Fuel

Hydrocarbon fuel supplies from fossil sources are finite To ensure future mobility, various alternatives tohydrocarbon fuels have been tested, such as solar power, batteries, hydrogen, alcohol fuel (such as methanol

or ethanol), and “flexible fuels,” which may contain up to 85% alcohol, but which also incorporate enoughhydrocarbon (at least 15%) to permit a cold start Suitable performance has been obtained in most cases.However, each type of alternative fuel may also have its unique disadvantages In addition, the transitionfrom one fuel to another often requires important modifications to any automotive materials that touch thefuel or its reaction products, in order to avoid incompatibility problems Thus, utilization of alternatives

to hydrocarbon fuel typically requires a significant development effort to ensure appropriate durability ofmaterials

A number of years ago, at a time of heightened interest in finding a substitute for gasoline, experimentsusing methanol-containing fuel were carried out by various car companies The fuel (termed M85)consisted of 85% methanol and 15% unleaded gasoline City and freeway driving tests were conducted

on a chassis dynamometer, so that each vehicle on test experienced exactly the same road and weatherconditions as the other test vehicles [18] A flexible-fuel vehicle that ran on gasoline was also part ofthe test, so that a quantitative assessment of the effects caused solely by differences in the fuel could beobtained The findings from that test were as follows

In freeway service there was no difference in the rate of engine oil degradation or engine damagebetween the methanol fuel and unleaded gasoline This suggested that under conditions in which thecombustion of the fuel is complete, the nature of the fuel was not a factor in the extent of engine oildegradation In city service, the engine oil used with gasoline degraded approximately 2.5 times fasterthan oil in the methanol-fueled vehicles that had been tested under identical conditions That is, methanolwas significantly milder to the oil than gasoline during city driving The reason for this difference in severitywas because the molecular weight of methanol is considerably lower than that of gasoline, so that theproducts of partial combustion of methanol boiled out of the oil and thus were not available to inactivatethe engine oil additives The investigators were surprised by this result, since at that time it was assumedthat alcohol effects on engine oil would be more aggressive than those of gasoline under all types of serviceconditions

In extremely cold short-trip winter service, methanol was harsher to the oil than was gasoline, sincecombustion of methanol produces approximately twice as much water per kilometer of service than doesgasoline Toward the end of short-trip testing with methanol fuel, the engines were being lubricated with

a mixture that contained less than 50% engine oil and slightly more than 50% contamination (water,fuel, and fuel reaction products) When gasoline fuel entered the engine oil during short-trip winterservice, the viscosity of the engine oil decreased When methanol fuel entered the engine oil, the methanolformed an emulsion, which caused the viscosity of the oil to increase Even though extreme short-tripdriving in a winter climate is not representative of most trips, it is desirable to learn about any potentialproblems before the vehicles are in the hands of the general public These results indicate that when using

a non-traditional fuel, an investigator must confirm that engine materials are not endangered

An additional concern that needs to be explored when using alternatives to gasoline is to make surethat engine and seal materials are compatible with the alternative fuel of interest For example, in studiesconducted by exposing polyacrylate, silicone, and nitrile seals to methanol fuel, it was found that methanolwas able to extract beneficial protective additives out of some seals, so that the susceptible seals mightbecome prone to hardening [19] To conduct such a seal test, a thin segment of the elastomer of interest

is immersed in the desired test fluid (here, M85 fuel) and allowed to remain in contact with the fluid at atemperature of interest for whatever duration the investigator deems important Analyses can determinewhether beneficial additives have been extracted out of the elastomer by the test fuel In addition, analternative fuel may enter a seal and soften it Thus, any seal that had become softened by an alternative

fuel would be at risk of accelerated wear or failure Even though some types of seals had become soft inthe short-trip study with methanol fuel, seal failure did not occur.

Solubility relationships between engine materials and engine fluids can usually be identified in books or tables incorporating titles such as“solubility parameters”or“cohesion parameters.” The solubilityparameters include three numerical terms One of the terms indicates the extent to which the substance

hand-of interest is soluble in hydrocarbons such as gasoline A second term indicates the extent to which thesubstance of interest is soluble in mildly polar materials such as a compound that has a chain of severalcarbon atoms linked to a polar atom such as chlorine at one end of the hydrocarbon chain A third termindicates the extent to which the molecule of interest is soluble in a hydrogen-bonding fluid such as water

If any engine material interacts adversely with a fuel of interest, either the engine material needs to bereplaced or the material needs to be coated or in some way protected Use of these solubility parameters todetermine material compatibility before attempting a vehicle test with an alternative fuel can sometimesallow an investigator to avoid an engine failure

If piston rings have a molybdenum fill (or another kind of potentially removable fill), it is worthwhile

to conduct a bench test to determine whether the fuel interacts with the fill material In the case studycited above, M85 fuel was capable of removing the molybdenum fill in piston rings, which caused therings to contact the cylinder bore on the sharp edges at the center of the rings The sharp edges of therings (where the molybdenum was removed) contacted and abraded the cylinder bore It is also important

to determine that bearings are not damaged in the presence of an alternative fuel A simple immersion

of the material of interest in the alternative fuel, followed by analysis of the elements that have enteredthe fuel by being extracted from the engine component, provides an assessment of compatibility Inaddition, the amount of fuel in an elastomer sample should be measured If analysis indicates a problem,

it becomes prudent to identify an alternative material composition for the component that exhibited theincompatibility

These driving tests with an alternative fuel provided results that were sometimes at odds with tional automotive wisdom Because of this divergence, the investigators were able to gain an enhancedunderstanding of several mechanisms of oil degradation and their relationship to engine wear and corro-sion for both gasoline and alcohol fuels In addition, these results highlighted the fact that it may be risky

conven-to make assumptions about durability of engine materials in both city and short-trip service, when using

an alternative fuel, since the alternative fuel may behave differently than hydrocarbons when in contactwith engine materials

The kinds of issues related to alternative fuels described above need to be investigated, understood,and resolved before alternative fuels and the engines in which they are used are placed in the publicdomain Useful information in this regard can sometimes be gained from bench tests (e.g., wear tests), butunanticipated interactions between the fuel and engine oil may occur under actual operating conditionsthat may be outside the domain of a simplified or single parameter bench test In addition, valuableinsights regarding fundamental causes of various engine effects may be derived when comparing testresults from different kinds of fuels

These tests indicate that, by paying attention to material compatibility and recognizing the needs thatare specific to a given fuel, alternatives to gasoline can become highly successful automotive power sources

If at some future date supplies of hydrocarbon fuels begin to dwindle significantly, many major automobilecompanies already have test results in their archives that will permit them to successfully utilize alternativefuels Ethanol fuel is currently in use in Brazil

1.6.4 Prolonging the Working Life of Engine Oil

A decade or two ago, many North American vehicles owner’s manuals indicated that a driver shouldchange engine oil at 5,000 km (3,000 miles) under most driving conditions other than freeway service,12,000 km (7,500 miles) was a recommended North American oil-change interval for freeway or longer-distance service, but possibly only 10 or 20% of the drivers drove under conditions that met the criteriafor the longer oil-change interval Oil-change intervals for vehicles that were developed to use “synthetic”

engine oil were approximately 2 times longer during freeway or autobahn service than was the case for anormal mineral oil.

Even though it was well known that oil quality and service type greatly influenced the rate of engineoil degradation, it has only been within the last two or three decades that automobile manufacturers havebegun to provide an in-vehicle warning to the driver that an oil change is needed Such a warning system isnow available on a significant fraction of current-model vehicles Some of these warning systems include

a sensor that may determine whether an engine oil has become excessively acidic or whether the engine

or oil has exceeded a reasonable value for some other parameter such as oil temperature or viscosity.Other systems incorporate a computer model that calculates the rate of engine oil degradation based onmeasurements that the vehicle manufacturer believes are important Examples include such values as thetemperature of the engine oil and the number of times the oil has been exposed to a combustion event

No matter which technique has been used as part of an oil-change indicator system, the end result is that,

in general, the oil-change interval indicated by the warning system often is longer than the values that hadpreviously been listed in an owner’s manual, since the presence of a sensor or a model eliminates much

of the uncertainty that a driver might have regarding the appropriate point at which to change engineoil It also reduces the probability that a driver may completely neglect to change the engine oil [15,17].Computer monitoring of all aspects of vehicle operation is likely to continue expanding in the future and

is expected to extend oil-drain intervals and prolong the availability of oil supplies

1.6.5 Minimizing Emissions and Pollutants and Ensuring Backward

Compatibility

Two important approaches toward minimizing automotive emissions are (1) improving the efficiency

of the engine so that fewer pollutants are produced during the combustion process and (2) reducingthe amount and type of chemicals in the fuel, engine, and engine oil that can adversely influence theeffectiveness of a catalytic converter [20] A tactic that is useful to improve the efficiency of an engine is toadjust engine parameters on an instantaneous basis during vehicle operation, so that combustion will beoptimized immediately and engine efficiency will be maximized [20] Another issue of great importancefor engine oils is the concept of backward compatibility Current engine oil formulations must provideadequate protection to older engines for which previous engine oil formulations were designed; otherwisethere is potential for generating engine problems and causing customer confusion

1.6.6 Future Investigations

Solar energy, batteries, and various types of alternative fuels have been developed to power vehicles, butonly a few alternatives to hydrocarbon-fueled vehicles have remained available to the public for morethan a relatively short interval of time Hybrid-electric vehicles represent a positive step toward reducingconsumption of hydrocarbon fuels, but such vehicles do not fall within the domain of “completelyrenewable.” Addressing mobility issues, including reliability and utilization of renewable resources (frommanufacture, to use, to environmentally acceptable recycling or disposal), will provide challenges to futuregenerations of lubrication engineers

For example, automotive lubricants in the future may have to differ measurably from those of today

to meet the demands of advanced vehicles The information needed to lubricate hybrid vehicles, fuelcell-powered vehicles, or whatever other type of automobile will be in use in the year 2050 or 2100will have to be gained experimentally Automotive engine oils will have to be formulated using baseoils and additives which do not cause deterioration of emission control system components, since it isimportant to move in the direction of reducing pollution as far as possible At the same time, customersare demanding more maintenance-free vehicles, so that fill-for-life lubricant systems will be a preferreddevelopment These desires will require novel approaches to lubricant formulation and revolutionary, asopposed to evolutionary, advances in additive chemistry Whereas we are currently pursuing low-SAPS(sulfated ash, phosphorus, sulfur) oils to enable current and near-term emission requirements to be met,

zero-SAPS oils will likely be required in the longer term Innovation in both the lubricant industry andthe automotive sector is desirable, such as designing lubricating systems that automatically sense theamount and condition of the lubricant, adjust fluid levels accordingly, replenish additives when necessary,and regenerate the oil by removing contaminants Such innovations represent significant challenges toindustries that have become accustomed to small, stepwise increases in performance over the years.

Acknowledgments

The authors thank James Spearot and Frank Caracciolo of General Motors Research and DevelopmentCenter, Robert Olree of General Motors Powertrain, Thomas Boschert of Afton Chemical Corporation,Larry Smith of Infineum, Ben Weber of Southwest Research Institute, Ewa Bardasz of Lubrizol, and GeorgeSchwartz of Electromechanical Associates for providing information relevant to the completion of thischapter

[5] H Kageyama, T Suzuki, and T Ochia, “Numerical Study on the Three Dimensional ContactPressure and Deformation of Piston Skirt,” SAE International Paper No 2001-08-0082, Warrendale,

PA (2001)

[6] K Funatani, K Kurowawa, P.A Fabiyi, and F.M Puz, “Improved Engine Performance Via Use

of Nickel Ceramic Composite Coatings (NCC Coat),” SAE International Paper No 940852,Warrendale, PA (1994)

[7] V.D.N Rao, D.M Kabat, D Yeager, and B Lizzote, “Engine Studies of Solid Film Lubricant CoatedPistons,” SAE International Paper No 970009, Warrendale, PA (1997)

[8] L.L Ting, “A Review of Present Information on Piston Ring Tribology,” SAE International Paper

No 852355, Warrendale, PA (1985)

[9] S.E Schwartz, S.C Tung, and Michael L McMillan, “Automotive Lubricants,” ASTM Manual 37 on

Fuels and Lubricants, chap 17, pp 465–495, West Conshohoken, PA (ASTM Headquarters) (2003).

[10] A Kapoor et al., Modern Tribology Handbook, Vol 2, Materials, Coatings, and Industrial Applications,

chap 32, pp 1187–1229, Boca Raton, FL: CRC Press (2003)

[11] S.E Schwartz and D.J Smolenski, “Development of an Automatic Engine Oil-Change IndicatorSystem,” SAE International Paper No 870403, Warrendale, PA (1987)

[12] S.E Schwartz, “A Model for the Loss of Oxidative Stability of Engine Oil during Long-Trip Service

Part 1 Theoretical Considerations,” STLE Tribology Transactions 35: 235–244 (1992).

[13] S.E Schwartz, “A Model for the Loss of Oxidative Stability of Engine Oil during Long-Trip Service

Part 2 Vehicle Measurements,” STLE Tribology Transactions 35: 307–244 (1992).

[14] P.J Younggren and S.E Schwartz, “The Effects of Trip Length and Oil Type (Synthetic VersusMineral Oil) on Engine Damage and Engine-Oil Degradation in a Driving Test of a Vehicle with a5.7L Engine,” SAE International Paper No 932838, Warrendale, PA (1993)

[15] D.J Smolenski and S.E Schwartz, “Automotive Engine Oil Condition Monitoring,” Lubrication

Engineering 50: 716–722 (1994).

[16] F.G Rounds, “Effect of Lubricant Additives on Pro-Wear Characteristics of Synthetic Diesel Soots,”

[19] S.E Schwartz, “Effects of Methanol, Water, and Engine Oil on Engine Lubrication System

Elastomers,” Lubrication Engineering 44: 201–205 (1986).

[20] N Canter, “Development of a Lean, Green Automobile,” Tribology and Lubrication Technology 60:

15–16 (2004)

Automatic Transmission Fluids

2.10 Specifications and Testing Requirements 2-10

2.11 Timeline of ATF Specifications 2-11

References 2-14

2.1 Introduction

Automatic transmission fluids (ATFs) are highly specialized lubricants designed to function in automatictransmissions found in mobile equipment exposed to extremes of ambient environmental temperatures.They function as heat transfer fluids, lubricants, hydrostatic hydraulic fluids, and power transfer fluids

To effectively perform these functions ATFs must have a carefully designed set of both physical propertiesand performance attributes

Viscosity is one of the key physical properties of ATFs The viscosity of a modern ATF, might be

low temperature fluidity of a modern ATF is also a critical design element and is generally under 20,000 cP

and EP/antiwear properties Other performance properties, also included in specifications, establish howcompatible the ATF is with seals, metal alloys, plastic components such as thrust washers and gears,electrical insulation, resistance to foaming, and resistance to viscosity shear down To provide all theseproperties there may be 10 to 20 different additive components required, making ATF one of the mostcomplex lubricants in the industry Additionally, ATF is generally formulated from a very high qualitymineral or synthetic hydrocarbon base fluid, and the additive content is generally between 10 and 20%.More recently, alternative types of automatic transmissions other than clutch-type step-shifttransmissions have been developed, which has resulted in several specialized types of fluids in the

2-1