MASTER'S THESIS
Tribological Performance of Diesel Engine Oils
Valon Gojani 2016
Master of Science in Engineering Technology Mechanical Engineering
Luleå University of Technology
Department of Engineering Sciences and Mathematics
VALON GOJANI
Tribological Performance of Diesel Engine Oils
Valon Gojani 2015
Master of Science (120 credits) Tribology
Luleå University of Technology
Department of Engineering Sciences and Mathematics
VALON GOJANI
Acknowledgments
I would like to thank God for helping me, giving me strength to continue my studies and work on my master thesis while I was going through most difficult time of my life due to a family tragedy.
I also want to thank my family, my friends and my Tribos program classmates Naveed, Tanmaya, Melkamu, Catur and Poorya for their unconditional help in my difficult times, and for providing me technical, professional and moral support.
I especially thank my supervisor and examiner Dr. Anders Petterson for his support, encouragement, inputs in research based on his industrial experience and his guidance regarding thesis writing.
I would like to thank Dr. Jens Hardell for his inputs on research methods and his very helpful advices, Professor Braham Prakash for his moral support, and Dr. Arash Golchin for his technical support and guidelines.
Last but not the least, I want to thank EM Tribos program coordinators Prof. Dr. Mitjan Kalin, Dr. Ardian Morina and Dr. Nazanin Emami for their help and encouragement through all this study program.
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Table of Contents
Acknowledgments ... 2
Abstract ... 4
1. Introduction ... 5
1.1. Internal Combustion Engines ... 5
1.2. Function of the lubricant in an internal combustion engine ... 5
1.3. Anti wear additives ... 8
1.4. Lubricant performance specification ... 10
2. Theory ... 13
2.1. Tribological and rheological parameters ... 13
2.2. Lubricants ... 17
2.2.1. Base oil ... 18
2.2.2. Additives ... 19
3. Research gaps ... 22
4. Objectives ... 23
5. Methodology ... 24
5.1. Test procedure and setup ... 24
5.2. Description of oils ... 27
6. Results and discussion ... 28
6.1 Viscosity ... 28
6.2. Friction characteristics ... 29
6.2.1. Static coefficient of friction ... 29
6.2.2. Dynamic coefficient of friction ... 30
6.3. Wear volume ... 32
6.4. OIL 10 Results ... 33
7. Conclusions ... 37
8. Future work ... 38
9. References: ... 39
Appendix ... 41
VALON GOJANI
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Abstract
In a tribological study which involves the contact between the two surfaces in relative motion, a lubricating oil plays a vital role in reducing friction and helps in protecting materials from wear. If the lubricating oil should meet these requirements, it is important that it has all required quality and reliability aspects which in many cases are overlooked in many available markets oils.
In this research work, tribological studies of various industrial diesel engine oils are performed to validate the stated quality and reliability aspects. These engine oils are selected randomly from the market and their comparison is drawn with the VDS approved Volvo oils which are used as reference oils. Tribological studies are performed at various temperatures using a ball and disc test setup configuration and tests are performed under reciprocating movement between the specimens. The tribological studies also analyze and validate the capability of the test rig to evaluate the quality of oils.
Results from the experiments show that all tested oils do not fulfill the claims of manufacturers and in some cases the oils do not qualify the standard requirements specially when it comes to viscosity. This research concludes that all available market oils do not fulfill the claims of manufacturer and also do not qualify the standard requirements. The experiment results concluded based on this test rig are sufficient enough to differentiate between a good and a bad oil.
1. INTRODUCTION
5
1. Introduction
1.1. Internal Combustion Engines
An engine is the machine that converts any energy into mechanical energy. It can convert different energies, but the most common is the heat, which then creates force. These engines are almost all the time combustion engines, either with internal or external combustion.
Internal combustion engines (ICE) are engines where the combustion of the fuel occurs with an oxidizer, in a combustion chamber, that is part of working fluid flow circuit. In ICE, the force comes from the expansion of high-pressure and high-temperature gases, produced by combustion. This force is applied to component of the engines, typically to pistons, turbine blades or a nozzle. This chemical energy is transformed into mechanical energy by the movement of a component over a distance.[1]
Both diesel and gasoline engines convert the energy of the fuel into mechanical energy by small explosion or combustion. The difference between these engines is the principle of combustion. In the gasoline engine, fuel and air mixture is compressed first, and then a spark ignites the combustion, while in the diesel engine the air is compressed initially to high pressure and temperature, and then the fuel is injected in to the hot compressed air which causes the ignition.
[2]
There is a relative movement between various components of ICE engine, which results in friction and wear. An engine oil is used to reduce these energy loses (friction and wear).
1.2. Function of the lubricant in an internal combustion engine
[3] Engine oils have many functions in the engines, besides tribological tasks. The oils contribute to the sealing of the cylinder, transport particles (the sludge, soot and other particles) to
1. INTRODUCTION
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the oil filter, etc. Some of the points inside the engine where lubricant influences the work are shown in Fig 1.1.
There are different movement, different lubrication regimes, different speeds, wide range of temperatures, micron tolerances in between contacting surfaces, and other parameters that have to be covered from lubricant from the tribological aspect.
An engine oil should reduce the friction and wear, even in extreme conditions, e.g. -400C and up to more than 1000C, high pressure, etc. It should flow in very low temperatures (in cold start up) to avoid metal to metal contact, and it should maintain the film thickness in very high temperatures, also resist the mechanical and thermal ageing.
Besides this, during the combustion process, the engine oil helps in sealing the cylinder and piston, while it should burn off the cylinder wall without leaving any dirt, it should cool the piston, and it has to halt the gases formed from the combustion, transport the dirt and ensure the filterability. Additionally, in case of water formation, it should protect the engine components from corrosion.
Modern high performance diesel engines are usually utilizing technologies like turbo charging, intercooler, high-pressure direct injection and extensive use of electronics. This has significantly improved the combustion and thereby reduced exhaust emission and improved the cost for operating the engine.
One of the requirements for diesel engine oils is extension of the oil changing interval.
For the commercial vehicles sector, the criteria are reliability and long-life. The main requirements are the dispersion of large concentration of soot particles and neutralization of sulfuric acid combustion by-products.
1. INTRODUCTION
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Fig. 1.1.Scheme of lubrication points in engine.
Due to continuous increase of requirements in specified oil performance, the manufacturers have increased the performance of the oils. This led to more use of semi-synthetic and synthetic formulations, reducing the use of conventional mineral oils. Prices of oils have increased, but also economic and ecological demands. The oils have to provide:
- longer life in spite of higher thermal and mechanical loads - reduced exhaust emissions by improving fuel efficiency - lower oil-related particulate emissions
- improved wear protection even in severe conditions.
To be competitive, modern engine oils have to offer both protection of engine components and contribute to reduced fuel efficiency. During the last 50 years, the combined effect of better oils and better engine design reduced the specific oil consumption (oil per unit of energy produced) eighth times. [8]
1. INTRODUCTION
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1.3. Anti wear additives
There are many publications pertaining to tribological performance of lubricants and one of the most important aspects is the additives used in the lubricant and their quantity. Depending on the base oil and the additives used, the lubricants differ in terms of the tribological performance.
[4]
One of the most important additives for protecting the interacting surfaces are antiwear additives in general, zinc dialkyl dithiophosphate (ZDDP) and, dispersants and detergents in particular (the latter are used as antiwear lately). Also soot particles will have a significant effect on friction and wear, as they are present in the oil used for diesel engines. [4]
ZDDPs have been used in internal combustion engine oils for over 30 years as antiwear and antioxidants additives. In the absence of ZDDP type antiwear additives, the highly stressed components can fail prematurely due to catastrophic wear. These components work usually in the 1m/s speed and 1GPa pressure region, where simpler AW additives cannot function. During 30 years, there has been research undertaken to understand the wear mechanisms of ZDDP.
Fig. 1.2.Chemical structure of zddp
ZDDPs decompose in oil predominantly by a hydrolytic mechanism, ultimately to zinc polyphosphate and mixture of alkyl sulphides. These two products are the precursors of the antiwear process of ZDDP.[4]
ZDDPs are used mainly for their antioxidant properties and their ability to prevent wear.
ZDDPs has shown to have a subsequent effect on friction, it can increase the friction in case of mixed lubrication regime. Primary ZDDP and secondary ZDDP have been investigated. For higher temperatures, the wear volume was higher.[5]
In mixed and boundary lubrication regimes, molybdenum dithiocarbamate (MoDTC) additives have resulted in a very low friction coefficient. MoDTC are most effective in reducing friction in high concentration and high temperature, but over time, the MoDTC effectiveness reduced and as a result the coefficient of friction (CoF) increased in the linear sliding, but in
1. INTRODUCTION
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reciprocating tests this is not the case. A substantial number of studies have focused on interactions between ZDDPs and MoDTC, and the results showed that MoDTC effect was higher when used together with ZDDP. [6]
Lower percentage of MoDTC reduces the friction but not as much as higher percentage of MoDTC, while there was not much difference in wear volume between high and low concentration of MoDTC. With the increase of MoDTC the ZDDP elements are reduced, which results in increase of wear. [6]
The effect of temperature is clearly seen in CoF and wear coefficient. In most of the oils, with the increase of temperature, the CoF reduced while the wear increased. Only a few oils have shown the opposite and this is attributed to the decrease of viscosity, while the opposite, can be attributed to use of ZDDP alone where the formation rate of phosphates increase and no longer polyphosphates are formed with the increase of temp to 100°C.[6]
The presence of ZDDP has reduced the wear, but with the use of MoDTC, the wear is increased compare to lubricant with ZDDP alone.[6]
Most particles matter produced by combustion process in a diesel engine is expelled with exhaust gases, but some goes to the oil. This changes the oil viscosity, and percentage in weight.
This affects the oil over time by modifying the performance of the oil and limit the engine service interval. Soot increases the oil thickness and increases viscosity and fuel economy and CO2
emissions. It also reduces the anti-wear effect and leads to more wear. [7]
A severe increase in oil viscosity is observed even at low soot levels, but this is not clearly understood.[17]
1. INTRODUCTION
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Table 1.1. Viscosity of the oil samples at 40 and 900C at the different soot levels Oil ID Temp. (°C) 0% Soot 2% Soot 4% Soot
WVU397 40 69.67 81.07 151.52
WVU398 40 81.79 95.45 182.95
WVU399 40 73.84 85.61 177.84
WVU400 40 89.18 96.2 202.27
WVU401 40 77.25 90.91 185.61
WVU402 40 89.74 108.34 202.27
WVU403 40 58.31 83.84 126.13
WVU404 40 90.12 101.52 200.74
WVU397 90 14.77 17.42 24.05
WVU398 90 16.67 20.08 26.89
WVU399 90 14.58 17.61 23.86
WVU400 90 17.05 17.05 18.37
WVU401 90 14.01 15.15 19.7
WVU402 90 16.48 20.08 21.78
WVU403 90 13.83 16.86 19.32
WVU404 90 16.86 18.37 20.26
Source: Effect of diesel soot on lubricant oil viscosity – S.George, S.Balla, V.Gautam, M.Gautam Another investigation on a Mack engine showed that in some cases viscosity was increased with increase of soot mass, in some is decreased with increase of soot mass. Viscosity correlates directly with surface area of soot-in-oil. Experimental studies and mathematical approaches have shown that increase in viscosity is depended in volume fraction occupied by soot. Effect of soot in lubricant varies with respect to oil temperature. These compounds degrade the oil. [7]
1.4. Lubricant performance specification
Worldwide, there are 1380 lubricant manufacturers, both small and big. About 180 are national and multinational oil companies, and 1200 of them are independent lubricant companies manufacturing lubricants. [8]
There are many standards worldwide, which specify the characteristics and performance that an oil has to fulfill to get in to the market, and the supplier have to produce the oil that fulfills
1. INTRODUCTION
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the standard requirements. But not all the oils are controlled before they get in to market, therefore their claims are not always proven there.
Knowing all these information, there are questions that arises:
1. What is the quality of the oils in the market?
2. Is the oil performance always as written in the description paper?
3. Can the engine users rely on the claims from the suppliers?
In the Figure 1.3 and Figure 1.4, a Volvo engine is shown after using an oil bought in market in a third world country, and the oil description showed that oil will fulfill the requirements of the engine. The oil has worked for a few hours, and after that it destroyed the whole engine, and as it can be seen, the oil has turned into a polymer and it is hard to remove it.
Fig.1.3. Consequences of low quality engine oil.
1. INTRODUCTION
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Fig1.4. Consequences of low quality engine oil.
The oils in market are obviously not always good enough to perform its functions. The paper description might be false claim from the supplier, and therefore, the engine user should not rely on that, else, it may destroy the whole engine.
2. THEORY
2. Theory
A lubricant has many functions in a tribological system, but most important are reduction of friction and wear, and there are cases when relative movement between two surfaces is impossible without a lubricant. Saving energy and resources and reducing emissions are other things achieved by use of a lubricant, thus, the lubricant is becoming more important and is playing a big role in industry of today.[9]
Lubricants on average consist of 93% of base oil and 7% additives and other components.[9]
Lubricants also contribute in pollution environment to certain extent. As a waste, use of heavy metals used in oils are quite hazardous to environment (eg. Pb). Diesel engine particulate emissions is a big polluter, about one third of it is caused by engine oil evaporation. Any effort in reducing the lubricant consumption or increasing the lubricant quality will help in reducing the environmental impact. [10]
2.1. Tribological and rheological parameters
Whenever there is a relative movement between two or more bodies in a system, there will be friction, which can generate wear, heat, etc. Such a system, is called a tribosystem.
The variables in the tribosystem are: the materials of the contacting surfaces, the interfacial medium (e.g. lubricant) and the surrounding atmosphere. In addition to this the type of movement, forces, temperature, speed and duration of stress will also affect the tribological response.
Tribological processes occur in the contact area between two interacting surfaces and these can be physical and chemical or physical-chemical. [11]
Friction is the mechanical force that resists the relative movement between sliding or rolling surfaces. It may be dynamic or kinematic friction (resisting movement) or static friction (hindering movement). The causes of friction are the microscopic contact points of two surfaces in contact.
The energy lost from friction is converted to heat or mechanical vibrations. [12]
2. THEORY
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Wear can be defined as progressive loss of material from a surface due to mechanical or chemical processes during contact with a body. Wear mechanisms are abrasion, adhesion, erosion, tribochemical reactions, fatigue, etc.
Abrasive wear [19] develop when two surfaces in contact are not of the same hardness, so the softer surface will be worn from a harder surface while in relative movement. Abrasive wear can be 2-bodies abrasive wear or 3-bodies abrasive wear. In the first case, there are only 2 surfaces in contact, in the other case, there are also particles included in the system, and in case of hard particles, they will wear off the soft surface.
Three main mechanisms of abrasive wear are: Plowing, Cutting and Fragmentation Adhesion wear refers to undesirable movement of particles or debris from one surface o another while these two surfaces are in frictional contact.
Erosive wear is generated by the contact of solid or liquid particles against the surface.
This is a very short slide motion of particles in the surface, and progressively while repeating they remove the material from the surface. [20]
Tribochemical reaction can be corrosion and oxidation wear which happens due to chemical reactions between the surface and the environment.
Fatigue is the weakening of material while loads are applied repeatedly. This may lead to a local damage.
Wear can lead to the failure of the components. It can be measured in different ways, two most used methods are measuring the weight loss and also examination of surface profile of sample. [13]
Viscosity is the ability of a liquid to resist movement and it may also be explained as internal friction in the fluid. When a fluid is poured in a tube, the particles near the wall will move slower than particles in the center axis of the tube. The higher the viscosity of the fluid, the slower is the movement of it in the tube. Viscosity may be shown as dynamic or kinematic. [12]
Dynamic viscosity can be explained by model of parallel fluid layers moving with different speeds.
Shear rate S, is the difference in velocity between two given fluid layers as shown in Figure 2.1.
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Fig.2.1 Shear stress rate
Kinematic viscosity is the ratio of dynamic viscosity and density of the fluid.[12]
With the rise of temperature in the system, viscosity of the engine oil drops as a result.
Figure 2.2 describes the behavior of kinematic viscosity of different oils (a, b and c) with respect to temperature.
When the temperature increases in the system the viscosity of the oil drops significantly.
In Figure 2.2 (a, b and c) is shown the kinematic viscosity of different oils as a function of temperature.
In Figure 2.2(A) there is a set of three oil samples from group 1 which have shown a decrease in (kinematic) viscosity while the temperature is increasing. The curve is exponentially decreasing.
While in Figure2.2(B) there is also plotted a set of three other oil samples from group 2, which are linear, and they also decrease in viscosity while the temperature increases.
Sample c in this case showed lowest sensitivity to temperature changes; as it decreases less when the temperature increases compared to samples a and b which have changed more rapidly with the change of temperature.
2. THEORY
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Fig. 2.2 (A) Showing the exponential change in kinematic viscosity with respect to temperature.
(B) Showing almost linear change in kinematic viscosity with respect to temperature.
Viscosity index is a measure of the effect of temperature on viscosity. The higher the viscosity index (VI) the lower is the change in viscosity. When this was introduced, the greatest was VI=100 and the lowest VI=0, but later there are liquids with higher VI than 100, and these are calculated as VIE. [14]
2. THEORY
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Fig. 2.3 Plot for the viscosity index.
Figure 2.3 shows the behavior of oils with different VI number and how the VI is calculated. [14]
Viscosity grades helps to classify the lubricants according to their application. ISO viscosity grades apply for industrial lubricants, while SAE apply for automotive engine lubricants and gear oils.
Oils fitting only in one viscosity grade are known as monograde oils, and these are the oils without viscosity index improvers, while oils that cover two or more grades, are known as multigrade oils, and these contain VI improvers, or the base oils with high natural VI.
2.2. Lubricants
In the global lubricant market, more than 60% of lubricants are engine oils, and in the foreseeable future, this will increase in the third world countries.[3]
In engine oils, the base oil plays an important role compared to other oils, so in engine oils the base oil is selected according the required performance and necessary viscosity.
2. THEORY
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Depending on the base oil used and requirements of the engine, the lubricant may contain different additives; up to 30 different and percentage vary from 5 to 25%. The largest percentage of additives is performance additives (antioxidants, anti-wear agents, detergents, dispersants, friction modifiers, etc.). [3]
2.2.1. Base oil
Base oil, depending on its production, can be mineral, synthetic or bio. Mineral oil is the oil refined directly from the crude oil, synthetic oil is produced through chemical synthesis and biodegradable oil is found in nature (vegetables, flowers, etc.). Base properties e.g. viscosity, comes from the base oil. Other properties are improved by additives. Base oils for lubricants are categorized by API group, according to their level of saturation, Sulphur and viscosity index, VI, see table 2.1.
Group I base oils, who is the most common for simple mineral based lubricants are normally produced by solvent refining and de-vaxig of crude oil distillates.
Due to low yield and performance, no new group I plants are build and global capacity are decreasing.
Group II to III are normally processed by hydro treating and wax isomerization of crude oil distillates. If process is run more severe, group III oils is obtained.
Group III oils of high quality can also be processed by Gas to Liquid processes, GTL. Those oil of superior quality and are often referred to as group III+ base oils.
Group IV are Poly Alpha olefins, PAO. They are processed by polymerization of 1-Decene in to larger molecules.
Group V consist of all other type of base fluids such as synthetic and natural esters, Poly-glycols and others.[15]
2. THEORY
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Table. 2.1. Groups of base oils
API BASE OIL CATEGORIES
Base Oil Category Sulfur (%) Saturates (%) Viscosity Index Mineral Group I (solvent refined) >0.3 <90 80 to 120
Group II (hydrotreated) <0.3 >90 80 to 120 Group III (hydrocracked) <0.3 >90 >120
Synthetic Group IV PAO Synthetic Lubricants
Group V All other base oils not included in Groups I, II, III, or IV
Source: http://www.machinerylubrication.com/Read/29113/base-oil-groups
2.2.2. Additives
[16] Base oil is not sufficient enough to satisfy all the needs of high performance lubricant, so additives are used to improve lubricant performances. Additives are chemical substances that are used to improve different parameters of lubricants. Additives may be classified in different ways, but considering how they affect the tribological system, they can be classified in types:
That influences the physical and chemical properties of base oil
Affect primarily the metal surfaces modifying their physicochemical properties.
Additives may be in different sizes, and are used for different purposes. The additives can assist each other but there are also cases where they hamper each other. Some additives can also be multifunctional products, and this reduces the possibility of additives to interfere each other.
The influence of additives in lubricant performance is very big, by improving the lubricant to fulfill the requirements for it. Not every property of the lubricant can be influenced by additives, but a large number of properties are influence by different additives. The most common additives are:
Antioxidants, Viscosity modifiers, Pourpoint depressants, Detergents and dispersants, Antifoam agents, Demulsifiers and Emulsifiers, Dyes, Antiwear additives, Extreme pressure additives, Friction Modifiers, Corrosion inhibitors.
Viscosity Modifiers are used to make the oil useful in both low and high temperatures. As the viscosity changes with the change of temperature, this can lead to a basic problem of the oil.
If very thick (during cold) it may stop flowing, and if very thin (during hot) it may collapse. So these additives help in reducing the change of viscosity on temperature variation.
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Mode of action: Viscosity modifiers are polymeric molecules, and these molecules are very sensitive to temperature. In the cold, the chain shrinks and so they do not affect the viscosity of the oil, and in hot temperature, the chain relaxes and this increases the viscosity. [21]
Antioxidants are used to reduce the oxidation process of the lubricant. All lubricants undergo oxidation in their working environment. Increasing temperature increase the rate of oxidation. The presence of oxidation catalysis such as metal ions, oxygen, NOX increases the oxidation rate. The antioxidants used in lubricants, decrease this rate, therefore the degradation of lubricant is decreased.
Mode of action: While organic peroxides help the oxidation of lubricant, an effective antioxidant is a chemical compound that destroys organic peroxides. A list of organic compounds includes phosphates, sulfides, amines, disulfides, phenols, selenides, etc.
Friction Modifiers are used to reduce friction in more severe contact areas. There are two groups of friction modifiers that work in different ways to reduce friction: organic FM, and organo- molybdenum FM. The organo-molybdenum FM do not work efficiently under light loads or at low temperatures. Friction modifiers are able to extend the working range of lubricants in terms of contact pressure, but only to a certain level, above which they become ineffective.
Mode of action: Friction modifiers are usually esters, natural and artificial fatty acids and other materials, molecules of which have a polar head and an oil-soluble tail. When used, the head is attached in the metal surface, while the tail stands in the opposite direction, and protect the metal from friction from the other surface (in contact) as long as contact is not very heavy (in which these molecules will brake).
Antiwear agents are used to reduce wear. These are typically Phosphor containing chemicals, of which the most used are zinc dialkyldithiophosphates (ZDDPs), organic phosphates and phosphites. These antiwear agents may increase the friction. Antiwear agents work by reacting with the metal surface under severe conditions, to form a protective layer. This is antiwear layer is believed to be formed with the degradation of ZDDPs.
Mode of action: ZDDP decompose under high temperatures and high pressure. This decomposition results in removable alkyl groups and sulfur atoms, and the long linear ZP molecules composed of Zn, P and O. These products coming from decomposition create a film in the metal surface which is a protection layer.
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Extreme pressure agents are similar with the Antiwear agents. They both prevent or reduce the wear, but under very high pressure, where antiwear agents do not work, extreme pressure agents are needed.
Mode of action: EP additives typically contain organic sulfur, phosphorus or chlorine compounds.
Under high pressure conditions, metal surface reacts chemically with these compounds.
Dispersants are used to remove the internally generated contaminants. Engine produces small particles of highly carbonaceous due to incomplete burning of the fuel.
Soot, together with fuel, water and acids that are precursors to sludge formation, can reach the crankcase as blow gases. These compounds can block the oil-ways, which can lead to oil starvation and equipment failure. The introduction of dispersants in the oil, help to handle this problem.
Detergents are used for different functions in the lubricants depending on the specific application. 1. Suspend oil-insoluble oxidation, 2. Neutralize inorganic acids, 3. Reduce corrosion, 4. Reduce deposit formation in high-temperatures, 5. Lately they are used also as antiwear additives, 6. Etc.
Rust and corrosion inhibitors are used to protect ferrous materials from corrosion.
Mode of action: Corrosion inhibitors operate by reacting chemically with the nonferrous metal components to form a protective film, which is corrosion resistant.
2. THEORY
3. Research gaps
The available literature regarding oils and their performances is based on standardized major providers such as Shell, Valvoline, Chevron, etc. but in the market we can find other providers who are not well known, and their products are not tested and explained.
Even their claims are such that the quality of their oils is similar to big companies’ oils, but there is no such literature to support those statements.
Other practical fact not found in literature is the fact of false providers, usually in third world countries, selling oils in so called “black market”.
4. OBJECTIVES
4. Objectives
The main objectives of this research project are to:
1. Compare the tribological and rheological performance of 10 commercially available engine oils using a VDS approved oil as reference.
2. Evaluate the reliability of the lubricants in the market
3. Evaluate the performances of the additive packages using tribological tests
4. Evaluate the effectiveness of a standard tribological test rig to screen the performance of engines oils.
6. RESULTS AND DISCUSSION
5. Methodology
A series of tests were carried out to determine the rheological and tribological performance of 10 commercially available engine oils using AISI 52100 high grade bearing steel specimens. A linear reciprocating test rig “Cameron Plint” was used for testing the oils. During the test, the coefficient of friction is measured, and after testing, the wear volume is measured by using a 3D optical surface profiler “Zygo NewView 7300”, and also the material surfaces were investigated using scanning electron microscope (SEM).
Testing was carried out using 10 commercially available heavy duty diesel engine oils, manufactured by different suppliers. Three of these oils are manufactured by Volvo, nine out of ten are Volvo approved oils, while one is bought from an unknown supplier. The oils contain different base oils and different additive packages.
Materials used as specimens for both surfaces are AISI 52100 high grade bearing steel as it provides stable and reliable results. The average surface roughness of the specimens were Ra=0.015μm.
5.1. Test procedure and setup
Dynamic viscosity is measured in Bohlins CVO rheometer for all oils at 40°C and 100°C.
The test rig can be used to concurrently measure shear stress and shear rate of a liquid sample. The ratio between shear stress and shear rate yields the dynamic viscosity. Viscosity index is calculated with the D2270 standard using 400C and 1000C viscosity results.
The tribological tests are carried out using a Cameron-Plint high frequency friction and wear machine to study the friction and wear characteristics. The principle of working of this rig can be seen in Figure5.1
6. RESULTS AND DISCUSSION
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Fig. 5.1 Cameron-Plint rig
In the upper holder, a Ø10 mm ball is mounted while in the lower holder a disc is mounted.
The ball oscillated against the disc which was stationary. Specimens were cleaned using industrial benzene and ethyl alcohol in an ultrasonic cleaner for 5 minutes each. A normal load is applied, temperature is controlled and movement was made with a low frequency. Selection of load and speed is done considering limitations of the test rig. Radius of the Hertzian contact was 0.15 mm and the calculated contact pressure was 2.19 GPa. Testing conditions are shown in Table 5.1 and other information relevant for the tests are shown in Table 5.2.
Table. 5.1 Testing conditions
Experimental Test Conditions
Normal Load 100 N
Frequency 4.3 Hz
Temperature 400C and 1000C Sliding distance 8 mm
Test Duration 3 h
Lubrication Fully flooded
6. RESULTS AND DISCUSSION
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Table 5.2. Other testing conditions
Material AISI 52100 high grade bearing steel
Configuration Ball on disc
Geometry Ball diameter = 10 mm
Disc d=25 mm h= 8 mm Calculated Hertzian contact radius 0.15 mm
Calculated contact pressure 2.19 GPa
Surface roughness Ra=0.15 μm for disc
Ra=0.48 μm for ball
Worn surfaces are observed using 3D optical surface profilometry (Zygo NewView 7300) where wear volume and wear depth was measured. After the test in Cameron-Plint, there is a wear scar in the surface of the disc, which is located with the help of Zygo, and then a reference surface height is set as 0 point. The groove below this 0 point is the wear depth. The total volume wear is Voldn-Volup.
Voldn is the volume of the groove, the volume of the material removed from this place, but not all the material is wear, the VolUp represents the material which is removed from the groove but not from the sample, this is due to plastic deformation. This material is still in the sample, but it is above the 0 point.
6. RESULTS AND DISCUSSION
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5.2. Description of oils
In this thesis ten different engine oils are investigate for their ant wear performance and frictional behavior.
The tested lubricant are from five different suppliers and are all stated to meet relevant approvals and to be suitable for use in modern diesel engines.
Oil number 1-3 content and performance is reviewed in detail, oil 4-9 is approved against the stated standards. Oil ten is most likely a false claim, due to inappropriate additivation, see chapter 6.4 for more details.
1. OEM CJ-4, 15W-40, VDS-4. Group II Base fluid
2. OEM CI-4, 15W-40, VDS-3. Group I Base fluid
3. OEM CI-4, 15W-40, VDS-3. Group II Base fluid, same additive package as no 2
4. Supplier 1, Top tier synthetic, CJ-4 5W-30
5. Supplier 2, Standard tier, ACEA E7 15W-40
6. Supplier 2, Standard tier, ACEA E7 15W-40 used for 500h
7. Supplier 3, Standard tier, CI-4, 15W-40, VDS-3
8. Supplier 4, Standard tier, CJ-4, 15W-40, VDS-4
9. Supplier 4, Standard tier, CI-4, 15W-40, VDS-3
10. Supplier 5, uncertain quality stated to be ”CI-4” 15W-40
The above mentioned oils are designated by unique reference number, that is oil 1,2,3 up to oil 10 and these numbers will be used for these oils in upcoming discussions and references.
6. RESULTS AND DISCUSSION
28
6. Results and discussion
During the tests, data were collected to understand the rheological and triboogical behavior of samples used in this work. In this section, these results will be shown and discussed.
6.1 Viscosity
In Figure 6.1 the dynamic viscosity of all samples is shown. This graph shows viscosity of the oils at both temperatures (400C and 1000C). At 1000C, all the samples gave similar values, while at 400C, oil 4 showed lower viscosity than the others, because it had different grade.
Fig. 6.1. Dynamic viscosity of 10 engine oils
As expected, the viscosity of all oils was lower at 1000C than at 400C, and the error was negligible (almost zero) with repeatability.
While SAE standards work with Kinematic Viscosity, data of oils in this project were calculated from Dynamic viscosity obtained in Bohlins CVO rheometer. Calculations were done simply by using
𝜈 =
µ𝜌, where
𝜈-
kinematic viscosity,µ-
ynamic viscosity and𝜌–
oil density.6. RESULTS AND DISCUSSION
29
The results gained from tests, showed that viscosity of all oils meet the SAE J-300 requirements (besides Oil 6 that is already used).
Table 6.1. Viscosity comparison between given samples and SAE
Oil name Kinematic Viscosity at 1000C (calculated)
Minimum Kinematic Visc. at 1000C (SAE)
Maximum Kinematic Visc. 1000C (SAE)
Oil 1 (W40) 15 12.6 16.5
Oil 2 (W40) 15 12.6 16.5
Oil 3 (W40) 15 12.6 16.5
Oil 4 (W30) 12 9.3 12.6
Oil 5 (W40) 15 12.6 16.5
Oil 6 (W40) 11 12.6 16.5
Oil 7 (W40) 15 12.6 16.5
Oil 8 (W40) 16 12.6 16.5
Oil 9 (W40) 15 12.6 16.5
Oil 10 (W40) 14 12.6 16.5
Source: Society of Automotive Engineers (SAE), December 1999
6.2. Friction characteristics
6.2.1. Static coefficient of friction
Figure 6.2 shows the values of static coefficient of friction for all the samples at both temperatures (400C and 1000C), including errors shown in repeatability.
6. RESULTS AND DISCUSSION
30
Fig.6.2. Static coefficient of friction for all 10 oils,
As it can be seen, for all the samples, static coefficient of friction is lower at 1000C. OIL 2 has a different base fluid group from OIL 1 and OIL 3, therefore its COF is higher at 400C. OIL 4, 5, 6 and 7 gave also lower COF, similar to OIL 1 and OIL 3. OIL 8 and OIL 9 gave higher COF than first 7 samples.
The surprise in this work was OIL 10. Even that its viscosity fits with the VDS standards, the performances of this oil was not stable during repetitions. Error was very high (value not shown in figure because is too high, so it is shown only the test error) in its performances in different test with same settings (error value is shown later). This will be discussed in a later section.
6.2.2. Dynamic coefficient of friction
Dynamic COF for all the samples at both temperatures (400C and 1000C) is shown in Figure 6.3. Other version of results is given in appendix. Unlike static COF, it can be seen that CoF is lower at 400C than at 1000C, besides OIL 2.
6. RESULTS AND DISCUSSION
31
Fig. 6.3.Dynamic coefficient of friction.
This different behavior can be attributed to the speed in relative movement between specimen and viscosity of oil. With low speed, the viscosity had a bigger effect on coefficient of friction, while for dynamic COF where speed is higher, viscosity did not change the result.
One likely the explanation for the difference in frictional behavior for oil 2 may be the difference in base fluid, Oil 2 use a API group I base fluid and number 3 that use the same additive package use group I. However, to be able to fully conclude that further investigations may be necessary.
6. RESULTS AND DISCUSSION
32
6.3. Wear volume
Figure 6.4 shows the wear volume of disc in mm3 for all the samples at 400C and 1000C, where is possible to compare them easily.
Fig.5.4. Wear volume of all 10 oils at 400C and 1000C.
For all the samples, wear volume was higher at 1000C, and this is due to lower viscosity of oil in higher temperature and the wear was more severe. OIL 5 and OIL 9 showed the opposite results, and this can be attributed to their chemical composition of additives.
As we can see here, OIL 10 gave the highest wear volume at 1000C, and also in repeated tests, gave unstable results. This will also be discussed together with coefficient of friction results obtained from OIL 10 later together with other analysis made for this oil.
OIL 5 and OIL 6 are the same oils, the difference is usage time. OIL 6 showed more wear volume than OIL 5 at both temperatures. This is because while OIL 5 is new, OIL 6 was used for 500hrs, and during this time, wear particles and other dust was introduced in this oil, and created extra abrasive wear, besides these, also additives in the oil have depleted on time.
6. RESULTS AND DISCUSSION
33
6.4. OIL 10 Results
The results from the wear and friction study clearly show that OIL 10 was different from other oils in its performances. To clarify the situation, and have some more clear reasons for this behavior, chemical analysis of all tested oils samples have been carried out using “Inductive Couple Plasma” (ICP).
ICP analysis determines the elemental concentration of the most common additive elements such as Zinc (Zn), Phosphor (P) and others. It also shows wear product elements and contaminants such as Iron (Fe), Silicon (Si), and others. For full results see table 6.2
The result shows that all of the investigated oils except oil 10 has normal and expected additive levels, oil 10 has approximately 10% of the expected concentrations of Zn, P and Ca. That is one clear evidence that oil 10 has no chance to fulfill the stated API CI-4 quality level and are a “false claim”. The insufficient additive levels explain why Oil 10 has unstable and bad results in the wear and friction test. If an engine is operated with oil 10 it is more or less guaranteed that engine damage will occur and risk for fatal failure are high.
Behavior of this oil in repeated tests is showed in following graphs.
Fig. 6.5. Two repeated tests of static coefficient of friction vs time, obtained using OIL 10.
As it can be seen in Figure 6.5., the behavior of this oil showed in graph (a) is very different from its behavior in (b), even the test parameters and settings were the same.
Also the surface profile of wear tracks was different during the test (with same parameters and settings), this is showed in Figure 6.6 (a) and (b).
6. RESULTS AND DISCUSSION
34
Fig. 6.6. (a) Wear scar profile, obtained using OIL 10.
This behavior where the COF was way much lower during one test, and very high during repeated test, and also wear track surface profile with different depth, is understood only after chemical analysis of the oils (shown in table 6.2), where the results showed that insufficient additives in chemical properties of OIL 10.
A comparison in wear scar and its surface profile between OIL 2 (as a referent oil in this work) and OIL10 (which showed false claims) is showed in Figure 6.7 (a) and (b) and Figure 6.8 (a) and(b);
Fig. 6.7(a)and (b) Surface Profile of the specimen when OIL 2 used.
6. RESULTS AND DISCUSSION
35
Fig.6.8(a) and (b) Scanning electron micrograph of the specimen when OIL 2 used.
Here it can be clearly seen that OIL 10 did not protect the surface from wear as good as OIL 2. It is also seen that on the specimen where OIL 10 is used, abrasive wear has occurred.
Table 6.2. Chemical analysis of all samples
Here it can be clearly seen that Oil 10 is a “scam”, and it gives a more clear understanding of its behavior, which in absence of sufficient additives, gives poor and unstable results.
6. RESULTS AND DISCUSSION
36
7. Conclusions
Tribological experiment using a reciprocating ball on disc set up together with rheological tests have been carried out to evaluate the performance of 10 different commercially available diesel engine oils. The salient conclusions from this study are as follows:
- All tested oils met the stated viscosities according to SAE J300 standard as new, if they still do that as used is not investigated.
- Dynamic coefficient of friction was lower than static coefficient of friction all the oils.
- Static coefficient of friction was lower at 1000C compared 400C except for OIL 2. No scientific explanation for that behavior has been found.
- OIL 5 showed less wear volume than OIL 6, because OIL 6 was used for 500 hours and therefore in the end of its service life
- OIL 10 was found to have false claims regarding its API CI-4 classification. It has shown very poor and unstable results.
- The used methodology in this study has the capability to determine viscosity, frictional behavior and wear volume for engine oils. However, the precision in the method is not sufficient to discriminate between different engine oils but are capable to identify oils that suffer from lack of anti wear additives.
8. Future work
If this study is to be performed again, a more precis method for viscosity measurements such as ASTM D445 will be useful. It would also be of interest to run low temperature viscosity such as Cold cranking simulator to determine it the tested oils meets the low temperature requirements according to SAE J300. If possible an additional tribo-test that address a different set of tribo pairs such as continuous sliding may add additional knowledge of the tested oils performance in an modern diesel engine.
9. References
:[1] "History of Technology: Internal Combustion engines". Encyclopædia Britannica.
Britannica.com. Retrieved 2015-03-21.
[2] http://auto.howstuffworks.com/diesel1.htm
[3] M.Fusch, The world Lubricants Market – Current Situation and Outlook, TAE Esslingen,
Tribology 2000 Plus, 12th international Colloquium, 11-13 January, 2000, Vol. I , p. 9.
[4] Theantiwear mechanisms of zddp’sH.Spedding and R.C. Watkins
[5] A review of zinc dialkalkyldithiophosphates (ZDDPs): characterization and role in the
lubricating oil - Allyson M. Barnes *, Keith D. Bartle, Vincent R.A. ThibonSchool of Chemistry, University of Leeds, Leeds LS2 9JT, UK
[6] ZDDP and MoDTC interactions in boundary lubrication – the effect of temperature and
ZDDP/MoDTC ratio – A. Morina, A.Neville, M.Priest, J.H.Green
[7] Investigating the Effect of Carbon Nanoparticles on the Viscosity of Lubricant Oil from
Light Duty Automotive Diesel Engines – A. Asango, A. La Rocca, and P. Shayler (University of Notingham) – 04.01.2014
[8] G. Lingg, A. Gosalia, The Automotive and Industrial Lubricant Market. Automotive and
Industrial Lubrication, 15th International Colloquium on Tribology, 17–19 January 2006, TechnischeAkademie Esslingen.
[9] M. Fuchs, The World Lubricants Market, 8th International Conference on Industrial,
Calcutta, 1997.
[10] M. Fuchs, The Global and Regional Lubricants Markets, Conference !Revitalizing the
Lubricants Business in the 21st Century’ Houston 1998.
[11] H. Czichos, K.-H. Habig, Tribologie Handbuch, Vieweg, Wiesbaden 1992.
[12] D. Klamann, Lubricants and Related Products, VCH Verlagsgesellschaft, Weinheim
1984.
[13] A. Gerv", Radioisotopes in Mechanical Engineering, Fourth United Nations International
Conference on the Peaceful Uses of Atomic Energy, AEC Conference 71-100-55, 1971.
40
[14] H. Holland, Information of the Institut fur Reibungstechnik und Maschinenkinetik,
TechnischeUniversit&tClausthal, Clausthal- Zellerfeld.
[15] http://globalindustrialsolutions.net/base-oil-definition.php (checked on 29.06.2015)
[16] D.W. Mackney, Nick Clague, Gareth Brown, Gareth Fish, and John Durham - Automotive
Lubricants
[17] Characteristics of diesel engine soot that lead to excessive oil thickening – C. Esangbedo, A.
L. Boehman, J. M. Perez
[18] Effect of diesel soot on lubricant oil viscosity – S.George, S.Balla, V.Gautam, M.Gautam
[19] Rabinowicz, E. (1995). Friction and Wear of Materials. New York, John Wiley and Sons.
[20] Stachowiak, G. W., and A. W. Batchelor (2005). Engineering Tribology. Burlington, Elsevier Butterworth-Heinemann
[21] Machinery lubrication March 2008 (http://www.machinerylubrication.com/)
Appendix
A. Friction measurements (Repeatability) A.1 Friction curves at 40
0C
Figure A.1 Oil 1
Figure A.2 Oil 2
0 0.02 0.04 0.06 0.08 0.1 0.12
0 2000 4000 6000 8000 10000 12000 [s]
test 1
test 2
test 3
0 0.05 0.1 0.15 0.2 0.25
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
42
Figure A.3 Oil 3
Figure A.4 Oil 4
0 0.02 0.04 0.06 0.08 0.1 0.12
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
0 0.02 0.04 0.06 0.08 0.1 0.12
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
43
Figure A.5 Oil 6
Figure A.6. Oil 10
0 0.02 0.04 0.06 0.08 0.1 0.12
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
44
A.2 Friction curves at 100
0C
Figure A.7 Oil 1
Figure A.8 Oil 2
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
45
Figure A.9 Oil 3
Figure A.10 Oil 4
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
0 0.02 0.04 0.06 0.08 0.1 0.12
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
46
Figure A.11 Oil 6
Figure A.12 Oil 10
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
0 0.05 0.1 0.15 0.2 0.25
0 2000 4000 6000 8000 10000 12000
Test 1 Test 2 Test 3
47
B. Wear track (SEM analysis) B.1 Wear track at 40
0C
Figure B.1 Oil 1
Figure B.2 Oil 2
48
Figure B.3 Oil 6
Figure B.4 Oil 10
49
B.2 Wear track at 100
0C
Figure B.5 Oil 1
Figure B.6 Oil 2
50
Figure B.7 Oil 6
Figure B.8 Oil 10
