Corrosion and Wear
Engine Oil Performance
Automatic Transmission Fluids
Most engines used in transportation are of the internal combustion type. These engines have high thermal efficiency and are lightweight relative to their power capability. The performance of engine lubricants is judged on their ability to reduce friction, resist oxidation, minimize deposit formation, and prevent corrosion and wear. To meet these functional requirements, engine lubricants must be supplemented with additives, as follows:
- Anti wear additives
- Friction modifiers
- Antirust additives
- Viscosity modifiers
- Antifoam agents
- Bearing corrosion
- Pour point
- Viscosity modifiers
- Dispersants inhibitors
Most problems associated with engine lubricants are related to lubricant decomposition and the entry of combustion byproducts into the crankcase. The major causes of engine malfunction due to lubricant quality are deposit formation, contamination, oil thickening, oil consumption, ring sticking, corrosion, and wear. (Lubrizol)
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The two main sources of lubricant contamination are blow-by from the combustion chamber, and gases and volatiles from the crankcase that are vented into the intake manifold as an anti-pollution measure. The various gases interact with one another and the lubricant to form soot, carbon, lacquer, varnish, and sludge.
Soot particles are hydrocarbon fragments partially stripped of hydrogen atoms. They also contain an appreciable amount of oxygen and sulfur. Commonly found in the combustion chamber, soot particles are strongly attracted to one another and to polar compounds in the oil, and they also tend to form aggregates, which have a soft and flaky texture.
Carbon deposits are hard and result from the carbonization of the liquid lubricating oil and fuel on hot surfaces. These deposits have a lower carbon content than soot and usually contain oily material and ash. They are commonly found on the piston top lands and crowns, in piston ring grooves, and on valve stems.
Lacquer and varnish form when oxygenated products in the lubricant are exposed to high temperatures. Lacquer is often derived from the lubricant and is generally water soluble. It is commonly found on pistons and cylinder walls and in the combustion chamber. Varnish, on the other hand, is fuel related and is acetone soluble. It is commonly found on valve lifters, piston rings, and positive crankcase ventilation valves.
Sludge is caused by lubricant oxidation, oxidation and combustion products in the blow-by gas, and the accumulation of combustion water and dirt. It can vary in consistency from a baked deposit to that of mayonnaise. Low-temperature sludge, most prevalent in gasoline engines, is watery in appearance and forms below 95°C. High-temperature sludge is more common in diesel engines and forms above 120°C. (Lubrizol)
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Oil thickening can result from lubricant oxidation, the accumulation of
insoluble's, and soot. Viscosity increases due to polymerization of oxygenated products and the suspension of fuel-derived
insoluble's in the bulk lubricant. (Lubrizol)
Oil consumption is related mainly to the lubricant that travels past piston rings and valves, and burns in the combustion chamber. The extent of lubricant consumption depends on a number of equipment and lubricant related factors, including viscosity, volatility and seal-swell characteristics. A certain minimum amount of oil is required to properly lubricate the cylinder walls and pistons. High oil consumption, however, indicates a problem such as cylinder wear, bore polishing, stuck piston rings or out-of-square grooves. These conditions increase the amount of blow-by gases entering the crankcase.
Lubricant volatility is another important factor responsible for oil consumption. Lighter base oils can leak past the piston rings more readily and be burned. (Lubrizol)
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The major cause of ring sticking is the formation of deposits in the piston grooves, resulting in the loss of an oil seal. This not only increases the potential for blow-by gases in the crankcase but also leads to poor heat transfer from the piston to the cylinder wall. Resultant thermal expansion of the pistons can lead to loss of compression and engine seizure. (Lubrizol)
Corrosion and Wear:
Diesel fuel with high sulfur content causes piston ring and cylinder wear. Corrosive wear is more commonly associated with combustion and oxidation products; it results from the attack of sulfur acids or organic acids on iron surfaces. This kind of wear is controlled by using lubricants with a base reserve. (Lubrizol)
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Engine Oil Performance:
In the U.S., the classification of and requirements for engine oils are established through a process led by API, AAMA, EMA, and CMA. These trade associations, together with ILMA, provide the framework for new engine oil categories. The technical societies ASTM and SAE verify the technical need for the new category and ultimately recommend the tests and performance limits to define the category.
In Europe, individual OEMs continue to be a major influence on engine oil performance requirements for both passenger-car and heavy-duty applications. The ACEA classification system consists of nine different sequences to define engine oil quality for European automotive service fill applications. The system is based on a schedule of physical, chemical and engine tests similar to those used in the U.S. but using both ASTM and CEC test methods. All performance claims against ACEA requirements must be supported by data generated under the European Engine Lubricant Quality Management System (EELQMS). This system consists of two codes of practice -- one developed by ATC and one by ATIEL -- and defines the process for developing, testing and reporting the necessary performance data.
In the U.S., API administers the licensing and certification of engine oils through a system that meets the warranty, maintenance and lubrication requirements of original equipment manufacturers (OEMs). Engine oil performance requirements, test methods, limits for the various classifications and testing processes are established cooperatively by the OEMs, oil marketers, additive companies and testing laboratories. These groups routinely review the classifications system and implement changes as conditions warrant. (Lubrizol)
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Gasoline Engine Lubrication:
Lubrication problems in gasoline engines, particularly in passenger cars, are associated with:
- Low-temperature or light-duty operation, resulting in excessive contamination of the lubricant by partially burned fuel fragments and other blow-by products,
- High-temperature oxidation, resulting in excessive engine rust and sludge that can lead, among other things, to excessive oil thickening,
- Valve train wear resulting from the high cam lift and spring loads required to provide high volumetric efficiency and high engine speeds, and
- The use of air-pollution control devices such as positive crankcase ventilation and exhaust gas recirculation.
Recognizing these problems, the automobile industry has sought to define the lubrication requirements of their engines in terms of engine dynamometer tests published by ASTM and involve:
- Sequence IID: Low-temperature deposits and rusting.
- Sequence IIIE: Oil thickening and valve train wear.
- Sequence VE: Performance of lubricants under stop-and-go driving conditions.
In North America, all licensed oils must meet the requirements of ILSAC GF-2 and API SJ. The procedures for licensing and certifying engine oils by API are complemented by the Chemical Manufacturers Association (CMA) Product Approval Code of Practice, which provides a statistically valid testing and approval system for testing candidate oils. A key provision of the Code is multiple-test acceptance criteria for the Sequence IID, IIIE, VE, VI-A and L-38 test. In Europe, all licensed oils for gasoline powered engines must meet the requirements of ACEA A1, A2, or A3. (Lubrizol)
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Diesel Engine Lubrication:
Lubrication requirements for diesel engines are being driven by emission control requirements. New engine designs are being introduced to meet the latest emissions regulations, which are listed below. These changes subject the lubricant to more hostile operating conditions. In reducing emissions, the lubricant has been identified as a direct contributor to hydrocarbon particulates. This occurs partly from the leakage of lubricant past exhaust valve guides and turbocharger seals, but mainly from consumption of the lubricant film on the cylinder liner during combustion. To reduce particulate emissions, new engines are designed to operate with a thinner lubricant film and to allow less lubricant leakage past the piston rings. Unfortunately, this can increase the potential for ring and liner scuffing, and can increase operating temperatures and the potential for deposit formation. In addition, the engine modifications and adjustments necessary to ensure compliance with 1994 exhaust emissions regulations also had the adverse effect of increasing lubricant soot levels, which can increase wear and oil viscosity.
In the U.S., oils intended for use in low-emissions diesel engines must meet the requirements of API CH-4. Deposit control performance of lubricants intended for low-emissions engines is evaluated in the Caterpillar 1P single-cylinder engine test. The Roller-Follower Wear, Mack T-8E and Mack T-9 tests measure a lubricant's ability to guard against serious soot-related problems such as valve train wear, viscosity increase and filter plugging. In addition, lubricants to be used in four-stroke diesel engines operating on high-sulfur fuels must meet the requirements API CF, while lubricants to be used in modern two-stroke diesel engines must meet the requirements API CF-2. Similar requirements are influencing the development of engine oil requirements in Europe, where ACEA has issued diesel engine oil specifications for both light-duty diesel and commercial vehicles. (Lubrizol)
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The U.S. Military diesel engine oil specification, MIL-PRF-2104G, covers engine oils suitable for lubricating spark and compression ignition internal combustion engines. Engine performance tests used for MIL-PRF-2104G qualification are the Caterpillar 1M-PC and 1N, Mack T-8, Roller Follower Wear and HUEI Oil Aeration Tests. MIL-PRF-2104G implements API CG-4, CF and CF-2 diesel engine oil requirements. The specification also covers power transmission fluid applications in combat/tactical service by including the transmission test requirements of Allison C-4 and Caterpillar TO-4. These requirements cover graphite, paper and bronze friction; friction retention; gear wear testing; and seal compatibility. It should be noted that compliance with the TO-4 test requirements in MIL-PRF-2104G does not constitute full compliance with the Caterpillar specification. (Lubrizol)
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The primary functions of a gear lubricant are the same as those for all other lubricants. However, particular emphasis might be placed on friction
reduction, which is not as difficult to prevent as with crankcase lubricants because no fuel degradation products are present. The principal types of additives used for gear lubricants are:
- Antiwear and extreme pressure additives
- Antifoam agents
- Antirust additives
The oxidation, rust and foam inhibitors used in gear lubricants are generally of the same type as those used in crankcase lubricants.
Of particular importance are antiwear and extreme pressure additives, which are activated only under specific conditions and are inactive under other conditions. This property is necessary both to preserve the reagents and to avoid extraneous reactions that might be detrimental to the system. Examples of harmful side effects are excessive wear on gear teeth, ball and roller bearing parts, and other differential components, as well as possible deposits in oil passages and other critical areas.
Combining the necessary additive properties in a single gear lubricant package is not a simple matter. A major stumbling block is the difficulty in reconciling high-speed and high-torque requirements. Some materials that enhance one type of performance can hinder the others. Because of the complexity involved, many full-scale passenger car and truck axle tests must be run both in the laboratory and on the road. These tests require the support of numerous chemical and physical bench tests to screen out the most promising candidates.
Passenger car rear axle lubricants require score protection as well as thermal, oxidative stability and rust protection. This is provided through the use of sulfur-phosphorus lubricants. Requirements for these lubricants are described in the API GL-5 specification.
The requirements of many equipment builders exceed those of the API specifications. As a result, SAE and ASTM have updated GL categories to reflect present and future needs. This action has promoted the development of gear lubricant categories API MT-1 and proposed PG-2, which are designed to meet the performance requirements of North American heavy-duty or commercial equipment.
API MT-1 designates oils for heavy-duty truck and bus manual transmissions. Its focus is on improved high-temperature cleanliness and stability, oxidation and anti wear control, and compatibility with oil seals and copper alloys. PG-2 is for heavy-duty truck and bus final drive axles using spiral bevel and hypoid gears. It also includes seal compatibility.
Many automobile manufacturers have proprietary test requirements for their factory-fill gear lubricants. As such, factory-fill oils provide unique performance characteristics that are critical for the satisfactory operation of a particular OEM's drivetrain. Their performance characteristics may include break-in, bearing pre load and limited-slip durability. However, API GL-5 lubricants are often recommended for service-fill applications. The quality of European gear oils is influenced primarily by the requirements of the major commercial vehicle and passenger car manufacturers. API GL-5 and MIL-L-2105D establish the minimum performance level for European commercial vehicles. Equipment builders then impose additional requirements to meet their specific needs for improved surface fatigue, component cleanliness, synchromesh durability and viscometrics. The majority of passenger cars now use a transaxle drive train arrangement, reducing the need for rear axle lubricants. These vehicles are filled for life at the factory.
Conventional API GL-4 lubricants are being replaced by more specialized manual transmission fluids. These fluids have excellent thermal stability and carefully tailored frictional characteristics to provide smooth synchronization and good shift quality.
A performance area not addressed by industry specifications is limited slip. Because of hardware differences among the various limited-slip differentials, no standard industry-wide test is available to evaluate a lubricant's ability to prevent chatter in this application. Lubricant requirements, therefore, are based on performance in an individual manufacturer's test rig or vehicle. (Lubrizol)
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Automatic Transmission Fluids:
The principal functions of automatic transmission fluids (ATFs) are:
power transmission in the fluid member (torque converter), hydraulic control medium, heat transfer medium, and lubrication of transmission parts such as clutches, gears, bearings, seals friction modification. The general types of additives used to enhance these functions are: antioxidants, dispersants, extreme pressure additives, friction modifiers or "oiliness" agents, pour point depressants, viscosity modifiers, seal conditioners, corrosion inhibitors, and antifoam agents.
A critical problem in developing an ATF is providing the desired frictional properties for proper clutch pack operation while still providing the other properties. Because of differences in transmission design among the major auto manufacturers, the required frictional properties vary considerably. One design may require a "slippery" fluid with a low coefficient of friction at lockup to provide a smooth shift without the noise and wear produced by stick/slip. Another might require a higher coefficient of friction to ensure fast clutch plate lockup that prevents wear due to excessive slippage. ATFs must also have a sufficiently high viscosity at elevated temperatures to ensure against excessive leakage in hydraulic and control systems. This would result in low hydraulic pressures and degradation of shift characteristics. In addition, too high a viscosity at low temperatures causes reduced fluid flow, which causes reduced fluid efficiency, pump cavitation, extended shift time, possible clutch plate failures, and reduced starting capability.
Modern vehicle and transmission designs place increased stress on the automatic transmission fluid. The drive to improve fuel economy has led to more aerodynamic car designs that permit less airflow around the transmission, thereby increasing operating temperatures. This, combined with reduced sump sizes, results in severe thermal stress on the fluid. Requirements in both the General Motors DEXRON®-III and Ford MERCON® specifications are aimed at avoiding problems caused by high-temperature operation. The 4L60 oxidation test in DEXRON®-III ensures that the fluid keeps transmission part virtually sludge free. In addition, the specification severely limits total acid number (TAN) increase. Likewise, MERCON® limits maximum viscosity increase in the aluminum beaker oxidation test to 40% to prevent high-temperature oxidation problems.
Automakers are offering longer drive train warranties; therefore, increased emphasis has been placed on improved durability and consistent shift quality. Providing the proper friction performance is a vital role for an ATF. The band and plate friction tests, and 4L60 cycling test in DEXRON®-III, and the MERCON® friction durability test ensure that a transmission retains its shift quality throughout the life of a vehicle.
Electronic controls are common in modern transmissions, and the smaller fluid orifices required for these controls challenge an ATF's theology, especially in cold weather. To ensure proper operation, particularly at startup, ATFs must have improved low-temperature viscosity. Therefore, both DEXRON®-III and MERCON® require a maximum viscosity of 20,000 cP at -40°C.
Another important property of ATF is compatibility with elastomer seals. The fluids can affect the tensile strength, elongation, hardness and volume of elastomers. Various immersion tests are generally used to evaluate the compatibility of ATFs with the different seal materials. Several other fluid properties are important for the proper functioning of an automatic transmission, including lubrication of the moving parts, foam resistance, low volatility, low pour point, and high flash and fire points. (Lubrizol)
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