Tribology and engine oil

What is Tribology?

Tribology is derived from the Greek word “Tribos”. Meaning of Tribos is Rubbing. Tribology is a science that deals with friction, lubrication and wear in all contacting pairs. Tribological knowledge helps to improve service life, safety and reliability of interacting machine components; and yields substantial economic benefits.

How do we test the friction reducing capabilities of ENVIRO₂PRO?

Shell 4 Ball Test rig.

In the trails the wear-reducing product “ENVIRO₂PRO” was examined according to DIN 51350 on its wear reducing capacities with an constant, respectively, increasing test force. The trails have been carried out with a four-ball test rig and a universal tribometer by the FH Lübeck.
Test results according to DIN 51350 part 4 / investigation welding forces:

The test-phase started with a test-pressure of 2.000N and at 4.800N the pressure was increased with steps of 200N. At loads which correspond with the friction welding point of the balls, the engine of the test rig stops after a certain time due to the fact that the welding balls join together.
However, during the test with ENVIRO₂PRO joining of the welding balls didn’t occur. Therefore more test-runs were carried out with pressures up to 12.000N. As the tested product was a ENVIRO₂PRO treated mineral oil, the actual welding forces should have been between 3.200 and 3.400N.

Test rig
(Example of a test rig. Photo: TU-Chemnitz)

When we compare the friction-values after increasing test forces from 300N up to 12.000N, it becomes clear that the friction-values increase from 3.400N upwards. However with increasing test forces (above 3.400N) the friction-values decrease and so the wear protection becomes evident. Under pressure and temperature ENVIRO₂PRO develops an elastic and firm adhesive layer, so we can assume that this layer ensures the actual wear protection between 3.400N and 12.000N.

Engine oil: technical overview in a nutshell.

Manufacturing modern engine oil is a precision operation. From the time that the crude oil goes into processing, until the finished lubricating oil is stored, careful control of temperatures, pressures and process time is exercised. Elaborate equipment takes undesirable components from the oil. Small, precise amounts of desired compounds are added at certain stages. Throughout the entire operation, extreme care is taken to keep contamination out of the product. Painstaking work is required to produce oils that will give first rate lubrication to engines of all types, under all conditions of operation and service. Such wide performance standards require that the oil be refined to meet exacting specifications such as viscosity, viscosity index, pour point, cloud point, flash point, fire point, ash, carbon residue, color and resistance to oxidation and corrosion.

When new oil is poured from its sealed container into an engine, it goes from the controlled environment of the oil refinery into a completely uncontrolled chemical factory - the engine. There is little control of temperature, pressure, time or contamination in the engine. The oil is subjected to temperatures from below zero to perhaps 1500°F (816°C) on the cylinder walls and to pressures from atmospheric up to thousands of pounds per square inch in the bearings. The length of time the oil is exposed to this uncontrolled environment depends simply on the whim of the operator with regard to operating conditions and drain intervals.

In addition to the variations in time, temperature and pressure, the oil now becomes exposed to a great variety of contaminants. The burning of the gasoline or diesel fuel itself yields a host of chemicals which find their way into the crankcase oil. When a pound of gasoline is burned, about a pound of water is produced. Combustion also creates oxides of nitrogen from the air, and these oxides may form acids. Small amounts of carbonic acid, sulfuric and sulfurous acids, lead sulfate, compounds of chlorine and bromine, and other complex compounds may be formed. If combustion is incomplete, aldehydes are formed, and when knock or detonation occurs, even more complex reactions take place. During combustion, unburned or partially burned carbon or soot also forms, and it forms more heavily when the air fuel mixture is rich.

Most of these compounds leave the engine through the exhaust, but a small amount goes into the crankcase from blow-by. Some fuels also form other undesirable decomposition products. These products are not only oil contaminants, but may interfere with the action of piston rings and affect engine cleanliness.

Further contamination occurs from the heavy ends of the fuel which reach the crankcase during cold engine operation. Dust and dirt find their way into the oil. The water, chemicals, heavy fuel ends, dirt , dust and the air in the crankcase are then completely mixed with the oil by the motion of the internal parts of the engine. The oil itself will tend to deteriorate because of oxidation. Even though it is well inhibited, high temperatures will speed its oxidation. This chemical union produces oil oxidation products which in themselves can be harmful. With these processes continuing over wide ranges of speed and temperatures between oil drain intervals, it is no wonder that the oil may eventually deteriorate as a lubricant. Let us examine the various types of engine operating conditions and the effects of some of these contaminants so that they can be brought under some degree of control. It is not possible to eliminate oil contamination nor to avoid the influence of temperature and time. However, it is possible to minimize their effects to some extent and thus prolong oil life.

LOW TEMPERATURE OPERATION - Wintertime operation of engines in stop-and-go service with considerable idling and very short runs, impose low temperature conditions on the crankcase oil. Under these conditions, the cooling water temperature may not exceed 120°F (49°C), and the crankcase oil temperature may not even be this warm. When the engine is started in cold weather, heavy fuel fractions enter the combustion chamber in liquid form and wash down the walls. Combustion is liable to be erratic. Contamination from water and other products of combustion is also higher during low temperature operation because of the condensing effect of cold cylinder walls. It might be pointed out here that low temperature operation is not specifically limited to extreme cold weather. Many times, even in warm weather and in southern parts of the country, the same problems exist with light duty engines that never get sufficiently warmed up.

Under low temperature operation the oil essentially deteriorates by contamination rather than by oxidation. The mixture of water, oil and contaminants forms a mayonnaise-like sludge which tends to settle out of the oil on the bottom of the oil pan and in other areas of the engine not sufficiently washed by the oil. After about 1500 miles in an average gasoline engine crankcase, the major contaminants may be as follows:

0.125% abrasives, sand and dirt 3.00% gasoline 0.25% soot and carbon

0.125% metals 0.50% water and glycol

0.50% tars, gums and resins

These low temperature effects can be reduced by any number of steps that increase the temperature of the cylinder walls and the crankcase oil. Water temperature can be increased by using higher temperature thermostats, blocking off a portion of the radiator and/or using a fan which is thermostatically controlled. A direct method of increasing oil temperature levels is to insulate the outside surface of the crankcase.

In addition to good oil filtering, adequate crankcase ventilation is helpful. Care should be taken to keep any breather openings clean, and if equipped with positive crankcase ventilation, the valves and tubes should be checked regularly to see that they are clean and free. One of the best answers to the problems of oil contamination under low temperature conditions is to drain the crankcase at more frequent intervals than usual.

HIGH TEMPERATURE OPERATION - Hot weather operation at sustained high speed and heavy loads will result in high crankcase oil temperature. It is not uncommon to find crankcase temperatures over 275°F (135°C). Under these conditions, oil deterioration is mostly the result of oxidation at high temperatures rather than by contamination. When thoroughly mixed with air, oils tend to oxidize faster and this rate of oxidation doubles with every 18°F increase in temperature. Oxidation tendencies are also increased by metallic contaminants which act as catalysts. The minute particles of iron that are scraped from cylinder walls accelerate oil oxidation at high temperatures.

Oxidation of the oil forms many compounds such as oil soluble acids, resins and varnish like materials. Some of these materials also occur from fuel decomposition. These compounds tend to adhere to hot metal surfaces producing the stain, varnish or lacquer coatings frequently seen on piston skirts. Under higher temperatures, they may completely plug oil control rings and piston ring grooves. Another class of products are the materials formed when oil strikes metal, which is hot enough to "crack" the oil. These materials are not oil soluble, and form the "coffee grounds" type sludge. The amount of oil deterioration through oxidation and the formation of undesirable compounds depends on the total combination of oil type, fuel type, operating temperature, engine design and oil drain interval. Under adverse conditions, even a slight reduction in operating temperature may relieve the situation, as well as the change to better quality oils.

DISCOLOURATION - Generally when crankcase oils are subjected to the extreme of service, as mentioned above, they will definitely change colour. They have probably become opaque and may range in colour from a dark chocolate brown to a light grey. Because of the contamination and gradual deterioration, the oil be-comes discoloured.

First of all, the detergent additives used in oil are designed to keep the engine clean by holding the contaminants and deterioration products as finely divided particles in the oil itself. This keeps the engine clean, but it does make the oil look dirty. If the oil is dark brown or wine coloured, it is usually due to the oil oxidation products and varnish-like materials already mentioned. Actually, it is much better to keep this debris in suspension in the oil than to have it drop out of the oil as varnish on the pistons, rings and grooves.

The grey discoloration which is seen frequently is due, in part, to road dust and dirt, and in the past due to the decomposition of lead compounds from fuels containing tetraethyl lead. Generally you do not see this grey discoloration in diesel engines. Where the oil is black and opaque, this may be attributed to slightly rich air-fuel mixture and to the formation of soot and carbon. While it is due to a rich air-fuel mixture, quite often you will find this type of discoloration in heavily loaded or overloaded engines.

FUEL DILUTION - Ambient temperatures, air-fuel ratios, fuel volatility and mechanical condition of the engine are factors that directly influence the amount of crankcase oil dilution. Low temperature operation and worn cylinder bores and rings will also produce a high amount of dilution.

Five or six per cent fuel dilution in gasoline engine crankcase oil is the maximum that should be allowed. This will reduce the SAE grade of the crankcase oil by one number. With improper engine operation, it is possible to dilute the crankcase oil sufficiently so that there may be metal-to-metal contact in the bearings. Excessive dilution in the crankcase should never be disregarded, as it means that something is wrong, either with the engine or the manner in which it is being operated.

DUST AND DIRT - The air in metropolitan areas may contain as much as four or five tons of dust and dirt per cubic mile. On unimproved roads, in dry weather, or on farms, this figure is greatly exceeded. Road and field dust is very abrasive and will cause very rapid wear of cylinder bores, rings and ultimately the bearings.

Air and oil filters are remarkably efficient when maintained according to manufacturers' recommendations. The average operator is prone to neglect these accessories and, consequently, they become less effective as mileage is accumulated. Both oil and air filters should be serviced regularly.

SOOT - Soot is generally formed by incomplete combustion of the fuel. This occurs when there are rich air-fuel ratios during starting in cold weather and also during intermittent operation at low temperatures. The soot is picked up by the oil on the cylinder walls and washed down into the piston ring belt and the crankcase. Oil ring plugging is due to soot mixing with other combustion products and oil deterioration products to form a soft mass in the ring area. The accumulation in the oil ring slots is then baked hard by higher temperatures, and plugs the oil ring. Oil ring plugging means severely increased oil consumption because of the lack of control on the cylinder walls. Soot formation is difficult to control because of rich air-fuel ratios needed for cold starting and idling. Engines used in stop-and-go service at low temperature seem to be particularly susceptible to plugging. Under these ~ conditions, the best answer is good crankcase ventilation, high operating temperatures and more frequent oil drain intervals.

CONCLUSIONS - The problems of oil contamination and deterioration have been subjected to a great deal of research by everyone involved with engines. Answers to the problems are being provided by improved motor oils, an additive like ENVIRO₂PRO and by engines designed to reduce contamination. The ideal combination of an oil that will not deteriorate and an engine that will not contaminate will probably never be reached. Therefore, the best is to drain the crankcase at reasonable intervals and to refill it with fresh oil. However, this reasonable interval is not easy to establish for a wide variety of operating conditions, but we have worked out recommendations for various types of oil, operating conditions and based on a great deal of experience.

Oil exchange interval aspects:

The reason for engine oil changes is that the oil is degraded in terms of viscosity and oxidation, and that solid or liquid contaminations become mixed or dissolved into the oil. The presence of contaminations in engine oil is generally undesired, as solid contamination particles potentially cause abrasive wear and liquid contaminations may cause corrosive attacks, tribo-chemical wear and viscosity changes. Undesired polishing of cylinder liners in diesel engines during operation, commonly called bore polishing, may occur if corrosive species and small abrasive particles are present in the lubricating oil. The contaminants in the engine oil build up over time. This disadvantage is normally maintained by replacing the contaminated oil with a new batch. The oil change intervals can be based on running hours, particularly in the case of smaller engines, or on the condition of the oil, as determined by oil sample analyses ¹ or by the response of a particle sensor in the oil circuit or tank. When establishing an oil analysis programme, the location and time for the sampling of used engine oil should be carefully considered, as this may have a strong influence on how representative the sample is in regard to the condition of the engine. In wet sump engines a special sampling point should be introduced between the oil pump and the oil filter. In dry sump engines, the return line can be used for sampling, provided the pressure is sufficient for allowing sampling, or a vacuum pump can be employed to assist the sampling. In this context, particular notice should be made to the fact that the oil at the piston rings is far more contaminated than the oil in the engine sump or oil tank.

¹ Oil analysis:

Oil analysis (OA) is the laboratory analysis of a lubricant's properties, suspended contaminants, and wear debris. OA is performed during routine preventive maintenance to provide meaningful and accurate information on lubricant and machine condition. By tracking oil analysis sample results over the life of a particular machine, trends can be established which can help eliminate costly repairs. The study of wear in machinery is called tribology. Tribologists often perform or interpret oil analysis data.

OA can be divided into three categories:

1. analysis of oil properties including those of the base oil and its additives,
2. analysis of contaminants,
3. analysis of wear debris from machinery,

The usage of ENVIRO₂PRO will extend the service intervals as dictated by the engine manufacturer and as mentioned in the operating instructions of the engine. To what extend the service intervals can be postponed is influenced by several external factors. We are happy to discuss your personal situation and guide you in steps you need to take in order to profit from postponed oil changes and engine service intervals.