MODERN SYNTHETIC LUBRICANTS FOR ENGINE OIL APPLICATIONS By: Richard G. Golembiewski, P.E. RIS Technical Editor --------------------------------------------------------------------------- MILWAUKEE, WI - RIS - There's been a great deal of interest, of late, in the performance of synthetic lubricants. Manufacturers have enticed the motoring public for a number of years now, with claims for increased fuel economy, reductions in fric- tion and wear, decreased oil consumption, better cold cranking performance, and extended drain intervals. Many motorists however, remain skeptical, as the price of synthetics is usually much higher than conventional petroleum based oils. In addition, a great deal if misinformation has circulated regarding them. Are these claims simply hype, or is there something here that the average motorist can be interested in? Let's take a look. Synthetic lubricants have been around for a long time. Synthesized compounds are the only thing that will continue to flow at the low temperatures found in the arctic or in outer space. During the past twenty years, some of these same benefits have been made available to the general public. In order to properly examine the role synthetic lubricants play, and their performance, we need to first look at the fundamentals of hydro-dynamic lubrication and lubricant properties and production. Fundamentals of Hydrodynamic Lubrication: As usually stated in engineering texts, and intuitively grasped by most laymen, a lubricant is inserted between two moving surfaces to reduce friction, and the resultant generation of heat and wear. "Hydrodynamic" lubrication exists when two surfaces are separated by a relatively thick film of lubricant. A high pressure is not required to separate the surfaces. In a typical engine, plain or grooved journal bearings are used to hold the crankshaft, piston rods, camshafts, and other machine components. Take out a deck of cards. Place the deck on a table, and with your hand, move the deck horizontally. Notice how the bottom card does not move, while the top card moves the most. Those in between move too, with the amount of motion dependant on the height from the table. What's happening here is that the friction between the table and the bottom card keeps it from moving. In fluid mechan- ics, we refer to the layer that doesn't move as the boundary layer. The type of stress you applied to the cards is called a "shear stress", and is equal to the horizontal force you applied divided by the flat area of a card. This shear stress also affects the velocity of each card. This relationship is directly proportional to the shear stress, and the distance from the table, and inversely proportional to a quantity we will call the "viscosity". (Actually, it's a bit more complicated than this, but it's a suitable simplification for our purposes.) What this simply means is that for a given distance from one surface, the velocity will be lower if the lubricant has a higher viscosity, and a constant shear stress is applied. Hence, the viscosity is a measurement of the internal friction of the fluid, and its resistance to motion. Our example used cards, but fluids are often modeled as infinitely thin layers. Thus if you drop a steel ball into a glass of molasses (a high viscosity fluid) it will drop slowly because of the internal friction of the fluid. Likewise, dropping the same steel ball in a glass of water, will cause it to drop rapidly because the fluid does not have a particularly high viscosity. We will not consider the methods used to measure viscosity, but rest assured that standard methods have been developed. One of the problems with this internal friction is that it produces heat. If we model the fluid molecules as a series of balls connected by springs, transfer of momentum takes place between the molecules and the amplitude of vibration becomes greater. This means that it takes longer for one molecule to randomly strike another, reducing the internal friction, and hence the viscosity. This means that the viscosity of a fluid generally decreases with temperature, and increases as the temperature drops. If a fluid's viscosity is a function only of temperature, then it is characterized as "Newtonian" after Isaac Newton. Unfortunately, the viscosity of many fluids, engine oils among them, drop with high shear rates. Such fluids are termed non-Newtonian. OK, so what's the fluid's internal friction got to do with the friction between a pair of parts, such as a crankshaft and a bearing? Plenty! First, we need to differentiate between thin-film and thick-film lubrication. Thin films are a problem. as the viscosity decreases, the lubricant is less able to withstand the loads placed on it. Heat is generated, reducing the viscosity even further. Surface-to-surface contact may occur. In thin films, the coefficient of friction between the two surfaces actually goes up as the viscosity decreases. Such films are termed "unstable". It is essential then to provide a film which is sufficiently thick to provide proper lubrication. If the film is thick enough, however, the coefficient of friction between the mating surfaces actually goes down as the viscosity of the lubricant drops. The temperature drops, and the viscosity of the fluid rises slightly. This acts as a stabilizing effect, and prevents loss of film thickness. The designer then, needs to specify the bearing/journal design and lubricant viscosity (for a given speed) in such a way as to prevent the formation of a thin film. This means that the viscosity has to be high enough even at high temperatures. However, the fluid still needs to flow at lower temperatures, and there is enough reduction in friction between mating surfaces at moderately low viscosities to warrant their selection. As an aside, the temperature rise can be controlled somewhat by providing a constant flow of oil to and from the bear- ings. While the temperature in the sump (where we usually measure it) seems high, it's significantly lower than at the contact surfaces themselves, and enough heat can be transferred to make a recirculating flow system desirable. If temperatures become too high, then an additional cooler can be added. Lubricating Oil Fundamentals: So what about the lubricant itself? What kind of specifications does it have to meet? The American Petroleum Institute (API), the American Society for Testing Materials (ASTM), and the Society of Automotive Engineers (SAE) have cooperatively developed specifications for lubricating oils. If you take a look at the top of a motor oil can, you'll find the following: SAE viscosity specification (such as 5W-30, which means that it is a multi-vis oil that meets both the 5W and 30 specifications.), an API service classification (such as SF/CD), and perhaps an "energy conserving" designation. The SAE viscosity designation, means that the oil meets SAE J300 specifications for cold cranking (if a "W" rated oil) and at 100 degrees Celsius (if without a "W" rating), when proper ASTM testing procedures are followed. The API service classification is a bit more complex. You see, an oil may initially meet the SAE viscosity specification, but when run at high temperatures for a period of time, its performance may deteriorate. The API classifications for most engine oils are set for spark-ignition engines (such as SF, where the "F" is a chronological designation), and compression-ignition (diesel) engines (such as CD). Several test sequences are run using a standard engine. For instance, rust and number of stuck lifters are rated, the viscosity increase over time (we'll talk about why this happens later) at, say 100 degrees F is measured, and the amount of sludge, varnish, oil screen clogging, and cam lobe wear is estimated or measured. These classifications are getting tougher. For instance, the SE rating for 1972 model cars allowed a maximum of 400% increase in the oil's viscosity when measured at 100 degrees F after 40 hours. However, the SF classification for 1980 model cars, allowed a maximum of 375% increase in the viscosity when measured at 40 degrees C after 64 hours, with subsequent reductions in the other categories. The new SG rating is even tougher. An engine may be designated as "energy saving" if they demonstrate reduced fuel consumption when compared to an SAE 20W- 30 Newtonian reference oil. With the coming of federally mandated CAFE requirements, most manufacturers are designating this type of oil for use in late model engines, and the EPA allows their use. Just what does an oil consist of, and how can it be compounded to meet these specifications? Let's look first at conventional oils. Crude oil as it comes from the ground is made up of a number of hydrocarbon compounds - primarily paraffins, but it also includes other compounds. Often, these compounds are separated by viscosity through a distillation process. Since different fractions of the crude have different boiling points as well as different viscosities, progressive boiling is used. Those fractions with lower boiling points are allowed to vaporize, and are collected and then cooled. These neutral fractions typically have lower viscosities, while the bright stocks (those with higher boiling points) generally have higher viscosities. As such, we can separate oils by viscosity. But here's a problem. If we compound an oil to have a relatively low viscosity (or a multi-vis oil with a significant amount of these lower boiling point/lower viscosity stocks) some of them will vaporize at high temperatures, resulting in higher oil consumption. What's left behind has a higher viscosity. Varnish and sludge are also present. If the decrease in viscosity, amount of sludge, varnish, and cam lobe wear are too high, it fails the API service test. That's why a 5W-30 oil that meets the SF rating represents a major step. Those oils are said to be "energy saving" since their lower viscosity at lower temperatures (with thick-film lubrication. Remember, if the viscosity is too low, surface-to- surface contact may occur resulting in increased friction and wear!) results in lower part-to-part friction. Yet by passing the SF rating, it shows that it's still pretty good. Now, there are many things in the average motor oil than various refined fractions of crude. Included are various additives, such as anti-wear agents, extreme pressure (EP) additives, anti-rust agents, corrosion inhibitors, detergents, dispersants, and friction modifiers. Most of these are self-explanitory. They are added to enhance the performance of an oil. The EP additives are put in to help the oil hold up between surfaces which feature high contact stresses such as those between the cam lobes and followers. Detergents and dispersants are put in to help remove dirt and sludge and hold it in suspension, until it's either removed in the filter, or the oil is changed. Also included are various oil modifiers such as pour point depressants, viscosity index (VI) improvers, and seal swell agents. Pour point depressants are added to inhibit wax crystal growth at low temperatures. This gives the oil better cold crank- ing performance. VI improvers are designed to help an oil's viscosity/temperature performance. Remember that at high temperatures, an oil's viscosity drops. If it drops too low, we lose film thickness, and are in big trouble! The viscosity index (VI) is a measurement of how an oil's viscosity changes with temperature, compared to reference oils. The higher the number, the better. VI improvers are polymer compounds with interlocking structures (polymers are long chain molecules). Because these chains are interlocked, they don't move as easily at high temperatures and resist viscosity loss. Unfortunately, they don't necessarily contribute anything to lubricity, and in fact begin to wear out under shear stresses. As they wear, the oil's VI deteriorates, and we're left with the old VI improver, which has to be held in suspension. This is another reason to change your oil frequently! The VI improver's sensitivity to high shear stress is significant in that if the shear stress is high enough, the oil may experience either a temporary or permanent loss of viscosity! Finally, an oil company may add various compounds which help protect the base stock, such as anti-foam agents, antioxi- dants, and metal deactivators. The antioxidants are important as they prevent the oil from reacting with oxygen at high temperatures and forming sludge, varnish and lacquer. So where do synthetics fit in? What are they? The term "synthesize" means to put together from small bits. Rather than separating crude into various fractions as is done with conventional oils, synthetic base stocks are made by reacting various organic chemicals together. For instance, if an acid an an alcohol are allowed to react, a compound known as an ester is produced. (As an aside, the aroma present in flowers is generally produced by an ester. Others include butter, lard, tallow, linseed, cottonseed, and olive oils - although I wouldn't substitute my favorite engine oil for any of them in my cooking, or vice- versa!) Other synthetic hydrocarbon compounds are also suitable for lubricating oils, and manufacturers may blend two or more compounds together to arrive at suitable properties. It should be noted that many additives are also made of synthesized compounds. First, though, let's compare a conventional oil to a synthetic. A synthetic may require considerably less VI improver to have the same viscosity index. Remember that the VI improver wears out. Synthetic's are also more thermally stable. Synthetic base stocks also have lower pour points - often below -50 degrees F, and require little or no pour point depres- sant. In contrast, bright stocks may stop pouring at 25-30 de- grees F, and need it. Still, synthetics are a bit more expensive, so compounding one to compete directly with a conventional oil may not make economic sense. That's why they are usually made to have superior properties. The extra performance is often worth the cost penalty. For instance, synthetics can be compounded with very low pour points. This gives good cold-cranking performance. They may also be compounded with slightly lower viscosities at lower temperatures (while still meeting SAE specifications). This helps to reduce friction, and results in less wear and better fuel economy. Now the 5W-30 "energy saving" oils will do the same thing, but as we've discussed before, to lower the viscosity, these oils may be compounded with fractions which have a higher volatility. After a period of time, they begin to boil off or oxidize, leaving behind an oil of higher viscosity. Now, that same oil may meet API SF specifications, but a synthetic may remain stable for a LONGER period of time. (Esters exhibit excellent performance in the API test. Other compounds are very, very good also.) That means that longer drain intervals are possible. A word on use. Some synthetic compounds are not compatible with conventional oils. However, most manufacturers, have recognized that one may add a quart of their product to someone else's, and have compounded them to be. To do otherwise would be to pass up their intended market! (As an aside, I try to avoid having to mix conventional oil, if I can help it. While they are also compounded to be compatible, the performance may not be the same when mixed together. It's ok in a pinch, but I don't make a habit of it.) Also, the lower friction resulting from the use of a synthetic lubricant makes them unsuitable for break-in. To sum up, synthetics provide an excellent alternative to conventional oils - especially if better performance is required. It's your choice!