OIL SPECIFICATIONS What is oil viscosity a.k.a. oil weight? What is the best motor oil viscosity?

Transcription

1 OIL SPECIFICATIONS What is oil viscosity a.k.a. oil weight? Viscosity is a fluid s resistance to flow. In case of lubricants the viscosity is very important because it affects the oil s ability to reduce friction and transfer heat. The viscosity is measured in mpa*s (millipascal-seconds) but in everyday life we don t use the actual measured viscosity, we use viscosity grades instead. These grades, also known as weights were set by the SAE (Society of Automotive Engineers) and in order for a fluid to fall in a given category it has to meet certain limits. These limits are listed in the SAE J 300 table: The numbers that have a W after them are the so-called winter viscosities while the numbers that don t have a W are the so-called summer- or operating temperature viscosities. Several decades ago motor oils either fell in one of the winter viscosities or in one of the summer viscosities. These products were monograde oils and car owners had to change oil at least twice a year: once for the winter and once for the summer. Thanks to the synthetic base oils and the viscosity modifier additives nowadays most motor oils are so-called multigrade oils that don t change their viscosity with temperature that much, so they meet the limits of a winter and that of a summer viscosity grade at the same time. These oils can be used regardless of season. Monograde oils are still used for special purposes but usually not as engine lubricants. What is the best motor oil viscosity? There is no single best motor oil viscosity. As a rule of thumb always follow the car manufacturer s recommendations. Older cars are usually ok with a 10W-30 oil, while newer cars usually like a lower viscosity oil, like 5W-30, 0W-30 or sometimes even 0W-20. It s important to keep in mind that viscosity is not the only important property of the oil: if there is any special specification (e.g. API SN or GM Dexos 2) required by the car manufacturer then the oil should meet that specification and should also be the correct viscosity in order to be used for that particular vehicle.

2 What is the viscosity index? Viscosity (actual viscosity, not the viscosity grade) changes with temperature. The hotter it is the lower is the viscosity, the colder it is the higher is the viscosity. However, the degree of this temperature-related change of viscosity is not the same in case of every oil. Some oils change their viscosity more when temperature changes some change less. Those that change less have a higher viscosity index, those that change more have a lower one. (Remember the discussion about the monograde and multigrade oils before? Monograde oils viscosities change more as temperature changes multigrade oils change less so multigrade oils have higher viscosity indices.) We like oils with higher viscosity indices more because it means that the oil is more stable viscosity-wise. For lubrication purposes there is an optimal viscosity and the less the oil deviates from this optimum, the better. Synthetic oils have higher viscosity indices making them superior to the mineral oils in this regard as well. Why change the motor oil from time to time? The motor oil costs only a small fraction of the total amount that you spend on your car but choosing the right oil can save you large sums. Low quality or overused motor oil is not capable of fulfilling its duties and can cause a series of problems. We are going to introduce the most important motor oil related problems, their usual causes and consequences. Deposit formation Possible reasons: additive depletion or contaminated motor oil. Possible consequences: pre-ignition, reduced power, higher emissions. Wear Possible reasons: abrasive physical particles in the motor oil, additive depletion, motor oil contamination or too low motor oil level. Possible consequences: engine component failure or engine breakdown. Motor Oil Viscosity Increase Possible reasons: additive depletion, motor oil oxidation and motor oil contamination. Possible consequences: motor oil circulation problems, wear of critical engine components, mechanical problems. Motor Oil Thermal Breakdown Possible reasons: additive depletion, motor oil oxidation, abnormally high engine temperature. Possible consequences: motor oil thickening, oil starvation, cold start problems, engine failure. Motor Oil Circulation Problems Possible reasons: motor oil pump malfunction, clogged oil passages, too low oil level. Possible consequences: low motor oil pressure, wear of critical engine components, mechanical problems. How to Avoid These Problems?

3 These problems can be avoided by using quality motor oils and by respecting the factory recommended oil drain intervals. A quality motor oil contains all the additives that are required to prevent these problems so there is no need for aftermarket additives, for engine flush or for changing the oil more frequently than recommended by the OEM. An exception from the last rule is when the operating conditions are tougher than normal, since those conditions require the shortening of the oil drain interval. What Are The Reasons of Oil Consumption? One of the dreaded things car owner face is oil consumption. For some people the only time when they have to do anything motor oil related is the time when they find out that its level is too low so it needs to be topped up. That s assuming that the regular oil change is done by the repair shop and not the owner. It s easy to think that the oil consumption is the oil s fault especially if you didn t notice it until the last oil change. The truth however is not so simple. Oil consumption has many reasons. Let s go through the most common ones quickly: Engine design. Some engines consume oil by design. From day 1. The consumption can be as high as 1 liter per 1000 kms (cca. 1.7 quarts per mile). This depends on the make and model of the engine so if oil consumption matters to you a lot then you are better off asking questions on the forums about it. Driving style. RPM affects oil consumption. The higher your usual RPM is the more oil will be consumed. That s because you put extra pressure on the seals and gaskets and some of the oil finds its way around and gets burned away in the combustion chamber. Seal and gasket condition. The more worn they are the more oil will be consumed. What happens is the same thing that I mentioned in the previous section: the oil finds its way around the seals and it gets burned away in the combustion chamber or it simply leaks out. Higher-than-normal engine temperature. The hotter your engine runs the thinner your oil will be. Once again it will be simpler for a thin oil to reach the combustion chamber and get mixed and burned with the fuel. Too-low viscosity motor oil. If you use an oil with a viscosity below the recommended by the engine manufacturer than you will experience oil consumption for the same reason as presented in the previous section. By viscosity I do not mean SAE viscosity only. The HTHS viscosity also counts. For example you could experience higher oil consumption with lower HTHS viscosity ACEA A1/B1 oil than with a higher HTHS viscosity ACEA A3/B3 oil. Low quality motor oil with high volatility. I mention this as the last one because this is the least common reason. If the oil s volatility is very high than it could be noticeable in the oil consumption rate.

4 There are also numerous other reasons including but not limited to high oil level, the damage or clogging of different engine parts and the fuel contamination of the oil. Ok, so these were the reasons, let s see what you can and can t do about it: This is the most important: do not automatically try a higher viscosity oil to combat oil consumption. First consider the following: o Higher viscosity means worse cold temperature performance, slower oil circulation, higher oil pressure, less heat transfer and sometimes, if the oil passages are thin worse lubrication even on operating temperature. o This does not mean that the lower the motor oil s viscosity is the better the oil. It just means that you should always stay within the range specified by the manufacturer and never use a thicker oil than allowed only to decrease oil consumption. Think about it: you could put grease in the engine instead of oil and there would be no oil consumption but your engine would fail in seconds or minutes. So no oil consumption does not mean better engine lubrication nor longer engine life. You could have your engine checked for worn seals and gaskets especially if it s a high mileage engine. If you had no oil consumption problems when your engine was new and now you have since it s old then this is the most probable cure for your engine. You could use an engine oil stop leak additive to reduce oil consumption but this should only be a temporary solution because it doesn t treat the underlying cause of the consumption it just hides it. You could change the oil to a fully synthetic type if you are not using such oil already. Synthetic oils have a lower volatility so this can also have an impact on the oil consumption. Change your driving-style to stay between 2-2.5k RPM most of the time. You will save oil and fuel at the same time. If you can t find the reason or the reason is none of the above then take it to a mechanic instead of getting creative and trying unorthodox measures, because doing so can cause more harm than good. Lubrication glossary

5 Additive A compound mixed to the base oil in order to modify its properties or performance. Base Oil Usually refined crude oil fraction or selected synthetic base stock. Additives are mixed to them during lubricant manufacturing. Total Base Number (TBN) The amount of acid required to neutralize the lubricant s basicity expressed in KOH equivalent. Detergent Additive An additive mixed to fuel and lubricants in order to keep the engine clean. The most used detergent additives in motor oils are metallic soaps that have enough base reserves to neutralize the acids formed during engine operation. Pour Point Indicates the fluidity of the oil or fuel on low temperature. It s the lowest temperature on which the fluid still flows. Dispersant Additive An additive, which keeps the solid contaminants in colloid suspension, thus prevents the oil sludge and varnish buildup on the engine parts. This additive is usually non metallic (ashless) and it s used together with detergent additives. DPF (Diesel Particulate Filter) The DPF is a physical filter that reduces the solid particle content of the exhaust gases. Ring Sticking Ring sticking occurs when the piston ring sticks into the groove due to excess contamination. EGR (Exhaust Gas Recirculation) A system designed to reduce the NOx emission. It recirculates the exhaust gas back into the intake manifold thus diluting the fuel/air mixture and reducing the temperature and NOx formation. Pre-ignition Pre-ignition is the combustion of the fuel/air mixture that occurs in internal combustions engines before the spark plug fires. Typical reasons are the hot fuel or the lubricant buildup in the combustion chamber. It reduces engine power and it can damage the engine. EP Additive Lubricant additive that prevents the seizing of the sliding surfaces under extreme pressure conditions.

6 Refining A series of processes through which the crude oil is converted into a petroleum product. Such processes include: thermal cracking, catalytic cracking, polymerization, alkylation, reforming, hydrocracking, hydroforming, hydrogenation, hydrogen treating, solvent extraction, dewaxing, de-oiling, acid treating, clay filtration and deasphalting. Anti foam additive An additive to reduce the foaming of petroleum products. Can be silicone to destroy the surface bubbles or polymer to reduce the count of small internal bubbles. Ash Metallic residue formed due to the high temperature in the combustion chamber or on other engine parts. Hydrofinishing Hydrogen treatment of crude oil in order to saturate the molecules and increase stability. HTHS (High Temperature High Shear) Viscosity A measure of a fluid s resistance to flow under conditions resembling highlyloaded journal bearings in fired internal combustion engines, typically 1 million per 150 C. Catalytic Converters Oxidizing catalytic converters remove hydrocarbons from the exhaust gas, while reducing catalytic converters reduce the NOx emissions. Both types contain noble metal (platinum, palladium or rhodium), which can be poisoned by lead or phosphorous present in fuel or lubricants. Lubrication The reduction of friction and wear using a friction-reducing layer. This layer can be liquid, solid or plastic. Kinematic Viscosity The fluid s resistance to flow when affected by gravity. Usually measured on 40 C or 100 C. Varnish Thin, insoluble non-wipeable layer on the internal parts of the engine. It can cause the sticking and breakdown of internal moving engine parts. Anti-Wear Additive An additive that creates a thin layer with good adhesive properties in order to prevent metal-to-metal contact at high-load spots. Corrosion Inhibitor An additive that protects the lubricated metal parts from the chemical attacks caused by water or other contaminants.

7 Flash Point Minimum temperature at which a fluid will support instantaneous combustion, but before it will burn continuously. The flash point is an important indicator when determining the fire and explosion hazard of a petroleum product. Oil sludge Thick, dark residue with mayonnaise-like consistency, which builds up on the non-moving inner parts of the engine. Insoluble contaminants in the oil accelerates its buildup. Oxidation Occurs when the petroleum product reacts with oxygen. The process is accelerated by heat, light, metallic catalysts, water, acids or the presence of solid contaminants. Oxidation stability A petroleum product s resistance against oxidation. Affects product life both in storage and in use. Total Acid Number The required amount of KOH to partially or fully neutralize the petroleum product s acidity. Friction The resistance that occurs when one object is moved on another object. The friction depends on the smoothness of the sliding surfaces and on the force used to press them on one another. Synthetic Lubricant Lubricant made of specifically composed chemically reacting substances that has pre-determined properties and characteristics. Pumpability A low-temperature property of the oil, which determines to what extent is the oil capable to flow to the pump on low temperatures. Multigrade Oil A motor or gear oil that meets the requirements of more than one SAE viscosity grade and thus it can be used in a wider temperature range. Tribology Science branch dealing with friction, wear and lubrication. Viscosity A fluid s resistance to flow. Viscosity index The relationship between a fluid s viscosity and temperature. Fluids with a higher viscosity index change their viscosity to a lesser degree due to the change of temperature.

8 Viscosity modifier Lubricant additive, usually a polymer that provides beneficial rheological properties to lubricating oils, such as reducing the tendency of an oil s viscosity to change with temperature.