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Lube basics

Probably no machine puts greater demands on its lubrication system than an automobile engine. Many engine parts, even those away from the direct heat of combustion, operate at temperatures of many hundreds of degrees Fahrenheit. Rotating and reciprocating parts run at close clearances with surface speeds of several hundred feet per minute. These conditions put severe demands on the lubricating ability of motor oil.

Motor oil does six important jobs in an engine:

1. It reduces friction.

2. It serves as a shock absorber to cushion reciprocating and rotating parts.

3. It forms an oil film between piston rings and cylinder walls to help seal the combustion chamber.

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4. It acts as a coolant to remove heat from lubricated parts.

5. It helps to clean the engine of sludge, varnish and dirt.

6. It prevents rust and corrosion.

Most motor oils are petroleum hydrocarbon compounds or mixtures of compounds. Natural petroleum oil, however, does not have all the qualities needed to do all the jobs listed above. To improve the properties of motor oil, refiners mix in many additives, such as:

*Antioxidants, which keep the oil stable at high temperatures to reduce varnish and carbon deposit formation.

*Rust and corrosion preventives, which counteract the harmful effects of water and acids formed by combustion blowby.

Detergent-dispersant additives, which reduce sludge and varnish formation by keeping sludge particles suspended in the oil so they can be removed when the oil and filter are changed. *Foam inhibitors, which reduce foam that's formed by the churning action of moving engine parts.

*Viscosity index (VI) improvers, which maintain uniform viscosity under different operating conditions and extend the viscosity range of an oil. That is, a 10V-40 oil has more VI additives than a 1OW-30 oil. We'll discuss viscosity in detail later.

*Pour point depressants, which coat the paraffin crystals naturally present in oil and keep them from sticking together at low temperatures, so the oil flows more freely at low temperature.

*Antiwear and extreme-- pressure additives, which improve an oil's ability to form a film on close-fitting parts under high pressure.

*Wetting agents, which help oil maintain a lubricating film on high-temperature surfaces.

*Friction-reducing additives, which lower friction between moving parts.

When all of these additives are mixed into a quart of motor oil, they often constitute a larger percentage of the finished product than the petroleum oil base stock.

Motor Oil Classifications

All engines have the same basic lubrication needs, but a diesel truck engine operates under different conditions than a passenger car gasoline engine. Similarly, a late-model gasoline engine has different lubrication requirements than one built 30 years ago, and an engine operated in Arizona has different needs than one operated in Alaska. Oil companies, therefore, make different kinds of oil for the specific needs of different engines and their operating environments. The two classification systems for motor oil are SAE Viscosity ratings and API service classifications.

SAE Viscosity Ratings

The Society; of Automotive Engineers (SAE) established the viscosity grading system that indicates how easily an oil flows at a specific temperature. The SAE viscosity grade numbers for motor oil are WV, 5W, 10WV, 15, 20W, 25W, 20, 30, 40, 50 and 60. Low-viscosity (thin) oils have low numbers and flow easily. High-viscosity (thick) oils have high numbers and do not flow as easily.

Oils tested for viscosity and or multiviscosity, oils. Oils with only one viscosity grade are single-grade, or single-viscosity, oils. Almost all oil companies market single-grade oils with one high-temperature viscosity number, such as SAE 30 or SAE 40. No company currently markets an oil with a single low-- temperature viscosity, such as 20W.

API Service Classifications

The American Petroleum Institute (API) has standardized a series of service classifications for motor oils. An oil is run in laboratory engines to determine its resistance to wear, oxidation, corrosion and deposit formation, as well as its overall ability to lubricate. Oils for gasoline engines have service classifications beginning with an "S" (for Service) and followed by a letter "A" through "L" to indicate increasing performance levels. Oils for diesel engines have service classifications beginning with "C" (for Commercial) and followed by a letter "A" through "I." Classifications CG, CH and CI are followed by the number 4: CG-4, CH-4 and CI-4. The "S" and "C" prefixes of the service classifications do not stand for "spark" (gasoline engines) and "compression" (diesel engines), as is sometimes thought.

Service classifications SA and SB never were suitable for automobile engines. You may find bottles of SA and SB oil on grocery store shelves, but these classifications are barely suitable for lubricating hedge clippers. Similarly, SC through SG oils have been superseded for use in modern gasoline engines. Oils for late-model gasoline engines carry service classifications SH, SJ, SL and the newer GF-1, GF-2 and GF-3. Service classifications SH (1996 and older), SJ (2000 and older) and SL (2001 and later) can be substituted for SF and SG oils. Some SL oils also meet the GF-3 energy conserving requirements. We'll cover GF ratings in more detail later.

Automotive Industry

Lube basics

Motor, Nov 2002 by Layne, Ken

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