Colin, You Asked for It
Colin, you asked about oil grades and the relative merits of synthetic lubricants compared to “natural” oil. Well, not to bore all you guys and gals out there, here is a brief summary.
Natural petroleum is a mixture of many different hydrocarbon molecules, most of which are in long chains with from 18 to 34 carbon atoms per molecule. Specifications for oils (and fuels such as gasoline and Jet A) are in terms of physical parameters (e.g., boiling point, vapor pressure, flash point, viscosity, etc.) and not in terms of chemical constituency. Therefore, natural lubricating oils may vary in makeup and performance from product to product. The SAE established and maintains the standards for lubricating oils.
The numbers such as 10W-30 refer to the viscosity of the lubricant. The lower the number, the less viscous the lubricant. Viscosity is measured in units called centistokes but for our purposes we can think of it as “thickness”. Molasses is “thick” (high viscosity) and ethanol is “thin” (low viscosity). In the simplest method viscosity is measured by heating a lubricant to a specified temperature and measuring how quickly it flows through a standard hole. The actual current method employs a cold crank simulator but produces similar numbers.
The viscosity of a lubricant is a strong function of the temperature of the lubricant. Most lubricants become less viscous as their temperatures increase. Multi-grade capabilities are obtained by mixing lubricants of different grades. The two numbers in the rating for motor oil lubricants refer to a low temperature viscosity and a high temperature viscosity. For example, a multi-grade lubricant with the number 10W-30 has a viscosity at -13 deg F equal to that of a lubricant with a rating of 10 and a viscosity at 212 deg F equal to that of a lubricant with a rating of 30. The “W” in the rating means that the lower number is the “winter” grade. In simple terms, the lubricant acts like a “thin” one at low temperatures and a “thick” one at high temperatures.
Newer automobile engines have very small clearances between the parts of their journal bearings (e.g., rod to crankshaft) and need “thin” oils to be able to get into the tight spots. Most wear on a journal bearing is experienced during either startup when the only oil present is the remnants of a film from previous running or during oil starvation. In both cases, the bearing has metal-to-metal contact rather than riding on a film of oil. Similar considerations apply for lubrication of sliding surfaces such as cams and valve stems (for OHC engines). Lubrication of a roller or ball bearing device follows the same principles but is somewhat more complex. A low viscosity lubricant provides better startup protection because of its ability to enter the bearing while a higher viscosity lubricant provides better running protection because it doesn’t “thin out” and become too thin to provide a suitable lubricant film.
You also asked if synthetics are better than petroleum lubricants. The answer depends on the engine and how it is used. If most of the driving is short, low-speed jaunts then petroleum-based lubricants are fine. If, however, the engine is driven on long, high-speed trips then synthetic lubricants are probably better. The major advantage of a synthetic is that it lasts longer before the molecules in it break down. This is primarily because the molecules in synthetics are shorter chains than petroleum-based lubricants. Of course, the temperature of the oil is the critical parameter and that temperature depends on the design of the engine as well as the way the engine is used. For engines that operate at low oil temperatures, the major reason for oil changes is that the lubricant becomes contaminated with things such as combustion products that “blow by” the rings of the pistons, water that condenses in the lubricant, and chemical reactions with these contaminants in the form of acids that the metals of the engine. For engines that operate at high oil temperatures, the primary reason for oil changes is to replace lubricants that have broken down as described above. In addition to these basic considerations, the number and characteristics of the additives is also an important factor to consider. To discuss the effects of all the additives that are used is beyond the scope of this blog.
Colin, the SAE has an aeronautical division as well as an automobile one and most of my experience has been in the aeronautical world of turbojets, turbofans, and turboprop engines. Bearing temperatures in aero engines tend to be a lot hotter than those in automobile engines (on the order of 350 to 500 deg F) so all the above concerns are heightened in aero engines.
Probably more than you wanted to know but here it is any way. By the way, I did work for Chevrolet many years ago in Warren, Michigan at the Chevrolet Engineering Center. The first 275 HP Corvette engine for the 1964-5 models was certified on my dynamometer in the summer of 1963—but that’s a story for another blog.
Sorry about the boring engineering stuff, boys and girls, but he DID ask.