E. R. Booser
Edited by Lawrence Kren
Makers of most European cars today suggest changing oil every 6,500 to 9,500 miles. Volkswagen and Peugeot plan to extend that figure to 18,000 miles or 24 months for its passenger cars, and Daimler-Benz may debut a "fill-for-life" vehicle by 2010.
Key to the success of such initiatives is better quality motor oils. Current synthetic oils let automobiles run 25,000 miles between oil changes or about five times longer than most conventional types. Despite the advantage, their relatively high cost has limited market share to about 3%. But emerging severe hydrocracked oils cost about half as much as synthetics and last just as long. This has major oil companies scurrying to supply the expected boon. What's in it for them: greater efficiencies and a more environmentally friendly, adaptable process.
Traditional refineries distill petroleum crude into mineral lubricating oils using a process largely unchanged since the early 1900s. Here, distillation separates hydrocarbon molecules by boiling point and their respective molecular size and viscosity. Added solvents selectively extract aromatics and other impurities such as sulfur, nitrogen, and oxygen compounds. These extracts are an environmental concern and are often burned as fuel.
Hydrocracking instead treats base oils with hydrogen rather than solvents. It yields more useful products while eliminating the environmental problems. Moreover, hydrocrackers can use less-expensive petroleum crude which helps refineries recoup installation costs. Production can readily shift from lubricants to automotive and diesel fuels that meet more demanding emissions regulations. And by-products from lube-oil production serve as high-end fuels for transportation vehicles.
Although hydrocracking may seem recent, it dates back to the 1930s. The first modern hydrocracker — a small 1,000 barrel-per-day unit — was activated by Standard Oil of Calif. in 1959 to produce improved transportation fuels. By 1972, Gulf Oil had built three severe hydrocrackers for lubricant production, but further installations by Gulf and others got sidetracked as production focused on environmentally improved gasolines and diesel fuels.
More recently, Chevron has become a major developer and operator of high-pressure hydroprocessors and has licensed the technology to Petro-Canada and Conoco. These large-scale installations cost about a half-billion dollars each and pose daunting engineering challenges. For example, catalytic reactors and some support hardware run at temperatures to 900°F and pressures to 3,000 psi. Steady improvements to piston pumps, catalysts, and other components exposed to the severe conditions help make the operations more reliable.
The hydrocracking process
Mineral oils derived from petroleum have three basic hydrocarbon structures: paraffins, cycloparaffins, and aromatics. Most desirable are paraffins in which hydrogen atoms completely saturate carbon-atom chains. These hydrogen-saturated paraffin chains can also form ring structures or cycloparaffins. The third type has a six-member aromatic ring structure with an unsaturated carbon-atom skeleton.
The high temperatures and pressures of the hydrocracking process destabilize hydrocarbon structures in crude petroleum. This leads first to hydrogen saturation of aromatic rings to form cycloparaffins, and finally to opening of cycloparaffin ringsto give chain-structured hydrocarbons.
So-called Group II base stocks are processed only to where most aromatic hydrocarbons are eliminated along with sulfur, nitrogen, and oxygen impurities. The remaining cycloparaffins provide sufficient solubility for most conventional additives. Group III base stocks undergo even more severe hydrocracking, nearly completing the conversion to branched-chain paraffins. Group III oils provides longer service life and a viscosity less sensitive to temperature changes as indicated by a very high viscosity index (VHVI – above 120).
Oil's worst enemy
Over time, oil reacts with dissolved atmospheric oxygen and breaks down or oxidizes. Oxidation starts a chain reaction that first forms hydroperoxides then progresses to other oxidation products, all of which increase acidity, viscosity, darken color, and leave surface deposits. Internal combustion engines that leak combustion gases into the oil (blow-by) accelerate the process. Oxidation-inhibiting additives slow the deterioration over a hundred-fold by eliminating the initial hydroperoxides and by interrupting the chain sequence.
Useful life continues through an induction period as the oxidation inhibitor is slowly consumed. Oxidation depends strongly on temperature as life generally halves for every 10°C (18°F) above normal operating temperatures. Although additional inhibitor delays life-ending oil breakdown, slow accumulation of oxidation products and contaminants such as wear particles and soot eventually signal an oil change.
The good news is, oils with a paraffinic molecular structure stave off oxidation. Hydrocracked oils have such a structure and last about three times longer than traditional solvent-refined oils at a given operating temperature. One reason is the absence of aromatics. While traditional solvent-refined base stocks contain up to 35% aromatic hydrocarbons, severely hydrocracked oils are often clear and colorless with less than 0.5% aromatics. This helps minimize varnish, dark deposits, insoluble sludge, and other problems that shorten oil life. The composition works in a variety of applications including premium turbine oils (used in gas and steam turbines, electric motors, compressors, and a wide variety of rotating machinery), high-pressure hydraulic oils, and extreme-pressure gear oils.
Measuring oil life
Laboratory bench tests are the traditional method to evaluate oxidation life. For example, the Turbine Oil Stability Test (TOST - ASTM D943) bubbles oxygen through an oil sample raised to 95°C and in contact with water and metal catalysts. But TOSTs can take several thousand hours to complete because of better base oils and additives. A more aggressive Rotary Bomb Oxidation Test (RBOT - ASTM D2272) boosts pressure to 100 psi and temperatures to 150°C. Both tests measure the initial induction period involving only slow oxidation. This induction period typically precedes much more rapid oxidation as measured by elevated oil acidity (TOST) or drop in oxygen pressure (RBOT).
In addition to these tests, automotive engine oils are put through accelerated wear and life evaluations in test engines. From here come standards including the new GF-3 motor oil classification. GF-3 is scheduled for adoption by the International Lubricant Standardization and Approval Committee, and then by the API as Service Classification SL. Oils meeting the new standard (Group II hydrocracked oils for instance) are less volatile which helps cut airborne components. They also have a relatively lower viscosity to improve gas mileage and cold-weather starting. Best of all, Group II oils extend time between oil changes and reduce wear of engine valve train and piston ring surfaces. Diesel engines get the new PC-9 classification which also relies heavily on hydrocracked base stocks.
Several lubricant suppliers in North America and Europe are marketing "synthetic" engine oils compounded with even more severely hydrocracked Group III base stocks. These oils are said to be equivalent to polyalphaolefin synthetics such as Mobil
1. VHVI oils should make major inroads as automakers push for reduced maintenance and longer oil-change intervals.
Limitations and fixes
The new Group II and Group III oils aren't without problems, however. One drawback is a lower solubility to additives. Automotive engine oils are particularly challenging because they contain a relatively large amount of oxidation inhibitors, antiwear additives, and detergents.
Additive separation can be a problem in some high-speed compressors. Centrifuging action in bearings slings additives outward, likely because of incomplete solubility in the base oil. One way to boost additive solubility is to blend Group III base stocks with synthetic ester fluid. Similar results are possible by adding Group I solvent-refined oils to Group II hydrocracked base stocks, but at the expense of potential longer life.
Paraffinic-hydrocracked oils also have different solvent properties that may compromise some gaskets, seals, and nonmetallic components. For instance, such oils can displace plasticizers in elastomers and plastic parts, possibly changing their dimensions and mechanical properties.
Although compatible replacements are typically available for new designs, existing diesel engines, automotive components, gear units, and other machines can encounter problems. The new oils also separate water more rapidly which may require modified lubrication systems. Improved filtration may also be needed to avoid build-up of contaminants and wear particles between longer oil-change intervals. All these issues need addressing if hydrocracked oils are to deliver on their promise of longer life at a lower cost.
Key benefits of hydrocracked lubricants
• Improves action of antioxidant additives.
• Lets some applications run at higher temperatures.
• Less carbon formation and fewer deposits.
• Improved low temperature fluidity.
• Separates water and releases foam more efficiently.
• Lower oil volatility for reduced emissions.
• Biodegrades faster.
• Lower impurities and toxicity make them suitable for use in some cosmetics and pharmaceuticals.
Background information for this article comes from Conoco, ExxonMobil, General Electric, and Petro-Canada. A source of further information on lubricant properties and performance in bearings is a new book by M. M. Khonsari and E. R. Booser, Applied Tribology: Bearing Design and Lubrication, John Wiley & Sons, New York. It's available at www.amazon.com