Imagine the flexibility of not having to change your engine oil so frequently. Not only will this have an impact on your pocket book, but also there will be an environmental impact as there will be millions of gallons less used oil requiring processing. It is no wonder that extended oil change periods have become an urgent priority. Automotive manufacturers are aiming for one year for initial servicing. Builders and users of hydraulic equipment, turbines, rotary compressors, gear units, and electric motors and generators are also looking forward to longer service without relubrication.
The possibility for longer-life automobile engine oils emerged over the past several decades with availability of polyalphaolefin (PAO) synthetics. These oils come with 25,000-mile suggested oil changes in passenger cars. For industrial machinery, water-white medicinal-quality mineral oil base stock was used by Amoco in formulating its Rykon turbine oils for extended life expectancy. Nevertheless, high cost limited the market share of these lubes: currently PAO synthetics amount to only two percent1of the total lube oil production (Table 1).
Table 1. Lubricant Base Oil Production and Relative Cost
But the push for extended oil changes has fueled rapid expansion of refineries producing hydrocracked oils that have similar performance characteristics to those of PAO synthetics, but at a cost that is closer to traditional solvent-refined base stocks.
The three general hydrocarbons present in mineral oils are: paraffinic chains, cycloparaffins (also known as naphthenes) and aromatics. Table 2 lists the general characteristics that these three provide in finished lubricating oils.
Table 2. Characteristics of Hydrocarbon Types
The initial stage of catalytic hydrocracking leads both to hydrogen saturation of aromatic ring structures to produce cycloparaffins as well as to the removal of sulfur and nitrogen impurities to provide nearly water-white Group II base oils. As indicated in Table 5, more severe hydrocracking then opens the ring structures to create Group III base stocks made almost entirely of paraffins. Subsequently, a catalytic dewaxing procedure is used to eliminate straight-chain, high-melting waxes. Finally, hydrofinishing saturates any remaining unstable double bonds in the hydrocarbon chain structures. Table 3 shows the composition, viscosity index and related requirements for the five API oil groups.
Table 3. American Petroleum Institute (API) Base Oil Categories
Steps To Hydrocracking
Since the early 1900s, simple distillation of crude petroleum has been the primary means of separating hydrocarbons based on their boiling point, corresponding molecular size and viscosity. This distillation has commonly been supplemented since the 1930s by solvent extraction for removal of sulfur, nitrogen and oxygen compounds, as well as some condensed ring aromatics.
Following initial refining with hydrogen in the 1930s, Gulf Oil eventually built three severe lubricant hydrocrackers by 1972, but focused on producing environmentally improved gasoline and diesel fuel. It is only in recent years that Chevron took a major step forward by developing high-pressure hydrocracking units for lubricant production and licensed the technology to Petro-Canada and Conoco. Additional hydrocracking units are also being added at refineries in Asia, Europe and the United States.
The severe hydrocracking needed to produce Group II and Group III base stocks involves unusually demanding engineering and material requirements. With hydrogen process temperatures up to 800ºF to 900ºF and pressures up to 3,000 psi, the initial capital cost for a hydrocracker (refinery) can be up to a half-billion dollars and involves severe demands on piston pumps, catalysts and reaction vessels. Despite this high cost, large-scale hydrocracking has rapidly increased Group II lubricant base-stock volume to about half of U.S. lubricant production. While Group III base stocks have found quite limited use in industrial lubricants, they have been increasingly replacing PAOs in synthetic automotive engine oils. This trend will likely accelerate as automobile manufacturers strive for the 25,000 to 35,000 mile, or one-year, oil change interval being advertised by some synthetic oil suppliers.
Increased Life With New Oils
Over time, oil tends to break down by reacting with dissolved atmospheric oxygen. This oxidation starts a chain reaction that first forms hydroperoxides and then progresses to other oxidation products - all of which increase acidity and viscosity, darken color, and leave surface deposits and varnish. By eliminating the initial hydroperoxides and by interrupting the chain sequence, oxidation-inhibiting additives slow this deterioration rate by more than a hundredfold.
Useful life continues through an induction period as the oxidation inhibitor supply is slowly depleted.
Deterioration rate depends strongly on temperature. Although adding an inhibitor delays life-ending breakdown, slow accumulation of oxidation products and contaminants such as wear particles and soot in engine oils eventually signal a need for an oil change.
The good news is that life expectancy is extended with the paraffinic structure of new hydrocracked Group II and Group III oils. Absence of aromatic hydrocarbons gives more effective oxidation inhibitor action, minimizes sludge and varnish deposits, and generally avoids related machinery problems.
Measuring Oil Life
Laboratory bench tests are traditionally used to evaluate oxidation life. For example, the turbine oil stability test (TOST-ASTM D943) bubbles oxygen through an oil sample in contact with water and metal catalysts at 95ºC. Because TOST time takes several thousand hours with better base oils and additives, the more aggressive rotating pressure vessel oxidation test (RPVOT) raises pressure to 90 psi at 150ºC. Both tests measure the length of an initial induction period involving only slow oxidation. This induction period typically precedes much more rapid oxidation as meas ured by increased oil acidity (TOST) or a drop in oxygen pressure (RPVOT).
Typical test lives in Table 4 indicate there is an approximate threefold increase with hydrocracked Group II base stocks compared to Group I for premium turbine grade and hydraulic mineral oils used in turbines, compressors, electric motors and generators, and a wide range of industrial applications.
Table 4. Oxidation Test Life with Solvent-refined Group I
and Hydrocracked Group II Oils
Automotive and diesel engine oils have much more complex requirements for providing wear resistance, ability to disperse wear and combustion products, and tolerance for water - in addition to oxidation resistance. For their life evaluation, candidate oils are put through accelerated tests in actual test engines. From these engine tests come standards such as the GF-3 automotive motor oil classification adopted by the American Petroleum Institute as service classification SL. These requirements have generally been met with oil formulations using at least in part Group II hydroprocessed base stocks and are used with suggested oil change intervals up to 7,500 miles. Even more demands are being set in GF-4 for the future new car market (rollout expected in third or fourth quarter of 2004). Corresponding requirements are being developed for a new PC-10 classification for higher performance diesel engine oils.
Table 5. Steps to Group II/III Base Oils
A growing number of lubricant suppliers in North America and Europe are taking another major step forward in marketing synthetic engine oils compounded with the more severely hydrocracked Group III base stocks with very high viscosity indexes (VHVI) above 120. Promoted as equivalent to PAO synthetic oils, VHVI oils will likely gain major inroads as automobile manufacturers strive for reduced maintenance and with some suggested oil change intervals up to 25,000 to 35,000 miles (one year maximum).
The typical service life increase to be expected with the hydrocracked oils in various applications is given in Table 6.
Table 6. Expected Oil Service Life
The difference in composition also leads to the following advantages for hydrocracked products:
Limitations and Fixes
Despite their longer life, Group II and Group III-based oils have encountered some problems. One drawback is lower solubility of additives. Automotive engine oils are particularly troublesome because they contain large amounts of detergents, antiwear additives and oxidation inhibitors.
Lack of additive solubility can even be a problem with Group II and Group III base stocks in R&O turbine oils because they contain only about 1.0 percent or so of rust and oxidation inhibitors. In high-speed rotary compressors, centrifuging action has separated the additives from solution in the oil … likely because of their incomplete solubility. To avoid this separation, modifications such as blending either with synthetic ester fluid or with Group I solvent-refined oil may be needed, sacrificing the potential for long life.
Altered solvency action with the paraffinic hydrocracked oils may also adversely affect some gaskets, seals, paint and coupling components. While suitable alternative materials are commonly available to match the needs of Group II and Group III oils for new machine designs, trouble could still be encountered in older diesel engines, gear units and other existing machines.
Upper viscosity is limited in hydrocracked base stocks, especially in Group III, by the reduction and even elimination of both aromatic and cycloparaffin ring structures that otherwise would increase internal flow resistance over that with simple hydrocarbon chains. While cycloparaffins remain in Group II, overall molecular size and structure of the remaining hydrocarbons limit these base stocks to ISO viscosity grades below about 320 to 460 cSt at 40ºC. Unfortunately, higher viscosity oils are required for severe demands in industrial gearing, large reciprocating compressors, and related applications. Because Group III base stocks are comprised primarily of paraffinic chain structure, their even lower viscosity restricts their normal use in automotive oils to 0W and 5W SAE grades. Higher viscosity grades depend on additions of more viscous PAOs or long-chain polymeric additives, or use of viscous bright stock from traditional petroleum refining.
Unfortunately, filtration has not uniformly improved to match the longer oxidation life available with Group II and Group III oils. Automotive oil filter changes are sometimes recommended based on the oil life now available. Steam turbines at power houses have been shifting to full-flow filtration. Diesel engine practices have been upgrading engine oil filtration to meet needs for long-term removal of soot from the lube system.
Likely next steps for these new oils to deliver on their promise of longer life at low cost include 1) improved filtration to avoid build-up of contaminants and wear particles between longer oil change intervals, 2) modification of additives and base oil blends for improved additive solubility, and 3) matching machine design and maintenance to take full advantage of the improved lube properties.