Synthetic lubricants can be cost-effective alternatives to traditional mineral oils in off-highway engine applications if the performance benefits outweigh the higher lubricant cost. Before making a decision to switch from mineral oil-based lubricants to synthetics, equipment operators should evaluate the operating condition of their engines, the physical environment in which they operate, the factors that condemn the oil - oil degradation or accumulation of contaminants - and conduct a cost/benefit analysis to establish target drain intervals for the synthetic lubricant. Regardless of the choice of lubricant types - synthetic or mineral - equipment operators should monitor oil condition with a quality-monitoring program to establish the optimum drain interval.
Off-highway operations are characterized by the following:
Engine oils have changed considerably over the last ten years. A major advance is greater availability of new, more highly refined base-stocks that significantly enhance lubricant performance under more demanding operating requirements. The American Petroleum Institute (API) identified five base oil categories (Table 1) that are used with the API Base Oil Interchange Guidelines to help assure that engine oil performance is not affected adversely when one basestock is substituted for another in a finished oil formulation.
Historically, refiners made basestock by solvent refining and dewaxing selected crude oil fractions (Group I). With the development of hydrotreating and hydrocracking technology, refiners introduced highly refined, low aromatics, low wax basestocks with improved oxidation stability in large volume (Group II).
More recently, higher viscosity index basestocks made by high severity hydrocracking of petroleum fractions have become available (Group III). Group III stocks differ from Group II products in the structure of the lube oil molecules that impart the higher viscosity index. Group III basestocks are limited to lower viscosities, typically, 4 to 7 cSt at 100°C.
API identified polyalphaolefins (PAOs) as a special class of basestock. PAOs are made by a chemical process and have the characteristics of uniform composition, very high oxidation stability, high viscosity index and no waxy molecules. By adjusting the manufacturing process, PAOs can be made in a wide range of viscosities, commonly from 4 cSt to 100 cSt at 100°C. For many years, PAOs and esters (Group V) were the only available premium basestocks for engine lubricants operating under extreme temperature and conditions.
Most synthetic engine oils made with PAO now contain a small amount of ester to give the basestock the same solvency power as typical mineral oils. This helps to assure that the synthetic lubricant will have the same seal swell characteristics as conventional oils.
Table 2 lists some typical properties of Group I through Group IV basestocks with similar viscosities at 100°C.
Critical base oil properties improve from Group I to Group IV. Also, less viscosity index improver (polymer) is required in multigrade engine oils with the higher VI basestocks. This leads to improved shear stability (Stay-In-Grade) and fewer deposits from polymer degradation.
Basestocks that have been severely hydrogenated to remove almost all aromatics (Group II and III) and chemically manufactured, 100 percent paraffinic PAOs (Group IV) have an added advantage in boosting the performance of dispersants in the additive package of fully formulated oils to hold soot in finely divided suspension in the oil. Soot forms in engine lubricating oil when fuel is not completely burned. If the soot is not properly dispersed, viscosity increase due to soot thickening can shorten the useful life of the oil. Also, soot particles can clump together and form deposits in critical parts of the engine. 1
The main barrier to using better performing Group III and Group IV basestocks is cost, Table 3. Group IV (PAO) basestocks used in conjunction with esters (Group V) are more expensive than other basestocks derived from crude oil. The decision to use engine oils made with PAO/ester blends has to be based on a cost/benefit analysis to justify the higher price of the lubricant against the anticipated benefits.
Definition of Synthetic Lubricants - Buyer Beware
For many years, PAOs and other chemically synthesized basestocks were the only lubricant products that could be advertised as synthetic. The National Advertising Division of the Better Business Bureau broadened the definition of synthetic lubricants to include products made with Group III basestocks in a 1999 ruling. This created confusion in the marketplace as to what the customer was actually buying when he specified “synthetic motor oil.” 2
Some lubricant marketers also promote semisynthetic engine oils that are a blend of synthetic basestock and conventional mineral oils. Semisynthetic engine oils could have as little as 10 to 20 percent synthetic in the formulation. These products can be purchased at a lower cost than full synthetics. Performance features, especially with respect to low temperature flow, high temperature evaporative loss and oxidation stability, are generally inferior to full synthetics, although they may be perfectly adequate for many applications. There is no requirement for oil marketers to specify the amount of synthetic basestock in finished semisynthetic oil, or to state whether the synthetic component is a Group III or Group IV base oil.
The point here is to be sure that the performance features are understood when purchasing a synthetic engine oil. Ask the oil marketer for product data sheets, test results, especially data that shows lubricant performance under stressed conditions, and field test reports in equipment that is relevant for your operations.
Why Use Synthetics (PAO/Ester) in Off-Highway Crankcase Applications?
Synthetic crankcase lubricants (PAO/ester) offer the following potential advantages over mineral oil-based products:
Maintenance engineers are concerned with protecting the investment in equipment and keeping it running economically. Synthetics offer an opportunity of extended drain intervals and longer time to overhaul, both of which may lead to lower maintenance costs. Balanced against this are the higher cost of oil, and possibly air and oil filters.
Under extreme temperature conditions, synthetics (PAO/ester) offer the only practical choice of lubricant because of the superior low temperature flow properties and low evaporative loss at high temperatures. If mineral oils are used in extremely cold weather climates, engines need to run continuously or heat must be applied to engines and oil sumps prior to startup. If equipment is operating in remote, relatively inaccessible locations, where the cost of equipment repairs is extremely high, synthetics provide a measure of insurance even if normal drain intervals, established with mineral oils, are maintained.
But for most equipment operators, the decision to switch to synthetic lubricants is based on straightforward economics. To offset the higher lubricant cost, operators need to see reduced maintenance costs that come from longer drain intervals or documented evidence that time to overhaul is extended. A simple calculation provides a first pass target to determine drain intervals with synthetic lubricants that give equivalent lubricant life cost with mineral oils (Figure 1).
For example, assume a sump size of 50 gallons, 2 gallons makeup oil, 4 hours downtime for maintenance, $50 per hour for maintenance, $100 per hour gross profit on the equipment, 50 gallons waste oil, $0.25 per gallon disposal cost, and a 1000 hour drain interval for mineral oil. Assume also that mineral oil costs $3.50 per gallon and synthetic lubricant costs $12.00 per gallon. The target drain interval for break-even economics with the synthetic lubricant is shown in Figure 2.
If maintenance is conducted on off-shifts, when equipment is not normally operated, lost operating profit on the equipment for maintenance downtime is zero, and the calculated break-even drain interval for the synthetic oil increases to 2120 hours.
Also, if oil consumption is high from leaks through auxiliary equipment or past the rings and valve guides into the combustion zone, the break-even calculated drain interval for synthetic lubricants increases relative to the drain interval for mineral oil.
Condemning Limits - Oil Deterioration or External Factors
The next step is to determine if the oil change interval is based on oil deterioration or external factors. Condemning limits based on oil quality include the following:
Base number depletion, soot loading and associated viscosity change, and accumulation of wear metals in the oil may not change significantly when a synthetic lubricant replaces a mineral oil. However, at high soot loadings, synthetic engine oils offer improved soot-handling performance over mineral oils with lower viscosity increase and fewer deposits at equivalent soot loading.
Burning high sulfur fuel generates corrosive acids that will accelerate base number (BN) depletion. Operating under high loads, lugging and low air:fuel ratios may increase soot loading in the oil. Abrasives in the oil that are not captured by filtration can accelerate the generation of wear metals. See reference No. 1 for an excellent discussion of factors that establish oil drain intervals.
External factors that contribute to deterioration of the oil and reduced drain intervals include:
In the wet, dusty or dirty environments frequently encountered in off-highway applications, oil may need to be replaced before the oil reaches its condemning limit based on oil decomposition and/or additive depletion. To counter unfavorable environmental factors, operators may need to install more efficient air and oil filters and change them out more frequently to take full advantage of the inherently higher quality of the synthetic lubricant. These costs need to be factored into the equation for equipment maintenance. If the cost of maintaining cleanliness of the synthetic engine oil is too high, it may be preferable to use less expensive mineral oil and change it out more frequently.
Regardless of the calculated choice to use a synthetic or mineral oil in off-highway engines, equipment operators should protect their investment with a high quality lubricant maintenance program, set oil condemning limits initially based on manufacturer’s recommendations and establish good maintenance practices. Data from an oil maintenance program provides the quantitative basis to establish “real world” oil change intervals, assess the operating condition of each piece of equipment and determines the economic benefits of replacing a mineral oil with a synthetic lubricant.