Saturday, November 1, 2003
The Mineral Oil Myth
Aircraft piston engines break-in just fine on ashless dispersant oil, so why do engine manufacturers still require break-in using straight mineral, non-ashless dispersant oil?
There is a pervasive myth in the aviation piston-engine world: the myth claims that in order to break-in a new or freshly overhauled engine or cylinder, straight mineral oil must be used during the break-in period. The term "straight mineral oil" refers to oil that does not contain the ashless dispersant chemicals found in AD or ashless dispersant oil.
Every mechanic has been told from the beginning of time that engines must be broken in on mineral oil. The reasoning behind this claim is that when an engine is run for the first time, it is necessary for the piston rings to seat themselves to the cylinder walls. Properly seated rings provide a fit that keeps compressions high and oil consumption low. Mineral oil, it is claimed, allows seating to occur because it isn’t so slippery that rings won’t be able to wear off the microscopic metal mountain peaks that populate the material that makes up the steel cylinder walls. This part is true. The rings do have to seat, by wearing smooth the metal peaks.
Ashless dispersant oil, the myth says, is too slippery and too good at preventing wear to allow the rings to seat properly. We have all heard this, it gets passed from mechanic to mechanic, and it even sounds kind of logical. Theoretically, AD oil contains some special chemical that gives it more lubricity and greater film strength than straight mineral oil.
But this simply isn’t true.
Unfortunately, this myth is not only passed along by mechanics, but it is also codified in the engine manufacturers’ maintenance data, in the form of service information. Both Lycoming and Continental make the exact same claim and insist that mineral (non-ashless dispersant) oil be used for break-in. Many engine overhaulers adhere to the OEMs’ advice and even state that their warranties are void if mineral oil isn’t used for break-in.
No matter what the OEMs and those overhaulers say, however, break-in occurs perfectly well with non-synthetic ashless dispersant oil. There is no scientific reason why this industry should continue to require operators to run mineral oil in a new or overhauled engine then have to switch later to ashless dispersant oil.
Any non-synthetic oil, even a multigrade, is mineral oil by default. The base oil is 100-percent mineral oil. So a typical non-synthetic aviation ashless dispersant oil is mineral oil. But it does contain the ashless dispersant chemical package. What is in this chemical package?
The ashless dispersant package consists of molecules with polar heads and nonpolar tails, according to a Phillips 66 lubrication specialist. These molecules surround dirty carbon and soot byproducts of combustion and suspend them in the oil. Without the ashless dispersant, these particles tend to group together and precipitate onto the guts of the engine. With the ashless dispersant, fresh oil quickly turns black after running for a few hours because the nasty combustion byproducts are suspended in the oil. This chemical has nothing to do with wear characteristics or capability (or, as the Phillips 66 specialist explained, lubricity and film strength). The presence or absence of the ashless dispersant package does not in any way affect the oil’s ability to retard or promote the seating of the piston rings to the cylinder walls. Another component of the chemical package, which makes up 5 to 10 percent of a typical quart of AD, oil is an antioxidant.
All that the ashless dispersant package does is pull undesireable combustion byproducts into suspension in the oil so that they get drained out when the oil is drained.
There might be a slight difference between wear characteristics of Shell’s straight mineral oil and ashless dispersant oil, according to Paul Royko, aviation technical manager for AeroShell Oil. In AeroShell ashless dispersant oil, some corrosion inhibitor chemicals have "surface-action chemistry," he explained, where chemicals can plate out on the metal surface and act as "sacrificial" layers. The sacrificial layers will be used up before metal-to-metal wear occurs, but the surface-action chemistry will not crowd out the actual oil itself, he added. The additive that Lycoming recommends for the H-series O-320–LW16702–is a good example of an antiscuffing surface-action chemistry agent. AeroShell’s Plus oils and its semi-synthetic 15W50 include the antiscuffing agent required by Lycoming for those engines, as do other oils such as BP’s new Castrol single-grade aviation oils.
But where does AeroShell come down on the issue of using ashelss dispersant oil for break-in? "I can’t say go ahead and do it," Royko said. "If you’ve done it and it works, that’s your business. We go with the recommendations of the engine builders. People have used W100 [AeroShell’s ashless dispersant oil] and had no problems."
Richard Fowler, president of America’s Aircraft Engines in Tulsa, Oklahoma, starts all of his overhauled engines on Phillips 20W50 X/C multigrade ashless dispersant oil and he has never had a break-in problem. Fowler overhauls about 250 small and large Lycomings and Continentals per year and he has been using ashless dispersant Phillips oil for break-in for the past 10 years. "It’s working fantastic," he said.
Why do Lycoming and Continental continue to insist that non-AD oil be used for break-in?
From Lycoming’s Service Instruction No. 1014M: "All other engines must be operated on mineral oil during the first 50 hours of operation, or until oil consumption has stabilized. If an ashless dispersant oil is used in a new engine, or a newly overhauled engine, high oil consumption might possibly be experienced. The additives in some of these ashless dispersant oils may retard the break-in of the piston rings and cylinder walls."
Yet, oddly, Lycoming goes on to state that: "All turbocharged engines must be broken-in and operated with ashless dispersant oil only." And in the same instruction: "O-320-H; O/LO-360-E series engines may be operated using either straight mineral oil or ashless oil."
Is there some different or special kind of cylinder wall metal used in turbocharged engines or the above two normally aspirated engines? The cylinders are all made of the same materials. So why does Lycoming allow ashless dispersant oil to be used on those engines?
Lycoming engineer Rick Moffett responded to these questions: "Many of our customers use ashless dispersant oils for break-in of their engines with factory approval, particularly turbocharged powerplants.
"Part of the basis for the present recommendation can also be ‘that’s the way our ancestors did it’ and it takes some effort to change it. Lycoming has had experience with several of our OEM customers that used AD oil for break-in and experienced some oil consumption problems due to glazed barrels. Whether or not they would have had the same number of problems with mineral oil is open to speculation. They solved the problem by operating at higher power to accelerate break-in.
"Lycoming specifies the use of AD oil on turbocharged engines and the 320-H and 360-E because we do not feel that they need any help with break-in, if in fact there is a difference in the break-in characteristics between the two oil types.
"It would be extremely difficult to set up a program that would decide the advantages of one oil type recommendation over the other. The sample size would involve many engines, many engine models, and many measurements. Most likely there is not much difference between the two, which is probably what the results would tell you. In the end, Lycoming cares less about what oil type is used, but rather that the results are acceptable. If a customer uses ashless dispersant oil, has satisfactory results, good day and happy flying. If a customer repeatedly has problems effecting a good break-in with AD oil, most likely they will get a recommendation to switch to mineral to see if it would improve the situation."
Teledyne Continental continues to recommend the use of mineral, non-ashless dispersant oil for break-in, according to an engine-break-in statement on TCM’s web site. "A straight-weight, non-dispersant mineral oil ...is recommended for the break-in period," the TCM statement says.