by Nigel Calder
There is an oft-repeated aphorism that “oil does not wear out.” So why, one may wonder, the insistent clamor from engine manufacturers, and indeed myself, and this magazine, about the need to regularly change engine oil? Well, oil may not wear out, but over time it becomes contaminated and can no longer protect an engine. In fact, the oil becomes a potential source of damage.
This contamination occurs in two different waysandmdash;what I will refer to (decidedly loosely and for want of better terminology) as chemical contamination, and physical contamination.
Chemical contamination both degrades the oil, causing a loss of lubricating properties, and also adds substances that chemically attack engine surfaces. In other words, chemical contamination breaks down the lubricating properties of the oil itself, or leads to the addition of other harmful substances in suspension or solution in the oil. Physical contamination adds abrasive particles that are carried around by the oil, mechanically attacking engine surfaces. The two are interrelated in as much as the less the lubricity of the oil, the more the damage that is likely to be done by abrasive particles, while some of the substances that are carried around in suspension (notably soot) can also be abrasive to a degree (soot, as a matter of fact, doesn’t fit my categories too well!). Nevertheless, it is useful for the purposes of the following discussion to keep the two broad distinctions in mind.
Chemical contamination occurs in normal engine service as a by-product of combustion. It takes a number of different forms. Heat, age, and other natural processes cause oil to oxidize, which, in turn, causes it to thicken and encourages the formation of sludges and varnish. Water is a by-product of combustion; a small amount condenses on cold engine surfaces and ends up in the oil, causing emulsification and reducing its lubricity. (This is frequently a problem with generators and sailboat engines if they are repeatedly operated at low loads and temperatures, or for short periods of time.) Sulfur is found in most diesel fuels and some of this finds its way into the oilandmdash;it mixes with the water to form sulfuric acid (particularly destructive to engine bearing surfaces). Unburned fuel works its way down cylinder walls, diluting the oil and lowering its viscosity. And so on.
The primary physical contaminants consist of metal particles (from internal wear in the engine) and dirt. Usually drawn into the combustion chambers through the air inlet manifold, dirt is then scraped down into the crankcase by the piston rings and goes into the oil. The most damaging part of dirt is silicon, which is extremely hard and abrasive. Soot formed in the combustion chamber is also scraped down into the crankcase and picked up by the oil; it both thickens the oil, lowering its lubricity, and also contributes to abrasive damage.
Chemical contaminants and soot are combatted by specially formulated additive packages that are put into the oil during its manufacture. Anti-oxidation agents, detergents and dispersants keep engines clean, hold soot and other by-products of combustion in suspension, and help prevent increases in the viscosity of the oil, while alkalinity agents neutralize acids. Over time, this additive package is steadily consumed until it fails to counteract one or other of the chemical contaminants. At this point, the oil needs to be changed.Removing contaminants
Physical contaminants are removed on all modern engines by a full-flow, pleated-paper oil filter which is installed between the oil pump and the engine oil galleries, and through which all the oil has to pass before circulating through the engine. Full-flow filters are not nearly as effective as they might be since in order to ensure a free-flow through the filter, the mesh sizes are typically relatively large (30 to 40 microns; one micron equals 0.00004). Microscopic particles below this mesh size pass through the filter and remain in the oil, increasing wear rates and, over time, building up to clearly damaging levels. In the meantime, the filter itself will slowly plug up with macro-particles.
Once a filter is partially plugged, the pressure differential across the filter paper increases, and the oil is driven through the smaller, as yet unplugged, surface area. Since the same amount of oil now must pass through a smaller opening, the flow rate increases through these areas, with the eventual tendency to rupture the filter. Some filters incorporate a pressure relief valve that opens and allows unfiltered oil to enter the engine should the filter becomes plugged beyond a certain point. Other filters simply rupture, flooding the engine with dirt. So, to prevent both a build-up of excessive contaminants in the oil, and also a failed oil filter, both the oil and filter need to be changed at regular intervals.
Thus, there are two standard, and distinct, lines of defense against oil contamination:
1. An additive package in the oil to chemically neutralize or hold in suspension undesirable substances.
2. An oil filter to physically remove undesirable particles larger than a certain size.
In theory, oil could be used indefinitely if, on the chemical side, the pollutants could be removed at the source or the oil cleaned up through a re-refining process and a fresh additive package introduced. And, on the physical side, if all the damaging particles could either be kept out of the oil, or periodically removed. In practice, of course, neither of these conditions are met. In fact, as soon as an engine is restarted after an oil and filter change, the contaminants, particularly the physical contaminants, begin to build up again so that even with proper oil changes the engine is, in effect, always running on “half-dirty” oil. Obviously, this is not the best thing for the engine.
Various studies conducted by filter and engine manufacturers have shown that of the microscopic particles that pass through a normal oil filter, the most damaging are those in the five to 10 micron range, particularly when it comes to piston ring wear. These particles are small enough to work their way between the rings and cylinder walls, or between bearing shells and journals (the surface of a shaft enclosed by bearing shells), and then large enough to do serious damage. Only slightly less harmful are particles from 10 to 20 microns, with those from 2.5 to five microns coming in a close third. Particles from 2.5 down to one micron in size are a distant fourth.
A tremendous amount of research has concentrated on evolving cost-effective methods for oil filtration down to five microns or less. Two approaches have proven both practical and economical: by-pass filters and centrifuges.By-pass filters
We noted above that in order not to place an undue restriction on the flow of oil through the engine, a full-flow filter must have a relatively large mesh size. A by-pass filter is always used in conjunction with such a full-flow filter. The by-pass filter is placed in the supply line to the engine bearings, after the full-flow filter, and simply “bleeds off” some of the excess oil pumped by the oil pump, dumping this oil back into the sump. In order to ensure that a sufficient volume of oil is still going through the engine, a restriction is built into, or downstream of, the by-pass filter, limiting the flow through the by-pass filter.
Since the full-flow filter is supplying all the oil needed by the engine, it doesn’t matter what kind of a restriction the by-pass filter creates in its line, and so mesh sizes can be made as small as desired. In practice, by-pass filters have a mesh that filters particles from 10 microns down to one micron, with a three to five micron filtering capability being the most common.
Typically, no more than 10% of the oil moved by the oil pump passes through a by-pass filter at any given time, but in the space of just a few minutes most of the oil in the engine will have been circulated through the filter, cleaning out the smaller particles not trapped by the full-flow filter. Once the oil is clean, the by-pass filter keeps it clean. The net result is dramatically reduced engine wear rates, and, as a result, considerably longer life between major overhauls.
A by-pass filter can be added to any engine (many large engines have them fitted as standard equipment). A supply line is tapped into the oil gallery at some point downstream of the full-flow filter, and taken to the by-pass filter. The return line is fed back to any non-pressurized point in the oil system (usually the pan). Combined full-flow and by-pass filters are available for some engines. These simply screw right onto the existing full-flow filter housing, with an internal circuit in the filter itself bleeding off part of the flow to the by-pass filter segment.Centrifuges
A centrifuge attacks the same problemandmdash;that of removing microscopic particles from the oil “dash” but from a different direction. The circuitry (plumbing, so to speak) is the same as for a by-pass filter, but instead of having a densely-packed, small-mesh filtering media, the centrifuge directs the by-pass oil through a bowl that is mounted on bearings, to small nozzles on the underside of the bowl. The oil is driven out of these nozzles under pressure from the engine oil pump, causing the assembly to spin at a high speed. The resulting centrifugal effect causes entrained dirt particles to be thrown out of the oil onto the outer housing of the unit, where the particles accumulate in a dense, black, rubbery mat. (I can still remember my amazement almost 20 years ago when I first opened one of these filters and saw a 3/4 inch thick solid cake of material that had come out of the oil.) At each regular oil change, or every second oil change, the outer housing is removed and the cake of compacted dirt is dug out of the housing and discarded.
The size of the nozzles on the turbine, combined with the engine oil pressure, determine the rate of oil flow through a centrifuge. The nozzles must be sized to limit the flow to a level that will not drop the engine oil pressure or starve the full-flow oil filter. If the centrifuge is set above the level of the engine pan, after exiting the nozzles, the oil will drain back into the pan (gravity return), but otherwise, some form of pressurized air is needed to push the oil out of the base of the centrifuge and back into the pan.
Centrifuges have proven extremely effective in removing dirt particles down to one micron in size, including some soot particles that would pass through just about any other filter. A centrifuge is initially more expensive than a by-pass filter, but thereafter requires no filter element replacement, so is cheaper to maintain. (Additionally, there aren’t the same disposal problems that are an increasing concern with old oil filters). Also, since the contaminants are thrown out of the path of the oil flow, a centrifuge does not slowly plug up, restricting the flow of oil through it, as does any filter.
Centrifuges have primarily been designed for large engines with high-volume sumps and high-pressure oil pumps. The most widely known centrifuge, the Spinner II (T.F. Hudgins, Houston, Tex., 77292; 800-231-7746), works best at 70- to 90-psi oil pressure, but will clean oil down to 30 to 35 psi. Below this pressure, efficiency declines sharply. The smallest model Spinner II requires an engine oil pump with a flow rate of at least 10 gallons per minute, which is well beyond that of many sailboat auxiliary diesel engines, but will be found on larger marine power plants.
In a marine application, where an engine is frequently operating at an angle of heel, a gravity-return model will need to be above the level of the engine pan at all angles of heel, while any model is best mounted on the centerline of the engine; this will minimize the gravitational effects of heeling on the oil return.Re-refining oil
A by-pass filter or a centrifuge will do a great job of removing microscopic particles that would otherwise pass through a full-flow oil filter, and thus minimizes physical contaminants in the oil. It will not, however, remove water or fuel, or nullify the effects of chemical contamination. As a result, although one half of the contamination problem is substantially solved, the oil and filters must still be changed at the same intervals. A by-pass filter or a centrifuge is a device for extending engine life by reducing engine wear – it is not a device for extending oil life. To extend oil life the chemical contaminants must be neutralized. So far as I know there is only one widely-available device on the market that claims to do this: the TF Purifiner (TF Purifiner Inc., Boynton Beach, Fla. 33426; 800-488-0577 or 800-336-9437).
The TF Purifiner is a by-pass filter with an internal heater. The oil first enters a metering jet, which reduces the oil flow to a slow rate (the slower the flow through the filter the better the retention rate), and then it passes through a filter element composed of densely-packed, long-strand cotton fibers that trap particles, including soot, down to one micron in size. The oil then passes over a metal plate in a chamber heated to 180 degrees F to 200 degrees F. The high temperature causes water, anti-freeze, and fuel to evaporate and the vapors are vented either to the atmosphere, or via a hose into the engine air inlet.
The manufacturer claims that the combination of liquid contaminant evaporation and microscopic filtration removes most of the substances that degrade the oil and consume the additive package. Without water there will be no sulfuric acid, and oxidation is slowed. And damaging losses of viscosity are prevented when fuel is removed. In addition, the ability of the filter element to trap larger soot particles reduces the work to be done by detergents, dispersants, and other additives. The net result is a stated dramatic reduction in the rate of what we have called chemical degradation and contamination of the oil, in addition to the reduction in the rate of physical degradation. Together, these things result in greatly extended oil life.
The TF Purifiner filter element is changed at the normal oil change intervals, but since it catches almost all the dirt in the engine, the regular full-flow filter rarely needs changing (it should, however, be changed every 500 hours, or at least annually since the paper and glue in the filter may deteriorate, leading to failure). According to Purifiner, by keeping the oil in a consistently clean state the TF Purifiner greatly reduces the rate of additive depletion. This is because the extra oil used to replace both the oil lost during filter changes, and that lost during normal engine operations, has sufficient new additives to compensate for the diminished rate of additive consumption.
To many in the engine business, this all seems too good to be true, but Purifiner has impressive data from extended truck-fleet trials to back its claims. Nevertheless, there are, of course, drawbacks. The first is the initial cost. The second is the cost of the replacement TF Purifiner filter elements, which typically are more expensive than a regular full-flow filter element. And the third is the fact that periodic oil analysis is required to ensure that the oil is still in an acceptable condition for continued use.
To offset these expenses, there are projected savings on oil, labor, and the cost of disposing of waste oil. Moreover, a periodic oil analysis is, in any case, advisable. In addition, with the TF Purifiner filter the engine will always be running on clean oil, with significant benefits in terms of reduced wear rates, so even on a small engine, the unit will probably be cost-effective. The larger the engine, and the heavier its use, the more attractive a device such as the TF Purifiner becomes.
A TF Purifiner should not be operated at extreme angles of heel. The manufacturer recommends a maximum 20 degrees angle, which is consistent with most engine manufacturers maximum recommended angles of heel. Since the oil returns to the sump by gravity feed, a unit will need to be mounted above the pan at all angles of heel. And, as with the Spinner II, mounting on the centerline of the engine is best, with the oil return line also on the engine centerline.Another wrinkle Finally, it is worth taking a look at one other heavily-promoted newcomer to the oil filter market. This is the Tattle Tale (Racor, Parker Hannifin Corporation, Modesto, Calif. 95353; 800-344-3286).
The Tattle Tale is a replacement for an engine’s full-flow filter. It consists of a housing containing a fine-mesh stainless-steel screen, with a pressure-activated warning light. The stainless screen replaces the pleated-paper element found in a normal full-flow oil filter. Various mesh sizes are available from 28 to 115 microns, with the 28 micron screen being the normal one in engine filtering use. This is a somewhat finer mesh than is found in a typical full-flow filter, and, as such, will catch some particles that would pass through a normal filter. However, it is not fine enough to catch the microscopic particles picked up by a by-pass filter or centrifuge. In other words, the Tattle Tale is a substitute for the full-flow filter, and not an alternative to a by-pass filter or centrifuge.
The principal advantage of the Tattle Tale over a conventional filter is the re-usability of the stainless steel filter screen. At oil change time, the screen is taken out, cleaned in a kerosene or diesel bath, and replaced. For a fleet operation there will be obvious savings in filter elements, and in the number of old oil filters requiring disposal. For an individual boat owner, however, cleaning the filter mesh is likely to be messier and more troublesome than changing a spin-on filter.
The larger an engine, and the heavier its use, the greater the potential benefit of an add-on filtration device, particularly a by-pass filter or centrifuge. In most instances, the cost will be recovered many times over in reduced engine wear, maintenance, and down time. This fact has been appreciated for many years by heavy-equipment and fleet operators.
The same sort of economics does not apply to small engines in occasional use. In the past, the typical sailboat auxiliary engine has rusted from the outside long before it wore out from the inside. The life of the peripheral equipment – alternator, starter motor, manifolds, and heat exchanger have, as often as not, determined the life of the engine, rather than the rate of wear on bearings, cylinders, and piston rings. But this state of affairs has been steadily changing. As the creature comforts on boats proliferate, driving up the electrical load, engines are increasingly run long hours for purposes other than propulsion. Whereas, at one time it was not unusual to put less than 100 hours a season on the engine, today it may accumulate several hundred.
In these circumstances, an add-on filter begins to become an attractive proposition on even quite small engines. At the same time, manufacturers of these devices have produced some down-sized equipment suitable for just about any engine. For, at the most, a few hundred dollars, it is possible to give a small engine an unprecedented degree of protection against the damaging effects of dirty oil, and, in the case of the TF Purifiner, to minimize oil changes altogether. For many of us, this is a subject worth further investigation.
Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.