Oil filtration
Although everyone today accepts the need for proper filtration of engine oil, few truck operators appreciate how complex the subject is, and how many variables and trade-offs are involved in decision making

by Paul Abelson, Technical Editor


Oil becomes contaminated from four sources: dirt, metal particles, combustion by-products and the breakdown of the oil itself.

Oil Problems

Oil performs many functions in an engine. It separates metal surfaces that might otherwise be in contact, reduces friction losses inside the engine and cleans the engine internally. It does other jobs, like cooling the engine and combating acids that form during combustion. To perform properly, the oil must be as free from contamination as possible.

Dirt (identifiable as silicon in an oil analysis) can enter the engine either through the air intake or while oil is being added, if filler openings are not kept clean. When air leaks in around air intake clamps and joints, dirt can enter the engine during operation in dusty environments (off road, around cement or sand, during dust storms, etc.). The dirt does not burn. It may exit out of the exhaust stack, or it may be picked-up by engine oil and carried to the oil pan.

Metal particles can be worn from engine surface in three ways. First, abrasion occurs when hard particles get stuck in spaces too tight for them to flow through. As the surfaces move against each other, separated only by an oil film, the particles chip out additional particles.

Fatigue damage occurs when a load presses a hard particle into a metal surface. This weakens the metal, allowing particles to pop out, much like the creation of a pothole in weakened pavement.

Adhesion occurs because no matter how smooth and mirror-like a metal appears, its surface has microscopic hills and valleys. When the hills of one surface cut through the oil film separating it from another, then contact the peaks of the second surface's hills, pressure on these pin-point surfaces causes them to adhere to each other and break-off from the base metal. Any of these particles can become the cause of additional wear as they remain suspended in the oil.

Combustion by-products are created when diesel fuel does not burn completely. There are several forms of soot, which is primarily carbon. Just as carbon can form diamonds (a hard abrasive) or graphite (a lubricant) it can form hard or soft soot. Hard soot acts like an abrasive. Soft soot is chemically active. It attaches itself to additives in the oil, reducing their effectiveness.

Soot has another property which leads to engine damage. It agglomerates, or joins to itself to form progressively larger particles. As soot particles too small to do any damage circulate in the oil, they become bigger and bigger until they abrade and fatigue engine metals.

Among the additives in modern oils, suspension agents help keep particles small and separate. Over time, however, they lose effectiveness, especially since modern reduced-emission engines are designed to capture soot in engine oil rather than releasing it out the exhaust.

CG-4 oil is designed to hold soot up to three percent of its volume. The new CH-4 oils are required to hold more than five percent, a huge increase. The specifications, however, put no limit on micron sizes. Remember, if a particle is larger than the oil film, it will damage the surrounding engine parts. As seen in the accompanying table, some oil film thicknesses are a half micron or less.

Oil break down occurs due to excessive heating or oxidation and forms sludge and varnish that, like soot, are held in suspension in the oil. It can deposit onto pistons, rings, valves and injectors, altering design tolerances and adversely affecting performance. As performance deteriorates, the process intensifies.

Filtration can be defined as the removal of these harmful particles from the oil by some mechanical means, before the particles can do any significant damage. Significant, as used here is a relative term. It relates to how long you keep your engine and how long you want the engine to last before an overhaul is needed. If you drive 100,000 miles a year and trade your truck every four or five years, standard filters and 15,000-mile oil drains should give acceptable life without significant wear, in on-highway, line-haul service. But if you operate off-highway, wear will be greater unless oil drains are more frequent or filtration is upgraded. Similarly, if you want to keep your engine for 1,000,000 miles, you may have significant wear before you reach your goal unless you shorten your drain interval, upgrade your filters, or both.

Diesel Engine Component Oil Film Thicknesses
(microns) Ring-to-Cylinder 3.0 - 7 Rod Bearings 0.5 - 20 Main Shaft Bearings 0.8 - 50 Turbocharger Bearings 0.5 - 20 Piston Pin Bushing 0.5 - 15 Valve Train0 - 1.0 Gearing 0 - 1.5

Typical full-flow filters, which filter all the oil before if flows through the engine under pressure, are rated at about 40 microns, or one/one-thousandth of an inch. Premium filters with synthetic media typically filter to 25 microns. The better ones are as low as 20. These filters offer protection against large particles that may enter the engine. Soot particles that agglomerate are usually removed by draining the oil before they reach 25 or 40 micron size, as are wear metals. Combination full-flow and bypass filters can improve overall filtration by removing particles down to 10 micron size from a portion of the oil. Bypass, in this context, means that a small portion of the oil, usually three to five percent, is sent to a special filter where it is cleaned and returned either to the main oil stream or to the sump, bypassing the oil system altogether.

Bypass has another meaning. When a filter is cold or plugged with dirt, flow can be restricted. Instead of the engine losing all lubrication, a valve opens to bypass the restriction, allowing unfiltered oil to circulate through the engine. Dirty oil is better than none at all, but don't confuse the two definitions of bypass. Ultra-fine bypass filters help keep oil cleaner in case the full flow filter has to operate in bypass mode.

There are bypass filters that use thick fibrous media to filter oil in depth. Because of their bulk, they are usually away from the engine and are connected by hoses. These bypass filters are also called off-line filters. The more dense the media, the more restrictive the flow, so tradeoffs must be made. If efficiency is constant, size must increase to get more flow. A 12- to 15-liter truck engine may flow 60 gallons of oil per minute. Bypass filters may flow only two to three gallons a minute, but they filter it much more finely (down to as little as one micron for some filters, five for some and 10 for others) depending on size and media type. Amsoil has a small, light bypass filter rated at five microns. A Luberfiner 750 cubic inch filter will flow three to five gallons a minute, filtering to five microns. A Como filter of the same size will flow less oil, about 1.5 to two gallons a minute, while filtering to under two to three microns.

Our industry is blessed with people like Donaldson's Arnie Carlson, a winner of The Maintenance Council (TMC) Silver Spark Plug Award, who has devoted his entire career to the subject and, as he says, is still learning about it every day.  Arnie has advised me on the subject over many years.   We obviously can't impart a lifetime of learning in one brief article, but we will try to provide enough basic knowledge to help you understand your choices, so you can make better decisions. 

Another form of micro-fine filter is the centrifuge. The Spinner II uses oil pressure to turn a central nozzle assembly, spraying oil outward. The oil contacts a spinning cup. Particulate matter is held by centrifugal force while the cleaned oil flows back to the oil sump. Flow rates and filtration are comparable to a Como cartridge filter.

The sooner particles are removed, the less wear each will cause. Early removal of soot, before agglomeration into larger size particles, reduces the severity of wear. Removal of particles in the oil takes place by draining the oil from the engine and replacing it, or by physically removing the particles through bypass filtration, leaving clean oil to function in the engine. But remember, bypass filters are for cleaning particles from oil before they harm the engine or degrade the oil. As a result, the use of micro-fine filtration also allows oil life to be extended, but the oil drain interval must be determined using oil analysis over time (see sidebar).

Progressive owner-operators use bypass filtration to protect engines. Payback can be achieved through extended oil drain interval and longer life to overhaul. One small fleet in Idaho and Montana currently drains oil twice a year, at 60,000 mile intervals using ultra-fine filters. Another, using a super-premium semi-synthetic oil is three-fourths of the way to evaluating a once-a-year (100,000-mile) interval, using the same Como filter.

Other new developments in filtration include cleanable full-flow filters from Webb and Racor, and "oil refiner" type compressed-fiber-element bypass filters. They include a heater element designed to cookoff liquid impurities. One caution about replaceable cartridge bypass filters that claim indefinite oil life is that most require frequent cartridge changes to head off channeling. Channeling occurs as fibrous materials become moist and are subjected to vibration and impact. Channels open throughout the fibers. They allow the passage of larger particles, often as large as the full flow filter, to pass. When changing these large cartridges, any settled fibers are removed before a passageway is created completely through the filter. Oil additives are likewise replenished by adding the volume needed to replace oil in the old cartridge. This replacement oil must be taken into consideration when calculating costs and return.

Technology already exists that may some day eliminate the need for oil drains. Both Webb and Cummins supply systems that periodically remove small quantities of oil from the engine sump and replace it from an on-board tank of make-up oil. The removed oil is filtered and mixed with fuel in the return line or tank, to be burned later to propel the truck. Oil is never totally clean, but is stabilized at a cleanliness approximately that of oil 40 to 50 percent of the way to drain interval. When using such a system, you must report oil purchases as fuel and pay tax on it.

Tradeoffs in selecting any oil filtration should be considered before deciding on any one system, no matter what glowing reports you get from your friends. Examine the frequency of oil drains with each new system, the initial cost of the system including installation, the filter element replacement frequency and its cost, the cost of makeup oil every time you change elements, and the cost of oil analysis to verify that the system is doing what it should be doing. Talk to friends who operate in a similar manner and who run various drain-extending filters. Don't forget that following engine makers' recommendations will provide adequate engine protection, although the economics can often be improved. But to take advantage of any improvement, you must do your homework. You can get more detailed information from TMC Recommended Practice RP 1403. Because it is written for medium-duty truck users, it contains more background and explanation than the similar RP written for heavy-duty operators. RP 1403 can be ordered from TMC at 703-838-1763. LL