Bottom Line
Your engine’s first line of defense
Dirt happens! So do water, soot, acid, bacteria and a host of lesser contaminants. They either enter the vital fluids of your engine, breed in its fuel or are created in the engine itself by the process of combustion. How can you protect your engine from these engine-destroying substances? The mechanisms that protect your engine from harm are its filters - your first line of defense.

by Paul Abelson, technical editor

In this article, I will not be making any specific recommendations about particular filters, nor even mentioning brands. Rather, I will describe what types of filters there are and how each operates. When you have an understanding of their function, you'll be in a position to evaluate the ads and any promotional literature you'll see, so you can make informed decisions.

Coolant Filters

In cast iron engine blocks, and on iron cylinder liners, rust can form on surfaces not fully protected by rust-preventing additives. Solder can react in brass and aluminum radiators. Coolant formulated with hard water can precipitate minerals and salts that coat and insulate parts of the engine, causing localized hot spots. Supplemental coolant additives, called SCAs, lubricate water pumps and also work to combat rust, solder bloom (a form of corrosion) and acid and also offer protection from liner pitting. When in too high a concentration, they can precipitate into solids that block cooling flow.

The first defense against cooling system problems is to maintain SCA level; not too little, and not too much, either. The next defense is a spin-on filter. Coolant filters remove all solids before they can clog passages or allow abrasive deposits to wear water pumps prematurely. Coolant filters come two ways: charged and uncharged. Charged filters have SCAs inside. When you change the filter, you replenish your SCAs. When you use uncharged filters, you have to check your SCA level (with test strips) and add liquid to your coolant. Charged filters may use one of several families of SCA, which are not chemically compatible. It is best to stay with one manufacturer. Uncharged filters must be purchased to fit your particular engine, but you can use any of the major brands. Avoid generic no-name or offshore products. You may not know what you're getting, and in case they don't work, you may not have any recourse.

Fuel Filters

Every time you refuel, dust, dirt and atmospheric moisture enter your fuel tanks. If you refuel at an off-brand or generic truckstop, you may be pumping contaminated fuel, with dirt, water or bacterial growth in it. Dirt can come from leaks in old piping, but most often, it's from poor housekeeping. Failure to clean the area around the filler caps for the in-ground tanks, or not cleaning the space around sunken fillers can allow dirt to enter when caps are opened. Water can enter when filling during bad weather or from water vapor that condenses from the air when tanks cool. Water is heavier than diesel, and will sink to the bottom of the tank. Certain bacteria and fungus feed on hydrocarbons (diesel fuel) and draw oxygen from water. They breed inside storage tanks, and form a slimy film that, if drawn into your fuel system, will start to clog filters. Usually, these harmful substances lie beneath the level of the fuel pick-up to be removed during periodic housekeeping at the truckstop. But if a new load of fuel has just been delivered, it will stir up the contents of the tank. You could wind up pumping these contaminants into your own tanks.

The same cycle of condensation and organic growth can happen in your tanks, and agitation occurs as you drive down the road. That agitation also traps air in the fuel.

In the days before electronic engines and emissions controls, screening out large particles of contamination would provide adequate protection for fuel systems. Then, tolerances were several thousandths of an inch and injection pressures were 2,000 to 3,000 psi. Today, pressures approach 30,000 psi and may go to 50,000 to meet more stringent regulations. Tolerances are in tenths of thousandths of an inch. Then as now, water was a great concern. Injector tips sit in the combustion chamber. Even though they have good cooling paths to dissipate heat, the tips (where fuel is injected) can reach well over 1,000 F. An errant drop of water finding its way through the fuel system could turn to steam so suddenly, with such explosive force, it can blow the injector tip from the body and imbed it into an aluminum piston top.

Virtually all fuel filters today include a fuel-water separator. For complete fuel system protection, you need a primary filter and a secondary filter with a fuel-water separator. The primary filter stops the big pieces, 40 or 50 microns or larger, that could wreak havoc with injector pumps or block flow. They are often found on the suction side of the fuel pump. In years past, when fuel would deposit wax on the filter surface and block flow, the only thing to be done was to change the primary filter. Today, there are chemicals that will dissolve the paraffin in a clogged filter.

Secondary filters, called that because they are the next filters along the line, are actually the primary protection for the fuel system. To protect today's high pressure, finely machined fuel injectors, they use paper elements to filter down to 10 microns or less. A micron, by the way, is one one-millionth of a meter, a thousandth of a millimeter. In standard terms, it's 0.000039 inches. Ten microns measure roughly four tenths of a thousandth of an inch.

Since diesel contains paraffin wax that can form the structure that gels fuel, a good fuel filter will have a heater element to warm the fuel so it can flow and be effectively filtered. The filter will also have a mechanism to remove water. Some have screens made of polymers that attract water to their surfaces but allow fuel to pass through. Others flow the fuel through tortuous passages made of similar material that literally pull water out of the fuel. The water flows together to form heavy drops that fall and collect in reservoirs.

Some filters have electrical contacts and warning lights that let you know when it's time to drain the filter. Most are visual. You have to check them, usually when you do a pre- or post-trip inspection. Virtually all trucks today come with some form of combination fine filter and water separator. Even if you don't specify, you'll probably get one, but you can specify by brand name. Some fuel filters combine added features, like de-aeration and temperature control to provide more constant fuel density and a more predictable burn.

Oil Filters

Filtering engine oil presents its own unique set of challenges. Like fuel, water can enter engine oil through condensation, but since oil quickly warms to 200 F or more in the crankcase, most of the water evaporates. Some contamination comes through the air cleaners (fine dust and dissolved road salts) and gets into the combustion chambers, but most is created in the engine itself. While electronic controls and high injection pressures have significantly reduced the levels of soot produced during combustion, soot is still created, especially during idling when the engine operates well below designed operating temperature. Today's engines are designed to capture soot in the oil and carry it to the crankcase, rather than have it go out the exhaust. Soot is the product of incomplete combustion of hydrocarbon material. Being mostly carbon, it comes in several forms. Hard soot, like diamond, is abrasive. Left unchecked, it can easily grind away any surfaces that rub together, such as cam lobes and valve lifters, valve stems and guides, piston rings and cylinder liners and, of course, bearings. Soft soot, like graphite, can be slippery. It is also chemically active, and can attach itself to additives in the oil, neutralizing their ability to function. Dirt and soot are the major solid contaminants produced during combustion. When they first form, soot particles are tiny. Some are less than one one-hundredth of a micron. But soot particles attract each other and agglomerate, or join together. They can grow to become 20 to 50 microns in diameter, especially if suspension additives in the oil are depleted.

Some metal-to-metal clearances, such as between gear teeth or cams and lifters, are less than one micron, often between one-half and one-tenth of a micron. Large soot particles abrade or weaken metal surfaces, causing metal chips to enter the oil. They, in turn, contribute to more wear in an ever-increasing cycle - unless the contamination can be removed from the oil. That is the oil filter's task.

Nitrogen and oxygen are the major components of air. Sulfur is found in diesel fuel and, to a lesser degree, in oil. It is naturally occurring in petroleum. During combustion, these elements combine to form oxides of nitrogen (NOx) and sulfur dioxide. They, in turn, can react with water vapor, also produced during combustion, to form nitric and sulfuric acids. Most NOx goes into the exhaust, which is why engine makers are working hard to find ways to reduce this heavily regulated precursor to smog. The sulfuric acid, however, is captured in the engine oil and carried to the crankcase. Most of it is neutralized by the alkalinity of the oil, as measured by the total base number (TBN). Acid is aqueous (water-based) and can be absorbed by cellulose found in filter paper and, to a far greater degree, in depth-type bypass filters.

Every engine comes with one or two spin-on, full-flow oil filters. They are called full-flow because every drop of oil from the oil pump goes through the filter. Because it protects from catastrophic failure, most full-flow filters are rated at between 25 and 40 microns. They are designed to screen out the "big" pieces. When the filter element, a paper screen pleated to provide much more surface area, becomes clogged, or if the oil is cold and thick, flow through the full-flow filter is restricted. Then these full-flow filters go into bypass mode. Valves open and let the oil bypass the filter element and flow directly into the engine. The theory is that it is better to have dirty oil going through the engine for a short time, than to have it run with no oil flow at all.

Don't confuse bypass mode with the term bypass filter. A bypass filter takes some of the oil pump output and diverts it to be filtered much finer than by a full-flow filter. Filtration varies with the type of filter, but many trap particles down to 10, five, three or even one micron. Because they filter so finely, their flow is very restricted. To keep size reasonable, they can only handle a small portion of the oil at a time. They super clean between 2 percent and 5 percent of the oil, then return it directly to the crankcase, bypassing the engine. Thus the name, bypass filter.

Even with such a small flow going through the bypass filter, virtually all the engine's oil is cleaned within 45 minutes, and kept clean thereafter.

Bypass filters come in many shapes and designs. Some use angled spray jets to spin the oil and the surface it is sprayed on. The centrifugal force from the spinning action draws soot and contaminants from the oil and impacts them on a special cup. At maintenance, the cake of soot can be cleaned from the cup, or the entire cup can just be replaced.

Combining the features of a full-flow filter with a centrifuge, another type of filter flows oil through a metal screen that has angled openings. The screen filters like a full-flow filter, but the spinning motion imparted by the angled holes causes finer particles to be removed by centrifugal force. The screen is cleanable. The maker claims particles as small as five to 10 microns are removed, but not how many at that size. All other bypass filters use some form of highly compressed media, usually cellulose fibers (paper, cardboard or cloth). By forcing oil to flow over, around and even through the fibers, extremely fine particles are trapped within the depth of the filter. Some filters use shredded wastes, such as newspapers, mattress ticking and cardboard. Some use cotton fibers. Several makes take un-dyed, virgin cotton mill ends and compress them tightly. Others use cotton twine and wind it in a crisscross pattern to build-up a depth of several inches. One manufacturer pioneered the use of stacked discs of cardboard to present a tortuous path for the oil through its compressed fibers. These stacked discs are often combined in the same spin-on housing as a full-flow filter, in order to get the advantages of both types without external hoses and connections. Paper is an effective filter medium, used for most full-flow and fuelfilters. Several manufacturers make canisters to hold rolls of paper towels or toilet paper. In bypass mode, they are surprisingly effective, but, like compressed fiber filters elements, they are prone to channeling. This takes place when a large (10 microns and larger) contaminant particle winds its way through the filter medium, leaving a channel in its wake. This allows particles to follow the path and flow through virtually unfiltered.

It takes a while for channels to develop. How long is a function of how tightly the media are compressed, how big and sharp the particles are and how clean the engine is. Engines that "burndirty" (that producemore soot), clog and channel more rapidly than clean engines. To prevent channeling all the way through, many bypass filter makers recommend changing at least their filter elements every 10,000 to 15,000 miles regardless of what oil analysis says about the condition of the oil. When a bypass element is changed, any channeling that has occurred internally is eliminated, literally thrown away. Also, new oil is added to replenish the one to four gallons removed with the old element. When calculating the effectiveness of a bypass filter, always consider the extra volume of oil the filter adds, and how often it gets replenished. Some filters that claim you will never need to change oil again actually have you replenish all the crankcase oil every 40,000 to 50,000 miles or less, a few gallons at a time.

Wrapped and wound filters are highly resistant to channeling. They are made of virgin filter paper, fabric or cord - very highly tensioned and wrapped around a mandrel. Some have filter elements bound in an oil-resistant cover that allows oil to flow only parallel to the axis, not radially. Their elements are used in housings designed to release dirty oil at one end and collect clean oil at the other. Elements can be used in traditional 750 cubic inch housings or newer, more efficient, 1,000 cubic inch housings. Five hundred cubic inch elements are available in throwaway containers that quickly connect to inlet and outlet hoses. Like most bypass filters, wrapped paper units can be mounted virtually anywhere, with hoses providing access to and from the engine. These elements are usually so tightly wrapped, they close in behind any particles working their way through the element, virtually eliminating channeling.

Several makes of bypass filters have special evaporation chambers and heater elements, said to drive off contaminants. Water-based contamination, such as acids and coolant, will be absorbed into the cellulose of their cotton or paper elements before they reach the heater. And although I've been asking at trade shows for many years, no one has answered satisfactorily why oil, which is in the crankcase at 195 F to 225 F, needs to be heated to 180 F in the filter housing to drive off impurities.


Armed with this knowledge about all the filters designed for the fluids in your engine, you should be able to make better decisions about spec'ing filter systems on new trucks and adding accessory systems to existing ones. I've tried to share general knowledge so each of you can evaluate sales literature, separate fact from hype and make up your own mind.

Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.

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