That layer of dust on your television screen is composed of particles about 40 microns in size. Seems harmless, but particles much smaller—between 5 and 15 microns—can destroy expensive components in a diesel-engine fuel system in a surprisingly short time. Add some water to the fuel, and the prospect of harming the system further increases. The moral of this story is that fuel filters and fuel/water separators are your primary defenses against fuel-system hazards, so be careful about economizing in this area.
At the risk of oversimplifying, diesel fuel systems have a low-pressure side and a high-pressure side. The low-pressure side pulls fuel from the tank via a transfer pump, conditions it by removing dirt and water, then delivers it at a specified pressure (usually less than 100 psi) to the high-pressure side.
The high-pressure side meters (measures) fuel, then controls its injection into the engine cylinders. Design details differ, but all systems use a high-pressure pump (ranging in capacity from, maybe, 8,000 to more than 30,000 psi) to push fuel through tiny orifices in the injection nozzles, creating an atomized spray in the combustion chamber.
The high-pressure side depends on extremely close tolerances between moving parts to achieve sealing. Microscopic abrasives, however, score these precision mating surfaces, allowing leakage that impairs fuel flow and causes low power and hard starting. Plus, sharp-edged bits of grit blasting through a nozzle tip erode the tip's orifices, disrupting carefully engineered spray patterns and jeopardizing fuel economy and emissions compliance.
Water is another culprit. Water breaks down diesel fuel's film strength, diminishing its lubricating properties and permitting mating parts to scuff together. Water takes two forms in diesel fuel. Emulsified water (5- to 10-micron droplets in suspension) is suspected of causing premature nozzle-tip wear, and free water (larger droplets) can explode nozzle tips and soak filters. Emulsified water is created by the shearing action of the transfer pump on free water coming from the fuel tank.
Inadequate pressure on the low-pressure side of the system, caused by restricted fuel flow, can result in air bubbles forming in injection nozzles, then imploding against precision surfaces and causing pitting. Inadequate fuel supply also may cause injectors to run hot and oxidize fuel, creating filter-plugging by-products.
A fuel filter's job is to stop harmful particles, without impeding fuel flow, and to do so reliably for a reasonable time period. Responsible for trapping debris is the filter's medium, typically a paper-like material of cellulose (plant and wood fibers), or a mix of cellulose and man-made glass fibers.
As fibers become smaller in diameter, more can be packed into a given space. For example, a filter medium made from 9-micron Esparto grass fibers, says Parker Filtration, contains 16 times as many fibers as a medium made from 40-micron wood fibers. Glass fibers are even smaller, from 4 to 0.50 microns. More fibers increase the medium's trapping ability, because particles are caught not only in the spaces between fibers, but also imbed themselves into fiber strands.
Fuel filters are seldom plugged by dirt in the fuel, but rather by gums and varnishes (asphaltenes) naturally present in diesel fuel. A high-quality filter medium with small-diameter fibers evenly distributed, says Parker, maintains better efficiency during the filter's life than a medium with coarser, unevenly distributed fibers. In the latter, asphaltenes close off small spaces between the fibers first, forcing fuel through larger and larger openings that have increasingly less trapping ability.
Quality filters with strong, well-supported media also help diesel fuel systems resist a vicious natural enemy—vibration, which can dislodge particles already trapped in the filter. Engine movement causes vibration, as do the reverse-pressure waves set up in the fuel system as it continually spills excess, high-pressure fuel from the injector nozzles. Some filter manufacturers, Caterpillar for example, use a spirally wrapped fiberglass band to stabilize filter pleats against vibration flexing.
The typical low-pressure side of a diesel fuel system uses a primary filter-fuel/water separator combination between the tank and transfer pump, then a secondary filter-fuel/water separator—or filter alone—between the transfer pump and the high-pressure side. In today's high-pressure fuel systems, the primary filter may be rated between 20 and 10 microns, and the secondary filter at between 7 and 2 microns. Some would say, however, that fuel-filter-efficiency ratings, usually expressed as the smallest particle (in microns) the filter can stop, lack uniformity. Buyers, thus, are cautioned to make apples-to-apples comparisons.
For example, many heavy-duty filter manufacturers use the Society of Automotive Engineers (SAE) test J1985 to rate fuel-filter efficiency and capacity. This is a "single-pass" test that measures the filter's ability to trap specific particle sizes as the test fluid (usually hydraulic oil) contaminated with test dirt makes one pass through the filter. A particle count before and after the filter allows a percentage-efficiency rating to be calculated for each size.
Other manufacturers use a "multipass" test, such as SAE J1858, which is essentially the same as the single-pass version, except that the test fluid is circulated multiple times through the filter. For a given filter, the efficiency rating typically is better when evaluated by the multipass test (versus single-pass), because the filter has more opportunity to trap particles. A "time weighted average" multipass test means that periodic particle counts have been taken during the test, then averaged to establish the overall rating, versus taking just one reading at the end of the test.
And if you encounter "Beta-ratio" ratings for fuel filters and use them for comparison, make sure you know the size of particle (in microns) for which the rating is given. The Beta-ratio rating is simply the number of particles (of a given size) counted in the test fluid before the filter, divided by the count after the filter. For example, if 2,000 particles 5 microns in size are counted before filtering, and 1,000 after, then the Beta-ratio rating is 2. You can convert this number to percentage efficiency by dividing it into 1 (in our example, that's 1 divided by 2, or 0.50), then subtracting the result from 1 (1 minus 0.50 is 0.50, or 50 percent).
Of course, researchers keep refining test methods in an effort to more objectively rate fuel filters. Southwest Research Institute (SwRI), for example, working with a consortium of filter and engine makers, has developed an index rating system based on the calculated wear a filter allows in a fuel system.
SwRI's "wear-index" test approximates actual filter working conditions by using diesel fuel as the test medium, by using contaminant sizes known to cause substantial damage (6, 10 and 14 microns), and by inducing vibration during the test. The test also simulates the effect of cold-engine starts on the filter's propensity to allow trapped particles to be re-entrained in the fuel flow. SwRI has established baseline numerical values that say, in effect, filters with this wear-index rating and this re-entrainment rating (or below) will provide adequate protection.
Fuel/water separators, which often also include a high-efficiency filter, employ one or more of three basic processes to extract water from diesel fuel: gravity separation, coalescence and absorption. Gravity separation slows down the fuel flow to allow free water to drop out. Various means are used to slow the fuel, including cone-shaped baffles or turbine-like blades that impart a gentle rotary motion to the fuel stream.
Coalescence (sometimes called "stripping") typically uses a "hydrophobic" barrier, that is, a filtering medium coated with a water-resisting substance like silicone. The barrier allows fuel to pass, but stops even very small water droplets, which then coalesce (join together) as they slide down the medium to a reservoir. "Depth-type" coalescing separators may use several stages of processing, including a thick hydrophilic (water-accepting) medium that allows water to coalesce within before reaching a final hydrophobic barrier.
In the absorption process, water-laden fuel flows into a super-absorbent medium that has great affinity for water and holds it, while allowing fuel to pass. Absorption-type devices work well on free water, less so on emulsified water.
When changing a fuel/water separator, replace it with exactly the same type of unit originally installed. Some types work best on the upstream (vacuum) side of the transfer pump; others work best on the downstream (pressure) side. And if you want to retrofit a fuel/water separator, seek advice about the type of unit and location that will work best for your machine.