Aerodynamic truck styling is one factor that has moved the air-intake system and filter under the hood instead of hung alongside it.

Aerodynamic truck styling is one factor that has moved the air-intake system and filter under the hood instead of hung alongside it.

There was a time when fleet managers and owner-operators specified many components on their new trucks and tractors, but that's been seriously curtailed by vertical integration — the practice of manufacturers controlling the design and spec'ing process using parts they build themselves or get from a few suppliers.

Among the no-longer-spec'd components are air filters. What remains is the need to strip 99.9% of impurities from air that will be ingested by a diesel engine, whose compression-ignition operation is extremely sensitive to the smallest contaminants.

Two other reasons for the demise of air-filter choice are aerodynamic truck styling, which places the air-intake system and filter under the hood instead of hung alongside it, and exhaust emissions standards, which truck and engine builders must meet and certify.

Buyers can't be allowed to change filter designs because that would affect engine performance, including what goes out the exhaust. Trying to satisfy the preferences of fleet managers and individual truck owners, then certifying the combinations with federal and state Clean Air authorities, would be far too expensive, explain suppliers and original equipment manufacturers.

Cummins, now the sole independent engine supplier to truck makers, establishes performance requirements, and its application engineers work with their counterparts and the OEMs to be sure that those requirements are met. OEM people are happy to work with Cummins, just as they do with their own proprietary engine suppliers, so their engine offerings comply with the regulations.

Making a filter

To understand how to get the most out of your air filters, it helps to understand a bit about their design. John Gaither, director of heavy-duty engineering for Luber-finer, now part of Fram Filtration, which supplies filter elements to builders and other suppliers, has been designing air filter elements for over 30 years. He explains how that process now works.

It starts three or four years before a truck model's introduction. “We're given some preliminary performance characteristics: efficiency levels, dirt capacity, pressure drop information, and others,” he says. “There are probably 20 total performance characteristics, but the important ones are initial efficiency — what percentage of dirt the filter will take out — then the full-life efficiency, and that's important because all filters perform best when there's a light coating of dust on them.”

Today, there are new materials that help filter engineers meet those desired performance characteristics.

“Years ago, there were only two fibers: hardwood and softwood,” Gaither says. “Hardwood has smaller diameter fibers, which are better at removing small particles, and softwood has larger fibers that remove larger particles. If we wanted to change the performance, we would change the mix from hard to softwood” so air flowed easier.

Now there are synthetic fibers: polyesther and microglass. Some fibers are very, very small, in the nano range. So there are different types of fibers, and manufacturers have the ability to make the medium — the material that makes up the filter element — thicker.

“There's an upstream and downstream side, and it's important to get the filter in the right orientation so you catch the large particles up high; then the air will move around those particles and into the medium without restricting the flow,” Gaither says.

Pre-cleaner benefits

Pre-cleaners are simpler devices, which, as their name implies, strip some impurities from the air before it enters the filter element. They're not as common on highway trucks as they once were, but are still a good idea, Gaither says.

“A pre-cleaner is a wonderful thing because it will take out a lot of the heavier, more massive contaminants. Most are centrifugal; they fling particles out to the side. They will take out about 95% of particles 30 microns and larger.”

Most air intake systems are underhood, and manufacturers design them for efficiency and to help meet exhaust emissions requirements.

Most air intake systems are underhood, and manufacturers design them for efficiency and to help meet exhaust emissions requirements.

Gaither describes most pre-cleaners as dry with plastic stands that cause air to swirl around and the particles fall to the bottom.

“There's no media, just a swirl action with a dust pan someplace. Some are integrated into the housing or filter element itself. Old oil-bath filters worked well, but were messy to clean.”

“Double-can” setups — one large filter canister on each side of the hood, which are still used on many traditionally styled conventional-cab models — slow the air and easily trap particles. They have long element lives because inlet air is split between them.

With a single filter, air is concentrated and must flow fast to satisfy the diesel's appetite. The smaller the filter's volume, the faster the flow. With high velocity, particles go much farther and bury deeper in the fiber, where they will impede the air flow, Gaither says. That's why element materials and design are so important.

Next Page: Servicing Concerns[PAGEBREAK]

Building the systems

Donaldson Co. began designing and making air filter systems 100 years ago, and now produces them for many heavy and medium trucks, according to Mike Anderson, director of product management for engine air business unit. Many filters now are smaller, he notes, because of the move to under-the-hood, very custom-designed systems.

To compensate, Donaldson offers more efficient and longer-lasting elements.

“Buyers can spec a pre-cleaner,” Anderson says. “The greatest challenge is to get air cleaners small enough and still meet the requirements. We have seen air flow increases along with demands for greater life. It's both efficiency and package size.”

Spinning of the air by pre-cleaners takes out larger particles and moisture, but there has to be a place for the moisture to go, Anderson says. In Europe, water separators are used; they're similar in design but have less restriction than U.S.-style pre-cleaners. The Technology & Maintenance Council of ATA suggests spec'ing a water separator to keep moisture out of the element and the engine.

Servicing concerns

Servicing remains a concern. “A lot of the systems continue to ingest water,” says Paul Bandoly, manager of technical service and customer training at Wix Filters. “So we've got to be cognizant of that — water ingestion and service life, because people are stretching service intervals way out.

So restriction gauges and air filter monitors have become more prevalent.” He believes they are as important on diesel pickups as on Class 8 trucks and tractors.

In addition, he says, owners should pay a lot more attention to the air induction systems, “There are lots of different kinds of clamping, lots of plastics, lots of modular stuff. Those things can become damaged and warped. So you've got to inspect the air induction system, from front to rear, and it's more critical than it ever was before. Clamps holding two molded devices together — they can change and loosen, and air can get in and bypass the filtration system.”

For many years, a common thing to secure the air filter was the wing nut and bolt, according to Bandoly. “Now you see the radial-seal air filter. If the filter's bent in any way, it won't fit into the housing properly and you'll get air bypassing the filter.”

How you handle the filter is important. “Don't buy one, throw it in the bed of your pickup and then throw something on top of it,” he says. Radial-sealing is good because it takes away the variability of tightness, but if the clamps are loose or ineffective, they will fail to seal properly.

Housings can get damaged; things get bumped and cracked because things happen in the shop. Things can loosen, so they need to be checked so that they're still secure, says Bandoly. Components are made to tolerate the underhood heat, but things can happen. “Now you're seeing air-induction sensors, mass air-flow sensors, and others that measure it, so air has to be cleaned at the high level of efficiency as it has been for years,” Bandoly says. “Some medium-duty diesels have programming that if they sense results less than what they expect, the Check Engine light can come on and they can go into limp mode.”

Don't dust the engine

Donaldson, which makes filter housings for OEMs and says it invented the radial seal 30 years ago, publishes posters showing mechanics the correct ways to open and reseal housings. It also has posted numerous video demonstrations on YouTube, which show steps to take to avoid dust damage.

“Dusting” a diesel begins when a careless mechanic yanks out an element and lets accumulated dirt fall further into the intake piping.

Upon startup, the engine sucks in the dirt, which damages the tur-bocharger, pistons, cylinder walls, injectors and other parts. Instead, the mechanic should carefully withdraw the element while using an oily rag to wipe dirt away from the exposed inlet, then wipe out the canister before installing a new element.

Reading the air restriction gauge, which many trucks today have, is the only way to know if the element needs changing, experts say. Looking at the element indicates nothing for sure, and seeing dirt on the element is not necessarily a sign that its life is over.

All elements work better after some dirt has adhered to its surface. And now the system has been opened and the engine put in danger of dusting, and someone is guilty of “over-servicing,” as defined by Recommended Practice 301C, published by the Technology & Maintenance Council.

“Restriction is simply resistance to flow,” the RP says. “Anything that directs the movement of the air (inlet accessory, ducting, housing, connections, elbows, etc.) imposes some restriction on the flow of air.

Restriction is normal and engines are designed to work with it. How much varies by engine size and output. Manufacturers publish acceptable and maximum restriction rates, as measured in inches of water. Watch the gauge to know how much the element is loaded up with contaminants, and change the element when the gauge says so.

“Once you get up to about 15 inches of restriction, it's time to service it, for ours, anyway,” advised Dino Marutsos, heavy-duty technical sales manager for K&N. “Some people say they don't work, but a static gauge isn't supposed to move. A dash-mounted gauge moves, and you have to watch for the highest restriction.”

Restriction gauges are properly read after the engine has worked at maximum output for a specified time, such as climbing an upgrade. At 15 inches, most dry elements are about 90% of capacity and servicing should be scheduled, the TMC recommended practice says.

Usually this means taking out the old dry element and replacing it with a new one. Some suppliers make washable filters that can be reused. One is K&N, best known for its low-restriction, high-performance elements for cars, light trucks and motorcycles. But its heavy truck filter elements, available since 2010, are instead designed for long life, Marutsos says. They can be cleaned in water or with low-pressure compressed air.

Filters and elements are tested according to standards set by the American Society for Testing Materials, which specifies the device and procedures for its use.

“The filter is placed in its own housing, and we shoot it with dirt and catch it in another area and measure it, explains Marutsos. “There's a larger test stand for the large, heavy-duty filters and a smaller one for the passenger car and motorcycle filters.”

If the goal is to take out 99.9% of contaminants, then the caught contaminants will add up to that percentage of the dirt thrown into the air stream. If not, it's back to the designer's computer screen.

About the author
Tom Berg

Tom Berg

Former Senior Contributing Editor

Journalist since 1965, truck writer and editor since 1978.

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