Not Your Father's Air Filter
Truck builders have moved air-intake systems under the hood, but buyers can still spec pre-cleaners and restriction gauges
March 2013, TruckingInfo.com - Feature
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-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.
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.
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