AIR INTAKE SYSTEMS

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Air cleaners: they’re more than just for show.

There’s hardly anything on a truck that’s simpler than its air-intake system. And not too many things – maybe only your right foot – that can have as much effect on fuel economy. A diesel engine needs very clean air in order to function efficiently, much more so than a gasoline engine does.

Air intake is one of those things that you don’t normally have to worry about spec’ing when you buy a new truck. The engineers have it pegged. But if you go from a nice clean over-the-road haul to pulling gravel out of a quarry and then down a few dirt roads to the highway, you’d do well to ask a question or two of the experts at your engine service shop.

And, as with many other components, if you buy a used truck it makes sense to find out if the air filter in there is the correct one – meaning sized properly to your engine – and if it suits the kind of work you’re doing.

Air Restriction

There is one spec’ing decision, regardless: most diesel-powered trucks come with air-restriction gauges nowadays, but make sure of it. You absolutely need it if you have a dry-type filter system, because you can’t just look at a filter element and see that it’s dirty. It may look like it needs a changeout when in fact it has lots of life left. Only the gauge would tell you. The filter could also look clean enough to keep but in reality be overloaded with fine dust. Again, only a restriction gauge could give you the true picture.

The gauge could be mounted in the dash (preferred) or under the hood, in the piping between the filter canister and the engine. Usually it’ll be a bar-type readout that shows restriction in inches (of water). It means ‘restriction’ literally – the difficulty that air has in getting through the filter, past the trapped dirt, and into your engine.

The contaminants involved are ordinary dust, carbon soot (from exhaust), and water by way of rain or snow or even fog. If dirt particles are ingested into the engine, they’ll do nasty things like damage the cylinder liners.
A normal restriction reading is 10 to 15 in., and as it moves to 20 in. or so it means there’s dirt plugging things up. The gauge will probably go up to 30 in., but the element needs changing at about 20 in a two-cycle turbocharged engine, at 25 in a four-cycle turbo.

Leave it longer and the engine won’t be breathing very well, and fuel economy will suffer as a result. In a turbo diesel you’ll use 2% more fuel at 30 inches, compared to 10, according to The Maintenance Council of the American Trucking Associations (TMC).

There’s not just dirt to contend with in limiting restriction. The ducting has an important role to play here too. Add too many elbows and bends, and you’ll also add restriction, perhaps in significant amounts. TMC says that an easy 45-degree bend will add a half inch of restriction, while a smooth-radius 90-degree elbow adds one inch. But create a sharp 90-degree bend and your restriction gauge will show an increase of two and a half inches.

So that’s clearly a spec’ing decision too, though it’s only likely to arise if you’ve asked for some special equipment or made an oddball modification of some sort. Luckily, the ducting normally doesn’t travel very far, so there are only so many elbows you could come up with anyway.

The air-cleaner canister can be outside the hood on either side, or on both sides of a traditional long-nose/narrow-cab conventional. These days, with aerodynamic conventionals predominant, it’s more likely to be under the hood. On cabover tractors, the filter canister will be one of two places – with frontal air intake it will sit inside, above the engine, and in a snorkel system it will be outside, behind the cab.

Oil-Bath Air Cleaners

Somewhat specialized, the oil-bath air cleaner is serviced strictly on a regular mileage basis. Restriction readings have no place here because the system traps contaminants in engine oil, and restriction may not change at all over the service interval.

Dirt and water are removed from the incoming air by a wire screen (sometimes removeable) and get deposited in an oil cup. The system needs service when the level of oil in that cup rises a certain amount above normal, usually half an inch. Beyond that, the dirt-laden oil would get drawn into the engine, with damaging results. Arbitrarily, you could also service the air-cleaner system when you change engine oil.

Service involves replacing the oil in the cup, with whatever lube is used in the engine, and at least once a year the entire system is removed and flushed with solvent or steam-cleaned.

Dry Filter Types

The dry system’s filter element, round or rectangular, sits inside a canister, and the system will draw air through the element and then into the engine. It can be mounted either directly on the engine or remotely.

The science of air cleaning has improved over the years, and these days most elements – or ‘cartridges’, as they’re called in some systems – are very sophisticated things made of specially formed and treated paper or cellulose (sometimes a synthetic material as well), called the ‘media’. They’re sometimes re-useable, but with newer types this is less common. Make sure you know what you’ve got, because washing and re-using an element designed for one-time use will cause you grief.

If you do have a re-useable element, limit it to one year’s life and no more than six cleanings. Cleaning can be done by either compressed air or washing in a special solution.

Most air-cleaning systems are single-stage mechanisms, but there are several two-stage types that have a pre-cleaner and a dust cup that collects contaminants before they reach the filter element itself.

Before you install a new filter element, make sure that you’ve got the right one. It’s possible to have the right diameter and the wrong height. It would fit, but there would be a gap between housing and filter, so the gasketing would never be able to make a seal.

The Water Problem

Moisture can wreak havoc inside an engine, of course, especially the salty water that flies around during the winter. That road salt can get in through a saturated filter or a leak in the system.

Deposits of road salt can build up inside the turbocharger’s compressor intake, warns TMC, reducing the throat diameter and increasing the temperature of intake air. The result? Power loss at least, and possibly valve-train damage, as well as an excessive load on the turbo’s bearings and potential bearing failure. Pieces of caked salt could also break off and damage the blower housing, lobes, and seals.

So every air-cleaning system must have a water-separating mechanism of some sort near the air intake. On conventional tractors with an outside air cleaner, the intake should be mounted away from the flow of water-laden air as it streams over the hood or fender. That’s not always possible, in which case a shield of some sort must be used. On cabovers with an over-the-roof snorkel intake, the air inlet should be 14 to 18 in. above the roof line.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to Trucknews.com.


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