Let’s face it: filters lack a certain glamour. As one of the simplest auto service procedures and one of the last bastions of the do-it-yourselfer, filter service has had an image problem going back to the days of the “oil bath”. While that’s understandable, given the low cost and ease of replacement, what’s inside a good filter is very high-tech, although much of it is at the microscopic level.
How they work
Perhaps surprisingly, filters aren’t simply strainers for the working fluid they keep clean. Fluids (air is also considered a working fluid) can be contaminated in several ways, but the primary factors are impurities that are dissolved in the fluid and those that exist as solids or liquids in a separate “phase” to the fluid. Getting dissolved substances out of air, oil or fuel is more a problem of chemistry than a filtration issue, although it is technically possible to separate contaminants at the molecular level. Catalytic converters react dissolved pollutants in its working fluid, exhaust gas, by promoting a chemical reaction on the surface of its interior honeycomb. Fortunately, additive packages in oil and fuel take care of most of the dissolved impurity issues, leaving automotive filters to take care of particulates.
Air filtration is the first line of defense against premature engine wear. Engines inhale air in very large quantities, about a quarter of a million cubic inches per minute for a typical 1.5 L four-stroke turning at 6000 RPM. At flow rates that large, suspended particles move at very high speeds, especially when constricted through intake runners and air boxes. The particles carried in the air stream have large amounts of energy, which can be used to advantage in the centrifugal air cleaners used in construction equipment and heavy trucks, but must be dissipated before the particles can be trapped in the filter. The “trapping” action happens as the particles bounce, decelerate and stick to the fibre structure of the media, allowing effective filtration even though the size of the void space between the fibres is larger than the size of the typical dirt particle. Fibres may be cellulose (wood pulp), polyester, or glass. The advantage is the ability to flow adequate air volumes into the engine with minimal pressure drop across the filter.
Fuel filters don’t have to decelerate fast-flying particulates, but they do operate immersed in solvents, under considerable pressure in fuel injected engines. Pressure drop across fuel filters is an important factor in system performance, particularly as the filter ages, and is one of the main reasons why experienced technicians often install OE-quality fuel filters as a starting point in fuel injection diagnosis.
For oil filters, fluid pressure, heat and in some designs, the need to bypass engine oil should the filter become clogged is an added complication. Where filters clear fuels and lubricants, porosity (or “permeability” to filter engineers) is the key to efficiency. Unlike air filters, to trap small particles, the media needs small holes. The challenge for oil filter designers is to cope with longer drain intervals, especially with synthetic oils, while packing enough surface of the right media in a very small package. The same is true for the new spin-on transmission filters, although ease of service should encourage regular changes. And if you’re a racer or fleet operator, and used to opening the “can” to search for metal particles, you may want to reconsider: by the time the silver gets noticeable, the engine is in serious trouble, so don’t rely on the filter media to warn of impending engine failure. Spectroscopic oil analysis is a better choice.
Is OE or Aftermarket best?
The correct answer to the question of which design is best for a typical vehicle may surprise: either one. Original equipment manufacturers specify filters by bench testing methods and life cycle analysis on test engines. In the case of air filters, a “standard dust” is introduced and flow and pressure drop is measured. Testing on running engines involves operations at levels of contamination far exceeding what would be experienced in normal driving with regular maintenance. With realistic change intervals, it’s possible to design aftermarket filters with more open media for longer life and lower cost without sacrificing performance in real-world conditions.
Is there a secret to good engine air and fluid filtration? Good installation practice is one, mainly by paying attention to filter orientation and the need to preserve the gasket. The “mail slot” technique is definitely a bad idea with air filters, as is drastic over tightening of oil filters with wrenches. Blowing out air filters with compressed air, or tapping the dirt loose is another procedure with diminishing returns. Perhaps most importantly, use the service reminder sticker, warranty book entry or shop management software to record the change date and mileage. Discolouration is not a reliable indicator of the end of a filter’s useful life, but age and mileage are. SSGM