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Filtration foundations

October 11, 2010
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In my time on this earth, I have come across many people who are car experts. People who have the skills to be MacGyver-esque when it comes to fixing virtually anything that goes awry in their vehicles and who seem to know all of ins-and-outs of the automotive industry.

In my time on this earth, I have also come across many more people who don’t know a crankshaft from an intake valve, let alone anything about how to change their own oil. This includes some carwash and lube owner/operators.

Then, there are those of us who fall in that grey — area those of us who know the basics and can generally decipher mechanic lingo. This is where most of the carwash and lube owner/operators fall.

Hopefully, you have a manager in the first category. Cheers to him and may he continue to serve as an inspiration to the rest of us. For the folks that fall in one of the last two categories, its time that we take the bull by the horns and bulk up our arsenal of automotive knowledge. Today’s lesson: oil filters. Glamorous? Perhaps not, but one’s knowledge on this subject can be life saving to your customer’s vehicle.

This is more or less a crash-course in engine oil filters: what they do, why they’re important and an overview of various oil filter medias and the strengths and weaknesses of each.

History of oil filters
The earliest incarnation of the modern oil filter came about in 1923, when Ernest Sweetland introduced his invention known as the Purolator.

Incorporated into the lubricating system after the oil pump and before the oil flows into the engine bearings (take a deep breath if you feel overwhelmed, it will get easier to understand), the original Purolator featured an upright series of seven twill weave cloth-covered, perforated plates encased in a heavy-duty cast container. It also had a sight feed glass on one side, enabling the owner to see the oil flow and change the filter when flow slowed to a trickle.

The Maxwell Chalmers Company saw immense promise in this new product, installing a Purolator on a Maxwell automobile that was test-driven on a round-trip from Detroit to the West Coast in 1924. The longer oil drain intervals, cleaner oil and reduced engine wear offered by the Purolator ensured endorsement by the automotive industry, and they soon became standard on many popular automobiles of the day.

In the late 1930s, cotton waste material was introduced as filtration media, providing the first filter replacement capability. Various woven fabrics were also used in some filter designs.

By 1946, as disposable filter models became the norm and interest in saving production costs increased, materials such as pleated paper and cellulose became the filtration media materials of choice, materials that are still widely used in today’s oil filters. The first full-flow oil filter, capable of filtering 100 percent of the motor oil, was introduced in 1943 and became standard on mass production vehicles by 1946.

Whew. The history lesson is over. Back to reality and back to the task at hand.

Oil filter functionality

The evolution of the oil filter spanned decades and oil filter technology continues to advance and mature to this day. The constant improvement of oil filters is a testament to the incredibly important role they play in the functionality of engines.

The next question, then, is just why are oil filters so important? Just what exactly do they do that warrants the need for extensive knowledge on the subject?

The answer to those questions seems simple enough — the oil filter filters oil. (Say that ten times fast.) The sole purpose of oil filters is to catch and hold the dirt and contamination that managed to weasel its way into the oil stream.

Sounds like the oil filter has a pretty laid-back job but when you consider that the number one killer of engines (no, this is not a public service announcement) is dirt, the job of the oil filter quickly rises up the ladder of importance.

Engines are subjected to contamination from both outside environmental sources and internal contamination produced in the engine by the combustion process. Because they are constantly at risk of falling victim to these contaminants, it is vital that the oil filter does its job.

Engine oil filters remove solid foreign matter, such as soot, wear particles and dirt, from the engine oil. They are installed between the oil pump and the body of the engine. All the engine oil is routed through the filter before it circulates in the engine.

Most oil filters are fitted with an oil pressure relief valve to assure a continuous flow of oil to the engine in the event of the filtering element becoming obstructed. To prevent oil starvation in such an event, unfiltered oil bypasses the obstructed oil filter and lubricates the engine.

The filter itself
Oil filters can be divided into two design groups: cartridge and the more popular spin-on. The major difference between the two is in their construction. In a cartridge filter, the filtering element is replaced by disassembling an external housing. In the case of the spin-on filter design, the filtering element is changed by replacing the entire cartridge and filter housing assembly. Both types of filters are available in a variety of sizes. The filtration characteristics of each type depend solely on the media used. The cost of both types of filters is also dependent on the type(s) of media used.

Filtering mechanisms
There are two basic types of filter mechanisms used in oil filters: surface and depth.

Surface filters are made up of a pleated paper and contaminants are trapped on the surface of the paper. The oil comes from the crankcase and enters the filter. The oil then flows over the surface of the paper and the contaminants are trapped while the clean oil is allowed to pass through the pores of the paper. The most popular type of surface filter is the spin-on filter discussed above.

Depth filters are essentially the same as the surface filter except for the filtering media. In depth filters, the filtering media is usually a blend of cotton thread and resilient supporting fibers that meet the flow requirements recommended by original equipment manufacturers (OEMs). Depth filters are limited in flow capability in comparison to surface filters of the same size. High flow rates in some engines may be too much for the depth filters to properly handle which is why this type of filtering mechanism is beginning to be phased out.

Inside the filter
So, we now understand (feel free to use the term “understand” loosely) the designs of oil filters as well as the mechanisms of filtration, but what exactly are these filters made of? There are many types of oil filter media, including cellulose, synthetic blend, stainless steel mesh and synthetic. The majority of oil filters are made of cellulose (paper) media and are economy filters. The fibers in the paper act as a type of a screen to block contaminants of an average size from acting as an abrasive when the oil recirculates. Cotton is sometimes added to the cellulose to improve its performance.

Synthetic blend is a media that is a combination, as is evident in the name, of the cellulose media and the synthetic media. The filters are typically made of 15 percent synthetic media and 85 percent cellulose media.

Stainless steel mesh media is the least-used oil filter media and is only used for very specific applications and efficiencies. This media is used when extremely high flow is necessary; for example in drag racing with high pressure pumps, seldom a concern for oil change professionals.

Synthetic media is generally regarded as the most advanced filtration media. Synthetic fibers in synthetic media are smaller, have a controlled size and shape and can pass more fluid through because there are more of them. They have small spaces between their fibers that prevents contaminants from loading in the depth of the media and plugging the oil-flow path, which could result in higher restriction and less capacity. The small fibers ensure high efficiency levels, maximum capacity and stellar durability.

Quality control
All filters have to undergo SAE (Society of Automotive Engineers) tests to prove that they meet the engine manufacturer’s requirements.

The SAE J806 test, for example, uses a single-pass test, checking for contaminant holding capacity, size of contaminant particles trapped, and ability to maintain clean oil.

As an amendment of the J806 test, the multi-pass test also looks for filter life in hours, contaminant capacity in grams, and efficiency based on weight.

There is also a new test, the ISO 4548-12 that provides both particle counting and gravimetric measurement to measure filter capacity and efficiency.

Congratulations! You made it through Oil Filters 101. Now go put your oil filter knowledge to use at your shop.


Sally Trnka is a Technical Writer for AMSOIL INC. She can be reached by phone at: 1-800-777-8491 or, by snail mail, at: AMSOIL INC., AMSOIL Building, Superior, WI 54880.