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How an Oil Filter Works

May 29, 2020

How an Oil Filter Works

It is often said that motor oil is the lifeblood of your engine. If clean oil is not circulating through all of the vessels and across the components inside an engine, it will soon die a very expensive death. If oil is the blood of an engine, the oil pump is its heart, making sure that the oil collected in the oil pan underneath your engine makes it all the way to the top so that everything is lubricated. As the oil is being pumped through your engine, it makes its way through the oil filter. Staying with the anatomy metaphor, the oil filter is a kidney that filters out waste. But unlike your kidneys, an oil filter eventually gets clogged up with debris from your engine and needs to be replaced.

Why the need for an oil filter?

A modern automotive engine is designed with extremely tight tolerances between its internal components. All of the pistons, valves, bearings, and other parts fit snugly in place. This makes for a more lightweight, powerful, and efficient engine. But it also makes lubrication more challenging.

When the pistons travel in their cylinders, or the valves open and close, or the camshafts rotate in their bearings, or dozens of other components move around in close proximity to each other, they create friction and heat. Motor oil works to mitigate friction. A thin layer of oil coats all of the parts and creates a sliver of separation. That way, no metal-on-metal contact occurs. If it did, the metal would quickly deteriorate and your engine would grind to a halt – literally welding itself together.

Because the spaces between engine components are so small, it is essential that the oil used be uniform in its composition, of the correct viscosity or grade, and remain clean.

Makers of modern motor oils strive to produce products that are uniform at a molecular level. This is where synthetic motor oil has a distinct advantage over conventional oil. Created artificially in a lab, rather than derived and distilled directly from crude oil, synthetic oil (such as Mobil 1™ Full Synthetic) is made up of molecules that are far more consistent in size and shape when compared to conventional oil.

Viscosity is the ability of a liquid to resist flowing – how thick it is. Motor oil is categorized not only by type (conventional, synthetic, synthetic blend, etc.), but by its viscosity, or “grade”. The codes 10W-30 and 5W-20 represent how thick the oil is when cold (the “W” stands for winter), and how thick it is when warmed up to operating temperature in your engine. Therefore, 5W-20 oil is grade 5 oil when cold (for easier starts) and grade 20 (“20-weight”) when warm. The higher the number, the thicker the oil. Since engines are made with smaller spaces, it is understandable that oil viscosities have thinned out over time. 30-weight oil was once common; today, oil as light-weight as 0W-16 is being used in some engines.

The “clean” comes into play in two ways.

First, oil only lasts for so long. A glance at your vehicle’s maintenance schedule will reveal that an oil change is probably the most frequent procedure recommended. Depending on the make and model of your car, that could be anywhere from every 5K miles (the shortest interval recommended by a manufacturer under normal conditions) to 10K miles or more. It all depends on your vehicle. Overall, oil change intervals have been extended from recommendations of the past. Still, some “special” driving conditions might warrant shorter times between changes. No matter what, your oil will not last forever. It breaks down (mostly due to heat and the shearing forces in your engine) and loses its lubricating abilities. It turns to sludge. So, it needs to be changed to keep it clean and fresh.

But it also needs to be kept clean in between oil change services. That is the job of the oil filter. All of those moving parts, lubricated as they may be, still manage to wear at one another, leaving behind fine metal debris. The combustion process also creates some impurities that circulate in the oil. These contaminants can cause damage if not removed. The oil pump forces the oil from the sump (oil pan) to the oil filter where the impurities are removed before heading off to the rest of the engine.

What’s in an oil filter?

An oil filter is essentially a cylindrical container that mounts to the side of your engine. Sometimes it is located toward the front of the engine compartment, sometimes on the side, accessed through a wheel well. At least one manufacturer conveniently mounts the oil filter near the top of the engine in plain sight when you open the hood to make it easier to replace. There is no standard location for an oil filter mount. And there is no standard size for an oil filter. There are hundreds of different filters, each applicable to a specific application.

Regardless of where it is mounted and how big or small it might be, all quality oil filters will have similar components and construction. Most engines are fitted with canister-style filters. These “spin-on” units are self-contained and consist of a metal canister that ranges in size from that of a can of Vienna sausages to a can of Progresso soup. A series of small holes form a circle around the perimeter at one end; the oil enters the filter through these holes. Once inside, the oil is forced through a porous filter medium where impurities become trapped. A large hole is located in the center of the same end and acts as the exit for the oil on its way back to the engine. This hole is threaded so it can mount to the engine. A round rubber gasket seals the filter so oil does not leak. When a canister filter has been used up, it is disposed of, like an old air filter or set of brake pads.

Some automakers use a slightly different style of filter on their newer engines: a cartridge filter. Not a new design – cartridge filters actually predate their canister relatives – but seen as an environmental benefit, this style works in the same way as a canister filter. It is just made a bit differently.

The cartridge filter relies on a non-disposable filter housing mounted to a flange on the engine. When it is time for an oil and filter change, only the filter element and rubber seal are replaced. Only the fabric filter material and gasket need to be disposed of, preventing millions of metal cans from making their way to the trash heap.

Both types of filters will eventually clog up with enough contaminants embedded in the filter media that oil can no longer seep through. So, filters are also fitted with a bypass valve to make sure that the flow of oil through your engine is not disrupted if the filter stops up. Naturally, if the oil bypasses the filter, it is no longer being cleaned.

In either case, that of a canister or a cartridge filter, the purpose and the working materials are the same.

Choosing the right filter

What is different (besides size and style, of course) is the quality of the filter you select. Yes, every filter has the same basic components, but the quality of those components is different from one brand to the next. Just as there are choices between ceramic, semi-metallic, or organic brake pads, you have a choice regarding the quality of your oil filter. You will find quite a wide variance in prices for the same model of the oil filter. Some are as cheap as a few bucks, and others cost five times the price or more.

Why the disparity? Oil filter media is made from both organic and synthetic fibers saturated with resin and folded into pleats for increased surface area. Filters that feature a higher amount of synthetic materials are more effective and generally cost more. The filter element in a quality product will also hold up, resist shredding, better than a cheap one. The metal in an inexpensive filter can also be of a lighter gauge. And the gasket can be less-than-capable. The gasket seals the filter for thousands of miles against constant heat and vibration. Gasket quality is essential, but the ones used on a low-price filter tend to become brittle.

With your oil and filter, you get what you pay for. So it pays to make an investment in quality. A lot of research and development goes into a quality motor oil and oil filter. With manufacturers recommending extended oil change intervals, the longevity of your oil filter is important.

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