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Understanding Oil Filters


 
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PostPosted: Mon Jan 14, 2019 4:40    Post subject: Understanding Oil Filters Reply with quote

Many misconceptions surround oil filters and many of them are played out on social media outlets such as You Tube and many others where people do things such as cutting them open and seeing what they contain, and suddenly state things such as filter A is better than filter B as it has a longer filter medium, or forums such as this one and many others, and their lack of knowledge and instant assumptions astound us as engineers as filters are much more complex than that, and this little piece is designed to explain many of them and to educate people as to what to look for and why?

Oil filters have a design life and this is usually as a vehicle mileage as this is the easiest and most consistent way for non engineers and the uninitiated to easily identify when they want or need changing, and when to change them; and often this is dictated by the vehicles engine manufacturer under "average conditions" as many traditional off road vehicles, as opposed to SUV's do actually do off road and work and such vehicles are given two sets of service intervals called average working conditions and severe working conditions, severe working conditions often stipulate that the oil and filter changes are done at much lower mileages than average working conditions.
Design life is dictated by many things and older oil filters on older engines have a short designed working life as their oil service intervals were much shorter than more modern engines with their much longer oil service intervals, and this means that older engines can have an oil filter life of just 3,000 miles or 5,000 kilometres and this is our first question:

What is our oil and air filter designed working life?

If you have an older vehicle with a service specification of 3000 miles and oil filter with a designed operational life of 3000 miles and you change your engine every 3000 miles then everything is fine but if you fit an oil filter with a designed operational life of 3000 miles to an engine with designed service intervals of 6000 miles than you have problems.

Oil filters have a specified oil viscosity range and a specified oil type and this is our second question:

What is our filters designed viscosity range and oil type?

Most oil filters have a variable working viscosity range and different markets fit different filters for differing viscosity ranges and ambient air temperatures, a hot country such as South Africa will have a different oil filter fitted to those in a Scandinavian country as one country is predominantly hot while the other is predominantly cold; and yes they are all manufactured for differing oil types such as synthetic, semi synthetic, or mineral oil as this can make a significant difference.
In simple terms mineral oil is thicker than semi synthetic oil and semi synthetic oil is thicker than full synthetic oil and if you have an oil filter designed for mineral oil then you can put oil other than what it was designed for through it, it will work with semi synthetic oils or synthetic oils, but if your filter is designed for full synthetic oil you cannot put mineral or semi synthetic oil through it without problems and generally these are expensive problems; but some engine and filter manufacturers do make filters which can work with either semi or fully synthetics, but check.

If we look at an oil filter and begin with the end cap, this contains a series of holes around the outside and these are where the unfiltered oil comes in directly from the oil pump, and in the centre there is a large threaded hole through which the filtered oil passes to the engine, and it is where the filter screws to the engine, so lets take an oil filter apart:

On the inside of this end cap is a non return valve and this can be of two main types which are a basic flap valve or a traditional sprung valve and in newer types of filter use a silicone flap valve as they are cheaper, simpler, and more effective during the engines first start from cold as they only require as little as 2 psi (or less) to open them and allow oil to flow through them, spring valves are much worse as they have a much higher spring pressure and they require much higher pressures to open them which means they take much more engine cranking to open them and they delay the oil flowing to the engine for much longer. In winter when we have sub zero temperatures as the oil is much thicker, the spring types of non return valve can literally take seconds to open.

Now we move to the inside of the filtering medium and see that it is housed over a fine mesh and this is generally a fine plastic mesh to hold the filtering medium in shape and stop it deforming under the oil pressure passing through it.

At the bottom or top, or even in the middle of this mesh there may be another valve called a by pass valve and common myths claim this is to allow oil to flow when the filter is blocked, this is a total misconception as while it may do this it is a secondary function and nothing more, and many filters do not have a bypass valve fitted and these are called full flow filters as all the oil flows through them as it cannot bypass the filtering medium; if a filter has a bypass valve this is for one simple reason and this is easily explained.
When a partial flow filter such as these are on a vehicle and you go out to work at 5.00am and its -10 outside you know your engine oil will be like treacle and gives very little lubrication at these temperatures and your oil pressure will be excessively high when compared to its normal operating temperature and oil pressures above 50psi are not uncommon at these low temperatures, these pressures open the valve and allow a proportion of engine oil to bypass the filter and travel through the engine and it warms the oil up much quicker and this is its function, to warm the engine oil up quicker. As the oil temperature rises it becomes thinner and the oil pressure drops and as it drops the bypass valve begins to close in proportion to the oil pressure and allows less oil to bypass the filter until it reaches the point where the oil temperature is sufficiently high to reduce the oil pressure to the point where the bypass valve is closed and the filter now becomes a full flow filter.

Filtering mediums are the most common area of misconceptions and this is due to a total lack of understanding.

Generally there are two types of filter medium and one is the cellulose coated paper and the other is the fibre filter and this is simply the types of materials they are made from, paper or fibre; there is a third filtering medium which is a hybrid of both cellulose coated paper and fibres, so our final question is:

What is our filtering medium made from?

Traditionally cellulose paper was always used as it offers a fair filtering medium which can be made cheaply and in huge quantities for cheaper machine manufacturing and it can be made accurately and have its surface area hugely increased by pleating it as with a cellulose coated paper filter, size really does matter.
Fibre filters use a fibre and these are termed as "active filters" and when you look at them under a microscope they look quite coarse when compared to a paper filtering medium and this is for god reason as unlike a static paper filter, the fibres wave about as oil passes through them and they knock a lot of the particulate matter they are filtering, by literally hitting them and they fall down the filter to the bottom, this gives them a partial cleaning action as well as a filtering action and they give a much better filtering action and a self cleaning action in a much smaller package. Active filters can give an increase in working life of over 350% in a package around half the size without even trying and in a package of equal size to a paper filter you can easily approach an increased working life approaching 1000% which is why we see many ordinary vans with oil services of 30,000 miles and more for many heavy lorries.

We have another version of these oil filters and if you open one up it looks to have two different filters in one can, the top half is of one type and there is a metal or plastic ring around the bottom of it, and below this there appears to be another filter; in these type of filters generally, the top half is the active fibre filter which cleans and filters the oil and it knocks particles to the bottom of the filter, the bottom of the filter is cellulose coated paper which traps and holds the filtered particulates.

As we can readily see it is material type which is more important than size.
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