As technicians, we know that oil filters are designed to filter out, or entrap solid particulate contaminants in engine oil, but what we often do not know is how efficient a particular oil filter is in removing solid particles of a given range of diameters. We do know however that oil filters are not created equal, but since we cannot test the efficiency of an oil filter in any way other than fitting it to an engine, deciding which oil filter is more equal than competing brands is no easy task. In this article, we will take a closer look at the properties, characteristics, and attributes high quality oil filters, such as Ryco oil filters must possess to satisfy the oil filtration demands of modern engines, starting with this question-
Automotive oil filters are generally of the full-flow type, which means that all of the oil in the engine is continuously passed through the filter in order for the filter to entrap solid contaminants for as long as oil is circulating though the engine. In most full-flow designs, the pressurised oil enters the filter from the outside of the filter medium, to be forced through the media towards the centre core, from which the oil leaves the filter and enters the engine. However, in a few cases, the oil enters the filter through the core, and passes through the filter medium towards the outside to improve the oil’s flow characteristics through the filter, and to reduce the physical size of the filter element.
Nonetheless, regardless of the direction of flow through the filter, the ultimate efficiency of all oil filters depends on several filtration mechanisms, but note that not all filtration mechanisms are present in all filters. Which mechanisms are present in any given oil filter depends on both the filter design, and the actual filter medium in the filter. Below are some details of the various filtration mechanisms that occur in most automotive oil filters-
Direct interception
As the term suggests, particles that are bigger than the smallest pores on the surface of the filter medium are intercepted, and held captive on the surface of the medium.
Depth entrapment
In some filters, the pores in the filter medium are not of uniform size, which allows some particles to penetrate the medium until the particles encounter pores they cannot pass through. Thus, particles can be trapped at various depths deep within the filter medium, and while this is a very efficient way to entrap solid particles, the flow rates of these filters generally begin to deteriorate sooner than filters that use surface interception/entrapment of solid particles.
Adsorption
This mechanism depends on the molecular and/or electrostatic attraction between the particles and the filter medium to capture solid contaminants.
Inertial impaction
Some particles are massive enough to be partially driven into the filter media by their motion when oil flow rates are high, and then to be held captive by the filter media by adsorption forces that are created by the oil flowing around the impacted/embedded particles.
Brownian movements
This is a complex physical process that causes particles (typically smaller than about 1 micron) to move around in the oil independently of the oil flow pattern. In practice, this process causes small particles to become entrapped in/by the filter medium when they venture close enough to the filter medium to be captured by adsorption forces. Note that the efficiency of this process is directly related to the temperature of the oil- the hotter and less viscous the oil becomes, the more particles are entrapped.
The filtration processes outlined above largely depend on the filter medium in any given oil filter, but in premium oil filters like Ryco filters, there may be more than one filtration mechanism present. However, in most “economy” oil filters, there is no guarantee that any of the mentioned filtration mechanisms have a high efficiency value. In fact, most economy oil filters are rated to remove solid particles that are bigger than about 40 microns, which is several times larger than the vast majority of solid particles in most car engines. Having said that, let us look at some of the things an oil filter has to deal with in terms of particle sizes, starting with this question-
As a practical matter, a micron is defined as one millionth of a metre, which is many times smaller than the smallest object that normal unaided human vision can resolve. Consider the diagram below-
This diagram shows some comparative diameters, but note while that the scale has been greatly exaggerated for clarity, the diameter of each circle is correct relative to all others. So what do these circles mean in terms of oil filtration? Consider this-
You might argue that it is extremely unlikely that any of the objects listed above will ever find their way into a car engine, but it is important to keep the diameters of these objects (as opposed to the objects themselves) in mind in the context of the effective micron ratings of oil filters, some details of which are given below-
Nominal Micron Rating
This rating refers to any given oil filters’ ability to remove solid contaminants of a specified diameter at an efficiency rating that is expressed as a percentage- usually somewhere between 50% and about 98%. In practice, this percentage is known as a filters’ nominal rating, and it means that if the filter is rated at 60% for particles that are say, 20 microns in diameter, that filter will remove 60% of all particles that are 20 microns in diameter.
Absolute Micron Rating
This rating means that a filter is capable of removing at least 98.7% of particles that have a specific diameter, as opposed to all particles in the oil, regardless of their diameter. This test is performed by passing oil that contains a known number of particles of a certain size through a sheet of filter media, and the rating is calculated based on the number of particles that made it through the media.
Beta Rating
This rating is obtained by passing oil that contains measurable particles of different diameters through a filter multiple times. In practice, the test uses very accurate particle counters both downstream and upstream of the filter, and the actual beta ratio is calculated by dividing the number of particles of the rated micron size upstream of the filter by the number of particles of the rated particle size downstream of the filter.
Thus, if a given filter is rated at 20 microns, and has a beta ratio of 10, it means that (on average) there will be 10 particles bigger than 20 microns upstream of the filter for every one 20-micron particle downstream of the filter. Therefore, since beta ratios are calculated with this formula - ((beta-1)/beta) x 100 - this particular filter is 90% efficient at removing particles that are 20 microns and larger from the oil.
However, it must be noted that although the beta ratio is the most widely used way to express the efficiency of automotive oil filters, this test method ignores actual operating conditions like changes in oil temperature and oil flow rates. Moreover, beta ratings also do not provide any indication of a particular oil filters’ ability to keep contaminants entrapped, the total contaminant load the filter can hold without impeding oil flow, or how consistently the filter will perform over its useful lifetime.
An additional problem involves the fact that few, if any oil filter manufacturers actually publish the effective ratings of their products, which means that unless we fit high quality oil filters from manufacturers such as Ryco, we have no way of knowing how efficiently the oil in any engine will be filtered, which brings us to the subject of-
Metal wear particles, as well as agglomerated carbon particles represent a significant percentage of the contaminant load in all engines, but the real problem is that air filters are not nearly as efficient in filtering out air-borne contaminants, as we would like to think.
The fact is that two of most common substances on Earth, silica and alumina, are both harder than good quality hacksaw blades, and both exist in the form of wind-blown dust grains that vary in diameter from sub-micron size, to about 30 microns and larger, and it would be tempting to think that high quality air filters are effective in filtering out these abrasive contaminants.
However, in the Australian context where dust levels are generally very high, silica dust particles form a substantial percentage of atmospheric dust, and particles up to about 10 micron in diameter pass through even OEM air filter elements with relative ease, to end up being suspended in the engine oil.
If we take this fact together with the fact that many economy oil filters are rated to only remove solid particles measuring about 40 micron and larger, it becomes obvious that silica particles can eventually turn engine oil into an abrasive paste that has the potential to defeat the lubricating action of even the best engine oil formulations.
On the face of it, objects that measure only 10 microns across should pass right through engine clearances without touching sides, but this assumption would be wrong. Consider the typical engineering clearances in a modern engine-
It should be noted however that the clearances listed above are operating clearances, and NOT dynamic clearances, which change continuously based on engine speed, engine load, and the viscosity of the oil.
In practice, most mechanical wear between sliding surfaces occurs when the size of a solid particle that is trapped between the surfaces falls within the same range as the thickness of the hydrodynamic oil film between the surfaces. Therefore, based on the examples listed above, it is entirely possible that 10-micron silica or other hard particles can form a bridge between two surfaces when the dynamic clearance between say, a crankshaft journal and the bearing material decreases to 10 microns or less when the engine is under a heavy load.
The result of the scenario described above is predictable, if not inevitable: increased rates of mechanical wear, loss of oil pressure that further increases wear rates, increased oil and fuel consumption, and ultimately, engine failure- all of which begs this question-
The answer to this depends on how one interprets the word “effective”; if it is taken to mean “perfect”, then no, but if it is taken to mean that a high quality Ryco oil filter will remove most solid contaminants most of the time, then yes, effective oil filtration is certainly possible.
Consider the results (given below) of a recent study of oil filtration vs. engine life that was performed by General Motors in the USA. This study was based on single-pass oil filters that were known to be 98% effective in removing solid contaminants of various diameters, and its purpose was to measure increases in useful engine life. An oil filter rated at 40 microns was used as a baseline reference, in which normalised wear and useful engine life each had a value of 1-
15-micron oil filter
With this filter, engine wear was only 29% of the normalised engine wear with a 40-micron filter, and engine life was extended 3.4 times.
8.5- micron oil filter
This filter reduced wear to 18% of the normalised engine wear with a 40-micron filter, and engine life was extended 5.5 times.
7-micron filter
This filter reduced engine wear to 0.14% of the normalised engine wear, and engine life was extended 7.1 times.
From the above it should be obvious that engine life is directly related to the diameters of the sold particles that are removed (or not) from the engine, but there is also a downside to this proposition. The fact is that the smaller the pores in the filter medium are, the shorter the useful life of a filter becomes, because the filter is clogged sooner. Moreover, modern engines demand high oil flow rates, which is difficult to accomplish with fine-pore oil filters when oil is cold and very viscous. In addition, oil filters have to provide reliable and efficient filtration during the entire oil/filter replacement interval period.
Satisfying all of these demands under real-world conditions becomes very difficult when oil filter manufacturers generally do not publish the operating parameters of their products. In practice, this means that unless we fit high quality oil filters to our customers’ vehicles, factors like expected engine life and oil filter performance is very much a guessing game.
Thus, to remove all the guesswork and ensure efficient oil filtration, any high quality oil filter should have certain qualities and attributes that are generally not present in economy filters. Below are details on some of the most important qualities and properties of high quality oil filters, all of which are present in premium Ryco oil filters-
Good pressure/flow profile
While not all Ryco oil filters are fitted with internal by-pass valves, those that do are extremely unlikely to activate the valve prematurely. As a practical matter, oil by-pass valves are calibrated to remain closed until a preset pressure differential across the valve is reached, this differential typically being in the 0.70 – 0.84 pressure differential /square cm range, which is high enough to allow full flow filter operation during the filter replacement interval- even on fine-pore filters.
Nitrile drain-back valve seat
Being made of soft, pliable nitrile-based materials, the drain-back valve seats on Ryco oil filters ensure a positive seal, which in turn, prevents the partial by-pass of unfiltered oil into the engine during normal engine operation.
Fully sealed filter media
All Ryco filters that use pleated paper elements are sealed against leakage past the media seam, as well as past the media and the end cap with a thick layer of adhesive that is guaranteed to be fully heat and oil resistant. Moreover, the filter media in Ryco filters are fully supported to withstand flexing and deformation during oil pressure surges, which is one of the main causes of filter failure on economy oil filters.
High dirt-holding capacity
While all oil filters are able to hold on to captured solid particles, some oil filters do this better than others. For instance, Ryco filter media are designed not to release captured particles, or to allow captured particles to be forced through the media during oil pressure surges.
High resistance to media migration
One of the most common problems with economy oil filters is that the filter media becomes brittle and starts to disintegrate either as the cause of engine vibrations, or because of extremely high differential pressures. Regardless of the cause though, the end result is the same- an additional load that the damaged filter cannot remove.
From the above it is clear that although perfect oil filtration is not possible, the use of premium quality Ryco oil filters creates conditions that comes as close to perfect as it is possible to get under real-world conditions.
