Post: What is an Oil Filter

An oil filter is a filter used to remove contaminants from engine oil, transmission oil, lubricating oil or hydraulic oil. They are primarily used in internal combustion engines of motor vehicles (on-road and off-road), powered aircraft, railroad locomotives, ships and boats, and static engines such as generators and pumps. Other vehicle hydraulic systems, such as those in automatic transmissions and power steering, are often equipped with oil filters. Gas turbine engines, such as those on jet aircraft, also require the use of oil filters. Oil filters are used in many different types of hydraulic machinery. The oil industry itself uses filters for oil production, oil pumping, and oil recovery. Modern engine oil filters tend to be either “full flow” (in-line) or “bypass”.
Early automobile engines had no oil filter, only a simple mesh screen placed on the oil pump intake. As a result, frequent oil changes were required due to the generally low quality of the oil. The Purolator oil filter was the first automotive oil filter; it revolutionized the filtration industry and is still in production today. The Purolator is a bypass filter where the majority of the oil is pumped from the sump directly to the working parts of the engine, while a small portion of the oil passes through the filter via a second flow path, filtering the oil over time.
.Bypass and Full Flow
.A full flow system will have a pump that delivers pressurized oil through the filter to the engine bearings, where the oil then returns to the sump under gravity. For dry sump engines, the oil that reaches the sump is sucked by a second pump to a remote tank. The purpose of a full flow filter is to protect the engine from wear.
.Bypass
Modern bypass oil filter systems are secondary systems, where the bleed from the main oil pump supplies oil to the bypass filter, which then does not flow to the engine, but returns to the sump or tank. The purpose of the bypass is to have a secondary filtration system to keep the oil in good condition, free of dirt, soot and water, providing much smaller particle retention than is practical with full flow filtration, which is still used to prevent any particles that are too large from causing significant wear or serious clogging on the engine. Originally intended for commercial and industrial diesel engines with large oil volumes, where the cost of oil analysis testing and additional filtration to extend oil change intervals makes economic sense, bypass oil filters are becoming increasingly common in private consumer applications. (The bypass must not affect the pressurized oil supply within a full-flow system; one way to avoid this effect is to make the bypass system completely independent).
.Pressure Relief Valve
Oil Filter Cartridge, Internal Structure.
Most pressurized lubrication systems incorporate an overpressure relief valve to protect the engine from oil starvation by allowing oil to bypass the filter if flow restriction becomes too great. Filter bypassing can occur if the filter becomes clogged or if the oil thickens due to cold weather. An overpressure relief valve is often incorporated into the oil filter. Filters that are mounted so that oil will flow out of them often incorporate an anti-drainback valve to keep the oil in the filter after the engine (or other lubrication system) has been shut down. This is done to avoid a delay in the buildup of oil pressure after the system is restarted; without an anti-drainback valve, pressurized oil must fill the filter before it can flow to the working parts of the engine. This condition can cause premature wear of moving parts due to the initial lack of oil.
.Oil Filter Types
.Mechanical
Mechanical designs use a filter element made of either bulk material (such as waste cotton) or pleated filter paper to capture and isolate suspended contaminants. As matter builds up on (or in) the filter media, oil flow is gradually restricted. This requires periodic replacement of the filter element (or the entire filter, if the element cannot be replaced individually).
.Cartridge and Spin-On
Volvo Replacement Paper Filter Element
Early engine oil filters were of cartridge (or replaceable element) construction, where a permanent housing contained a replaceable element or cartridge filter. The housing was either mounted directly on the engine or remotely, and connected to the engine by supply and return lines. In the mid-1950s, the spin-on oil filter design became available: a separate housing and element assembly that could be unscrewed from its bracket, discarded, and replaced with a new one. This made filter replacement more convenient and potentially less cumbersome, and quickly became the mainstream type of oil filter installed by automakers around the world. Retrofit kits were available for vehicles originally equipped with cartridge filters. In the 1990s, European and Asian automakers in particular began to move back to replaceable element construction because less waste was generated each time the filter was changed. American automakers also began to move back to replaceable cartridge filters, and retrofit kits were offered to convert spin-on filters to cartridge filters for popular applications. Commercially available automotive oil filters vary in design, materials, and construction details. Filters made entirely of synthetic materials, with the exception of the metal drain canister inside, are superior and more durable than the traditional cardboard/cellulose/paper filters that still dominate. These variables affect the efficacy, durability, and cost of the filter.
Motorcycle oil filters on Kawasaki W175. Old (left) and new (right).
.Magnetic
Magnetic filters use permanent magnets or electromagnets to capture ferromagnetic particles. The advantage of magnetic filtration is that maintaining the filter is simply a matter of cleaning particles off the surface of the magnet. Vehicles often have a magnet in the automatic transmission oil pan to isolate magnetic particles and extend the life of media-type oil filters. Some companies are manufacturing magnets that attach to the outside of oil filters or magnetic drain plugs (first invented in the mid-1930s and used on cars and motorcycles) to help capture these metal particles, although there is still debate about the effectiveness of such devices.
. Sedimentation
Sedimentation bed filters, or gravity bed filters, allow contaminants that are heavier than the oil to settle to the bottom of the container under the force of gravity.
. Centrifugal
A centrifugal oil purifier is a rotating sedimentation device that uses centrifugal force rather than gravity to separate contaminants from the oil in the same manner as any other centrifuge. Pressurized oil enters the center of the housing and then enters a drum rotor that rotates freely on bearings and seals. The rotor has two nozzles that direct the oil flow onto the inner housing, which causes the drum to rotate. The oil then slides to the bottom of the housing wall, leaving behind the particulate oil contaminants that adhere to the housing wall. The housing must be cleaned regularly, otherwise particles will accumulate to a certain thickness causing the drum to stop rotating. In this case, unfiltered oil will be recirculated. The advantages of centrifuges are that (i) the cleaned oil is separated from any water that is heavier than the oil, which settles to the bottom and can be drained away (provided the water is not emulsified with the oil); and (ii) they are much less likely to clog than conventional filters. If the oil pressure is insufficient to rotate the centrifuge, a mechanical or electrical drive can be used instead.
Note: Some separator filters[11] are described as centrifugal, but they are not true centrifuges; rather, the oil is directed in such a way that there is a centrifugal vortex that helps contaminants adhere to the outside of the filter.
.High Efficiency (HE)
A high efficiency oil filter is a bypass filter that is claimed to extend the oil change interval. High efficiency oil filters typically have a pore size of 3 microns, and studies have shown that this can reduce engine wear. Some fleets have been able to extend the oil change interval by 5-10 times.
.Placement of filters in the oil system
Determining how clean the oil needs to be is important, as the cleaner the better, the higher the cost. After determining the optimal target cleanliness level for a pollution control program, many engineers are faced with the challenge of optimizing filter placement. To ensure an effective solids intrusion balance, engineers must consider factors such as whether the filter is being used for protection or pollution control, ease of maintenance, and whether the performance of the unit under consideration meets the challenge of target setting.