Farmer's Weekly (South Africa)
HOW AN OIL FILTER WORKS
Amazingly, a slightly dirty oil filter is more efficient than a clean one! Jake Venter explores this and other interesting facts about this crucial engine component.
In the early days of motoring, filters were unknown; carburettors simply sucked in dirt and dust. These particles collected in the oil and petrol and caused considerable component wear; the life of an engine was typically a fraction of what we expect of an engine today.
The various filters used in modern engines developed gradually, spurred on particularly by research carried out during the two world wars.
FILTER CONSTRUCTION
By 1910, most engines had a strainer on the intake side of the oil pump to collect relatively large pollutants, an adequate system in the days when oil was changed every 2 000km.
The first cartridge-type oil filters appeared in the late 1920s, but they were mostly of the bypass type. Engineers were concerned that a blocked oil filter might cause oil starvation, so they came up with a filter arrangement that allowed only a portion of the oil to be routed through the filter at any particular time. All the oil was eventually filtered, but it took a bit longer. The modern full-flow oil filter appeared a few years later. In this design, all the oil flowed through the filter on its way from the oil pump to the engine. The filter material therefore had to be less restrictive than before and a relief valve was required to prevent oil starvation in case of a filter blockage. Most modern engines have full-flow filters, but some heavy duty engines are fitted with both types of filter. The bypass unit is filled with denser material that can remove extremely small dirt particles from the oil. Both types are available in either spin-on or removable cartridge design.
Oil filters usually have the following components:
• An O-ring or other type of seal to prevent oil leaking from the filter housing/ cylinder block interface;
• An anti-drain valve to stop oil draining out of the filter when the engine stops. Some filters are mounted nearly horizontally so that such a valve is unnecessary; • A filtering element, usually a good-quality treated paper, pleated and scalloped to increase the area exposed to the oil;
• A bypass valve on the inlet side so that a blocked filter will not stop the oil flow;
• A rigid steel core that supports the filter element;
• A steel cover designed to be strong enough to cope with the maximum pressure that the oil pump can deliver.
REPLACEMENT
early cars had no oil filter, so their engines didn’t last long
An oil filter should be replaced at the interval mentioned in the owner’s manual, or at least every second oil change. These intervals are determined by the engine manufacturers working in consultation with oil filter manufacturers.
One of the tests conducted by engineers consists of passing air though a filter. Controlled amounts of specially formulated dirt are fed into this air and the pressure drop is then measured. This drop is an indication of the degree to which the filter is blocked. The most interesting part of such a test is the conclusion that the filter becomes more capable of stopping small dirt particles when it is partially blocked. The trapped dirt helps to trap more dirt so that it’s actually harmful to change a filter too soon.
ol der vehicles
The older an engine gets, the more carbon particles are produced during combustion. This means that the oil filter in an older car should be replaced more frequently.
The oil should also be changed more often if you indulge in what the car manual calls ‘severe motoring conditions’. Typically, these are motoring for long periods at close to full throttle, or under very hot ambient temperatures, or travelling less that 10km every day so that the engine seldom reaches normal operating temperature.
• Jake Venter is a journalist and a retired engineer and mathematician. Email him at jacobventer77@gmail.com. Subject line: Auto engineering.