Even new engines sometimes contain small particles of metal left over from the manufactu-ring process, or grains of sand which have not been removed from the crankcase after casting. Old engines continu-ally deposit in the sump tiny fragments of metal worn from highly-loaded components such as the piston rings. To prevent any of these lodging in close-fitting bearings or blocking oil drillings, engine oil is filtered.

The simplest filter is a wire mesh strainer that prevents solids from entering the oil pump—most engines have one of these.

 In addition, many engines have an extra filter that traps very fine particles. The most common type has a pleated paper or felt element, and pumping oil through it removes all but microscopic solids from the lubricant.

 Another way of separating particles is to pump the oil into a fast-revolving cylinder. Centrifugal force then throws the particles to the outer edge while uncontaminated oil passes through the centre.

 Most engines use a full-flow system to filter all the oil after it leaves the pump. The most popular method is to pump the oil into a bowl or canister containing a cylindrical filter. From the inner walls of the bowl, oil flows through the filter and out from the centre to the main gallery.

Full-flow filtration works well provided the filter is renewed at regular service intervals. If it is left in service too long it may become blocked. When this happens, to prevent oil starvation, the build-up of pressure of oil inside the filter forces open a spring-loaded relief valve in the housing and the oil by-passes the filter. The valve prevents immediate engine failure, but the engine will be lubricated with unfiltered oil until the filter is renewed.

 Rarer now is a by-pass filtration system in which only a proportion of the oil pump output goes through a filter. The remainder is fed directly to the oil gallery.