Through the filtering of the main oil-flow in the four-stroke engine all the points to be lubricated are supplied directly with filtered oil. In the oil sump or in a special container the oil is then sufficiently cooled and can once more be pumped into the circulation.
In the case of the pressure circulation lubrication, the oil reaches the oil filter directly, using one single pump. In the filter flange the bypass valve allows the oil, should the filter be congested, to flow directly to the points to be lubricated. The pressure is calculated in such a way that also the most distant lubrication points, (e.g., the cylinder head) receive enough lubricating oil. In the case (nowadays only rare) of secondary-flow filtering, only a small part of the oil is tapped off from main oil-flow. This oil can be filtered more intensely due to the lower flow-volume. The rest flows unfiltered to the lubrication points. One recognises a congested secondary-flow filter by the fact that it is no longer warmed up by the oil flow.
In todays modern engines changeable filters with steel casings are only rarely used. To reduce disposal problems (casing, valve and filter paper) only the paper-insert is now changed. These are called case-filters (see picture).
On its way from the pump to the oil sump, lubricating oil can only reach either the cylinder head or the crankshaft. The difficulty with the pressure circulation lubrication, is the oil supply to the connecting rod and the cylinder sleeves. Here e.g., drilled holes in the crankshaft or spray-jets in the con-rod shaft, provide for the further transportation. From all lubrication points can the oil drip back into the oil sump, where it is taken, from the coolest possible place, back into circulation. To effectively manage high thermal demands, oil sumps made of aluminium with a special oil guidance, thermo-statically controlled oil coolers or just a larger amount of oil can be helpful. In a cold engine the oil pressure can become very high, for this reason there are the oil pumps which are specially regulated to avoid such pressure 'peaks'. The normal oil pressure at operating temperature should be at least 2 bar. 08/11