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 Engine Oil-Finder

Lubrication 2

Nr.Type of lubrication
1Dry friction
2Mixed friction
3Fluid friction

Right up to the 1980s, the operational safety was the top priority as far as the development of oils was concerned. Then, in addition, the respective engineers started to concentrate on the efficiency. Nonetheless, the layout of, e.g., the oil-pump and the feed-line network is still very important, perhaps even more so due to the decreased viscosity of the oils. One speaks of possible consumption advantages of up to 4 percent.

That however, does not alter the fact, that the above shown diagram of the friction course of oil lubricated bearings (Striebeck graph), is still valid, whereby, the hydrodynamic lubrication is still imperative. As the name says: 'dynamic'. There is e.g., the engine start-up after an oil filter change or the running of the still cold engine before the oil pressure has been sufficiently built up. Also the piston dead-centers or two mating surfaces with only a minimal of friction surface can lead to mixed friction.

The cooling is yet another area which becomes increasingly important for the lubrication system. As prime examples, technical literature mentions the lubrication of the turbo-charger bearings and the splash cooling of the pistons, mostly also in charged engines. Indeed, this is of course, valid for all the points, where oil has been and has provided lubrication. Oil comes into contact with places in the engine, where there is a lot of heat development, coolant on the other hand, does rather not.

The use of oil-hydraulics, e.g., to tension the timing chain(s) or it's application for regulation tasks, is relatively new. What is meant here, is the whole, in the meantime, pretty complex controlling of the valve drive, from the hydraulic tappets to the camshaft adjustment and the the variable valve-lift. This regulating has to provide a lot for the eficiency, because of the, approx. 1 bar higher oil pump pressure, some of it is lost again.

At this point we come to the oil pressure itself. It is highest directly at the pump, it decreases however, with each resistance. Thus, e.g., the lubrication of the connecting rod can be difficult, because on the way through the crankshaft from the main- to the con-rod bearing hardly any pressure remains. There must be a well sized relief valve, to take the pressure decrease into consideration. By the way, this is also influenced by the age of the engine, the larger the play is, the higher the pressure has to possibly be.

Apart from calculating the oil pressure, practical testing is absolutely necessary when checking the lubrication system. Thus, e.g., a Diesel motor car engine could be run on a test bench at 1500, 3000 and 4500 RPM, and at each of these engine speeds, very little-, half- and full strain. That would be then be nine measuring points. In the case of the petrol engine, one would have to spread the range wider apart. You can imagine the amount of effort required before any relevant information, e.g., the fuel consumption, can be determined, particularly if one has to also run all the test points using reference oil.

The oil pressure alone is not enough to make the engine happy. There, where the oil-line is blocked, the oil pressure is at it's highest, indeed, the lubrication point after the blockage runs dry. Thus, the through-flow of the oil must also be considered. By the way, the cylinder head, which is furthest from the oil pump, now becomes interesting. Hot thin oils, e.g., provide a higher through-flow rate than do high revs, also because, not only with aluminium, heat increases the tolerance levels. 01/12

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