Thoughts on oil pressure

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Guy Croft
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Thoughts on oil pressure

Post by Guy Croft » Mon Jul 28, 2008 7:25 am

Lifted from a post by me in 'Readers' Cars', mildly edited/enhanced and listed here for refererence:

Some thoughts on oil pressure.

Every engine has a unique oil pressure 'signature' dependent on the size of the engine, the architecture (size, length, shape, placement) of the feed galleries, the running clearances, number of oil-fed ancillaries (oil sprays, turbocharger, cooler, lines etc etc), the size and type of the oil pump. The free volume the pump is capable of if venting to atmosphere is huge, but restrict the exit orifice by feeding the oil to a small pipe (ie: complicated oil gallery with fine clearances) and you build up pressure in the delivery line. So down goes the volumetric flow rate but up goes the pressure, so you can see they are closely related. Naturally the further from the pump and the bigger the clearances, greater the losses due to bends and sharp edges the lower the line pressure and the lower the chance that the oil will get to where it's needed.

I say 'up goes the pressure' but in reality there will be a limit on how high it goes dependent on the characteristic of the pressure relief valve and of course the clearances in the pump itself. On a wet sump unit, the oil pressure relief valve allows oil that is pressurised in the main casing - by the gears or rotors - to bleed back into the pickup pipe. Restrict the pump exit completely by locking it up or feeding to an engine gallery that is too small, well, something in the pump is going to break because oil is more-or-less incompressible and the higher the pressure in the casing the harder the pump is to turn. It was common in the days when oil pumps were driven off aux driveshafts for the pump shaft to snap. Restrict the cross sectional area of the gallery adjacent the pump and sure, you will get high pressure between the pump and gallery but the gallery won't fill (at all , or quick enough) because the volumetric flowrate is too small. By the same token, it's vital that the volumetric flowrate and pressure are high enough to keep the oil flowing to where it's needed, remembering that it's constantly bleeding out at the final destinations (bearings etc). That is known as the 'bleed-down' (or leak-down) rate. The response time between bleed-down and replenish should be zero, or the oil film is going to rub dry, but that 'recovery rate' won't be met if the pump can't keep up - because, say, the oil is simply too hot/thin (and lost its viscosity) or the pickup is feeding air/gas (oil starvation). Or because the pump is simply too small or the relief valve setting is too low and the pump can't deliver enough pressre.

Do you need volume or pressure? Well, in short you need both, but there is a finite limit to both given correct oil temperature and, ignoring power loss (which you can certainly get by forcing the pump to deliver too much pressure) going under/over that limit is likely to lead to engine failure in one mode or another.


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