Oil Pressure Rise Over Crank?

[HarleyE30]

Hi everyone,

I need to know the oil pressure rise over the crank, and also through the connecting rod (drilled rod).

Firstly for the crank:

Let's say main bearing oil pressure is 2 bar(g). Engine speed is 1000 RPM (104.7 rad/sec) and the crank drill length is 55mm. Oil specific gravity is 0.88 kg/litre

I'm struggling to derive an equation? I'm assuming I need to be looking at centrifugal effects here?

Many thanks for any advice

 

[MikeHalloran]

No, it's much more difficult than that.

The oil pump in an engine is only responsible for putting the oil where it is needed, and maintaining a flow so that it will not overheat. The 'plain' bearings in an engine, thanks to eccentricity and rotation, basically inhale that oil and pressurize it to many bar to support the applied loads. The oil pump has nothing to do with that pressurization process.

You need to be looking at 'hydrodynamic lubrication'.


Mike Halloran
Pembroke Pines, FL, USA

 

[HarleyE30]

Thankyou for the reply.

That would be exceeding the scope of the project I'm working on. Is there a quick method of roughly calculating the change in pressure ? It need not be accurate at this stage; just an approximation would suffice.

 

[Tmoose]

Hi Harley,

Like Mike said, the actual support for the rod bearing is hydrodynamic in nature. Delivered pressure does not have much effect on that.

Are you the thinking about the resistance the oil entering the crank's main bearings meets (and must overcome)due to the centrifugal effect on the column of oil in the first inch or so of typical drilled oil passage? That is the one that extends from the center of the main journal to the main journal OD. Oil in the other half of the drilled oil passage, from the juornal center out to the rod journal, is assisted in flowing "outward" by the cetnrifugal pressure profile.

http://static.cargurus.com/images/site/2012/11/21/10/44/pic-5717446344398985354.jpeg

http://www.fordfe.info/Images/CrankOiling1c.jpg

I thinks some of the oil fed to a early Porsche 911 cranks is introduced to a small journal diameter in the snout.

http://forums.pelicanparts.com/uploads/crank1A.JPG

I have no clue whether the reduction in centrifugal back pressure was considered a plus, or a coincidence.
Seems like everybody else just cranked up the pump pressure a few psi.

 

[HarleyE30]

Thanks for that Tmoose.

I need to know the oil pressure at the top of the rod. I've dug out some old calculations (from 1954) which provide the following steps:

"The pressure of the oil supply st the piston is the sum of the static supply pressure at the main bearings and the dynamic head due to acceleration of the oil column" The following equations are then shown:

 

[Tmoose]

Hi Harley,

So the rod is drilled for oil up thru the I-beam, from big end to pin bushing, like these Jag XKE rods?

http://classicjaguar.com/cj/6289st1.jpg

I think in a semi modern engine the oil pressure in the block is fed to a groove in the upper main bearing so a single oil hole in the crank main journal communicates with it (and is pressurized) just half the time. If either the crank mains are "cross drilled" or full groove main inserts are used the rods >could< receive full time oiling. However there is often just on,e or at most 2 oil feed holes in the crank's rod journal.

If one hole, it is positioned away from TDC a bit. I >think< leading TDC to introduce oil into the bearing clearance space at a time of relatively low load, and enforced clearance.

http://media.appliednanosurfaces.com/2013/04/crankshaft.png

http://www.carzi.com/wp-content/uploads/crank-781.jpg

If there are 2 holes in a rod journal they are often "cross drilled" , each about 90 degrees from TDC.

http://www.alfabb.com/bb/forums/att...actory-threaded-2-0-crankshaft-aut68670_1.jpg

In any case at TDC the oil hole in the journal is quite a ways from the rod centerline, where I picture the rod Big end pin oiling hole in your con rod being, like those Jag rods.
So, even if the crank oil drilling is fully pressurized at TDC, the exit won't be anywhere near in line with the entrance to the oil drilling in the rod.

I think I've seen vintage rods with a circumferential groove behind the rod insert, and notches at the insert parting line, or holes in the insert, which is what it would take for oil to be actively pumped to the pin bearing at the odd moments when the planets align.