Oil delivery

[badmotogoozer]

 

I love engine design - everything is a compromise. Nothing is absolute.

 

 

Sounds like the model of relativity to me. Watch out, you're gonna be in Ratchet's bad books!

 

Rj

[luhbo]

I'd like to correct or add one basic and important thing: the oil pump does not create the oil pressure that keeps for instance the rods "surfing" on the crank surfaces.

 

This is a dynamic effect, purely based on the relative movement between gliding surfaces (of proper quality). The resulting pressure achieves values a hundred or thousand times higher than these poor 3 bars the pump may deliver. In fact, if the gap between the surfaces is small enough the pressure theoretically can reach infinite values! This happens every time the bearing is under load, the gap in the loaded area becomes very small with a resulting very strong but thin oil film, on the opposite side it gets bigger and the pressure falls.

 

One should understand that the oil is not pressed between the gliding parts (as long as they move), it's very harshly sucked into these gaps or slots.

 

So all the pump has to do is to make sure that there's always enough oil at all the important places for getting sucked away.

 

At least two things can be derived from this: rising the oil pressure does not automatically improve the lubrication, but always costs serious power

 

and second: a flickering oil light or even oil light on does not automatically mean a totally broken lubrication.

[mike wilson]

I agree with your first conclusion to some extent but not your second. As you say "So all the pump has to do is to make sure that there's always enough oil at all the important places for getting sucked away." In a low gear at high revs, which is the situation we are talking about, oil requirement is as high as it gets. If you have no effective oil pressure, you are going to run out of hydrodynamic wedge very rapidly. There is, as far as I can see, no reservoir of oil for the big end bearings to aid the situation apart from what is in the crank galleries. At 6000rpm, 3 seconds is approx 100 revolutions. If you assume 0.5cc per revolution for the big end, it needs 50cc to be available for that time. Bet there's not that much in the crank gallery....

 

Some people have discovered how little lack of lubrication plain beariings will tolerate.

 

mike

[luhbo]

That's right, yes. Anyway, we talk here of perfect conditions, perfect surfaces. In reallity we don't have them unfortunately, so the engine, its parts are always moving on each other under conditions I would translate for now as something like "mixed friction".

 

Of course everything else but good, but also not really a disaster should the red light come on one day. If it stays on, then it's a different story, but even then you have a good chance to go away with new shells only.

 

Hubert

 

I forgot: High revs, low gear ==> low pressure on the parts. That's what engines like most.

[pete roper]

At 6000rpm, 3 seconds is approx 100 revolutions. mike

 

 

Errr, Mike. 6,000 divided by sixty= 100. That's the reason I chose 6,000 RPM in my original piece, it makes the sums real easy. ay 6000RPM the crank is spinning 100 times a second.

 

Hubert said;

 

*This is a dynamic effect, purely based on the relative movement between gliding surfaces (of proper quality). The resulting pressure achieves values a hundred or thousand times higher than these poor 3 bars the pump may deliver. In fact, if the gap between the surfaces is small enough the pressure theoretically can reach infinite values! This happens every time the bearing is under load, the gap in the loaded area becomes very small with a resulting very strong but thin oil film, on the opposite side it gets bigger and the pressure falls*

 

This is one of the things that I'm going to cover in my next little rant, it is probably the mostcritical factor governing plain bearings' superiority over rolling elements BUT As previously stated there is NO SUCH THING AS SUCK and while residual lubrication is a wonderful thing it certainly isn't sufficient to protect heavily loaded surfaces like big ends at full noise, even for a brief period. I've got a couple of hours spare this afternoon and I'll go through it and present MY argument on the subject. Whether people agree or not is up to them but I think that they will find the logic incontrovertable.

 

Pete

[pete roper]

Oh Boy! OK, lets try tackling this one

 

Perhaps the best way to start is by looking at what forces are catually going to be working on the oil in the bearings. We'll keep it very simple, we won't go into Brake Mean Effective Pressure, Volumetric Efficiency or Thermal Efficiency, we'll just look at some very basic figures.

 

A four stroke engine has, not surprisingly, four strokes. An induction stroke where a new charge is pushed into the cylinder, a compression stroke where that mixture is compressed, a firing or power stroke where the work is done and an exhaust stroke where the spent gas and waste heat is expelled from the cylinder. All this happens in TWO revolutions of the crank or 720 degrees of rotation.

 

Now we'll ignore valve timing, harmonics, self-supercharging and scavenge, fricional losses betweeen piston and bore etc. end gasses and the like and just work on absolutes.

 

As the piston reaches the bottom of it's induction stroke the cylinder will be full of new charge at atmospheric pressure, OK so it sits there with 14PSI, (One atosphere.) both above and below the piston. The inlet valve closes and the piston moves mack up the bore. Because it's easy we'll say we have a 10 to 1 compression ratio so when the piston reaches the top of it's stroke the pressure ill be ten times what it was when the piston began it's upward movement so at TDC compression there is already 140PSI acting on the top of the piston. Then the mixtrure is ignited and the pressure rise is immeiately immense, lets say it rises by a factor of ten! So now, all of a sudden, you have a force of 1,400 Lbs per square inch pushing on top of the piston! That's a lot of force and it is that acting through the connecting rod to the crank that makes the crank spin! The thing is that before it can impart thrust to the crank pin it has to work on that thin film of oil that is separating the bearing in the connecting rod from the journal.

 

So what is happening to that oil and what does it have to be able to do to perform it's task.

 

Lets look at friction, you know, what makes your hands warm when you rub them together on a cold night.

 

When two surfaces are in contact there is an opposition to relative movement between them and this is what friction is. If the surfaces are clean and dry the force needed to overcome the friction depends on.

 

a.) the matrial from which the parts are made.

 

b.) The surface finish, if it's rough or smooth or polished.

 

c.) the load pressing the bits together.

 

For any clean dry surfaces the co-efficient of friction can be calculated by

 

Resistance to movement divided by the load pressing the two together.

 

When the static friction is overcome *work* is done and an eqivalent amount of heat is generated. In any bearing where the bits actually touch this heat has to be got rid of somehow otherwise it will build up and the bearing will rapidly fail.

 

So what is the answer? Obviously there is one otherwise all our plain bearings would fail very early in the piece with 1400Lbs/In2 acting on them 100 times a second!

 

Well it's very easy really. You simply don't let the bearings touch! And THIS is where the Yak Fat comes in!!!!!

 

Lubricant! Beautiful, glossy, slippery, golden Yak Fat!!!! A liquid that can be pushed in between the two parts that are moving relative to one another! The thing is that all the energy that otherwise would be going to cook the bearings still has to be dissipated and got rid of, it's just that it is not done by being absorbed by the material of the bearings, (The alloy or babbit of the shells or the journal.) but it's absorbed by the Yak Fat! And the HUGE advantage of this is that because it is a liquid it GOES AWAY! That's right! It takes the heat and f@cks off with it, meaning the bearings don't overheat! Brilliant!!!!! What governs how good it is at this is the "Viscosity" of the fat!

 

Viscosity is the property of the fuid by which they will resist flow: The greater the viscosity the greater the resistance to flow and vice versa, but also there is a direct correlation between this and the fat's ability to transport heat. Thus the friction in the lubricated bearing is dependent on the lubricant's viscosity but it is a careful ballancing act depending on a variety of factors. The forces imposed on the bearing, the oil's viscosity and ability to transfer heat, the flow rate of the oil through the bearing when it is *squeezed* by the forces acting upon it and the size of the bearing and journal that is having to carry the load. Obviously if you use a higher viscosity oil it will be able to carry away more heat. But if say you use an oil with a viscosity that will allow you to take away say??? 20 joules of energy in ten seconds at a certain flow rate you are still going to be going out arse first compared with if you have an oil that will take away 10 joules in ten seconds but it's flow rate is four times faster. Does that make sense?

 

OK, think about that for a bit and I'll continue and we'll look at the formation and properties of the Hydro-Dynamic wedge.

 

Pete (Rushing off to his selection of crappy old outdated text-books again )

[pete roper]

Pete (Rushing off to his selection of crappy old outdated text-books again )

 

I wasn't kidding! I opened my Hillier and Pittuck to do a bit of revision and what looked suspiciously like a mummified human ear fell out of the' Functions and Properties of Lubricants' chapter. Now, my admittedly failing memory tells me that this is either;

 

a.) The result of a fight I indadvertently was dragged into with a mob of Milwall supporters in Morden after a particularly nasty grudge match with Wimbledon in '86 OR,

 

b.) A bit of a particularly nasty kebab, also found in Morden at about the same time.

 

While the absence of blood makes me think 'Kebab' there are also bits of human hair. Mind you, knowing the kebab shops in Morden in '86 the presence of human hair definitley doesn't rule *kebab* out of the equation!

 

Sometimes I'm glad I'm not young any more!

 

Pete

[mike wilson]

Errr, Mike. 6,000 divided by sixty= 100. That's the reason I chose 6,000 RPM in my original piece, it makes the sums real easy. At 6000RPM the crank is spinning 100 times a second.

Pete

 

My brain was fried by the 36degrees in this room, guvnor.

 

But, even better, that means that 150ccs is needed to keep the bigend happy for the duration of oil starvation. Assuming that 0.5cc/revolution is a viable figure. I suspect it is rather low.

 

mike

[pete roper]

Sorry people, I will continue with this but I've been having to deal with a family crisis in England in the last week as well as doing everyday *work* and stuff. Incidentally if anybody is around in the SE/ East Anglia region between the 10th and 28th of September I'll be in Cambridge visting my Mum if anyone wants to save me and take me out for a beer! I'll generally be very busy as I'm going to have to try and get my Mum to accept it's time for here to move into some sort of sheltered accomodation, (She's 92!) and it ain't going to be easy or pleasant and my brother is NO help at all, even though he lives there

 

Pete