Main bearing diameters and engine rpm 1

[Deividas]

Hi! In this article (

http://www.hotrod.com/how-to/engine/hrdp-0401-torque-horsepower-guide/

) i read, that at high rpm, smaller bearing diameters is better than larger, "Some engines aren't happy at high rpm-and never will be. Their bearing diameters are too large, their strokes too long, and their head-flow capacities too poor to really work upstairs." Maybe someone can explain, why larger main bearing diameters is worse than smaller at high rpm's? It's all about centrifugal forces, tangential velocity or what?

 

[MikeHalloran]

HOT ROD used to be a good magazine, like when they taught us how to put 'juice brakes' on our '32, and the b/w photos clearly showed dirty hands that clearly belonged to the writer.

Now, it's just stuff like "We took a new car down to Joe Expensive's Go-Fast Shop, and he bolted on some stuff that he claimed would make the car go faster. We sorta wish we had actually tested it, but that would be, like, work."

IOW, what you read is secondhand or thirdhand, probably misquoted, and the original source may have been making up stuff anyway.

I know that if I designed engines, and had a set of internal rules of thumb for proportioning them, I wouldn't reveal those rules to just anyone.

I'd like to think that today's engine designers don't need rules of thumb because they have a lot of zoomy tools, but I've analyzed enough structures (not engines) to recognize that _all_ the parts of an engine are flexible, the internal loads are pretty large, and the dynamics of their interactions should be really, er, interesting, probably too much so for the zoomy tools to handle without simplifications in the math models, i.e., rules of thumb. ... which are too commercially valuable to reveal to a magazine writer.





Mike Halloran
Pembroke Pines, FL, USA

 

[Deividas]

I found another interesting article (

http://www.wallaceracing.com/enginetheory.htm

), it seems more informative than HOT ROD. There are some interesting theory:

Main bearing diameter also has an effect on maximum engine rpm limits. A large bearing diameter allows a large crankshaft cross-section which means greater strength, but it also means increased oil pump pressure to overcome the centrifugal force of the oil in the main journal.

There is graph, that shows minimum oil pressure at mains vs. journal diameter and engine rpm.
If it's true, then is it only reason, why smaller main bearing diameters are better than larger at high rpm's?

 

[RossABQ]

I've always heard that it had to do with the increased effective surface velocity in the bearings with larger diameters creating more heat due to increased friction.

I couldn't agree more with your points on Hot Rod and for that matter, all similar magazines. Aftermarket vendors seem to write the stories for them.

But they aren't the worst...Want to see some real journalism? Read here:

http://www.ehow.com/list_7446810_79-ford-truck-specs.html

 

[gruntguru]

Quote:
Main bearing diameter also has an effect on maximum engine rpm limits. A large bearing diameter allows a large crankshaft cross-section which means greater strength, but it also means increased oil pump pressure to overcome the centrifugal force of the oil in the main journal.

Does anyyone believe this?

je suis charlie

 

[Deividas]

gruntguru - you think it can't be true?

 

[carwhisperer]

I think that makes sense. Bigger journal = more surface area and more rotational mass. What am I missing?

 

[Tmoose]

The effect of main journal diameter ( radius really ) centrifugally opposing oil flow from the block is discussed in some moderately respectable circles.

A first cut at the centrifugally induced pressure at the surface of a 2.5" diameter journal spinning 7000 rpm is about 55 psi (due to over 1700 gs of acceleration.)
What the longer oil column (from the main journal out to the rod journal) is contributing is a bit of a puzzle to me.

 

[ivymike]

that longer oil column turns the crankshaft into a centrif. pump...

 

[Tmoose]

Hi ivymike,

"that longer oil column turns the crankshaft into a centrif. pump..."

But pumping liquids generally requires pressure. I'm thinking the longer oil column can not "pull" the shorter oil column like a robin pulling a fat worm out of the flower bed.
Unless the oil pressure is high enough to overcome the short worm's insistence and push it backwards to get things flowing in the right direction, a giant cavitation bubble or perhaps even a scary black hole will form at the center of the crank ( assuming the oil hole passes thru the center.

Some high revving engines have gone to the effort to feed oil into the nose of the crank, in part to reduce pressure requirements supposedly per the Internetz. I was thinking Porsche 911z had some nose/snout/small diameter oiling features but could only find fairly traditional schematics on the Internet.

Back in the 50s Chevy presumably thought about there was some effect too, according to the "New V8 " SAE paper.

 

[Deividas]

ivymike - what you mean by saying that longer oil column turns crank into centrifugal pump? You mean longer oil column sends more oil to rod bearings?

 

[BrianPetersen]

The longer oil column turns the crank into the opposite of a centrifugal pump followed by a centrifugal pump.

Before the oil can get to the passageway that leads to the adjacent rod bearing, first it has to get from the surface of the main bearing to the center of that journal where that passage connects. Once the oil is past that point and into the crank throw THEN it will get flung out, but first it has to get to that point.

In my experience, over-revving toasts a rod bearing before it ever cooks a main journal.

 

[tbuelna]

The optimum main bearing journal diameter for any given engine application involves a compromise of many factors. It is basically true that a larger diameter bearing journal will have higher losses than a smaller diameter bearing journal, all other things being equal. This is due to shear within the hydrodynamic oil film occurring at a greater radial offset from the axis of rotation. However, most engines are designed with efficiency in mind so they typically use the smallest main bearing journal diameter and width combination that meets requirements for load capacity, service life, structural stiffness, etc.

The concerns about extra power required to drive the oil pump if the pressure were raised 20-30 psig to compensate for larger main bearing journal diameters is unwarranted. It would probably increase the power to drive the oil pressure pump by less than 1 hp. Here's a graph from the article linked above.

However, the friction losses produced by several larger diameter main bearing journals at high rpm would likely be greater. And since journal bearing oil flow rates are mostly based on cooling/heat rejection requirements, the higher losses in the larger diameter bearings would also require increased oil mass flow for cooling.

 

[gruntguru]

Ah - now I get it. They are talking about feeding the oil from the main bearing INTO the centre of the crank, on its way to feed the crankpin journals. (The wording doesn't say that). And yes - there are other ways to get oil to the crank pins.

je suis charlie

 

[Tmoose]

Horsepower = Pressure (psi) x Flow (GPM)/1714.
Typical automotive oil pumps are gear type, so are simplistically constant volume.
When delivered volume is greater than permitted by engine internal clearances the rest is bypassed thru the pressure relief valve.
So I believe a high(er) volume pump will deliver a greater volume at the set pressure. Just more of it will be jettisoned thru the bypass

Legend has it Installing a higher pressure pump on various primitive V8 engines reportedly can make life noticeably difficult (and shorter) for the drive gear on the distributor shaft (which drives the oil pump too).

Mellings (a reputable engine parts manufacturer) claims otherwise.

http://www.mellingengine.com/portals/5/pdf/pdf_catalog/high-volume-pumps.pdf

 

[Lou Scannon]

FWIW, I find that article to be unscientific and inaccurate. Very simplistically, under the same boundary conditions, if more oil volume is pumped at the same rpm, the required power is higher, hence the required torque to drive the pump is higher.
My take on oil pump upgrades is, if you want to operate the engine at a higher than intended rpm, you need a higher pressure pump (typically via higher relief spring pressure), in order to meet the pressure demand at the higher rpm.
On the other hand, if you are keeping the peak rpm the same, but are adding more area where oil can escape the oil supply system back to the crankcase (e.g. larger bearing clearances), then you need a higher volume pump. In my particular case, I upgraded to a high volume pump in order to supply the extra volume needed for under piston oil jets that I added to my engine (and incidentally, experienced a drive gear failure).
Whether higher pressure or higher volume, for a given rpm, the drive power and hence torque is obviously higher.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[PackardV8]

Right angle gears, we learned via many failures, are a bad means to drive oil pumps and distributors. Today, many oil pumps are driven directly off the crankshaft and the ignition is triggered the same way. No gears to fail, no slack to load and unload.

jack vins

 

[Lou Scannon]

Right angle gears, we learned via many failures, are a bad means to drive oil pumps and distributors. Today, many oil pumps are driven directly off the crankshaft and the ignition is triggered the same way. No gears to fail, no slack to load and unload.

Quite so.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[MikeHalloran]

Technically, they're called skew gears.
They look like helical gears, but because the axes are crossed instead of parallel, they have basically point contact instead of line contact with each other.
So, yes, the power capacity is limited, and adding a bigger oil pump results in a shorter life.
But they were almost always good enough to get through the warranty.

I think they disappeared because going to distributorless ignition removed a labor step of timing the ignition, and then going to crank-driven oil pumps reduced the parts count.




Mike Halloran
Pembroke Pines, FL, USA

 

[dicer]

The simple answer is surface speed.
For a set rpm a smaller diameter journal or drill or what ever it is, will always have a lower surface speed that a diameter that is larger. With the higher surface speed comes more viscous friction thus more heat. I guess tbuelna pretty much covered that.