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Main bearing diameters and engine rpm

Main bearing diameters and engine rpm

(OP)
Hi! In this article ( http://www.hotrod.com/how-to/engine/hrdp-0401-torq... ) 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? :)

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

(OP)
I found another interesting article ( http://www.wallaceracing.com/enginetheory.htm ), it seems more informative than HOT ROD. There are some interesting theory:

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.
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? :)

RE: Main bearing diameters and engine rpm

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-spe...

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

(OP)
gruntguru - you think it can't be true? :)

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

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.

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

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.

RE: Main bearing diameters and engine rpm

(OP)
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?

RE: Main bearing diameters and engine rpm

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.

RE: Main bearing diameters and engine rpm

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.

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

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_cat...

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

Quote (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.
Quite so.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Main bearing diameters and engine rpm

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

RE: Main bearing diameters and engine rpm

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.

RE: Main bearing diameters and engine rpm

"gruntguru (Mechanical)18 Mar 16 04:27
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...."

Honda fed it through the nose of its 20,000 rpm V10. But, the oil path down through the entire crank relied on the fact that the V10's journal overlap was great owing to the very short stroke and drilled passages could be done. The reduction in maximum centrifugal radius was small, but centrifugal resistance was lower. Also, not having bearing clearances as part of the oil path reduced oil supply loss. This method would seem to be very difficult to achieve for the usual road car V8. Even if overlap is sufficient, the number of straight drillings -with plugs- would be much greater.

RE: Main bearing diameters and engine rpm

The benefit of feeding oil into the end(s) of the crankshaft is that it provides a more uniform/consistent delivery of oil to the rod journals. The issue with feeding the rod journals from the main journals is not the additional pressure required to push the oil within the gallery inward against an opposing dynamic force. The issue is that supplying a consistent flow of oil from the main journals requires an arrangement of grooves and holes that often compromises the performance of the main bearings. You don't want grooves or holes at a bearing/journal surface location that is subject to high oil film pressures.

Here is a picture that shows an example of a crankshaft with end fed drilled oil galleries. Note that the oil galleries zig-zag across the axis of rotation, which means they still must deal with dynamic rotational forces. However, there is an unobstructed flow at the end feed and no grooves are required at the main journals.

RE: Main bearing diameters and engine rpm

"Note that the oil galleries zig-zag across the axis of rotation, which means they still must deal with dynamic rotational forces."

Nevertheless the lowest pressure at any point (neglecting losses) is gallery-pressure at the axis of the shaft. For galleries fed from the main bearing radius the pressure at the shaft axis will be significantly less than gallery-pressure and potentially less than atmospheric - at which point you need to consider the vapour pressure of hot oil. Any lower than that and you have vapour lock. Beyond that (probably at ridiculous speed or journal diameter), there is the potential for a vacuum bubble to form at the shaft axis (When the centrifugal pressure at main journal radius is greater than absolute gallery pressure).

je suis charlie

RE: Main bearing diameters and engine rpm

"tbuelna (Aerospace)22 Mar 16 01:14
The benefit of feeding oil into the end(s) of the crankshaft is that it provides a more uniform/consistent delivery of oil to the rod journals. The issue with feeding the rod journals from the main journals is not the additional pressure required to push the oil within the gallery inward against an opposing dynamic force."

Honda found that the anti-centrifugal pressure requirement was a big problem and that a feature of their end-fed crank was that the radial excursion of the path was less than the radius of the mains in their very short stroke crank. In the above engine it is considerably (about 50%) greater than the radius of the mains. This is owing to the much longer stroke of this engine. So, for overcoming centrifugal force this engine would need a higher oil pressure than if it fed oil from main galleries. However, the other advantages that you mention are significant as well as the reduction of oil supply pressure fluctuations, something that Honda also benefitted from.

RE: Main bearing diameters and engine rpm

My little Honda single cylinder motorcycle engine (with rolling-element bottom end) has an end-fed crankshaft and camshaft. The oil pump provides circulation only, there's no relief valve, there's no oil filter, the lubrication system operates at a very low (unregulated) pressure.

RE: Main bearing diameters and engine rpm

" This is owing to the much longer stroke of this engine. So, for overcoming centrifugal force this engine would need a higher oil pressure than if it fed oil from main galleries."

I don't understand that statement. With oil feed at the axis, the pressure anywhere else in the crank drillings will be higher than pump-pressure.

je suis charlie

RE: Main bearing diameters and engine rpm

gruntguru, the oil path has "downhill" and "uphill" sections with respect to the rotation center. In the 4-cyl engine shown in the diagram the minimum uphill goes from about 1 1/2 times the main bearing radius inward toward the center. In the Honda V-10 crank that same slope is from about 0.8 of the mains radius. The 20% improvement was significant for them at 20,000rpm.
Unfortunately for Honda, the construction of the center-fed crank required welding of three sections and was outlawed along with their welded hollow-beam rods.

RE: Main bearing diameters and engine rpm

The current craze of turning V8 journal diameters down 1/8 or even more is based in part on allegedly less friction from lower surface speed.

Supposedly recent NASCAR engines rod journal diameters are a mere 1.87 inches.
http://www.epi-eng.com/piston_engine_technology/co...

That is down from 2.1 inch for the 350 crank in nearly every 350 Small block Chebbie.

RE: Main bearing diameters and engine rpm

140A. I still don't agree that any engine will need higher pump pressure if centre fed. The opposite is true.

je suis charlie

RE: Main bearing diameters and engine rpm

gruntguru, another contributor to oil pressure loss is the main bearing clearance space, so center feed could result in a lower pressure requirement in practice even in spite of greater centrifugal resistance.

RE: Main bearing diameters and engine rpm

HI 140 Airpower,

You said - "another contributor to oil pressure loss is the main bearing clearance space..."
Could you explain that a little more?

thanks,

Dan T

RE: Main bearing diameters and engine rpm

Dan, this is what it looks like to me comparing to center feed:

With center feed from the end(s) of the crank, oil pressure inside the crank is relatively more constant with relatively full pressure oil draining out through the rod and main clearance spaces.
With main web feed, oil entering the crank at the main bearings has some pressure drained away through the main clearance space combined with the fact that the oil feed into the crank is interrupted as the crank turns. Oil pressure to the rod bearings may be reduced and with worse pressure variations compared to center feed.

Therefore, center feed may require less pressure than main web feed regardless of a greater radial excursion and a higher centrifugal resistance to pump against -as in the 4-cyl engine diagram.

RE: Main bearing diameters and engine rpm

Having consistent/predictable oil pressure at the main/rod journal feeds is very important. The mass flow of oil to journal bearings is mostly a function of cooling requirements. More precise control of oil flow allows reduced design margins and a more optimized bearing.

RE: Main bearing diameters and engine rpm

Here is a cool diagram of a 4AGE crank.

People talk about running more oil pressure for higher RPM and I never really got why. The pressure that protects the bearings is generated by the oil between the bearing and the journal, not by the oil pressure. I can see how the centrifugal force on a journal fed crank like the 4A makes it harder to push the oil in at that point but since the rod journals are further out as has been mentioned here it would create essentially a centrifugal siphon. So I guess that the critical goal would be to keep the oil pressure high enough at the journal that it stays above atmospheric pressure and does not suck air in? If that is the case we are still talking about flow and not pressure. The 70ish PSI the motor makes is still well above atmospheric unless there is a restriction before that. Or is the critical concern that if there was air in the passage that the oil pressure would be high enough to get the oil all the way to the center?

RE: Main bearing diameters and engine rpm

Quote (yoshimitsuspeed)

The 70ish PSI the motor makes is still well above atmospheric
...true, especially when talking about gauge pressure, which I aSSume we are here... wink

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Main bearing diameters and engine rpm

140 airpower said,

"With center feed from the end(s) of the crank, oil pressure inside the crank is relatively more constant with relatively full pressure oil draining out through the rod and main clearance spaces."

i have not seen many end feed cranks, but my hazy recollection is they don't bother to perforate the main journals.
There are good and bad places to introduce oil to a main bearing, and it remains in a constant location relative to the block, so back feeding out a hole in the rotating crank would be less effective than a standard block oil feed.

RE: Main bearing diameters and engine rpm

Haha fair point Hemi.
I was thinking more clearly than I was typing.
My thinking was that at 70 PSIG at the pump it seems very unlikely it would drop to atmospheric pressure or below at the journal. The centrifugal force on the oil would logically reduce the oil pressure at the main but it just doesn't seem like it should drop it significantly with such a restriction at the rod bearing.
On the other hand if somehow the crank lost oil in the passageway and lost the centrifugal siphon it would certainly take a lot more oil pressure to push it back into the center of the crank.

RE: Main bearing diameters and engine rpm

Interesting, Tmoose. Are you implying that a center feed crank still should get main journal oil from the webs and that the center feed only serves the rod throws?

RE: Main bearing diameters and engine rpm

Hi 140 Airpower,

Again, my hazy recollection of end feed cranks is some don't, but recent Google searches, etc, suggets some do.

RE: possible examples of un-drilled main bearing oiling with end-feed cranks.
1 - Tbuelna's post shows drilled oil passages at the rod journals, but seems to show none at the main journals.
It looks to be a patent drawing, so perhaps never existed in steel.
http://patentimages.storage.googleapis.com/US83078...

2 - Sketchy blog about F1 crankshafts, with hole-less, unholy assumption about what is seen in one crank picture.
https://www.highpowermedia.com/blog/2921/crankshaf...

3- Cosworth FI V8 crank cutaway. Some extra activity is seen at some of the holes in the vicinity of the mani journals. Some look to be intersections of 2 holes drilled past each other, but it isn't real clear to me if there are hole in the main journals or not.
http://www.cosworth.hu/content/vegyes/cosworth_uk/...
http://www.cosworth.hu/?p=97

3 - Porsche 911/964 crank - no holes visible in any of the main journals as best I can tell, except the groovy rear (front?) main.
This is what I imagine I think I saw a few decades back to cause my belief.
http://i.ebayimg.com/images/g/b7AAAOSwhcJWMo-1/s-l...
http://www.ebay.com/itm/Porsche-911-964-3-6-OEM-St...

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