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?

 

[140Airpower]

"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.

 

[tbuelna]

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.

 

[gruntguru]

"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

 

[140Airpower]

"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.

 

[BrianPetersen]

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.

 

[gruntguru]

" 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

 

[140Airpower]

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.

 

[Tmoose]

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/comparison_of_cup_to_f1.htm

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

 

[gruntguru]

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

je suis charlie

 

[140Airpower]

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.

 

[Tmoose]

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

 

[140Airpower]

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.

 

[tbuelna]

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.

 

[yoshimitsuspeed]

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?

 

[Lou Scannon]

The 70ish PSI the motor makes is still well above atmospheric

...true, especially when talking about gauge pressure, which I aSSume we are here...

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Tmoose]

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.

 

[yoshimitsuspeed]

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.

 

[140Airpower]

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?

 

[Tmoose]

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/US8307804B2/US08307804-20121113-D00000.png

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/crankshaft-oiling

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/cosworth_4.jpg

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-l1600.jpg

http://www.ebay.com/itm/Porsche-911...ash=item35f4887396:g:b7AAAOSwhcJWMo-1&vxp=mtr