Aluminum block thermal expansion 3

[BobSP]

Hello everyone, i'm trying to figure out if there are any problems with thermal expansion of an aluminum block regarding the crankshaft. As the block heats and expands, the main bearings should move (stretch) more than the journals, putting some lateral pressure. Is that correct? Is there any special design allowance for that, or they just live with the pressure? Or perhaps the components are designed to have the proper clearances when hot, so there's only pressure when they're cold?

 

[patprimmer]

The crankcase end float is controlled in one bearing only. The rest allow the crank to float lengthwise to accommodate expansion differences. The clearance re end float should be enough to avoid seizure at the tightest conditions, but not have to much so that the engine becomes noisy.

Regards
Pat
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[ivymike]

I'm not exactly sure what you're asking, but it doesn't sound like Pat is answering the question I think you're asking. The main bearings are installed with a particular amount of crush which applies a radial force between the bearing shell and the mating surface of the block. This pressure is selected based on manufacturer's experience to give a bearing shell which doesn't bulge/buckle nor rotate in service. The pressure variations and block motions in operation have inherently been included in this experience.

If you're asking whether the openings in the block contract on the crankshaft and constrict its motion, then the answer is no. The holes get bigger when they get hot.

The other interpretation of your question that comes to mind is Pat's, and he gave a good answer.

 

[BobSP]

Thanks Pat and ivymike, my question was regarding expansion along the axis, not radial. Pat's answer i what i was imagining, there's enough clearance to allow small movement between bearings and journals.

 

[dicer]

Are you concerned about HP loss of Al vs Fe blocks?

 

[BobSP]

dicer, that's not my main concern, ehat im worried about is the possibility of increased stress on the crankshaft or block.

 

[tbuelna]

BobSP,

With blocks that have aluminum main caps and saddles, the main bearing/crank journal clearance and bearing shell OD/saddle/cap fit can be a problem due to the material thermal expansion mismatch. There is also the material stiffness mismatch to consider.

When you consider that the necessary radial clearance tolerance between the bearing shell and crank journal is just a few micro-inches, it becomes clear just how big an effect the thermal mismatch with an aluminum block main bore and steel crank journal creates.

Regards,
Terry

 

[patprimmer]

Further to what Terry says, aluminium rods have a locating dowel to help the bearing resist spinning in the big end of the rod.



Regards
Pat
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[pontiacjack]

Terry- "... a few micro inches..." tolerance of mains clearances? I'd say my aluminum block/main-caps competition engine would easily tolerate an additional five hundred(!) micro inches of mains clearances; that would open them from 'cold' .004" to 'hot' .0045". And differing thermal expansion of main bores from bearing inserts is a non-existant problem. I've always run normal crush on all bearing fits (also aluminum rods) with never any evidence of fretting. The main inserts in it right now are still perfect after three full competition seasons (900 HP, up to 9,000 RPM). Look at state-of-the-art race engines- more and more are now switching to aluminum main caps, where a few years ago the norm was steel caps on aluminum blocks. Among other disadvantages, steel caps on an aluminum block can be a hindrance to high-precision align-honing of the bores.

 

[BobSP]

Thanks everyone

 

[KeithDyer]

Another "For What It's Worth" . . . . . .

I have heard that the folks at Sunset Racecraft in Lubbock (top drag race and circle track builder in this part of the country) have some extensive data on the subject of horsepower loss with an aluminum block vs. cast iron block with the exact same internals.

About 50 HP less with the aluminum block, all variables the same.

Take care, k

 

[Tmoose]

It would be interesting to know a little more about the results, and the test. And theories as to why. Supposedly cylinder finish can measureably affect HP via ring "friction"

Those inertia chassis dynos will mistake reduced rotating weight for a HP increase every time. I don't expect they were rotating the block for this testing, but if it was calculated based on quarter mile speed or time or something, I can imagine an aluminum block directly or subtly changing weight distribution and transfer, ride height, cylinder wall temperature or ? X ? all of which might change performance.