Aluminum block thermal expansion
Aluminum block thermal expansion
(OP)
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?





RE: Aluminum block thermal expansion
Regards
Pat
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RE: Aluminum block thermal expansion
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.
RE: Aluminum block thermal expansion
RE: Aluminum block thermal expansion
RE: Aluminum block thermal expansion
RE: Aluminum block thermal expansion
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
RE: Aluminum block thermal expansion
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Aluminum block thermal expansion
RE: Aluminum block thermal expansion
RE: Aluminum block thermal expansion
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
RE: Aluminum block thermal expansion
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.