[Land-speed] high speed engines/tight clearances
saltfever at comcast.net
Thu May 15 15:07:41 MDT 2008
I have been struggling with this one ever since it was posted a week ago.
Kind of sounds like something a journalism major would naively accept
without looking into it further.
If you use a coefficient of thermal expansion (CTE) of 11 millionths for
aluminum and about 5 millionths for various steels (YMMV) you get ambient
build tolerance of about .002 inches. IOW, a 3.5 in. dia. Piston will grow
about .0038in. with a 100 temp rise. A Steel or iron bore will only grow
about .0017 in. per 100 degrees. That means you could assemble a 3.5 in.
dia. piston in an 80 deg room, with as little as .0025in. bore clearance
built in. At 180 deg you are pretty close to seizing but play with the
numbers a little and you see what I mean. For a 100 deg temp rise you are
only going to loose around .002 -.0025 in clearance. Then playing with
alloys like hypereutectics will further reduce the CTE.
As Neil mentioned in the olde days we use to lap-in our two-cycle model
airplane motors. Nobody ever measured the bores, it was just the feel of the
thing. Since we were running Don Francisco's nitro laced fuel with thick,
gummy castor oil the thing sealed pretty good. I hardly think F-1 is running
pistons without rings and is using a viscous seal (gummy oil). Can you
imagine the HP robbing friction in a 19,000 RPM motor if you ran it that
way! In fact, thinking conventionally, can you just imagine how much
horsepower is lost to friction in a bore with as little as .0005 in.
clearance as suggested in the F-1 scenario! I know little about F-1
technology but something just doesn't add up. I can see valid reasons for
assembling a complex engine hot; but bore seizing is not one of them. -Elon
From: "Ed Weldon" 23.weldon at comcast.net
Sounds good on paper but I think asking an assembly robot of suitable
envelope size to hold closer than .005 is a bit much. Build something like
that into a 200-250 degree oven and try to control temperature everywhere
and you've got a million dollar piece of equipment. (Snip . . . )
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