I got brain dead on this reply. Looks like no one has nailed me though. I
knew there was something wrong with these calculations but hadn't thought
it through. The problem is the compression ratio is just that - a ratio.
You can't apply the calculation to one side but not the other. 10:1 is 10:1.
David
67 BGT
71 BGT
At 11:22 PM 7/5/2000 -0600, David Councill wrote:
>I don't think it works quite like this. I was going to do some similar
>calculations but then decided it was too much work to worry about. And I
>try to avoid getting too scientific on this list although I've probably
>over stepped my bounds a time or two. I only have to mention the infamous
>discussion of batteries on cement floor discussion a few years ago as an
>example that some will shudder to remember. Well, in retrospect, the list
>has been doing really good this year after we survived that discussion,
>cats, drivelines, and other forgotten topics.
>
>But to make a long story short, there is the pressure decrease from the
>altitude, lower temperatures, the need to compensate the fuel mixture for
>high altitude. Probably too complex to worry about. I was hoping it would
>be simplified in a book kind of like the "ideal" gas law (engineering
>students learn that its not so ideal after all). Might make a good research
>project for an engineering student though if it hasn't been done before.
>
>David
>67 BGT
>71 BGT
>
>At 10:07 PM 7/5/00 -0700, Bullwinkle wrote:
> >Andrew:
> >
> >My brother and I spent several hours searching for a formula that would
> >convert
> >sea level compression ratio, to that at a different altitude. We couldn't
> >really
> >find anything. We found a few formulas that would give pressure for a
> >given CR
> >but it would take several formulas, and calculations to get any results.
> >However the following might provide some insight.
> >
> >Air pressure at sea level is 14.7 lbs/sq. in.
> >Air pressure at 10,000 ft is 10.2 lbs/sq. in.
> >
> >Air density in % at 10,000 ft is (14.7-10.2)/14.7 or 31% less. If an
> engine is
> >using a 10:1 CR at sea level, then at 10,000 feet the compessed air's
> density
> >would be 31% less. I would think a compression ratio of 6.9:1 would
> compress
> >the air 31% less than the 10:1 ratio.
> >
> >So my guess is that a 10:1 CR at sea level equates to 6.9:1 at 10,000 feet.
> >
> >The decrease in air pressure as the altitude increases is not a linear
> >function. The correction at 5000 ft wouldn't be half of the above. As a
> >rough
> >guess, I would say that decrease in air pressure from sea level to 5,000 ft
> >would be approximately 2 lbs./sq. in. So the equivalent CR of a 10:1
> might be
> >about 8.7:1.
> >
> >This is just a guess.
> >
> >Blake
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