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<< Lancair Builders' Mail List >>
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In a message dated 1/4/2002 5:46:05 AM Pacific Standard Time, gwbraly@gami.com writes:
<<<...It was because I perceived that Jack, and to some extent, you, were only
focused on the issue of the absolute value of the BMEP...>>>
George:
I think you're right (that we agree more than we disagree). IF my original response to Mr. Casey suggested that I thought that BMEP (AKA "torque-per-cubic inch x 151") was in any way an absolute, then I communicated my thoughts poorly. It was NOT my intention to suggest that BMEP is an absolute, but rather a very good comparative quantifier, and, in fact, quite accurate when comparing engines which incorporate roughly comparable design, material-science, and combustion technology.
<<<... I tried to point out some examples of an engine operating at exactly the
same BMEP, but with entirely different combustion pressure characteristics
that will have VERY VERY different consequences with respect to engine
longevity....>>>
That point was well-taken, but clearly inapplicable to the case in point. The data you presented are from your experiments, with "Case B" using technology implemented, as I understand it, with a control loop closed on instantaneous cylinder pressure, which requires some pretty sophisticated computer technology to implement. IF you are achieving the same BMEP (on the same engine) with lower peak pressure, then no argument, the integral of the pressure curve is the same, and longevity will be better. However, the point which, I think, became obscured in the exchange is that the technology you are marketing is advanced, and definitely not commonplace. When that technology (to lower the peak and displace it further past TDC) is implemented, either by computer magic, or by less complex means, then it certainly provides the opportunity for greater reliability at the same BMEP, or the same reliability at higher BMEP, or some of both.
(BTW, did I miss your list of heavily-boosted, 175-BMEP, SI engines which run 3000 hours before needing top end work??) In fact, ONE of the reasons we have achieved the targeted reliability on our development engines (at over 220 BMEP on pump gas) is a result of advanced combustion technology, implemented by proprietary (new design) cylinder heads, intake system, and fuel injection components.
That being said, however, the use of BMEP as a yardstick in the cases presented during this exchange was indeed valid, because (a) with "no high-tech parts" (as Mr. Casey bounded the problem) it is unlikely that any of the abovementioned technology is implemented in his engine, (b) 45" MAP on a crate motor ("no high-tech parts") with 9.0 to 1 calculated CR isn't likely to produce the kind of reliability that I'd fly behind. <<<...However, in the context of the use of high powered piston engines for aircraft like the Lancair, a Malibu, or a Mirage, it is MY OPINION...that the term is often used in an inappropriate and broader than warranted manner...>>>
Well, thanks for making my point here. The 350-HP TSIO-550 used on the Lancair and Malibu (not necessarily the same model) ran at about 187 BMEP for takeoff (350 HP at 2700) and 158 BMEP at 75% cruise (262 HP at 2400 RPM). Now, I consider 158 to be a fairly conservative number, yet many have experienced less than laudable reliability from those engines, yes??
<<<...Unfortunately, it is sometimes used, with other technical jargon, to try to
make an argument appear ... to be more persuasive than the combustion science would support...>>>
I wonder if anyone will get all indignant over your (admittedly more subtle) snides?
In closing, I'd like to say that, as a result of all the whining about "kinder and gentler" postings, I reviewed the offending document again, and found nothing in it that I thought to be particularly offensive, and definitely nothing "ad hominem" (with the possible exception of my reference to certain well-known snake-oil salesmen). Regarding that reference, I had ABSOLUTELY NO INTENTION of even suggesting that you were in any way similar to them. If I offended you, I do apologize, and hope this hasn't hindered future communication.
Regards,
Jack Kane
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