lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> George: Thank you for your erudite response. Now, I suggest that you go back and re-read my letter, because it is clear that you have responded to perceived claims and issues which nowhere appeared in that text. While your explanation of the original derivation of BMEP is correct, it is clear from other comments in your letter that you (and many others) don't have a full grasp of the true significance of BMEP. Indeed, the mean cylinder pressure required to achieve a given torque doesn't yield much information about the infinite number of different waveforms which could produce the same net torque. But, in point of fact, BMEP was never intended to do so, and no one who understands it's significance would read any such inferences into a BMEP number. People commonly argue against "high" BMEP on the basis of reduced reliability based solely on the miscomprehension that BMEP is somehow related to real cylinder pressure distribution. Someone who understands BMEP would realize that one can make significant improvements in the BMEP of an engine by making changes which have absolutely NO bearing on the combustion process AT ALL, but which reduce the magnitude of the FMEP (such as increasing the efficiency of the coolant pumps, reducing the parasitic losses in the reciprocating and rotating machinery, etc.). The real significance of BMEP lies in the ability to quickly determine a rough quantification of comparative efficiencies and the reality of various claims, since, as I am sure you already know, BMEP is simply a different means of stating "torque per cubic inch of displacement". (BMEP = 151 x lb-ft-Torque-per-cubic-inch). In fact, if I have correctly interpreted the scrambled chart in your response (which, coupled with its subsequent weak conclusions, I take to be a transparent promotion of your PRISM system), the data given to define the "Condition A" and "Condition B" are deficient of critical specifics, as is often typical of anecdotal argument. For example, you claim that both engines (Condition A and Condition B) are making the same power, but you conveniently omit the remainder of the defining parameters, including: (a) whether or not they have the same displacement, (b) whether or not both have identical power-section geometry, and (c) whether or not they are being evaluated at the same RPM. (Also, in view of the simplicity of making a dyno say pretty much anything you want, I am highly suspect of power claims based on any dyno other than my own. Davy Blanton could "document" 230 HP out of a pathetically-poor engine which, on it's best day would be lucky to make 160 SAE-standard HP on any correctly calibrated dyno.) If, however, I assume that all three of those factors (displacement, geometry, RPM) are the same in both A and B engines, then clearly both are operating at the same BMEP, even though the pressure distribution for Condition B is clearly different from the (more peaky) distribution in A. What that revelation shows is that brake torque (the difference between the indicated torque and the friction torque) is, as has been known for over a century, a function of the integral of the "combustion pressure v. rotation" curve, and that many different curves can produce the same integral. However, if one engine produces the same power on less fuel than the other (better BSFC), then clearly one is more efficient than the other, and BMEP is not indicative of that difference. Nor was it offerred as such. While you do state a few disjoint facts, your argument against the usefulness of the BMEP yardstick is based upon small and infrequent exceptions in an attempt to invalidate a basic engineering procedure of comparative analysis. (To label that an epistemological exercise is, I think, a bit of a self-important hyperbole.) There are several specific points in your letter to which I would like to respond further. <<<...There are about 400 million hours of highly successful engine operating experience, with aircraft spark ignition piston engines operating at cruise BMEPs in the range of 175 psi. Those engines normally went well past 3000 hours between overhauls and or jug changes. Oh! And by the way, they were heavily boosted engines....>>> Regarding your statements above, it would greatly enhance clarity if you would state the specifics and sources of your data. Which engines, what ratings, what's your defininition of "heavily boosted"? I find your statements to be characteristic of those often posed by fast-talking salesmen: sweeping claims without any hint of supporting data. Since there is no mention of any specific engine type which operated at around 175 cruise BMEP and lasted for 3000 hours, I can only revert to that which I have experienced. My experience would define "heavily boosted" as more than 55" hg MAP. I can't really think of a "properly engineered" engine, operated in a regime I would call "heavily boosted" making ONLY 175 BMEP. Perhaps you could help me with some hard data. Here are some specific examples of what I would call "well-designed, properly engineered, heavily boosted" engines. The P&W R-2800-32W C-series engines ran at 396 BMEP at takeoff; Some models of the Merlin ran over 300 BMEP on 65" MAP; The Wright R-1820-82A ran at a BMEP of 237 at takeoff power and somewhere around 200 at cruise. However, I don't recall many of those engines making 3000 hours between overhauls. As for numbers in the neighborhood of what you stated, a normally aspirated 300 HP Lyc IO-540 (K1A5 as just one example) makes about 163 BMEP from about 2300 to 2700 RPM, and can be brought up to 175 without too much difficulty. A turbocharged 540, (TIO-540-V2AD, for example) makes almost 200 BMEP at takeoff and at 75% (2300 RPM) makes about 168 BMEP, and I don't recall too many of them going to 3000 hours, nor many GTSIO-520's (200 BMEP takeoff, 175 BMEP cruise) for that matter. Is it your position that all those are improperly engineered? Or could it be that perhaps you've conveniently omitted several of the original design parameters in order to argue a point? <<<....so "longevity" is not an issue in a turbo-charged engine that is properly engineered....>>> NEVER said it was. <<<....In piston engine combustion science, it is a fundamental (and common) epistemological error to assume that all BMEPs of 175 PSI cause the same kinds of stress (distress) on the engine....>>> Search as I may, I can't find anything in my original letter which even SUGGESTED that "...all BMEPs of 175 PSI cause the same kinds of stress (distress) on the engine...." However, as for the usefulness of BMEP as a quick comparator among engines, see arguments presented above. <<<...The "Brake Mean Effective Pressure" term is an artifact. It is purely a calculated number, from other numbers...>>> Recognized experts in the field of internal combustion engines (C.F.Taylor, Sir H.R. Ricardo, Gordon P. Blair, and many others) would seem, from their research, writings, and accomplishments, to disagree with you. I can only humbly attempt to follow their leadership. Further, would you, by the same (specious) reasoning, also label HORSEPOWER to be an artifact because it also is a purely calculated number (torque x RPM / 5252),? <<<...simply throwing out some arbitrary BMEP number as a basis for comparing one engine condition to another, hoping to "conclusively" prove to somebody some issue with respect to piston engine design, operation, or durability...>>> Again, I must suggest that you go back and re-read my letter. For your convenience, I'll summarize by stating that nowhere therein did I "throw out some arbitrary BMEP number", nor were my numbers offerred to "conclusively prove anything". The BMEP numbers I used were either calculated from Mr. Casey's claims, or learned from lots of experience designing, building and testing high reliability, high output piston engines. They were intended to provide members of the experimental community a means of evaluating the potential reality of wishful thinking. (Note that they have proven historically that (a) they will buy pretty much anything and (b) they are easily convinced that the basic laws of math, chemisry and physics, covered extensively in high school, have recently been repealed by some snake-oil salesman. <Bede, Rahm, Blanton, etc. etc.>, ) <<<...Further, it is, as its name indicates, an "average" of a real world engineering parameter that is much more important when considered in its detailed characterization than it is useful when reduced to is "mean", where it becomes only somewhat useful, but capable of being highly misleading...>>> Assuming that I understand the above excerpt, then I beg to differ with you. The concept of "mean" can be very useful when used properly. Obviously, a mean number was never intended to suggest information about the infinite numbers of combinations which can yield that value, and BMEP was never intended to represent the shape of the combustion pressure v. rotation curve. However, from your apparent lack of understanding of the usefulness of BMEP demonstrated above, I can see where you might feel the need to resort to Clintonian semantics to rebut the initial proposition. I think it's unfortunate that you might not have had the opportunity to study statistical methodology in order to appreciate the usefulness of not only simple concepts such as "mean", but even more useful concepts such as "rms" (root-mean-square), "standard deviation", "poisson distributions", "surface modelling", and lots of others. <<<...an experienced person evaluating the structural integrity of the engine, Condition A) would suggest a very short operational life...>>> Again, I have to differ with your assertion. A truly experienced engineer would draw no such conclusion without having examined much more data about the engine (such as the design configurations, materials, heat treatments, and surface conditions of the critical parts). In fact, if you are interested, I can provide you with specifics on engines which operate for VERY long lives at peak cylinder pressures in excess of what you have quoted for Condition A. (I also find it somewhat Clintonian that you offer some extreme example (A) in your scrambled table, then state that your real life example operated somewhere around 1/3 of the way from A to B.) And for the sake of clarity, I am sure you understand that the concept of increasing net torque by reducing negative pre-TDC torque by delaying ignition, resulting in lowered peak pressure, made possible by highly-homogeneous charge characteristics and fast-burn chamber technology is neither new nor revolutionary. While closing the control loop on combustion pressure by means of difficult-and-EXPENSIVE-to-certify computer magic is certainly an interesting way to proceed, others have chosen to achieve those goals by simpler (certifiable) means. Regards, Jack Kane LML website: http://members.olsusa.com/mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please remember that purchases from the Builders' Bookstore assist with the management of the LML. Please send your photos and drawings to marvkaye@olsusa.com.