X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [70.9.49.102] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0.5) with HTTP id 907449 for lml@lancaironline.net; Sat, 31 Dec 2005 16:59:41 -0500 From: "Marvin Kaye" Subject: Re: Where has all the power gone? To: lml X-Mailer: CommuniGate Pro WebUser v5.0.5 Date: Sat, 31 Dec 2005 16:59:41 -0500 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for Gary Casey : One issue is the definition of "theory." Otto described a theoretical process and predicted that the highest efficiency came with all the combustion at TDC, but of course his "theory" neglected a few real-world concerns, namely that combustion can't occur instantaneously and there are heat transfer, leakage and friction concerns. Create a theoretical model with all those considerations (and some that I neglected) taken into account and you will have a best economy and best power point with the thetaPP at about 16ATC. One can then say that theory and practice match; it's just that one person's theory may not be the same as another's. Ott's was very simplified and he said "compress the charge, add heat(burn) and then expand the charge." One very good point that George makes is that while the BSFC is not particularly sensitive to changes in thetaPP the peak pressure and thermal load on the engine go up dramatically when the thetaPP is advanced. I'm also concerned that with increases in peak pressure the shape of the pressure curve also becomes sharper, inputing more of a pulse to the engine structure. This could have a bad effect on crankshaft torsional vibrations (sometimes called "harmonics"), so for engine durability I have a suspicion that we should favor the retarded side of the peak. True? Regarding the RSA system, it works by balancing the fuel pressure drop across a fixed orifice with the air pressure drop in a fixed venturi. There is a small offset spring, but at high fuel flows this is a good representation of how it works. Both orifices operate on the turbulent-flow equation of mass flow being proportional to the square root of rho times DP. A simplification that helps keep it simple and doesn't hurt accuracy too much is that the air is incompressible, the mach number being relatively low. A very short explanation is to note that the air changes density while the fuel does not and since the densities are inside the square root the error will be proportional to the square root of the air density change. To use more math, one can create the equation for air/fuel ratio(mass flow of air divided by mass flow of fuel), leaving out the fixed coefficients and assuming that the DP of the air is the same as the DP of the fuel (a good assumption). Now air/fuel ratio is proportional to the square root of the ratio of density of air divided by the density of fuel. The fuel does not change density so that value can be lumped in with all the other constants and what you have left is that the air/fuel ratio is proportional to the square root of air density. I'm no good at all the equation symbols in text format, so I hope this makes sense. On the subject of the LSE system I assume that the timing scheme is constant with rpm and varies linearly with manifold pressure, advancing by about 20 degrees with a 20-inch drop of manifold pressure. Those of you with his system and with the display option, is this what you see? Gary