Return-Path: Received: from [65.33.163.227] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 4.3c3) with HTTP id 815046 for lml@lancaironline.net; Mon, 21 Mar 2005 23:42:44 -0500 From: "Marvin Kaye" Subject: Re: [LML] Re: superchargers To: lml X-Mailer: CommuniGate Pro WebUser Interface v.4.3c3 Date: Mon, 21 Mar 2005 23:42:44 -0500 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="windows-1252"; format="flowed" Content-Transfer-Encoding: 8bit Posted for "George Braly" : I have compressor maps for lots of aircraft centrifugal compressors - - and only one of them gets to 80% at any point on the map. It is unlikely that the fixed rpm ratio version of that unit would operate at that optimum efficiency for more than a small portion of its operating life. >>Typical turbine/compressor combinations I have seen operate at around 70% >>combined efficiency. Of course a fraction of that efficiency is reclaimed from >>the proverbial "waste heat" of the exhaust. Centrifical compressors are >>commonly 80% or better. << I have compressor maps for lots of aircraft centrifugal compressors - - and only one of them gets to 80% at any point on the map. It is unlikely that the fixed rpm ratio version of that unit would operate at that optimum efficiency for more than a small portion of its operating life. >> 4) To get the air temperature back down to tolerable (detonation margins) ranges requires an intercooler All sorts of engines run without intercoolers, without damage. Detonation is a function of temperature AND pressure in the cylinder at, or near, full compression, not the temperature in the intake. Besides, if you are flying up high, where an add on system makes sence, the air starts out colder so it isn't all that much warmer than it would have been at ground level. The principle reason for adding an intercooler is the same as for adding a supercharger, to make the intake air denser and increase the mass flow through the engine. << Well, after reading that, I am sort of beginning to wonder that NACA never knew what it was doing. The primary reason in an aircraft to use an intercooler is not to gain more charge density. It is a LOT easier to do that with the compressor. The primary reason is to provide adequate detonation margins at higher power settings. If you think that one of the primary component causes of detonation is NOT the temperature of the induction air - - - then I would suggest that you take another look at the hard data. I have been teaching this precise subject matter on Saturday mornings every other month for the last 34 months. This includes about 600 mechanics and pilots over the last couple of years, including as recently as last Saturday morning when I had the definitive NACA data on induction air temperature vs detonation free BMEP plotted and up on the over head screen in the class room. In addition to teaching the subject, I have measured the results, directly, on the test stand on a TIO-540J2BD engine - - with and without an intercooler. Regards, George