Return-Path: Received: from fed1rmmtao06.cox.net ([68.230.241.33] verified) by logan.com (CommuniGate Pro SMTP 4.2b7) with ESMTP id 310010 for flyrotary@lancaironline.net; Sat, 10 Jul 2004 00:42:37 -0400 Received: from davidandanne ([68.111.224.107]) by fed1rmmtao06.cox.net (InterMail vM.6.01.03.02 201-2131-111-104-20040324) with SMTP id <20040710044206.NZPC24672.fed1rmmtao06.cox.net@davidandanne> for ; Sat, 10 Jul 2004 00:42:06 -0400 From: "DaveLeonard" To: "Rotary motors in aircraft" Subject: RE: [FlyRotary] BOV more/less air flow Date: Fri, 9 Jul 2004 21:42:09 -0700 Message-ID: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_001A_01C465FD.95FB4460" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2910.0) Importance: Normal In-Reply-To: X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1409 This is a multi-part message in MIME format. ------=_NextPart_000_001A_01C465FD.95FB4460 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Ed, Great write up! In general I very much agree with you on all points but I want to free flow some thoughts that may modify people's feeling about a BOV. I am even less of a turbo expert than you, but here are some thoughts to consider. First, I think we have to figure out exactly what is causing John's turbo's to fry. IMHO we have 3 general candidates 1) overheating, 2) turbo overspeading, or 3) compressors into the surge region. While it is possible that there is a combined cause, and probable that more than one factor plays a partial role, most likely ONE factor is primarily responsible for JTD (John's Turbos' Demise). IMHO that one factor is overheating. We know that heat is a factor while according you the map you posted we are probably not in surge or overspeed region most of the time. Also, turbo overspeading and compressor surge are likely to present with very different symptoms like surging pressures or a disintegrated turbo. In contrast, overheating weakens the metals involved and would behave much like what John Describes. In this case it seems to be turbine axle and possibly the blades that are not up to the task. There are 2 ways to improve the situation. Reduce the heat, of course, and reduce the forces acting on the metals. The BOV can be very instrumental in reducing the force acting on that axle, and possibly also slightly reduce the heat transfer. My BOV (which I assume is fairly standard) is not an on/off valve, but a spring loaded valve that will open more or less depending on the pressure in the plenum, the ambient pressure, and the pressure differential across the throttle body. There is no way it will instantly drop the pressure significantly. In normal operation it would never go from full closed to full open suddenly, but rather just start leaking more and more air as the combination of those three pressures changes. The only time it would suddenly open is when the throttle is suddenly closed. While on one hand it does seem very inefficient to compress air only to have it bleed out before entering the engine, I believe it is far more inefficient and potentially harmful to have the compressor constantly fighting against a large pressure differential. That pressure differential is the largest determinant of the force on the axle. Further, if the turbine wheel is free to turn a little easier (faster) it places less back pressure on the exhaust and less heat will be transfeffed. It is true that the faster RPM increases the radial stresses on the blades, but radial stresses are tolerated much better than the axial stresses caused by high back pressure. (Since I have decided that heating is the problem and not overspeeding.) There is no doubt that we both agree on the BEST solution: a larger turbo with an even larger waste gate placed perhaps a little further from the engine. But until I change my set-up to fit such a turbo and waste gate I think the BOV is a helpful addition. Further, the BOV does provide some measure of protection against over boost like what happened to Todd. Dave Leonard ------=_NextPart_000_001A_01C465FD.95FB4460 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Ed,=20 Great write up!  In general I very much agree with you on all = points but I=20 want to free flow some thoughts that may modify people's feeling about a = BOV.  I am even less of a turbo expert than you, but here are some = thoughts=20 to consider.
 
First,=20 I think we have to figure out exactly what is causing John's turbo's to=20 fry.  IMHO we have 3 general candidates 1) overheating, 2) = turbo=20 overspeading, or 3) compressors into the surge region.  While it is = possible that there is a combined cause, and probable that more than one = factor=20 plays a partial role, most likely ONE factor is = primarily responsible for=20 JTD (John's Turbos' Demise).
 
IMHO=20 that one factor is overheating.  We know that heat is a factor=20 while according you the map you posted we are probably = not in=20 surge or overspeed region most of the time.  Also, turbo = overspeading and=20 compressor surge are likely to present with very different symptoms like = surging=20 pressures or a disintegrated turbo.  In contrast, overheating = weakens the=20 metals involved and would behave much like what John Describes.  In = this=20 case it seems to be turbine axle and possibly the blades that are not up = to the=20 task.
 
There=20 are 2 ways to improve the situation. Reduce the heat, of course, and = reduce the=20 forces acting on the metals.  The BOV can be=20 very instrumental in reducing the force acting on that axle, and = possibly also=20 slightly reduce the heat transfer.  My BOV (which I assume is = fairly=20 standard) is not an on/off valve, but a spring loaded valve that will = open more=20 or less depending on the pressure in the plenum, the ambient pressure, = and the=20 pressure differential across the throttle body.  There is no way it = will=20 instantly drop the pressure significantly.  In normal operation it = would=20 never go from full closed to full open suddenly, but rather just start = leaking=20 more and more air as the combination of those three pressures = changes.  The=20 only time it would suddenly open is when the throttle is suddenly=20 closed.
 
While=20 on one hand it does seem very inefficient to compress air only to have = it bleed=20 out before entering the engine, I believe it is far more inefficient and = potentially harmful to have the compressor constantly fighting against a = large=20 pressure differential.  That pressure differential is the largest=20 determinant of the force on the axle.
 
Further, if the turbine wheel is free to turn a little easier = (faster)=20 it places less back pressure on the exhaust and less heat will be=20 transfeffed.  It is true that the faster RPM increases the radial = stresses=20 on the blades, but radial stresses are tolerated much better than the = axial=20 stresses caused by high back pressure.  (Since I have decided that = heating=20 is the problem and not overspeeding.)
 
There=20 is no doubt that we both agree on the BEST solution: a larger turbo with = an even=20 larger waste gate placed perhaps a little further from the=20 engine.  But until I change my set-up to fit such a turbo and = waste=20 gate I think the BOV is a helpful addition.
 
Further, the BOV does provide some measure of protection = against over=20 boost like what happened to Todd.
 
Dave=20 Leonard
 
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