Return-Path: Received: from [24.25.9.101] (HELO ms-smtp-02-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 4.2b7) with ESMTP id 310009 for flyrotary@lancaironline.net; Sat, 10 Jul 2004 00:26:55 -0400 Received: from EDWARD (clt25-78-058.carolina.rr.com [24.25.78.58]) by ms-smtp-02-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id i6A4QMNs020825 for ; Sat, 10 Jul 2004 00:26:24 -0400 (EDT) Message-ID: <001401c46636$10d1f980$2402a8c0@EDWARD> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: BOV more/less air flow Date: Sat, 10 Jul 2004 00:26:25 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0011_01C46614.890B3A50" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1409 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1409 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0011_01C46614.890B3A50 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hi Bob, Some good questions are being raised and I am scratching my head = over them. Its taking a bit of research as I have no experience with = BOVs. I believe the answer to your question about the power surges is = that there is hysteresis in the action of the BOV in that it doesn't = just open at say 1.5 and close again at 1.5. While it may lift off at = 1.5 the pressure would likely have to reduce to some lesser value say = 1.45, 1.475 before the spring would actually force the valve shut again = against the flow. Otherwise the valve would likely chatter as you = hypothesized. But I would imagine if the boost was not brought under = control by reducing the throttle or some other means that it might well = continue to open and shut or simply stay open depending on how effect it = has on the boost equation. Eric also raised a question concerning my conclusion about the effect of = the BOV on air mass flow. I am working on a couple of hypothesis to = that question. It appears that there are two possibilities. Either the = BOV effect does not materially affect the air mass flow directly by its = action of venting of some of the boost air or it does. How about that = for nailing it down {:>). =20 By directly I mean - does the amount of air mass the BOV vents amount = to an significant amount of the total air mass flowing through the = system. If it does not vent an appreciable amount of the total mass = flow then I do not believe its the diverting of air mass that causes the = boost to drop back to its set point, but the fact that a relative small = amount of air vented affects the pressure ratio causing it to decrease, = causing the density to decrease causing the mass flow to decrease. =20 I guess the problem I have with possibility that the BOV diverts = sufficient air mass to affect the boost is that the engine can be though = of as an 80 cid cup that is ladling 80 cid of air 100 times a second at = 6000 rpm. That is a lot of air being removed from the manifold, yet the = small compressor wheel has no problem keeping the manifold pressurized = against that type of outflow. So unless the BOV vents a considerably = amount of air mass I just don't see right now how that could materially = affect the boost ratio or the air mass flow. But, I will post my = analysis for consideration tomorrow as Its getting a bit late. I may = see the light tomorrow.{:>) Its certain good to have folks jump in with such questions.=20 Ed Anderson RV-6A N494BW Rotary Powered Matthews, NC ----- Original Message -----=20 From: Bob Perkinson=20 To: Rotary motors in aircraft=20 Sent: Friday, July 09, 2004 10:10 PM Subject: [FlyRotary] Re: BOV more/less air flow So what happens when the BOV opens. The pressure on the intake = manifold side starts immediately venting to the atmosphere reducing the = pressure inside the manifold - this is of course what the BOV is = designed to do. The compressor wheel has been spinning at 90,000 rpm = with 1.5 boost pressure ratio. Now the boost pressure ratio may be 1.2 = or less in a very short duration of time (milliseconds). This = immediately reduces the backpressure on the compressor wheel caused by = the manifold pressure being less than it before the venting. . The = engine has not yet reacted to this change, as it is still combusting the = boost density air it had ingested into the combustion chamber before the = BOV opened. Neither has the exhaust gas flow been affected - yet. Ed, In the situation that you describe what would prevent the BOV from = alternately opening and closing causing noticeable power surges?=20 IMHO I would also think that the turbine that had the larger = compressor would be the one whose shaft would suffer from the sudden = attempt to accelerate and the increase in vibration as the speed = increases when the back pressure is lost. I would also like to take this opportunity to thank you and all the = others on this list for all the information that is freely imparted. Bob Perkinson Hendersonville, TN. RV9A If nothing changes Nothing changes =20 ------=_NextPart_000_0011_01C46614.890B3A50 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hi Bob,
    Some good questions = are being=20 raised and I am scratching my head over them.  Its taking a bit of = research=20 as I have no experience with BOVs.  I believe the answer to your = question=20 about the power surges is that there is hysteresis in the action of the = BOV in=20 that it doesn't just open at say 1.5 and close again at 1.5.  While = it may=20 lift off at 1.5 the pressure would likely have to reduce to some lesser = value=20 say 1.45, 1.475 before the spring would actually force the valve shut = again=20 against the flow.  Otherwise the valve would likely chatter as you=20 hypothesized.  But I would imagine if the boost was not brought = under=20 control by reducing the throttle or some other means  that it = might=20 well continue to open and shut or simply stay open depending on how = effect it=20 has on the boost equation.
 
Eric also raised a question concerning = my=20 conclusion about the effect of the BOV on air mass flow.  I am = working on a=20 couple of hypothesis to that question.  It appears that there are = two=20 possibilities.  Either the BOV effect does not materially affect = the air=20 mass flow directly by its action of  venting of some of the boost = air or it=20 does.  How about that for nailing it down=20 {:>).  
 
 By directly I mean - does the = amount of air=20 mass the BOV vents amount to an significant amount of the total air mass = flowing=20 through the system.  If it does not vent an appreciable amount of = the total=20 mass flow then I do not believe its the diverting of air mass that = causes the=20 boost to drop back to its set point, but the fact that a relative small = amount=20 of air vented affects the pressure ratio causing it to decrease, causing = the=20 density to decrease causing the mass flow to decrease.  =
 
I guess the problem I have with = possibility that=20 the BOV diverts sufficient air mass to affect the boost is that the = engine can=20 be though of as an 80 cid cup that is ladling 80 cid of air 100 times a = second=20 at 6000 rpm.  That is a lot of air being removed from the manifold, = yet the=20 small compressor wheel has no problem keeping the manifold pressurized = against=20 that type of outflow.  So unless the BOV vents a considerably = amount of air=20 mass I just don't see right now how that could materially affect the = boost ratio=20 or the air mass flow. But, I will post my analysis for = consideration=20 tomorrow as Its getting a bit late.  I may see the light=20 tomorrow.{:>)
 
Its certain good to have folks jump in = with such=20 questions. 
 
 
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From:=20 Bob=20 Perkinson
Sent: Friday, July 09, 2004 = 10:10=20 PM
Subject: [FlyRotary] Re: BOV = more/less=20 air flow

So what happens when the BOV opens. The = pressure on=20 the intake manifold side starts immediately venting to the atmosphere = reducing=20 the pressure inside the manifold =96 this is of course what the BOV is = designed=20 to do. The compressor wheel has been spinning at 90,000 rpm with 1.5 = boost=20 pressure ratio. Now the boost pressure ratio may be 1.2 or less in a = very=20 short duration of time (milliseconds). This immediately reduces the=20 backpressure on the compressor wheel caused by the manifold pressure = being=20 less than it before the venting. . The engine has not yet reacted to = this=20 change, as it is still combusting the boost density air it had = ingested into=20 the combustion chamber before the BOV opened. Neither has the exhaust = gas flow=20 been affected - yet.

 

Ed,

In the situation that you describe = what would=20 prevent the BOV from alternately opening and closing causing = noticeable power=20 surges?

IMHO=20  I would also think that the turbine that had the = larger compressor would be the=20 one whose shaft would = suffer from=20 the sudden attempt to accelerate and = the=20 increase in vibration as the speed increases when the back pressure is = lost.

I would also like to take this opportunity = to thank=20 you and all the others on this list for all the information that is = freely=20 imparted.

Bob Perkinson
Hendersonville, TN.
RV9A
If = nothing=20 changes
Nothing changes
 

 
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