X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 01 Feb 2007 09:23:27 -0500 Message-ID: X-Original-Return-Path: Received: from imo-m23.mx.aol.com ([64.12.137.4] verified) by logan.com (CommuniGate Pro SMTP 5.1.5) with ESMTP id 1803543 for lml@lancaironline.net; Thu, 01 Feb 2007 02:12:37 -0500 Received-SPF: pass receiver=logan.com; client-ip=64.12.137.4; envelope-from=Sky2high@aol.com Received: from Sky2high@aol.com by imo-m23.mx.aol.com (mail_out_v38_r7.6.) id q.c17.1058b07b (40522) for ; Thu, 1 Feb 2007 02:11:38 -0500 (EST) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Thu, 1 Feb 2007 02:11:36 EST Subject: Re: [LML] Re: Altitude Switch --Fuel pressure confusion. X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1170313896" X-Mailer: 9.0 Security Edition for Windows sub 5358 X-Spam-Flag: NO -------------------------------1170313896 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit I am a bit confused by Bob M. and Jeff E. In a carbureted aircraft engine configuration, the only use of an adjunct pump (hand or electric) is to supply some priming fuel directly to the intake side of a cold engine before start. The engine driven pump generally provides a constant pressure fuel supply at about 4-6 psi (maybe less) to the bowl until the float says stop. The carb doesn't care if the fuel is sputtering into the bowl with vapors since the fuel level in the bowl is all that counts and that level is kept ahead of the actual carb's needs. Oh yeah, gravity supplied fuel to a carb works too - as long as there is fuel in the bowl. On the other hand, a fuel injected engine, say a Lyc 320 with an RSA style fuel system, generally requires that some level of fuel pressure must be maintained so the stinking throttle body can match it to the air via pressure measured by some weenie tubes in the induction stream so that ultimately it can deliver the correct fuel flow to the injectors. The ideal pressure is about 26 psi, but the system only gets upset if the pressure drops below 12 psi (fluctuations between those values don't seem to make much difference). How can such variation happen? Well, one way is vaporization of the fuel because of heat and thus cavitation in the engine driven pump, especially if that pump is also heat saturated from the engine operation. How can that be fixed? Cool fuel could help or maybe raise the system fuel pressure (read boost pump) to try to keep the fuel from vaporizing before you actually would like it to vaporize, like near the intake valve. The "hi" boost pump in my airplane could maybe get the fuel pressure up to 28 psi, not much higher than the engine pump operating on cool fuel but high enough to get the fuel back to a liquid state. Another way to solve a pressure problem traced to the engine pump might be to duct some cool intake air over the engine pump - if that is the source of fuel pressure loss/fluctuation during flight. The only times I have problems with vaporization is during a hot start or idling/taxiing very slowly on a really hot day - my red idiot light labeled lo fuel pressure comes on and is backed up by the fuel pressure gauge dipping below 12 psi, a condition that has been resolved by turning on the boost pump. BTW, the boost pump is on for takeoffs and landings as a backup. Scott Krueger AKA Grayhawk Lancair N92EX IO320 SB 89/96 Aurora, IL (KARR) A man has got to know his limitations. -------------------------------1170313896 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
I am a bit confused by Bob M. and Jeff E.
 
In a carbureted aircraft engine configuration, the only use of an=20 adjunct pump (hand or electric) is to supply some priming fuel=20 directly to the intake side of a cold engine before start.  The engine=20 driven pump generally provides a constant pressure fuel supply at about= 4-6=20 psi (maybe less) to the bowl until the float says stop.  The carb doesn= 't=20 care if the fuel is sputtering into the bowl with vapors since the fuel= =20 level in the bowl is all that counts and that level is kept ahead of th= e=20 actual carb's needs.  Oh yeah, gravity supplied fuel to a carb works to= o -=20 as long as there is fuel in the bowl.
 
On the other hand, a fuel injected engine, say a Lyc 320 with an RSA st= yle=20 fuel system, generally requires that some level of fuel pressure must be=20 maintained so the stinking throttle body can match it to the air=20 via pressure measured by some weenie tubes in the induction stream = ;so=20 that ultimately it can deliver the correct fuel flow to the=20 injectors.  The ideal pressure is about 26 psi, but the system onl= y=20 gets upset if the pressure drops below 12 psi (fluctuations between those va= lues=20 don't seem to make much difference).  How can such=20 variation happen?  Well, one way is vaporization of the fuel becau= se=20 of heat and thus cavitation in the engine driven pump, especially if th= at=20 pump is also heat saturated from the engine operation. 
 
How can that be fixed?  Cool fuel could help or maybe raise the sy= stem=20 fuel pressure (read boost pump) to try to keep the fuel from vaporizing befo= re=20 you actually would like it to vaporize, like near the intake valve.  Th= e=20 "hi" boost pump in my airplane could maybe get the fuel pressure up to 28 ps= i,=20 not much higher than the engine pump operating on cool fuel but high enough=20= to=20 get the fuel back to a liquid state.
 
Another way to solve a pressure problem traced to the engine=20 pump might be to duct some cool intake air over the engine pump=20 - if that is the source of fuel pressure loss/fluctuation during=20 flight.
 
The only times I have problems with vaporization is during a hot start=20= or=20 idling/taxiing very slowly on a really hot day - my red idiot light labeled=20= lo=20 fuel pressure comes on and is backed up by the fuel pressure gauge dipping b= elow=20 12 psi, a condition that has been resolved by turning on the boost pump.
 
BTW, the boost pump is on for takeoffs and landings as a backup.
 
 
Scott Krueger=20 AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)

A m= an=20 has got to know his limitations.
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