X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from poplet2.per.eftel.com ([203.24.100.45] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTP id 3691687 for flyrotary@lancaironline.net; Mon, 22 Jun 2009 19:28:23 -0400 Received-SPF: none receiver=logan.com; client-ip=203.24.100.45; envelope-from=lendich@aanet.com.au Received: from sv1-1.aanet.com.au (sv1-1.per.aanet.com.au [203.24.100.68]) by poplet2.per.eftel.com (Postfix) with ESMTP id 09822173571 for ; Tue, 23 Jun 2009 07:27:45 +0800 (WST) Received: from ownerf1fc517b8 (203.171.92.134.static.rev.aanet.com.au [203.171.92.134]) by sv1-1.aanet.com.au (Postfix) with SMTP id 7DCD1BEC01E for ; Tue, 23 Jun 2009 07:27:45 +0800 (WST) Message-ID: <95C85C871CD7462CA1C858D051B803DB@ownerf1fc517b8> From: "George Lendich" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Duty Cycle for Injectors Date: Tue, 23 Jun 2009 09:27:47 +1000 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0017_01C9F3E4.DEAFDC50" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.5512 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 X-Antivirus: avast! (VPS 090622-0, 06/22/2009), Outbound message X-Antivirus-Status: Clean This is a multi-part message in MIME format. ------=_NextPart_000_0017_01C9F3E4.DEAFDC50 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Cary, Very interesting, thank you for that info. Bendix is a low pressure injector system is it not? I have been reading about the benefits of low pressure systems - are = they coming back in vogue? George ( down under) ----- Original Message -----=20 From: Gary Casey=20 To: Rotary motors in aircraft=20 Sent: Monday, June 22, 2009 9:44 PM Subject: [FlyRotary] Re: Duty Cycle for Injectors Just a little historical anecdote: When we at Bendix were developing = the very first US production fuel injection system (remember the = Cosworth Vega?) we sized the injectors for about 80% on-time at maximum = manifold pressure, minimum air temperature and full power. This is a = compromise, of course - a low-flow injector is desireable to have the = most accuracy at idle when the pulse width is very short. But if the = injector approaches 100%, there will be a pulse width less than 100% = when the injector will stay fully open, creating a step in fuel flow. = This is a normal occurence during cold engine operation when the = enrichment algorithm commands more flow, but it turns out that a max = engine speed there is little or no need for cold enrichment regardless = of temperature, so the effect is pretty much ignored. The idea of = sizing the injectors for 80% duty cycle had nothing to do with racing = applications, but it applies there, too. The obvious problem = encountered when undersizing the injectors is that once 100 duty cycle = is reached the flow is limited - the engine will lean out if the rpm or = air flow goes higher. Not a good thing. Inceidentally, there is no concern for injector relaibility or = durability. It is a simple solenoid valve and running at 100% duty = cycle would theoretically make it last longer - it is the total number = of open-close cycles that wears the injector. As injectors wear the = flow generally increases and most modern injectors are rated for a = billion cycles with a flow increase of less than 3%. The coil is cooled = by the fuel itself, so overheating is not a concern. Gary Casey -------------------------------------------------------------------------= ----- -----Inline Message Follows----- Ed, I remember reading that the 80% duty cycle rule (of thumb)was = originally developed for the racing industry. Given that probably 100% will never be reached in the day to day = vehicle use - the 80 % duty cycle most probably doesn't apply for = sizing. However bigger injectors are standard in vehicles where the RPM = limit is much higher than what we may experience in our Aviation = application, even if for only short periods of time. Given that our RPM is on the high end for climb and take-off, but not = as high as it could be (not max RPM), and cruise is for the most part = only 6,000 rpm, do we really have to held to the 80% DC rule, where = perhaps 90% DC (for short periods of time) may well give sufficient = safety margin to maintain longevity of the injectors. Not that there is a major benefit in having borderline injector duty = cycle, as you so rightly pointed out - it's just that it would be = interesting to know! George (down under) ------=_NextPart_000_0017_01C9F3E4.DEAFDC50 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Cary,
Very interesting, thank you for that=20 info.
Bendix is a low pressure injector = system is it=20 not?
I have been reading about the benefits = of low=20 pressure systems - are they coming back in vogue?
George ( down under)
----- Original Message -----
From:=20 Gary=20 Casey
Sent: Monday, June 22, 2009 = 9:44 PM
Subject: [FlyRotary] Re: Duty = Cycle for=20 Injectors

Just a little historical anecdote:  When we at Bendix were=20 developing the very first US production fuel injection system = (remember=20 the Cosworth Vega?) we sized the injectors for about 80% on-time at = maximum=20 manifold pressure, minimum air temperature and full power.  This = is a=20 compromise, of course - a low-flow injector is desireable to have the = most=20 accuracy at idle when the pulse width is very short.  But if the = injector=20 approaches 100%, there will be a pulse width less than = 100% when the=20 injector will stay fully open, creating a step in fuel flow. =20 This is a normal occurence during cold engine operation = when=20 the enrichment algorithm commands more flow, but it turns out that a = max=20 engine speed there is little or no need for cold enrichment regardless = of=20 temperature, so the effect is pretty much ignored.  The idea of = sizing=20 the injectors for 80% duty cycle had nothing to do with racing = applications,=20 but it applies there, too.  The obvious problem encountered when=20 undersizing the injectors is that once 100 duty cycle is reached the = flow is=20 limited - the engine will lean out if the rpm or air flow goes = higher. =20 Not a good thing.
 
Inceidentally, there is no concern for injector relaibility or=20 durability.  It is a simple solenoid valve and running at 100% = duty cycle=20 would theoretically make it last longer - it is the total number of = open-close=20 cycles that wears the injector.  As injectors wear the flow = generally=20 increases and most modern injectors are rated for a billion cycles = with a flow=20 increase of less than 3%.  The coil is cooled by the fuel itself, = so=20 overheating is not a concern.
 
Gary Casey

-----Inline Message Follows-----

Ed,
I remember reading that the 80% duty = cycle rule=20 (of thumb)was originally developed for the racing = industry.
 
Given that probably 100% will never = be reached in=20 the day to day vehicle use - the 80 % duty cycle most = probably=20 doesn't apply for sizing. However bigger injectors are standard in = vehicles=20 where the RPM limit is much higher than what we may experience in our = Aviation=20 application, even if for only short periods of time.
 
Given that our RPM is on the high end = for climb=20 and take-off, but not as high as it could be (not max RPM), and cruise = is for=20 the most part only 6,000 rpm, do we really have to held to the 80% DC = rule,=20 where perhaps 90% DC (for short periods of time) may well give = sufficient=20 safety margin to maintain longevity of the injectors.
 
Not that there is a major benefit in = having=20 borderline injector duty cycle, as you so rightly pointed out - =  it's=20 just that it would be interesting = to=20 know!
George (down=20 under)

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