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Date: Mon, 22 Jun 2009 04:44:15 -0700 (PDT)
From: Gary Casey <casey.gary@yahoo.com>
Subject: Re: Duty Cycle for Injectors
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
In-Reply-To: <list-3690684@logan.com>
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Just a little historical anecdote:=A0 When we at Bendix were developing the=
 very first US production=A0fuel injection system (remember the Cosworth Ve=
ga?) we sized the injectors for about 80% on-time at maximum manifold press=
ure, minimum air temperature and full power.=A0 This is a compromise, of co=
urse - a low-flow injector is desireable to have the most accuracy at idle =
when the pulse width is very short.=A0 But if the injector approaches 100%,=
 there will be a=A0pulse width less than 100%=A0when the injector will stay=
 fully open, creating a step in fuel flow.=A0 This=A0is a normal=A0occurenc=
e=A0during cold engine operation when the enrichment algorithm commands mor=
e 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 muc=
h ignored.=A0 The idea of sizing the injectors for 80% duty cycle had nothi=
ng to do with racing applications, but it applies there, too.=A0 The obviou=
s
 problem encountered when undersizing the injectors is that once 100 duty c=
ycle is reached the flow is limited - the engine will lean out if the rpm o=
r air flow goes higher.=A0 Not a good thing.=0A=0AInceidentally, there is n=
o concern for injector relaibility or durability.=A0 It is a simple solenoi=
d valve and running at 100% duty cycle would theoretically make it last lon=
ger - it is the total number of open-close cycles that wears the injector.=
=A0 As injectors wear the flow generally increases and most modern injector=
s are rated for a billion cycles with a flow increase of less than 3%.=A0 T=
he coil is cooled by the fuel itself, so overheating is not a concern.=0A=
=0AGary Casey=0A=0A=0A=0A=0A________________________________=0A-----Inline =
Message Follows-----=0A=0A=0AEd,=0AI remember reading that the 80% duty cyc=
le rule (of thumb)was originally developed for the racing industry.=0A=A0=
=0AGiven that probably 100% will never be reached in the day to day vehicle=
 use -=A0the 80 % duty cycle=A0most probably doesn't apply for sizing. Howe=
ver bigger injectors are standard in vehicles where the RPM limit is much h=
igher than what we may experience in our Aviation application, even if for =
only short periods of time.=0A=A0=0AGiven that our RPM is on the high end f=
or climb and take-off, but not as high as it could be (not max RPM), and cr=
uise is for the most part only 6,000 rpm, do we really have to held to the =
80% DC rule, where perhaps 90% DC=A0(for short periods of time) may well gi=
ve sufficient safety margin to maintain longevity of the injectors.=0A=A0=
=0ANot that there is a major benefit in having borderline injector duty cyc=
le, as you so rightly pointed out - =A0it's just that it would be interesti=
ng to know!=0AGeorge (down under)=0A=0A=0A      
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<html><head><style type=3D"text/css"><!-- DIV {margin:0px;} --></style></he=
ad><body><div style=3D"font-family:times new roman, new york, times, serif;=
font-size:12pt"><DIV>Just a little historical anecdote:&nbsp; When we at Be=
ndix were developing the very first US production&nbsp;fuel injection syste=
m (remember the Cosworth Vega?) we sized the injectors for about 80% on-tim=
e at maximum manifold pressure, minimum air temperature and full power.&nbs=
p; This is a compromise, of course - a low-flow injector is desireable to h=
ave the most accuracy at idle when the pulse width is very short.&nbsp; But=
 if the injector approaches 100%, there will be a&nbsp;pulse width less tha=
n 100%&nbsp;when the injector will stay fully open, creating a step in fuel=
 flow.&nbsp; This&nbsp;is a normal&nbsp;occurence&nbsp;during cold engine o=
peration 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.&nbsp; The=
 idea of sizing the injectors for 80% duty cycle had nothing to do with rac=
ing applications, but it applies there, too.&nbsp; The obvious problem enco=
untered when undersizing the injectors is that once 100 duty cycle is reach=
ed the flow is limited - the engine will lean out if the rpm or air flow go=
es higher.&nbsp; Not a good thing.</DIV>=0A<DIV>&nbsp;</DIV>=0A<DIV>Inceide=
ntally, there is no concern for injector relaibility or durability.&nbsp; I=
t is a simple solenoid valve and running at 100% duty cycle would theoretic=
ally make it last longer - it is the total number of open-close cycles that=
 wears the injector.&nbsp; As injectors wear the flow generally increases a=
nd most modern injectors are rated for a billion cycles with a flow increas=
e of less than 3%.&nbsp; The coil is cooled by the fuel itself, so overheat=
ing is not a concern.</DIV>=0A<DIV>&nbsp;</DIV>=0A<DIV>Gary Casey<BR></DIV>=
=0A<DIV style=3D"FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, t=
imes, serif"><BR>=0A<DIV style=3D"FONT-SIZE: 12pt; FONT-FAMILY: times new r=
oman, new york, times, serif"><FONT face=3DTahoma size=3D2>=0A<HR SIZE=3D1>=
=0A</FONT>-----Inline Message Follows-----<BR><BR>=0A<STYLE></STYLE>=0A=0A<=
DIV><FONT face=3DArial size=3D2>Ed,</FONT></DIV>=0A<DIV><FONT face=3DArial =
size=3D2>I remember reading that the 80% duty cycle rule (of thumb)was orig=
inally developed for the racing industry.</FONT></DIV>=0A<DIV><FONT face=3D=
Arial size=3D2></FONT>&nbsp;</DIV>=0A<DIV><FONT face=3DArial size=3D2>Given=
 that probably 100% will never be reached in the day to day vehicle use -&n=
bsp;the 80 % duty cycle&nbsp;most probably doesn't apply for sizing. Howeve=
r bigger injectors are standard in vehicles where the RPM limit is much hig=
her than what we may experience in our Aviation application, even if for on=
ly short periods of time.</FONT></DIV>=0A<DIV><FONT face=3DArial size=3D2><=
/FONT>&nbsp;</DIV>=0A<DIV><FONT face=3DArial size=3D2>Given that our RPM is=
 on the high end for climb and take-off, but not as high as it could be (no=
t max RPM), and cruise is for the most part only 6,000 rpm, do we really ha=
ve to held to the 80% DC rule, where perhaps 90% DC&nbsp;(for short periods=
 of time) may well give sufficient safety margin to maintain longevity of t=
he injectors.</FONT></DIV>=0A<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;=
</DIV>=0A<DIV><FONT face=3DArial size=3D2>Not that there is a major benefit=
 in having borderline injector duty cycle, as you so rightly pointed out - =
&nbsp;it's just that i</FONT><FONT face=3DArial size=3D2>t would be interes=
ting to know!</FONT></DIV>=0A<DIV><FONT face=3DArial size=3D2>George (down =
under)</FONT></DIV></DIV></DIV></div><br>=0A=0A      </body></html>
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