Date: 06 Sep 95 13:25:00 PDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: BOWMANR%em.EDW@mhs.elan.af.mil (Mr.Roger Bowman)
Subject: Running FI engine empty- long reply
--PartBoundary Wed Sep 6 14:33:22 8214
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Form: Memo
Text: (85 lines follow)
On Thu, 31 Aug 1995, Harvey Chao wrote:
> Running out of gas in a fuel injected engine, gas or diesel is
> a real "NO-NO".
I've heard this before. Could someone please explain if this is in fact=20
true, and if so, why and what could be damaged? (Besides one's ego,=20
slogging down to road in the rain on foot with a gas can...).
Tobin
_______________________________________________________________
well:
Both diesel and FI use fairly specialized fuel pumps to move all that fuel =
=0D
around. Diesels, in particular, are very specialized; these pumps time a =
=0D
VERY specific amount of fuel at a VERY specific time (relative to piston =0D
position....5=BABTDC, for example) under HIGH pressure (350 PSI? maybe more=
) =0D
directly into the combustion chamber. These pumps are precision parts, ver=
y =0D
expensive.....and they are lubricated by the fuel traveling through them. =
=0D
Diesel fuel is somewhat more lubricative then gasoline, and this scheme =0D
works our pretty well.....until you run out of lube oil (fuel). At this =0D
point, all those precision parts under high stress start scraping against =
=0D
each other, and well....I can't continue.
Gasoline FI is very different. In a gasoline engine (with VERY few =0D
exceptions, and no production examples I know of...) the fuel pumps puts =0D
about 30-50 PSI (depending of pulsed or continuous FI) on the fuel, which i=
s =0D
metered by injectors located int he intake manifold....lower pressures, les=
s =0D
precision of timing.....less stress on fuel pump, cheaper fuel pumps (I kno=
w =0D
FI pumps are expensive....but diesel pumps are more so. Gasoline pumps are=
=0D
what...$100? Diesel pumps are probably $500, rebuilt, with a $100 core =0D
charge....hope yours is rebuildable, by the time you get into this arena, =
=0D
the rebuilders know what to look for, and track the pumps through thte =0D
system, and back charge....)
Gasoline is a much poorer lubricant, so I can;t believe that the fuel in =0D
gasoline FI is doing very much lubrication, the pump must be designed so =0D
that it is "self" lubricated, or just doesn't need it. So I do not think =
=0D
that running out of gas would cause the pump to fail as a result of losing =
=0D
the lubricant.
Gasoline, however, being a fluid, is probably a good conductor of =0D
heat.....and that is where the presence (or absence) of fuel in the pump =0D
could be a problem. With all ($100) worth of precision parts whirring =0D
around, they must get pretty warm, warm enough to damage the wearing =0D
surfaces due to friction without the cooling influences of the fuel flowing=
=0D
through on a fairly regular basis.
FI systems have an interlock, tying the ignition signal to the FI pump. =0D
Running out of gas means the engine stops, no ignition signal.....and the =
=0D
pump stops after a few seconds, not enough in my opinion to damage the pump=
. =0D
Of course, if you are coasting down hill, in gear, and run out of fuel, th=
e =0D
engine will continue to produce spark, the pump will continue to =0D
run.....that is a problem. Don't do that. It would be Very Bad for your =
=0D
fuel pump.
Diesels, on the other hand, I would never run out of fuel for any reason. =
=0D
Pumps are expensive, and without some means to replace the fuel up to the =
=0D
pump (Some Mercedes Diesels have a little hand pump to transfer fuel up to =
=0D
the injection pump in such a case) It occasionally will completely drain th=
e =0D
battery trying to repressurize the system after running out of =0D
fuel....again, not good.
Bottom line: for gasoline, don't make a habit of it. If you are running =
=0D
close to the edge, feel for the edge with your right foot, and as soon as i=
t =0D
becomes clear that ALL the fuel is gone (as much as a couple of mile after =
=0D
the first stumblings of fuel deprivation) push in the clutch, turn off the =
=0D
ignition, find neutral, and coast the rest of the way downhill into the gas=
=0D
station that you have been praying for (!) I don't believe any harm would =
=0D
result.
For Diesel, don't do it if you can possibly avoid it. ever. Harm (expensiv=
e =0D
harm) could result.
Bear in mind the idiot principle; surely the designer would have accounted =
=0D
for this condition (our of fuel) to present itself sometime during the =0D
equipments lifecycle. One would like to think it can happen...once or =0D
twice...and no harm would come of it. Don't, however, push your luck.
bowmanr%em@mhs.elan.af.mil
Live Smart. Think for yourself. Transform the future.
90 Westie Synchro...FI, run out of gas on purpose 1 time.
bowmanr%em@mhs.elan.af.mil
Live Smart. Think for yourself. Transform the future.
Use Proportional Font: true
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