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Date: Mon, 28 Jan 2013 10:21:07 -0500
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From: Robert R Pastusek <rpastusek@htii.com>
Subject: RE: [LML] Re: Also, does the Primer work?
Thread-Topic: [LML] Re: Also, does the Primer work?
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George,
I'll add a bit to Colyn's post:  Some (but not all) injected Continental en=
gines are equipped with a separate, solenoid-controlled primer circuit that=
 1) powers the electric boost pump and 2) opens the solenoid, dumping raw f=
uel into the intake manifold when activated. I don't have this system on my=
 TSIO-550E, but use the boost pump and mixture control to achieve a similar=
 result. Some believe that dumping the fuel into the intake manifold via th=
e primer circuit is better; I really don't know. My engine dumps fuel direc=
tly into the cylinders via the injectors when the boost pump is on and the =
mixture control is in; works well for both hot and cold starting--with diff=
erent techniques.

I've not actually run a big Continental with a failed engine fuel pump, but=
 the folks at the Continental engine class that Colyn and I attended last w=
eek advise that the engine should run at partial power when the aux pump is=
 on "low boost" and will likely need to be modulated by the mixture control=
 when on "high boost." The "should" means that this would be the expected p=
erformance; they were rather reluctant to commit to any specific operation =
not knowing how our fuel systems are installed/plumbed/configured.

Bob

George,   I don't have the whole answer but maybe one of the engine experts=
 will chime in.
I do know:
- engine pump does have a spring loaded bypass so that the boost pump can d=
rive fuel through it if it fails.  (We learned that at the Continental Fact=
ory course this week).
- High boost is a speed function on the boost pump.  I have never seen a IV=
 that didn't have one.
- I'm not sure what the primer button does (I have both) but I think it has=
 special plumbing.   Many planes don't have this.
- I need to do some research to understand the relative outputs of the engi=
ne pump vs. the boost pump and how that will affect mixture in the event of=
 an engine pump failure.   My expectation is low boost will not be enough a=
nd high boost will probably require a mixture adjustment.

On Jan 27, 2013, at 12:30 PM, George Wehrung wrote:

As a continuation to my previous post, referencing the build manual chapter=
s 14 and 23:

Chapter 23: wiring:  denotes a Single Pole Double Throw rocker switch for t=
he LOW-OFF-HIGH positions of the fuel boost pump.

Chapter 14: Firewall Forward, depicting the fuel flow does not show a separ=
ate routing for the fuel primer circuit.  So how does the primer function w=
ork?  Is it merely pushing a higher volume of fuel through the circuit?  I =
know it's necessary for hot starts to quickly circulate the cooler fuel to =
the manifold but does it somehow bypass the gascolator and the engine drive=
n fuel pump?  This would make sense if it did as some aircraft call for pus=
hing in the primer and possibly holding it in to keep the engine running in=
 the event of an engine driven fuel pump failure.

So then why do I only have a low selection of the boost pump?  Should I rep=
lace my boost pump switch with a three-position LOW-OFF-HIGH switch.  Shoul=
d I have the boost pump on LOW for takeoff, descent and landing?

Thanks again

George


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