levashov
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Joined: Sun Aug 22, 2010 10:33 am

Solenoids. What Are They For In Fuel Sys. Of B76?

Tue Jun 14, 2011 11:58 pm

Hello!

I've just started my multy commercial training with Beechcraft 76 Duchess. While im going thru the fuel system i found on the system schematic that in the priming pipe between auxiliary pump and primer there is an "primer solenoid", as well as in pipe going to the heater is "heater solenoid".

In the system description of engine priming: The battery master and the aux fuel pump (l or R) switches must be turned on prior to activating the fuel priming system. The push to prime switch may be actuated in either the both or start position. Pushing in on the magneto/start switch activates a solenoid, located behind the firewall in each nacelle, and supplies fuel directly into the no. 1 2 and 4 cylinders for engine priming.

The questions are:

What are those solenoids for? My instructor said that the main purpose of solenoids is to drive the fuel from the tank into the cylinders (or to heater in cabin heating system). But im quite unsure with his answer, because it seems to me not logical. My guess is that that solenoids is like a shutoff valve. They are not driving fuel itself (it must be done by pump), but opening valve in pipe so fuel can go to cylinders when priming activated, or heater system switched on.

Could you, guys, who familiar with what are solenoids, what are they for in fuel system for, explain me this. And if you have pictures or schematics of how that kind of solenoids looks like to share a link.

Thank you very much!
 
wingscrubber
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RE: Solenoids. What Are They For In Fuel Sys. Of B76?

Wed Jun 15, 2011 3:15 am

I'm not intimately familiar with the Duchess fuel system, but you are correct in presuming that 'solenoid' in fuel or hydraulic systems usually infers 'solenoid actuated shut-off valve'.

I'm only a PPL, but 'priming' the engine prior to start on the 172 is a matter of switching on the fuel pump to pressurize the system, then opening the mixture to allow some fuel to flow before closing it again, and then turning the pump back off.

On the Duchess, seems as though you activate the pump to pressurize the system, and then either with the priming switch or mag switch, the solenoid opens and allows fuel to flow into the intake manifold of the engine before you start?

Also, if the engines are fuel injected, or maybe if fuel is not gravity fed, then a fuel pump must run continuously to deliver fuel to the engine. I think sometimes this might be mechanically driven on old airplanes...

http://en.wikipedia.org/wiki/Solenoid_valve
http://ma3naido.blogspot.com/2007/10...raft-fuel-systems-fuel-primer.html

Hope that helps...?
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levashov
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RE: Solenoids. What Are They For In Fuel Sys. Of B76?

Wed Jun 15, 2011 3:38 am

Oh yes, thank you very much. That what i was looking for!
 
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zeke
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RE: Solenoids. What Are They For In Fuel Sys. Of B76?

Wed Jun 15, 2011 6:39 am

Quoting Wingscrubber (Reply 1):

On the Duchess, seems as though you activate the pump to pressurize the system, and then either with the priming switch or mag switch, the solenoid opens and allows fuel to flow into the intake manifold of the engine before you start?

I think on the dutches the prime line is downstream of the aux fuel pump. Pushing the mag switches in opens the solenoid valve with fuel under pressure from the aux pump directly to the primer nipples into 3 cylinders.
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Fly2HMO
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RE: Solenoids. What Are They For In Fuel Sys. Of B76?

Thu Jun 16, 2011 2:20 am

You can think of solenoids as large electrical relays, which usually provide some sort of mechanical movement.

Solenoids are often used to actuate valves. They are on/off normally.

The Piper Seminole has a similar system. The priming system uses solenoids as well IIRC

Quoting levashov (Reply 2):
Oh yes, thank you very much. That what i was looking for!

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