Aux fuel pump

popaviator

popaviator

Well-Known Member
I was hoping to get an in depth answer as far as why and how the aux fuel pump works in a 172. A friend of mine was saying that in fighter jets the aux fuel pump wouldn't suffice, way different than a 172, but I'm just wondering the differences.There isn't much said about it in the 172 poh.

Thanks
 
I don't remember what type of aux pump the 172 has, but the difference between it and the fighter is recip versus turbine. The turbine engine is going to require a higher fuel pressure than the recip will. If you have a carbed engine, then you don't need pressure, you just need the pump to provide flow.
 
It's a simple vane type pump IIRC. And like the previous poster says, it's only needed if the engine driven pump fails (although you probably know this). Fuel injection is just that, the fuel is injected under pressure, rather than drawn into the engine like in a carburetor engine. The main reason why it is more efficient is because the volume of fuel into each cyl. can be more precisely controlled, rather than just going where it wants to in a carburetor. Think of it like this, in an injected engine, you have 6 cyls., 6 points of fuel introduction to each cyl. In a carburetor engine, 6 cyls., 1 point of fuel introduction to 6 cyls. And it atomizes the fuel much better as well, which promotes a more even burn, less unburned out the exhaust, so less need to be introduced, which equals fuel savings.
 
popaviator said:
I was hoping to get an in depth answer as far as why and how the aux fuel pump works in a 172. A friend of mine was saying that in fighter jets the aux fuel pump wouldn't suffice, way different than a 172, but I'm just wondering the differences.There isn't much said about it in the 172 poh.

Thanks
Click to expand...

Normally the gravity feed plus the engine driven fuel pump will do the trick, that said, if you lose the engine driven fuel pump, and it wasn't a failure in the accessory gearbox that did it (which would probably kill the mags, so you'd be a glider) then you could click on the electric pump, and it should provide enough pressure. That said, in bigger turbine motors that require a lot of fuel, the aux pump may or may not do it. I remember that if you lost the primary fuel pump in the 1900C (the 800PSI pump) you'd have an engine failure, because the aux pump only provided something like 15PSI, and wasn't enough to meet the flow demands of the big pump.

That said, there are smaller pumps in some turbine motors, which are used to prevent cavitation of the fuel. PT6 is an example that comes to mind.
 
ppragman said:
Normally the gravity feed plus the engine driven fuel pump will do the trick, that said, if you lose the engine driven fuel pump, and it wasn't a failure in the accessory gearbox that did it (which would probably kill the mags, so you'd be a glider) then you could click on the electric pump, and it should provide enough pressure. That said, in bigger turbine motors that require a lot of fuel, the aux pump may or may not do it. I remember that if you lost the primary fuel pump in the 1900C (the 800PSI pump) you'd have an engine failure, because the aux pump only provided something like 15PSI, and wasn't enough to meet the flow demands of the big pump.

That said, there are smaller pumps in some turbine motors, which are used to prevent cavitation of the fuel. PT6 is an example that comes to mind.
Click to expand...

Turbine equipment typically has the aux pump only to prevent cavitation, and a motive flow pump to move the fuel to the engine.
 
First keep in mind that the only reason a 172 has fuel pumps at all is because the fuel injection system needs a bit more pressure than gravity can give. Older 172s that are float-carbureted don't need pumps, gravity does all the work of filling the carburetor bowl. The aux fuel pump on a 172 is either a vane type pump or a centrifugal (not sure that the book says, and I've never seen the innards of one). It puts out probably somewhere in the neighborhood of 7 psi. The engine driven pump is known as a diaphragm style pump and probably only does about 3-5 PSI on a good day. So in this one, your aux pump is actually designed as a backup for the engine-driven pump. Hence the term "auxiliary pump" rather than "boost pump".

As others have said, the injector nozzles in a turbine engine need a relatively high pressure of fuel to 1) overcome the high air pressure in the burner can (in the neighborhood of 300 psi) and 2) atomize the fuel as it comes out of the nozzle. The "boost pumps" on these engines are really only designed to get fuel from the tanks to the high-pressure pump. On some engines on some fuel systems, gravity feed MAY do the job if a boost pump fails-that could be the source of the 737 legend mentioned.
 
Roger said:
First keep in mind that the only reason a 172 has fuel pumps at all is because the fuel injection system needs a bit more pressure than gravity can give. Older 172s that are float-carbureted don't need pumps, gravity does all the work of filling the carburetor bowl. The aux fuel pump on a 172 is either a vane type pump or a centrifugal (not sure that the book says, and I've never seen the innards of one). It puts out probably somewhere in the neighborhood of 7 psi. The engine driven pump is known as a diaphragm style pump and probably only does about 3-5 PSI on a good day. So in this one, your aux pump is actually designed as a backup for the engine-driven pump. Hence the term "auxiliary pump" rather than "boost pump".

As others have said, the injector nozzles in a turbine engine need a relatively high pressure of fuel to 1) overcome the high air pressure in the burner can (in the neighborhood of 300 psi) and 2) atomize the fuel as it comes out of the nozzle. The "boost pumps" on these engines are really only designed to get fuel from the tanks to the high-pressure pump. On some engines on some fuel systems, gravity feed MAY do the job if a boost pump fails-that could be the source of the 737 legend mentioned.
Click to expand...

Thanks for the answer :beer:
 
kiloalpha said:
I was told that the 737 can be gravity fed... Any truth to this statement from 737 drivers?
Click to expand...


Just a WAG, but I bet the 73 can gravity feed becuase the engine has three pumps. An engine driven mechanical, and an electric pump to feed that pump. And then on top of that, a boost pump for that electric pump. There is probably enough flow to supply the machanical pump at low power settings. Now ask for some work from the engine, and the gravity can't keep up, you get an air buble and loose prime, and the mechanical pump never recovers?

Or I could be way wrong? When I was working on diesel engines, in some of the older Cumming's engines, if you lost prime, you either had to crack the lines open and bleed the injectors while cranking, or you had to use starting fluid to generate enough boost from the engine driven pump to re-prime the lines(about 1000rpm).
 
You must log in or register to reply here.
Back
Top