Tip: Electric fuel pump setup

[frankh]

Originally Posted by David-aviator

For sure it will work mechanically. From the electric point of view, are you going with 2 batteries? Not sure I understand "....the e-bus which is schottky-diode isolated from all power sources...."


Going electric has hits appeal but I will tell you it is not bullet proof. I had one engine shut down in flight with the Subaru due to a tripped poly fuse on EXPBUS and a friend had the same thing happen a couple weeks ago. In both cases the engine restarted by going to backup battery which uses different circuits but it sure gets your attention. I am presently looking forward to flying with an engine that will keep running if the electric system goes TU and am about 3 days away from firing the beast up.

Ok a couple of points

1) Nothing is bullet proof no matter what it is. And as I pioneered this system I of course am biased..

2) This sytem uses two pumps running at the same time for take off an landing. If I remember correctly from the Egg system it used a duty/standby pump..I.e one pump running and if it quit a second one would start. I personally don't think this is a desirable solution because that that split second delay in a critical moment (like near the ground) can lead to sucking a lot of seat cushion!

3) You never rely on a single point of failure with this sytem...well you could argue the battery buss is a single point I guess but hard to see what the failure mode is. My system uses a switch for each pump and a seperate fuse of course and two alternators,..mainly because it flys in IMC.

4) Are the risks of vapour lock more than total electrical failure?..Good qustion, personally when running autofuel I think they are.

When I really looked at this I came to the conclusion that running a mechanical pump with two wingroot pumps added extra failure modes and complexity...and with high vapour pressure fuel was undesirable anyway.

remember the electric only pump solution is only as "risky" as a Lycoming running two electronic ignitions...if your electrical system goes TU you'll come down just as fast with two Ligthspeeds as you will with 2 electric fuel pumps. There are plenty of airplanes flying about this way and nobody seems to have a problem with it.

Frank

[David-aviator]

Originally Posted by frankh

Ok a couple of points

1) Nothing is bullet proof no matter what it is. And as I pioneered this system I of course am biased..

2) This sytem uses two pumps running at the same time for take off an landing. If I remember correctly from the Egg system it used a duty/standby pump..I.e one pump running and if it quit a second one would start. I personally don't think this is a desirable solution because that that split second delay in a critical moment (like near the ground) can lead to sucking a lot of seat cushion!

3) You never rely on a single point of failure with this sytem...well you could argue the battery buss is a single point I guess but hard to see what the failure mode is. My system uses a switch for each pump and a seperate fuse of course and two alternators,..mainly because it flys in IMC.

4) Are the risks of vapour lock more than total electrical failure?..Good qustion, personally when running autofuel I think they are.

When I really looked at this I came to the conclusion that running a mechanical pump with two wingroot pumps added extra failure modes and complexity...and with high vapour pressure fuel was undesirable anyway.

remember the electric only pump solution is only as "risky" as a Lycoming running two electronic ignitions...if your electrical system goes TU you'll come down just as fast with two Ligthspeeds as you will with 2 electric fuel pumps. There are plenty of airplanes flying about this way and nobody seems to have a problem with it.

Frank

The vapor lock issue is the #1 reason I prefer electric pumps with one running continuously. That area from the firewall to the mechanical pump is most vulnerable. Pressurized fuel coming through the firewall really secures that area. The early Subaru install had the pumps on the forward side of the firewall out of concern for a pressure line failure in the cabin but that risk soon paled after a couple guys had vapor locks where the fuel came through the firewall. Pumps are now in the cabin. I still don't like pressure lines in the cabin but it is IMHO less of a risk than vapor lock, and as you say, especially with mogas.

You are absolutely correct about the plumbing complexity with electric and mechanical pumps in series. The mechanical pump can not suck fuel through the electric pump so a by-pass with a check valve has to be in place.

I am not knocking your creation....just coming off the totally electric operation and currently feeling good about it.

[frankh]

Originally Posted by David-aviator

The vapor lock issue is the #1 reason I prefer electric pumps with one running continuously. That area from the firewall to the mechanical pump is most vulnerable. Pressurized fuel coming through the firewall really secures that area. The early Subaru install had the pumps on the forward side of the firewall out of concern for a pressure line failure in the cabin but that risk soon paled after a couple guys had vapor locks where the fuel came through the firewall. Pumps are now in the cabin. I still don't like pressure lines in the cabin but it is IMHO less of a risk than vapor lock, and as you say, especially with mogas.

You are absolutely correct about the plumbing complexity with electric and mechanical pumps in series. The mechanical pump can not suck fuel through the electric pump so a by-pass with a check valve has to be in place.

I am not knocking your creation....just coming off the totally electric operation and currently feeling good about it.

Nope totally understood and 900 hours of faultless operation is testament to the fact it works...I can understand your reluctance to go all electric again...but..

Jan Eggenfelner eventually threw me off his feedback forum when I was trying to point out design flaws with his system..

One of those reservations was this duty/standby pump setup...I.e I think it was a flawed system. (the other was the pumps north of the firewall)

back then I had several years of success with my carbed zodiac and this system. I suggested a redesign of the sube system that looks very much like mine does now..Jan dismissed it immediately..mainly i think because he did not understand it..Supposition on my part. But this is where the design for the FI'd setup came from..yes I was considering an Egg engine at the tim.

But it was a wingroot mounted pump with the pressure relief at the pump..The challenge with the soob was this need for the continual return from the firewall..Well I still don't think that is necessary and the Lycoming seems to have proven that to be the case...Would also avoid the need for the 6 port Andair selector valve.

Anyway having two pumps that can run continuously is a far better solution I think..as I guess you discovered..Not that a pump failing at altitude won't get your attention, but at least its not dangerous.

Frank

[rocketbob]

Originally Posted by David-aviator

Not sure I understand "....the e-bus which is schottky-diode isolated from all power sources...."

The system is two Odyssey PC-680's, two master contactors, two battery master switches, one alternator. Then onto the main bus fuseblock, a switched avionics fuseblock, and an essential-bus fuseblock. The e-bus, which only has fuel pumps and ignitions, is fed off of the main bus via one side of a Ixys DSS2x121-0045B schottky rectifier. The e-bus alternate feed switch is an SPDT switch with both sides of the switch going to each battery, with another Ixys rectifier (which has two rectifiers in one package) in between the switch wires going to either of the two batteries.

So if I had one battery dead and the master off, the e-bus rectifier will isolate it, and prevent back-feeding it.

[grantcarruthers]

Thanks for all the effort it took to share this. I'm very pro-auto fuel and leaning toward all electric so some day way too far in the future I'll be looking at a system like this. It seems very nicely done and thoroughly what if'ed within reasonable failure modes.

Question, hydraulically speaking, how much safer is it from a VL perspective to have the pumps in the root vs the tunnel forward of the stick?? How much higher would the pump sit, ie how much higher a hill would the fuel have to be sucked up to reach the pump if located in the tunnel. The concern being of course VL pre-pump.

Other reasons for wing root vs tunnel?

Thanks much

[frankh]

Well not THAT much difference to be honest..The line sizes are relatively large and while there are a few bends and elbows hydraulically speaking the losses would be small. The other advantage to the tunnel is that you could retain the selector valve and suck from either tank.

However...(There is always at least one of those) if you were slow in changing from one tank to the other you could in theory vapour lock one or both of the pumps...

Also the because whenever you suck on fuel you are dependant on not sucking so much that you reduce the pressure on the fuel to below the vapour pressure of the fuel, so if something whacky like you trod on a fuel line for example..you could still end up in a VL situation.

The engineering purist in me says, "If I'm going to do it right..Lets do it right" I.e place the pumps as close to the source as possible.

It also removes the selctor valve which is what I prefer.

But you could certainly design a system where both pumps were more conveniently located...Albeit you now have to run the return lines to the tanks if the engine is fuel injected.

Frank

[Tom Hunter]

I have FI using a standard Bendix system on a 200 HP Lycoming. No return line is needed.

I have both a mechanical pump on the engine and NO pump on each wing.

What I have found in 525 hours is that the engine pump will PULL the fuel from each wing. I only use the electric boost pump in take off and landing. Unlike your system, I only have one electric pump and my feed is either left or right tank.

I guess what this all proves is there are more than one ways to skin a cat. My goal was to achieve the simplest system as possible. So there are no cross feeding, or check valves, etc.

At first I thought it wouldn't work without the electric pump running, but I soon found on ground tests, that wasn't the case and with only 5 gals of fuel in the left wing, it (the mechanical pump) pulled the fuel fine and the fuel pressure was 26 psi sitting and running on the ramp.

The only problem I have had so far, is the electric pump (which I don't use all that much..go figure) died after only 400 hours tach time. And it was one of those expensive pumps to boot!

Tom Hunter

[airguy]

Your design is the standard setup, it will work just fine, and does every day for thousands of aircraft.

The problem with that setup, that we are trying to avoid, is the vapor lock exposure caused by the pump PULLING the fuel from the tank up to the pump. This produces an area of low pressure on the intake side of the mechanical pump, which also happens to be a high-temperature area due to heatsoak from the engine block. These two factors both combine to push the fuel closer to vaporizing and reduce the margin of safety. To eliminate that issue, you need to either remove the heat source or increase the pressure.

Putting the fuel pumps in the wingroot instead of the engine block accomplishes BOTH and results in the same steady fuel supply to the engine. It also eliminates the need for a fuel valve to switch tanks - some say this is a good thing, some say bad - personal choice there.

Turning on your fuel boost pump will accomplish the same thing as using a wingroot pump - deliver pressurized fuel into the FWF to prevent vaporlock, in addition to covering against a mechanical fuel pump failure at low altitude. Trouble is, as long as you've got the heatsoak exposure from the mechanical pump, your at some non-zero risk of vapor lock. Taking that pump out of the equation just adds more safety margin and brings you back to the same level of complexity as the standard system. Both methods use two fuel pumps, if one fails the other one saves you.

[majuro15]

Frank, this is a five year old thread that is still providing excellent information! Thanks for all the time and patience in describing the system. I'll be following your lead for my -10 in the coming years to help mitigate cost, LL worries, and help out the tree huggers!

For the return line, where does it come off of the FWF area? I'm imagining just before the servo for the FI system? Then what does it look like, any need for a check valve or just run another line back to a tank? Should the exit in the tank be low or hi or does it matter?

[airguy]

It depends on what type of pump you have, but the majority of electric fuel pumps actually use the fuel itself to cool the sealed electric motor as it flows by. Since the pumps move more fuel than the engine needs, some of it goes forward to the engine and the rest is bypassed via a return line. You don't want to take that return line and just route it back into the suction of the pump, since you'll end up simply recirculating the majority of the fuel during low-power operation and it will get hot as it makes repeated loops through the pump cooling the electric motor each time - that's exactly the problem we are trying to get away from. A return line back to the tank for the bypass flow solves this problem, allowing the return fuel to mix with the fuel in the tank. On my airplane I took that return line and fed it into the second bay in the tank, so that I wouldn't have any problems with splashing or foaming of the fuel in the first bay where the fuel pickup is.