Power switching for SDS fuel pumps and ECUs

SabreFlyr · #1 01-23-2019, 06:57 AM

I'm planning a dual SDS ignition and injection system for my RV-14's IO-390. While not absolutely set on this, I am planning to use the Vertical Power Primary Power System and VP-X Pro in a dual-alternator, "single" battery system. I would also use the SDS backup battery that Ross is planning for the EM-6 and a TCW 6Ah IBBS and a G5 with internal battery for avionics backup.

For further redundancy, I am considering an emergency bus contactor direct from the main battery that would power one fuel pump and one ECU (with the others being powered as necessary from the SDS battery). I haven't gone through various failure scenarios yet and I'm sure that will impact this decision, but I'm wondering just how I will switch the power to the pumps and ECUs. Under normal operation, I believe that both pumps and ECUs would be powered by the main bus through the VP-X. But, with two fuel pumps and two ECUs each having two possible power sources, I'm wondering about the possible proliferation of switches. Besides selecting a power source for each, there's the matter of selecting one fuel pump or the other and, possibly, removing power from one ECU (is this ever done? I haven't studied these systems enough to answer that). I envision, first of all, two switches selecting main or backup power for each side of the engine system. But, each of those has to control both a fuel pump and an ECU (DPDT won't cut it, right?). Then there would be two 3-position selector switches: fuel pump main/both/stby and ECU1/both/ECU2.

If there's a system that will drive me mad in this build, it will be the electrical system!

airguy · #2 01-23-2019, 08:19 AM

Having a direct battery-to-fuel pump (or EFI) wire/switch would assume the event of a major electrical foul-up that would require hitting the master switch to shut down the entire electrical system - like smoke in the cockpit from burning wiring, that sort of thing. In that case the situation becomes much simpler - everything goes dark that does not have its own internal battery like the G5. Then you can have a single wire to a single switch (preferably guarded switch) to the fuel pump. If you use a diode through your NORMAL fuel pump voltage supply, then the current from the emergency supply cannot back-feed through the normal supply wire (from their common connection at the fuel pump power feed) back to the bus leading to the original shorted component, so you'll be fine even if you forget to turn off the NORMAL fuel pump power switch before activating the emergency.

Alternatively, you could use a 3-position switch for the fuel pump, for NORMAL/off/EMERGENCY where the normal wiring is interrupted anyway, and you're just switching the source supply - but that puts a single point of failure (the switch) that takes the fuel pump out beyond any hope of use.

rocketman1988 · #3 01-23-2019, 08:32 AM

Want a headache?

After reading this thread you will have one...this has been discussed ad nauseam, and one thing is for sure, everyone has an OPINION...

http://www.vansairforce.com/communit...m+architecture

SabreFlyr · #4 01-23-2019, 09:42 AM

Quote:

Originally Posted by airguy

If you use a diode through your NORMAL fuel pump voltage supply, then the current from the emergency supply cannot back-feed through the normal supply wire (from their common connection at the fuel pump power feed) back to the bus leading to the original shorted component, so you'll be fine even if you forget to turn off the NORMAL fuel pump power switch before activating the emergency.

Alternatively, you could use a 3-position switch for the fuel pump, for NORMAL/off/EMERGENCY where the normal wiring is interrupted anyway, and you're just switching the source supply - but that puts a single point of failure (the switch) that takes the fuel pump out beyond any hope of use.

I guess I was uncertain about reliability of switches vs diodes. So I guess that I would have four diodes (one for each fuel pump and ECU) and two guarded switches (one to select the SDS batt for pump 1 and ECU 1 and one to select the emer bus for pump 2 and ECU 2)?

Quote:

Originally Posted by rocketman1988

Want a headache?

After reading this thread you will have one...this has been discussed ad nauseam, and one thing is for sure, everyone has an OPINION...

http://www.vansairforce.com/communit...m+architecture

Been in there and I think I'm still trying to get over that headache! I guess this thread is just my attempt to get it sorted out in my own mind.

rocketman1988 · #5 01-23-2019, 09:57 AM

It will definitely give you a headache.

Realize that there is NO perfect solution; it is impossible to design a mechanical systems such that there are no possible faults. The best you can do is to try and minimize the impact that any single failure can have on the entire system.

It is really about risk mitigation, and that varies with each individual as to how much risk they are comfortable with...