Power system architecture for EFI
So I'm getting closer to being able to start my systems installation and hoping to make some progress on my electrical diagram while I'm laid up next week. I've been looking over some thoughts on electrical systems and trying to figure out how best to go about providing reliable power to an SDS EFI system on an IFR-equipped aircraft.
I've done all my planning around the idea of a dual-battery, dual-alternator system--effectively, the Nuckolls Z-14, albeit with minor changes to the cross-tie wiring and switching (I don't want my start switch and crosstie switch combined), and adding some form of ground power input.
Under such a system, I'd wire half of the EFI directly to each of the battery buses, with a fuse and a switch for each component. However, this leaves me with at least 10 switches just for the engine components, as I'd have to have some way of turning them all off. Unless the coils, injectors, and injector relay don't draw power at all when they aren't firing? This also means I lose power to half the injectors if a whole bus goes down, and I don't think that would be a good thing for the engine.
Now, I also considered taking the same basic system, but instead of splitting items among two buses, I'd have them feed a single "engine bus" through a diode arrangement straight off the batteries. This reduces problems of having to deal with "which bus is powering things" but adds parasitic drain from the diodes and still keeps all the switches.
A variation on that would be to put a switch in line between each battery and the engine bus and keep the diodes; this would eliminate a lot of the individual component switches and leave me with just the fuel pumps. Everything else would be on whenever engine power was on.
I will try to get some block diagrams up soon to illustrate these ideas, but for now, does anyone have feedback so far?
The coils and injectors don't draw any power unless the ECU(s) are running and the engine is turning over.
You'll get plenty of opinions on the best way to wire this setup.
The more diodes and relays you have, the more potential points of failure.
The injector relays only switch the injector paths between ECUs when using dual ECUs. They are necessary in this instance because we need primary isolation as well as backup connection for this strategy. Contacts are normally closed with no power so they draw nothing in the primary position. This is a normal design strategy in most critical systems, for us at least.
I'm a big fan of ATO fuses rather than breakers for most parts of the EFI. Lighter, cheaper, more reliable and take up less space. We pick a fuse rating of around 3 times the nominal current draw of the device. If that fuse blows, it's because there is probably a short in that wire and you wouldn't want to reset a breaker anyway.
Fuses are generally there to protect the wiring, not the device.
Ok, I've put up a couple of basic block diagrams below.
I've received some feedback from others; haven't yet had time to process those, but thank you!
I'm aiming to keep this alive as my plan evolves; the more eyes, the better...
One design I really liked was the following:
Three buss systems.
Primary: This is your 60A system, includes MFD, landing lights....
Essential: This is your 20A system, Nac/Com 1, PFD....
Engine: Ran the Plasma III systems (dual).
Three batteries; Two large batteries for the primary and essential bus. A third small NiCad (I think) battery for the engine bus.
Starter connected to primary bus.
Primary connected to Essential via one way diode.
Primary connected to Engine via one way diode.
Essential connected to Engine via one way diode.
Engine bus will maintain power, until you lose both alternators and all three batteries. Downside, you lose Essential bus Alt in flight. You do not find out until you test on ground at startup or shutdown by turning off Primary bus first.
So I've done lots of thinking in the past several months and have come up with another idea.
This starts off as a Z-14 (dual alt, dual batt) with the following major addition/change:
A separate engine bus is fed off the main and aux buses through diodes. If either bus is up, the engine has power. I'd set the alternator on the main bus about 0.5V higher to give it "priority" in feeding the engine bus.
For additional redundancy, I'd have an "emergency power" switch that directly connects the engine bus to the batteries through diodes (or, two switches that individually connect each battery to the engine bus). This way, I can kill the entire system and then feed just what I need to run the engine. In such a scenario I'd rely on the EFIS backup batteries for instrumentation as they're likely to last about as long as the ship's batteries will power the engine.
As before, I'd be using fuse blocks for power distribution.
The only other notable change I could see would be either separating the crossfeed and start switches, or using a crossfeed switch and a separate "start battery selector" rotary switch. But that might be getting too fancy.
Ground power input will just be battery charger connections under the cowl.
More comments invited. Thanks!
Edit: link should be viewable now
I actually liked your Z14 Mod 2 setup. I'm planning for SDS and using an Engine bus with a back up alternator and two batteries and VPX. Shoot me an email and I'll send you the power schematic that I'm working on.
On your mod 2, I'd recommend just adding a switch for the second battery. I have a diode keeping my second battery charged, but it remains isolated until I throw the "oh ****" switch.
Honestly, it depends on what your failure modes are and what you want your flow. Personally, I'm doing an avionics master through the VPX to quickly load shed for alt/s failure. For a main batt failure, I'll kill the master and flip on the "oh ****" switch which will power back up the engine buss as well as the IFD440, PFD (with ems). The G5 has it's own battery and will remain on even if everything else craps out.
At some point, you'll arrive at situations you just can't have a redundancy for and either accept those risks or stop flying (for example, electrical fire, engine failure).
A good design has no "Oh ****" switch to keep the engine running. Engine electronics need dual power or auto switching.
The airplane is at 200 feet, the departure end just went under the wing, and the engine quits. Right then, nobody has enough spare brainpower to diagnose an electrical problem.
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