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Old 07-15-2017, 12:34 PM
rmartingt's Avatar
rmartingt rmartingt is offline
 
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Default 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?
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  #2  
Old 07-15-2017, 01:21 PM
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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.
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  #3  
Old 07-15-2017, 02:03 PM
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rmartingt rmartingt is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
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.
Same for the injector relays, I assume (listed on the sheet for 2A fuses)?



Quote:
Originally Posted by rv6ejguy View Post
The more diodes and relays you have, the more potential points of failure.
That's why I'm trying to go through this exercise. I'd like to keep the engine power supply as basic as possible but also minimize switchology and operational complexity.
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  #4  
Old 07-15-2017, 09:10 PM
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rv6ejguy rv6ejguy is offline
 
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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.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW- 415.8 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


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  #5  
Old 07-17-2017, 08:26 AM
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rmartingt rmartingt is offline
 
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Ok, I've put up a couple of basic block diagrams below.

Block diagrams

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...
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Last edited by rmartingt : 07-17-2017 at 08:29 AM.
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  #6  
Old 07-17-2017, 09:58 AM
tspear tspear is offline
 
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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.

Tim
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