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nuckolls.bob(at)aeroelect
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 Posted: Fri Feb 14, 2020 12:56 pm Post subject: Always hot power for fuel injectors and ignition coils |
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| Quote: | I'm currently planning the electrical architecture for my RV-8 with EFII and I've been following the recent discussions of fuel injector power with great interest. Does anyone connect the positive side of the injectors and coils to the always-hot battery bus and leave them powered all the time, or are they generally always switched?
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What manufacture of EFII system and what
are the recommendations of the manufacturer?
| Quote: | I'm leaning towards a dual-feed bus with diode-protected feeds from the main contactor and a relay on an aux battery. I'll post a schematic when I get done drawing it.
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What powers the rest of the airplane?
What kind of engine. What architecture?
Bob . . .
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cofford
Joined: 01 Jan 2018
Posts: 9
Location: Puget Sound, WA
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| Posted: Sat Feb 15, 2020 3:11 pm Post subject: Re: Always hot power for fuel injectors and ignition coils |
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The system is FlyEFII System32. The manufacturer recommends using their own bus manager system, which provides a diode-protected, arguably dual-feed switched bus (the terminal on the bus manager is dual-fed, but the engine components are all fed by a single wire). I'm sure the manufacturer's recommendations are fine in most cases. That being said, I'm an engineer in the manned spaceflight industry, and I don't like black boxes and easily avoidable single-point failures.
The aircraft currently has no electrical system as I'm about to begin installing one. I've been considering both Z12 and Z19 architectures without an endurance bus.
Thanks,
Casey
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cofford
Joined: 01 Jan 2018
Posts: 9
Location: Puget Sound, WA
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| Posted: Sat Feb 15, 2020 6:53 pm Post subject: Re: Always hot power for fuel injectors and ignition coils |
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I had a chance to draw up some schematics. This is the dual battery system I've been kicking around for a while. This would be a system based on Z19.
https://drive.google.com/open?id=1vKln4Zl0CC24Si8QUW78MCfymVRQejvZ
As I've given this more thought, a Z12 architecture with one battery and two alternators makes quite a bit of sense. In that case, the schematic looks like this:
https://drive.google.com/open?id=1Q32syGUfgtm7dR-bksQex80r2olKD_Al
Goals here were:
1. Any single component failure does not result in loss of power to the engine bus.
2. Any single failure does not require pilot action to keep power to the engine bus.
3. Any redundancy must have the ability to be checked during preflight.
Comments are most welcome.
Thanks!!!
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Sat Feb 15, 2020 7:32 pm Post subject: Re: Always hot power for fuel injectors and ignition coils |
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On the single battery system:
If the EFIS has two voltmeter inputs, connect one to the main bus and one to
the engine bus. Switching the aux power relay on and off while comparing
the two voltmeters will confirm that the aux power relay is working.
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Joe Gores |
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nuckollsr
Joined: 24 Mar 2009
Posts: 95
Location: Medicine Lodge, KS
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| Posted: Sun Feb 16, 2020 12:04 pm Post subject: Re: Always hot power for fuel injectors and ignition coils |
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A manufacturer of ANY accessory should be cautious about
venturing outside their product into issues of aircraft architecture
and systems integration.
As we've studied over the years, off-hand recommendations
or even citations of, "This is what I fly in my airplane" are
seldom examples of the lowest parts count, lowest cost
of ownership and minimized risk across the full spectrum
of potential installations. Further, when offered by individuals
of limited experience, the potential for designed in risk goes up.
For decades my teachers said, "draw a box around the
gizmo for which you are writing a specification. Describe all
inputs, outputs and expected performance for what goes
on INSIDE your box . . . then quit.
In the case before us, the 'box' is your EFIS system. It's
a perfectly valid request that one or more inputs require,
"Failure tolerant source of power from x.x to xx.x volts
DC at x.x amps".
The selection of architecture and operating philosophy are
a separate specification embracing design goals that
are seldom known by the appliance manufacturer. Your
design goal is to minimize risk to the WHOLE system which
starts with you, includes a complex machine with lots of
potential for failure all topped off the the environment
through which you are flying.
No matter how many multi-feed busses, standby batteries,
extra alternators you design into your project, demonstrated
realities tell us that the greatest single cause for engine
failure is running out of gas . . . and the biggest risk
for hitting the mountain does not arise from a blown
fuse on the gps.
Engine not withstanding, there are lots of electro- whizzies
that we would like to keep lit up. Keeping the engine running in
case everything else has gone dark only improves the probability
that you'll hit the rocks under power. I suggest that adding an
'engine bus' is redundant and unnecessary for Z12 with one
battery (well maintained . . . of course) and two engine driven
power sources .
Keep in mind that a 'battery bus' is an always hot
distribution device located adjacent to the battery and
fitted with protective devices consistent with crash worthiness
design goals. A bus located elsewhere powered through
any form of switch gear and 'protected' with distribution-
rated limiters or fusible links is not a battery bus . . . it's
just another bus with its own purpose/nomenclature.
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cofford
Joined: 01 Jan 2018
Posts: 9
Location: Puget Sound, WA
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| Posted: Sun Feb 16, 2020 7:04 pm Post subject: Re: Always hot power for fuel injectors and ignition coils |
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Hi Bob,
Thanks for your thoughts. I plan on mounting my battery on the firewall, and the engine fuse block on the cabin side, near the battery and pass-through. The relay would be mounted to the battery box. I was not planning on adding any circuit protection devices to the engine bus relay feed.
Here's the whole architecture as of today:
https://drive.google.com/file/d/1asF6lxJcRF4q06fiD3ytd3ktgnxzHuLJ/view?usp=sharing
Thanks,
Casey
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