Well;
the worry is this. Safe wiring practice requires being able to protect
the wire, especially if it is in a bundle or close proximity with other wires,
against any failure which would overload the wire. Running the numbers
says that the maximum safe current for a 16 awg wire is 15 amps. If I
interpret you correctly; it is possible for my coil power lead to have to carry
27 amps (for 3 coils) for a short time, which would trip a breaker, or blow a
fuse. Under these conditions one can not properly protect the wire
against a potential short – unless a much larger wire is used. My
wires run in a wiring duct, which isn’t exactly a bundle, but the
analysis says for 27 amps in a bundle you need about a #10 wire.
Further;
I went to great lengths to maintain the redundancy that you also strived for in
the ECU with the dual controllers and the isolation switches. I have the
engine critical bus connected to the dual battery system, and isolated by 40
amp diodes. I think you’re suggesting that it’s possible for
the coils to momentarily draw 54 amps (6 x 9), plus pump and injectors; maybe
up to nearly 70 amps. Ain’t
gonna work.
So
what am I misunderstanding here, or how do we get around this? Under what
temporary condition can this high current exist? Is there an operational
sequence to avoid this possible condition?
Thanks,
Al
Al,
not Tracy, but I think what Tracy is saying that in the milliseconds or
so it takes the EC2 to calculate the proper coil dwell time it is possible for
a coil(s) to draw up to 9 amps (or perhaps even 27 amps in your
case). The instantaneous
power in that case would be 27*12 = 324 watts IF that
current drain remained for as much as 1 second (actually probably less because
when cranking you probably would not have more than 10 volts). However, I
believe the duration of that surge is much shorter duration. IF
I am correct then I suspect your 16 gauge wire is more than adequate because of
the very short time interval in which this happens (I'm guessing that situation
does not last for even 100 milliseconds, Tracy??).
If
that is the case then the power (which is what we are really concerned
about) per second would be 1/10*27*12 = 32.4 watts average power consumed
during that time. The very conservative Power transmission (bundled) for
16 gauge is 3.7 amps x 12 volts = 44.4 watts. Therefore, IF the surge is l00
milliseconds duration or less then it would appear you wiring is more than
adquate. Now if the surge condition can actually last for as long a
second then its a different story.
Ed
Ed;
You’ve got that right, but I guess
I wasn’t really clear on what my concern is. I’m not necessarily
worried about the #16 wire being seriously overheated by the draws of the
coils. I’m worried about being able to safely protect that circuit
against a different failure – like a short – and not have a
condition that will continue to blow the appropriately sized fuse, or trip a
breaker.
On the broader view, I think a potential
spike of close to 70 amps on my engine critical bus is not acceptable; so I
think we need to hear from Tracy before carrying this any further, and see that we are not off the
track here. And I don’t want to be redesigning my electrical system
at this point.
In my particular case; the engine
critical leads are protected by fuses; for the reasons John paraphrased from
Bob Nuchols; but also for maximum reliability. The power to my breaker
buses goes through the master switch solenoid. Go directly from the battery(ies)
to the engine critical bus and eliminate a couple of single point failure modes.
My search for data led me to conclude that MTBF for properly sized fuses is
greater than for CBs. I wouldn’t try to convince anyone that one way
is better than another; just that my logic lead me to this approach. I
would assert; however, that there should not be a circuit in my airplane;
unless it is one wire all by itself that could not damage anything else if it
melted, that is not suitably protected against a possible short circuit. Not
having protection in the engine critical circuits, or having one breaker for
all the circuits doesn’t might that criteria. So there is a fuse
for each circuit; pump, leading ign, trailing ign, primary injectors and
secondary injectors.
Al