Power system architecture for EFI

[RandyAB]

Originally Posted by Carl Froehlich

I don?t know what the deal is on having two power feeds to a ?dual ignition bus?. Why not have one ignition on each battery - no ignition bus or diodes?

Carl

I'm won't argue against that as an option, however, I'd still like to understand exactly how this particular builder could have prevented the problem that ultimately killed him with the bus that he had set up. Just learning the pitfalls.....

[AdamB]

Originally Posted by RandyAB

I'm won't argue against that as an option, however, I'd still like to understand exactly how this particular builder could have prevented the problem that ultimately killed him with the bus that he had set up. Just learning the pitfalls.....

Hey Randy,

From what I recall, is that it has to do with having fuses upstream of breakers.
Breakers blow much slower than fuses. A fault on one of the ignitions caused both upstream fuses to blow quickly and way before the breakers that were attempting to protect the wires for the individual ignitions could take effect. Both fuses blowing caused both ignitions to lose power.

The basic premise is to only have one protection device in any given circuit and don't serialize them especially don't serialize two different types of protection devices that have different blow characteristics.

Hopefully that explained it well enough.

[johnbright]

Originally Posted by RandyAB

For learning purposes and to clarify the teaching moment, would this unfortunate incident have been mitigated by having a much larger breaker and appropriately sized diode fed wire running between the battery and the bus?

The answer is in the videos at the link in post #279. http://www.aeroelectric.com/Referenc...eb2008_LA-IVp/

[KatanaPilot]

Originally Posted by Carl Froehlich

I don?t know what the deal is on having two power feeds to a ?dual ignition bus?. Why not have one ignition on each battery - no ignition bus or diodes?

Carl

Exactly what I'm doing on my SDS equipped RV-10. Dual bus, dual alternator, dual battery. Yes, there is a cross-tie relay, but it will only be used for start. One bus is dedicated to a single ECU, fuel pump and ignition.

Maybe a bit overkill and obviously adds weight, but for my first electrically dependent airplane, I'm trying to be conservative (at least as far as ensuring electron flow is concerned). I know some will say conservative means magnetos and mechanical fuel injection.

[RandyAB]

Originally Posted by KatanaPilot

Exactly what I'm doing on my SDS equipped RV-10. Dual bus, dual alternator, dual battery. Yes, there is a cross-tie relay, but it will only be used for start. One bus is dedicated to a single ECU, fuel pump and ignition.

Maybe a bit overkill and obviously adds weight, but for my first electrically dependent airplane, I'm trying to be conservative (at least as far as ensuring electron flow is concerned). I know some will say conservative means magnetos and mechanical fuel injection.

I'm going to go with what Ross has suggested for system architecture. If each circuit off of the bus bar is fuse or c/b protected, and the bus bar has two sources of potential power input, why is that better other than having two physical bus bars? I don't think bus bars themselves are prone to failure. I think this a rehashing of the discussion much earlier in the thread, at least how I understand it.

Edit: After reviewing the Nuckolls' videos referenced in post 281 I'm going to reconsider what I just said.....

[RandyAB]

Originally Posted by AdamB

Hey Randy,

From what I recall, is that it has to do with having fuses upstream of breakers.
Breakers blow much slower than fuses. A fault on one of the ignitions caused both upstream fuses to blow quickly and way before the breakers that were attempting to protect the wires for the individual ignitions could take effect. Both fuses blowing caused both ignitions to lose power.

The basic premise is to only have one protection device in any given circuit and don't serialize them especially don't serialize two different types of protection devices that have different blow characteristics.

Hopefully that explained it well enough.

Thanks for that Adam. So I understand correctly, the main problem is that both the injectors breaker and the upstream fuse were rated the same and for the reason that you outlined, the fuses from both battery sources blew first when the injector took a dump. It seems strange to me that the wiring and fuse from the battery was the same amperage of the injector which tripped. That wire should be powering the whole bus, not just one component. That implies that it should have been rated significantly higher If that had been a 30 amp breaker with appropriately sized wire, this shouldn't have happened. That is the way I am understanding it.

Edit: Reviewing the videos referenced in #281 answers this well.

[RandyAB]

Originally Posted by johnbright

The answer is in the videos at the link in post #281.

Thanks. Very informative.

[rv6ejguy]

I've gone back a ways in this discussion to the schematic in the links posted by Dan of the accident aircraft.

The 10A fuse recommended in the schematic for the 2 coil packs and 4 injectors is plain scary.

We know that each coil pack draws around 2A and the injectors about an amp each in round figures- AVERAGE current. That's around 8 amps.

However the momentary surge current on these devices is MUCH higher. The coils can draw 10A each and the injectors almost 2 amps each for a few milliseconds or so. This is one reason why we have big cautions for breaker sizes when using a VPX which responds to PEAK current, not average current as traditional breakers and fuses do.

SDS sizes fuses and breakers at at least double the average current of the device if it doesn't have high inductance and around triple for things like fuel pumps, injectors and coils which do have high inductance and high surge current.

For injectors, we recommend a 5A breaker or fuse for EACH injector
For coil packs, we recommend a 10A breaker or fuse for EACH coil pack
For fuel pumps, we recommend a 15A breaker or fuse for EACH fuel pump (this value is stamped right into the Walbro pump bodies BTW)

Please remember that fuses and breakers are just there to protect the wiring from going up in smoke. Generally they won't protect the device. As in most things to do with electronics, running a component near its max rating is likely to shorten its lifespan.

A marginally sized breaker or fuse will run hot and may eventually oxidize and fail after a bunch of hours. There is no good reason to use marginally sized circuit protection here. The results can and have been fatal as I mentioned earlier in this post. I helped the TSB here investigate that fatal accident and found the cause for them pretty quickly- wrong breaker size used to feed the same components we are talking about here.

Lots of people in this thread who have no experience (and some who apparently do) with EFI and its components, have posted their opinions here while ignoring the most fundamental aspects of reliable circuit design- fusing. Bluntly, I caution people about taking the advice of people with no experience in this field. In this case, bad advice can kill you...

[RandyAB]

Originally Posted by rv6ejguy

I've gone back a ways in this discussion to the schematic in the links posted by Dan.

The 10A fuse recommended in the schematic for the 2 coil packs and 4 injectors is plain scary.

We know that each coil pack draws around 2A and the injectors about an amp each in round figures- AVERAGE current. That's around 8 amps.

However the momentary surge current on these devices is MUCH higher. The coils can draw 10A each and the injectors almost 2 amps each for a few milliseconds or so. This is one reason why we have big cautions for breaker sizes when using a VPX which responds to PEAK current, not average current as traditional breakers and fuses do.

SDS sizes fuses and breakers at at least double the average current of the device if it doesn't have high inductance and around triple for things like fuel pumps, injectors and coils which do have high inductance and high surge current.

For injectors, we recommend a 5A breaker or fuse for EACH injector
For coil packs, we recommend a 10A breaker or fuse for EACH coil pack
For fuel pumps, we recommend a 15A breaker or fuse for EACH fuel pump (this value is stamped right into the Walbro pump bodies BTW)

Please remember that fuses and breakers are just there to protect the wiring from going up in smoke. Generally they won't protect the device. As in most things to do with electronics, running a component near its max rating is likely to shorten its lifespan.

A marginally sized breaker or fuse will run hot and may eventually oxidize and fail after a bunch of hours. There is no good reason to use marginally sized circuit protection here. The results can and have been fatal as I mentioned earlier in this post. I helped the TSB here investigate that fatal accident and found the cause for them pretty quickly- wrong breaker size used to feed the same components we are talking about here.

Lots of people in this thread who have no experience (and some who apparently do) with EFI and its components, have posted their opinions here while ignoring the most fundamental aspects of reliable circuit design- fusing. Bluntly, I caution people about taking the advice of people with no experience in this field. In this case, bad advice can kill you...

Ross, in post 281 there is a link to five very short videos done by Nuckolls that seem to indicate that there are dangers associated with using one single bus, even appropriately fused for each component. The main jist is that there is a potential difference in amps from each of the battery sources that could blow fuses or breakers. I don't quite understand why the current draws he shows are so different but I'm have a request in to an electrical engineer buddy of mine (also an RV10 builder (Charrois here on VAF)) to review and help clarify for me. If you get the chance to look at them, your feedback would be extremely valuable.

[rv6ejguy]

I am referring to post #251 with this schematic: https://dms.ntsb.gov/public/61000-61...262/615998.pdf

Single 10A breaker protecting 2 coils and 4 injectors is a bad idea.