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Electrical Architecture for EFI & EI

dave wolfe

Well-Known Member
Supporting Member
Oct 23, 2010
Here's been my thought process for EFI & associated electrical architecture. My project is a long way from flying but I intend to do an EFI & EI conversion on a VW powered Sonex. The engine has a marginal single alternator (20 amp motor cycle stator & rectifier - regulator). My mission is day vfr here in the midwest, and my idea of 'safe enough' for this aircraft is ~45 minutes of powered flight after an electrical failure of some sort, and I BELIEVE that I'm comfortable with the failure of a single cylinder. I'll have a single engine controller taking care of fuel injection and ignition, single plug per cylinder thru a coil near plug arrangement, and two electric fuel pumps. My setup will NOT windmill so I need the ability to use the starter after an in flight engine failure.

So the need for two batteries is a given. This is where I drew up blanks in terms of good electrical systems to copy. Some I saw missed the mark in terms of single point electrical failures bringing down the engine, others a lot better in this regard but also becoming a sea of switches to navigate on the panel. I imagined myself flying 20 hrs a year trying to keep track of cross tie busses and emer busses and such and figured I'd probably crash if I had an issue unless I could simplify simplify simplify. This brought me to Concept A in the picture. Have a single switch that turns on EVERYTHING needed for the engine to run from one of the batteries, and another switch for the other battery. In addition to a master switch, there would be another switch that selects which battery is selected when the master switch is turned on. I purposely do NOT want to be able to connect the two batteries together, there are common source failures that will cause both to fail simultaneously.

EFI power concepts.png

I think this concept is good enough. You would take off with both engine switches ON, then turn one switch OFF after reaching a safe altitude to preserve the charge in the battery not connected to the alternator and hence not being charged. Somehow the pilot would need to manage the state of battery charge. The big issue I have with this setup is the potential to mismanage the switches so the aircraft is flown extensively using the battery not being charged, leading to the engine turning off.

That's where Concept B comes in. It's just the first concept, but an "auto" mode is incorporated into the engine controls so that when AUTO is selected, the engine is running off of the battery that is selected via the master switch. In auto, the battery powering the engine is normally being charged. The other switch position is TO/EMER and just energizes both batteries at the same time for those higher risk times such as takeoff or during an emergency. It comes as close as I could get to be similar to your car's operation, and NOW I think it's at the point where the operation is very very intuitive for any other pilots jumping into the seat.

So here's the resulting architecture, please forgive my flinstonian use of the free digikey schematic tool & my total beginner level of electrical drawings. Feel free to critique, offer improvements, tell me why it's flawed, etc, that's why I'm putting it here for discussion. Im an engineer in the aviation industry but this is NOT in any areas of my expertise, this is a very rookie level effort.