Is there a preferred method for shutting down an EFI Lycoming? Just turn off the master like on a car, or shut the fuel pump off?
Obviously both will shut the engine down. I was interested because of a wiring logic I would like to use.
IMO opening the master contactor should not kill the engine. In case of fire in the cockpit one should be able to open the master contactor and still have engine power and if the master contactor fails one should still have engine power. Stop the engine with coil power, fuel pump power, or ECU power your choice but kill them all within a few seconds of one another.
Is there a reason you can't just pull the mixture to cutoff first?
Any downside to just killing the fuel pump first?
How does every car manufactured in the last 30 years do it?
If my battery BMS turns my battery off, my master contractor fails "open", etc, I would like the engine to continue running off the alternator.
Caution - here there be dragons.
What you propose is possible with some setups - but certainly can result in widely varying voltage swings on the system with a battery to damp them out, and your high-dollar electronics are exposed to all those swings. You'll want to test this, and in the testing you are risking your electronics - so caution is advised.
I accidentally tested this myself in my airplane by bumping the master BATT switch off to open the master, with the ALT switch still on - and my airplane reacted well to that, but there are no guarantees.
I've done this with my ND clone in my lathe test stand. Removing the battery bumped the charging voltage from 14.5 to 15.5, but I only had a small cooler hooked up as a load, only 12V thing I could find I didn't mind if I fried.
More testing to follow but the analog and digital meter I had didn't seem to show voltage variance even when I was cycling the load on and off.
I think you'll find that as you increase the load the output voltage will remain more stable. I've got a full glass panel and electric-only fuel pumps, so I'm pulling a good 15-18 amps constantly and my voltage held pretty steady in the mid-13 to high 14 range. All those are steady loads, as you bring on things that are temporary or pulsing like strobes and radio transmissions, that's where you'll get voltage swings.
Apart from providing a load to hold the generator voltage in check the battery also provides a low source impedence for the switching regulator power supplies in the power circuits of most glass display systems (Dynon, Garmin etc.) Switching power converter control loop stability is negatively affected by an increase in source impedence from the power source as when running on the alternator without a battery. This may not damage the switch mode power supply but could possibly damage the display electronics and result in unreliable operation. One solution to this problem is to install a large capacitor (22,000 microfarad rated at 50 volts) in the bus circuit. This will also damp the high frequency voltage variation from the alternator rectifier diodes. Asking Dynon tech support for regulator control loop stability margin information got the same response as asking for definition of the two port parameters for the EMS-220. It was like speaking a unknown foreign language and expecting a rational answer. Garmin may provide more information and a least understand the reason for the concern.
KT
They do it by turning the master off (ignition key off, or stop button). Hence my question...
I see the functionality of the automotive ignition switch differently. In the car it's a single switch with a "chassis" function (the accessory position) and an "engine" function (start/run). In my airplane the master switch controls the airframe functions and another switch controls the engine functions. I have an airframe buss and battery and an engine buss and battery. My engine stops when I turn off the "engine switch", killing power to the entire engine buss (ECU, coils, injectors, fuel pump)
Airframe On/off; engine on/off. Simple.