johnbright
Joined: 14 Dec 2011
Posts: 166
Location: Newport News, VA
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 Posted: Thu Nov 18, 2021 10:48 am Post subject: EFI injector and other solenoid inrush current |
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Speaking of EFI+I, be it FlyEFII or SDS, someone mentioned recently that fuel injectors have an inrush current that should be considered when sizing fuses.
I'd like to point out that this is incorrect AFAIK.
The injectors and coils in an EFI+I system are pulsed so can their fuses or circuit breakers be hammered?
* I don’t imagine the short pulse widths of fuel injectors and ignition coils will hammer a fuse but I haven’t analyzed this. (Beyond my pay grade? Not worth the time?)
* Speaking to my SDS EM-5 four-cylinder application, what I do is follow SDS' recommendation of a 5A fuse or breaker for each injector and a 10A fuse or breaker for each coilpack.
* Injectors are 14.5 Ohms so they draw 1A when on and 0.84A average at 14.4V and 85% duty cycle.
* To my knowledge, the four-cylinder coilpacks ramp up from 0 to 6.5A in a sawtooth 4.3 mS wide twice per rev for an average current draw of 1.1A at 2,400 RPM. Disclaimer: The sawtooth height (current) and width (time) did not come from SDS but the resulting current draw calculation matches what SDS says per coilpack "a little over 1A at 2,400 RPM". I imagine the six-cylinder coilpacks will simply draw 50% more average current because they operate 50% more cylinders. (FlyEFII System 32 is different because it uses three four-cylinder coilpacks in six-cylinder applications whereas SDS uses two six-cylinder coilpacks.)
* Ref attached oscilloscope images.
* Beware ECBs. This from the SDS install manual re VP-X ECBs: “It has come to our attention that the VP-X electronic switching/circuit breaker box does not use average current draw over several seconds to trip breakers like conventional thermal breakers do. It may trip during normal operation due to peak current transients over only a few milliseconds.” I’m told by an electrical engineer OBAM builder this is not inherent to ECB technology but evidently it is an aspect of certain designs. To me an FMEA would include the possibility of an ECB system failing so electrically-dependent engine loads would not be put on it in any case.
A magnetic solenoid is an inductor and in an inductor current lags voltage. This applies to the single-coil and no-current-regulating-driver solenoids we comonly use like relays, contactors, saturated-core fuel injecors, and also to inductive coilpacks.
Some solenoids that do have an inrush current:
*Dual-coil solenoids that reduce current after the armature moves and closes the magnetic air gap. This because the solenoid produces more force when the air gap is closed so it can be held closed with less current. Ref for instance Bob Nuckolls "Let's Talk About Starter Solenoids": http://www.aeroelectric.com/articles/strtctr.pdf
* Peak-hold fuel injectors that use current regulating drivers to overdrive the solenoid to make it close faster then back off the current once the injector is open aka solenoid air cap closed.
* Contactors and relays that have peak-hold current drivers built in. I have not seen these recommended by Aeroelectric Connection but they do exist.
* Alternating current solenoids that have a moving armature have an inrush current because inductance increases when the air gap closes. But we are talking about direct current in OBAM aircraft.
I doubt peak-hold injectors are used on direct-drive aircraft engines because the advantage is increased dynamic range (due to faster opening and closing times) of fuel delivery from idle to full power high RPM and 2,700 is not high RPM. The saturated-core fuel injectors that came with my SDS system have 14.5 Ohm coils and the peak-hold injectors I remember from the 1990s were 2.5 Ohms so that is a way to tell the difference.
I'm using the term solenoid generically, it being a coil of wire with a magnetic core.
* Solenoid: A solenoid that provides force to actuate a separate device, like moving the pinion gear in a starter motor to engage the ring gear on the flywheel or electric door locks on a car or the hammer of a xylophone style door bell.
* Contactor: A high-current relay with integrated solenoid actuator.
* Relay: An electrical switch with integrated solenoid actuator.
* Fuel injector: An on-off hydraulic valve with integrated solenoid actuator that is pulse-width modulated to control flow.
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John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
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