If we are converting an engine that is already equipped with modern electronic ignition (EI) and electronic fuel injection (EFI) systems, what are the options for adding economical, lightweight, reliable redundancy for assuring fuel and spark continue to arrive? I understand and agree that these modern systems are highly reliable. Maybe from an objective standpoint redundancy isn't warranted (after all, there are plenty of other single points of failure on any engine: we don't have a redundant crankshaft, water pump, oil pump, throttle cable, etc). But if we nonetheless want redundancy in fuel and spark, what's the best way?
I'm thinking of the Aeromomentum Suzuki right now: 1 plug per cylinder, it has an aftermarket ECU (Microsquirt) that provides sequential FI (one injector per cylinder) and ignition.
Options I can see:
1) Add another Microsquirt ECU that has a completely redundant set of sensors (crank position, throttle position, temperatures, MAS, etc) and injectors, with a set of pluq wires going to the single set of plugs. Turned off in normal ops (except for a check before takeoff), and toggled on when needed.
PROS: Fully functional backup that results in normal performance
CONS:
- Expensive, complex, and not very light (for all the additional injectors, sensors, wires, etc)
- Electric power will still be required by both systems, so true redundancy requires redundant power
2) As above, but the auxilliary ECU is very minimalist: A single injector in the throttle body, the bare minimum of sensors, set up to function in "limp home" mode.
PROS: Cheaper and lighter than Option 1
CONS:
-- Degraded performance in "limp-home" mode
-- Same as Option 1 regarding electrical power
3) Caveman approach:
-- Fuel induction would be from a simple carb throat/venturi ahead of the throttle body (or, simpler, a Revflow/POSA/AeroConversions style "metered leak" that drips fuel into the intake based on the position of a cone-shaped pin. The pin would move in and out with the throttle all the time, but no fuel would flow because the mixture is turned off mechanically.) With either a venturi carb or "drip," in case of engine failure the main FI is turned off (either with a separate switch of a microswitch that is activated when the mixture knob on the backup system is move from "off).
-- Spark: Magneto, or the fixed-advance magnetron system used on Sonex (and B&S lawnmowers ).
PROS: -Technologically simple
- Will provide fuel and spark in case of total electrical failure (if gravity feed of carb is available)
CONS:
-- Lots of things to individually engineer and fit into the existing engine package. So, not very simple in practice
-- Can a spark gap be found that will work well with both the "main" EI and a much lower energy magnetron?
-- Engine will not be running efficiently in backup mode. Fuel delivery is bound to be too rich or lean until adjusted, magnetron ignition does not provide spark advance, etc.
-- Probably fairly heavy (carb body, new fuel lines, magneto, etc)
Based on the little I know, I'd lean toward Option 2. I don't know what a minimum set of "limp home" sensors would be, however.
Opinions (and spears) are welcome and expected . . .
I'm thinking of the Aeromomentum Suzuki right now: 1 plug per cylinder, it has an aftermarket ECU (Microsquirt) that provides sequential FI (one injector per cylinder) and ignition.
Options I can see:
1) Add another Microsquirt ECU that has a completely redundant set of sensors (crank position, throttle position, temperatures, MAS, etc) and injectors, with a set of pluq wires going to the single set of plugs. Turned off in normal ops (except for a check before takeoff), and toggled on when needed.
PROS: Fully functional backup that results in normal performance
CONS:
- Expensive, complex, and not very light (for all the additional injectors, sensors, wires, etc)
- Electric power will still be required by both systems, so true redundancy requires redundant power
2) As above, but the auxilliary ECU is very minimalist: A single injector in the throttle body, the bare minimum of sensors, set up to function in "limp home" mode.
PROS: Cheaper and lighter than Option 1
CONS:
-- Degraded performance in "limp-home" mode
-- Same as Option 1 regarding electrical power
3) Caveman approach:
-- Fuel induction would be from a simple carb throat/venturi ahead of the throttle body (or, simpler, a Revflow/POSA/AeroConversions style "metered leak" that drips fuel into the intake based on the position of a cone-shaped pin. The pin would move in and out with the throttle all the time, but no fuel would flow because the mixture is turned off mechanically.) With either a venturi carb or "drip," in case of engine failure the main FI is turned off (either with a separate switch of a microswitch that is activated when the mixture knob on the backup system is move from "off).
-- Spark: Magneto, or the fixed-advance magnetron system used on Sonex (and B&S lawnmowers ).
PROS: -Technologically simple
- Will provide fuel and spark in case of total electrical failure (if gravity feed of carb is available)
CONS:
-- Lots of things to individually engineer and fit into the existing engine package. So, not very simple in practice
-- Can a spark gap be found that will work well with both the "main" EI and a much lower energy magnetron?
-- Engine will not be running efficiently in backup mode. Fuel delivery is bound to be too rich or lean until adjusted, magnetron ignition does not provide spark advance, etc.
-- Probably fairly heavy (carb body, new fuel lines, magneto, etc)
Based on the little I know, I'd lean toward Option 2. I don't know what a minimum set of "limp home" sensors would be, however.
Opinions (and spears) are welcome and expected . . .