In an effort to teach the operation of the Rotax fuel injected engines, the following is my first attempt to simplify this complex subject. Any comments, suggestions are appreciated.
Operation and failure modes of the 912 fuel injected generators and backup generator switch.
Generator A alternating current goes into regulator A on the fusebox, regulates the power to direct 12 volt and into the fusebox. From the fusebox, power is sent out to each fuel pump and to the Engine Control Unit (ECU). The ECU supplies power to the engine, interprets sensors and controls all engine functions. Alternator A is 12/14 volts DC, 16 amps/about 240 watts, and the ECU/engine and fuel pumps use around 150 watts to run the engine. This power is supplied from the fuse box to the Engine Control Unit (ECU).
From the Engine Control Unit (ECU), two separate Lanes, A and B come out which operate two completely separate systems with two separate sets of spark plugs, two separate sets fuel injector valves, two separate manifold pressure readings, and two separate air manifold air inlet temperature probes, etc.. Additionally, feedback sensors for pressures, temperatures (air and EGT), throttle position, engine speed/position, and provide feedback information to the ECU to adjust the spark advance and fuel injected mixture for the engine for optimum operation. In summary, for normal operation, all engine electrical power, operation and fuel pump power from is from Generator A.
The second Generator B alternating current goes into regulator B on the fusebox, regulates the power to direct 12 volt and into the fusebox. Generator B has about twice the power output of Generator A since it has greater need. Generator B output is 30 amps/about 420 watts output. Generator B powers/charges the battery, and airframe power systems. Typically, the master switch, sometimes a relay and powers the main aircraft main power buss from the battery. This supplies the panel, avionics, lights, autopilot and all aircraft electric systems (minus the engine and fuel pumps powered by Generator A).
Typically before you start the aircraft, you should be reading battery voltage which should be around 12. 5 volts. When you start the engine, the starting sequence is to connect the battery to the ECU and hit the starter to engage the starter and turn the engine, start the engine so Generator A starts to generate voltage with the engine running to now power the ECU and run the engine. Your voltage is still reading battery voltage, about 12.5 volts until you increase the RPM to 2500 for five seconds which connects Generator B to the battery so you should read above 13 volts indicating your battery is being charged by Generator B. It should be noted that the fuel injected Rotax likes a little throttle, to start well above low idle, about 2500 RPM is a guideline.
Too clarify so there is no misunderstanding, during normal operation, Generator A provides the power to run the engine including both Lane A and Lane B. Generator B supplies the power to run the aircraft (minus engine) and charge the battery.
Failure modes.
Overall strategy: In any case if you suspect you have a generator failure, indicated by decreasing voltage below 13 volts, you would turn off all non essential items such as lights, and other non essential systems to keep from draining your battery. Some EFIS systems have backup batteries if your electric system fails.
If generator A fails, the fuse box relay immediately and automatically feeds the power from Generator B to the output/system of Generator A so the fuel pumps and the engine/ECU now have power and the engine keeps running. Now with Generator B power diverted to the Engine/ECU and fuel pumps, to run the engine, there is no power going to the battery/airframe so this system must run off the battery and the battery voltage should start to decrease from the 12.5 volts as the battery is drained.
If generator B fails, the engine will keep running since it is running off Generator A, but the power to the battery/airframe will not be functioning so this system must run off the battery and the battery voltage should start to decrease from 12.5 volts as the battery is drained.
Note that in either of these cases with generator A or B failing, both Lanes coming out of the ECU will be happy since they get power from either A or B (if A has failed) coming into the ECU. Unless there is some reading in the avionics installed that there is a generator failure, both lanes will continue to function as if nothing happened. The Lane lights will not typically illuminate and show a steady or flashing red, they are happy. However, you will see a reduction in voltage as the lead/acid battery starts getting drained. With the new lighter weight Lithium battery many use, they stay at a reasonably high voltage, rather than slowly dropping voltage, until they drain than a very dramatic drop and they are dead, less warning.
What happens in the remote event that both generators fail at the same time, the engine dies. The battery backup or emergency power stitch, what ever you call it on your aircraft, switch is moved to on and connects the battery to the ECU and fuel pumps and you can start and run the engine directly off the battery rather than the generators. This battery backup or emergency power switch is a third item that can be used to power the engine.
So the battery backup or emergency power stitch connects both generators and the battery together in parallel. All are connected as one. If you knew that your generator A failed and you were running the engine on just Generator B, and you see your voltage start to go down, if you turn on the battery backup or emergency power stitch, you connect the generator B with the battery system, you can power your battery and keep it charged. Does generator B have enough power to run the engine, charge the battery and run the airframe/avionics? Probably if you shed most of your loads and minimize radio transmissions. However, this scenario depends on how long you were running off the battery and how discharged it is and your electric load after you shut down essential items such as lights. Note Generator B has significantly more power 30 amps than Generator A 16 amps.
If it is Generator B that fails, and you turn on the battery backup or emergency power stitch, then that could be a problem since Generator A does not have that much excess power to power the engine and now the additional load to charge the battery and power the aircraft. So with Generator A failing, it is less advised to use the battery backup or emergency power stitch to charge the battery. The last thing you want to do is not have enough power to keep the engine running.
How do you know if it is Generator a or Generator B that has failed? That is a question in which each aircraft is different.
The Sling LSA POH Alternator/Charge System Failure 3.7.3 states that if Generator A fails, Lane light A will come on and be steady and if Generator B fails, the Lane light B will come on steady. For either Lane lights on, cycle to reset and if still steady on, ECU backup on and shed loads. (I am not sure I believe the lane lights will come on with either generator failure so this is still a question for me.) Any feedback on this specifically would be appreciated.
For most situations, simply running the battery out, losing your panel, instruments, losing communications, and if you were trained properly, you can simply fly the aircraft and land it without any electricity as long as that engine keeps running. Each situation is unique but understanding the generator systems helps make good decisions while flying.
