COMPLETE UPDATE.
With all the greatly appreciated comments from the industry experts, four pages now, I have completely updated and expanded my first and second attempt to this which I feel is now accurate. This is not for experts but for pilots trying to understand the complicated Generator and Lane situation. Additionally, I want to provide who ever reads this with the best information to avoid confusion.
Here it is updated version 3:
Description, operation and failure modes of the Rotax (9xxiS) fuel injected series engine Generators.
Description for normal operation and startup generators.
The Rotax fuel injected engine has two internal generators. These operate from the flywheel on the back side of the engine opposite the propeller side.
Generator A - Description and Operation
Generator A – When the engine is rotating, three wire AC (alternating current) from the generator goes into regulator A on the fuse box which converts the AC and regulates the power to 12/13 volt DC (direct current) and feeds into the fuse box. From the fuse box, power is sent out to each fuel pump and to the Engine Control Unit (ECU). The fuse box supplies power to the engine, while the ECU controls the fuel injectors/spark plugs, interprets sensors feeding the ECU thus computer controls all engine functions. Alternator A is through fuse box regulator A, is 13+ volts DC, 16 amps/about 240 watts. 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) and fuel pumps.
From Generator A, through the fuse box, to the Engine Control Unit (ECU), two separate systems called Lanes, A and B, come out of the ECU which operate two completely separate systems/lanes. These separate Lane systems have 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, shared feedback sensors (for ambient pressure/temperature, throttle position, engine speed/position, EGT) are additionally fed back to the ECU. All the feedback information to the ECU allows the ECU to adjust the spark advance/timing and fuel injected mixture/timing for the engine providing optimum operation.
In summary, for normal operation, all engine electrical power, operation and fuel pump power is from Generator A, fed to the ECU, with Lane A and B separate systems coming out of the ECU.
Generator B - Description and Operation
The second Generator B, similar to generator A, three wire AC (alternating current) goes into regulator B on the fuse box, regulates the power to DC (direct current) 13+ volt and into the fuse box. Generator B has about twice the power output of Generator A. Generator B output is 30 amps, about 420 watts output. Generator B powers/charges the battery, via main electric bus thus airframe power systems. Typically, the master switch, sometimes with a relay, powers the main aircraft power bus from the battery. This airframe bus supplies the panel, avionics, lights, autopilot and all aircraft electric aircraft systems (minus the engine and fuel pumps powered by Generator A). This is also used for the starter typically with a relay for the high current engine starter.
in summary for normal operation, Generator B powers the battery and airframe electrical system (minus the engine/fuel pumps which is powered by Generator A).
Generator A and B normal operation for startup
Typically, before you start the aircraft, with the master on, you should be reading airframe battery bus voltage which should be around 12. 5 volts. This is the resting voltage of the battery and varies slightly depending the battery and your voltage sensing instrument. When you start the engine, the starting sequence is to connect the battery to the Engine Management System (EMS)/ECU with the “Start Power” momentary switch, connecting the ECU with the battery and firing the fuel pumps. You hit the starter button to engage the starter and turn the engine. This engine rotation activates generator B to provide power to run the engine. Immediately after the engine starts and runs, you release the “Start Power” momentary switch and starter engage switch. Now Generator B is powering/feeding the engine (EMS/ECU/fuel pumps). You typically want the engine speed to be around 2200 RPM.
Note: After the engine starts, MAKE SURE to disconnect the Start Power/battery connect switch from the EMS/ECU/fuel pumps. Typically, this is a momentary switch so releasing both start power and starter button accomplishes this.
If you reduced the engine RPM to below 2500 immediately after startup, your airframe/panel/bus voltage is still reading battery voltage, about 12.5 volts since Generator B is running the engine and not charging the battery. In this phase of starting, you are simply checking the operation of Generator B as it runs the engine. You increase the RPM to 2500 for five seconds which tells the ECU to switch the engine power to Generator A. Now Generator B connects to the battery so you should read above 13+ volts indicating your battery is being charged by Generator B. Simply, a charged battery is about 12.5 volts and when the battery is being charged the charging voltage is about 13.5 volts. Note the engine has gone through a sequence to check Generator B first and Generator A during startup.
This is in normal operation. Generator A feeding the engine/fuel pumps, Generator B feeding the battery/airframe. Both the temporary “Start Power” and ECU backup switches should be turned off so everything is fed with the separate generators. Since Generator B is feeding/charging the battery the system voltage should read about 13.5 volts.
Note: It should be understood that the fuel injected Rotax likes a little throttle to start, well above low idle for initial startup, but should be lowered immediately to about 2200 RPM so you can check generator B is operating the engine and not charging the battery until you run the throttle above 2500 RPM. Then you can check generator B “charging voltage”, as the engine in now running on generator A.
In summary for normal operation:
During initial startup and running at 2200 RPM Generator B is running the engine and not charging the battery and you are seeing battery voltage of about 12.5.
Once the RPM is 2500 for more than 5 seconds, the ECU has Generator A running the ECU/engine and Generator B is charging the battery supplying the airframe bus at 13+ volts.
Generator failure modes of the Rotax fuel injected generators?
Overall strategy: In any case, if you suspect and/or have a generator failure, indicated by decreasing panel voltage, at or below 12.5 volts, you should turn off all non-essential items such as lights and other systems with minimum radio transmissions to minimize draining your battery. Some modern Electronic Flight/Engine Information Systems (EFIS) systems have backup batteries if your electric system fails.
If generator A fails, the ECU recognizes this failure and commends the fuse box relay to immediately and automatically feed the power from Generator B to the output/system/ECU of Generator A at the fuse box output. This way the fuel pumps and the engine/ECU now have power and the engine keeps running on generator B. Both Lane A and Lane B lights will start blinking as immediate warning for this specific problem (generator A failing). 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 the airframe system must run off the battery and the battery voltage should be 12.5 volts since it is not being charged and start to decrease from the 12.5 volts as the battery is drained. Some systems can go 30 minutes in this situation but this depends on your aircraft.
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 airframe/panel/buss power must run off the battery and the battery voltage should start to decrease from 12.5 volts as the battery is drained. No Lane lights will come on and no warning. Some systems may have a low voltage warning that could be an indication when the battery goes below the programmed low voltage threshold (maybe 11 volts).
Either way, with Generator A or Generator B failing, you will see a reduction in bus voltage as the battery starts getting drained. With the new lighter weight Lithium battery many use, it is said they stay at a reasonably high voltage, rather than slowly dropping voltage, until they drain with a more dramatic drop and they are dead, less warning.
If generator A and B fails, in the remote event with both generators fail at the same time, the engine dies. The “battery backup” or “emergency power ECU switch”, 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 source that can be used to power the engine. Depending on your battery and other loads, the battery might go for 15 to 30 minutes running the engine.
To describe this in another way, the “battery backup” or “emergency power ECU switch” connects both generators on the 12 volt side and the battery all together in parallel. Both generators and the battery 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 do this right away and the battery voltage has not drained and you have 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 non-essential items such as lights. Note Generator B has significantly more power, 30 amps than Generator A 16 amps. It has been known that connecting all three sources with the emergency power switch is not advised since there have been some known cases of both generators failing running with the ECU backup switch turned on, so this is not advised.
If Generator A fails, and you turn on the “battery backup” or “emergency power ECU switch, then that could be a bigger problem since Generator A does not have that much excess power to power the engine and now the additional load to charge the battery/power the aircraft. So with Generator A failing (both Lane lights blinking), it is advised to not use the “battery backup/emergency power ECU switch to charge the battery. The last thing you want to do is not have enough power to keep the engine running and/or burn out both generators.
Overall
For most situations, simply running the battery out, losing your panel, instruments, communications, airspeed, altitude, etc… and keeping that engine running is the best option. If you were trained properly as a pilot, you 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.
What is the purpose of the Lanes and the associated Lane lights. What is the process if a lane light starts blinking or comes on solid?
The lanes are two separate systems coming from the Engine Control Unit (ECU) each having separate systems: four spark plugs, four fuel injector nozzles, a manifold pressure sensor, and a manifold temperature sensor. The engine has shared Exhaust Gas Temperature (EGT), ambient temperature/pressure, RPM, throttle position sensors. These are two completely separate systems which are called Lane A and Lane B. The aircraft can fly if one system is not operating so it can fly with just one Lane in case one Lane fails in flight. However, for safety/redundancy, both systems are checked to make sure both Lane systems are operational before takeoff so you are flying with two systems for reliability.
It should be noted that the generators A and B do not correlate with Lanes A and B. Typically Generator A supplies the ECU and the two separate lanes, A and B, come out of the ECU.
On startup, if there are any Lane lights on after 5 seconds, you can reset the Lane by cycling it (turning it off for 5 seconds and back on). If that does not work, shut EVERYTHING down and restart. It is a computer and resetting is sometimes needed just like your home computer.
Generally speaking, blinking lane lights indicate some sort of minor sensor error. Solid lights indicate a more severe system failure.
As an example, if you are doing a long descent at idle in cold weather, the EGT probes in front of the engine have more cold air blowing on them and cool faster than the rear ones. If you continue, the temperature split from the front to the back EGT can get past the alarm level determined by the ECU and give a flashing Lane light, a sensor error. This is irritating since it will stay blinking even after you fix the problem. Giving it some throttle to equalize the EGT probes and resetting the Lane can clear the blinking Lane light. If you were situationally aware as you were descending and see the EGT’s start to split, you could give it some throttle and avoid the EGT’s splitting and the blinking/warning Lane light coming on in the first place.
Generally, if you are flying and a lane light comes on solid or blinking, you should immediately check engine systems, battery voltage and see if there is anything indicated as to what the problem might be.
Generally, with any solid or blinking light, you can shut that Lane off, wait for 5 seconds and turn it back on and it may clear itself. If it does not, then fly to your airport and figure out what you need to do to clear that light on. Sometimes it is just a loose connection. Warning, if you get a solid blinking light, do not shut off a lane that has no solid or blinking light, since that is a lane that is operating good.
Remedies for solid or blinking lane lights. First, wiggling/pressing on the electric connections may temporarily fix the problem as a start. If the problem persists, you need to down load the data with the Rotax/Buds/dongle and have the information analyzed by a Rotax expert to find the problem.
