Basic description of Generator and Lane system operation

Hi all

I hesitate to do this but can only tell you I will not make additional comments as this PDF is a technical document in as much as for someone who understands such schematics.   If you are so incinded then you need to take the specialty classes that explain the system in detail.  

The PDF is a high resolution (tap it to expand what you want to look at) of the 912 iS.  

Cheers

Jon Tensfeldt wrote:

Nachshon Kurtz wrote:

Hi Paul, 

I think there is some nuance to the Lanes that should be elaborated on further to clarify the “Generator A three…” and “From Generator A…” paragraphs.

To properly understand power distribution, it’s important to realize that the ECU does not power the ignition coils or the injectors; it controls them (via low-side switching, etc.). If you look at the fuse box diagram in the MMH (76-20-00), you’ll see that there are fuses for the Lane A and Lane B circuits. Lane A and B in the 912iS refer to more than just the different computers in the ECU and their associated sensors. They are actually two independent circuits/buses, each powering the components associated with that lane (though both circuits/buses always get their power from a single source - Gen A, Gen B, or the airplane battery). The engine components - ignition coils, injectors, and fuel pumps - are split between the circuits/buses and get their DC power directly from the circuits/buses. Additionally, the Lane A computer in the ECU gets its power from the Lane A circuit/bus and the Lane B computer gets its power from the Lane B circuit/bus. In normal operation, only a single lane computer actually controls all the engine components. This is possible because both lane computers have control wiring to all the engine components. However, even when, say, Lane A is controlling Lane B’s components, Lane B’s components still get their DC power from their circuit/bus. 

Many people, after learning that during normal operation only a single lane is controlling all the engine components, have a hard time understanding why shutting off a lane turns off half the spark plugs (ignition coils) and injectors. They wonder why the other lane can’t do what it normally would do and still control all the components. Their confusion lies in thinking of the lanes only in terms of the ECU computers. Shutting off a lane isn’t just turning off its computer - it’s actually shutting off power to that lane circuit/bus (which, of course, removes power from that lane’s ECU computer as well…). This means that the components in that lane no longer have power and so even though the remaining lane’s computer could control those components if they had power, it can’t because they don’t. 

The two separate systems - Lane A and B - therefore, start at the fuse box. It’s clear in the MMH (24-00-00) that the X1 wiring harness connector is for the Lane A circuit components and the X2 connector is for the Lane B circuit components. It is not the case that there is single circuitry up until the ECU and then things split from there. The fuse box is where power distribution is controlled. X1 wiring distributes power to all the different engine components associated with Lane A (ignition coils, injectors, fuel pump 1, Lane A ECU computer, and has control connectors for HIC A for Lane A switches, etc.) and X2 has the same for all the components associated with Lane B. 

Seems to me that both fuel pumps continue to run with one lane switch off.

I will have to check that again next time I go out.

The fuel pumps are independent of the ECUs. The fuel pumps are powered directly from the Rotax fuse box. The 12+ lead to pumps 1 & 2 originate at a fuses F4 and F5 respectively and run directly to the pumps. The 12- lead to each pump originates at the regulator A ground buss, and each (-) lead is switched by a fuel pump switch in the cabin. All that is required for the pumps to run is power to the fuse box, either by start power, alternator A, or alternator B if A fails. If both alternators fail the emergency power switch ties the battery to the Rotax fuse box and will run the pumps. 

It seems I am mistaken in my understanding of how the lanes interact and what exactly the lane switches control. My main point was regarding power distribution, but it seems I may be wrong about that as well. I apologize.

Nachshon Kurtz wrote:

It seems I am mistaken in my understanding of how the lanes interact and what exactly the lane switches control. My main point was regarding power distribution, but it seems I may be wrong about that as well. I apologize.

Nachshon,

No need to apologize, this forum is about exchanging ideas and learning. This is a complex subject and there is no technical document explaining these details. I believe you are right about much of this. The fusebox does provide power to the injectors and coils and the ECUs control the low (-) signal to these devices to activate them. But from what I can see the fusebox is not affected by a lane being switched off.  

What’s not clear from the wiring diagrams is if each ECU can control the outputs of the other when one is shut off. I believe the answer is no, and that the outputs to coils and injectors can only be controlled by the associated lanes they are wired to. This is evident by the fact that a problem with a coil or injector can be found by switching off a lane, thus exposing a problem on the remaining lane. In other words, I believe each lane and its associated injectors and coils operate independently and redundantly, so when one lane is shut off, so are the injectors and coils that are associated with it. That would be the only way to make sure you are testing all components associated with each lane when performing a lane check. 

If I’m right about this, it brings up other questions about dual lane and single lane operation that I have not seen answered. Assuming that during normal operation both plugs and both injectors are firing at each cylinder, then each injector would be delivering 1/2 the required fuel pulse. So then when one lane is shut off the remaining injector pulse width would need to be doubled to deliver the full correct fuel load. That seems easy enough for the ECU to accomplish.

 I have seen it suggested that in normal operation the Lane A and Lane B injectors and coils take turns, and each fire only every other cycle (I’m not in agreement, but open to being wrong). This would mean that when one lane is switched off the remaining lane goes to double duty and fires every cycle. This would explain why there is viturally no RPM drop when doing a lane check, but it seems overly complicated from a diagnostic point of view. This would be easy enough to figure out with an oscilloscope, but I have never taken the time to do that. I guess Rotax does not think we need to know these details… maybe they are right!  

Hi Jeff

The ECU can tell when an injector fails by the resistance.  If one fails the other will double pulse, effectively maintaining the correct air fuel ratio.  You will not see a lean cylinder in that case. 

Cheers