05-06-2017 12:05 PM
The NOx sensor is for emission control but it does that job by telling the ECM that the air fuel ratio is correct. The target NOx for a 0.5 g NOx emission is 60 ppm NOx. So when the ECM has warmed up the NOx sensor (this takes about 7-8 minutes) and switches to the NOx feedback mode, the ECM is looking to use the NOx signal to keep the engine air fuel ratio correct for the target NOx. If the NOx sensor is not working correctly, the ECM cannot maintain a correct air fuel ratio because it has no reference, so the engine operation could be unstable.
The engine has to be running over 900 rpm and have at least 25% load (about 20 psi inlet manifold pressure) for the ECM to start the NOx sensor warmup process. Then it takes 7-8 minutes for this warmup cycle to complete; you can watch it on the remote panel display. When it is complete, then the ECM switches to the NOx feedback mode. You can see the ECM take over and the actual NOx should begin to match the desired NOx on the screen. If you have no NOx value, then it is possible that the NOx sensor is bad. I would think that you would also have a fault code but you might not; the ECM just would not go into the NOx feedback mode. It would still be in the no feedback mode and if the engine setup is not correct, the engine might be unstable in this mode as well.
If there is no NOx value, then you need to determine if the NOx sensor or the wiring from the NOx sensor is bad. If it is, this must be repaired before the engine can be adjusted correctly. If you just unplug the NOx sensor, the ECM will see that it is disconnected and give a fault code that it cannot communicate with the sensor. The engine might run in the no feedback mode but if the adjustments are correct, the engine might be unstable and the emissions will not be correct.
I will PM you might phone number if you want to discuss with me
05-06-2017 11:41 AM
Thank you for the reply, Al.
You mentioned that after the engine & NOx sensor warms up, the ECM takes over and moves into the NOx feedback mode. Looking at the CAT panel in the fuel system section, I noticed that the NOx sensor readings are zero, all over the map, or sometimes even asterisks.
Would a NOx sensor that is bad (or going bad) cause erratic RPM issues? I was under the impression that NOx sensors were only for emissions control. Also, could I unplug the NOx sensor from the wiring harness and run the unit for a day or two as a troubleshooting measure without damaging the unit?
Thank you again for your help.
05-05-2017 11:18 PM
Not knowing anything about the history of the engine, my first thought is to tell you that I would perform engine setup again, according to the latest SM setup procedure. The latest one KENR6892-10 SOTA has the correct procedure beginning on page 65. It is important to do this first and correctly to get the engine setup correctly to run when starting up in the no feedback mode. When the engine warms up a bit and the NOx sensor has warmed up and is operational, the ECM takes over and moves into the NOx feedback mode.
After this setup procedure, you should load the engine to normal level and adjust the wastegate to get the compressor bypass 10-15% open at rated load. Rated load can be compared using the inlet manifold pressure and compare to the engine performance data sheet. This is the best comparison for engine horsepower. If you are running less than 1400 rpm or less than rated load, then the SOTA has some different values to compare for throttle plate position and compressor bypass positions.
In addition, you might have to reduce the governor dynamic settings a bit but I wouldn't go overboard with reducing these settings.
Doing these items in order should put you back in business for this late G3516B ULB. It should have the later flash file to correct some of the issues early engines had.
You might have some issues due to compressor piping configuration. Many compressors impose too much load on engines during startup and because of this the ECM operating software cannot compensate enough to get the engine to settle down. If this engine has more than 35% load on it at start up, then I suspect this is a large part of the problem. This might be made worse by changes in operating pressures or compressor configuration that might have been made recently.
If you have further questions, let me know and we will work through it.
05-05-2017 09:27 PM
We have a G3516 CAT engine (serial no. N6E00122) driving a natural gas compressor, commissioned in late 2015. Engine has 8157 hours of operation. We've been operating this unit with relatively no issues until recently. The unit starts up fine, but the engine RPM will begin to swing, approx. between 700 - 1200 RPM until operations personnel finally shuts down the unit (issue does not self correct). These swings could occur anywhere from 30 minutes after startup, to a day or two.
I know this isn't a bad magnetic pickup, as I can hear the engine surging and can see the discharge flow rate swing as well when this is happening.
Any ideas would be very helpful and much appreciated. Please let me know if any additional info is needed. Thanks in advance for your time.