Re: throttle surging when starting unit

Yes sir, that makes sense to me and how I kinda had it figured.


I should have mentioned that the compressors these engines drive are 3 stage units. the main bypass is between the final discharge and the suction line and it does fairly well at unloading the compressor except that the 1st stage still compresses into the 2nd stage causing the higher load. the manual bypass I also referred to is on the 1st stage discharge line. when that is opened, the units act much more calmly as I think they should. The problem is, that manual valve isn't exactly ergonomic either by location or design.


The desired port temp setting has been scrutinized and it does help but the main thing I was asking about was before exhaust feedback mode. I think it is called No Feed Back mode but not sure. Up until just a few weeks ago and really trying to dig deep on the cause of this surging, I had never even seen a choke actuate. When it did it, I had the 1st stage bypass open to see how much better it would run. That was also the first time I saw the air pressure below atmospheric.


Once it switches over to Exhaust feedback, and it wasn't previously swinging the RPM, it will run smooth at about any load. If it was swinging beforehand, it doesn't seem to calm down until it goes into Comb feedback.


again, thanks for your input, it is always greatly appreciated!

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Re: throttle surging when starting unit


The problem you describe is a common problem when the compressor bypass has been designed incorrectly for the engine.  Cat assumes a starting load of 25% and the operating strategy is developed around this general starting condition.  When the compressor bypass imposes a higher load as you have described and it exceeds the 40% level, then the engine operating strategy wants to move to the next operating strategy.  The ECM uses a lot of engine system inputs to determine when to change from the "starting mode" to the "no feedback mode" to the "exhaust feedback mode" and finally to the "combustion feedback mode".  When the conditions that signal the ECM what the engine status is, then the ECM tries to move to the next mode of operation.  If the engine is cold then the engine does not respond and generally becomes upset durin operation.


Cat tried to address this situation with the core software of the 2.0 flash family for the G3600 A3.  This change was to hold the operating system in the "exhaust feedback mode" for a maximum of 300 seconds, even though the calculated load was greater than 40%, which signals the ECM to move to the "combustion feedback mode".  This delayed time parameter is a choice that is entered into the Configuration screen by the technician.  After the delay time has expired, the ECM moves the engine into the "combustion feedback mode".


There are no other similar flash file changes that I know of to address this kind of problem.  It is a situation caused by the conditions the engine is operating, primarily the compressor bypasses being incorrectly designed.  My experience is that the compressor bypass circuit should be the same pipe diameter size as the compressor discharge size.  By designing the compressor bypass to this flow capacity, the bypass will not impose an artificially high load on the engine at start-up and warm up (no feedback mode).


You can improve the engine operation some during the warmup (no feedback mode) and exhaust feedback mode by carefully adjusting these parameters; 1) the desired port temperature value in the config screen (adjust the desired port temp to get the FCF near 100 in this mode), 2)  the maximum choke position (to prevent the ECM from overchoking the engine in no feedback mode and exhaust feedback mode), and 3) the best time delay hold in exhaust feedback (what time delay value allows the engine to warmup with minimum misfires before going into combustion feedback).


Fine tuning these settings will help the engine "get ready" more quickly and steadily before the ECM moves the engine into "combustion feedback mode."


It is difficult to convey all of this in an email.  The tech certainly must understand the G3600 A3 engine.  Even being around a G3600 A3 may not result in the level of understanding of the engine systems operation needed to work on this fine tuning.

If something is unclear, I would be happy to talk to you to clear up your questions. 


Best Regards,

Al Hunt

Accepted Solution

throttle surging when starting unit

We have about 50 G3606 A3's here in the field and we have always noticed that when starting them cold or at least after a day or more of downtime, they surge the rpm sometimes pretty hard. we have been able to correct some of the issue with new actuators and making sure the rod ends are in good shape along with being set at the correct lenghts and angles but the particular problem I am talking about has not been remedied yet.


The only way we seem to have some success with a smoother rpm and no misfires is to unload the 1st stage bypass on the compressor. We have a main bypass valve that is how we load the compressor but that 1st stage bypass valve is an extra valve added for possibly what we are having problems with or getting coolers thawed out.


If we start the engine with the 1st stage bypass closed, we tend to see about 40-45% engine load right off the bat. within just aa few seconds of starting, the wastegate will begin to close and ramp up turbo speed. I am assuming that the throttle postion is suggesting a rich condition and air is being added. When this happens, we start to get cyl misfires and the problem gets worse. right after we start to get misfires, the throttle tends to start actuating and the engine rpm will start to swing heavily. None of this is how we would like to start up a relatively cold engine.j


Now, if we open the 1st stage bypass valve, quite a bit of these problems seem to be avoided and we get much smoother operation and load percentage is closer to 30%. The problem is, the 1st stage bypass valve is not the easist thing to operate and most of our operators are not going to mess with it unless they cant get enough cranking rpm to start the enging and even then, one of us mechanics would likely be called out.


Is there any flash files that help with heavier loaded cold starts? possibly one that allows the engine to run slightly richer before it swtiches over to Exh Feedback mode?


Just wanted to ask in case there were something out there instead of asking our operators to open the bypass valve when starting the unit as I doubt it would ever happen.