Diesel Generator Underloading

Boonies · Tuesday at 4:17 PM

I have received some great help on this forum in the past. Now I have some general questions about underloading diesel generators. I have seen warnings, in this forum and other places too, about running diesel generators at low load. I am aware of the problems this can cause.

There are some questions I have not been able to find the answer to. I have read not to run a diesel generator at low load for “extended periods of time”. How long is an extended period of time? Is it hours, days, or what? How do you define low load? I have seen different numbers like <30% full load and <50% full load. If the application requires that it must run at low load, but you still need the full capacity of the generator at times, is there any way to compensate for the low load periods of operation without causing long term damage?

I think we might have a somewhat unique situation here. Our generator is used seasonally and it is integrated into an automatic system where it will typically run for about an hour or so and then turn off for about 2 hours. It will cycle like this 24/7 for several months. I will spare you details of why and how this works unless someone specifically wants to know. When it is running the load can vary significantly. It might run the entire hour at about 15%-20% full load or it might be closer to 85%-95% full load, and it can be anywhere in between for any portion of that hour. I can make modifications in the automated system so I can insert run times at any % of full load I want. However, these “special” run times will need to “fit in the break period” when the generator would normally be off for 2 hours, and I cannot change the load profile during the automated 1 hour run times.

In addition to the automatic cycling described above, there are infrequent times when the generator needs to run for longer periods of time. It could be several hours, and in rare circumstances maybe even days. During these less frequent times it would have the more erratic load profile described above. I could attempt do manual load adjustments during these times, but it would be more difficult to keep from overloading the generator and the timing could vary significantly. If I were to attempt this what % of full load should I shoot for, how often, and for how long? How critical would it be if the some high load periods were missed when trying to do it manually?

Thanks for the help.

Ray70 · Tuesday at 4:35 PM

For me, I consider light loads to be around 25-30% or less.
"Extended periods of time" to me means a week+ of constant running at light load.
Don't think a few hours here and there will cause any damage and if you start to wet stack the machine a little, those periods of heavier load should clean it up without a problem.
Wet stacking on a liquid cooled diesel Gen is a REAL thing, but it doesn't happen over night and can be easily reversed if you don't let it go on for hundreds of hours with out any attention.

Mullaney · Tuesday at 4:38 PM

Boonies said:
I have received some great help on this forum in the past. Now I have some general questions about underloading diesel generators. I have seen warnings, in this forum and other places too, about running diesel generators at low load. I am aware of the problems this can cause.

There are some questions I have not been able to find the answer to. I have read not to run a diesel generator at low load for “extended periods of time”. How long is an extended period of time? Is it hours, days, or what? How do you define low load? I have seen different numbers like <30% full load and <50% full load. If the application requires that it must run at low load, but you still need the full capacity of the generator at times, is there any way to compensate for the low load periods of operation without causing long term damage?

I think we might have a somewhat unique situation here. Our generator is used seasonally and it is integrated into an automatic system where it will typically run for about an hour or so and then turn off for about 2 hours. It will cycle like this 24/7 for several months. I will spare you details of why and how this works unless someone specifically wants to know. When it is running the load can vary significantly. It might run the entire hour at about 15%-20% full load or it might be closer to 85%-95% full load, and it can be anywhere in between for any portion of that hour. I can make modifications in the automated system so I can insert run times at any % of full load I want. However, these “special” run times will need to “fit in the break period” when the generator would normally be off for 2 hours, and I cannot change the load profile during the automated 1 hour run times.

In addition to the automatic cycling described above, there are infrequent times when the generator needs to run for longer periods of time. It could be several hours, and in rare circumstances maybe even days. During these less frequent times it would have the more erratic load profile described above. I could attempt do manual load adjustments during these times, but it would be more difficult to keep from overloading the generator and the timing could vary significantly. If I were to attempt this what % of full load should I shoot for, how often, and for how long? How critical would it be if the some high load periods were missed when trying to do it manually?

Thanks for the help.

.
Well... It seems already (based on your question) that you have already been running this generator for some time? What you need to watch for is "wet stacking". That tends to clog up the muffler and start spitting out chunks of carbonized material when the load increases. Have you seen any wet stacking happening?

2Pbfeet · Tuesday at 8:54 PM

Boonies said:
I have received some great help on this forum in the past. Now I have some general questions about underloading diesel generators. I have seen warnings, in this forum and other places too, about running diesel generators at low load. I am aware of the problems this can cause.

There are some questions I have not been able to find the answer to. I have read not to run a diesel generator at low load for “extended periods of time”. How long is an extended period of time? Is it hours, days, or what? How do you define low load? I have seen different numbers like <30% full load and <50% full load. If the application requires that it must run at low load, but you still need the full capacity of the generator at times, is there any way to compensate for the low load periods of operation without causing long term damage?

I think we might have a somewhat unique situation here. Our generator is used seasonally and it is integrated into an automatic system where it will typically run for about an hour or so and then turn off for about 2 hours. It will cycle like this 24/7 for several months. I will spare you details of why and how this works unless someone specifically wants to know. When it is running the load can vary significantly. It might run the entire hour at about 15%-20% full load or it might be closer to 85%-95% full load, and it can be anywhere in between for any portion of that hour. I can make modifications in the automated system so I can insert run times at any % of full load I want. However, these “special” run times will need to “fit in the break period” when the generator would normally be off for 2 hours, and I cannot change the load profile during the automated 1 hour run times.

In addition to the automatic cycling described above, there are infrequent times when the generator needs to run for longer periods of time. It could be several hours, and in rare circumstances maybe even days. During these less frequent times it would have the more erratic load profile described above. I could attempt do manual load adjustments during these times, but it would be more difficult to keep from overloading the generator and the timing could vary significantly. If I were to attempt this what % of full load should I shoot for, how often, and for how long? How critical would it be if the some high load periods were missed when trying to do it manually?

Thanks for the help.

Sounds like an interesting load cycle!

I think that there is a reason that you don't find "rules" on enough load. I think that "wet stacking" depends on the engine, and its fuel adjustment, how cold the environment is, and exactly how it is being run. Low loads for weeks is not good for sure. I think many surplus generators turn up wet stacked because someone needed some fluorescent lights on for a few weeks, and only had a bigger generator.

Generally, if you are getting to 85-95% of the capacity some of the time for an hour or so, my experience is that should warm everything up to the point the generator has a chance to burn off any excess carbon or wet stacking. High loads push more heat out that with a lean burning diesel will gradually burn off any unburned fuel or carbon in the exhaust manifold or muffler.

The proof is in the pudding though. If your machine has been doing this for some time, and it isn't already wet stacked, my bet is that it will be fine.

2Pbfeet

msgjd · Tuesday at 9:01 PM

2Pbfeet said:
The proof is in the pudding though. If your machine has been doing this for some time, and it isn't already wet stacked, my bet is that it will be fine.

2Pbfeet

well-said !

Guyfang · Wednesday at 7:30 AM

If it makes you feel better, turn off your new control system, load the set to 80-90% and let it run a few hours. Get a lawn chair out, and see what comes out of your exhaust. If you get a lot of hard black carbon coming out, make sure nothing flammable is in the exhaust cone.

Tinstar · Wednesday at 11:21 PM

This might help.

Scoobyshep · Thursday at 1:41 PM

Guyfang said:
If it makes you feel better, turn off your new control system, load the set to 80-90% and let it run a few hours. Get a lawn chair out, and see what comes out of your exhaust. If you get a lot of hard black carbon coming out, make sure nothing flammable is in the exhaust cone.

One of my favorite things to do (although my local fire dept doesnt find the load bank burn off nearly as amusing) .

Boonies · Thursday at 2:04 PM

Thanks guys. Lots of good info.

Guyfang · Thursday at 2:28 PM

One of my favorite things to do (although my local fire dept doesnt find the load bank burn off nearly as amusing)

The German fire department didn't find id funny at all when we set the weeds on fire!!

But a bottle of fine sippen licker smoothed it all out.

Scoobyshep · Thursday at 2:41 PM

Guyfang said:
One of my favorite things to do (although my local fire dept doesnt find the load bank burn off nearly as amusing)

The German fire department didn't find id funny at all when we set the weeds on fire!! But a bottle of fine sippen licker smoothed it all out.

I usually do it during the rainy season (most of the year here in Florida). They tend to be less angry if: there is a proper fire extinguisher visible (i have an ABC on standby and a fire blanket) and more importantly the grill going with various cooked meats available.

Diesel Generator Underloading

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