Generator load

Greetings,
I've read many times that it is not a good idea to run a generator without a load. How long is too long to run one unloaded? 5 min.? 1 hour? or? How little load need one apply?
Thanks.

I don't really know but my boat has a 18 kw that came out of another boat don't think that with everthing. On I could get anymore that 50 present load I wouldn't worry to much about that

Read the owner's manual would be my advice. The key word is "no". My old Onan's manual was insistent to avoid running with no load, however, is even more insistent to run for three to five minutes with no load before shutting it down.

With my 5KW gen-set, especially nearing the end of a battery charging cycle, I load it up with an electric space heater. If it's a warm day I just sit it out in the cockpit. Also when needed I turn on the electric hot water heater, together they add about 2,500 watts of load.

SteveH

Running for 5-10 minutes should not be a problem. But running for an hour or more does become a problem.

Generators are unique because they run at a constant rpm irrespective of the load. When a generator is running with no load, it is sucking in the same air volume as with 100% load, so it will stay cool at no or light load with all of that air blasting through.

The cool running is what causes the problems combined with rings not sealing well at low cylinder pressures. Diesel doesn't burn completely at low temps and runs down the cylinders diluting the oil and glazing the cylinder walls. Partially burned fuel causes soot build up in the head and exhaust manifold.

But running a genset at 50% or greater load after an hour of no load should quickly clean up any problems. I suspect it takes hundreds of hours at no or light load to cause any damage that can't be cleaned up after a couple of hours of heavy load.

Tony Athens once said over on boatdiesel that his treatment for gensets that run lightly loaded all season in the Caribbean (or the Sea of Cortez in his area) is to hook up an 80% load and run it all day. That usually cleans them up.

David

According to my Onan / Cummings dealer 25% of full load was the Onan minimum requirement. As already mentioned, glazing of the cylinders, soot build up and unburned fuel wicking past the rings into the lube oil were all issues.

So I built a dummy load for my generator. Ended up building a raw water distribution box that the generator draws off of. Included in the box are two 1,000 watt stainless immersion heater elements (truck oil pan heaters). The elements will be wired to the breaker panel and allow me to add 1 or 2 thousand watts of load to the generator as needed. Elements are cooled by raw water flow going to the generator, and have high temperature shutoffs in the event that I try to use them on shore power while the generator isn't running. Will post some pics when I finish the last of the install. Know this was way over the top, but for a few hundred $ in materials and a bunch of my time, I have a variable dummy load for generator to eliminate this potential problem. Besides, like most things on my boat, I had fun engineering and building it.

Ted

I have worked on many gensets over the last 20yrs, and only ran into two cases of glazed cylinders, both Northern Lights about 20kW. Complaint was high oil use. That's it out of many hundreds of other problems.

I don't think it is preferred to run them unloaded, but in my experience it is not a big deal.

In the winter I often leave mine on overnight for heat. The load is pretty light. In the morning I load it up with water heater and air compressor, it blows a little soot and then clears up. About 5000hrs on the unit, 5kW NL.

I think it is most important to load it up when new. That was not done on the ones that actually got glazed cylinders.

FWIW. In running large nuclear power plant DG's the low load concern was the buildup of carbon in the exhaust and cooling down the lube oil too much in the winter. The turbo wheels were the size of car tires. If we were less than 25% load for >4 hours, then we had to load it to >75% for at least one hour to burn the carbon off.
110F was minimum temp for the straight 40 weight oil to allow the engine to start in 10 seconds and fully load to 4400 Kw in less than 130 seconds reliably. Lube oil heaters were required.
We always loaded them and unloaded them gradually during routine maintenance testing with the exception of loss of power tests.

Greetings,
Thanks all. In anticipation of heading south I've been running the 3 diesels on a daily basis. Just wanted a general idea of how poor my practices were.

I wonder what percentage of diesel engine failures are due to underloading, not a very big number I think.

Greetings,
Mr. S. "... underloading, not a very big number I think." Uh-oh now ya gone an' dun it...

I accidentally ran my Onan MDKD 6.5 for several hours no load, not realizing the compressor unit for the fridge and separate freezer had died. It emitted blue smoke and using oil, presumably the bores were glazed.
I was able to reverse that by adding a doubled dose of snake oil (Nulon Heavy Duty Diesel Engine Treatment, Nulon being a large supplier of lubricants in Australia), as advised by Nulon, to an oil change. Scoff at the remedy by all means, but please remember it worked. I continue a standard dose each oil change, as insurance against unwitting underloading.

High Wire said:

In running large nuclear power plant DG's the low load concern was the buildup of carbon in the exhaust and cooling down the lube oil too much in the winter.

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When we bought the boat, proper generator loading wasn't explained to us very well and while there was no damage the engine, after a couple of seasons we started experiencing high temp shutdowns. We discovered the carbon buildup inside the exhaust elbow had mostly blocked off the water injection flow into the exhaust riser. Some serious scraping, grinding and a new gasket solved the problem. We now run one of the heaters, any tiime the generator is running and haven't had that problem since.

DEATH from under loading??

As noisemakers on small boats simply get replaced , not sent out for rebuilding, how would an owner know if it died from high time and was simply worn out , or killed with poor operating practices.?

Most are decades old , and few think they didn't get their moneys worth , from a used unit that might be 30 years old.

running gennies and main engines at light or no load before shut down is important to allow cool down. Idling into the dock takes care of the mains but just shutting down the genny after it has ben under heavy load will burn the lube oil in the turbo.

bayview said:

... but just shutting down the genny after it has ben under heavy load will burn the lube oil in the turbo.

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Turbos on marine recreational generators? We've had 3 different generators and none have had a turbo.

Larry M said:

Turbos on marine recreational generators? We've had 3 different generators and none have had a turbo.

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Some Kohlers in the 15-20kW range have turbos. Tiniest turbo you will ever see!

Ski in NC said:

Some Kohlers in the 15-20kW range have turbos. Tiniest turbo you will ever see!

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I was going to ask the same question. Then I saw who posted and changed my mind.

Sent from my Nexus 4 using Trawler

Well, I have a Onan 15KW MDJF converted to radiator cooling on a trailer, that gets used for standby power during outages. It has been known to run at NO LOAD for 7 days straight. That's 168 hours. It regularly runs for 24 hours a day and is very lightly loaded for maybe 8 of those hours. Mostly it keeps the computer running in my Brother in laws office when the power is out. The oil is always clean and clear. No smoke, starts instantly,no problems whatsoever. But then it only had about 5000 hours on it when we converted it, and we've maybe used it another 1000 hours,It gets loaned out amongst family a lot, and there is a 225 Lincoln welder and a cutting torch on the trailer so its kinda popular. When used for welding the power draw is sporadic at best, maybe 90% no load then a small load for awhile. I expect that it will only last maybe another 10,000 hours. Running with no load is tuff on them.

bayview said:

running gennies and main engines at light or no load before shut down is important to allow cool down. Idling into the dock takes care of the mains but just shutting down the genny after it has ben under heavy load will burn the lube oil in the turbo.

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A turbocharged genset?
The Onan MDKD (normally aspirated) Manual prescribes 5 minute no load operation before shut down. I keep a timer at the helm to remind me when time is up.

caltexflanc said:

Read the owner's manual would be my advice. The key word is "no". My old Onan's manual was insistent to avoid running with no load, however, is even more insistent to run for three to five minutes with no load before shutting it down.

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My Westerbeke also says to run for 5 minutes with no load to allow it to cool down.