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Wet Stacking a diesel generator

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immortl

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Joined
Oct 12, 2024
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34
Location
Western MT
With regard to wet stacking a diesel generator - something like a Kubota Lowboy II 11kw or 14kw generator, my understanding is the engine runs at a constant 3600rpm on these, or am I wrong on this point? if I'm wrong the premise of my question is incorrect. Is not constantly running at 3600 rpm enough to keep them from wet stacking, regardless of the electrical load from the generator portion? Based on my diesel pickup truck experiences, unless I've been doing it wrong, one kicks up the idle speed via a controller to prevent wet stacking when idling for an extended period of time. The high idle raises exhaust gas temps despite not much of an actual load on the engine and the higher temps indicated by this is enough to burn the fuel and prevent wet stacking. If my understanding is correct and the Kubota runs at a steady 3600 rpm, ought that be enough to prevent wet stacking regardless of the actual electrical load?

As I'm planning and building out my off grid solar system, I'm looking forward a bit and want to put in a decent diesel generator to primarily charge batteries and possibly also simultaneously feed the AC in on a pair of Victron Quattro 10ks. For right now I have a predator 7000 running watt non inverter gas generator that came with the property. This is sufficient to run 1 chargeverter. 11 or 14kw is likely overkill for my current electric needs today, but as I build a proper house, shop, greenhouse, assorted outbuildings etc... for the future I am thinking 11 or 14kw diesel would be sufficient to run 2 (11kw) or 3 (14kw) chargeverters and simultaneously feed some minor loads. Could end up being undersized but seems like a happy medium to start with. Starting battery storage will be 64kwh via 4 Midnite MNPowerflo16s. I foresee at least doubling if not tripling that over the next year or two. Winter tends to be fairly overcast where I am at in mountains of Western Montana.

Thanks,
Joe
 
Some articles say it is due to idle, some due to light loads. I'm not sure if running full RPM but light load avoids it or not.

With generator auto-started by inverter, so long as inverter by itself is large enough and generator is just to make up for low battery, you ought to be able to size the system to load it well.

If using lead-acid battery, a couple hours absorption is required. Best if PV can finish off the charging after generator brings it near full.
With LiFePO4 which can be kept below 100% you can just have the generator start, run at optimum load, stop.

You might want an 1800 RPM diesel.

A lot of energy being dumped as waste heat. Consider heat exchanger and use it for shop & domestic heating.
 
As you have time on your side, look for an 1800 RPM propane generator. They come up in auctions from time to time sometimes complete with an AC system and air compressor all on a nice trailer, courtesy of AT&T or Verizon etc
 
Why would you ever idle it unloaded, or run it unloaded for long periods? It will take 5 hours of running (with no ac load) to recharge your batteries.
I don't think you'll have much of a problem with wet stacking if you have a well adjusted and maintained engine.
 
If memory serves, those kubota lowboy's are 1800rpm, not 3600. They're fairly quiet as well. That was on my list when my gas gen kicked the bucket.
 
My Perkins diesel genset runs at 1800 rpm. I'd also be surprised if a diesel genset runs at 3600.

OP, regardless of rpm, the diesel needs to have a load, say 50% - 80%, to not wet stack
 
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immortl said:
my understanding is the engine runs at a constant 3600rpm on these, or am I wrong on this point?
Click to expand...
I have never seen a diesel genset running at 3600 RPMs. The Kubota's that I have seen were all running at 1800RPMs.
immortl said:
As I'm planning and building out my off grid solar system, I'm looking forward a bit and want to put in a decent diesel generator to primarily charge batteries and possibly also simultaneously feed the AC in on a pair of Victron Quattro 10ks. ...... for the future I am thinking 11 or 14kw diesel would be sufficient to run 2 (11kw) or 3 (14kw) chargeverters and simultaneously feed some minor loads.
Click to expand...
If you have 2x Quattro 10K inverters, I would just run the gen into the AC input of the Quattros. Then you can set how many amps you want to draw from the genset, and the inverters will dynamically adjust charge rate and even do gen assist if needed, to avoid overloading the gen. When configured properly, the Victrons are exceptionally responsive to surge loads, and will very quickly drop charge amps to avoid overloading the gen. As long as your gen outputs a nice steady frequency (and it should, if it's a Kubota with a decent electronic governor setup!) you really won't have any need for the chargeverter at that point.
 
Overall the intent driving my question was a general, outside of actual use cases, "theoretically does the diesel engine need a load to not wet stack or is the 3600 rpm enough?"

I will continue to research and keep an eye out for an 1800rpm generator. Per the published specs for the Kubota Lowboy II the engine rpm is 3600. I understand the principle behind 1800rpm being better than 3600rpm, but even at 3600rpm, these Kubota engines seem to be reliable and last a long time.

I don't imagine ever running it without a load - I was more thinking if the generator was oversized for my current utilization I would be pretty low in it's rated output so below the 'optimal range'. But as I think about it more, running it through the Victron to charge the batteries would put a decent load on it.
 
Wet-stacking has to do with operating engine temperature. It's not specific to low load or idling - but those are the two main causes. Any condition that keeps the engine temperature below optimal for an extended period will cause wet-stacking, even if the engine is under heavy load. The cause is when the cylinder internal temperature gets below optimal, the fuel doesn't completely burn, and is sent out the exhaust as a mist or liquid, which collects on the internals and combines with exhaust soot. Too much and things start to perform badly. And unfortunately, wet-stacking causes wet-stacking, so you really have to take care of it quickly or prevent it where possible. A small amount of it is not bad, and can be blown out by loading up the engine for "a few minutes".

So, as long as you are not running the generator without any load, or too light of a load for a long time (not sure how long is long), you should be fine. This is certainly one reason to not over-spec your diesel generator for your system. In your case, I would avoid the 11K or 14K generator until you need one. Stick with the Predator 7k and a Chargeverter until you need more than that. If you do oversize your generator now, make sure when you are running it, that it's loaded up to some percentage that keeps it running nice and hot.
 
Koyaanisqatsi said:
Wet-stacking has to do with operating engine temperature. It's not specific to low load or idling - but those are the two main causes. Any condition that keeps the engine temperature below optimal for an extended period will cause wet-stacking, even if the engine is under heavy load.
Click to expand...
That's a helpful explanation. A thermostat that is stuck open, esp. when running in cold temps is an example of a condition that can cause wet stacking. Water temp and oil pressure gauges at a minimum are helpful when monitoring performance.
 
@pvdude Is that an old screenshot? Or do you have a direct link to that generator for that price? Looks to be $9795 when I found it on Central Maine Diesel's site.
 
Semi-related…generator manufacturers will attempt to upsell you on the size. For my moms house the largest load is a 3 ton AC, and Generac wanted to spec a 18kw or 22kw unit. Would be running under minimal load most of the time.

I purchased a 14kw and did the mechanical install myself. The 14kw grunts a bit to start the compressor but has no problem running. If there was a problem I would get a soft starter.

I would suggest downsizing as much as possible to keep the load up, and add soft starters for the largest motors.

Good paper to read: The Impact Of Generator Set Underloading
 
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Delmar said:
Semi-related…generator manufacturers will attempt to upsell you on the size. For my moms house the largest load is a 3 ton AC, and Generac wanted to spec a 18kw or 22kw unit. Would be running under minimal load most of the time.
Click to expand...
Oh goodness, Generac sales people add up all the circuit breakers add 20% and sell that monumentally oversized generator even though the max load will be 30% of that...and people buy the crap out of them. Then their generators run during power outages when they're 2,000 miles away and the cabin has been closed for the winter. Oy.
 
Cmiller said:
I have never seen a diesel genset running at 3600 RPMs. The Kubota's that I have seen were all running at 1800RPMs.

If you have 2x Quattro 10K inverters, I would just run the gen into the AC input of the Quattros. Then you can set how many amps you want to draw from the genset, and the inverters will dynamically adjust charge rate and even do gen assist if needed, to avoid overloading the gen. When configured properly, the Victrons are exceptionally responsive to surge loads, and will very quickly drop charge amps to avoid overloading the gen. As long as your gen outputs a nice steady frequency (and it should, if it's a Kubota with a decent electronic governor setup!) you really won't have any need for the chargeverter at that point.
Click to expand...
my denyo which is a 10kw welder genset turns at 1800 but my friends 8kw denyo turns at 3600 the biggest difference is his is a straight 240 split phase genset and has the 2 cylinder kubota where as mine has the 3 cylinder kubota, so a part of it boils down to the size of the diesel engine and genset.
 
Daddy Tanuki said:
generac would be the last gen set I would ever buy there are more horror stories about them then any other stationary genset on the market.
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Early on as I started my research I was thinking a Kohler propane generator and I've read your posts in other threads about Generac and other not great posts elsewhere about Generac. They never made it to the list.

Learning to weld has been on my list of things to learn. My brother learned 2 years ago and has helped me out with a few things. With this new property I'm realizing I really need/want to learn how to weld. What makes a genset a 'welder genset'? That is something I ought to work into my research and consideration.
 
immortl said:
Early on as I started my research I was thinking a Kohler propane generator and I've read your posts in other threads about Generac and other not great posts elsewhere about Generac. They never made it to the list.

Learning to weld has been on my list of things to learn. My brother learned 2 years ago and has helped me out with a few things. With this new property I'm realizing I really need/want to learn how to weld. What makes a genset a 'welder genset'? That is something I ought to work into my research and consideration.
Click to expand...
its designed so that it has terminals to hook up welding leads to, or you can use the output side to power loads so it has connections for both. usually they are three phase though so it takes some mental gymnastics to figure out how to best use them in an offgrid setup. if the genset is big enough you can run three fo the chargverters to charge batteries and then set them using a clamp amp meter to ensure that you are not exceeding the gensets capability, or you can use tow fo the chargeverters as i did, one running 208/240 the other running 120. L1, L2 to the 208 and then L3 and neutral for the 120 the 120 unit of course will need to be set at 50% or lower per the manual. i use a clamp meter to balance all three leads to ensure that I am pulling equally. that or buy a dedicated 3 phase charger. but its kind of difficult to feed 3 phase to a house as US 240 is actually "split phase" where each sine wave is 180° out of phase with each other. 3 phase the legs are 120° out form each other so even though the voltage is 220 or so, it consists of two legs 120° apart with a bit dead spot int he middle, not good for some things, OK for others.

i have seen smaller welder gensets that put out 240 split phase vice 3 phase you would be more apt to find a 240 split phase welder generator in the states than here in japan as while they have split phase here its not as ubiquitous as it is in the states.

the yellow circle shows 4 terminals for 3 phase power, the red circle is 4 leads for two welders to use simultaneously
 
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gotta try again cannot get the photo to attach, hre look at this webpage form denyo: https://www.denyo.co.jp/english/products/welder/


4 terminals on the left are for the welding cables for two (2) people to weld at same time. the four small temrinals labeled U V W O is the connections for three phase power. U, v, or W + O is 120 single phase, U + V is 208 120° single phase 200, V+W is same, use all three to access 3 phase power.
 
SparkyJJO said:
3600 RPM sounded high for diesel to me (but what do I know).
Click to expand...
Lots of small diesels can run over 3000 rpm. Vw diesels redline 4200rpm. At least the older ones did.
5.9 Cummins run 2800rpm in certain applications... guy are pushing them over 4000rpm.

But any small diesel usually will rev pretty high, fyi those small diesels a usually very high compression like 20-24:1 and often indirect injection. One reason why they are good on fuel
 
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