Wet stacking, RPM, I wonder aloud too much

[OverkillTASF]

I know it has been asked and discussed a lot, but I'm curious... The guideline I mostly recently saw was something like "Make sure to periodically run at at least 80% load".



Given the above power curve... and some engines this may be even more pronounced... If someone wanted to run the engine for maximum fuel efficiency, they would run it at around 2600 RPM. Max power is 3600 RPM. Which yields:

Max fuel efficiency, 2600 RPM, 4.2HP output. 3.36HP load or more to prevent wet stacking.
Max power output, 3600 RPM, 4.9HP output. 3.92HP load or more to prevent wet stacking.

Now the "simple" question: Is wet stacking about the engine's max possible output, or just about the max possible output at a given RPM? Or is it about something else altogether, and the 80% guideline doesn't tell the whole store?

If I were designing a generator where noise and fuel efficiency were my primary concerns, and I could choose to run this engine at 2600 RPM, if I mated it with a proper generator or alternator head that could make use of its full 4.2HP at that RPM, I wonder if I'd have to run it at 3.92HP or 3.36HP load "periodically".

 

[gimpyrobb]

Your post is kinda vague since no motor is listed. Most wet stacking occurs on water cooled engines, but can happen with air cooled. Also, many gensets are rpm dependent for Hz.

Got any more specifics?

 

[TNriverjet]

Don't these MEP engines only run at 1,800 RPM? At idle the system provides more fuel than is really needed to keep the engine running. This accounts for sudden loads and helps the governor keep the engine at a constant RPM when loads switch on and off. I think under a loaded condition, the fuel is combusted more completely, AND the engine moves more air through the system. You can hear and feel the air pressure changes at the exhaust under different loads. This increased exhaust pressure can even dislodge previous deposits and propel them into the air over your site. My first few runs of my 802A yielded carbon deposits in my hair that were found much later that day.

Disclaimer: I'm not condoning the measurement of exhaust pressure with your bare hand...

 

[OverkillTASF]

Got any more specifics?

That particular chart is for one of the Yanmar L70 sub models. I wonder if it is more rare on air cooled engines because they're generally smaller capacity and easier to "fully load". Also I assume they run hotter... But, I'm mostly curious in general!

Don't these MEP engines only run at 1,800 RPM?

Probably the bigger ones!

TNRiver, that sounds like the intuition I had... under load at a given RPM, there shouldn't be leftover fuel if it's working hard unless the mix is too rich... But with simple injectors and fuel metering, you can't really tweak fuel based on load, only for a given RPM? So of course you want it to have enough fuel for the max power output at that RPM... which means at lower power outputs you end up piling on too much fuel which also cools things off... I FEEL like with wet stacking, it's the max power output at a given RPM that matters; Not the max power output of the engine itself... but maybe this is bad science. Check exhaust pressure with hand, got it.

 

[JRM]

https://www.youtube.com/watch?v=4ApPm-LFzrM

 

[Poccur]

I would say that smaller engines are less likely to wet stack as they tend to spin at higher rpm and run hotter....60Hz, two pole (usually smaller) gen generally spins at 3600 rpm....whereas 4 pole (usually larger) generally spins at 1800rpm...
Revving an engine quicker increases the temperature in the piston so wet stacking is less likely to happen or cleared...loading a generator makes the pistons hotter too but in the absence of load, and if you are not worried about frequency, revving the engine can help clear it....

With the larger engines I use I work on 40% to 50% of max capacity as the minimum loading to ward off wet stacking... and if you start to see it forming, load the generator up and sweat it out....

 

[OverkillTASF]

if you start to see it forming, load the generator up and sweat it out....

But... this is the crux... If in its application it's being run at a lower RPM, do I need to "load it" based on the HP output at that RPM, or at the HP output of the max power RPM?

Also, hello over there in Roanoke!

 

[Poccur]

Hello back from Roanoke..

I would (respectfully) say you may be over thinking it....it is a function of the minimum exhaust temperature that the engine is designed for. Below that point the engine starts to wetstack....whatever load gets you to the temperature at the desired running speed is the right setup.

As for designing a generator to run at 2600rpm...to what end? What frequency are you aiming for? If you are chasing 60Hz it is typically a multiple of 1800 for your RPM...

 

[OverkillTASF]

Mostly thinking of either a belt driven generator, or running an a/c compressor, or rectifying the 3 phase AC to DC for battery bank charging. You know...

I do tend to overthink things... But without access to Yanmar's engineers to give go/no-go on hare-brained schemes, I'm left trying to decode conventional wisdom. So now you've got me wondering what the minimum acceptable exhaust temp should be...

 

[Poccur]

what is your engine?

 

[OverkillTASF]

Yanmar L70 in this example.

 

[Poccur]

Are you working with a MEP 016?

 

[OverkillTASF]

I've got a few 817A's, one (hopefully just one) with a bad inverter... so began my wondering.

 

[Poccur]

Not familiar with 817..

 

[OverkillTASF]

317A. Oops. On a conference call. Heh.

 

[OverkillTASF]

831A. I am smoking crack. 317A is a road I used to ride my dual sport on.

 

[Poccur]

A see a little 1.5kW resistive load bank in your future

 

[Daybreak]

Howdy,
First - you should put some information in your post.

example; I have a MEP-831A that has 10 hours on it. Does a generator like this with a Yanmar L70 engine have wet stack issues? Is there a way to run this unit as to not have wet stack issues?

Has any other member had issues with long running a MEP-831A?

**************
And you also search the forums for others which talk about the MEP-831A

Just remember this, it is a diesel motor. If it starts to wet stack, run long and hard a while to clean it up. OK, your good.

 

[OverkillTASF]

My question wasn't about the MEP 831A or L70 engines specifically, but about the phenomenon of wet stacking in general. I used the power output chart from the L70 simply as an example. Kind of like asking about how addition works, providing am example of 2+5 and someone saying "7, duh!"

 

[Daybreak]

Howdy,
I am puzzled.
Because I still have no idea what your looking for.