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Old 02-11-2015, 07:25   #1
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Light Loading of Diesels -- How Harmful?

I ran my main engine for about 40 minutes during the weekend with no load other than my newly fixed alternator (pulling a few horsepower), at a high idle speed of about 1200 RPM.

Before shutting it down, I ran it up under load (as the manual says to do), and was amazed that it emitted billows and billows of thick smoke. This doesn't seem good at all -- wet stacking?

My Yanmar 4JH3HTE is very smoky anyway. Has always been. I almost didn't accept the boat because of the smoke. Finally did, after a crack surveyor told me that the engine was in good condition and that this smokiness is sometimes characteristic of Yanmars. Meanwhile I had made the sellers send in injection pump, turbo, etc. for overhaul. I have since even changed the injectors, but nothing ever changed -- in more than 1500 hours since then, the engine has run perfectly, starts instantly, and -- smokes. It's the best starting diesel engine I've ever encountered -- even in below freezing weather, it starts in apparently less than a revolution. I've never used the air preheater. That's supposed to be proof of good compression.


But these billows were far beyond the usual smoking. It cleared up after a few minutes of running hard under load.

So, WTF?

I guess there just can't be any doubt that these engines don't like being run like this.

However, the U.S. military did a very extensive study, and claim that wet stacking, while annoying, is not actually harmful:

http://www.dtic.mil/dtic/tr/fulltext/u2/a151273.pdf


Any comments? I don't really know what to think.
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Old 02-11-2015, 07:46   #2
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Re: Light Loading of Diesels -- How Harmful?

Not so sure how valid that test is, I believe it was conducted at idle speeds between 1800 and 2000 RPM, or full operating RPM of a genset, just at little to no load?
Your not going to kill a Diesel from slobbering or wetstacking right off, but I do believe it's bad on one, can lead to bore glazing, which is different than bore polishing BTW in that bore glazing may can at least be partially recovered from.
Your not going to hurt one from occasional low load running it, but if you get up every morning and run it a couple of hours at 1000 RPM or so to charge batteries and do this extensively, then yes I believe it is bad on the engine.
Every Military truck there is gets run weekly in the motor pool for a half hour or so, rarely do they ever move, and they aren't dead yet, but I don't think it's good for them.
I did something stupid when I was a kid and left our International 500 Bulldozer running at idle over the weekend. It's a little 3 cyl a lot like our boat motors, anyway it smoked like the devil, even had liquid spray out of the stack, but once that burned off it was fine.
I've been told that lots of Diesels up in the Great White North spend extended times at idle, they don't turn them off. I believe it shortens their life, but apparently doesn't kill them right away.
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Old 02-11-2015, 07:56   #3
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Re: Light Loading of Diesels -- How Harmful?

Oh, on Diesels the "oil cooler" is often more of an oil heater, it helps to get the oil up to 180F, the temp usually accepted as what is required to cook off the moisture. Even turbo motors, the turbo at lower RPM and load isn't putting much heat at all into the oil.
Point is Diesels at lower RPM and light load just don't generate that much heat. I believe a lot of that is because a Diesel doesn't have a throttle valve, each cycle it draws in a full cylinder of air, but burns only a tiny bit of fuel, so not much heat.

I have a pyrometer in my Duramax Diesel and at idle I believe the exhaust temp in the manifold is only about 250F or so
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Old 02-11-2015, 08:24   #4
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Re: Light Loading of Diesels -- How Harmful?

Quote:
Originally Posted by Dockhead View Post

Any comments? I don't really know what to think.
Continue to run your engine the way you are and the way the manual says to and stop worrying about. This is just the way it is and the only way to not have it happen is to never run the engine at idle, which of course isn't practical.
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Old 02-11-2015, 13:58   #5
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Re: Light Loading of Diesels -- How Harmful?

Hi Dockhead,

The military study you linked seems thorough. Its summary states that they found no instances of diesels run at high idle being damaged or suffering reduced life (directly attributable to the high idle).

I like the reports recommendation to increase intake air temperature. Is there a way that you can do this? On my installation, to address this same issue, I routed my engine air intake to the top of the engine compartment. It is hot enough up there to help make up for the lower temps that go along with low power operation.
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Old 02-11-2015, 14:51   #6
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Re: Light Loading of Diesels -- How Harmful?

Hey Dockhead , we have the same motor , mine being the 75 horse version. I agree with you about the easy start , wow did I push the button ? Never had any black smoke from mine , I get the usual white unburned fuel smoke at idle but under load or even if i rev it with no load , no black smoke ? Seems strange you are getting that I have roughly 3000 hrs on my Yanmar . I have a yanmar genset as well , non turbo 3 cyl . it does not start up as the other one does but two or three rotations of the crank and she sets in at 1800 rpm . This little motor runs without any white or black smoke at all , mind you it only has 120 hrs on it. But so far it has been super clean running .

Maybe has something to do with your exhaust track , back pressure , to many bends ? Hmmm mysterious.

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Old 02-11-2015, 15:23   #7
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Re: Light Loading of Diesels -- How Harmful?

Funny thing about the Mil test was that 60KW and larger generators had load banks on them, great big resistance heaters to load the engine when it wasn't being used to run an electrical system or something.
But Yanmars have the reputation of being smoky, mine nasties up the transom, I hope that will go away now that I had all the nozzle tips replaced, but they are I believe not known to be clean burning engines.


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Old 02-11-2015, 15:27   #8
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Re: Light Loading of Diesels -- How Harmful?

Aircraft, older cars when they had carburetors ran their air over the exhaust manifold to heat it and prevent carburetor ice.
But the rest of the world is going for cold air induction as the colder the air, the more dense. And if you have a turbo, trust me, the intake charge is well heated, trying to cool it down is what an intercooler is all about.


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Old 02-11-2015, 15:34   #9
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Re: Light Loading of Diesels -- How Harmful?

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Originally Posted by typhoon View Post
Hey Dockhead , we have the same motor , mine being the 75 horse version. I agree with you about the easy start , wow did I push the button ? Never had any black smoke from mine , I get the usual white unburned fuel smoke at idle but under load or even if i rev it with no load , no black smoke ? Seems strange you are getting that I have roughly 3000 hrs on my Yanmar . I have a yanmar genset as well , non turbo 3 cyl . it does not start up as the other one does but two or three rotations of the crank and she sets in at 1800 rpm . This little motor runs without any white or black smoke at all , mind you it only has 120 hrs on it. But so far it has been super clean running .

Maybe has something to do with your exhaust track , back pressure , to many bends ? Hmmm mysterious.

Regards
I have never had any black smoke with my 4JH3 HTE. It's white unburned fuel smoke. I bothers me.

Funny you should mention the generator. Me too! My experience is exactly like yours. It's a 1000cc 3 cylinder Yanmar 3TN, and it does not start easily. It always starts, but unlike the main engine, sometimes needs some cranking. But it doesn't emit the slightest bit of smoke of any kind. Go figure.

By the way, neither Yanmar burns any oil.
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Old 02-11-2015, 17:01   #10
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Re: Light Loading of Diesels -- How Harmful?

Colder intake air can increase max horsepower, no doubt about it. But Dockhead is trying to get clean burning at low horsepower output. In that type of operation both diesel and gas engines do better with heated intake air. The hotter intake air helps vaporize the gas in a gas engine. In a diesel it increases the combustion chamber temperature which can give a cleaner, better burn. Blown gas engines also need to guard against detonation, and colder intake air helps a lot there. But on a diesel this is a non-issue, and aftercooling is only needed when horsepower output is being pushed to the level where charge density becomes important. Usually, on the kind of engines we run in sailboats, this is not an issue.
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Old 02-11-2015, 17:31   #11
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Re: Light Loading of Diesels -- How Harmful?

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Colder intake air can increase max horsepower, no doubt about it. But Dockhead is trying to get clean burning at low horsepower output. In that type of operation both diesel and gas engines do better with heated intake air. The hotter intake air helps vaporize the gas in a gas engine. In a diesel it increases the combustion chamber temperature which can give a cleaner, better burn. Blown gas engines also need to guard against detonation, and colder intake air helps a lot there. But on a diesel this is a non-issue, and aftercooling is only needed when horsepower output is being pushed to the level where charge density becomes important. Usually, on the kind of engines we run in sailboats, this is not an issue.
???

I'm trying to figure out what this means.

Charge density always matters, if I am not deeply mistaken. The whole point, in fact, of volumetric efficiency.

And sailboat engines with aftercooling are not so uncommon. My engine has an aftercooler, sea water cooled. Turbocharging without aftercooling makes much less sense, since the mass of air inducted into the cylinders is compromised by elevated temperature. More pressure doesn't do as much good, if it comes with higher temperature. Increasing the mass of air in the cylinders is the point of turbocharging, and aftercooling makes it actually work.


In the early '80's, I owned a Ruf-tuned 930 Turbo -- a 3.0 liter, one of Alois' first ten cars. It was lime green metallic, but I digress. With no intercooler (aftercooler). All of that Ruf magic -- including his famous Dampfrad -- or manual boost regulator -- was worth nothing compared to the giant boost in volumetric efficiency, and hence, horsepower, which came with the intercooler of the 3.3. Which I understood, to my disappointment, when I drove a bone stock 930 3.3.

If you're going to have a turbo, you might as well go the whole hog and have an aftercooler too.
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Old 02-11-2015, 17:44   #12
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Re: Light Loading of Diesels -- How Harmful?

This thread is directed at running an engine better at a LOW horsepower output, as per the title - Light Loading of Diesels. At high horsepower levels charge density in the cylinder is a primary factor in torque. Aftercooling of course increases charge density. But it is of no advantage when the engine is being run at a small fraction of its rated output, which is what you described in the original post. When running a diesel under light loading it is advantageous for the intake air to be hotter.
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Old 02-11-2015, 17:52   #13
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Re: Light Loading of Diesels -- How Harmful?

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This thread is directed at running an engine better at a LOW horsepower output, as per the title - Light Loading of Diesels. At high horsepower levels charge density in the cylinder is a primary factor in torque. Aftercooling of course increases charge density. But it is of no advantage when the engine is being run at a small fraction of its rated output, which is what you described in the original post. When running a diesel under light loading it is advantageous for the intake air to be hotter.
No argument. But turbo diesels don't even produce any boost, if there's not load on and only a small amount of fuel being injected. So you don't even get to the question of aftercooling under those conditions.
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Old 02-11-2015, 18:01   #14
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Light Loading of Diesels -- How Harmful?

Instead of trying to modify an engine for hot air intake etc., just don't run the thing at low load.
If you have no other means to charge the batteries, get a Honda, or put it in reverse and let it tug against the anchor.
Dockhead I believe has a built in genset, so no reason to run at low load, except maybe motorsailing with an autoprop, if then just turn the wick up a little to prevent stacking.

I really think it's only an issue for those that run them every day to charge batteries and nothing else.

Hey, at least we know they are after coolers and not intercoolers That says a lot

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Old 02-11-2015, 18:16   #15
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Re: Light Loading of Diesels -- How Harmful?

Well, actually you do a bit. Although the turbo isn't making any boost, as you say, the intake air is still going thru the aftercooler and so any attempt to heat the intake air (to improve the light loading combustion) would be negated by the aftercooler. It's hard to provide hot air to the engine when there's a cooler installed in the intake tract.

If you were of an experimental frame of mind it might be interesting to disconnect the aftercooler water lines, provide a source of hotter intake air, and see how this affects the lightly loaded running condition of your engine. I would expect it to run cleaner.

PS - I got into engineering originally because I was building racing engines and wanted a better grasp of the theory behind engine dynamics and design. I do understand high horsepower. I have a bike in the garage that runs mid 11's and is a ball on a twisty road. I bet you had a lot of fun with your 930. Sweet cars.
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