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Old 05-08-2020, 10:42   #1
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Are low rpm's hard on a Diesel engine?

Once in a while l fish/troll using a down rigger at two knots which is around 850 rpm's. I am wondering if motoring this slowly is likely to have a detrimental effect on the engine, as I've heard that diesels last longer operating at higher rpm's. My usual engine speed when motoring is around 1800 - 2000 rpm. Should l rev the beast any higher than this when underway?

Thanks in advance,

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Old 05-08-2020, 10:53   #2
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Re: Are low rpm's hard on a Diesel engine?

I know in my yanmar manuals it says that if you're going to run at low speed you should put it in neutral every 30 minutes and rev to max 5 times in a row. Presumably this is to blow anything that might be building up somewhere in the exhaust out.

Generally speaking, I like to run my engines at 75-80% and occasionally I'll max them out for 5 minutes just to run them hard. I've always been told by old timers that this is the way to do it, and I believe them.

Full disclosure- I'm not a mechanic, this is just how I do it and I've got many thousands of hours on small diesels without a failure.
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Old 05-08-2020, 11:49   #3
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Re: Are low rpm's hard on a Diesel engine?

I have run my Yanmar 4JH3E at 1800-2400 rpm for 2500 hours. Every few hours I run the engine up to 3000+ for 30-60 seconds.

This thread will become endless "I hear, I was told, I read" posts.
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Old 05-08-2020, 12:13   #4
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Re: Are low rpm's hard on a Diesel engine?

Replies are appreciated. Thank you for your expertise and patience.

I just wanted information, not to unleash endless threads. So sorry to have tweaked the dragon by asking a question. Next time I'll ask Siri, not CF.

Aloha.
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Old 05-08-2020, 12:54   #5
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Re: Are low rpm's hard on a Diesel engine?

seems an overreaction since you did in fact get "answers"

but a thread search would get you pages and pages
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Old 05-08-2020, 13:05   #6
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Re: Are low rpm's hard on a Diesel engine?

2000 is a great RPM for my diesel. listen for the engine to smooth out and go to a steady happy speed. my old yanmar was happy at around 2400 RPM.
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Old 05-08-2020, 13:30   #7
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Re: Are low rpm's hard on a Diesel engine?

Some people say that if you run the engine at low rpm, the combustion is incomplete and deposits build on the cylinder walls. I personally think it has to do with whether the engine reaches design operating temperature which for fresh water cooled diesels is controlled by the thermostat. There are millions of Diesel engines in cars that typically run at 15-20% of rated power on the highway and it is never a problem. If you have a salt water cooled diesel then you may not be reaching optimal temp at low rpms. That could be a problem but salt water diesels are a problem anyway
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Old 05-08-2020, 13:39   #8
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Re: Are low rpm's hard on a Diesel engine?

Quote:
Originally Posted by partingclouds View Post
Once in a while l fish/troll using a down rigger at two knots which is around 850 rpm's. I am wondering if motoring this slowly is likely to have a detrimental effect on the engine, as I've heard that diesels last longer operating at higher rpm's.
I'll just leave this here

Quote:

To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less.


https://www.sbmar.com/articles/low-s...arine-diesels/

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Old 05-08-2020, 13:58   #9
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Re: Are low rpm's hard on a Diesel engine?

It’s improper loading that’s more detrimental than anything else, but even that is questionable, especially if an engine is warmed up and run enough to clean out after hours of low load.
Best example I know of was OTR Trucks were routinely idled overnight, pretty much every night, and all night, and they lasted decades and excess of a Million miles wasn’t unheard of.

US Army routinely runs everything they have weekly during “motor pool” for a half to an hour or so, and then it’s shut down. This goes on often for long times.
A study was commissioned years ago to determine the extent of damage that was being done and pretty much concluded it wasn’t harming the engines, yes they woud build up oil and unburied fuel in the exhaust and this caused “stacking” or “slobbering” but it wasn’t harmful, just made a mess.

Every now and again I find it to post a link, here it is
https://apps.dtic.mil/dtic/tr/fulltext/u2/a151273.pdf

The screen shot is of conclusions page.

My personal opinion / observation is if you follow Yanmar’s recommendations you will be fine, and likely all your doing then is cleaning out the exhaust so when you do finally shower down on the power it doesn’t smoke like a train for a minute, but even if does, it’s not harming anything, but smoke is upsetting so blow it out every now and again and prevent the smoke.
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Old 06-08-2020, 08:52   #10
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Re: Are low rpm's hard on a Diesel engine?

By constantly going at the same RPM for long periods, you are actually scoring the inside of the cylinder. So, it is a good practice to occasionally vary the RPM to avoid this. Learned at the Mack Boring class for Yanmars a number of years ago. Also, after going very slowly into a marina and docking, I always rev the engine in neutral 3 times before shutting it down, to clear out any carbon build-up.
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Old 06-08-2020, 08:54   #11
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Re: Are low rpm's hard on a Diesel engine?

Cummins (for a 6BTA 5.9 engine) says to keep the temperature up in the normal range to prevent unburned fuel in the cylinders. They recommend not idling the engine for more than 10 minutes without a load because temps will drop. As long as you have enough load on the engine to keep the operating temperature up to engine specs you should be fine.
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Old 06-08-2020, 09:00   #12
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Re: Are low rpm's hard on a Diesel engine?

This question always get two camps to answer, and the usual CF argument; Yes and No.
I can tell you this, commercial diesel makers rate the same engine at different RPMs. A pleasure yacht engine may be rated for use at 3400 rpm, and a "continuous duty" use of the same block may be rated to run at 2400 rpm. That should tell you something. But there are other factors too...
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Old 06-08-2020, 10:01   #13
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Re: Are low rpm's hard on a Diesel engine?

Quote:
Originally Posted by sailing_gal View Post
By constantly going at the same RPM for long periods, you are actually scoring the inside of the cylinder. So, it is a good practice to occasionally vary the RPM to avoid this. Learned at the Mack Boring class for Yanmars a number of years ago. Also, after going very slowly into a marina and docking, I always rev the engine in neutral 3 times before shutting it down, to clear out any carbon build-up.
???
Did Mack Boring explain this claim? I can't think of a single reason for that to be true. If it was, every Genset ever made would suffer cylinder wall/liner scoring in every cylinder.
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Old 06-08-2020, 10:03   #14
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Re: Are low rpm's hard on a Diesel engine?

Quote:
Originally Posted by a64pilot View Post
It’s improper loading that’s more detrimental than anything else, but even that is questionable, especially if an engine is warmed up and run enough to clean out after hours of low load.
Best example I know of was OTR Trucks were routinely idled overnight, pretty much every night, and all night, and they lasted decades and excess of a Million miles wasn’t unheard of.

US Army routinely runs everything they have weekly during “motor pool” for a half to an hour or so, and then it’s shut down. This goes on often for long times.
A study was commissioned years ago to determine the extent of damage that was being done and pretty much concluded it wasn’t harming the engines, yes they woud build up oil and unburied fuel in the exhaust and this caused “stacking” or “slobbering” but it wasn’t harmful, just made a mess.

Every now and again I find it to post a link, here it is
https://apps.dtic.mil/dtic/tr/fulltext/u2/a151273.pdf

The screen shot is of conclusions page.

My personal opinion / observation is if you follow Yanmar’s recommendations you will be fine, and likely all your doing then is cleaning out the exhaust so when you do finally shower down on the power it doesn’t smoke like a train for a minute, but even if does, it’s not harming anything, but smoke is upsetting so blow it out every now and again and prevent the smoke.


This condition is also called “wet stacking”. It frequently occurs in diesel powered generators when the genset is run at much less than full load. It is corrected by installing a load bank on the genset output and running it at full load until the slobbering clears up.

I think it’s more related to incomplete fuel combustion at less than full load than low engine temperature. The thermostat should keep the ten relatively constant if your engine is so equipped.

Many diesels run best and most efficient at a certain rpm/load and fuel delivery. Get much below that and the unburnt oil turns to goo and collects on rings, grooves and cylinder walls. This is them pushed through the exhaust ie slobbering.
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Old 06-08-2020, 10:16   #15
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Re: Are low rpm's hard on a Diesel engine?

If you run at normal cruise speed out to the fishing grounds, troll for a few hours and then run at normal cruise speed back to port...don't worry about it.

The runs in and out will get the engine up to operating temp and burn any deposits out.

Where something like this could be problematic is a cruiser living for months at anchor only running the engine for an hour or two just to charge the batteries resulting in very low load and the engine never getting up to full operating temps. Even then if they motor the boat around for an hour or so each week, even that is unlikely to cause any issues.

I would be willing to bet 95% of cruising boat engines die from lack of maintenance and corrosion as opposed to some sort of wearing out process.
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