Are low rpm's hard on a Diesel engine?

[FPNC]

There are untold numbers of quality small diesel engines used in generators that are run continuously at 1800 RPM for 60 Hz AC power. They never operate at any other RPM. These generators rack up many thousands of hours. The issue is more about a quality engine than what every diesel engine needs. If a diesel cannot be run a low RPM's without causing an engine failure, that is a design problem.

[Shaneesprit]

Originally Posted by partingclouds

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,

Partingclouds

Hi I always see a lot of opinion and both good advice and misguided opinions on these posts.
What I have to say is not oh I think this or that, but the actual advice that’s relevant - I will try and answer some of the other answers I read in the replies.

nce in a while l fish/troll using a down rigger at two knots which is around 850 rpm's. oK well this is detrimental to your Engine how detrimental depends on many factors
1800 - 2000 rpm. Should l rev the beast any higher than this when underway? YES
Having said what your doing is detrimental there is no reason to stop doing it, you just control how long and use a standard method for cleaning up the Engine afterwards.
So first cleaning up the Engine - basically 20mins to an hour on hard load depending on how long since you have done this.
Why is it detrimental, well that’s to do with the operating temperature of the Engine.
What’s the operating Temperature? Well it has NOTHING to do with coolant temperature! It’s the combustion Temperature!
OK but you can’t measure the combustion temperature, well you can depending on Engine type but there is little need. Yes it helps to detect early problems but it’s expensive.. so we rely being on high load.
So what is high load? Well it has NOTHING to do with RPM although you do get access to different amounts of power at different RPM..
so how do you know if your on High load if 3000RPM can be without load? Well your in gear and travelling well - assuming your prop is reasonable then you will be fine - let’s not also get into Props.
A Note if you can’t reach Max RPM underway then you have either reached Max load or you have other issues.
If you really want to measure loads the easiest way is to install a manifold pressure sensor.
So what is detrimental about low load? Well an Engine is designed to work at 500C roughly speaking, so if it’s not reaching this temperature then, your oil maybe be to cold, oil also need to be hot, your pistons do not seal the gap in the cylinder because the rings don’t expand, diesel collects on the cylinders and runs into the oil, now you have fuel in your oil and the bore becomes glazed.
In the worse case a runaway engine that explodes, although this is extremely rare, that it’s ever that bad.
One other thing is the carbon this builds up in the head and in the exhaust and I have seen exhaust fires after returning Diesel engines to high load after being run without load until the exhaust carbon restricts the exhaust so much that the engine can no longer reach its normal loads.
So use your engine as you need, but just every now and then give it some hard running to clean it up,
I know people that will do this 5 mins in every hour or 1 hour in every 24.
Up to you but don’t stress over it.

[Bill_Giles]

It is of course true that lots of gensets run as you say. That is what they are designed to do! But not our marine diesels which are designed to be made to work. See this which makes it perfectly clear http://www.partinfo.co.uk/files/SB21...ONSUMPTION.pdf

[GoneDiving]

Originally Posted by Lepke

I got more hours than others out of my engines, but I kept the oil clean. Long idling will put more soot into the oil. A better oil filter or more frequent oil changes solves the problem.

It's amazing how much contaminants can be held in engine oil.

In a past life I would often hire small diesel powered pickups whose normal day hire was just short runs around town at city speeds doing collections or rubbish runs. My hire was typically to drive several hundred kilometers per day non stop at highway speeds. It was standard practice to have to add a litre or two of oil after the first day to replace any boiled off contaminants. From then on no oil was needed.

[a64pilot]

Originally Posted by FPNC

There are untold numbers of quality small diesel engines used in generators that are run continuously at 1800 RPM for 60 Hz AC power. They never operate at any other RPM. These generators rack up many thousands of hours. The issue is more about a quality engine than what every diesel engine needs. If a diesel cannot be run a low RPM's without causing an engine failure, that is a design problem.

Even lower, 1500 if eurospec 50 Hz, and those that swear you have to run the snot out of a diesel just ignore the simple fact that nothing lasts longer than a generator engine wise.

Then several, who know nothing at all about engines say, well it’s because generator engines are “special” generator engines, they are different. Which is of course nonsense.

[a64pilot]

Originally Posted by Bill_Giles

It is of course true that lots of gensets run as you say. That is what they are designed to do! But not our marine diesels which are designed to be made to work. See this which makes it perfectly clear http://www.partinfo.co.uk/files/SB21...ONSUMPTION.pdf

Bore polishing is a phenomenon that if it occurs it will do so very early in an engines life, before its broken in, not after break in.
After the first oil change if the engine was broken in properly and has had several full power excursions, polishing just doesn’t happen.

Generator engines and “marine” engines are the same, there just isn’t any difference.
Many if not most “marine” engines are actually mostly used on forklifts and tractors etc. they become “marine” when they are marinized.

[FPNC]

Originally Posted by a64pilot

Generator engines and “marine” engines are the same, there just isn’t any difference.
Many if not most “marine” engines are actually mostly used on forklifts and tractors etc. they become “marine” when they are marinized.

Pretty much all small diesel engines are "the same". Some are better at doing some tasks than others. Some have a better design and build quality. With that said, if a diesel engine will not work well as a generator engine, I would not want it for a propulsion engine in a sailboat.

[Newf1]

Originally Posted by partingclouds

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,

Partingclouds

I troll at least 90% of the time with my 2014 VP D1-30. RPM's are 850 to 950 to depending on currents and a normal day of trolling is 5 or 8 hrs. I have a 15-20 minute run to my fishing areas and usually go and come at 25-2800 rpms.
Every now and again I will run 3200rpm for a short time but that is not the norm. At 1400hr hours I checked the heat exchanger and exhaust elbow. There was no carbon build up in the elbow,,,just a light coating of soot. There was a light build-up on the outside of the tube bundle and an accumulation of "soft goop" near the discharge end. ( not sure what that was from) Gave the bundle and elbow a muriatic bath. On the discharge end I had a corrosion issue on the exchanger casting but that was from a known o-ring issue and nothing to do with low rpms. Today at 2200hrs I inspected again and the outside of the tube bundle was very clean and again, just a light layer of soot in the exhaust elbow. So I seem to have no exhaust build-up issues as a result of trolling rpm's but I am on my second set of engine mounts which would certainly be affected by the low rpm's and the higher vibration. I'm always worried about what will pop up with all the hours at these low rpm's but so far so good.

[EmeraldCoastSailor]

Probably nothing that an occasional Italian tuneup won't fix.

[parkstone bay]

Going back to the OP 's question, it's clear his trolling is only part of a package of work his engine is being asked to do ("once in a while" he goes fishing (presumably when SWMBO has allowed him out of the house!)). His normal work is at 1800 - 2000 rpm; an hour or so, even a a full day's fishing, won't be an issue. He still won't catch fish, but he'll have a
great day on the water drinking beer with a mate.

[tasssos8]

Originally Posted by Brewgyver

???
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.

You are totally right! I am sure those are tips heard from worthless marina dogs “mechanic enthousiasts” Steady speed.. most commercial boats go for days at the same Rev speed! And revving before shutting down is plain silly and it works completely the opposite. It is bad and builds up unburnt carbs! Here in the inland canals of Netherlands we motor for hours and days slowly at around 3-5 knots and the engines live a normal life like all! I am nearing the end of the season and I haven’t ever reached even the 2/3 of my Kubota’s power. Not everyone is crazy with revving and speeding you know. People make tulles of their own but nothing to do with real mechanics. Just follow manuals and logic. No tricks.