Are low rpm's hard on a Diesel engine?

[Lepke]

As a commercial fisherman, I trolled at 700 rpm for as long as 14 hours. If I was far out to sea I would shut down and drift at night, then troll again the next day. 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.

[montyp]

Maybe a little off topic, but some of the concerns about running diesels at low speed are related to modern trucking and machinery emissions equipment needing high exhaust temperatures to burn-off particulate that accumulates. Some tractor ECUs will require you to operation at high rpm for a certain length of time periodically.

[NorthCoastJoe]

Sailorboy nailed it. I never worry what rpm I am running at. Before I shut down I run full rpm for a bit then shut down.
My Yanmar is happy, purrs like a kitten and roars like a lion, my prop is properly matched to the boat and engine.

[Lancerbye]

A lot of this depends on the type of engine. Low speed or high speed. Yanmars are high speed diesels while many other marine engines are low speed diesels with max rpms in the 2100 to 2600 rpm range. My Perkins 4-236 is rated at max 2600rpm I usually cruise at 1400rpm. and troll at 700 rpm. As Lepke stated it is more dependent on maintenance than speed. I also commercial fished using a Chrysler-Nissan 6 cylinder diesel which ran long days at 600 rpm. Salmon trolling. Put more hours in 10 years than the average cruiser will put in a life time. One rebuild in 10 years.

[a64pilot]

Originally Posted by sailing_gal

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.

More urban myth, I’d be interested what the thought process is behind this?
Often during break in steady state RPM is to be avoided, this is I believe because you want to have full throttle pulls during break in, the closer to being broken in, the longer the pulls.

Lots of “Knowledge” is published by supposed “experts” the silliest one I’ve read is avoid high TBN oil, because it will polish your bores, and the expert has proof, because it happened to him.
However he’s a proclaimed expert and many believe whatever he prints is gospel, and those evil oil manufacturers are ruining untold numbers of Yacht engines I guess with their premium oils.

Another expert will sell you a product that seriously exceeds the manufacturers limits for the engine and I guess he tells you to not worry about it, they don’t know what they are talking about?

If you have a turbo motor and rev it up several times and shut it down, it can be bad for it.
It’s bad for two reasons, first you get the turbo up on boost and spinning fast, then shut down and remove the oil with it still spinning.
Secondly by revving it several times your dumping heat into the turbo and then shutting it down, removing the cooling oil and the oil left inside of it can form coke.
So if you want to rev it up, do so, but then let it idle for a minute to cool down and let the turbo slow down or some even stop at idle.

If it’s not a turbo then revving it up and shutting it down isn’t likely to do much, just as a habit from long past I believe in warming up a motor slowly and cooling one down the same.

[sailorboy1]

Originally Posted by a64pilot

If it’s not a turbo then revving it up and shutting it down isn’t likely to do much, just as a habit from long past I believe in warming up a motor slowly and cooling one down the same.

It's right in my Yanmar 4JH3E manual.

I wonder why people chase down "I hear, was told, reads" instead of reading the manual for their engine etc.

[Glacier Crews]

We have a Cummins QSB 5.9 480 HP Diesel and fish a lot. We installed a trolling valve and have been trolling for 6 years without issue. Hope this helps.

[Martkimwat]

It's not so much the low revs, as the fact it is usually not working hard, which will glaze the bores causing blow by and increased oil consumption and reduced compression. Diesels like to be worked hard so on a boat that can only be done by revving up to close to maximum or running at around 70~80% of maximum revs. On a vehicle, you can make it work harder by putting it into a higher gear and it will work hard at low revs but we don't have this option on boats.

[clakiep]

What surely would be detrimental is the combination of low RPM and high torque. Depends on the load You are towing.

[Reefmagnet]

Originally Posted by a64pilot

More urban myth, I’d be interested what the thought process is behind this?
Often during break in steady state RPM is to be avoided, this is I believe because you want to have full throttle pulls during break in, the closer to being broken in, the longer the pulls.

Lots of “Knowledge” is published by supposed “experts” the silliest one I’ve read is avoid high TBN oil, because it will polish your bores, and the expert has proof, because it happened to him.
However he’s a proclaimed expert and many believe whatever he prints is gospel, and those evil oil manufacturers are ruining untold numbers of Yacht engines I guess with their premium oils.

Another expert will sell you a product that seriously exceeds the manufacturers limits for the engine and I guess he tells you to not worry about it, they don’t know what they are talking about?

If you have a turbo motor and rev it up several times and shut it down, it can be bad for it.
It’s bad for two reasons, first you get the turbo up on boost and spinning fast, then shut down and remove the oil with it still spinning.
Secondly by revving it several times your dumping heat into the turbo and then shutting it down, removing the cooling oil and the oil left inside of it can form coke.
So if you want to rev it up, do so, but then let it idle for a minute to cool down and let the turbo slow down or some even stop at idle.

If it’s not a turbo then revving it up and shutting it down isn’t likely to do much, just as a habit from long past I believe in warming up a motor slowly and cooling one down the same.

I was told years ago that a constant speed engine will eventually create a ridge (or maybe it was carbon build up??) in the bore at the top of the stroke and suddenly running the engine at a higher rpm can ever so slightly raise the tdc position of the piston, causing it's topmost ring to contact this ridge with an associated risk of breaking. There's also the issue of bore glazing which results from running a diesel engine lightly loaded for extended periods of time.


As for giving the engine a burst of power immediately prior to switch off, this is definitely something I remember many old timer truckies doing for reasons I don't know but possibly makes some sense on an engine with a wet exhaust system as it would help (I think) to force out any excess cooling water which may have collected in the exhaust circuit.

[a64pilot]

Originally Posted by Reefmagnet

I was told years ago that a constant speed engine will eventually create a ridge (or maybe it was carbon build up??) in the bore at the top of the stroke and suddenly running the engine at a higher rpm can ever so slightly raise the tdc position of the piston, causing it's topmost ring to contact this ridge with an associated risk of breaking. There's also the issue of bore glazing which results from running a diesel engine lightly loaded for extended periods of time.


As for giving the engine a burst of power immediately prior to switch off, this is definitely something I remember many old timer truckies doing for reasons I don't know but possibly makes some sense on an engine with a wet exhaust system as it would help (I think) to force out any excess cooling water which may have collected in the exhaust circuit.

That was always the theory of don’t buy a car from the little old lady that only drove to church on Sundays.
However if the wear ring is so pronounced that it can break a compression ring, the motor is slap wore out anyway.

Some of the procedures in manuals are for reasons other than making an engine last a long time, I wouldn’t be surprised if Yanmars recommendation to race it up a few times before shutdown is there as an attempt to reduce the smoky start up Yanmar’s are famous for. I don’t believe it’s in my old engines manual.

The older Yanmar’s combustion chamber is actually not real efficient, it’s not a high swirl chamber, high swirl enhances fuel mixture and combustion and reduces smoke, the Yanmar combustion chamber is an old low swirl design, at least my old 4JHE is, so Yanmar has a reputation for smoky engines. Boosting Cetane cleans up the smoke a lot.
I would expect the newer common rails designs are different, and have high swirl designs and don’t smoke.

Oh, and bore glazing on an engine that’s broken in and the fuel system is in good shape just doesn’t happen.
If it did then every OTR truck that idled every night, all night long would have been glazed, and they weren’t.
Idling all night long is much more low RPM / low load than battery charging.

[Tellie]

From my old diesel mechanic days when I was taught by one of the old school guys. The four C's to always remember with diesels: 1) Clean air 2) Clean fuel 3) Clean oil, and 4) Don't Cill the damn thing with kindness. Well, he didn't spell so good but as a mechanic there were none better. Gas engines and diesel engines are two different beasts. If you run them both hard, one will wear out sooner than later and the other will be happy as a sow in it's own excrement while toasting you with rum runners for years.

[Mr B]

It depends on the load on the motor,
If your idling and no load, It will glaze up the barrels,
If your running low revs and the motor has a load on it, It wont matter,
Your motor is loaded, Working,
Your boat motor even when set at a constant speed, Is still loading up and down as you go thru waves, Its working, Not Idling with no load,

I sold my AS 400 welder for that reason,
I was using small electrodes, The diesel was idling, with no load, Even when welding the motor did not even notice it,
It was destroying a very good diesel, Glazing the inside of the barrels,

Truck and car diesels are forever going up and down in the rev range as they go up and down hills. Even slow moving diesels,

Most wear in any motor is when you start it up, Its running with out oil, Till the pump pushes the oil around the engine, and the crank shaft flicks oil up the cylinder walls,

Farmers use their diesel equipment once a year and then they are worked hard,
The rest of the year they sit in a shed, Unattended,

[teaguy]

Being from Vancouver, we tried trolling with a new downrigger one summer. The problems we had were not the engine, but the transmission. At low RPM going from forward to reverse the transmission just starting slipping and pounding as it caught from time to time. Had the local Yanmar guy have a look and said we polished our cone clutch causing it to slip. Lack of load on the transmission caused us to polish the clutch. It was fixed, they used lapping compound to rough up the surface. Last time we did that, went out and got a long leg Tohatsu and troll with that. So now we have a back up propulsion system. We have two downriggers now and catching lots of Salmon.

[kmacdonald]

Maybe, maybe not.