Low RPM cruise

[sailorboy1]

Originally Posted by Exile

When it comes to our boats, I've read in other CF threads that seawater is a much more efficient medium for cooling and so may be another explanation why our marine diesels are possibly more vulnerable to sooting up than over-the-road diesels.

What difference would the seawater being a better cooling medium than air? The engine is controlled at the same temperature either way.

[a64pilot]

The fuel theory for power boats came about as a result of what a Turbo does to the lifespan of specifically a DD. It’s not actually how much fuel is used, but how hard the engine is run, and those correlate pretty close. Of course many power boat people run their engines routinely HARD and go through engines in short order. Not so much anymore, cause fuel cost has hit even them, but some don’t care about fuel costs and buy new engines as necessary.
Then literally almost 100 years ago there were several papers that the Brits published on RPM vs engine wear. They were at one time pretty much the best engine builders there were, anyway engine wear seems to closely follow the amount of kinetic energy of the moving parts, which makes sense, double the energy of the piston and connecting rod assembly and have it reverse directions thousands of times per min and wear increases. Engine design plays into this as a short stroke engine can turn considerably higher RPM, but have a lower piston speed than a long stroke. Then often max RPM is simply a number that allows the engine to achieve rated power, a de-rated motor, those handle close to redline operation much better than a motor who’s redline is closer to the structural limit.
Increase power output of an engine by whatever means and the stresses of the parts and therefore the wear increases.

The problem with using EGT to determine load, is to know what numbers are acceptable. EGT will vary of course based on exactly where it’s taken. Often times it’s taken very close to the exhaust valve, except for those times that the limiting factor is how much heat will the turbo take, then you take it right at the inlet of the turbo, and then don’t exceed the max temp the turbo can tolerate. This is how it’s done in the majority of turbo charged aircraft engines, you lean to the temp the turbo can handle, the engine could take higher.

Turbos work of course by recycling heat, and on those motors where your trying to maximize the HP from a turbo, the exhaust is insulated.
Watercooling the exhaust like we do isn’t a good idea for a turbo motor, but of course for safety it must be done on a boat.

[Suijin]

It’s worth pointing out this is turbo diesel thing, not a diesel thing, in case some are conflating them as far as general advice goes.

[Exile]

Originally Posted by sailorboy1

What difference would the seawater being a better cooling medium than air? The engine is controlled at the same temperature either way.

Good point. On our seawater/freshwater cooled diesels the thermostat setting controls. Maybe these comments I read in prior threads were more about the scenario where people are only idling their engines for charging batts., etc., so engine temps are nowhere near the 180º or whatever set point the thermostat opens at. Or maybe they were just wrong.

[Exile]

Originally Posted by Suijin

It’s worth pointing out this is turbo diesel thing, not a diesel thing, in case some are conflating them as far as general advice goes.

To some extent it may also apply to non-turbo diesels, but it sounds like turbo diesels may be more sensitive to chronic idling. When I bought my first pickup truck with a Cummins turbo diesel I was specifically advised against excessive idling.

[ontherocks83]

So am I getting this correct, is the general consensus that underloading causes cylinder glazing but running faster than needed to prevent glazing causes premature wear?

If so then (and I'd be curious to figure out how to determine this) each engine and application would benefit from figuring out what is the minimum load or RPM while in gear required to prevent glazing and then you have best of both worlds.

[Suijin]

Originally Posted by ontherocks83

So am I getting this correct, is the general consensus that underloading causes cylinder glazing but running faster than needed to prevent glazing causes premature wear?

If so then (and I'd be curious to figure out how to determine this) each engine and application would benefit from figuring out what is the minimum load or RPM while in gear required to prevent glazing and then you have best of both worlds.

The rough guide is that running at 80% of max RPM will give you the longest engine life.

That said I know a former merchant marine engineer who while circumnavigating over 15 years put 11,000 hours on his Yanmar. He said most of those were charging hours. He had no solar but did have a wind gen. You could eat off his engine, looked brand new, and still had good compression.

[Teknishn]

Originally Posted by ontherocks83

So am I getting this correct, is the general consensus that underloading causes cylinder glazing but running faster than needed to prevent glazing causes premature wear?

If so then (and I'd be curious to figure out how to determine this) each engine and application would benefit from figuring out what is the minimum load or RPM while in gear required to prevent glazing and then you have best of both worlds.

I have a little 1GM and have been trying to figure this out myself. I have 3 different maintenance and repair manuals for my engine which were published over a 17-year period and they each give conflicting information (oil changes for example: every 50 hours, 100 hours, or 250 hours depending on which manual is referred too).

I typically motorsail at around what I "feel" is 1700-1800 RPM's as I don't have a tach. Every hour or so I ramp up the RPM's for 5-minutes or so. I'm the first to admit that this is all guesswork on my part but the last time I pulled the exhaust elbow and head to clean the valves (my personal 6-month maintenance inspection) I found minimal carbon build up and virtually no glazing. So I guess that this is what works for my little one-banger... :-)

[a64pilot]

Originally Posted by ontherocks83

So am I getting this correct, is the general consensus that underloading causes cylinder glazing but running faster than needed to prevent glazing causes premature wear?

If so then (and I'd be curious to figure out how to determine this) each engine and application would benefit from figuring out what is the minimum load or RPM while in gear required to prevent glazing and then you have best of both worlds.

Yes, my opinion is whatever RPM your engine requires when under load to come close to being fully warmed up. If your regular cruise temp is 180f then whatever it takes to get to or close to there.
It’s not an RPM thing either, heat comes from combustion mostly, and it takes very little fuel to raise a Diesel to a high idle, so it’s not getting warmed up, you want it working, which usually means driving the boat of course.

It’s my belief that Yanmar issued the race the engine up every two hours in neutral for the majority of boats that were idling them for hours a day charging batteries. Yanmar like all manufacturers primarily is concerned with getting their product through the Warranty period, and I’d bet they were getting warranty claims on newer motors in the charter fleets, who are big customers and need taking care of, or they may switch motors on their newer boats.
It’s not meant for those motoring or motorsailing between say 1500 and 1800 RPM, or if it is, what this race it in neutral about?

Running an engine at 80% of max thinking that is good for it is nuts.
That’s right at 2900 RPM on a 3600 RPM motor.
That is getting close to how the powerboat people run theirs, and their motors don’t last nearly as long as ours, and in truth theirs are usually better built more robust motors than ours too.
Aircraft engines that have a TBO of between 1500 and 2000 hours are only run usually at 85% of max power for a maximum continuous power, that’s not suggested, that’s if you exceed this You will break it power setting.
Most that own their own airplane and do their own work only cruise at 65% of power.

No, I think it’s like every other item on a boat, size or so that your using it mostly in the middle 1/3 of its capability and you’ll get good service out of it.

[Dockhead]

Originally Posted by a64pilot

The fuel theory for power boats came about as a result of what a Turbo does to the lifespan of specifically a DD. It’s not actually how much fuel is used, but how hard the engine is run, and those correlate pretty close . . . anyway engine wear seems to closely follow the amount of kinetic energy of the moving parts, which makes sense, double the energy of the piston and connecting rod assembly and have it reverse directions thousands of times per min and wear increases. . . .
Increase power output of an engine by whatever means and the stresses of the parts and therefore the wear increases.

I think the "fuel used system" is exactly a proxy for the "kinetic energy of the moving parts", or at least the amount of work the engine is being asked to perform. Just like BMW used for decades fuel consumed as the way to determine when maintenance is required, not miles or hours or whatever. I think it makes perfect sense, and I think that the engine wear must be related most closely, to the amount of work the engine has been asked to do, much more than hours, so an hour of use under less load and lower RPM will cause less wear, right down to the point where the combustion chambers aren't getting warm enough, then engine wear goes up again, but for different reasons.

Originally Posted by a64pilot

.. . Running an engine at 80% of max thinking that is good for it is nuts.
That’s right at 2900 RPM on a 3600 RPM motor.
That is getting close to how the powerboat people run theirs, and their motors don’t last nearly as long as ours, and in truth theirs are usually better built more robust motors than ours too.. ..

I agree.

[Dockhead]

Originally Posted by Teknishn

.. . I typically motorsail at around what I "feel" is 1700-1800 RPM's as I don't have a tach. Every hour or so I ramp up the RPM's for 5-minutes or so. I'm the first to admit that this is all guesswork on my part but the last time I pulled the exhaust elbow and head to clean the valves (my personal 6-month maintenance inspection) I found minimal carbon build up and virtually no glazing. So I guess that this is what works for my little one-banger... :-)

Same here. I mostly use my engine at below 2000 RPM, and I have never had any excess carbon build up, never needed to clean out my exhaust elbow, and the hot side of my turbocharger has always been squeaky clean.


But I do run the engine up to near the redline, regularly, at least for a few minutes every hour or two, and I back down on my anchor for some minutes running at redline.

[SV DestinyAscen]

Reading all the posts - am I understanding correctly that those encountering issues with soot are getting them due to incomplete combustion (underloaded) & glazing occuring because of too low temp (piston ring not sealing & hot gases getting thru and causing glazing)?

If I'm understanding this correctly then underloaded can contribute to low temperature - but low temperature (assuming the wax thermostat is functioning) isn't probably an independent event if you're running the diesel at any length of time. I guess what I'm trying to say is - put a sufficient load on the compression ignition engine and you're golden? Probably harder for a turbo diesel with higher torque at lower rpm/high air-fuel mixtures?

[sailorboy1]

Originally Posted by ontherocks83

So am I getting this correct, is the general consensus that underloading causes cylinder glazing but running faster than needed to prevent glazing causes premature wear?

No I don't feel it's a "general consensus" as much as it's one of those internet "everyone knows" things. It probably only makes a difference at the extreme end of the idle/hard scales. I.E. you don't idle for days and you don't run at max for days. In between is no man land where changing the oil probably makes a bigger difference.

This is just IMO and I'm not going to battle over it.

[Exile]

Originally Posted by Dockhead

But I do run the engine up to near the redline, regularly, at least for a few minutes every hour or two, and I back down on my anchor for some minutes running at redline.

For better or worse I do not do this regularly with my non-turbo diesel, but the few times I have (mostly when testing rpm with a newly pitched prop) the coolant temp has shot up quite dramatically, i.e. to the point where I feel I need to back down after only a minute or two. Under normal running the coolant temp stays where it should. I've confirmed gauge accuracy with a laser heat gun, and have done the usual checks on both sides of the cooling system.

Anyone else have this experience? Is this normal?

[Wotname]

Originally Posted by Exile

For better or worse I do not do this regularly with my non-turbo diesel, but the few times I have (mostly when testing rpm with a newly pitched prop) the coolant temp has shot up quite dramatically, i.e. to the point where I feel I need to back down after only a minute or two. Under normal running the coolant temp stays where it should. I've confirmed gauge accuracy with a laser heat gun, and have done the usual checks on both sides of the cooling system.

Anyone else have this experience? Is this normal?

I would say it was abnormal. Yes, you might get a temperature increase but not a dramatic one and not one where you have to back off after only a minute or two.

IMO, you have cooling issue, either a design issue or a defect.