Low RPM cruise

[StuM]

Originally Posted by a64pilot

So Ken if your motoring at 2500 RPM, every two hours, you place the transmission in neutral and race your engine five times?

Did you read that manual page?
"When the engine is operated at a low idling speed (below 1000rpm)....."

[sailorboy1]

Originally Posted by StuM

Strange, everyone that I've seen giving actual data gets much better mpg at

Either you don’t really know everyone or you are saying I’m a lair

luckly for me it matters not for which

[model 10]

On my last cruise I did a bunch of motor sailing. I found that I could have full sail up, with the engine at about 900 rpm and the wind vane would still work. Any more than 900 rpm would cause the wind vane to loose it. It took a while for the water temp to come up but it did. I think I was using about .3gph.

[a64pilot]

Originally Posted by StuM

Did you read that manual page?
"When the engine is operated at a low idling speed (below 1000rpm)....."

Seeing as how I posted it, it’s likely that I read it.
However that is for my engine, not his, and as I said the point of the thread is to determine what low speed is. My old manual defined it, either the newer manuals don’t, or people are not reading the manual.
Pretty sure most of our manuals say do X when operating at low speed, but then don’t define what low speed is, my manual does, newer ones do not, I assume.
I bring this up as there seems to be a preponderance of people that think for longevity of a Diesel, you have to run the snot out of it, something that having operated Diesels and other engines all of my life I find to be not true.
Plus I have never run into any recommendation of racing the engine in neutral for any other engine other than a Yanmar, and apparently they didn’t use to recommend that.
My suspicion is that there are a whole lot of charter boats out there, that every day spend hours at idle recharging batteries, and Yanmar put a statement in their manual to cover that use.
That has been misinterpreted by others to mean that if I’m at a low power cruise that I have to race the engine every two hours, which I don’t believe to be the case.

[smith8273]

OK, I have read all this — but does the Yanmar manual mention which is the best anchor?

[Kenomac]

Originally Posted by StuM

Did you read that manual page?
"When the engine is operated at a low idling speed (below 1000rpm)....."

My turbo plugged up from low revs under 1800, the turbo kicks in at around 1800, so I’m not willing to chance messing up another. I personally know of three others who had the same problem. I’m doing what the manfacturer service rep told me to do in order to avoid problems.

You do whatever you like with your engine, it’s your money.

[Uncle Bob]

Originally Posted by Kenomac

My turbo plugged up from low revs under 1800, the turbo kicks in at around 1800, so I’m not willing to chance messing up another. I personally know of three others who had the same problem. I’m doing what the manfacturer service rep told me to do in order to avoid problems.

You do whatever you like with your engine, it’s your money.

Ken, how dare you be so damn sensible and logical and have the audacity to follow the manufacturer's guidelines. Damn you I say.




Just kidding

[sailorboy1]

Originally Posted by a64pilot

and as I said the point of the thread is to determine what low speed is.

That has been misinterpreted by others to mean that if I’m at a low power cruise that I have to race the engine every two hours, which I don’t believe to be the case.

Based on operating MY 4JH3E I would say low rpm is anything less than about 2400 rpm. I say this because for MY engine I have found than if I run less than than that (and I almost always do) than running the engine up to 3000+ for a minute every few hours makes a noticeable difference to the engine.

[GaryBode]

It's true. Do what the engine manufacturer recommends.

[coopec43]

Diesels thrive on hard work and if run while not fully loaded problems will occur.

"Diesel engines can suffer damage as a result of misapplication or misuse, such as internal glazing (occasionally referred to as bore glazing or piling) and carbon build-up. Ideally, diesel engines should be run at least 60% to 75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis. Another potential problem is cylinder cavitation and erosion."(Wiki)

https://en.wikipedia.org/wiki/Diesel_engine_problems

Clive

[Warren Drifter]

For fuel consumption, this is in line with the old common “cube rule” for consumption of bunker for ships. Basically, to double the speed, it takes eight (8) times as much fuel. And this old rule can apply to small engines on yachts as well but to see it in reality, one would have to be prepared to make some fairly accurate measurements starting at a slow speed and taking several various higher speeds then plotting them.

[Dockhead]

Originally Posted by a64pilot

. . .I bring this up as there seems to be a preponderance of people that think for longevity of a Diesel, you have to run the snot out of it, something that having operated Diesels and other engines all of my life I find to be not true..

I don't think that any kind of preponderance of people think this.


I think it's true and everyone agrees that diesels don't like to run without a load. They are different from petrol engines because the mass of air flowed at a given RPM is constant at different loads -- only the quantity of fuel is changed. A petrol engine varies the mass of mixed fuel and air, and pulls a vacuum to modulate the mass of mixture. So the combustion chamber temperature will be more or less the same in a petrol engine, even at idle with no load, but a diesel engine will cool off massively if there's no load, as there is still the same amount of air flowing through it, even as the quantity of fuel has been cut right back.


So in my opinion the only problem is running with no or extremely little load, especially at higher than idle speeds (where more air is being drawn through). I think that charging batteries at a fast idle speed might be harmful if the alternator is small. But as long as you are driving a propeller, I can't imagine that any RPM is harmful, provided that you blow the engine out from time to time as Yanmar recommend.


I wrote in another thread -- I think the acid test is exhaust gas temperature. If the EGT is reasonable (over 275F?) then I can't see how any harm is being done. But a turbo engine needs a good load periodically to blow the carbon out and keep the turbo free.


I've put more than 2500 hours on my 4JH3 HTE (since my purchase at 830 hours), and I often run it at 1300 -- 1500 RPM when motor sailing. This engine was smokey when I bought it and still smokes, but starts and runs perfectly, doesn't use oil, and starts and runs exactly the same now, as when I bought it. I really don't think that it is true, that diesel engines need to be run at a large percentage of their maximum power, to stay healthy. On the contrary, I've a read power boat oriented literature on diesel engines which predict the service life of main engines based on the quantity of fuel consumed per liter of displacement, implying an inverse linear relationship between loading and engine life -- i.e., the lighter loaded you run it, the longer it will last, which makes sense to me, right down to the point where EGT is too low to promote a healthy atmosphere in the combustion chambers.


That's what I believe, and I don't claim that it is scientific or definitive, but I haven't read anything to convince me otherwise.

[Warren Drifter]

Another thing to think about what is read in those manuals is how and who done the translations from the Japanese or Dutch engine factory to other languages and back again. How many times was the same paragraph deciphered snd rewritten. Was it a real engine specialist who made the comment in the first place or just a exchange student earning a bit of spending money. Old testiment? New testiment?

[Dockhead]

Originally Posted by Warren Drifter

Another thing to think about what is read in those manuals is how and who done the translations from the Japanese or Dutch engine factory to other languages and back again. How many times was the same paragraph deciphered snd rewritten. Was it a real engine specialist who made the comment in the first place or just a exchange student earning a bit of spending money. Old testiment? New testiment?

There is a school of thought that says that diesel engines like to be run hard, and should be run hard most or all of the time, as in a significant percentage of their maximum power.



I can't prove it, but I just don't think this is true, and I have never heard any coherent argument for this position.



Car engines typically operate at what -- 10% of maximum power? With regular but short term excursions to 40% or 50%, and very rare use at 90% or 100% (except in Germany where you might occasionally get to drive for an hour or two near maximum speed)? Half the car fleet in Europe is diesel powered, and there is no data anywhere that diesel engines run like that suffer from short life, turbo problems, or anything like that.


One thing I think really reflects health of a diesel engine -- don't laugh -- is what it sounds like. If the engine sounds happy, then I think it has to be. My engine sounds happiest at something like 1800 to 2000 RPM, just when you can hear the turbo working, and sounds distinctly less happy at 2800 to 3200 RPM, where some of these "diesels need to be run hard" schools of thought would have us believe the engine needs to be run for health. But 1800 to 2000 is less than half of maximum RPM, and on propeller curve that's only something like 20% of maximum power.



I don't claim to have definitive or scientific knowledge about this, and would love to be proved wrong, but I haven't read anything anywhere with objective facts which contradict what I've written here.

[Exile]

Originally Posted by Dockhead

I think it's true and everyone agrees that diesels don't like to run without a load. They are different from petrol engines because the mass of air flowed at a given RPM is constant at different loads -- only the quantity of fuel is changed. A petrol engine varies the mass of mixed fuel and air, and pulls a vacuum to modulate the mass of mixture. So the combustion chamber temperature will be more or less the same in a petrol engine, even at idle with no load, but a diesel engine will cool off massively if there's no load, as there is still the same amount of air flowing through it, even as the quantity of fuel has been cut right back.

I've observed this -- from years of owning both diesel-powered vehicles & my own boat -- but never quite put it together that this was the explanation. Makes sense. My own boat's diesel will run anywhere from slightly under 180ºF with little load to a bit over 190º at closer to max cruising rpm. In contrast, my diesel generator will run at exactly 180º regardless of load, presumably because it maintains the same (1800 rpm) idle speed.(?) These observations seem consistent with Dock's explanation.

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.