In a discussion on the sailboatowners.com forum, I had posted that it was bad to run a diesel at idle. I was quickly rebuked on this point by some knowledgeable/experienced boaters, including one very well respected moderator/featured contributor. I have also seen this subject come up on the Cruisersforum.com.
First thing, lets establish the definition of idling because this was of some issue. In the case of this discussion it would be running your motor in neutral. Even if you bring the RPMs up to 1,000 you are still idling your motor. For instance, the idling range for my Universal M25-XPB is 1,00-1,200 RPM.
Now, I have always operated under the notion that idling is bad for your diesel engine. For instance, running your diesel at anchor to charge your batteries and heat hot water. My owner’s manual tells me such:
But here are some quoted responses I got during the discussion on Sailboatowners.com.
For millions of sailboats running the main propulsion diesel is de rigueur for charging batteries, including everything from affordable, 20-something footers to 1/2 million dollar plus coastal cruisers (from Morris Yachts, for example). So while you may have a theoretical point, in practical terms it is meaningless. To paraphrase you: in general, it doesn’t matter.
I don’t idle mine, I usually run it at 1000 RPM or slightly higher. I don’t get enough of a difference in output at higher RPMs to make it worth the extra noise. At idle (700 RPM) it doesn’t make enough voltage to charge beyond a float level.
The only potential negative effects are glazing of the cylinder walls over time, that may cause some smoking. Mine has not exhibited this in 29 years of operation (at about 3000 hours). And, if it becomes necessary it is correctable: Westerbeke used to run generators on a dyno at 80% load with a teaspoon of feldspar in each cylinder to roughen up the cylinder walls and stop the smoking! (maybe they still do). You can break the glaze with a hone pretty quickly when it’s time, like every 40 years or so. Probably won’t even need new rings.
A big charging load, taking conversion efficiencies and drag into account is about 3 or 4 HP, which is about 15 to 20% of max rated output for this engine; so, hardly idling.
A bonus is that it heats the domestic water, too…
From Maine Sail:
My buddy Darren owns a good sized excavating, irrigation and landscaping company in Colorado and we talk diesels quite a bit. Just got to see him last week.. He has a good sized fleet of them and a couple of them have over 20k hours with no rebuilds. The last time I spoke with him about engine longevity he had one Yanmar block and one Mitsubishi block with over 20k hours. He bought both of these machines used with about 5k hours on them back in the late 90’s.
Most of his smaller engines are either Yanmar or Kubota but he does have a few Mitsubishi’s too. His engines run all day and never shut off and they idle for long, long, long hours. Now granted these are not in a marine application but in well over 500,000 hours of combined run time on his fleet he has yet to rebuild an single small diesel engine. All his machines run Shell Rotella and it gets changed regularly. Of course he buys his oil in 55 gal drums and I buy it by the gallon…
If heavy equipment running Yanmar, Mitsubishi and Kubota blocks can rack up10k to 20k hours, while doing hundreds and hundreds of hours of idling per year, with no rebuilds then a well maintained marine diesel should be able to do the same.
When we had the discussion about not letting diesels idle a few years ago he just laughed about the glazing the cylinder walls. His sarcastic comment was something like “Sh&t I better let my guys know not to let them idle”. Course he’d already been doing it for 20 some odd years, with no failed engines or rebuilds needed, so he was surprised to find out his engines were going to die soon…
Our engine has idled perhaps half or more of its 3600+ hours. It burns ZERO oil, has cross hatching in the bores that looks like new and she purrs like a kitten. We have Sea Frost and often sail with the engine idling or will let it idle to chill the plate if we are alone and not disturbing others. Our boat also did a five year 24/7 on-the-hook almost circumnavigation. She had no generator and only the factory alt and a single solar panel. The batteries lasted six years and were still kicking.
When this came up on the Cruisersforum.com, the following replies were posted:
Everything I have read and have experienced supports the notion that a diesel is best run in the 75% to 90% (or thereabouts) power output range. An occasional period of idling will not be detrimental if followed by a period of running at a good load (for example starting the engine and letting it idle for a while to warm up before heading out). The frequent in and out of the harbour short hops are murder to the poor diesel.
I can not explain why automotive diesels, which spend a lot of time idling, survive. I could speculate though that they would last a lot longer if run like a semi trailer road transport truck – much harder and much longer.
A modern diesel should last 20000 to 30000 hours yet they get replaced in boats at a fraction of the expected service life. IMHO that is, amongst other things, related to the unfavourable working conditions of a sail boat auxiliary engine.
Diesels like to run fairly hot (180 – 190 F range). Running colder and/or idling a lot causes more wear and tear, carbon buildup and soot buildup.
I am sorry that I can’t produce a link to something official looking.
I don’t know if this applies to the small diesels (non turbo/super charged), but the diesel generator that I have a lot of experience running had 36 cylinders displacing 645 cubic inches each, and it had an interesting gear driven (at low load) turbo (at full load) charger/compressor.
This diesel had very specific directions against running at low load (less than 30%).
The reason given for the load restriction was lower cylinder pressure at low load. The piston rings are designed to operate at rated load combustion pressures, this ensures normal lube oil consumption rates. Light and no load operation promotes “souping”, which is excessive oil escaping past piston rings into combustion chamber. Souping causes smoking exhaust and potential exhaust manifold fires.
The recovery direction, if the engine was run at low load, was to follow that with at least a half hour at greater than 50% load.
Based on what I have seen running this engine, I would suggest that it is not a good idea to do extended runs at low load.
[Interesting side note, jviss and Jd1 both have Catalina 36s.]
So by posting this (and cross posting it to several different areas) I am hoping to get some feed back that will help me determine if idling is bad for my diesel. If you comment on this, please try to provide something to support your opinion.
Cross posted on Sailboatowners.com and Cruisersforum.com.