Running a Diesel at Idle

[JK n Smitty]

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.

From the Universal Diesel Operations Manual - February 2010

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.

From jviss:

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:
From Jd1:

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.

From nes:

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.

Fair winds,

Jesse

Cross posted on Sailboatowners.com and my blog.

[tropicalescape]

Good post ,I run my yanmar at idle speed all the ,for hrs at a time and I have had no probs..As long as truck drivers idle there trucks for long hrs at a time(24 hrs at the truck stop ) ,I will do the same..again good post...

[zeehag]

lol i was gonna say what te did----if idling a diesel is so bludi bad for it--why is it trucks always are at idle in truck stops and they havent any trouble--oh--not all diesel trucks ar owned by big corporations---there are many independents also idling theirs.... they do not shut down at dinner time--idle entire time..yet this is allegedly BAD fro boating??? what yachtie made that rule out of sky blue????

[DeepFrz]

It seems to me that the vast majority of engine problems posted on this forum are related to fuel and cooling/exhaust problems. There are some, but relatively few, problems relating to the basic engine itself or how that engine has been run. Keeping the fuel clean and fresh, making sure that water does not back up into the exhaust system, keeping the exhaust riser clear of carbon buildup, maintaining the raw water pump/system (including the heat exchanger) will almost always ensure a long lasting and well running diesel engine. The operative word here is Maintenance.

[Wraith_Mac]

I agree with many above posters.
I was going to quote a personal history with diesels but Maine Sail and DeepFrz covered all that needs to said in that regard.
Idle for as long as necessary, just run it up to 80/90% - even briefly redline it - when you can, supply clean fuel, top grade oil, regular changes of oil and filters is all you should need to do.
The ONLY time idling is bad for a diesel, indeed just about any engine, is in a break in period when new, or rebuilt, you have referenced that in your owner's manual snippet above, perhaps you misunderstood it?
Probably the main difference between terrestrial diesels and their marine cousins is that on the land/roads the engines get worked through the rev range more and most boaters I've seen try to pick a 'sweet spot' and stay there forever.
I don't favor driving a boat engine for sometimes days on end at the same rev setting and would recommend a bit of variety [higher revs] every 8 or so hours for an hour or so.
I do not have any scientific evidence [a link] to support this, it's just what we do and it has worked for decades, can do no harm.
Cheers,
Mac

[ontherocks83]

Not to start trouble or offend anyone but the old adage of I've been doing it for years and have never had a problem really doesn't hold water. Just because you don't notice anything does not mean that the engine is running as good and efficiently as it could be.

first I would say follow the manufactures guide lines, (they put them there for a reason), most that I have read say to run at 80-85% of max rpm for most of the operation, if idling for a while give it a full speed run for a while etc... etc... this is primarily because diesel runs best under high compression and high load (which is how diesel fuel ignites by pressure and heat not spark)

2nd trucks, tractors etc that have diesels are not supposed to be idled for long periods of times either. the reason that they handle it better is because they can operate at higher temperatures and burn off more of the carbon much more easily than a marine engine. diesels love heat (obviously within operating specs) the hotter the engine the more efficiently they run, and even though they handle it better it doesn't mean that they don't load up and have small detrimental effects to running at idle. comparing how a truck runs to a marine engine is apples and oranges although both "fruit" they operate under different temps. loads, conditions, etc.

3rd the problems that you "haven't noticed" can be burning more oil than you would have been had the cylinders not been glazed over. or running slightly less efficiently etc etc. in short idling leads to glazed cylinder walls and carbon build up in the cylinders and possibly into the exhaust. unless you disassemble your engine your not going to see what it is actually doing to the inside of it.

call me crazy but im going to run my engine how the manual says to (the people that wrote it are a lot smarter than me) and if I owned a mack truck I might do what everyone else does because they run hot and hard enough to mitigate most effects of idling all of the time. Your boat engine does not.

[Matt sachs]

"Breaking in" is not the same thing as normal operation.

[Cheechako]

Originally Posted by zeehag

lol i was gonna say what te did----if idling a diesel is so bludi bad for it--why is it trucks always are at idle in truck stops and they havent any trouble--oh--not all diesel trucks ar owned by big corporations---there are many independents also idling theirs.... they do not shut down at dinner time--idle entire time..yet this is allegedly BAD fro boating??? what yachtie made that rule out of sky blue????

Yep, It's been posted many times and is a great example of why idling is not an issue for an engine used normally. maybe the confusion is if all you do is idle the engine. Many boaters idle their engines to do things like charge batterys, make water, heat water etc. But they also might spend 12 continuous hours at 2800 rpm under load. Alot like those guys at the truck stop.... or a lot like those fishing charter boats that spend much of the day idling out on the fishing areas after cruising out there.

[ontherocks83]

that's all and good but let me ask you with the cost of engine repairs and or the cost of an eventual replacement why would you risk causing any long term harm (even if it is minor over the short run).
the one thing I can guarantee is long term idling causes carbon build up and carbon build up leads to problems (and over time even after hard runs you wont be able to clear everything out). I don't know about you but I don't want to be wasting money repairing what I consider to be one of the most important parts of my boat (not to mention one of the most expensive).
Engines should be treated and maintained like the mother of your children not some cheap prostitute you thrash on the weekend. but hey that's me. id rather spend way more effort on preventative maintenance and taking care of what I own then being lazy (i don't mean lazy to be offensive to anyones maint habits just in general)towards it now and paying for repairs later.

[Cheechako]

Originally Posted by ontherocks83

that's all and good but let me ask you with the cost of engine repairs and or the cost of an eventual replacement why would you risk causing any long term harm (even if it is minor over the short run).
the one thing I can guarantee is long term idling causes carbon build up and carbon build up leads to problems (and over time even after hard runs you wont be able to clear everything out). I don't know about you but I don't want to be wasting money repairing what I consider to be one of the most important parts of my boat (not to mention one of the most expensive).
Engines should be treated and maintained like the mother of your children not some cheap prostitute you thrash on the weekend. but hey that's me. id rather spend way more effort on preventative maintenance and taking care of what I own then being lazy (i don't mean lazy to be offensive to anyones maint habits just in general)towards it now and paying for repairs later.

You make the assumption that it is bad if done as part of normal usage. I think the question posed here is... is it?
My opinion is charter fisherman and truckers wouldnt do it if it was. Many commercial operators are of the opinion that turning the engine off is bad... but from a practical standpoint... would you leave your engine running all the time if it's not necessary?

[MarkSF]

My belief is that idling is unlikely to do any damage, as long as the engine is regularly run at operating rpm too. If you only ever idled the engine, condensates would build up in the oil, carbon in the cylinders, and so on. Regular use at a high percentage of maximum output is indicated.

The same is true for cars that only do very short trips. They need to do one or two trips a week where the engine is thoroughly warmed up.

[Cheechako]

Yep, short trip cars are notoriously bad buys. The engines dont heat up thoroughly, leaving residue and acids inside. I once bought a vintage car from an 84 year old lady. Her family made her quit driving. In 20 years the car had only accumulated 32,000 miles on it. It looked like a new car inside. The carbon and gum was so thick inside the engine you could scoop it out with a spoon!

[Suijin]

Originally Posted by JK n Smitty

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:
[URL="http://svsmitty.files.wordpress.com/2013/09/idling-from-universal-deisel-operation-manual_february-2010.jpg"]

As noted, the break in period is very different from normal operation. Generally you want to run your engine at progressively increasing loads, Ideally with a heat temp gun to gauge your intervals.

It would be one thing in you were idling your engine for the better part of every day, day in and day out. Idling your engine for a few hours in the morning to charge up, interspersed with running under normal load, is not going to appreciably shorten the life of your engine. If you need to run it more than that for power generation, then get a gen set ffs.

[ontherocks83]

again you cant compare to trucks, cars, commercial etc. you are talking about much bigger hotter running engines and it is still detrimental to them it is just that they can recover from it better than your small 2-4 cyl inboard.
I will clarify that obviously engines have to idle during their life time however I would not make regular practice of idling it for hours on end time after time. it is detrimental to the engine (increased oil consumption, lower compression, exhaust fouling etc..)
as for cars needing to be run up to temp if all you do is short trips you are absolutely correct, however I have seen time after time (i'm in the car business) other problems arise from those short trips even if they get weekly highway runs, they will go through catalytic converters faster, foul plugs, destroy o2 sensors, and if you ever have to break down one of those engines there will be significantly more carbon build up on key components.
so yes full runs will help but there are always going to be other effects that you cant get rid of just by doing a full speed run

[sailorboy1]

I run my Yanmar engine the the way the manual says to run it. Which means I can run it at at light load/idle as long as I run it up in rpm every few hours. I also run it a fairly light load (2000 rpm) most of the time when motoring because that is the sweet spot of speed/fuel use, but when I do this I also run it up to full load every few hours for a few minutes.

I bet there are 2 main things that wear out engines: 1 - running them, 2 - not running them