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Old 04-10-2020, 14:23   #1
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Curious about diesels..

Why is it that big ships and semis, etc can let their engines idle for days on end but it's not okay for our little baby diesels to run without a load for long periods? I just had to ask after hearing a cargo ship nearby idling for 4 days now.
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Old 04-10-2020, 14:25   #2
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Re: Curious about diesels..

Cargo ships don’t idle, those are generators.

Semis spend most of their time with loads on the engine in the big picture.
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Old 04-10-2020, 14:52   #3
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Re: Curious about diesels..

Okay, I'll buy that about cargo ships but it sure sounds like an idling engine... Trucks spend every night idling all night long with no load. So my question still, why can't we let our engines run? I'm just curious what the difference is.. big engines are designed to run nonstop?
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Old 04-10-2020, 15:01   #4
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Re: Curious about diesels..

I don't have any big truck experience, but aren't those "idling" trucks actually running refrigerator/freezer compressors. I'm sure their not just using their fuel without a purpose.
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Old 04-10-2020, 15:10   #5
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Re: Curious about diesels..

The semis idle overnight because the drivers need to get good sleep. A lot of times they need heat or AC to sleep well. Poor sleep leads to accidents or driving tickets so the take the issue seriously. Trucks stops are starting to offer heat and/or AC which is cheaper for the drivers and better for the engines long term.
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Old 04-10-2020, 15:17   #6
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Re: Curious about diesels..

The correct answer is: Its better not to have the engine go thru a complete cool off /heat up again cycle if it was shut off than to let it idle for 3-4 hours --they consider that to be more debilating to the engine than idling! Sounds like a old wives tale to me ---but Im not a trucker.
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Old 04-10-2020, 15:19   #7
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Re: Curious about diesels..

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I don't have any big truck experience, but aren't those "idling" trucks actually running refrigerator/freezer compressors. I'm sure their not just using their fuel without a purpose.
reefer trucks have their own small diesels separate from the main truck engine
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Old 04-10-2020, 15:29   #8
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Re: Curious about diesels..

I know why the big engines are allowed to run., That's not my question! I completely understand the idea behind it. I'm asking why our little engines are different. Or are they?
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Old 04-10-2020, 15:50   #9
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Re: Curious about diesels..

Bore glazing occurs on big diesels too. However big rigs work their engines hard over a wide range of revs , whereas yacht engines tend to run at fixed revs for long periods , this doesn’t help to break up bore glazing , it’s also a problem for farm tractor engines left idling for long periods
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Old 04-10-2020, 16:05   #10
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Re: Curious about diesels..

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I know why the big engines are allowed to run., That's not my question! I completely understand the idea behind it. I'm asking why our little engines are different. Or are they?
They aren't much different. It is about duty cycle and % power.

Operate at say 5% load for 80% of time and then 20% load for 20% time = bad.

Operate at say 5% load for 20% of time and then 20% load for 60% time and then 85% load for 20% of time = OK

Operate at say 100% load for 100% of time = bad

These aren't hard and fast numbers, rather they are to give you the idea of the continuum between no load all time and max load all the time (i.e. both at bad); there is a sweet range in the middle. The engine designer dictates the range and location of the sweet spot!
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Old 04-10-2020, 16:12   #11
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Re: Curious about diesels..

The use is different more so than the engines.

Big Rigs, might idle over night but then they run 8-12hr under heavy load. Any build up on the cylinder walls gets burnt off each day, so it never becomes an issue.

If you use your boat engine for a couple hours to charge one day and then the next day you motor for a few hours, you will be doing essentially the same thing and it generally won't be an issue.

Where you can get in trouble is if you are at anchor and you idle the engine for hours each day for weeks on end to charge the battery while never actually loading up the engine.

Of course with modern solar systems and generators, most boats don't idle their engines to charge the batteries any more.
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Old 04-10-2020, 17:04   #12
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Re: Curious about diesels..

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Originally Posted by goboatingnow View Post
Bore glazing occurs on big diesels too. However big rigs work their engines hard over a wide range of revs , whereas yacht engines tend to run at fixed revs for long periods , this doesn’t help to break up bore glazing , it’s also a problem for farm tractor engines left idling for long periods
So where have you been? Just woke from a coma?
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Old 04-10-2020, 17:17   #13
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Re: Curious about diesels..

One reason you don’t let small boat diesels idle is the water injection elbow in the exhaust clogs up. The elbow on my diesel clogged with less than a hundred hours because it was only run at idle for an unknown number of years. PO’s had aged out and died and title problems after meant the boat was only run periodically at idle by well meaning friends For a number of years. The elbow blocked up in a few hours after I bought the boat
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Old 04-10-2020, 18:25   #14
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Re: Curious about diesels..

I’ve been thinking about this again too.

I had a Luger 3 cylinder in my northern lights generator. It saw 30 amp loads, microwave loads and tiny loads at the tail end of battery charging.

My 6 cylinder Cummins straight 6 is at like 1/10 throttle 80% of its life. It idles plenty and is taxed hard fairly infrequently. No problems.

Makes you wonder why people are so paranoid about marine engines dying from the same.
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Old 04-10-2020, 18:51   #15
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Re: Curious about diesels..

I wonder the same thing...
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