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Old 09-09-2016, 00:55   #1
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Why are high RPMs bad for diesels?

I know the big criticism on charter boats is the excessive amount of hours on the diesel engines. I have heard that in itself is really not a problem for diesels, it is the high rpms the tourists use to push the boat to maximize their vacations.

What exactly is the damage caused by running the diesels engines above 2500 or 2800 rpms?
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Old 09-09-2016, 01:07   #2
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Re: Why are high RPMs bad for diesels?

Diesels work on compression to create the combustion rather than a spark plug to help create the combustion. The compression exceeds as the engine revs increase and pushes the internals inside the engine to their maximum stress levels.

Its based on low burn rate of diesel plus the longer stroke of the diesel engine.

First you must understand the difference between petrol and diesel engines, the diesel works on purely compression of fuel , heating and generating bang to produce power, the gasoline on the other hand is natively twitchy and needs a spark to explode and produce power on its own.

That said,

Think of the petrol engine as a cheetah,it has lightweight bones and has a streamlined body thus is very fast but is not powerful enough to kill a lion or a rhino.

The Diesel engine is like an elephant, its slow and has a lot of strength, but needs heavier legs to support the huge body mass thus it cant run as fast as the cheetah but has a lot of strength.By strength of the elephant I mean torque in the engine.

The petrol engine natively combusts so it does not need heavy duty parts to withstand the explosion , yes you are compressing the fuel but its not nearly as much as the diesel, that is why 100cc motorcycles rev like crazy they have lightweight construction(cranks, pistons).

The diesel does not combust as much as petrol , it needs to be compressed to a much higher extent to combust thus the cylinder head , the piston, the crank everything needs to be heavy duty to withstand the compression explosion thus slowing down the speed of the piston.

Diesel burns slowly compared to petrol and for most diesel engines the limit is around 4800 to 5000 RPM.

Adding clarity to the above point, in a petrol engine almost 95% of the fuel is burnt in every stroke but in the diesel engine not all of the diesel is burnt in each stroke due to the slow combustion rate some diesel remains in the cylinder before the next stroke so no matter how hard you push , that small quantity of diesel will not burn before the piston is ready for the next stroke thus limiting the speed or redline.
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Old 09-09-2016, 01:08   #3
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Re: Why are high RPMs bad for diesels?

I've ever heard that before. My understanding is that diesels like to be run at high load and high rpm. Keeps carbon buildup to minimum and even burns it off.
Much much worse is running the diesel at idle or low rpms with little or no load for any longer periods.

edit: poster #2 beat me in replying. my reply is to op's post #1.
cheers.
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Old 09-09-2016, 01:23   #4
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Re: Why are high RPMs bad for diesels?

Any engine if constantly ran at excessive rpms is not going to last as long as if ran at recommended rev range. Charter boat people i guess hop in and think its not their engine so why not push it to the limit for the 5 days we have the opportunity to.
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Old 09-09-2016, 02:16   #5
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Re: Why are high RPMs bad for diesels?

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Originally Posted by nauticalnomad View Post
Any engine if constantly ran at excessive rpms is not going to last as long as if ran at recommended rev range. Charter boat people i guess hop in and think its not their engine so why not push it to the limit for the 5 days we have the opportunity to.
nauticalnomad's explanation is easy to understand.


The only way to explain better needs discussing design factors of diesel engines, specifically the bore/stroke ratio, the type of fuel injection, and the timing of fuel injection.


Many diesel engines are designed for long life at low engine speed. They then to have long piston strokes compared to the bore diameter of their combustion cylinder. Such engines usually do not have computerised (or electronic) management of the timing of injection. And are mechanically fairly simple - one of the implications being that they have a preferred engine speed range.


Other diesel engines, such as auto engines labelled tDI for "turbo direct injection", are designed for the other end of the scale - for high engine revs with a peak of torque and horsepower at a much higher engine speed than that first category. Such engines almost always have electronic management that, as its name suggests, manages when and how much fuel is injected. Electronic chips can be swapped out of that electronic management unit to change the preferred engine speed range, at what speed the torque peaks, and at what engine speed max horsepower is generated. All that delivers performance for a car on the road. Very few small marine diesels are in this second category.


Most marine diesel engines in the sailing fleet are in between those two ends of the diesel engine spectrum.


So what happens to the charter fleet: people who are only used to driving a spark ignition engine, have never maintained a diesel engine, have never read that book about Zen and the art of motorcycle maintenance.




So charter people often (but not always) drive the boat like they stole it (or if you prefer, they drive it like they drive a spark ignition engine or a tDI engine).


Which sure does blast any carbon deposits out.


And get the engine to its operating temperature (and some more) fast.


They don't cool the engine down by letting it idle to cool, but just keyswitch it off like they would a car, trapping heat in the engine.


And so on ... all ways of increasing wear and tear.
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Old 09-09-2016, 02:47   #6
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Re: Why are high RPMs bad for diesels?

Turbo diesels like to be run at higher rpm. If run continuously at low rpm, be prepared to replace the turbo which will eventually plug up, overheat and be ruined.
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Old 09-09-2016, 03:44   #7
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Re: Why are high RPMs bad for diesels?

Two things mainly. Wear and fatigue. An engine running at 2400 rpm instead of 1200 rpm will wear more. The piston in that example will cover double the distance per hour. With fatigue of all stressed parts in the engine, they will suffer more fatigue the higher the stress. With metal parts the closer you go to the stress limit, the more proportionally that fatigue life is shortened. An engine at high revs will have very much more load than at low revs.
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Old 09-09-2016, 04:06   #8
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Re: Why are high RPMs bad for diesels?

Boats must be harder on engines than heavy equipment!
We run our equipment wide open 10 hours a day, on a 60 hp Kubota skid steer thats about 2800rpm. Summer ripping pavement, winter plowing snow and loading salt. Adds up to about 2000+ hours a year, deal them after 5 years.
I have never run a piece of equipment that wasn't designed to run wide open. LOL a bulldozer has a decelerator pedal, runs full speed until you step on that pedal to go from forward to reverse.
Maybe it's a maintenance issue, I admit I don't have much diesel boat experience....
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Old 09-09-2016, 04:21   #9
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Re: Why are high RPMs bad for diesels?

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Originally Posted by poiu View Post
Two things mainly. Wear and fatigue. An engine running at 2400 rpm instead of 1200 rpm will wear more. The piston in that example will cover double the distance per hour. With fatigue of all stressed parts in the engine, they will suffer more fatigue the higher the stress. With metal parts the closer you go to the stress limit, the more proportionally that fatigue life is shortened. An engine at high revs will have very much more load than at low revs.
I agree. It is an item of popular wisdom that diesels like to be run near maximum RPM and load, but in my opinion this is bullocks.

Diesels don't like to be run at very low LOADS -- because they flow a constant amount of air at any given RPM (unlike petrol engines), and with very low load, the governor injects a very small amount of fuel, and the combustion chambers don't get warm enough, and there's not enough pressure to seat the rings. You get blow by, bore polishing, wet stacking, etc.

That's why these problems occur so often with constant speed diesel generators -- too little load for the RPM. A small load might be ok at idle speed, but is not ok at 1800 RPM or whatever.

But like any mechanism, as Poiu says, the more stress you put on them, the more they will wear, so operating near max RPM or load will wear them out much faster than somewhere in the middle of their range.

Turbo engines are better with smaller loads, because when the turbo is off, they are in effect a small capacity engine, and better tolerate smaller loads, than a larger capacity non-turbo engine would tolerate the same load. You just have to be sure to run them up hard periodically to keep the turbos from coking up -- follow the manufacturer's instructions. Yanmar says once an hour for a few minutes.

My father always ran his Perkins 4-108 at 2000 RPM, and got more than 20,000 hours out of it with zero problems or work on it, ever, and it was still running perfectly when he sold the boat. I'm not sure he even ever adjusted the valves, but he did change the oil religiously.

I run my turbo Yanmar at 1600 to 2300 depending on the situation, mostly at around 1800. I follow the instructions and run it up once an hour when operating it below 2000 RPM. It runs like a top, starts instantly, doesn't burn a drop of oil. It runs very well at max load and RPM -- I can get about 9.5 knots at 3900 RPM, turbo whistling merrily. But mostly I run it at 1800 RPM, where it uses about 1/10 the amount of fuel as it does at 3900.
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Old 09-09-2016, 04:29   #10
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Re: Why are high RPMs bad for diesels?

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Originally Posted by lindabarzini View Post
What exactly is the damage caused by running the diesels engines above 2500 or 2800 rpms?
Aside from what everyone else said...

The actual RPM numbers vary per engine. Over 2500-2800 for a high-revving Yanmar, which might top out at 3600 RPMs, isn't necessarily a big deal. 2600 on our Cummins engines and several of the comparable CATs is WOT, good to check occasionally, but not rated by the manufacturer for continuous operations.

IOW, to define "high," the exact numbers for a given engine is something to get direct from the manufacturer.

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Old 09-09-2016, 04:57   #11
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Re: Why are high RPMs bad for diesels?

My bet is that a charter boat that has been run often and hard if propped properly is more likely to outlast a dock queen that is run at low rpms and a few times a year. Load in most cases is the determining factor of engine wear
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Old 09-09-2016, 05:45   #12
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Re: Why are high RPMs bad for diesels?

The generic answer I have gotten from different boat mechanics is marine diesel engines like to be run at approximately 80% of max rpm. So for a 2600 rpm red line you are looking about 2100, for a 3000 rpm red line 2400 rpm, and so on.
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Old 09-09-2016, 05:51   #13
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Re: Why are high RPMs bad for diesels?

OK, where to start?
First RPM of any reciprocating engine isn't the issue, it is piston speed, meaning long stroke engines must be run at lower RPM in order to keep piston speed within reasonable limits, this is why performance motorcycle motors can be run at very high RPM all day long with no harm, short strokes.
The RPM limit of a Diesel has nothing at all to do with structural limits, it has to do with fuel delivery, but sometimes a designer will accept the fuel delivery RPM limit and spec a long stroke that closely matches the fuel limit to enhance Torque.
A spark ignition engine for example can continuously spray fuel onto the backside of the intake valve, then when the valve opens, the engine will inhale the fuel, but combustion does not begin until the spark plug fires. This is one very good reason a spark ignition engine can run at such low fuel pressure as in 30 PSI or so, it has all day to deliver the fuel.
I Diesel engine delivers fuel directly into the combustion chamber, whether it has a pre-chamber or not, and fuel burn begins the instant fuel becomes available, so you have to begin an injection pulse at exactly the right moment and get the fuel in there fast, as the piston isn't waiting on you. This interval gets shorter and shorter as RPM increases, pretty quickly even with insanely high fuel pressures, there is just not enough time to get the fuel in there, that is what limits Diesel RPM.
A common rail Diesel will run fuel pressures above 30,000 PSI, yes that is not a mis-type, 30,000 PSI and they are THE hot rod Diesels, I have a 6.6 Diesel truck I have had for years, a common rail and let me tell you compared to our Grandfathers Diesels, it is a completely different animal.
On it's Tachometer, it has two distinct redlines, it goes hashed at 3,500 RPM, that is the fuel delivery limit, but during heavy downhill braking, its acceptable to allow the engine to turn up to 5,000 RPM, now we are getting close to the mechanical limit.

In short usually an engine design will allow limited high power which equals high RPM cause HP = RPM x Torque if you need it, My Yanmar 4-JHE for example allows operation at 3600 RPM for one hour, 44 HP, but continuous only 3500 or 40 HP, usually limiting factor may be heat.

Then there was a study done previous to WWII by the Brits that showed engine wear increased non linearly, meaning if you say double RPM you more than triple wear, presumably it due to the higher kinetic forces of a recip engine, remember that piston has to stop and reverse direction at every stroke, that stopping and starting has tremendous energy that has to be absorbed by the bearings, plus of course higher RPM means everything is happening more often in a given time frame, so of course higher wear.
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Old 09-09-2016, 06:05   #14
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Re: Why are high RPMs bad for diesels?

Then we get into what happens if I run her at low load / RPM?
Well some answers are it doesn't get hot enough, and what that means specifically is it takes heat to burn off combustion products that if you don't can lead to bore glazing, where deposits fill the cross hatching in a cylinder bore and you end up with a glass smooth bore.
Sound good having a smooth bore right? Well it's not and the reason it's not is the cross hatching's job is to hold oil, this oil lubricates the piston and rings, carries away wear material and provides a ring seal.
A smooth bore will increase cylinder wear as there is no oil, it will increase cylinder blow by which will cause the oil to overheat and fill with soot, and soot is carbon and I think you know how well carbon lubricates. Hint, it's like sand.
Additionally if you don't get your oil up to about 180 F, you won't cook off the moisture, moisture will combine with combustion by products and form acid, and you don't want acid in your oil for obvious reasons. Actually most oils are formulated to neutralize acid, sometime zinc is used, if you do an oil analysis, one of the numbers will be TBN, which stands for I believe Total Base Number, or the measure of how much acid can the oil handle, forever a high TBN was considered good, but newer oils I believe have a much lower TBN as the metals used to neutralize acid are not friendly to pollution control devices, I believe zinc will poison a catalytic converter for instance.


So what does this mean? It means that if you want to make your Diesel last as long as possible, change oil and filter often and run it hard enough to get the oil and cylinders well up to operating temp, but don't run her too hard as with anything, running the snot out of it continuously will shorten its life.
A bad thing to do is go down to visit your boat and run it in the slip for fifteen min or so without even moving the boat, although I see that often. If your going to do that, put it in gear and go on a dock cruise for fifteen min or so with the engine at least low normal cruise RPM.
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Old 09-09-2016, 06:13   #15
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Re: Why are high RPMs bad for diesels?

Interesting no mention of improved oils. The oils have improved greatly in the last 20 years, less breakdown, better lubrication, suspends more particals.

Also in the last 20 years engine block, pistons, cranks are all machined to closer tolerances.

Living with 2016 technology and machinery is so much better than when I was a kid. Everything works better, very few mechanical breakdowns although engine software components could fail.

I remember back in the day. If something worked I was surprised. Now I expect things to work.

But I also agree high RPM's near the red line does not help longevity of even an engine that was designed to last a long time.


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