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Old 02-03-2011, 06:10   #1
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Engine wear at different RPM's

DefJef wrote the following in another thread (http://www.cruisersforum.com/forums/...ean-55985.html):

"The other point we don't see discussed with respect to engine wear is the nature of the engine wear. A diesel run at no load and idle will show more wear than one run under load at a few thousand RPM. So few entire rotations can equate to MORE engine wear. I would guess that the same would apply running the engine constantly at the "red line". Each of those three engines running the same time would show different wear with the idle and the redline engines showing more wear.

I'd be curious to see some sort of plot of engine speed, load no load vs engine wear. Are there any out there?

And why don't we have more sophisticated metering showing total number engine revs (as opposed to hours)? I noticed that my hr counter was wired to the key switch and so if the key was in the on position without the engine turning over it was counting hours! I fixed that one. "


I think it's a fascinating question.
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Old 02-03-2011, 06:18   #2
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Re: Engine wear at different RPM's

Could you link to the previous thread where DefJef is posting?
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Old 02-03-2011, 06:19   #3
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Re: Engine wear at different RPM's

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Could you link to the previous thread where DefJef is posting?
\Opps I see it
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Old 02-03-2011, 06:46   #4
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Re: Engine wear at different RPM's

It is an article of faith among sailors that diesel engines will carbon up, polish the bores, or fly apart (depending on who you talk to) if you run them for a long time with a low load and/or at low RPM. That you had better keep them loaded up. I was even warned not to use my genset at less than 50% of its capacity.

Now this just doesn't make any sense to me. Why would this be the case? At zero load and idle -- that I can see. Because a diesel engine, contrary to a petrol engine, doesn't produce much heat if it is not doing any work. So at idle and zero load, the engine might not be able to maintain a reasonable operating temperature, and running a cold engine for hours at a time might indeed cause carbon deposits.

But a fresh water cooled marine engine has a thermostat. So as long as such an engine is producing more heat than is lost through radiation into the engine compartment, it will be able to maintain operating temperature. On my Yanmar, that temperature is 80 degrees C, much warmer than raw water cooled engines.

As long as the engine has operating temp, and is turning fast enough to have normal oil pressure, why would it carbon up or polish the bores? And at any speed at all when you're making way, you will have a certain load on the engine anyway.

My Yanmar manual says nothing about not running at low loads. In fact, there is an optional "trolling throttle" for extended operation at under 1,000 RPM.

Can it be that the popular wisdom about not using marine diesels at low loads comes from raw water cooled engines? Because a raw water cooled marine diesel is a different kettle of fish, so to speak. It has no thermostat, so it will have optimum operating temperature (and that is quite a bit lower than the standard operating temp of a fresh water cooled engine) only close to full load. The lower the load, the lower the temperature, and pretty soon the engine is so cold that lubrication is not working properly and carbon deposits can form.

Could it be that this old rule was made for raw water cooled engines, and is not really applicable to fresh water cooled engines?
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Old 02-03-2011, 07:13   #5
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Re: Engine wear at different RPM's

I've never really believed that low load/speed will greatly increase wear on the engines (not counting starting and really low idle).

I do believe it a good practice to run the engine up under load once in a while. But to me this would only really be needed if I was rountinely running the engine just to charge the batteries.

In practice diesel truck engines (and I doubt the cyclinders and pistons of a truck engine are really any difference to a marine one) idle all night to provide heat etc while the drivers sleep. And these last along time so why wouldn't a marine engine?

I think this falls into the "everyone knows this" area where people believe sometime that isn't really true. Lets see some proof; like a tech paper from the engine manufacturer's.
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Old 02-03-2011, 07:36   #6
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Re: Engine wear at different RPM's

I'm at home right now and my engine manual is on the boat, so this may not be a perfect quote, but my manual says that the ideal cruising RPM should be 80% of Maximum Continious RPM. It also states that if operating for long periods below this speed that the engines should be run at this speed for 5 minutes every hour to prevent carbon buildup and cylinder glazing. Now it may be that the evil empires are just trying to get you to wear your engines out faster to sell you a new one, but I don't think so. It seems to me that even they recognize that if their engines get a reputation for wearing out rapidly they won't sell very many.
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Old 02-03-2011, 07:55   #7
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Re: Engine wear at different RPM's

Detroit Diesel experts have told me that running at low RPM's does not cause wear, but will allow a carbon build up that could cause problems if not addressed. The solution is simple. All that is needed is to run at 70 to 80% RPM's for 15 minutes after running at low RPM's for hours.

Even though the engines cooling circuit will be at normal operating temperatures at low RPM's, the combustion and exhaust components will not get to temperatures that will eliminate carbon.

They also advise that wear relates to total fuel consumed rather than running time. This means that slower speed equals less wear per hour.
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Old 02-03-2011, 08:02   #8
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Re: Engine wear at different RPM's

Whoops accidentally submitted before the second paragraph.

Strictly speaking, carbon build up and cylinder glazing are not wear. Carbon buildup can choke the exhaust, foul the valves, and creating hot spots in the combustion chambers causing improper fuel burning, reduced power, and severe maintenance headaches. Cylinder glazing is a film of unburned fuel residue that ruins the ability of the rings to seat properly causing increased oil consumption and poor compression, which reduces power output. It's pretty well known in internal combustion engine engineering that higher RPMs always equals more wear, that is higher friction and more erosion of moving metal parts. The nature of diesel fuel and diesel combustion actually reduces the wear rate compared to a gasoline engine, but wear is not the only issue to consider when considering the useful life of an engine. If my exhaust ports are clogged with carbon, the exhaust valves fouled, the cylinders glazed and my 40 hp engine is only putting out 8 hp, consuming almost as much oil as fuel and smoking like a old chimney, the cost of putting right will essentially be the same as a rebuild on a worn engine. The fact may be that the metal parts may only have worn 10% of their useful lives, but my wallet won't be able to tell the difference.
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Old 02-03-2011, 08:08   #9
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Re: Engine wear at different RPM's

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Cylinder glazing is a film of unburned fuel residue that ruins the ability of the rings to seat properly causing increased oil consumption and poor compression
I thought this was nonsense, and went to find some information to post about it, and found out that this is really true! You learn something every day. I had not known that unburned fuel played any role in bore glazing.

Here is a great resource on bore glazing and bore polishing:

Bore glazing

"Bore glazing occurs at low speed and light load operations, particularly resulting from blow-by in new engines. Exhaust gases passing piston rings in newly built or re-built bores can react with oil and wear products, forming a golden-coloured varnish glaze. Most engine manufacturers warn against the potential problem. Sabb and Lister-Petter are in surprising agreement on the subject, as shown in their operator's handbooks. In each case the wording is identical: “Long periods of light or no-load running early in the engine's life may lead to cylinder bore glazing and high lub oil consumption.”

The authors suggest that bore glazing occurs mainly during the break-in period, and that broken-in marine diesels which have the right type of oil are very unlikely to suffer from bore glazing under any conditions of load or speed. They say that any load at all will prevent glazing:

"An engine that is driving the boat will never suffer from either glazing or polishing, even if it is at anchor or on a mooring."
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Old 02-03-2011, 09:03   #10
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Re: Engine wear at different RPM's

Again, run your diesel like it is designed to and the manufacturers suggest or kill it with kindness. Really, there is no Cabal to sell you new engines because you ran it like they suggest.
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Old 02-03-2011, 10:06   #11
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Re: Engine wear at different RPM's

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Originally Posted by Dockhead View Post
[...] Now this [engine wear (mrm)] just doesn't make any sense to me. Why would this be the case? [...]
Temperatures are only one aspect of what is going on. Another one is too light pressure on piston rings. Rings in diesel engines are designed in such a way that, when combustion occurs, the pressure that results has easy access to space behind rings and pushes on them from behind, which, in turn, increases pressure of the rings on cylinder walls. This increased ring pressure serves two purposes:

1. is to help seal the combustion space and reduce blow-by;

2. ensures, that there is a constant and uniform wear pattern to the rings and cylinder walls.

Small loads produce not enough of (2) and glazing develops.
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Old 05-03-2011, 05:12   #12
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Re: Engine wear at different RPM's

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Originally Posted by Dockhead View Post
DefJef wrote the following in another thread (http://www.cruisersforum.com/forums/...ean-55985.html):



And why don't we have more sophisticated metering showing total number engine revs (as opposed to hours)?
IIRC, diesel trucks I worked with years ago had tachometers with a meter recording revolutions (like an odometer), called "hours". I believe one "hour" was 100,000 revolutions. At 1667 rpm (low for those engines) one "hour" would take one hour, so reasonably close.
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Old 05-03-2011, 05:47   #13
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Re: Engine wear at different RPM's

Talk to English narrow boat owners they rarley use 10 - 15% full power.
Their motors last a life time!!!!!!!!!!!!!
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Old 05-03-2011, 06:47   #14
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Re: Engine wear at different RPM's

I've heard that the number of years an engine will last is based on the amount of fuel comsumed. Everything being equal I suppose that just might be the case. I have a 25 year old Perkins 4-108 and it still is running well even though it is/was over propped for all those years. I run most of the time at no greater than 80% of the WOT however my WOT is much less than the Perkins suggested number of 3600 rpm. I can acheive the Perkins suggested WOT in neutral but not when the gear is engaged thus my pitch is too high. I don't think that I'm wearing the engine and causing problems with the cylinder bores. People compare performance of a truck trying to take a large hill in the upper gears and the result is lugging the engine however I don't think it would be lugging if the truck can take a hill without going to full throttle in that particular gear. IMHO
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Old 05-03-2011, 07:17   #15
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Re: Engine wear at different RPM's

You can't have these conversations without separating engine types. (APPLES TO ORANGES)... A truck engine, or slower Perkins and the like, are very different from modern, lightweight, high RPM engines, like Yanmars. This group, unlike the other two, likes to be run under load, and mostly at high RPMs. This prevents carbon build up, and increase lifespan.

It seams counter intuitive, I know... But there it is. If you call Yanmar America's engineers, or Mack Boring's, etc, they all agree.

I run my 2 GM 20-F at 2,900 RPM all day long.

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