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Old 01-28-2019, 08:43 AM   #1
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Engine Longevity

The January/February issue of PassageMaker has an interesting article on diesel engine life by Steve Zimmerman. It is well worth reading particularly since there is one subject he raises that runs counter to many popular notions - engine loading.

Steve offers up that to maximize engine life (meaning hard iron innards) the engine should be operated at 35% load, as measured by fuel burn, or less, except for bursts to 80% load to clean things out. He defines how load is calculated, M ratings, prop curves, old vs new engines and general maintenance.

A read of the article should keep in mind Steve Z is not your average weekend boating warrior. He is a step or two above most of us. In my case the engines cruise along at 30% load with a few minutes per day above or below. On TF it is common to read that diesels are meant to be run hard, heck even at 80% load continually.

The PM article gets one to thinking,
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Old 01-28-2019, 09:40 AM   #2
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Most diesels do best at or near their torque peak which varies somewhat from engine to engine. Also aspiration needs to be taken into account. Some engines may indeed do well at 35% load and others not so such as engines rated at continuous duty. It's important to have a manufacturers graph of your engine to really understand your particular engine Here's a brief article on ratings that may illustrate better.Frontier Power Products - Making sense of diesel engine specs
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Old 01-28-2019, 10:04 AM   #3
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Engine percent load is a weird concept. 35% load on a N/A Lehman 120 or Perkins 135 is about 80 hp. But that 80 hp is only 17% load on a Cummins QSB5.9 480, an engine with about the same displacement as those. Now don't get me wrong as I think that the QSB can easily produce the 170 hp continuously that it takes to get to 35% in fact that is low for continuous duty for that engine.


I prefer to think in terms of hp per liter, the concept that Tony Athens uses when talking about getting best life from a diesel. He believes that you should run at no more than 35 hp per liter for a high output turbocharged diesel (which doesn't apply to the N/A Lehmans or Perkins). That value is about 210 hp for the Cummins or 44% load. Also note that that same engine mechanically also comes in 380 down to 230 hp versions and the pct load on each will be even higher.


Also the era of the engine is important when talking about how much hp you can pull out continuously. The older Lehman and Perkins didn't have piston lube oil cooling, after cooling, better machining, metallurgy and lube oils. All of which lets you pull more hp with less wear.


Read Tony's article on the subject here: https://www.sbmar.com/featured-artic...t-perspective/



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Old 01-28-2019, 10:22 AM   #4
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Engine percent load is a weird concept. 35% load on a N/A Lehman 120 or Perkins 135 is about 80 hp. David
Your 80hp number seems very high for these two engines. Steve Z's methodology load calculation is to look at prop curve. If max HP developed is at 6 gph, then 35% is a bit more than 2 gph or 38 hp.
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Old 01-28-2019, 11:28 AM   #5
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Your 80hp number seems very high for these two engines. Steve Z's methodology load calculation is to look at prop curve. If max HP developed is at 6 gph, then 35% is a bit more than 2 gph or 38 hp.

You are right, I made a mistake. I somehow got 35% of 130 = 80. It doesn't obviously as it is about 38 as you say. Makes the 35% of 480 hp even worse by comparison for about the same displacement doesn't it?


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Old 01-28-2019, 11:31 AM   #6
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There's too many other factors that come into play to go by just a % of load calculation.
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Old 01-28-2019, 11:46 AM   #7
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You are right, I made a mistake. I somehow got 35% of 130 = 80. It doesn't obviously as it is about 38 as you say. Makes the 35% of 480 hp even worse by comparison for about the same displacement doesn't it? David
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Many of our engines are/were used in gensets. An interesting comparison to the marine application is the genset Cummins 5.9 or 6.7. For genset use the base engines are fueled to sit pretty close to the 30 - 40% load range as originally noted on their marine application.
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Old 01-28-2019, 11:48 AM   #8
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My cruise speed of 7 knots burns 2 GPH at close to 1,500 RPM. That 2 GPH equates to 40 HP which is 30% of the 135 HP rating for my JD 4045tfm75. The only shortcoming is that I'm 300 RPM below peak torque. Unfortunately there isn't a transmission available with a 3:1 ratio and a down angle to increase the RPM for same speed. Still very happy with the setup, performance, and hopefully longevity.

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Old 01-28-2019, 11:51 AM   #9
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I have long questioned the "run it hard" mythology. The history of it seems to come from large, lightly loaded mechanically injected generators, which suffer various maladies from old injection technology, like after dribble and leakage etc.

There are many more diesels in trucks than in boats (and often the boat engines are adapted from trucks). These engines are not run at 80% load often, and do not suffer the consequences boaters attribute to low loads. Especially true of common rail engines where injection efficiency is not tied to rpm or load. It is not uncommon for truckers to idle their engines all night to keep HVAC going in the cab, without harm.
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Old 01-28-2019, 12:25 PM   #10
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I think it was Tony Athens

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I have long questioned the "run it hard" mythology. The history of it seems to come from large, lightly loaded mechanically injected generators, which suffer various maladies from old injection technology, like after dribble and leakage etc.

There are many more diesels in trucks than in boats (and often the boat engines are adapted from trucks). These engines are not run at 80% load often, and do not suffer the consequences boaters attribute to low loads. Especially true of common rail engines where injection efficiency is not tied to rpm or load. It is not uncommon for truckers to idle their engines all night to keep HVAC going in the cab, without harm.
Who said the less fuel you burn for given distance, the longer your engine will last. Going faster and burning more fuel while not necessarily bad for the engine, will shorten its life.

He also talks about the importance of quickly getting the engine up to temperature to reduce the amount of unburned fuel that gets into the oil, or the possibility of fuel washing lubricant out of cylinders. He says we should start the engines just before throwing the last two lines and letting the engines warm under load.

I have always suspected the 80 rule. An over the road truck typically runs at 30 percent or less. We got a million miles from our fleet of trucks.

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Old 01-28-2019, 12:31 PM   #11
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Who said the less fuel you burn for given distance, the longer your engine will last. Going faster and burning more fuel while not necessarily bad for the engine, will shorten its life.

He also talks about the importance of quickly getting the engine up to temperature to reduce the amount of unburned fuel that gets into the oil, or the possibility of fuel washing lubricant out of cylinders. He says we should start the engines just before throwing the last two lines and letting the engines warm under load.

I have always suspected the 80 rule. An over the road truck typically runs at 30 percent or less. We got a million miles from our fleet of trucks.

Gordon
That million miles average for semi engines translates to around 20,000 hours or around 100 years the way most people use their boats.
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Old 01-28-2019, 01:53 PM   #12
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I read the PM article also and was so pleased to see a picture of the Ford Lehman Super 135 - said if maintained ok they would run 10,000 to 20,000 hrs between over hauls - and I have two of those - 1988 models with around 5,000 hrs each, for 30 yrs of use. Simple, NA and easy to maintain Now its up to me to continue the maintaining
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Old 01-28-2019, 02:38 PM   #13
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Maybe for recreational or light duty rated engines. While I don't think running at 30% load will harm a diesel, I know running at 80% of hp will double the life of most engines, compared to running flat out.

Heat and dirt are the real life killers of engines. Clean oil causes very little wear. But heavy loads of soot and other contaminates will start wearing bearings, rings and cylinder walls or sleeves. As the bearings wear the clearances open and let oil escape causing a lowering of oil pressure. Cross hatching on the cylinder or sleeve wall will wear away faster with dirty oil, eventually leaving smooth walls. Once the cross hatching is gone, ring and wall wear happens very fast. Then the engine becomes hard to start, burns and passes oil in the exhaust. Cross hatching is the tiny scratches purposely made in the walls during the honing process. It carries oil to the upper cylinder, lubing the rings. See pic of Detroit sleeves. Most full flow oil filters bypass oil once the filter becomes clogged with dirt. Then oil circulates w/o any filtering. Full flow filters only catch dirt to about 30 microns when new. 30 micron dirt will cause wear. Adding a bypass filter or centrifuge will capture debris down to 1 micron, maybe smaller. Stopping almost all wear.
Too much heat, measured in exhaust gas temperatures, especially turbo engines, will break down the metal alloys in cylinder components - valves, rings, pistons, cylinder walls or sleeves. Running a turbo engine at 80% of hp will greatly extend the engine life by lowering temps inside the cylinder. In my experience, it will more than double the engine life (along with clean oil).
Heavy duty engines with continuous ratings probably get the best hp/fuel usage when fully loaded, Like ship, tug and other commercial use engines. But they're made differently. Most continuously centrifuge the engine oil. And the oil is heated before starting. Some have hydraulic accumulators that store oil at pressure that is released before the engine is started to pressurize the bearing surfaces. Most wear occurs at startup. HD engines have heaver castings, thicker rings, special expensive alloys requiring special machining that typical recreational engines don't have. And heavy duty engines are designed to be rebuilt in place many times.
You want to extend your engine life? Add a bypass filter or centrifuge and keep the rpm down to 80% of HP.
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Old 01-28-2019, 03:06 PM   #14
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Heavy duty purpose built engines are a different animal with different economics. If the application requires a certain constant power output, for longer than the lifetime of an engine, then total power output x time between replacement / cost is the equation. Therefore it may make sense to run at 90% power for a shorter time at less replacement cost, than 30% power on a much larger engine for longer, but with higher replacement cost.

Recreational boat engines, especially trawlers have a different mission and different economics. 1st, most diesels in this service are killed due to mistreatment, long before they would have worn out. Unless you acquire it late in it's life cycle, it will likely last longer than you own it, whether run at 80% or 30%.

I'd agree with Tony, the less fuel you burn the longer it will last - but I suspect it will outlast my ownership regardless.
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Old 01-28-2019, 03:38 PM   #15
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Couldn't imagine running ours at 80%.
40 lph extra fuel burn for a 3 knot increase in speed simply ain't worth it.
Much happier at our less than 30% for 8 knots
I reckon the engine and running gear is happier as well.
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Old 01-28-2019, 03:54 PM   #16
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"Maybe for recreational or light duty rated engines. While I don't think running at 30% load will harm a diesel, I know running at 80% of hp will double the life of most engines, compared to running flat out."


So you believe that a QSB5.9 480 hp engine will last a long time running at 80% of 480 or 380 hp????


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Old 01-28-2019, 04:32 PM   #17
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I hate to read such blanket statements by someone claiming to be an expert. Engines are rated for different uses and manufacturers will give you guidelines on how to best run them. This goes from continuous use work engines designed to run at WOT or near it 24/7 to high performance engines designed for light loads and occasional use.

We run all ours at a maximum of 67-80% load depending on boat and engine. But we also vary speeds and run them at 30-60% a good bit. We also occasionally run at WOT mostly for a performance and operating test.

Now, I think fuel used is a good measure for servicing and for the life of an engine, but not by itself, the care of it is extremely important. Based on fuel usage, clearly a boat run slower will use less and theoretically the engine will last longer. However, there are exceptions to that too. I'll toss one out. 65% load can run 23 knots and use 96 gph. Can run at 28% load and get 15 knots and use 42 gph. Let's go 1000 nm. Boat 1 will take 43.5 hours and use 4176 gallons. Boat 2 will take 66.7 hours and use 2801 gallons. So, you run the engine 53% more hours and use 33% less fuel. Will Boat 2 really last longer? Will it even though the engines were specified for faster running and designed for 65-80% load? I'm not convinced it's that simple. I've seen people regularly run a boat that we run at 70% load at about 12% load just barely above displacement. They put twice the hours on them. We've had no problems and they have had some. Now, I won't blame it on the speed. I'll say maintenance and care or could just be luck.

Zimmerman's statement is probably true for a few boats, but it says on most that you should ignore the builder and the engine manufacturer and run the engines at less load than that designed for. I don't like that advice.
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Old 01-28-2019, 04:36 PM   #18
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3208 NA Power Curve

Just so i understand load correctly, it is the HP for a given RPM as a % of total HP according to the power curve such as one below?

And for a trawler would you use the displacement one vs the second one?

For example, at 2200 RPM would it be 102/210 or 49% load?

Or would it be 184/210 or 87% load?

Makes a big difference, not sure i understand why there are two versions...
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Old 01-28-2019, 05:42 PM   #19
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Just so i understand load correctly, it is the HP for a given RPM as a % of total HP according to the power curve such as one below?

And for a trawler would you use the displacement one vs the second one?

For example, at 2200 RPM would it be 102/210 or 49% load?

Or would it be 184/210 or 87% load?

Makes a big difference, not sure i understand why there are two versions...
The article is on the Passagemaker web site. Your boat appears to have a semi-planing hull. As I understand it, you divide the fuel flow at your selected cruise RPM (on the dotted line prop curve) by the fuel flow at maximum rpm. So for your example of 2200 rpm it would be about 6gph/11.9 gph... roughly 50%. If you cruise at say 1700 rpm, the fuel burn on the dotted line looks like about 3gph (2.8 per the chart)...so 2.8/11.9 is about 23%. That said, the note on the chart in your attachment says this particular prop curve is for displacement hulls. If you have a prop chart for a planing hull, or better yet a semi-planing hull, you'd probably get a more accurate set of numbers.
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Old 01-28-2019, 05:51 PM   #20
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One should run an engine so as to not cause excessive wear. But hard enough to keep the temps up (mostly lube oil) to minimize the buildup of deposits corrosive and otherwise.

The ideal engine speed and load varies (maybe considerably) from engine to engine.

Also I don’t think there’s anything magic about max torque rpm.

I’ve always run my trawler at very close to 50% load.
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