Under loading.

I liked this article, although I'm sure others may debate the specifics.

The Perils of Chronic Under-Loading | Steve D'Antonio Marine Consulting

Jim


Jim
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Overall a good article. Think most diesels can live happy long lives around 50% of rated power though.

Ted

Many diesel marinizations do well in boats as they carry an auto style rating , which is nonsense for a vessel use.

3 cubic inches per HP will keep most folks in the ball park.

I am not sure from reading Steve's article at what point "chronic under loading" occurs. He says 75% load is optimum, but the pictures of a Man engine's instruments show 28% load and he says that the coolant and oil temperatures at that load are "ideal".


So, is it 75% or is it 28% or is it ??? Mr. D'Antonio?


Frankly, I believe a lot depends on the engine. A 6 liter Lehman 120 should be able to run just fine all day long making 25% load or 30 hp. Thousands and thousands cf cruisers have run them that way with no harm done. OTOH my 5.3 liter Yanmar 370 will probably run too cool at that hp loading. Why? It has a cooling system that is designed for three times the power of the Lehman and it has unregulated sea water cooled oil. Both take more than 30 hp for the oil and water to get up to operating temperature.


On the combustion side, the Lehman's injectors are sized for 120 hp and mine are sized for 370. When those injectors on my engine are only operating at 1/12 load they can't have as nice a spray pattern as injectors operating at 1/4 load.


All of which means I run my Yanmar to produce 60 hp minimum or about 16% of max. I do try to run it up to 75% load for fifteen minutes every dozen hours or so to blow out the carbon build up. So far it has worked for me.


David

I don't think running at super low load is optimum, but in twenty years in the engine business I have only run into two examples of glazed cylinders. Both on generators. Never seen it on propulsion engines, and lots just putt along at very low load. Including mine.

Ski in NC said:

I don't think running at super low load is optimum, but in twenty years in the engine business I have only run into two examples of glazed cylinders. Both on generators. Never seen it on propulsion engines, and lots just putt along at very low load. Including mine.

Click to expand...

Ski in NC said:

I don't think running at super low load is optimum, but in twenty years in the engine business I have only run into two examples of glazed cylinders. Both on generators. Never seen it on propulsion engines, and lots just putt along at very low load. Including mine.

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Thanks Ski. Your experience in this matter gives me piece of mind. Once at operating temperatures, I normally set my Lehman 135 at 1800 and leave it there for the most part, and that gives me 7.5 kts. I rarely go above 2,000. WOT is 2650, I beleive, and according to one of the engine surveys I have, this indicates the boat is properly propped.


Jim
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Ski in NC said:

I don't think running at super low load is optimum, but in twenty years in the engine business I have only run into two examples of glazed cylinders. Both on generators. Never seen it on propulsion engines, and lots just putt along at very low load. Including mine.

Click to expand...

Ski

My sense is that too many people get paid to write articles that are of interest and appeal to the general boating public. That does not mean the articles are factual for all engines. My friends in the same business as you find under loading a "modern" propulsion engine is overhyped. Does your experience find the DD 2 strokes suffer any under loading issues?

A comment about loading. I have always looked at %load at a given rpm as being relative to the maximum horsepower that the engine can produce at that rpm. For example at my normal cruising rpms of 2000, my 3200 rpm rated diesel is loaded at approximately 30% (based on comparing the calculated propeller curve with the engine output curve. At 2400 rpms (75% of rated engine rpms), my engine is only loaded to about 42% NOT 75%. I only get to 75% load if I run up at 2900 rpms.

Basically engine load % should to be looked at in terms of the propeller curve (with add ons for alternators, etc.) compared to the engine maximum output curve. Calculating load percent that way, few boats ever run at 75% load and then only briefly.

Now another consideration. My wife HATES the noise level when I run above 2500 rpms. Frankly I would rather buy a new engine a few years early than listen to her complaining.

TDunn said:

A comment about loading. I have always looked at %load at a given rpm as being relative to the maximum horsepower that the engine can produce at that rpm. For example at my normal cruising rpms of 2000, my 3200 rpm rated diesel is loaded at approximately 30% (based on comparing the calculated propeller curve with the engine output curve. At 2400 rpms (75% of rated engine rpms), my engine is only loaded to about 42% NOT 75%. I only get to 75% load if I run up at 2900 rpms.

Basically engine load % should to be looked at in terms of the propeller curve (with add ons for alternators, etc.) compared to the engine maximum output curve. Calculating load percent that way, few boats ever run at 75% load and then only briefly.

Click to expand...

Funny you mention that. My John Deere has a load gauge on the panel. The computer determines load percentage based on RPM and fuel consumption. The gauge displays the percentage of load for that RPM not the engine at maximum output. JD is more concerned with the percentage at the cruising RPM. There are warning alarms for too low and too high a percentage based on time. As an example, if I were to run over 80% for 30 minutes or at 100% for 10 minutes, the alarms start going off. 40 to 70% for a given RPM seems to be the design operating range.

Ted

djmarchand said:

Frankly, I believe a lot depends on the engine. A 6 liter Lehman 120 should be able to run just fine all day long making 25% load or 30 hp. Thousands and thousands cf cruisers have run them that way with no harm done. OTOH my 5.3 liter Yanmar 370 will probably run too cool at that hp loading. Why? It has a cooling system that is designed for three times the power of the Lehman and it has unregulated sea water cooled oil. Both take more than 30 hp for the oil and water to get up to operating temperature.


On the combustion side, the Lehman's injectors are sized for 120 hp and mine are sized for 370. When those injectors on my engine are only operating at 1/12 load they can't have as nice a spray pattern as injectors operating at 1/4 load.


All of which means I run my Yanmar to produce 60 hp minimum or about 16% of max. I do try to run it up to 75% load for fifteen minutes every dozen hours or so to blow out the carbon build up. So far it has worked for me.

Click to expand...

Very similar to the strongly-emphasized advice I got in my first year of cruising with our 260hp Volvo KAD44P. Long term idling was to be avoided, as the cooling system would over-do it. But cruising a bit above hull speed, such that the coolant temp was 175-180 degrees, should keep the oil sufficiently warm, and evenly enough too. Occasional runs up to normal planing cruising speeds recommended.

Seems to have worked - more than 4,000 hours doing mostly slow cruising so far.

Maybe just for fun I'll check the oil pan temp.

DJMarchand, good catch. This MAN has an oil thermostat, so its oil temp is regulated unlike most others. When that photo was taken, during as passage I made from Scotland to Iceland, aboard a Fleming 65, the vessel was laboring in a heavy sea so the the helmsman was constantly varying load as she climbed up waves and then surfed down them. I'm certain the average load was somewhere near 50% during that part of the run.

I've taken apart and repaired many diesel engiens (and genset engines) and while it's anecdotal, and no one rule fits all of them, those that are chronically under loaded clearly have the greatest level of carbon build up on rings and valves, and in turbos. Oil analyses also bears out this theory, I see scores of these reports every year and chronic under-loading nearly always results in higher fuel dilution and additive depletion. Those findings, as well as boat builder's penchant to over-power vessels, are what inspired this and several other articles I've written on the subject.

The good news is it's pretty easy to mitigate with periodic run ups. When I suggest clients do this on more than a few occasions they report back after having done so to say, "I ran up to 75% and after a few minutes the engine began to overheat, so is this a problem for the engine?" The answer is yes, but the problem is not with the engine, it's with the cooling system, a clogged heat exchanger or injection elbow, damaged impeller or worn pump cam, the run up simply exposed it.

TDunn said:

Now another consideration. My wife HATES the noise level when I run above 2500 rpms. Frankly I would rather buy a new engine a few years early than listen to her complaining.

Click to expand...

At least when the engine gets around to wailing you know there's actually a problem.

Had a mailman lock up his car one time because he idles it everywhere for work. The head had built up so much carbon that the piston couldnt reach full height.

RCook said:

Maybe just for fun I'll check the oil pan temp.

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This is what I do with an IR gun during routine ER checks when cruising. My engine loading as estimated by fuel burn has to drop below 15% before oil temps fall below 175F.

Steve DAntonio said:

. When I suggest clients do this on more than a few occasions they report back after having done so to say, "I ran up to 75% and after a few minutes the engine began to overheat, so is this a problem for the engine?" The answer is yes, but the problem is not with the engine, it's with the cooling system, a clogged heat exchanger or injection elbow, damaged impeller or worn pump cam, the run up simply exposed it.

Click to expand...

Add to the causes for high temperatures - the boat is over propped. Sometimes straight out of the factory as I have encountered during a few sea trials.

IMHO this is why propping to allow rated RPM is important, then a high temperature can be chased along the lines of your suggestions.

sunchaser wrote;
"My fuel burn has to drop below 15% before oil temps fall below 175F.

Now that is significant information. Never woulda thunk.

But what you're saying sounds like something a gas engine would do.

Someone alluded to coolant heating the lube oil a few posts back and that's the only possible explanation for the oil temp being that high w only a 15% load on. What do you suppose is the path of all that heat from the coolant to the oil? Is there an lube oil cooler running off the cooling system keeping the lube oil temp artificially high just like the coolant? Then to be a cooler at some point the heat transfer will need to reverse. Something in my thinking must be bass akwards.

manyboats said:

Is there an lube oil cooler running off the cooling system keeping the lube oil temp artificially high just like the coolant?

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Yes, a plate heat exchanger. This is not uncommon on many engines., as also mentioned by others. Many diesels operate under sporadic light loads and are designed accordingly, think generators. If the basis of one's marine engine is a genset or stationary unit, you likely have a good one.

Too bad RickB isn't on board for this discussion. He not only knows this stuff inside out but has designed and invented systems to deal with load shedding and exhaust "clean up."

Steve:


Thanks for your explanation. I think all of us could use an objective set of criteria for determining when we are under loaded, or maybe more useful- criteria that would indicate than an engine is properly loaded. So let me propose some, taken from your article and your posting above:


1. Coolant temperature at 160 F or above.
2. Oil temperature at 180 F or above.
3. Exhaust turbo only has a light dry soot coating (we can't easily see injector tips or ring carbon, but turbo soot should be a decent indicator).
4. Oil analysis shows less than 0.5% fuel content.


I believe that these criteria will tell you if you are loaded decently. On my Yanmar 370, I meet all of these criteria with the possible exception of oil temperature while running at about 1,600 rpm and producing roughly 60 hp with occasional high power runs to blow out the accumulated soot.


David

Thanks Steve. Bob Smith said we should be taking the our engines up to WOT occasionally, as if it can't get there something is wrong.



Jim
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One more potentially dangerous generalization. While I don't argue with anything in the article, ultimately it only applies directly to the single engine under observation. What age engines are we talking about? Where do common rail engines come into play? What type use was the engine set up for initially? And for the answer, back to the manufacturer.

We know temperatures are important. We also know warm up and cool down. We know there is a best load and an acceptable load. I personally believe in varying the speed. In fact, I look back at break in instructions of not running at WOT over x period of time and varying speeds. However, you can take the exact same base engine and it can be built for uses from light fast boats to continuous use slow heavy boats. Then the same engine converted to a generator.

I was recently in a discussion on generators. Specifically someone quoted a Northern Lights sales rep at a show saying not to run under 50%. Well, that's not what Northern Lights says in their literature and that's an old, outdated statement. Engines change. Today's Lugger or John Deere isn't the one from 30 years ago. On that engine, NL says "a generator should never run with less than a 25% load. 35% to 70% is optimal." Makes a huge difference in sizing and in flexibility.

This came up for us in determining what size second generator to use in a boat where the primary is 21.5 kw. Do you go with a "half gen" of 9 to 11 kw or go with two identical 21.5 kw. The 9 to 11 argument is that even at low load you'll be over 50%. However, it turns that generator into only a very occasional use and we've seen boats where the primary generator had 4000 hours and the small gen had 140 hours. So we looked at our loads. Maximum load requirement we have is 16 kw. Minimum which is around 8 kw. Well, either of those is in a reasonable load level for the 21.5. With a 9 to 11 kw generator we could only run minimal items and the moment some others came on it would overload it. Now, we do monitor temperatures, load and other factors.

Know your own equipment. Know what is recommended for it. Know the recommended temperatures and loads. Steve's article is great in that it gives some real examples of what he has seen in engines run constantly and consistently at very low load. Just take that knowledge and combine it with what your engine manufacturer says as to what is appropriate for your engine and ways to monitor it.

BandB said:

One more potentially dangerous generalization.

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Not sure if you were referring to me, but Bob Smith is supposed to know Lehman's as he was the guy that developed the marinization of them and he presumably developed his ideas after spending considerable time at the factory and working with the engineers there.

I looked through my notes I took over the two times I took his course and he mentioned that boats boats in the PNW have the potential to operate "too cool". That discussion came up recently during another thread. He talked about having a "piss line" [what he called it] to another thru hull, that would bypass some of the raw water flow away from the heat exchanger but that option was never developed. So, he said to just run the engine at 1800. I remember pressing him on this topic.


Jim
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sunchaser said:

Ski

My sense is that too many people get paid to write articles that are of interest and appeal to the general boating public. That does not mean the articles are factual for all engines. My friends in the same business as you find under loading a "modern" propulsion engine is overhyped. Does your experience find the DD 2 strokes suffer any under loading issues?

Click to expand...

To butt in a bit, as an experienced DD user, who has had the good fortune to know three excellent and experienced DD mechanics and many fellow users:

No, they don't. The protocol is to run them up to about 80% of WOT for a little while if they have been at low speeds for several hours; in most cases they will smoke a bit and clear up. The key is not to run them at no load idle for more than a few minutes.

One thing I like about modern electronic engines is they are more tolerant of low speed operation. Whether that applies to no load, I don't know.

JDCAVE said:

Not sure if you were referring to me,

Click to expand...

I wasn't. Actually I was referring to the potential of people taking Steve's article or any of what has been typed here and assuming that it applies to their engines in the exact same way.

I have na Hinos I run around 30% to 65% while cruising mostly 30-35%, at the end of the day I run up to wot for maybe 5 minutes. This tells me that the engines will reach wot and all is well with the hull,running gear and the engines aren't being overloaded.


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I am still confused as to how to determine % Load. I know it isn't rpm but how? Is it the amount of hp being produced, i.e. If the engine is dated for 300hp and based on manufacturers curves show you are producing about 100hp is that 33% loading?

The above graph is one of many obtained by google "fuel map for diesel engine"

Yes ,its for a car engine , but the concept is correct for any diesel.

Setting the vessel up to cruise in the bulls eye will be the most efficient that boat can do with that engine.

Yes it will have sufficient load not to die early.

Comparison between engines can be done , which is why boat engine folks do not supply this style info.

A tutorial,

https://www.morganscloud.com/2015/06/02/understanding-an-engine-fuel-map/

djmarchand said:

Steve:


Thanks for your explanation. I think all of us could use an objective set of criteria for determining when we are under loaded, or maybe more useful- criteria that would indicate than an engine is properly loaded. So let me propose some, taken from your article and your posting above:


1. Coolant temperature at 160 F or above.
2. Oil temperature at 180 F or above.
3. Exhaust turbo only has a light dry soot coating (we can't easily see injector tips or ring carbon, but turbo soot should be a decent indicator).
4. Oil analysis shows less than 0.5% fuel content.


I believe that these criteria will tell you if you are loaded decently. On my Yanmar 370, I meet all of these criteria with the possible exception of oil temperature while running at about 1,600 rpm and producing roughly 60 hp with occasional high power runs to blow out the accumulated soot.


David

Click to expand...

The only criteria I would add to this is exhaust gas temp above 450-500F. An exhaust gas probe would tell you this, but that's not something most folks have or will add. Alternatively you could shoot the dry portion of the exhaust elbow immediately after the elbow with an IR pyrometer for a rough reading on EG temp.

The coolant should always reach and ultimately run at the thermostat's rating. If it's more than a few degrees lower while under load something is wrong.

The turbo soot accumulation is pretty subjective, one man's build up is another's patina. And, unless it's disassembled, you can only see the intake turbine on the turbo, not the portion that operates in the exhaust gas stream, and that's where soot, and un-burned fuel would accumulate.

Additionally, fuel dilution is dependent on the lube time, the number of hours on the oil. Some engine manufacturers limit the absolute fuel dilution to 5%, however, if that occurs after 70 hrs then that's problematic (and the oil would need to be changed), while if it occurs after 200 hours then it's far less of an issue. I have clients running displacement trawlers, powered with common rail engines, using synthetic oil, doing oil changes at 450 hrs (this is within the manufacturer's spec's provided you use syn oil), practicing the periodic run up, with no appreciable fuel dilution or other contamination showing up in the oil analysis other than a slowly falling TBN number, which is normal, the threshold for which is 2.5.

dhays said:

I am still confused as to how to determine % Load. I know it isn't rpm but how? Is it the amount of hp being produced, i.e. If the engine is dated for 300hp and based on manufacturers curves show you are producing about 100hp is that 33% loading?

Click to expand...

In most cases, if your engine doesn't have a load % indicator, then you can rely, roughly, on % rpm of rated wide open throttle (WOT). For instance, an engine rated at 3000 rpm WOT, would be run at 2250 for 75% load. Load and rpm curves are not superimposed on each other, they diverge but not significantly. Bottom line is the 75% number isn't terribly critical, it need not be exactly 75%, that's a figure I use to ensure the engine is loaded while on sea trials, for realistic temperature measurements.

Before doing any of this, however, make certain your engine reaches rated WOT under load. Ideally it should exceed this rated number by 20-40 rpm, i.e. using the above example 3030 would be ideal, which tells you the engine isn't overloaded (acutely, this can kill an engine far quicker than under-loading), and that your prop is properly matched to the engine. If your engine doesn't reach rated WOT rpm find out why, and propeller adjustment should be the last option. Before that check injectors, valve adjustment, fuel supply and exhaust back pressure. I've encountered excessive back pressure on vessels fresh from the builder, so don't rule that out on a new vessel. In many cases, a prop pitch reduction only masks other problems, and when this is done it essentially de-rates the engine, you aren't getting the HP you paid for.

One aspect not mentioned in the article is this problem is magnified by raw water (seawater), as opposed to jacket water (coolant), cooled after coolers. Raw water after-cooled engines have broader performance capabilities, however, they require cooler combustion air to meet nitrogen oxide emission guidelines, which means the intake charge air is often too cool for ideal combustion at light loads, exacerbating wet stacking, carbon fouling and soot accumulation.

Lots of advice, contradictions and guesses on this thread, some of it hopefully accurate. To cut to the chase, and gensets aside - provided the vessel is propped correctly, relevant temperatures are achieved, 65 to 75% power (rated fuel burn not RPM) achieved now and then and maintenance is up to snuff - it seems under loading issues can be set aside.

Confusing things a bit more, does anyone remember all the discussion a year or so ago instigated by a different marine writer about the benefits of over propping?