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Old 10-23-2011, 08:25 PM   #1
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A new and excellent post on underloading

This post from BoatDesign.net is very recent and sums up a lot of the mysteries of underloading damage or potential damage. It dos'nt address the whole issue from where we stand as operators of old trawler yachts but will tend to enlighten all that read it.*
<table class="tborder" style="background-color:#d1d1e1;color:#000066;width:100%;border-width:1px;border-color:#0b198c;border-style:solid;" border="0" cellspacing="1" cellpadding="6" align="center"><tbody><tr><td class="alt1" style="font:normal normal normal 10pt/normal verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;background-color:#e4e4e4;color:#000066;">From a diesel generator technical manual


"Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines - ideally diesel engines should run at least around 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings. This causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze, which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface. which are there to hold oil and return it to the crankcase via the scraper ring.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures. Un-burnt fuel leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.

The problem is increased further the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required. Under loaded running inevitably causes not only white smoke from unburnt fuel but over time is joined by the blue smoke of burnt lubricating oil leaking past the damaged piston rings, and the black smoke caused by the damaged injectors. This pollution is unacceptable to the authorities and any neighbours.

Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures, allows the piston rings to scrape glaze off the bores and allow carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves this will not have any effect.

The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines.

For emergency only sets, which are islanded, the emergency load is often only about 1/4 of the sets standby rating, this apparent over size being necessitated to be able to meet starting loads and minimizing starting voltage drop. Hence, the available load is not usually enough for load testing and again engine damage will result if this is used as the weekly or monthly load test. This situation can be dealt with by hiring in a load bank for regular testing, or installing a permanent load bank. Both these options cost money in terms of engine wear and fuel use but are better than the alternative of under loading the engine.
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Old 10-23-2011, 09:30 PM   #2
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A new and excellent post on underloading

How does this translate?

At max throttle at 2400 RPM, fuel,consumption is 4.5 GPH for my particular engine (JD 4045D).* At two-thirds power, does that translate to 3 GPH, or about 2150 RPM?


-- Edited by markpierce on Monday 24th of October 2011 06:50:51 AM
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Old 10-24-2011, 03:48 AM   #3
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RE: A new and excellent post on underloading

Measuring fuel flow is a viable method of estimating diesel loading.
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Old 10-24-2011, 07:40 AM   #4
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RE: A new and excellent post on underloading

I wonder how common noticable damage from idleing or underloading diesel engines is, and at how many hours has it occurred? I have never heard of one damaged by it although they may be out there. Are any statistics available?
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Old 10-24-2011, 08:22 AM   #5
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RE: A new and excellent post on underloading

Quote:
Steve wrote:
I wonder how common noticable damage from idleing or underloading diesel engines is, and at how many hours has it occurred? I have never heard of one damaged by it although they may be out there. Are any statistics available?
Steve W.
*If you read boatdiesel.com Tony A will say that he has never seen one damaged that way. Only by overloading.
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Old 10-24-2011, 10:21 AM   #6
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RE: A new and excellent post on underloading

Quote:
jleonard wrote:Steve wrote:
I wonder how common noticable damage from idleing or underloading diesel engines is, and at how many hours has it occurred? I have never heard of one damaged by it although they may be out there. Are any statistics available?
Steve W.
*If you read boatdiesel.com Tony A will say that he has never seen one damaged that way. Only by overloading.

*Somewhat agree..... I have read several sources that say once a diesel is broken in...the chances of damage from underloading are slim...especially with main engines because they operate all over their rpm range sooner or later...

I can understand a severely underloaded genset having a "somewhat" premature death...but most do anyhow from TOTAL underuse...not necessarily underloading.
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Old 10-24-2011, 03:14 PM   #7
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A new and excellent post on underloading

Posting this comment on underloading I was not stating or implying that your engines were going to die from underloading. Seen enough evidence to the contrary. At this point I'd even buy a 32 Nordic Tug w 220 Cumins and drive it around mostly at 7 knots. This thread was intended as basically for your information only. So many seem so vague about what actually happened when underloading damage did occur I thought this post would make the mystery much more clear.*

Mark, 2/3 load at 3gph yes. 2150 rpm *....have no idea. 2/3rds load is wherever you burn 3 gph. Probably is about 2150 now that I think of it and I think you do have a flow scan and if accurate it should confirm that. And I'd say that's about perfect loading for your regular cruising most of the time.*


-- Edited by nomadwilly on Monday 24th of October 2011 03:23:31 PM
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Old 10-24-2011, 04:20 PM   #8
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RE: A new and excellent post on underloading

Good post Eric.

I've seen this happen on a yacht with no genset and the main was used to charge the batteries every day. When it finally departed the anchorage, it was accompanied by clouds of blue smoke -*definitely an oil burner with glazed bores.

However, on its return a few days later the exhaust was absolutely clean. The owner reckoned it cleaned up after about six hours continuous running. Maybe you can deglaze by putting a load on - or maybe he was just lucky?
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Old 10-24-2011, 05:11 PM   #9
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RE: A new and excellent post on underloading

Quote:
nomadwilly wrote:

Mark, 2/3 load at 3gph yes. 2150 rpm *....have no idea. 2/3rds load is wherever you burn 3 gph. Probably is about 2150 now that I think of it and I think you do have a flow scan and if accurate it should confirm that. And I'd say that's about perfect loading for your regular cruising most of the time.*
*But Eric, I can about double my range at 1600 RPM consuming 1.5 GPH at*one-third power.* Here's hoping that won't cause damage.* Perhaps it would be good to have occasional brief (15-minute) periods of two-thirds power?
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Old 10-24-2011, 05:57 PM   #10
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RE: A new and excellent post on underloading

Yes, diesel generators are frequently damaged by continuous underloading. We see it all the time on larger yachts which quite often have generators installed that are 30 to 50 percent larger than required. The phenomenon is a fact, it is not myth or superstition or a mystery. If someone has never seen one damaged (greatly accelerated wear and failure to make power or pick up the load) it is because they have never spent much time around generators that run nearly constantly.

Don't make the mistake of confusing generator underloading with propulsion engine "underloading" as these are two completely separate issues.

A generator operates at a constant maximum rated speed which depends on the synchronous frequency. The engine turns at that speed regardless if it has zero load or 110 percent load. This produces a different set of mechanical and combustion related issues than relate to a propulsion engine.

A propulsion engine produces low output because it is connected to a propeller that absorbs little power at low rpm. It is a separate issue.
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Old 10-24-2011, 07:50 PM   #11
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RE: A new and excellent post on underloading

Quote:
RickB wrote:
Don't make the mistake of confusing generator underloading with propulsion engine "underloading" as these are two completely separate issues.
..
A propulsion engine produces low output because it is connected to a propeller that absorbs little power at low rpm. It is a separate issue.
*RickB, please don't leave us "hanging."* What is the preferred loading for a propulsion engine?
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Old 10-24-2011, 08:20 PM   #12
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RE: A new and excellent post on underloading

Quote:
markpierce wrote:*What is the preferred loading for a propulsion engine?
*Any point within its rated power envelope. It is illustrated on the power graph supplied by the manufacturer.

If it is sold by the manufacturer as a marine engine and is* installed in a boat with a fixed pitch propeller, it is shown by the propeller curve.

*

RTFM
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Old 10-24-2011, 11:18 PM   #13
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RE: A new and excellent post on underloading

Quote:
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*Any point within its rated power envelope. It is illustrated on the power graph supplied by the manufacturer.

If it is sold by the manufacturer as a marine engine and is* installed in a boat with a fixed pitch propeller, it is shown by the propeller curve.
*OK!* So for the JD 4045D, that's 1000 RPM and above, compared to its idling speed of 750.
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Old 10-25-2011, 04:11 AM   #14
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A new and excellent post on underloading

A propulsion engine produces low output because it is connected to a propeller that absorbs little power at low rpm. It is a separate issue.

With a CPP , or a knowledgable owner with a "Cruising prop" there is no problem.

The big problem is "fast trawlers" or semi plaining boats that putter at 6K most of the time , barely hi idle.

While the problem can exist on a displacement boat , in most cases it would never be noticed.

An engine that might go 10,000 hours on a properly loaded gen set might get 3000 underloaded.

at 100 to 200 hours a year for most recreational power boats , 10,000/200 is a lot of years.

Most will be dead from poor maint before then.

The other big help is the usual built slow boat will not have a HD* industrial diesel, usually a taxicab or agricultural "marinization" that is happy to suffer low loading as its mfg HP rating is very optomistic.

*


-- Edited by FF on Tuesday 25th of October 2011 04:14:28 AM
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Old 10-25-2011, 04:38 AM   #15
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RE: A new and excellent post on underloading

"... barely hi idle."

High idle is the rpm it reaches at full throttle with no load. It is (hopefully) the highest rpm you will ever see the engine produce.
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Old 10-25-2011, 10:59 AM   #16
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RE: A new and excellent post on underloading

Mark wrote;
<table class="forumline borderline" style="font-size:12px;background-color:#0053a2;border-collapse:collapse;margin-bottom:10px;width:100%;border-width:1px;border-color:#0053a2;border-style:solid;" cellspacing="0" cellpadding="0"><tbody><tr class="tr tr-odd"><td class="row1 borderline comment-content" style="font-family:Verdana, Arial, Helvetica, sans-serif;border-collapse:collapse;background-color:#eff2f9;border-width:1px;border-color:#0053a2;border-style:solid;padding:4px;" valign="top"><table style="font-size:12px;width:1125px;padding:0px;margin:0px;"><t body><tr><td style="font-family:Verdana, Arial, Helvetica, sans-serif;width:1121px;" colspan="2">
*"But Eric, I can about double my range at 1600 RPM"

Where are you going Mark?

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Old 10-25-2011, 12:56 PM   #17
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A new and excellent post on underloading

Quote:
nomadwilly wrote:*

*
<table class="forumline borderline" style="background-color:#0053a2;width:100%;border-collapse:collapse;margin-bottom:10px;font-size:12px;border:#0053a2 1px solid;" cellspacing="0" cellpadding="0"><tbody><tr class="tr tr-odd"><td class="row1 borderline comment-content" style="font-family:Verdana, Arial, Helvetica, sans-serif;border-collapse:collapse;background-color:#eff2f9;border-width:1px;border-color:#0053a2;border-style:solid;padding:4px;" valign="top"><table style="margin:0px;width:1125px;font-size:12px;padding:0px;"><tbody><tr><td style="font-family:Verdana, Arial, Helvetica, sans-serif;width:1121px;" colspan="2">
*

Where are you going Mark?

</td></tr></tbody></table></td></tr></tbody></table>
*Same question my GF*asked nine*years ago when I got a passport.* (She already had one.)* After 20+ countries, I'll need to renew next year.


-- Edited by markpierce on Tuesday 25th of October 2011 12:59:28 PM
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Old 10-25-2011, 02:57 PM   #18
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RE: A new and excellent post on underloading

Going to Portugal?
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Old 10-25-2011, 04:26 PM   #19
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RE: A new and excellent post on underloading

Quote:
Steve wrote:
Going to Portugal?
Steve W.
*Just got back earlier this month.* Visited both Lisbon and the Azores again, just like last November.
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Old 10-25-2011, 05:09 PM   #20
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RE: A new and excellent post on underloading

Idling in gear all day could be called "underloading." As long as the engine is operated within its rated power envelope and reaches operating temperature it should be happy. Recreational boat engines, generators included, just don't run enough in their lives to suffer from the underloading boogeyman.

Read the horror stories, they die from flooding a cylinder through the exhaust by overcranking or some other failure, they suffer from bad injection caused by rust and dirty fuel, they croak when a heat exchange rots out and fills the crankcase with seawater or coolant, they overheat from overloading related to dirty props and bottoms, pump failures or leaking hoses, but they rarely ever just wear out prematurely because they spent too much time at low power.
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