Underloading a Diesel

Discussion in 'Diesel Engines' started by Paul F, Oct 3, 2011.

  1. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    well at fast speed the 4 stroke and the 2 stroke cost the same to run
    at slow speed the 2 stroke creates 50% less CO so its environmently more friendly
     
  2. Submarine Tom

    Submarine Tom Previous Member

    Big question could be: How far to the fishing grounds?

    Powerabout, did you really mean CO or CO2 perhaps?

    -Tom
     
  3. DCockey
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    DCockey Senior Member

    I assume you meant CO2. A 2 stroke has much better fuel efficiency than a 4 stroke at slow speed. Hmmm. The opposite of what I've understood.
     
  4. powerabout
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    powerabout Senior Member

  5. Frosty

    Frosty Previous Member

    The concern was not the co2 or any green tree hugging stuff, which would be impossible to discuss about a 175 HP 2 stroke engine.

    The point was would a large engine like that troll for 6/8 hours without fouling.

    I said the 4 stroke would be better but better still would be another engine say 20 HP to do the troll work.
     
  6. powerabout
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    powerabout Senior Member

    They will both be fine I just find the 4 strokes seem to run very rich at idle and really stink and of course a small engine would be better as you say
    You can get a 25 HP E Tec full electronic but still pull start and no battery if need be
    Both bigger engines are very quiet at slow speed
     
  7. FAST FRED
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    FAST FRED Senior Member

    Lifted from another board.

    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.
    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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  8. tranmkp
    Joined: May 2002
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    tranmkp "wherever you go. there you are"


    My God what a sweet installation you have.
     
  9. rubenova
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    rubenova Junior Member

    That sounds like a pretty good engine/generator combination. Please share the brand names described lasting 30000 hours , sounds like the reputation Gardner Oil and Arrow oil field engines have.
     
  10. rubenova
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    rubenova Junior Member

    Sorry, the last question is in response to responce 15 by CDK
     
  11. mydauphin
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    mydauphin Senior Member


    All this is very true, but I wonder, if you use bio-diesel instead of regular diesel, and make sure your thermostat is performing well and keeps temperature over 200 degrees, then perhaps a diesel can run much better being underloaded.
     
  12. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    then perhaps a diesel can run much better being underloaded.

    The lack of combustion pressure will not be changed by a different fuel , If the pressure is too low the rings don't seal, period.

    Different rings -square cut , are better for low loads than trapizoidal.

    And the usually higher basic compression ratio of a non turbo engine also help.

    FF
     
  13. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "Dicussion last night at the bar. Guy wants to get to the fishing ground fast with a 175 Outboard then troll for 6-8 hours ".

    175 to get there 4 stroke 10-20 hp to put around , and as get home.
     
  14. Frosty

    Frosty Previous Member

    25% load is/ was considered minimal. In the interest of fuel economy I intend to run my 44 ft power cat with one 250HP at just a tad over idle at say 1200rpm.

    Glazing of the cylinders will not be my concern.

    Glazing of cylinders was not only with diesels and basically this old wifes tale should have blown away with the wind along with white bread and poor oils.

    I supposes you've just go to have something to worry about to keep irritable bowel Syndrome happy.
     

  15. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    IBS arhh I see ....hence the reputation hey Frosty
     
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