Underloading a Diesel

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

  1. Brian@BNE
    Joined: Jan 2010
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    Location: Brisbane, Australia

    Brian@BNE Senior Member

    Wouldn't it be better to have EGT pyrometers, and run at enough load to get to a minimum working temperature? This shouldn't be too hard - with only one engine of a twin system operating that prop is way overpitched, but not a problem at low-moderate throttle.

    And have folding props if you're gonna only use one engine.
     
  2. Frosty

    Frosty Previous Member

    There is a thermostat that I have confidence in keeping head temps to the correct manufacturers specifications.

    Quote--with only one engine of a twin system operating that prop is way overpitched.

    Not an ideal scenerio but thats running one engine. and thats why you just run at above idle. At that RPM I am probably getting prop slipping 50%

    26x26 Folding props for 250Hp surface drives !!!, have you seen any?
     
  3. Brian@BNE
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    Brian@BNE Senior Member

    Coolant temp wont tell you about load, and you need some load!

    Here's an article that has a surprising number of choices for that HP, and also surprising performance of the 'folders' compared to fixed prop. But none to replace surface drives...

    If you can lift the unused prop out of the water you don't need folding ones.
     

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  4. Frosty

    Frosty Previous Member

    A thermostat does not read temp but controls it and keeps it to withing manufactures specifications. Thermostats were discarded in some misguided belief that running cooler improves engine life, fortunately this thinking has gone along with glazing diesels cylinders by unloading.

    ( with only one engine of a twin system operating that prop is way overpitched)

    (Coolant temp wont tell you about load, and you need some load!)

    One post im overloaded the next im not.
     
  5. WestVanHan
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    WestVanHan Not a Senior Member

    I run my Cummins QSM-11 635s at low rpms all the time.

    But: my oil is continuously centrifuge cleaned,and I open them up to maybe 1/3 or 1/2 for a couple/few minutes every hour or two to beat ferries,run currents, beat someone else to a good anchorage,avoid whales,etc.

    The modern electronic injection has a higher efficiency,so that likely also helps.

    In 550 hours have zero problems.
     
  6. Brian@BNE
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    Brian@BNE Senior Member

    There is a gap between max power curve and propeller demand curve, often called reserve power. With just one prop turning you eat into this a bit. Given enough rpm the prop demand will become higher than the engine can produce = overloaded.

    But down at your 'just above idle' rpm you could be underloaded with the one prop. If your engine manuf says that its OK to spends hours at that rpm then you will be OK - they do set min rpm's, but tractor manuf seem to be the only ones to talk about it. Running at low rpm's that are within the manuf range will not be a problem.

    And yes thermostats are very important, but you are missing the point about EGT, cylinder pressures and ring design, as Fast Fred often posts.
     
  7. Brian@BNE
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    Brian@BNE Senior Member

    How low is 'low rpm's'?
     
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  8. WestVanHan
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    WestVanHan Not a Senior Member

    Often 900+ but usually 1000 to 1100 will give me 9 to 10 knots,but maybe it's not that "low" after all as she winds out at 2300- so I'm running just under half.
     
  9. Brian@BNE
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    Brian@BNE Senior Member

    West
    Yes I agree, not that low. Nice speed level, and you would no doubt get great fuel economy also.

    I looked at an operating manual for your engines online, but could not find any reference to a minimum operating rpm. But it could be in the ECU menu/info - seems like that's where all the detail is.

    For JD 9 litre tractors, the operating range is 1400-2100 rpm. But I think the 1400 lower limit is related to the shape of the torque curve and capacity of the governor to respond to load spikes that can occur in agriculture usage. On a boat those won't happen. I'm starting to think that low rpm isn't an issue for boats at all, provided there is some loading but no overloading. The engine designers have probably figured out a way to mitigate the potential glazing problem. Cummins' third ring is obviously part of the solution.
     
  10. WestVanHan
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    WestVanHan Not a Senior Member

    Hey Brian thanks. I lived in the Brissie area for a while, had friends in Redcliff and Clontarf.
    It's fun to go to google and do street view of places I lived in,Noosa and Surfers's.

    Anyways,what I consider the worst thing to do is to let the engines warm up from cold at idle. They're cold,not working,not making much heat,and all that unburnt fuel is hanging around.
    I see guys idling to warm up for 15-20 minutes.

    I start up,let them run a minute and then start out at ~800 rpm with little load..and they're up to temp within a minute or so.
    Tried idling at the dock once..after 8 minutes they still weren't up to temp,so I took off.


    I've idled into the dock anyways,so i just switch off but see guys pull in,and let them "cool down" for 5-10 minutes.
     
  11. Frosty

    Frosty Previous Member

    quote There is a gap between max power curve and propeller demand curve, often called reserve power. With just one prop turning you eat into this a bit. Given enough rpm the prop demand will become higher than the engine can produce = overloaded. quote.

    I dont wish to be rude but you understanding of props does not apply to surface props that slip as much as 50% and propbably more untill they are running in the optimum water.

    Many people ask if it a jet as thats what it looks like from the rear. The prop tips are just 2 inches above the water at rest. I have therefore no loading problems or need to ventilate.

    The props do'nt work well until they are 'dried out' and then slippage reduces to 15to 20%. You cant load the engine like a normal prop on a normal boat.

    Glazing of cylinders is not a problem with todays modern combustion chamber design , temperature control, modern clean fuels and modern oil for Diesel engines.

    Rimula +1 is the oil for todays engines. Glazing is not the massive problem being discussed here.
     
  12. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Last night I was on some other boat engine forum and this topic came up.

    One guy who has a marine engine rebuilding co. said that of the thousands of marine diesels he's rebuilt, the most common reasons in descending order: overloading,known problems with the engine design,poor maintenance,and cooling system failure.

    Has not had one that was glazed up and ruined from underloading.

    I'd make a claim if you are in a trawler and crossing the Pacific for a month being very underloaded you'd have problems.

    Or running a gen set for a longgggg time with no load.
     
  13. powerabout
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    powerabout Senior Member

    There is a Singapore based megayacht that was designed to have a major hydraulic installation on the gen sets and it ended up with the pumps on the ME gearboxes.
    the MTU gen sets have been rebuilt twice due to running with no load and the boat is now in a yard having the gen sets replaced for smaller ones.

    All the dynamic postioning drill ships/rigs all have a mode to set up thrusters to oppose each other to burn power to keep load on the gen sets when plenty are online for drilling but not being used.
    I was looking at a new one the other day and the bias button for the thrusters had a KW input so you choose how much power to absorb
    ( I hope they find oil as they sure as well burn it)

    I think its a large engine problem not high speed pleasure craft issue
     
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  14. Frosty

    Frosty Previous Member

    You would not start a gen if you had no use for it would you. You do need a small gen for small jobs. Unlike an oil company this is done for economy , both fuel consumption and the price of the equipment.

    Having a 100KW gen running a fridge is not only waisting fuel but is a design problem.
     
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