Brake specific fuel consumption slow running marine diesels.

Discussion in 'Diesel Engines' started by Lemans, Feb 21, 2016.

  1. Lemans
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    Lemans Lemans

    A few day's back I was asked in an other thread about the specific fuel combustion of a 'modified' 4-stroke cycle.
    We know from manufacturers information that the real big ones have BSFC around 160gr/kWh.
    To figure out the BSFC of an engine, you bring it to a laboratory, mount it on a test stand and apply load on a running engine - I have difficulties imagining the same approach for an engine that size.

    So, does anybody knows how the BSFC is tested/calculated/estimated for these 4 stores high engines?
     

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  2. baeckmo
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    baeckmo Hydrodynamics

    Oh, the giants are tested thoroughly, don't worry about that! The most common brake for big power is the hydraulic type, with a rotor inside a housing.

    Since the power of a turbomachine rotor is proportional to the diameter^5, the brake is generally much smaller than the engine. In some cases, you "bring the lab to the Engine"; the brake is attached to the engine still resting on its assembly floor. The absorbed power is cooled off, either by air in cooling towers or by injection of cold water into the circuit.

    In addition to the braking, a number of additional measurements are recorded, just as in any engine test. In fact, the low speed and big dimensions make those additional recordings easier than with, say a car engine.
     
  3. Lemans
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    Lemans Lemans

    Thanks!
    I had no idea.
     
  4. FAST FRED
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    FAST FRED Senior Member

    IF you build an engine your local auto rebuilder for the racing folks will have or know of a Dyno to use to see how your engine is operating.

    Big truck and bus tranny rebuilsers also have the gear,
     
  5. Lemans
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    Lemans Lemans

    Up to 100 HP I have my own test bench (water-brake). I was just surprised of the size and power of these slow running marine diesels.
    The question is now, what engine should I build to see how well this split-intake will perform?
    What is the major decision factor if for a client if he is up for a new marine engine?

    I believe, for marine purpose, we can rule out petrol engines.
    So for a diesel engine? Power / BSFC / size / weight / displacement ?

    My answer to that question would be - specific fuel consumption - but a have stopped assuming that everybody see things the same way as I.
     

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  6. Barry
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    Barry Senior Member

    The BSFC changes throughout the engine's rpm range. You may design an engine that produces 100 horsepower at maximum horsepower but have a poor BSFC number at that horsepower level. ( and say for discussion purposes, this max hp occurs at 3000 rpm)
    Perhaps the best BSFC number for your 100 max hp engine at 3000 rpm, occurs at 1500 RPM, but the engine produces only 50 HP.

    To your question "What is the major decision factor if for a client if he is up for a new marine engine?"

    I would decide what is the horsepower needed to push the boat at its usual operating speed and then try to design the engine that will produce the amount of horsepower (net of prop losses) required with the least amount of fuel burn per hour.

    In other words. If you want the best BSFC number, perhaps you need to build an engine that could produce 200 hp at maximum, but you need only 100 hp, so this engine would be operating much below max hp, but would give the client the lowest fuel cost per mile/nm/km.

    So the goal COULD be "the least amount of fuel burn to produce 100 HP"

    Your comment "my answer to that question would be -specific fuel consumption"
    could be changed to " my answer to that question would be -to provide the lowest specific fuel consumption at the 100 HP target requirement"

    Which is what I am saying but am including the HP requirement that your client needs
     
  7. Lemans
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    Lemans Lemans

    I agree that normal operating conditions at best BSFC number is the smartest thing to do.
    I found a very interesting paper on 'downspeeding the diesel engine' and the benefits of such a setup.

    http://publications.lib.chalmers.se/records/fulltext/147782.pdf

    Just assume that the engine runs at 1000 rpm in normal operating conditions.. How fast will it run at full power? I'm not a boat designer but I don't think a boat will go that faster to let the engine spin at 1500 rpm. I know that this is a very tricky question that nobody can answer without the correct specs of an engine. Let's assume a flat torque curve?
    100Hp@1000 up to 150HP@1500 rpm.. ( sorry if this is a stupid question :) )
     
  8. Barry
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    Barry Senior Member

    It is a general question but a general answer is that if the engine produces 50 more hp, the boat will go faster.

    (there are quite a few variables, boat hull shape, weight, etc)

    Our own vessel, with 700 hp available at max ,

    1000 rpm 6.0 nmph 2.90 nmg

    1500 rpm 8.7 nmph 1.74 nmg


    nmph nautical miles per hour

    nmpg nautical miles per gallon
     
  9. Lemans
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    Lemans Lemans

    Hi Barry,

    These figures are confusing me.
    At first glance, your vessel need only 40 HP for 6 nmph and 80 HP for 8.8 nmph.

    What fuel is used and what is the top speed of your vessel?
    (also quit possible that my calculations are wrong... )
     
  10. FAST FRED
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    FAST FRED Senior Member

    Using fuel consumption from ship style engines is not useful.

    Just as diesel has more weight and BTU per gallon than gasoline , the #6 bunker fuel is denser than diesel and has more BTU per gallon.

    The cruising trawler folks might be a market as there are few efficient engines at the 30-60 hp required .
     
  11. Barry
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    Barry Senior Member

    The point that I was making is that an increase in available horsepower increases the speed. As per your comment that an increase in hp might not increase the boat speed


    The vessel is a planing hull, 40 feet, 32,000 pounds, wet weight, cruising speed, 23 knots
     
  12. Lemans
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    Lemans Lemans

    The question that intrigues me the most is how much the rpm of an engine is responsible for the BSFC figures. On one side we have the fast running automotive engines with best figures around 195gr/kWh (2000/2500 rpm) and on the other end, the low running marine diesels with 160gr/kWh (100 rpm).
    If I want to test a low running engine and the 'split-intake' cycle with the same engine I need to build one from scratch.
    Best 'power' results for crank-case pressure is a bore/stroke ratio of 2/1.
    The engine speed I have in mind for constant use is 500/1000 rpm.

    If we go for the 30 to 60 HP range, the engine needs a displacement of 3000cc (183 cubic inches), a 3 cylinder looks logic. Bore/stroke 136mm/69mm.
    At that low engine speed, a piston with reasonable hight (+100mm) connected with a 180mm rod...
    Average piston speed (and also friction) down to 2 m/sec.

    Can overall weight become an issue?

    Are separated mono cylinders used in the past? (bolted together to form a 2/3/4/.... cylinder)
    If not, why? It looks obvious that such a setup bring down production costs and customers could replace the broken mono-block or add one as more power is required. ( every mono-block 10 HP)
     
  13. Lemans
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    Lemans Lemans

    OK, this clears all.
    I just had an other kind of vessel in mind.
     
  14. Barry
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    Barry Senior Member

    Just another thought, with such a low horsepower target that it appears that you are looking at and the high compression of the diesel and the low rpm, I am wondering what the mass of the flywheel that you might need.
    In the old days, ( 1950's prox) John Deere tractor built twin cylinder diesels which ran a very large flywheel to aid the engine on the compression stroke. With you compressing the crankcase, you will be bleeding off some of the momentum of the reciprocating mass and might need a very large flywheel to smooth things out.
    Or more cylinders to get more power strokes per rev. etc
    Fast Fred, your comments as well to this please
     

  15. FAST FRED
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    FAST FRED Senior Member

    Smoothness at idle might be a concern , as well as power to shift..

    The engine needs to be able to start the shaft and prop in motion when shifting.

    A 1 -nch shaft and 12 inch prop is easy , a 2 1/2 and up shaft with a 30+ inch prop is a different matter of momentum to start...

    At 25+K any loss of engine momentum from pumping the cylinder probably could not be noticed.
     
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