Diesel Efficiency vs. Gas Efficiency

Discussion in 'Diesel Engines' started by Northwester, Feb 4, 2009.

  1. Northwester
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    Northwester Junior Member

    Doing research for repower options, I've noticed that fuel consumption curves for marine diesel engines seem to be readily available from manufacturers. Whereas, marine gas engine manufacturers don't very often provide such information, if ever.

    I would like to compare gas engine fuel consumption in terms of grams/kwhr or some other standard measure.

    Where can I find some numbers to compare different gas engine offerings available in the market today. I would like to compare all the different GM marine engines to determine which is most efficient at a modest cruising RPM:

    3.0l
    4.3l
    5.0l
    5.7l
    6.2l
    7.4l
    8.1l

    Is such an analysis possible?
     
  2. Guest62110524

    Guest62110524 Previous Member

    we were taught as diesel apprentices that the diesel has a mech efficiecy of 35% roughly and gasoline much less, thought it was 15% , but things have changed, the modern diesel is now much more efficient so google up mechanical efficiancy of internal combustion engines
    until electronic injection came for gas, then it was all a bit hit and miss, no such things as accurate fuel rate delivery,for ships cross head 2 cycle engines, they just seem to refine the things more and more so the consumption grams K/w hr comes down and down
     
  3. speedboats
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    speedboats Senior Member

    This would be hard to determine as fuel consumption will vary depending on the load.

    This means things aside from RPM.

    An engine swinging say a 19* prop at 4500rpm will be working harder than the same engine swinging a 17* prop at the same rpm. Working harder means higher fuel consumption.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I'll take the long way round to explain, hopefully it won't get too messy...

    Modern small diesel engines' efficiency arrives to 0.40-0.41 and more, whilst the gasoline engines' efficiency hardly go over 0.27-0.28 (based on fuel mass). So, their efficiencies based on fuel masses are related by the formula:
    (1) Efficiency_mass_gasoline = 0.68 * Efficiency_mass_diesel

    Gasoline's and diesel's densities are 0.74 kg/l and 0.84 kg/l, respectively. So their efficiencies based on fuel volume will be proportionally smaller:
    (2) Efficiency_volume_gasoline = 0.62 * Efficiency_volume_diesel

    Now, if you analyse the declared data for various diesel engines, you'll find out that their approximate fuel consumption at max power can be calculated with the formula (in litres per hour):
    (3) FC_diesel = 0.19 x HPmax

    Knowing the relation between diesel and gasoline's efficiencies (the formula n.2 above), the formula for gasoline engine fuel consumption will be, approximately:
    (4) FC_gasoline = 0.30 x HPmax

    You can find some power and consumption curves for marine gasoline engines here:
    http://www.bpm-marine.it/english/engines.htm

    Take their V12 620S model as an example.
    You will find out that at max power it delivers 630 HP and consumes 219 gr/HP/h = 138 kg/h of gasoline.
    The energy content of gasoline is 46 MJ/kg ( http://en.wikipedia.org/wiki/Fuel_efficiency ), so the raw input power from the fuel is 1762 kW = 2361 HP.
    Therefore, this engine's efficiency is 630 HP / 2361 HP= 0.27 which confirms the efficiencies I've assumed at the beginning .

    Now, back to those 138 kg/h of gasoline consumption. It corresponds to 186 l/h (litres per hour). If you try to calculate it from the formula (4) you'll find:
    FC= 0.3 * 630 HP = 189 l/h
    which confirms the validity of the formula used.

    Of course, there will be differences between various engine manufacturers, but they will be very small. Both diesel and gasoline engines technology have reached a point of developement where it has become very difficult to make such a big progress to substantially change those numbers.

    Now, that analysis is for max. RPM and max power. If you need to analyse reduced (cruise) RPMs, there you have a problem which is called ECU (Engine Control Unit - find more here: http://en.wikipedia.org/wiki/Engine_control_unit ).
    It is a programmable electronic control system which controls the engine behaviour at various regimes, ambient conditions, fuel characteristics etc. It can be programmed (mapped) to control and regulate the engine in nearly any way desired and therefore is a big unknown variable for any partial-RPMs analysis, unless you have equipement for reading or re-mapping the ECU.
    So you can't perform that kind of analysis without direct help and data about your engine's current ECU mapping from the manufacturer.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    As a rough guide, a diesel is about 200~210 g/kWh, a GT is around 270~300g/kWh.

    Main difference is that as you pull back the revs of a diesel, the fuel consumption changes, ie gets better. Whereas with a GT, not much change, the fuel consumption graph is not exactly horizontal, but certainly not anywhere near the same as a diesel.
     
  6. apex1

    apex1 Guest

    Although the above figures are about right (as a rule of thumb), I totally have to disagree with the latter statement! Its wrong!
    The Diesel engines efficiency to which the 200g rule applies is provided in a very, very narrow band of revs only! Usually at a setting around 80%. If you go above or below this optimal setting, the (specific) consumption increases dramatically! That unfortunately applies especially for the very low rev / low load band, where consumption can increase to 370g kw and above!
    The gas guzzler has not such problem with low rev / low load consumption.

    Regards
    Richard
     
  7. FAST FRED
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    FAST FRED Senior Member

    The Diesel engines efficiency to which the 200g rule applies is provided in a very, very narrow band of revs only! Usually at a setting around 80%. If you go above or below this optimal setting, the (specific) consumption increases dramatically! That unfortunately applies especially for the very low rev / low load band, where consumption can increase to 370g kw and above!
    The gas guzzler has not such problem with low rev / low load consumption.

    This is the hardest problem for one attempting to powerr a cruiser.

    Sure the Mfg will show a useless Prop HP curve , and might publish a graph of HP aviliable at different rpm

    BUT the real tool to efficiently set up a vessel is a FUEL MAP , which are guarded inhouse.

    A fuel map looks like a bulls eye or series of clouds , with the most efficient operation in the bulls eye and every ring outside is lower efficiency.

    FF
     
  8. apex1

    apex1 Guest

    right
     
  9. Northwester
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    Northwester Junior Member

    Thanks for all the information and discussion. What I am trying to accomplish with my research is to mate the most fuel efficient engine with the resistance curve for my small planing hull vessel.

    I'm working with classic deep-V Bertram 25' hull which weighs about 5500 lbs. fully laden. The deadrise for the planing portion of the hull aft is about 24 degrees. The beam of the planing section is about 8.0 feet. If anyone has published a good resistance curve for this boat, please point me to it. From my own observations, a cruising speed of 22-25 knots seems to be the best.

    In a perfect world, I would want to know how many KW of power needed to drive the vessel at the target cruising speed (where the resistance is at it's least point) and then find an engine that will produce that power while consuming the least amount of fuel in term of g/KWHR.

    Am I on the correct track?
     
  10. marshmat
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    marshmat Senior Member

    And it's flat out stupid of the manufacturers to hide these things.
    They're fairly simple to read, there's no proprietary information and you can't reverse-engineer the motor from a fuel map. The reason for making the fuel maps (or performance maps) in the first place- basically just curves of constant specific fuel consumption, plotted against rpm and bmep (or torque)- is so that the guy responsible for selecting and setting up the engine can get it running in the conditions it's best at. So why are they so hard to find?
    If any sketchy 3rd party wants to reverse engineer the thing, all they have to do is buy one and get out the micrometer and ECU scanner. Performance curves and fuel maps exist only to help set the thing up at its best operating point.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    whenever I've request these maps, I've always been supplied them, for verification on sea trials.
     
  12. marshmat
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    marshmat Senior Member

    I think what Fred and I are saying is that performance maps should be easily available when trying to select an engine in the first place. To be fair, I haven't yet had reason to heckle manufacturers to provide them- but if I were selling engines, I'd put the performance maps on the website right along with the tech specs, dimensional drawings, etc.
     
  13. daiquiri
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    daiquiri Engineering and Design

    That's all correct, but, like I said before, while the fuel consumption at max power is pretty much limited by the mechanics and internal fluid dynamics of the engine (and is quite comparable between different engines and related by formulas I've written above), the partial-load behaviour can be varied pretty much by ECU programming strategy.
    So the fuel maps will vary significantly between various engines and various manufacturers. It will vary significantly even for the same engine with different ECU mapping. That makes a generalized analysis for partial engine loads nearly impossible.
    So, as I see it, Northwester could do something like this:
    - decide which engine manufacturer he wants to use
    - phone them and ask if they will send him performance maps for different engines he wants to analyse
    - get back here with those maps and so we'll be able to give him an advice for those selected engines.
    ;)
     
  14. Northwester
    Joined: Sep 2007
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    Northwester Junior Member

    I've selected an engine for discussion. Cummins 6BTA 330HP@2800 RPM.

    I've attached a PDF of the manufacturers marine performance curve.
     

    Attached Files:


  15. Northwester
    Joined: Sep 2007
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    Northwester Junior Member

    I've also attached estimated performance from boat diesel calculator.
     

    Attached Files:

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