Are electric horses really bigger?

Discussion in 'Electric Propulsion' started by DennisRB, Apr 9, 2016.

  1. DCockey
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    DCockey Senior Member

    What does torque at zero rotational speed have to do with matching a propeller to a motor/engine? A propeller at zero rotational speed is producing no thrust if the boat is not moving, or drag if the boat is moving. That's a fundamentally different situation than a wheel drive land vehicle where "thrust" is directly proportional to torque on the drive wheels.
     
  2. essenmein
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    essenmein Junior Member

    I agree it presents no real advantage in marine use, I`m just highlighting fundamental engine differences. The flat 100% rated torque curve is a big improvement over gasoline engines and only a marginal improvement for most diesels, what you potentially gain with diesels is efficiency by decoupling engine rpm from prop rpm.
     
  3. proptop
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    proptop New Member

    Maximum Torque from the beginning

    This should be of interest:

    Attached the engine power- and torque curve of a "75 hp" diesel D2-75.
    Compare this with a 50 kW electric shaft drive motor "DB 80i" which offers maximum torque from the beginning. At about 1700 rpm the maximum power is reached which is why torque goes down.

    May be we have some prop experts who can comment on this?
     

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  4. The Q
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    The Q Senior Member

    They are, technically the pistons (and associated parts) in a locomotive are the engine, not the Whole locomotive.
    On many locomotives they have divided drive, that is some pistons to one Axle, other pistons to another axle so you have two engines providing thrust to separate axles which normally, but not always, were linked by a coupling rods.
    More than one type of Locomotive was known for being able to start up with driving wheels going in the opposite directions, if the driver / engineer got it wrong.

    Of course there is another type of ganging up, where multiple locomotives pull and push a train. In the UK putting two locomotives together at the front is called double heading. Putting a locomotive at the rear is callled banking, because it's mostly used to add a Locomotive just to pust a train over a bank (incline)
     
  5. daiquiri
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    daiquiri Engineering and Design

    Hi Proptop,
    such comparisons have already been discussed - check this post, at the page 3: http://www.boatdesign.net/forums/propulsion/electric-horses-really-bigger-55435-3.html#post772775

    What more can be said? Perhaps this - it is always useful to create a graph which combines all the curves of interest, so that they can be immediately compared.
    Like this two, where I have put the data of the two motors from your post:

    Graph 1:
    DiesElecComp.gif

    Graph 2:
    DiesElecComp2.gif

    (the diesel torque curve appears bumpy because Excel has connected the discrete data points with a polinomial curve - but it is just a visualization issue).

    You can observe following salient points:
    1. The shape of original torque curves is actually comparable for both engines. The diesel engine has a torque which is apparently highly variable in the original graph, but flattens a lot when placed on this graph. The torque variability (min-max) of this diesel is within 15%.
    2. The electric motor works in a lower RPM range. But it gives a nearly the same maximum power.
    3. When a hypothetical 1.67:1 reduction gear is clutched to the diesel motor output shaft, the max. power occurs at the same RPM for both engines, and the diesel gives a higher torque then the electric motor. That is the effect of the RPM scaling.
    4. When a hypothetical 1.35:1 reduction gear is clutched to the diesel motor output shaft, the two torque curves become pretty much identical. In this case, the diesel gives the max. power at a slightly higher RPM, and the rest of the power curves is exatly identical to the electric one.
    5. Finally, the electric motor is more suitable for boats which need to operate at trolling speeds, since it has a speed modulation which starts from zero. In order to obtain the same capability, the diesel requires a trolling valve to let the prop spin below 700-800 RPM in these graphs.
    Hope it helps.

    Cheers
     
  6. essenmein
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    essenmein Junior Member

    Then if you compare the fuel consumption for that same diesel, you can see why diesel electric is an interesting proposition. At anything other than full speed, efficiency is reduced dramatically. The amount of fuel needed to make one mechanical KW doubles below about 5kn (in this example). Not really an issue for long distance boats that run at one speed the whole time, but interesting for boats that vary their speed a lot.
     

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  7. DennisRB
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    DennisRB Senior Member

    This is from another thread but thought it would fit better in here.

    More BS from the marketing companies.

    Install regular diesels that have 1/3 the power to make it have equal sustained performance to their system then tell me how much less fuel thier system uses and how much lighter it is.

    A marine transmission might have 5% loss. The electro mechanical conversion probably wastes 20% at the gen set, 20% at the elec motor and if they size the cable well they might loose 4%. So there is your big gain over the 5% transmission loss they are probably referring to in the fine print.
     
  8. Joakim
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    Joakim Senior Member

    You can get better efficiency at some operating point with a diesel electric system. And you can get it much better, if you use big enough battery bank and have an optimized system, which automatically controls when to charge and when to use the batteries. But most likely the system marketed like that is not that good.

    Here are some reports of an EU project that went quite deep into this subject:
    http://www.hymar.org/upload/LIBS_130110.pdf
    http://cordis.europa.eu/publication/rcn/15923_en.html

    Note that the best system they could come up with used a clutch system, which enabled to use the diesel to drive the propeller directly at higher power and the electric motor was used at low power, for charging and as a boost. They didn't do well with a "normal" diesel electric system.
     
  9. DennisRB
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    DennisRB Senior Member

    Thanks for the links. Usually there is a crossover point where the diesel electric uses less fuel. This is in the low power range where large diesel engines will be producing a small fraction of thier total aviliable power. In this case all the associated elctro-mechanical conversion inefficiencies outweigh the inefficiency of a very lightly loaded diesel. So, yes it is possible.

    However you just use a smaller diesel that is not highly under loaded it will use less fuel in every case. This is pretty much admitted when they agree it is more efficient to use direct drive at higher power.

    If the end result with the hydrid system is very low power electric motors and/or very low continuous power supply from a gen set. It can be argued the fuel consumption should be compared to direct drive diesel engines of the same power the hybrid system can supply, rather than the much higher power diesel engines that would usually be fitted to the vessel.

    As Daiquiri showed in his graphical representation. Its invalid to claim that low HP electric motors will provide more propulsion. Therefore if one is willing to have 10kw electric motors one should be willing to have 10kw diesel engines and compare weights, costs and fuel economy to them. Something the manufactures never do by claiming their 10kw engines are as "power ful" as a regular 35hp diesel or other such nonsense.
     
  10. rwatson
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    rwatson Senior Member

    Preofessional Boatbuilder December/January 2017 has a great article on Hybrid power, and they make exactly this point.
     
  11. DennisRB
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    DennisRB Senior Member

    Good to see a magazine actually telling it how it is. Usually they just talk up whatever the advertisers pay them to.
     
  12. powerabout
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    powerabout Senior Member

    it will always be apples to pears unless the rating standards are the same or somebody puts electric motors on a braked dyno
     
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  13. DennisRB
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    DennisRB Senior Member

    True, but with the info we already know, we can tell with great accuracy that a 10kw electric motor has no where near the same propulsion potential as a 30hp diesel engine as suggested by the adds. Putting the 10kw electric motor on a dyno would probably only make it look even worse, not better.
     
  14. groper
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    groper Senior Member

    The rating standard is already the same, and has been for years - 1kW compares perfectly to 1kW :p:p oh, and for our imperial friends 1hp compares perfectly to 0.746kW :p:p
     

  15. groper
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    groper Senior Member

    i think the reason for the misconception is that all too often we size a diesel engine and gearbox much bigger than is needed whereas we size an electric motor exactly to the work we require from it.

    Take your average 40ft cruising cat for example - most typically use a pair of diesels of around 30hp. 60hp is more than double the power that is required to propel this type of boat at 8kts and we rarely ever try to push these boats harder than 8kts... so why do we size the diesels at 30hp when 10kW will do?

    But when we sell an electric drive for the same boat, we sell a 10kW system and it does what we said it would - push the boat to 8kts and hey- electric horses are bigger than diesel horses!

    We are also ignoring the fact that once we hit the hump speed of around 8kts - the power/speed curve rises exponentially and it looks like the electric powered boat does 8 kts yet the 30hp diesels only manage 9 kts... so electric horses must be bigger of course :p
     
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