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Natural gas vehicles (NGVs, CNG)

Discussion in 'Cars and Transportation' started by vfx, Apr 18, 2009.

  1. smorgasbord

    smorgasbord Active Member

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    Got a question:

    In the past year, the low price of Natural Gas has meant that some electricity generation has shifted from coal to natural gas. Now, I've never seen a car that can directly use coal as a fuel, but there are cars that use natural gas as a fuel.

    So, what's the cost and efficiency comparison between an EV using electricity generated 100% by natural gas and a car that uses CNG?

    TIA
     
  2. doug

    doug Administrator / Head Moderator

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    That's an interesting question. I think I worked it out at some point, mainly because I was trying to compare HFCVs to NGVs. And I don't think I saved my notes. I guess the easy thing to do is just look up the gas mileage (so to speak) of an existing vehicle so it's less theoretical.

    The Honda Civic GX claims 27 city and 38 highway MPGe.
    http://automobiles.honda.com/civic-natural-gas/

    GGE is 33.4 kWh
    http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent

    So Tank-to-Wheels that's 1222 Wh/mile city and 868 Wh/miles highway.

    With that you can begin to compare to other types of transport. You'll have to make some assumptions in terms of Well-to-Tank efficiency.
     
  3. smorgasbord

    smorgasbord Active Member

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    Thanks for the start. The 33.4kWh number is theoretical, not the actual powerplant produced energy per gge, right? So, how efficient are NG powerplants?

    Even so, at over 800Wh/mile best case, a Tesla at 350Wh/mile easily achievable case is twice as efficient, and maybe more so given powerplant inefficiencies. More reason for EVs instead of CNGs.
     
  4. doug

    doug Administrator / Head Moderator

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    The 33.4kWh can be viewed as a thermodynamic limit of how much chemical energy is contained in a gallon of gasoline. So it's not necessarily the electricity you can get out of it. kWh is just a unit of energy which is easy to relate to electricity. If we weren't including electricity in the conversation, BTUs or Joules might be better units.

    Looks like the efficiency of a NG power-plant can range from 30 to 60%.
    http://en.wikipedia.org/wiki/Fossil-fuel_power_station#Gas_turbine_plants
    http://www.naturalgas.org/overview/uses_eletrical.asp

    Say the transmission efficiency of the grid is 93%.
    http://en.wikipedia.org/wiki/Electric_power_transmission#Losses

    That gives you an idea of power plant to wall efficiency. That doesn't include charging efficiency. (It also doesn't include energy used to get the natural gas out of the ground on to the plant, but lets call that a wash when comparing to CNG cars).

    Anyhow including Well-to-Wall losses, you can multiply the the EV Wh/mile usage by a factor of two or three.
     
  5. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The motivation--and I think financing--for developing the diesel engine was to use coal (liquified) as a fuel.
     
  6. ElSupreme

    ElSupreme Model S 03182

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    Well it is about as direct as you will get a coal powered car. And if you put some wood on it two you have yourself a hybrid vehicle. :wink:

    Cugnot-s-original-steam-car.jpg
     
  7. Doug_G

    Doug_G Lead Moderator

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    I think that would be "Flex Fuel"...
     
  8. doug

    doug Administrator / Head Moderator

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    Ye olde external combustion engine.
     
  9. Robert.Boston

    Robert.Boston Model S VIN P01536

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    Another data point: an efficient gas-fired power plant uses about 7.1 MMBtu/MWh at the generator bus bar. If we assume 20% line losses, conversion and charging losses, so let's call that 8.5 MMBtu per delivered MWh.

    - - - Updated - - -

    Here's the opening section of a paper I'm working on this point:
     
  10. doug

    doug Administrator / Head Moderator

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    Nice. Do you include SMR hydrogen in your analysis as well? HFCVs supposedly should compare well in this special case.
     
  11. RDoc

    RDoc S85D

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    Actually Diesel originally intended to use powdered coal blown into the cylinder by compressed air, compression ignited. That didn't work so he went to liquid fuel in desperation as I understand it.
     
  12. TEG

    TEG TMC Moderator

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    Hmmm... They don't quote MPG or power output for these vehicles. Engine tuning for CNG compared to gasoline is better off optimized for one or the other. For instance, do they use the same compression ratio for both types of fuel? (CNG works better with higher compression.)
    The article states that these are not electrically powered at all, rather just "bi-fuel" ICE vehicles that can switch from CNG to gasoline automatically when the CNG runs out.

    This doesn't feel like a step forward. I think plug-in gas-electric hybrids would be a better solution. Among other things, with an electric hybrid system you get efficiency improvements from regen braking, engine auto stop at low speeds, etc. Efficiency is much better when running electric drive, and the home "refueling/recharging" is more simple with plug-in electric than with CNG.
     
  13. doug

    doug Administrator / Head Moderator

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    Agreed, I didn't see much here to be impressed by.
     
  14. qwk

    qwk Model S P2681

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    Wow, talk about manufacturers trying to desperately hold on to their ICE lineup.
     

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