Please explain MPGe

Discussion in 'Prime Fuel Economy & EV Range' started by Pizza Driver, Oct 20, 2016.

  1. Pizza Driver

    Pizza Driver Active Member

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    I don't understand all the implications of this number. Please explain.
     
  2. jdenenberg

    jdenenberg EE Professor

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    A plug-in vehicle uses electricity as a power source. The MPGE rating is an attempt to estimate the overall energy efficiency of the vehicle and compare it to a standard gasoline driven car. I have always questioned the validity of these estimates as they seem to err in favor of electrical power.

    JeffD
     
  3. Lee Jay

    Lee Jay Senior Member

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    The EPA sets an electricity equivalent of a gallon of gasoline at 33.7kWh. This number is an overestimate most of the time, especially if you usually use 10% ethanol fuel. However, it's only an overestimate by a few percent.

    That MPGe number is how far the car will go (in some EPA driving cycle) on 33.7kWh of electricity *delivered from the wall during charging*. Thus, it includes all losses including those from the charger, the in-and-out losses of the battery, the power electronics in the car and the electric motor.
     
  4. Mister MMT

    Mister MMT Active Member

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    I found here, this video explaining it well



    Jan
     
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  5. GT4Prius

    GT4Prius Active Member

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  6. bisco

    bisco cookie crumbler

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    it's good for comparing one car to another, but you don't get a readout when driving the pip, not sure about prime.
     
  7. Trollbait

    Trollbait It's a D&D thing

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    Wikipedia also has a decent article on it. Miles per gallon gasoline equivalent - Wikipedia

    Short form is that EV efficiency is measured by the watt or kilowatt hours of electricity used. MPGe just converts that value into something that can easily be compared to a gasoline car's fuel economy.

    Since MPG is measured from the amount of gas that is put into car, MPGe also uses the electricity amount put into the plug in. This means it accounts for the energy lost to the charging process.
     
  8. jdenenberg

    jdenenberg EE Professor

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    Note that they are comparing about $0.07 worth of electricity to about $2 worth of gasoline (classic apples to oranges comparison). I do not agree with the equivalence unless you do a full efficiency analysis from energy source (coal, nat. gas, hydro, wind. solar for Electricity vs. crude oil for gasoline) to motive power. I did this once to compare my kerosene burner to an electric heater (both are about 100% efficient) and the kerosene burner won easily due to the delivery inefficiency of the power grid (even assuming the fuel cost for the power grid was lower)

    JeffD
     
  9. Lee Jay

    Lee Jay Senior Member

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    No, 33.7kWh is not 7 cents anywhere.

    The delivery efficiency of the power grid is over 92%. You must have been including the thermal efficiency of the power plant, which is very hard to define when the sources are renewable.
     
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  10. jdenenberg

    jdenenberg EE Professor

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    A slip of 2 decimal points ... point taken

    JeffD
     
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  11. Trollbait

    Trollbait It's a D&D thing

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    These numbers aren't meant for such level of analysis. They are meant to be used by consumers as a way of comparing the fuel efficiency of two cars with different fuel sources in the process of selecting one or the other. The cars themselves have no influence on the efficiency of how the electricity or gasoline was made and got into the car, so has no direct relevance on on these numbers.

    As for cost, the window sticker that these figures are posted on includes the annual fuel cost as well as the cost to go 25 miles for the car using national averages. So does Fueleconomy.gov, and that site lets the user adjust those average values for more personal figures.
     
  12. HypersonicPrime

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    Is it just me or does the Prime not have a good display of overall (gas+electric) energy efficiency? The listed MPG is misleading because it doesn't include energy from the grid. The "Drive Monitor 2" miles/kWh doesn't include energy from gasoline. The only method I can envision to calculate an overall energy efficiency (MPGe) is the following: EV, HV, MPGe calculations... | PriusChat
     
  13. Oniki

    Oniki Active Member

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    I'l tell you what is is NOT:

    It is not a useful way to compare an EV to an ICE
    Doing so gives you wrong answers if your exercise is money, GHG emissions or criteria taipipe pollutants.

    ICE vs ICE ? sure
    EV vs EV ? Clumsy, but go ahead
     
  14. Zythryn

    Zythryn Senior Member

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    MPGe is very good at what it is. A measurement of the efficiency of plugin vehicles.
    I do agree it is terrible if measuring cost, or carbon or pollution.
    Just as a ruler is terrible at measuring something's weight.
     
  15. pilotgrrl

    pilotgrrl Senior Member

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  16. Oniki

    Oniki Active Member

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    I've been trying to come with an analogy for a while. How about this:

    Is a pound of fibre equal to a pound of sugar ?
    Both are one pound
    Both are carbs
    They have similar embedded calories

    And yet to a human they have very different utility. Fibre is good for a bulky stool; sugar will move you.
    And while fibre can be broken down to sugar by some animals, it takes energy to do so. That energy is not reflected in our initial comparison.
     
  17. pilotgrrl

    pilotgrrl Senior Member

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    Sorry, hit Send instead of backspace...

    ~$0.07 is about what I pay for 1 KWh of electricity. It's pretty exciting to me that that is about equal to $2 worth of gasoline.

    It's the little things in life that make me happy...

    Posted via the PriusChat mobile app.
     
  18. Zythryn

    Zythryn Senior Member

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    Sure, they are equivalent weight. So a scale would be exactly correct, stating they are the same weight.
    If you are trying to measure utility, weight is a very poor measurement.

    MPGe measures pure efficiency. Don’t try to use MGPe to measure something else.
     
  19. Lee Jay

    Lee Jay Senior Member

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    I don't know how you got that, but it's certainly not.
     
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  20. pilotgrrl

    pilotgrrl Senior Member

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    I got it from #8: "Note that they are comparing about $0.07 worth of electricity to about $2 worth of gasoline".

    Posted via the PriusChat mobile app.
     
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