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Thread: MPGe discussion

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    MPGe discussion

    Warning this is off topic and long. Don't blame me for boredom.

    So if you look up cars with the best MPG lately all you see are electrics with fancy triple digit MPGe ratings. I wanted to delve into how this was calculated and how a large, heavy car, with massive tires, and mediocre aerodynamics could be getting such great MPGe ratings.

    MPGe is the abbreviation for “miles per gallon of gasoline-equivalent.” It’s an energy efficiency metric that was introduced by the Environmental Protection Agency (EPA) in 2010 to compare the amount of energy consumed by alternative fuel vehicles to that of traditional gas-powered cars. If a vehicle uses non-liquid fuels that aren’t burned and gets its power from electricity or compressed natural gas, it’s rated in MPGe.

    According to the EPA, burning one gallon of gas produces 115,000 BTUs (British thermal units). To generate the same amount of heat by way of electricity, it takes 33.7 kilowatt-hours (kWh). Kilowatt-hours is the standard energy unit for electricity.

    In simplified terms, if an electric vehicle can travel 100 miles on 33.7 kWh of electricity, the EPA rates it at 100 MPGe.
    This definition felt a bit unsettling for me as for the same reason cars don't do 115k BTU worth of mechanical work on a gallon of gas, neither should electric vehicles. An article on Reuters I found pins the average modern new gas engine to a 40% thermal efficiency while Nissan is working on a 50% thermal efficiency engine and some very inefficient vehicles clocking in at 20%.

    This begs the question how thermally efficient is commercial scale electric? As it turns out, not very efficient. In 2020 in the US 36.47 quadrillion BTU of electric was generated. Among losses to this are conversion losses, plant electric use, transmission loss, and delivery losses. Coal power has among the lowest thermal efficiency, while renewables are significantly better. Despite this less than 20% of the US grid is renewable and only 12.96 quadrillion BTUs of electric was used in 2020. This equates to about a 64.5% loss or only about 35.5% efficiency. This seems to suggest a modern internal combustion engine can have better efficiency than wall power.

    Then the next aspect is charging inverters. EV chargers comes in Level 1 (slowest) to Tesla supercharger. According to the IEEE level 1 charging efficiency for electric cars is only averaging 85.7%. Level 2 and original superchargers could be up to 90% efficient. Supposedly Supercharger V3 could be 95% efficient but I haven't seen any real world data. So of the 35.5% efficiency another 5-15% of this number is lost in the battery charging process. This brings down efficiency overall to between 30%-33.7%.


    According to the EPA the Tesla model 3 long range is rated at 134 MPGe. With the previous set of efficiency calculated this puts the average MPG down to between 40-45MPG depending on slow charging or ultra efficient supercharger V3. This number is a long ways off from the publish MPGe figures. With this math a Chevy volt could get as little as 35MPG vs its published 118 MPGe rating or a Nissan leaf at only 33.6MPG.

    While the raw thermal efficiency may not be as good, this does little to address the two other factors, of costs for fueling and environmental cleanliness. As it stands with costs I went into detail on my electric car rant how the math doesn't add up, so what is left is further investigating the environmental impacts. I will save that part for another day. Let me know if I mathed wrong or forgot something.


    Update: After thinking about this some more, some energy is lost in the refining of gas at around 10%. This number seems to be guessed at often but not really known. Also while some electric is used to refine gas most fuel refineries use isn't electric itself. Additionally there are some transportation losses delivering the gas to a station that haven't been calculated in.


    Last edited by Mirageman38; 05-11-2021 at 01:05 PM.

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    My brain is too tired to comment much tonight! You're making some excellent points, however. I will let others fact check you.

    I did notice this article the other day that stated -

    "According to this study, which looked at California EV owners specifically between 2015-2019, 18% of electric vehicle owners switched back to a gas-powered car. For plug-in hybrid owners, 20% of them flipped back to a car solely powered by an engine."

    "The research found owners were 53% less likely to buy another EV if they did not have access to convenient, at-home charging."

    Taken from -

    https://www.cnet.com/roadshow/news/e...s-power-study/

    I can't say that I know anyone who owns an electric car. I had a good friend who owned an electric Polaris Ranger. He hated it. Traded it for gas one within a year or two.

    Does anyone know someone who owned an electric vehicle and upgraded to second one?

    I find plugging in my engine block heater during cold winter nights to be a slight extra hassle.

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    Quote Originally Posted by Mirageman38 View Post
    Warning this is off topic and long. Don't blame me for boredom.

    So if you look up cars with the best MPG lately all you see are electrics with fancy triple digit MPGe ratings. I wanted to delve into how this was calculated and how a large, heavy car, with massive tires, and mediocre aerodynamics could be getting such great MPGe ratings.



    This definition felt a bit unsettling for me as for the same reason cars don't do 115k BTU worth of mechanical work on a gallon of gas, neither should electric vehicles. An article on Reuters I found pins the average modern new gas engine to a 40% thermal efficiency while Nissan is working on a 50% thermal efficiency engine and some very inefficient vehicles clocking in at 20%.

    This begs the question how thermally efficient is commercial scale electric? As it turns out, not very efficient. In 2020 in the US 36.47 quadrillion BTU of electric was generated. Among losses to this are conversion losses, plant electric use, transmission loss, and delivery losses. Coal power has among the lowest thermal efficiency, while renewables are significantly better. Despite this less than 20% of the US grid is renewable and only 12.96 quadrillion BTUs of electric was used in 2020. This equates to about a 64.5% loss or only about 35.5% efficiency. This seems to suggest a modern internal combustion engine can have better efficiency than wall power.

    Then the next aspect is charging inverters. EV chargers comes in Level 1 (slowest) to Tesla supercharger. According to the IEEE level 1 charging efficiency for electric cars is only averaging 85.7%. Level 2 and original superchargers could be up to 90% efficient. Supposedly Supercharger V3 could be 95% efficient but I haven't seen any real world data. So of the 35.5% efficiency another 5-15% of this number is lost in the battery charging process. This brings down efficiency overall to between 30%-33.7%.


    According to the EPA the Tesla model 3 long range is rated at 134 MPGe. With the previous set of efficiency calculated this puts the average MPG down to between 40-45MPG depending on slow charging or ultra efficient supercharger V3. This number is a long ways off from the publish MPGe figures. With this math a Chevy volt could get as little as 35MPG vs its published 118 MPGe rating or a Nissan leaf at only 33.6MPG.

    While the raw thermal efficiency may not be as good, this does little to address the two other factors, of costs for fueling and environmental cleanliness. As it stands with costs I went into detail on my electric car rant how the math doesn't add up, so what is left is further investigating the environmental impacts. I will save that part for another day. Let me know if I mathed wrong or forgot something.
    Good work. Not to mention the pita of charging on a long trip and even at home. Say you forget to plug it in and you go out in the morning and you're not going anywhere lol.

    Plus the added cost and complexity.

    When I bought my car I thought for about 3.67 seconds about the Corolla hybrid but the extra 4k didnt make sense as I drive about 10k a year maybe! So to re coup that money would take me like 20yrs and I'd have a more complex car. Electric was never thought about even though I'd be a perfect candidate for 98% of my driving i.e. in town under 10mi trips and park in the garage nightly so access to electricity.

    I'd consider electric when there is a much more robust charging network and in turn electric cars will be that much better in say 10-15 more years when my current car will be ready to unload. Will see.

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    Quote Originally Posted by Mark View Post
    Does anyone know someone who owned an electric vehicle and upgraded to second one?

    I find plugging in my engine block heater during cold winter nights to be a slight extra hassle.
    I know of two people who have Tesla's. One guy started off with a Model S, then picked up a model 3. I believe they both had lifetime supercharging. Last I heard he got rid of the Model S and picked up a model X. He rents them on Turo. He cooked the motor and battery in the Model S racing an Audi on the highway. Got it covered under warranty and they upgraded it to a higher kw battery. It had other problems like the micro switches in the handles wouldn't work but he was able to fix that himself. He bought the model s and 3 used from out east, not sure about the x. He drives a lot and saved a ton of money on fuel over the years charging for free. He owns his own business so can afford to go sit in a parking lot charging for an hour or could even have customers come to him and work while his car charges. Super nice guy, always shows me new things it does whenever I see him. Offered me to take one for a drive but I was on the clock and in greasy coveralls so kinda couldn't.

    The other tesla guy I know is my brother in laws friend. He bought(financed) a new model X when the came out. The guy is a total douche and looks down on non tesla owners and others who spend less than $600,000 on their house. He was in an accident, (unfortunately not hurt) and the thing sat in a body shop parking lot for at least 6 months waiting for parts.

    I think this started when 7milesout stated the literal fact that Volts can get infinite literal Miles per gallon of gas, not eMPG. If all those facts above are true, wow! That's pretty wasteful. But I wonder how electricity production efficiency compares with gasoline. Likely way to many factors to take into consideration but I don't think 1/2 of all gasoline produced goes mia.

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        click to view fuel log View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)


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    Yeah, that is what got me started thinking about MPGe in general. I added a disclaimer for refining losses but it's hard to pinpoint these numbers. It's just crazy to think you need be burning over 3.3x coal equivalent to your EVs battery size and how the MPGe is a bit misleading.

        __________________________________________

        click to view fuel log View my fuel log 2019 Mirage ES 1.2 automatic: 38.8 mpg (US) ... 16.5 km/L ... 6.1 L/100 km ... 46.5 mpg (Imp)


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    MPGe has nothing to do with the ecological impacts of a vehicle and everything to do with cost.

    To drive an electric vehicle with 100 MPGe one mile will cost approximately half of what it costs to drive a conventional car with 50 MPG the same distance. That's it.

    I say approximately because of the differing costs of fuel and electricity based on region.

    Also, in your calculation of relative efficiencies, you forgot to take into account the 65-180 million years to create crude oil, then the energy and materials to pump it up, then transport it to a refinery, then refine it, then transport it to your local gas station.

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    Quote Originally Posted by PityOnU View Post
    MPGe has nothing to do with the ecological impacts of a vehicle and everything to do with cost.

    To drive an electric vehicle with 100 MPGe one mile will cost approximately half of what it costs to drive a conventional car with 50 MPG the same distance. That's it.

    I say approximately because of the differing costs of fuel and electricity based on region.

    Also, in your calculation of relative efficiencies, you forgot to take into account the 65-180 million years to create crude oil, then the energy and materials to pump it up, then transport it to a refinery, then refine it, then transport it to your local gas station.
    Well according to the official definition by the EPA this isn't the case. I actually ran some numbers in a different thread where if your electric costs enough, and gas is cheap enough you could actually potentially save money with fueling with gas. Obviously if you have lifetime free supercharger this changes things but when you paid $60k more for the vehicle that itself buys a lot of gas.

    As for the materials and time to create the same could be said partially for the electric grid. Most energy comes from coal and natural gas, these are still fossil fuels. Also I never calculated in the effects of delivering these resources to the powerplants.

    The point of this is more to make aware how MPGe is calculated, how you can better compare it with normal MPG, and to explain why MPGe numbers are so high. Not to justify anything in particular.

    Obviously if the electric grid was much cleaner there would be a bigger difference between the two but this isn't the reality right now. Also even if you have an electric car it's beneficial and more environmentally friendly to have a smaller electric car, that weighs less, and has smaller tires, and less wind resistance. Making it electric doesn't get around the rules of physics.

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        click to view fuel log View my fuel log 2019 Mirage ES 1.2 automatic: 38.8 mpg (US) ... 16.5 km/L ... 6.1 L/100 km ... 46.5 mpg (Imp)


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    Quote Originally Posted by PityOnU View Post
    MPGe has nothing to do with the ecological impacts of a vehicle and everything to do with cost.
    I'd say driving and ev has nothing to do with ecological impacts of a vehicle and has everything to do with spending less for fuel.

    Here's hoping this doesn't turn into one of "those" threads".


    Quote Originally Posted by Mirageman38 View Post
    Obviously if you have lifetime free supercharger this changes things but when you paid $60k more for the vehicle that itself buys a lot of gas.
    This is why I have my car. It'd take a long time for me to recoup even $30-whatever thousand for a new bolt. Right now my fuel bill is around $60/month. And now I'm spending over $300 in fuel this weekend going camping/quading. Yippy!

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)


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    Quote Originally Posted by Fummins View Post
    I'd say driving and ev has nothing to do with ecological impacts of a vehicle and has everything to do with spending less for fuel.

    Here's hoping this doesn't turn into one of "those" threads".

    This is why I have my car. It'd take a long time for me to recoup even $30-whatever thousand for a new bolt. Right now my fuel bill is around $60/month. And now I'm spending over $300 in fuel this weekend going camping/quading. Yippy!
    Yep. The greenest you can be (in terms of money and the environment) is to just keep your old car running as long as possible. Even better just don't own a car at all. But that's not possible for some people.

    Honestly lithium-ion power cells in electric vehicles are a bit **** of a solution all around but they're "good enough" to make them at least viable.

    I think the hydrogen fuel cells are the better solution long term but until they scale it out and figure out a way to reduce the costs of generating it we're stuck with what we got, which is gasoline.

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    From Wikipedia - Miles per gallon gasoline equivalent
    Miles per gallon gasoline equivalent (MPGe or MPGge) is a measure of the average distance traveled per unit of energy consumed. MPGe is used by the United States Environmental Protection Agency (EPA) to compare energy consumption of alternative fuel vehicles, plug-in electric vehicles and other advanced technology vehicles with the energy consumption of conventional internal combustion vehicles rated in miles per U.S. gallon.

    The unit of energy consumed is deemed to be 33.7 Kilowatt-hours without regard to the efficiency of conversion of heat energy into electrical energy, also measured in Kilowatt hours. The equivalence of this unit to energy in a gallon of gasoline is true if and only if the heat engine, generating equipment, and power delivery to the car battery are 100% efficient. Actual heat engines differ vastly from this assumption.
    I'll have to read the article one day and hopefully get a better understanding of this. But from these two paragraphs I am under the assumption that the heat generated by combustion does nothing for propulsion (well, maybe not nothing...) and thus is wasted. Electric drivetrains don't suffer these losses.


    Last edited by Eggman; 05-12-2021 at 01:49 AM.

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