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Old 04-26-2009, 11:40 PM   #11 (permalink)
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There's a good discussion of this topic on Wikipedia, at Miles per gallon of gasoline equivalent - Wikipedia, the free encyclopedia
You could make a case for heat-equivalent, extractable energy equivalent, dollar-equivalent, or CO2-equivalent methods for comparing gas to electric. I'm inclined to favor CO2 methods, using the average US carbon intensity of electricty, which I think is .6kgCO2/KWh. With gasoline at 2.421kgCO2/gal, you have 4KWh/gal.

So I'd say Ryland is getting 14mpg(CO2equiv, US average electric mix).

As I've said in other threads, electric cars don't reduce CO2 emissions by themselves. Low-carbon electric generation is required.

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Old 04-27-2009, 12:48 AM   #12 (permalink)
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So from a Co2 stand point I would be better off charging my car off a gas generator, rather then plugging it in to the wall outlet?
From a cost stand point I figure my electric car costs a little less then $10 per month in electricity, where my gas car costs me around $20 per month in gas, it's still a big chunk of my electric bill, almost a quarter of it.
There are also the days I charge my car off wind and solar, last time I tried to do that with my gas car it didn't work out so well.
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Old 04-27-2009, 12:59 AM   #13 (permalink)
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Converting mi/kWh to mpg

Hello, Ryland,

You asked for my take on what your EV car would do as a gasser. Okay, here goes--

Just to repeat myself, there is no right answer for this. My view is that you get a fair estimate of what it will do by multiplying by 17, like this--

3 to 4 miles /kWh x 17 = 51 to 68 mpg

Where did the other 50% go? I threw it away when I converted from heat to electricity-- I assumed a 50% conversion, a little optimistic.

Now, I have a second approach that you might find interesting. When I calculate the mpg of a car using drag, rolling resistance and some drive efficiencies, I can do it for BOTH an EV and a gasser. I looked up one of those calculations and compared the EV numbers to the gasser mpg numbers, and these are the results--

For the very same car, both ways--

3 miles /kWh is equivalent to 45 mpg as a gasser
4 miles /kWh is equivalent to 59 mpg as a gasser

To get these, I used the following drive system efficiencies:

Electric car: 70% of the electricity at the socket reaches the motor shaft
Gasser car: 30% of the gasoline energy is converted to work at the shaft

There is an additional assumption that the car is exactly identical in the two cases in all other respects. For example, I assumed 85% transfer from the motor shaft to the wheels in both cases (a standard assumption for gassers). This assumption can easily be off for an EV with wheel motors or no transmission. (Effects are "backwards" here--lower transfer in the gasser means the equivalent mpgs are lower.)

I hope this actually helps to clarify.
/Ernie Rogers

Quote:
Originally Posted by Ryland View Post
yes, that light blue vehicle is one two electric cars I own, it's a 1975 Seabring Vanguard, Citi-car, like what TomEV drives, the electric car that I drive every day is a later version, of that car, a 1981 Comuta-car, I don't have any good numbers on the energy usage of it because my batteries are 2nd hand and in rough shape so I've been leaving them on the charger more then I would normally to keep them topped off and to equalize and desulphate the plates, I'm also using the original on board charger from 1981 that does not turn off, it just tapers the charge slowly as the batteries get closer to full, this makes it hard to get exact numbers on energy usage, but if I had to make an educated guess based off Kill-a-watt meter readings at the outlet.
Ernie I would like to see, based off your math, what kind of mileage you think my electric car gets, with my current lead acid batteries I would say I can go one mile on 250-300 watt hours, or about 3-4 miles per KWH as it comes out of the outlet, before the charger.
So what does that come out to in MPG? and at what point am I losing that 50%?
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Old 04-27-2009, 01:39 AM   #14 (permalink)
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So why are making wild assumptions and just tossing out 50% of the energy that is being used? just because we feel like it!
After all, gas cars have to run off a heat engine, so lets use their poor performance to drag down the math on electric cars too, after all I bet no one who owns an electric car has ever charged it using solar or wind and that doesn't work in to our nice simple "lets just toss out 50% of your energy" theory, so we will just ignore it all together.
If you were to pole electric vehicle owners I would bet that a large percentage of them charge with renewable energy.
I thought that the reason for finding a common factor like BTU was so you could make the math more reliable and if reliable math is what we are looking for tossing in randomness then makes total sense.
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Old 04-27-2009, 11:17 AM   #15 (permalink)
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I'm sorry, Ryland,

I see I have hurt your feelings. There is a wild assumption here--the presumption by EV enthusiasts that their cars are naturally superior, and they "prove" that by the rediculus assumption that 34 kWh of electricity is exactly equal to the 116,000 BTU of heat contained in a gallon of gasoline. I am just pointing out that the idea of expessing EV efficiency in "miles per gallon" is absurd and should be abandoned.

Any crappy, poorly-designed EV looks good using bad math, which I confess is the normal practice everywhere, including by the EPA. You do your brotherhood a disservice by continuing this.

Ways to avoid this problem are--

1. Just admit there isn't a fair way to compare EVs and gassers, and provide separate lists of ranking.

2. We can all agree that electric drives are very efficient. We could concentrate on car efficiency from the motor shaft to the wheels, where all cars share roughly the same parts. Then we can compare merits of cars by their common elements of design such as rolling resistance, regen. braking, and aero drag.

3. We can go to the energy source in determining efficiency-- this is called "well-to-wheels" efficiency. (Which just gives us new things to argue about).

I have offered a reasonable compromise, that we simply acknowledge that heat and work (electricity) are fundamentally different, and I suggested that we connect them using the second law principles. This is what normally happens at an electric power plant anyway.

Ernie Rogers

Quote:
Originally Posted by Ryland View Post
So why are making wild assumptions and just tossing out 50% of the energy that is being used? just because we feel like it!
After all, gas cars have to run off a heat engine, so lets use their poor performance to drag down the math on electric cars too, after all I bet no one who owns an electric car has ever charged it using solar or wind and that doesn't work in to our nice simple "lets just toss out 50% of your energy" theory, so we will just ignore it all together.
If you were to pole electric vehicle owners I would bet that a large percentage of them charge with renewable energy.
I thought that the reason for finding a common factor like BTU was so you could make the math more reliable and if reliable math is what we are looking for tossing in randomness then makes total sense.
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Old 04-27-2009, 11:38 AM   #16 (permalink)
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Quote:
Originally Posted by Ernie Rogers View Post
3. We can go to the energy source in determining efficiency-- this is called "well-to-wheels" efficiency.
Who in their right mind would evaluate the energy chain of an EV (or a biodiesel) using such a phrase?!? Try again.
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Old 04-27-2009, 12:05 PM   #17 (permalink)
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Oh, I had overlooked your message earlier, Robert,

Excellent post, thank you.

Ernie Rogers

Quote:
Originally Posted by RobertSmalls View Post
There's a good discussion of this topic on Wikipedia, at Miles per gallon of gasoline equivalent - Wikipedia, the free encyclopedia
You could make a case for heat-equivalent, extractable energy equivalent, dollar-equivalent, or CO2-equivalent methods for comparing gas to electric. I'm inclined to favor CO2 methods, using the average US carbon intensity of electricty, which I think is .6kgCO2/KWh. With gasoline at 2.421kgCO2/gal, you have 4KWh/gal.

So I'd say Ryland is getting 14mpg(CO2equiv, US average electric mix).

As I've said in other threads, electric cars don't reduce CO2 emissions by themselves. Low-carbon electric generation is required.
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Old 04-27-2009, 01:38 PM   #18 (permalink)
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Not seeing a violation of any thermo laws...

The BTU figures are used as a comparison metric. Electric power trains aren't constrained by the btu content of gasoline. We could report in terms of mi/MJ - a unit that I've seen pop up more and more frequently...

Miles per MegaJoule

1KwH = 3.6MJ
1 gallon of gasoline (125,000btu) = 131.9MJ

3-4 miles/kWh - lets say 3.5 miles - this would be socket to wheel figures....

That's 3.5/3.6 mi/MJ = .97 mi/MJ

Compared to a 50mpg car - pump to wheel....

That's 50/131.9 mi/MJ = .38 mi/MJ


But doing so is missing the point. If I were completely new, and potentially open to the concept of driving more efficiently, I would feel alienated by this awkward unit. If I'm comfortable, I'm probably more likely to continue reading.



Does anyone have any literature with respect to an electric motor and the carnot heat cycle? That is, what's the equivalent/comparable isotherm compression stage for an electric motor and etc.? I've had red flags pop up in my head while reading that comparison. I even dusted off the old thermo book and heat transfer - nothing
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Old 04-27-2009, 02:05 PM   #19 (permalink)
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I personally think the numbers are fair. As long as, they use 1 US gallon equals 125,000 btu's, 115,000 btu's equals 33.7 kWh (from wiki linked above in another person's post). Using these numbers, 36.6 kWh equals 1 US gallon of gasoline. An electric motor has heat loss just the same as the gas engine. It is just much less.
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Old 04-27-2009, 05:15 PM   #20 (permalink)
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carnot and heat transfer have no bearing whatsoever on electric motors. period.

carnot is specifically regarding heat to work. Electricity is not heat. If you were to do something stupid like run it through a resistor and use that heat to power the an expansion engine then carnot applies.

Its literally like comparing force to work. They are different.

Also a gallon of gasoline produces 9kg of CO2. the US is powered by something like 53% by coal(22 lbs(8-9 kg) of CO2 per KWh). So a gallon of gas produces 18.odd pounds of CO2 and a KW in the US produces(or in the act of producing it, x CO is released into the atmosphere) 11 lbs(the other half is generated mostly through hydro and nukular(side note why does everyone always show the cooling tower as the evil face of nuclear power?)).

Thats a fair CO2 conversion.

Also it takes 4 GGE to make 5 gallons of gasoline(refining) So if you really want to whine about it being fair you had better add that in. Add in 40 more lbs of CO2 just for the electricity to refine the gasoline and already the electric is kicking gasoline out the door.

Ryland, how much electricity does it use while charging in total kwh and then how far does that get you. If its in your fuel log for the 100+ mpger I couldn't decipher it. Sorry I'm retarded I know.

Gasoline costs also emits an enormous amount of serious pollutants during refining. Coal drops NOx SOx, and some mercury, while refineries dump substantially more pollutants and far more carcinogenic compounds than a power plant.


Last edited by theunchosen; 04-27-2009 at 05:24 PM..
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