Auto X Prize throws water on GM's 230 mpg claim, offers mpge calculator
With all of the attention being paid to the 230 mpg number that the Chevy Volt will apparently be granted by the EPA, the Automotive X Prize thought it was time to weigh in on the subject of calculating fuel efficiency for vehicles that use energy sources other than gasoline. They don't like it. Instead, the AXP prefers MPGe, a "rigorous and more neutral measure" of fuel efficiency. The AXP's John Shore walked us through how the long-running competition thinks about MPGe. They've been at it for a while.
First, let's define MPGe. MPGe stands for miles per gallon equivalent, and measures fuel economy based on the energy content of a gallon of petroleum-based gasoline. For those who like formulas, the AXP defines MPGe as (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)]). Understanding and using MPGe is important, now more than ever, Shore said, because MPG is no longer particularly useful from the consumer's point of view. "It is obsolete," he said. Shore gave three reasons for moving away from MPG:
- The growing popularity of gasoline alternatives. When everyone was using gas (or diesel), MPG offered a decent way to compare the efficiency of different vehicles, especially if one drove prudently and understood how the ratings were calculated. But putting, for example, E85 into the tank changes the whole equation.
- We are now seeing vehicles that can use two power sources, most obviously plug-in hybrids that use electricity and a liquid fuel. MPG really loses its meaning when there is more than one fuel and only looking at the liquid fuel allows PHEV advocates to claim they get 100 or 150 mpg, which is kind of true but also deceptive, Shore said.
- Lastly, as soon as we introduce plug-in vehicles, the efficiency of the vehicle – no matter how you measure it – becomes a very strong function of how far you drive. This issue is not addressed by MPGe, but should be considered by people who want to understand fuel efficiency better. MPG is MPG no matter how long you drive it for. But a plug in hybrid changes depending how long it's been since the last charge.
[Source: Auto X Prize]
Since MPGe is the "figure of merit" that the AXP is using for the competition, they're confident that it's the best way to compare apples to pineapples. Shore noted that the natural gas industry faced a similar problem when they introduced CNG cars, and also chose to use MPGe (then called GGE, gallon of gas equivalent).
Shore and the AXP realized that most customers don't have any intuition about what kilowatt hours mean, so he added a simple calculation to the spreadsheet they use to calculate MPGe that includes the number of hours you plug in your vehicle and which type of outlet you're plugging into. This allows users to see how long it takes to get a set amount of equilavent energy from a gallon of gasoline. For example, a vehicle plugged into a 110 V circuit for eight hours gets about the same energy as what is in a third of a gallon of gasoline. Shore was surprised by this, and said that, "This shows why gasoline has remained king. in five minutes, you can put a sh*tload of energy into a car."
Shore added that, while the AXP thinks that MPGe is part of the answer, consumers will need to know three things about fuel efficiency numbers in the future: the MPGe, what sort of driving was done to achieve those numbers and, if the vehicle has a plug, how far the vehicle was driven. Armed with this information, Shore said, consumers will be able to accurately compare how efficient different vehicles, no matter what type of powertrains they have.
Reader Comments (Page 1 of 3)
Mark Kiernan 12:03PM (8/21/2009)
They are right the MPGe is a good standard to follow.
On a side note.
I thought the X-Prize was something that would create a new fleet of EV or something. It seems to be just an excuse for people to put lawnmower engines on baby buggies and canvas tops on bike frame and claim they are cars.
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Dave 4:31PM (8/21/2009)
http://www.ftc.gov/opa/2007/08/elabel.jpg
BonusOnus 3:19PM (8/24/2009)
The BTU numbers from electricity are still a little misleading from an environmental and energy point of view for two reasons. Firstly, the fuel mix (nuclear, coal, hydro etc) for the electricity determines the base amount of BTU that goes in to generating that kwh. So while 1kwh out of the wall is 3412 BTU, it might have come from significantly more energy at the power plant. Secondly, and somewhat closely related is the effect from transmission line losses and energy conversion costs. From raw fuel to the electrical outlet this loss can be as much as 90%, so just looking at the conversion efficiency of the engine itself is not entirely accurate. One needs to look at the efficiency of the entire system that turns fuel (coal, natural gas, or even uranium) into energy/movement of the car.
Ghen 12:16PM (8/21/2009)
IMO they still don't get it. MPG right now directly correlates with saving money. The higher the MPG the less you will spend per year traveling the same distance. Until they get a spec that takes the cost of the fuel into concern there will be no worthwhile equivalency.
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Boyprodigy1 2:34PM (8/21/2009)
on the contrary, if you are dumb enough to not understand that electricity is cheaper than gasoline, then i think that you probably shouldnt be allowed to drive in the first place. MPGe calculates efficiency. In every form of energy, the more efficient your vehicle is, the less it costs to drive. I think this idea is a win.
Dave 4:25PM (8/21/2009)
Ghen -
You are correct - consumers really are concerned with miles per dollar. And that is what consumers need on the sticker in some form or other. Something like the energyguide you see on a new washing machine.
However, the Environmental Protection Agency is concerned (theoretically) with protecting the environment. And there are also political agendas that give credit for avoiding foreign oil and for the ethanol farce. So the CAFE number isn't really a number for consumers. (And GM shouldnt be using it for advertising)
Michael 3:28AM (8/23/2009)
Auto X Prize is way off base.
The BTU content of the fuel is interesting trivia, but the fact is that electric cars actually convert far more of the BTUs you put into motion than internal combustion engines.
Their spreadsheet has created a measure that's BIASED AGAINST ALL BUT THE LEAST EFFICIENT processes that convert BTUs to motion.
NOT GOOD.
In fact, it's counterproductive to their own goal!
spookthehamster 9:07AM (8/24/2009)
@Michael
Energy used is energy used. It doesn't matter how efficient each process is when all you're measuring is the total used.
If you put 5l of fuel in a car, some may be wasted through heat etc, but you still burnt all 5l of it.
Ghen 7:25AM (8/25/2009)
@michael
Actually it should be biased towards whichever costs the least or whichever is best for the environment. If I waste 12 gallons of corn syrup to go 100 miles but it cost me $3 to fill the tank and rainbows come out of my tailpipe then it should compare very nicely to gasoline that has efficiency 4x-5x better per gallon.
Peter 12:18PM (8/21/2009)
This is a step in the right direction as it allows comparisons of energy consumption across different fuel types. However, I'm not convinced that the amount of energy used is the right metric to concern ourselves with. Energy consumption in and of itself does not have a negative affect, rather the CO2 associated with the energy does. And different fuels create very different amounts of CO2 per gallon equivalent: from 8.79 kg CO2 for gasoline down to zero for an electric vehicle charged by solar.
So an EV could get 1mpge but have a negligible effect on climate change compared to a 50mpg gasoline car.
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paulwesterberg 12:46PM (8/21/2009)
Note to GM: electricity is not free.
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paulwesterberg 1:41PM (8/21/2009)
Btw using the x-prize mpge conversion, and considering that GM has said they use 8kWh of the 16kWh pack to travel 40 miles:
If the volt gets 60mpg in charge sustaining mode then:
over 100 miles it will get 80mpge.
over 200 miles it will get 69mpge.
If the volt gets 50mpg in charge sustaining mode then:
over 100 miles it will get 69mpge.
over 200 miles it will get 58mpge.
The GM volt is not an automotive x-prize competitor because it cant do 100mpge.
why not the LS2LS7? 12:53PM (8/21/2009)
MPGe is still not all that useful when it comes to a dual energy source vehicle like a plug-in hybrid.
Furthermore, since the government uses a distorted figure for mpge (as seen on the Leaf), using mpge is opening yourself to a lot of misinformaton.
Furthermore, I can't believe the X Prize people made an energy calculator that uses BTUs as its unit! Use Joules, guys! Maybe I'd expect that from a guy who doesn't know that the reason you have to plug your EV in for so long is because there is so much energy in a gallon of gas. Sheesh, he works on the automotive X-Prize! Great to see you guys catching up!
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Shock Me 1:13PM (8/21/2009)
I suggest cost per 60 miles traveled in CD and CS modes plus a number for the all electric range at 60 mph on level ground.
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Tim 1:06PM (8/21/2009)
GM was premature in their 230-MPG announcement and should have waited until a new standard is adapted by the Society of Automotive Engineers and the entire automotive community. Then again, the Auto-X prize is not GOD either!
This controversy has NOT helped GMs credibility nor has the resulting turmoil helped with the transition from petroleum to EVs (range-extended or not).
Act in haste... repent in leisure!
Look before you leap!
Measure twice, then cut only once!
(and so on)
Then again to err is human and to forgive, divine!
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john 1:06PM (8/21/2009)
A PHEV needs only two numbers for range and for efficiency. It would look something like this:
Gasoline-only Range: 250 miles
Electric-only Range: 50 miles
Gasoline-only Efficiency: 32 mpg
Electric-only Efficiency: 5.4 mi/kwh
Simple.
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Shock Me 1:25PM (8/21/2009)
So calculating HWY mileage for a Volt with an 8 gallon tank would look something like this:
Gasoline-Only Range = 400 miles
Electric-Only Range = 40 miles
Gasoline-Only Efficiency = 50 mpg
Electric-Only efficiency = 5 mpkWh
Then a cost per mile figure in both modes with an assumed price 13 cents per kWh and $2.50 gallon of gas.
$ 0.05 per mile on gas
$ 0.02 per mile on electricity
itof500 1:21PM (8/21/2009)
The equivalent I like best is CO2 production per mile. Here in Indiana generating a kWh of electricity from our coal fired plants releases about 2 lb of CO2. Burning a gallon of gasoline (in any state) creates 20 lb of CO2. So the equivalence of kWh to a gallon of gasoline is 1:10 in this measure, rather than 1:30 using BTUs.
An example; The Volt is reported (cnn money) to use 10 kWh to drive its maximum 40 miles without using gasoline. In Indiana that would release 20 lbs of CO2, or the equivalent of a gallon of gasoline. So, from a CO2 standpoint it is getting 40 mpg. Not bad, but not triple digits either.
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ivor.pdx 1:54PM (8/22/2009)
I haven't heard about including a CO2 measurement into these comparison ratings, but it makes sense... CO2 accumulation is the 800lb gorilla that's about to smack us on the head, and the sooner people start thinking about associating CO2 with survival, the better off we'll be. Obviously these ratings need to demonstrate to the public why driving a lightweight Aptera is going to be better for all concerned than driving Lutz's massive electric Cadillac concept car.
meg 3:12PM (8/22/2009)
I really like this kind of chart much better than basic MPG which is no longer applicable.
The problem with the CO2 idea, itof500, is that it varies in different parts of the country. Here in the midwest we're belching coal, but there are plenty of places with cleaner electricity. I would say that the CO2 rates would be something local conservation groups should make fliers and websites for.
Additionally, much of the non-green crowd doesn't care about CO2 even if they should. They do, however, care about money. So while a CO2 rating might be nice, I don't think it will be interesting to that many people.