The AXP Blog (Automotive X Prize) has established a formula for converting gallons of gasoline to an equivalent energy number. Basically they asked, “How much energy was delivered to the vehicle, and how far did it go?” The result is MPGe (Miles Per Gallon equivalent).
Ultra capacitors combined with standard deep-cycle batteries comprised the energy storage in an experimental, flex-fuel, plug-in, Saturn Vue converted by AFS Trinity Power Systems of Bellvue, WA.
Different fuels have different pros-and-cons, but in all cases it’s valuable to increase efficiency (increase MPGe), which conserves energy. This is even true, for example, if the fuel is electricity generated from alternative energy sources. Alternative energy is not infinite energy. Increasing electric vehicle efficiency will result in more energy available for other purposes.
MPGe is an attractive figure of merit because it’s a direct measure of overall “pump-to-wheels” efficiency, because it’s technology-neutral, and because it relates nicely to consumer intuition – i.e., it reduces to the familiar MPG if the fuel is in fact gasoline.
MPGe is also attractive because it applies if a vehicle is powered by more than one fuel, such as plug-in hybrid electric vehicles (PHEVs), which typically use electricity plus a liquid fuel (often, but not necessarily, gasoline). Here’s how to compute MPGe for this important case:
Advocates argue that no breakthrough is needed; it now is possible to drive a plug-in hybrid car that does not need gas, or at least not much. At national average electricity prices, PHEVs would cost the equivalent of roughly 75 cents per gallon to drive when operating on their electric motors. (When charging, a plug-in hybrid car draws roughly the same amount of electricity as a home space heater.)
MPGe = EG / (g*EF + e*EW)
where
m = miles per gallon of liquid fuel used (MPG)
g = 1/m = gallons of liquid fuel used per mile (GPM)
e = plug-to-wheels electrical energy used per mile (Wh/mi)
EF = BTU per gallon of liquid fuel used (not necessarily gasoline)
EG = BTU per gallon of gasoline = 116,090
EW = BTU per Watt-hour (Wh) of electricity = 3.412
The formula above can also be used for pure battery electric vehicles (BEVs) and for pure liquid fuel vehicle – for BEVs, set g =0, and for pure liquid fuel vehicles, set e = 0.
Informal published values for BEV and PHEV fuel economy abound, but they can be inadvertently misleading. For example, some results report gasoline usage but not electricity usage (electricity usage is harder to measure). Also, not all results are well-documented, so accurate comparisons can be difficult.
One problem in applying the MPGe conversion formula above is that by definition MPGe is “pump/plug to wheels”, whereas the electricity usage (Wh/mi) data reported may be battery-to-wheels, which ignores the conversion loss that results from charging the battery via an AC (grid-connected) outlet. It’s not always obvious from test results whether or not the Wh/mi are measured from the plug or from the battery.
Bearing these issues in mind, I have computed MPGe for some real examples based on published data. I include example results below. You can see the details and other examples in the spreadsheet available here, which can also be used to explore additional examples. To account for cases where the Wh/mi measurement is battery-to-wheels, the spreadsheet includes an option for applying a plug-to-battery conversion factor.
Of the PHEV results, the main thing that stands out is that the Google.org data yield a significantly lower MPGe than other test data. This likely reflects the fact that the Google.org data are collected from actual daily driving by multiple drivers, whereas the other data are from fixed test cycles. Also noteworthy is the significant variation in the MPGe results from fixed test cycles, even for the same PHEV conversion tested over a similar range.
Overall, the results reflect a basic underlying problem – the difficulty of establishing test procedures that are not only well-documented and repeatable, but that reliably predict the fuel economy that consumers would experience. Indeed, that’s why DOE and EPA are intensively developing next-generation test procedures.
AXP Blog Commentator Zane Selvans observes:
It is interesting to note that that the additional fuel savings (in actual gallons) for increases in fuel efficiency beyond say, 50 MPGe, is close to negligible, when compared to the amount of fuel that can be saved by simply upgrading the most inefficient vehicles. For instance, one saves more fuel by going from 13 to 17 MPG, than by going from 50 to 500 MPG. For 1000 miles of driving, the transition from 13 to 17 MPG saves 21.368 gallons. For the same distance, the transition from 50 to 500 MPG saves only 18.1 gallons.
If the ultimate goal of high efficiency vehicles is to reduce global warming gas emissions, and our dependence on foreign fossil fuels, we would do better to focus on improving the efficiency of the worst vehicles, not the best.
Unfortunately, that’s more a cultural and political problem than it is a technology problem, which means it’s hard for a few people to really change things by themselves.
2 Comments
Although Mike Millikin failed to provide such a warning, After Gutenberg, even in its commentary is commited to responsible blogging, and thus has issued a SEGO (Serious Eyes Glazing Over) warning for the following cited article. After all, GCC invited an EV geek to speak on batteries. ‘Nuf said.
In a thoughtful, honest analysis of plug-in hybrid technology for Green Car Congress, guest geek Ron Gremban propounds that “the very most important measure of a PHEV is the extent of its ability to displace liquid fuels, to do so during normal US driving cycles, and to do so cost effectively.” Thus, “what we want,” argues Gremban, “is, on average, the most displacement of liquid fuels for the least incremental cost.” And, he further suggests use of the US06 combined drive cycle as a much more realistic standard cycle than other driving cycles used for comparison.
Sebastian Blanco opened a can of worms when he relayed to readers of Autoblog Green an observation by two management professors at Duke University. Richard Larrick and Jack Soll suggested that “thinking in terms of miles per gallon doesn’t give people a good understanding of a vehicle’s real efficiency when compared to other vehicles.”
Among the ABG
rabblecommentators, dhofmann was the first to suggest Mega Joules rather than Gallons, “so people can compare between different fuel types.”ABG commentator Doug observed:
The above observation prompted me to comment:
The Observatory at the New York Times has more on the discussion and via NPR, the Duke University Press Release follows:
Gallons Per Mile Would Help Car Shoppers Make Better Decisions
DURHAM, N.C., June 19 /PRNewswire-USNewswire/ — Posting a vehicle’s fuel efficiency in “gallons per mile” rather than “miles per gallon” would help consumers make better decisions about car purchases and environmental impact, researchers from Duke University’s Fuqua School of Business report in the June 20 issue of Science magazine.
Inspired by debates they had while carpooling in a hybrid car, management professors Richard Larrick and Jack Soll ran a series of experiments showing that the current standard, miles per gallon or mpg, leads consumers to believe that fuel consumption is reduced at an even rate as efficiency improves. People presented with a series of car choices in which fuel efficiency was defined in miles per gallon were not able to easily identify the choice that would result in the greatest gains in fuel efficiency.
For example, most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).
These mistaken impressions were corrected, however, when participants were presented with fuel efficiency expressed in gallons used per 100 miles rather than mpg. Viewed this way, 18 mpg becomes 5.5 gallons per 100 miles, and 28 mpg is 3.6 gallons per 100 miles — an $8 difference today.
“The reality that few people appreciate is that improving fuel efficiency from 10 to 20 mpg is actually a more significant savings than improving from 25 to 50 mpg for the same distance of driving,” Larrick said.
Soll noted that replacing a large vehicle that gets 10 mpg with one that gets 20 mpg reduces gas use per 100 miles from 10 gallons to five, a 5-gallon savings. Replacing a small vehicle that gets 25 mpg with one that gets 50 mpg reduces gas use per 100 miles from 4 gallons to 2, a saving of only 2 gallons.
“Miles per gallon is misleading and can play tricks on our intuitions,” Soll said.
“For families and other owners of more than one type of vehicle, the greatest fuel savings often comes from improving the efficiency of the less efficient car,” Soll added. “When fuel efficiency is expressed as gallons per 100 miles, it becomes clear which combination of cars will save a family the most gas.
“We believe that everyone should try to be as fuel efficient as possible. For some people, that may mean driving the most efficient car available, such as a small hybrid car, but for others it may mean finding the most efficient option possible within their chosen class of car,” Soll said. “There are significant savings to be had by improving efficiency by even two or three miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers.”
The authors recommend that consumer publications and car manufacturers list efficiency in terms of gallons per 10,000 miles driven, which is already the standard in many other countries. “This measure makes it easy to see how much gas one might use in a given year of driving and how much gas, and money, can be saved by opting for a car with greater efficiency,” Larrick said.
Larrick and Soll’s research was funded by Duke University.
Miles Per Gallon Gallons Consumed per Gallons Consumed per
100 Miles Driven 10,000 Miles Driven
10 10.00 1,000
15 6.67 667
20 5.00 500
25 4.00 400
30 3.33 333
35 2.86 286
40 2.50 250
45 2.22 222
50 2.00 200
One Trackback
[...] earlier post (with links to Adelman and Jensen podcasts) uses the same MPGe designation. This blog previously noted that a formula for converting gallons of gasoline to an equivalent energy number has been [...]