MPGe

The AXP Blog1 (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).

Experimental, flex-fuel, plug-in, Saturn Vue


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:

Felix Kramer


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.)

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.

Continue reading here: The Hansen Test

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