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The term miles per gallon gasoline equivalent (MPGe or MPG_ge) is a measure of distance traveled per unit energy consumption. It is meant to be useful for comparing the energy consumption of alternative fuel vehicles to those of gasoline based vehicles in terms of miles per US gallon, a unit familiar to US consumers. MPGe is based on the energy content of the fuel delivered to the vehicle. However, overall energy consumption of an alternative fuel vehicle also may have to consider the energy consumed to produce the fuel and deliver it to the vehicle, yielding a so-called well-to-wheel measure of efficiency, depending on the purpose of the energy comparison.

Description

The miles per gallon gasoline equivalent is based on the energy content of gasoline. The energy obtainable from burning one US gallon is 115,000 BTU. Thus one mile per gallon gasoline equivalent is equal to 115,000 BTU per mile. For alternative fuels, energy required to manufacture the fuel may also be considered. To convert the mile per gallon rating into other units of distance per unit energy used, the mile per gallon value can be multiplied by one of the following factors to obtain other units:

1 MPGE	= 8.55 miles/ million BTU
	≈ 0.0292 miles/kW·h
	≈ 0.0182 km/kW·h
	≈ 0.005 km/MJ

Conversion to MPGE

MPGE is determined by converting the vehicle consumption per unit distance, as determined through computer modeling or completion of an actual driving cycle, from its native units into a gasoline energy equivalent. Examples of native units include W·h for electric vehicles, kg-H2 for hydrogen vehicles, gallons for biodiesel vehicles, cubic feet for compressed natural gas, pounds for propane or Liquefied petroleum gas vehicles, and gallons for liquefied natural gas vehicles. Special cases for specific alternative fuels are discussed below, but a general formula for MPGe is:

${\displaystyle MPGe={\frac {total~miles~driven}{\left}}}$

Depending on the purpose, overall energy consumption for the vehicle may also need to include the energy used in the production of whatever energy carrier is used for the vehicle and the energy used in filling the "tank". For example, with electrically powered vehicles, a full accounting of all energy consumption would include the efficiency factor for conversion of primary fuels into electricity and the efficiency factor of charging the battery from the electrical plug.

Basic values for the energy content of various fuels are given by the defaults used in the Department of Energy GREET (Greenhouse gases, Regulated Emissions, and Energy used in Transportation) model , as follows:

Note, however, that - except for electricity - the energy content of a particular fuel can vary somewhat given its specific chemistry and production method. For example, in the new efficiency ratings that have been developed by the U.S. Environmental Protection Agency (EPA) for battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) – see below – the energy content of a gallon of gasoline is assumed to be 114,984 BTUs.

History and Usage

The origin of the general form of MPGe is uncertain . For the special case of a single alternative fuel, MPGe reduces to what is known as gasoline gallon equivalent (GGE), which was first introduced for compressed natural gas (CNG). In 1994, the U.S. National Institute of Standards and Technology or NIST defined "gasoline gallon equivalent (GGE) means 5.660 pounds of natural gas." In April, 2007, as part of Draft Competition Guidelines released at the New York Auto Show, MPGe was announced as the main figure of merit for the Progressive Insurance Automotive X PRIZE, a competition developed by the X PRIZE Foundation for super-efficient vehicles that can achieve at least 100 MPGe. In February, 2009, Consumer Reports announced that, as part of a partnership with the X PRIZE Foundation, they plan to report MPGe as one of several measures that will help consumers understand and compare vehicle efficiency for alternative fuel vehicles. In November, 2010, the EPA began including MPGe as one figure of merit on its new sticker for Fuel Economy and Environmental Comparisons (see below).

Electric and plug-in hybrid electric vehicles

Increasingly important today are vehicles fueled by electrical energy obtained from the grid (via a wall plug) and stored in an on-board battery – including both pure battery electric vehicles (BEVs), which use only electricity, and plug-in hybrid electric vehicles (PHEVs), which use electricity together with a liquid fuel obtained from a pump or other source and stored in an on-board fuel tank. For PHEVs, the liquid fuel is commonly but not necessarily gasoline (it might be diesel, biodiesel, ethanol, etc.) For such electric vehicles, the general formula for MPGe (above) reduces to

${\displaystyle MPGe={\frac {E_{G}}{\left(G_{M}*E_{F}+E_{M}*E_{E}\right)}}}$

where

${\displaystyle E_{G}=}$ energy (BTU) per gallon of gasoline = 116,090

${\displaystyle G_{M}=}$ gallons of liquid fuel consumed per mile (with ${\displaystyle G_{M}=0}$ for pure BEVs)

${\displaystyle E_{F}=}$ energy (BTU) per gallon of liquid fuel consumed (not necessarily gasoline)

${\displaystyle E_{M}=}$ plug-to-wheels electrical energy consumed per mile (Wh/mi)

${\displaystyle E_{E}=}$ conversion factor for BTU per Watt-hour of electrical energy (BTU/Wh) = 3.412

And for pure BEVs (${\displaystyle G_{M}=0}$), it further reduces to

${\displaystyle MPGe={\frac {E_{G}}{E_{M}*E_{E}}}={\frac {34,024}{E_{M}}}}$

This is based on the GREET assumption for ${\displaystyle E_{G}=}$. If the current EPA assumption for ${\displaystyle E_{G}=}$ is used, the result is

${\displaystyle MPGe={\frac {E_{G}}{E_{M}*E_{E}}}={\frac {33,700}{E_{M}}}}$

Note that reliably measuring the efficiency of electrical vehicles – i.e., ${\displaystyle E_{M}}$ (Wh/mi) - is difficult because it depends strongly on the so-called drive cycle (distance and speed profile). That’s also true of traditional gasoline vehicles (measuring gallons consumed per mile), but it’s more difficult and less reliable for electrical vehicles because they typically use regenerative braking to recover a portion of the expended energy. This makes driving style an important factor in the efficiency of electric vehicles, which is one of the reasons hypermiling is popular with drivers of electric vehicles.

Practical examples

In November 2010 the EPA began including MPGe in its new sticker for Fuel Economy and Environmental Comparisons (see example shown for the Chevrolet Volt). They rated the Nissan Leaf electric car with a combined fuel economy of 99 MPGe, and rated the Chevrolet Volt plug-in hybrid with a combined fuel economy of 93 MPGe in all-electric mode, and an overall fuel economy rating of 60 mpg combining power from electricity and gasoline. For both vehicles EPA calculated the MPGe rating under its five-cycle tests using the formula displayed earlier with a conversion factor of 33.7 kW-hr of electricity being the energy equivalent of a gallon of gasoline.

Controversy

The above MPGe rating methods contrived by the EPA are misleading and erroneous as they fail to fulfill the requirements of the previously stated governing equation:

${\displaystyle MPGe={\frac {total~miles~driven}{\left}}}$

The EPA's formulas fail to account for the "total energy of all fuels consumed" to power the vehicle. They ignore entirely the fuel burned in generating the required electricity.

In America about 70% of electricity comes from burning fossil fuels, mostly coal, to power large steam turbine generators. Thus most electric cars in America are in truth powered by coal or oil, and so that fuel must be accounted for in the EPA's MPGe formulas if they are to be credible.

Most power plants are around 33% efficient, meaning they consume three times as much energy as what they produce in electrical form. For example, if an electric car requires 40 KWH to recharge the battery pack, then a full 120 KWH worth of coal energy must be burned to provide that amount of electrical energy. The formulas invented by the EPA fail to account for that total amount of fuel. They only account for the 33% that is transformed into electricity.

Properly (honestly) accounting for the total energy of all fuels consumed, the Chevy Volt's BEV equivalent fuel efficiency, which is disingenuously listed as 93 MPGe by the EPA and GM, would in truth be just 31 MPGe or less; less because we've not considered transmission line inefficiencies.

Conversely, in cases where electrical power is generated from non-fossil fuel powered generators, like hydro-electric, or wind, or solar, then the MPGe might justifiably be near infinite since arguably very little chemical fuel is being burned to generate the electrical power.

Correct Formulas for True MPGe

Accounting for fuel burned to create the electricity to power the vehicle and assuming a 33% power generation efficiency from generator fuel to plug outlet yields the following corrected MPGe formulas:

${\displaystyle MPGe={\frac {E_{G}}{\left(G_{M}*E_{F}+E_{M}*E_{E}*E_{P}\right)}}}$

where

${\displaystyle E_{G}=}$ energy (BTU) per gallon of gasoline = 116,090

${\displaystyle G_{M}=}$ gallons of liquid fuel consumed per mile (with ${\displaystyle G_{M}=0}$ for pure BEVs)

${\displaystyle E_{F}=}$ energy (BTU) per gallon of liquid fuel consumed (not necessarily gasoline)

${\displaystyle E_{M}=}$ plug-to-wheels electrical energy consumed per mile (Wh/mi)

${\displaystyle E_{E}=}$ conversion factor for BTU per watt hour (Wh) = 3.412

${\displaystyle E_{P}=}$ efficiency of fuel to plug electrical power generation = 0.33

For pure BEVs (${\displaystyle G_{M}=0}$), the equation reduces to

${\displaystyle MPGe={\frac {E_{G}}{E_{M}*E_{E}*E_{P}}}={\frac {11,000}{E_{M}}}}$

This is based on the GREET assumption for ${\displaystyle E_{G}}$. If the current EPA assumption for ${\displaystyle E_{G}}$ is used, the result is

${\displaystyle MPGe={\frac {E_{G}}{E_{M}*E_{E}*E_{P}}}={\frac {11,200}{E_{M}}}}$

Conversion using GGE

The same method can be applied to any other alternative fuel vehicle when that vehicle's energy consumption is known. Generally the energy consumption of the vehicle is expressed in units other than W·h/mile, or Btu/mile so additional arithmetic is required to convert to a gasoline gallon equivalent (GGE) of 115,000 BTU/mile.

Hydrogen example with GGE

The 2008 Honda FCX Clarity is advertised to have a vehicle consumption of 72 mi/kg-H
₂. Hydrogen has an energy density of 120 MJ/kg (113,738 BTU/kg), by converting this energy density to a GGE, it is found that 1.012 kg of hydrogen is needed to meet the equivalent energy of one gallon of gasoline. This conversion factor can now be used to calculate the MPGGE for this vehicle.

${\displaystyle MPGE=vehicle\ efficiency\times {GGE}}$,

${\displaystyle MPGE=72{\frac {mi}{kg-H_{2}}}\times {1.012{\frac {kg-H_{2}}{gallon\ gasoline}}}=72.8}$

Life cycle assessment

Tank-to-wheel

MPGe when listed alone, as with MPG, does not describe a vehicle's full cycle, or well-to-wheel, fuel economy. Rather, MPGe is used in the same manner as MPG, and therefore, the value preceding MPGE is taken to be the tank-to-wheel energy consumption. This tank-to-wheel consumption is the EPA rating reported on the Monroney sticker.

Controversy

The MPGe as formulated by the EPA on the Monroney sticker is not a relative tank-to-wheel measure of fuel efficiency as it fails to account for the fuel "in the tank" that is burned at the electrical power generation plant. That is a serious error in the science of reporting relative fuel efficiency of an electric vehicle and is very misleading to the public, creating artificially elevated MPGe values as much as three times greater than is true.

Well-to-wheel

To calculate a vehicle's well-to-wheel consumption, the vehicle consumption is multiplied by a lumped well-to-tank efficiency factor (e_fuel), for the production and distribution of the fuel. The D.O.E. reports that the lumped average efficiency of electrical production and transmission in the United States (e_elec) is 0.303 while the efficiency of refining and distributing petroleum (e_gasoline) is 0.830.

For example, a vehicle like the Chevy Volt would have a MPGE_{well-to-wheels} as follows:

${\displaystyle MPGe_{well-to-wheel}={MPGe}\times {e_{fuel}}}$,

${\displaystyle MPGe_{well-to-wheel}=93\times 0.303=28.1MPGe_{well-to-wheel}}$.

Comparison to Monroney sticker

It is also important to determine if the vehicle consumption quoted for a particular vehicle is that of the EPA combined drive cycle. If not, the quoted value can not be directly compared to the value listed on another vehicle’s Monroney sticker.

See also

• Alternative propulsion
• Corporate Average Fuel Economy (CAFE)
• Energy density
• Energy conversion efficiency
• Fuel efficiency in transportation
• Gasoline gallon equivalent

References

1. Bioenergy Conversion Factors
2. GREET model retrieved 2011 01 20
3. "Handbook 44 Appendix D - Definitions" (PDF). NIST. 2007. Retrieved 2009-01-02.
4. "Press Release: Automotive X PRIZE Announces Draft Guidelines for Competition to Inspire Super-Efficient Vehicles". X Prize Foundation. Retrieved 2010-12-01.
5. "Press Release: Consumer Reports to Adopt MPGe Measure for Fuel Economy". X Prize Foundation. Retrieved 2010-12-07.
6. Nick Bunkley (2010-11-22). "Nissan Says Its Electric Leaf Gets Equivalent of 99 M.P.G." New York Times. Retrieved 2010-11-23.
7. David Bailey and Kevin Krolicki (2010-11-24). "Chevy Volt tops Prius in fuel economy rating". Reuter. Retrieved 2010-1124. {{cite news}}: Check date values in: |accessdate= (help)
8. "Volt receives EPA ratings and label: 93 mpg-e all-electric, 37 mpg gas-only, 60 mpg-e combined". Green Car Congress. 2010-11-24. Retrieved 2010-1124. {{cite web}}: Check date values in: |accessdate= (help); Cite has empty unknown parameter: |1= (help)
9. Fred Meier (2010-11-24). "Volt is rated 93 mpg on electricity alone, 37 mpg on gas generator". USA Today. Retrieved 2010-1124. {{cite news}}: Check date values in: |accessdate= (help)
10. Honda FCX Clarity Fuel Cell Vehicle Lease Program Begins with First Customer Delivery, Honda, 2008, retrieved 2008-12-02
11. HFCIT Hydrogen Storage: Basics, United States Department of Energy, 2007, retrieved 2008-12-02
12. Electric and Hybrid Vehicle Research, Development, and Demonstration Program; Petroleum-Equivalent Fuel Economy Calculation; Final Rule (PDF), retrieved 2008-12-02

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