Miles per gallon of gasoline equivalent

Miles per gallon of gasoline equivalent (MPGe) is a unit of measurement that relates efficiencies of different systems to the traditional unit of measurement for fuel efficiency (miles per gallon of gasoline).

Description

MPGe is based on the quantity of heat energy that can be obtained by burning a US gallon of gasoline (115,000 BTUs). [ [http://bioenergy.ornl.gov/papers/misc/energy_conv.html Bioenergy Conversion Factors ] ] The equivalent in terms of another fuel is the amount of such other fuel that would produce that same amount of heat. That other fuel equivalent is then the unit that enables mileage per that unit. On this basis MPGe is a meaningful measurement.

Energy conversions

See the following table for energy conversions. (Note that electricity is a means of transferring mechanical energy from one rotating machine to another, thus an equivalent unit of electrical energy must represent the equivalent heat energy used by the machine that produced it.):

*Data for electricity is based on data for 2005 for total US electricity production by fuel types.
*Lower heating value (net) is used for this table because the latent heat of vaporization of water in the fuel and the reaction products is not recovered in power applications such as transportation.

Electricity equivalence

(The appropriate choice to use for the type of electricity is not in general agreement. However, the economic advantage is for solar, wind, or hydro, if these sources are not already fully utilized. The fuel used to generate electricity in response to an added load, such as that of an electric car, will have to come from existing unused capacity. While most unused capacity is in natural gas fired facilities, the far more economical unused capacity is in coal fired facilities. It also must be noted that EIA indicates significant expansion of coal facilities over the next 20 years.)

Attempts are often made to equate the heat that electric energy can produce with the heat needed to produce that electricity. It is not a two way conversion and the fact that the units suggest that it is, is an unfortunate point of confusion. Many try to avoid this confusion by referring to heat in units of BTU and electric energy in units of kilowatt hours. It might be reasonable to ignore this problem if it was just a matter of ignoring small inefficiencies of energy transfer and conversion, but in this case we are talking about heat engine effects. Because these are subject to the second law of thermodynamics very large losses are very difficult to avoid. The reality of electric power production in the US from burning fossil fuels is that for every unit of electric energy that is produced, about twice as much heat energy is lost. [ [http://www.miastrada.com/analyses Miastrada Motors - Analyses ] ] The same reality applies to all combustion (heat) engines used in vehicles of course.

Cogeneration plants can recover some "waste" heat from generating electricity which may not be possible in a vehicle based generator or motor. This speaks to the possible economic and useful energy to GHG advantage of electric vehicles over conventional ones.

The maximum theoretical efficiency of fuel cells is 83% but current efficiencies are closer to 40%. [ [http://www.evworld.com/article.cfm?storyid=730 EVWORLD FEATURE: Fuel Cell Efficiency: A Reality Check:FUEL | CELL | FUEL-CELL | HYDROGEN | DIESEL | HYBRID | REALITY | EFFICIENCY | H2 | ] ]

The book "Handbook of Transportation Engineering" By Myer Kutz states that the thermal efficiency of the diesel [locomotive] engine is about 33%. [http://books.google.ca/books?id=fMZlnmS5YDMC&pg=PT589&lpg=PT589&dq=diesel+%22thermal+efficiency%22+locomotive&source=web&ots=nfEhSACtJk&sig=dyOtW8jpDJtZAjqSc_Dh983EIYg&hl=en#PPT589,M1] These are very large diesel engine which output the equivalent of about 11 kWh per gasoline gallon equivalent (GGE). [33.4 kWh (gasoline BTU equivalent in kWh) × 33% thermal efficiency ~= 11 kWh]

Cost

If for a given electric vehicle, we assume that coal is the energy source of choice for comparison, from the table above, we get about 11 kWh to a GGE of fuel so we can assign costs to a GGE from an electric source.

Since one US gallon of gasoline equals one GGE from the table above, using U.S data, given the 2007 average price of $2.84, [ [http://tonto.eia.doe.gov/dnav/pet/hist/mg_tt_usA.htm U.S. All Grades All Formulations Retail Gasoline Prices (Cents per Gallon) ] ] we get $2.84 per GGE/11 kWh per GGE = $0.25/kWh. As long as the cost of power is below $0.25 per kWh, we have better cost economy for an electric vehicle compared to an equivalent (in MPGe) gasoline vehicle.Fact|date=March 2008

Carbon dioxide emission

The carbon dioxide emissions generated from burning one U.S. gallon of gasoline and equivalent emissions from burning other fuels (for electricity) can be easily compared through the MPGe conversion.

When done as described here, MPGe allows a fair comparison of the energy efficiency of vehicles using different fuels.Fact|date=March 2008 The results seem sensible since the real efficiency of vehicles is mostly determined by the mechanical energy needed to push the car down the road.Or|date=March 2008 Most cars are subject to drag forces that are roughly similar. For example, the Tesla Roadster electric car achieves 4.7 mi/kWh (132 Wh/km) or 52/66 MPGe (coal/natural gas). (Not 158 MPGe as some would calculate it on the basis of incorrect representation of electrical energy).Fact|date=March 2008The 2005 Diesel Volkswagen Beetle 5-speed achieved mpg|41 highwayFact|date=March 2008 or 36 MPGe.The 2007 Honda Civic GX averages mpg|39 highwayFact|date=March 2008 "(X MPG of CNG?)" or 52 MPGe.A Flexible-fuel vehicle at mpg|30 on E85 is getting 43 MPGe, or it would get mpg|43 on gasoline.Fact|date=March 2008The Honda Civic VX which travels km to mi|82.08 for every U.S. gallon of gasoline used.

(Data is still not entered for all the carbon dioxide emissions of the listed fuels.) (The amount of energy needed to produce the fuels is also relevant, but it is at least very roughly balanced out when the calculation is done as prescribed above.)

Example

Mode Efficiency per passenger. Avg occupancy.

See also

* Fuel efficiency in transportation

Notes

* http://www.eia.doe.gov/oiaf/1605/coefficients.html
* http://www.miastrada.com/analyses

References