Gasoline gallon equivalent

Gasoline gallon equivalent (GGE) or gasoline-equivalent gallon (GEG) is the amount of alternative fuel it takes to equal the energy content of one liquid gallon of gasoline. 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."^[1]

GGE allows consumers to compare the energy content of competing fuels against a commonly known fuel—gasoline. Compressed natural gas (CNG), for example, is a gas rather than a liquid. It can be measured by its volume in Standard cubic feet (ft³) (volume at atmospheric conditions), by its weight in pounds (lb) or by its energy content in joules (J) or British thermal units (BTU) or kilowatt-hours (kW·h). It is difficult to compare the cost of gasoline with other fuels if they are sold in different units. GGE solves this. A GGE of CNG and a GGE of electricity all have the same energy content as one gallon of gasoline. CNG sold at filling stations is priced in dollars per GGE.

One important point that somewhat clouds the practical utility of a GGE for comparing different fuels to each other is that machines which run on them produce usable energy from different fuels at different efficiencies. For example a 2012 Nissan Leaf has a battery capacity of 24 kWh, or a GGE size of 0.72 gallons. A standard small gasoline-powered car with 25 MPG efficiency can go 18 miles on this much fuel. But the higher efficiency Nissan Leaf can go 80–100 miles on this much battery charge.

Another important point in considering the possible limitations of GGE is brought to light by recent real-world testing that has shown MPG results of alternative fuels that vary from GGE predictions. These tests suggest that efficiency improvements may be achieved if a vehicle’s engine is optimized to run on any given alternative fuel blend. The findings may show that other factors besides energy content contribute to a fuel’s performance, and, if confirmed, may yield further implications for a fuel’s emissions performance.^[2]

GGE - Gasoline Gallon Equivalent (US Gallons) tables

Rates per kWh for residential electricity in the USA range from $0.0728 (Idaho) to $0.166 (Alaska) and $0.2783 (Hawaii) ^{[14] [15]}

Compressed natural gas

One GGE of natural gas is 126.67 cubic feet (3.587 m³) at standard conditions. This volume of natural gas has the same energy content as one US gallon of gasoline (based on lower heating values: 900 BTU/cu ft of natural gas and 115,000 BTU/gal of gasoline).^[16]

One GGE of CNG pressurized at 2,400 psi (17 MPa) is 0.77 cubic foot (21.8 liters). This volume of CNG at 2,400 psi has the same energy content as one US gallon of gasoline (based on lower heating values: 148,144 BTU/cu ft of CNG and 115,000 BTU/gal of gasoline.^[16] Using Boyle's Law, the equivalent GGE at 3,600 psi (25 MPa) is 0.51 cubic foot (14.4 L or 3.82 actual US gal).

The National Conference of Weights & Measurements (NCWM) has developed a standard unit of measurement for compressed natural gas, defined in the NIST Handbook 44 Appendix D as follows: "1 Gasoline [US] gallon equivalent (GGE) means 2.567 kg (5.660 lb) of natural gas."^[17]

When consumers refuel their CNG vehicles in the USA, the CNG is usually measured and sold in GGE units. This is fairly helpful as a comparison to gallons of gasoline.

Ethanol and Fuels Like E85

1.5 gallons of ethanol has the same energy content as 1.0 gallon of gasoline.

The energy content of 1.0 US gallon of ethanol is 76,100 BTU, compared to 114,100 BTU for gasoline. (see chart above)

A flex-fuel vehicle may experience as much as 25% lower MPG when using E85 (85% ethanol) products. This is in part because the engine's compression ratio is fixed mechanically and electronic sensors can only modify the timing of the spark and/or instruct the fuel injection system to provide more of the reduced energy-content fuel.^{[citation needed]}

Efficiency

A concept closely related to the BTU or kWh potential of a given fuel is Engine Efficiency often called Thermal Efficiency in the case of internal combustion engines.

Generally speaking, an electrical motor is more efficient than an internal combustion engine at converting potential energy into work - turning the wheels that may move a car down the road.^{[citation needed]}

A diesel cycle engine can be as much as 40% to 50% efficient at converting fuel into work, where a typical automotive gasoline engine's efficiency is about 25% to 30%.^{[citation needed]}

The efficiency of converting a unit of fuel to rotation of the driving wheels includes many points of friction loss and heat loss through the exhaust or cooling system. Friction inside the engine happens along the cylinder walls, crankshaft rod bearings and main bearings, camshaft bearings, drive chains or gears, plus other miscellaneous and minor bearing surfaces. Friction outside the engine includes loads from the generator / alternator, power steering pump, A/C compressor, transmission, transfer case (if four-wheel-drive), differential(s) and universal joints, plus rolling resistance of the pneumatic tires.

The MPG of a given vehicle starts with the thermal efficiency of the fuel and engine, less all of the above elements of friction.

Miles per gallon of gasoline equivalent (MPGe)

Plug-in electric vehicles (and plug-in-hybrids) present a paradigm shift in articulating the monetary costs of moving a passenger vehicle down the road.

Charts on this page show variable costs of electricity for the BTU equivalent of a Gallon of Gasoline at select local retail prices. That does not address the relative thermal efficiency of an electric traction motor (80% to 99%) vs the thermal efficiency of an internal combustion engine (15% to 25%). This is a significant difference.

The US EPA has addressed this difference with a new standard for electric vehicles. In simple terms, it compares the work delivered by a gallon of gasoline in a passenger vehicle, to the work done by a quantity of electrical Kilowatt Hours, that costs the same as that gallon of regular. The standard is called Miles Per Gallon of gasoline Equivalent or MPGe.

If a gallon of unleaded regular costs $3.499 (Q4, 2012) and electricity $0.119 per KWh (national average,) MPGe is the distance you'll travel on $3.499 worth of electricity.

Under this standard, a 2012 Nissan Leaf delivers an MPGe of 92-106.

The methodology of this calculation is evolving. Depending on contract rate, the cost of electricity may vary significantly, as in the case of off-peak electricity. The cost of gasoline also shifts constantly, but this type of standard needs to stay fixed for about a year, so weekly cost shifts need to be addressed at some point. Calculations may or may not include losses from converting "grid" power to "battery charging" energy, or energy losses during battery charging and storage.

See also

• Miles per gallon of gasoline equivalent (MPGe)
• Engine Efficiency
• Thermal Efficiency
• Potential Energy
• Work - Thermodynamics
• Work - Physics
• Diesel Cycle Engines
• Efficiency
• Friction

References