GGE
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GGE (or Gasoline gallon equivalent) is the amount of alternative fuel it takes to equal the energy content of one liquid gallon of gasoline. We can use compressed natural gas (CNG) as an example here. Since CNG is a gas rather than a liquid, its volume is measured in cubic feet (CF), rather than in gallons. Therefore, GGE is a way of comparing equivalent amounts of fuel based on their energy content (here we will use British Thermal Units (BTU).
One GGE of natural gas is 127.77 cubic feet. This volume of natural gas has the same energy content as one US gallon of gasoline (based on lower heating values: 900 BTU/CF of natural gas and 115,000 BTU/gallon of gasoline).[1][2]
One GGE of CNG pressurized at 2,400 psi is 0.77 cubic feet. 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/CF of CNG and 115,000 BTU/gallon of gasoline.[3] Using Boyle's Law, the equivalent GGE at 3,600 psi is 0.51 cubic feet which corresponds to 14.5 liters or 3.82 actual US gallons.
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."[4]
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, as another example, burns more slowly and at a lower temperature than gasoline. This makes it easier to extract the chemical energy stored inside it. While a gallon of gasoline has about 50% more energy than ethanol, cars that run on ethanol can get very similar mileage to gasoline powered vehicles when they are optimized to take advantage of ethanol's higher octane rating. This optimization involves increasing the gasoline engines compression ratio from a normal of 9 or 10 to one, to as high as 16 to one. This involves significant and expensive refitting with different pistons (to reduce the combustion chamber size) and mechanical alterations to assure valve to piston clearance. When this optimization is completed, the engine is no longer suitable to operate on 100% gasoline, as the high compression ratio will cause severe knocking (which is very harmful to the engine). Ordinary consumers driving a "flex-fuel" vehicle can expect a substantial drop in fuel mileage when using 85% ethanol products (the compression ratio is fixed mechanically, and electronic sensors can only modify the timing of the spark and allow the electronic fuel injectors to provide more of the reduced BTU value fuel).
Notice that energy content cannot be fully converted to mechanical energy due to the limited thermal efficiency of a heat engine. Electricity is a carrier of energy, usually from one mechanical device to another. Thus an electric vehicle that is said to operate at around 90% efficiency (Tesla Roadster) actually operates in conjunction with a utility electric generating system that operates at 34% (US national average for fossil fuel generation). Typical automobiles are said to operate at around 15% efficiency.[5] However, the specially designed gasoline engine in the Prius is said to achieve better than 30% efficiency. Thus, the previously referenced Tesla Roadster actually operates at a system thermal efficiency that is modestly better than the Prius efficiency, after making some allowance for the electric part of the Prius system.
