How can I compare the energy content of alternative fuels and gasoline or diesel? What implications does this have for overall fuel and vehicle comparisons?
Alternative fuels have varying energy densities and are measured using a number of different units, which can make comparing them tricky. The gasoline gallon equivalent (GGE) unit allows drivers to make apples-to-apples comparisons of a given quantity of energy from alternative fuels and assess which fuel best suits their needs. Understanding the energy content of fuels can help inform comparisons of fuel prices and vehicle driving range.
What is a GGE? How about a DGE?
A GGE is a standardized unit used to compare the energy content of all fuels. This unit quantifies the amount of alternative fuel that has the equivalent energy content of one gallon of conventional gasoline. For medium- and heavy-duty vehicle fuel applications, diesel gallon equivalent (DGE) is often used.
How are GGE and DGE values determined?
Energy content is measured in British thermal units (Btus) per gallon of fuel, and is often referred to as the lower heating value of the fuel. To calculate GGE and DGE, the energy content of one gallon of gasoline or diesel is divided by the energy content of the comparison fuel. For example, conventional gasoline has an energy content of 116,090 Btus per gallon, while propane has an energy content of 84,250 Btus per gallon. As such, 1.38 gallons of propane has the same amount of energy as one gallon of conventional gasoline.
The table below displays the energy content, GGE, and DGE values of conventional and alternative fuels.
|Fuel||Energy Content*||Quantity of Fuel in 1 GGE||Quantity of Fuel in 1 DGE|
|Gasoline||116,090 Btu/gallon||1.00 gallon||1.11 gallon|
|Low Sulfur Diesel||128,488
|0.90 gallon||1.00 gallon|
|Biodiesel (B20)||126,700 Btu/gallon||0.92 gallon||1.01 gallon|
|0.97 gallon||1.07 gallon|
|Compressed Natural Gas (CNG)||923 Btu/cubic foot (ft3) or
|Liquefied Natural Gas||21,240 Btu/lb||5.47 lb||6.05 lb|
|Ethanol (E100)||76,330 Btu/gallon||1.52 gallon||1.68 gallon|
|Ethanol (E85)**||88,258 Btu/gallon||1.32 gallon||1.46 gallon|
Btu/kilowatt hour (kWh)
|34.00 kWh||37.64 kWh|
|Propane||84,250 Btu/gallon||1.38 gallon||1.53 gallon|
*Lower heating value. Source for CNG and hydrogen (Btu/ft3): Transportation Energy Data Book, Edition 35. Source for remaining values: Alternative Fuels Data Center (AFDC) Fuel Properties.
** E85 that is sold in the United States today actually contains, on average, approximately 70% ethanol. Therefore, E85 energy content calculated as [(.70) x (E100 energy content)] + [(.30) x (gasoline energy content)]
*** Electric vehicles are more efficient (on a Btu basis) than combustion engines, which should be taken into account when calculating and comparing miles per GGE (see below).
The values in the table above can help standardize fuel amounts for comparisons. For example, if you have 10,000 ft3 of CNG, you can determine the equivalent number of GGEs by dividing by 125.77 ft3 to get 79.5 GGE. Similarly, to determine the number of DGEs, you would divide by 139.21 ft3 to get 71.83 DGE.
How are GGE and DGE used to compare fuel prices?
Fuel prices can be represented in dollars per GGE or DGE for consistency in pricing between fuels. For that reason, the Clean Cities Alternative Fuel Price Report shows prices on an energy-equivalent basis (Table 3 in recent reports, http://www.afdc.energy.gov/publications/search/keyword/?q=alternative%20fuel%20price%20report). If values for price per GGE or DGE are not available, you can do the calculation on your own. For instance, if one gallon of E85 is $2.04, you would multiply by 1.32 (see table above) to find that this price equates to $2.69 per GGE after adjusting for energy content.
What are the factors that impact how far I can drive between fill ups?
The energy content of fuels is one factor that affects driving range. Filling up with a less energy-dense fuel often means that you will not be able to drive as far. However, tank size and vehicle efficiency also play a significant role.
Some alternative fuel vehicles (AFVs) have similar tank sizes to conventional vehicles, while others have larger fuel tanks to compensate for the difference in energy content. For example, vehicles that run on propane and biodiesel typically have similarly sized fuel tanks as their conventional fuel counterparts. As you can see in the table above, both of these fuels have lower energy densities than their conventional fuel counterparts, which subsequently can result in lower fuel economy and shorter range per tank. In the case of propane, bi-fuel vehicles are available that can operate on both conventional fuel and propane for extended driving range. In addition, propane and biodiesel offer many other benefits that can offset this difference.
CNG and hydrogen vehicles, on the other hand, often have larger tanks to offset the lower energy densities associated with these fuels. Fleets and drivers purchasing a CNG vehicle may have the option to install an additional CNG storage tank onboard the vehicle. Alternatively, bi-fuel CNG vehicles are also available to extend the range. As for hydrogen, these vehicles tend to have larger fuel tanks overall.
Tank size is not the only other factor that affects range; vehicle efficiency also plays a role. For instance, all-electric vehicles (EVs) are significantly more efficient than conventional gasoline vehicles. According to FuelEconomy.gov, EVs use anywhere from 59% to 62% of the electricity from the grid to power the vehicle, while conventional gasoline vehicles can only convert 17% to 21% of the energy from gasoline to power the vehicle (https://www.fueleconomy.gov/feg/evtech.shtml). This is one reason why EVs have such significant fuel economy advantages over conventional vehicles, even when you are comparing the fuels on an energy-equivalent basis.