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Liquid fuels: a question of efficiency EIA says that people use gasoline, diesel as vehicle fuels because no other fuel contains as much energy in the same fuel volume Alan Bailey Petroleum News
Liquid hydrocarbon fuels — gasoline and diesel — dominate the field when it comes to powering the vehicles that people use for road transportation. But, with interest in the use of electrically powered or natural gas fueled vehicles, for example, how do alternative fuels compare in efficiency with liquid fuel use? And what might be the efficiency tradeoffs in switching to alternative fuels?
A Feb. 14 report from the Energy Information Administration, or EIA, using information from the National Defense University, compared different fuels that have been used or that are contenders for use in transportation. In making its comparisons EIA particularly focused on the energy density of each fuel, in other words the amount of energy in the fuel in proportion to the cost, weight or volume of fuel needed to be carried on a vehicle.
A second issue is the fuel efficiency of a vehicle’s engine, the efficiency with which the engine converts energy from fuel into the propulsion of the vehicle.
Fuel comparisons Using gasoline as a reference point, EIA said that diesel has a slightly higher energy density in terms of volume, while having an almost an equal energy density in terms of weight. Put another way, a diesel-fueled car would require a slightly smaller fuel tank but carry the same weight of fuel as a gasoline-fueled car with the same mileage range and an engine of equal fuel efficiency.
Compressed propane, by comparison, has a somewhat lower energy density in terms of volume, while being about equivalent to gasoline in terms of weight: Propane-fueled vehicles presumably require fuel tanks that are a bit larger than those of gasoline-fueled vehicles.
Liquefied natural gas only contains 75 percent of the energy content per unit volume compared with gasoline, while being a little more efficient than gasoline in terms of weight. Ethanol, a material associated with biofuels, contains less energy per unit volume than liquefied natural gas while also being a bit heavier to carry than gasoline of an equivalent energy content.
Compressed natural gas, being less dense than liquefied natural gas, has a substantially lower energy density in terms of volume than the liquefied gas.
Hydrogen Hydrogen, which has been proposed as a potential transportation fuel, is relatively light and wins out over gasoline in terms of the energy it carries per unit weight. But liquefied hydrogen contains only about one-quarter of the energy per unit volume of gasoline, while compressed hydrogen gas fares even worse in terms of volumetric energy density.
As energy repositories, various types of battery, presumably including those used to power electric cars, come at the bottom of the energy efficiency ratings, being very much less efficient than gasoline both in terms of their volume and their weight. And although the fuel efficiency of an electric car is much higher than that of a gasoline fueled car, the fuel efficiency does not compensate for the very low energy density of the car’s batteries, EIA said.
Gasoline and diesel The high energy density and ease of on-board storage of gasoline and diesel fuel explains why 99 percent of fuel consumed by light vehicles in the United States in 2012 consisted of gasoline, with half of the remaining one percent of fuel consumption being diesel, EIA said.
“Fuels that require large, heavy or expensive storage can reduce the space available to convey people and freight; weigh down a vehicle, making it operate less efficiently; or make it too costly to operate, even after taking into account cheaper fuels,” EIA said. “Compared to gasoline and diesel, other options may have more energy per unit weight, but none have more energy per unit volume.”
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