Cars and trucks are far from the only vehicles that burn fossil fuels, obviously.
Jet aircraft too burn large amounts of fuel, and that's become a point of concern for a particularly large and powerful entity.
So the U.S. military has undertaken a new research effort, the Adaptive Engine Technology Development (AETD) program.
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Its goal is to develop an "adaptive" jet engine that can cut fuel consumption without sacrificing performance, according to a Brookings Institution report on the project.
This engine design would allow for seamless transitions between operational modes, enabling either greater thrust or increased range, depending on the situation.
It achieves this by manipulating two factors: bypass ratio and fan-pressure ratio.
Jet engine schematic [Image: Wikimedia Commons]
The bypass ratio is the amount of air passing around an engine core, compared to the amount of air passing through it.
A higher ratio allows for greater fuel efficiency, but also lower thrust, which negatively impacts performance; the reverse is true of a lower bypass ratio.
The fan-pressure ratio represents the pressure of air discharged by the fan at the front of the engine, relative to the pressure of the air being drawn in.
In this case, a higher ratio means more thrust and lower fuel economy, and vice versa.
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Typically, these ratios are fixed, but an adaptive engine uses ductwork and adjustable fans to change the way air moves through it.
This will theoretically allow an engine to temporarily increase thrust during takeoff, or under acceleration, and switch to a more fuel-efficient configuration for cruising.
Engines tested as part of the AETD program will also incorporate "hot section" components--heat-resistant materials designed to improve engine efficiency.
The result could be 25 percent lower fuel consumption, 30 percent greater range, and 10 percent greater thrust compared to a conventional engine, estimates say.
F-35 Lightning II [Image: U.S. Air Force via Flickr]
Even if deployed on a small scale, that could add up to major cost savings for the Pentagon.
In 2008, the Air Force spent $7.56 billion on jet fuel, and its operations likely contributed appreciably to global greenhouse-gas emissions as well.
Nonetheless, AETD is already attracting controversy. Critics worry that the improved engines will be used on the much-delayed F-35 Lightning II fighter--which they say should either be fully deployed or scrapped, not provided with even more development time.
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So far, the Air Force has not discussed exactly which aircraft would get adaptive engines.
Like many research programs undertaken by the military AETD, may not yield anything with a real-world application at all.
There are also concerns the project could be axed amid persistent calls by Congress for decreased spending--by both the Pentagon and the Federal government in general.