This type of fuel-saver gadget can be found for both diesel and gasoline-powered vehicles, and often attaches somewhere along the line between the fuel pump and injector. Its creators often claim that it produces an ionic field, which separates fuel molecules from each other as they pass through it. This, they say, causes the fuel to form a more vaporous cloud in the combustion chamber, making it easier for the fuel to burn quickly and cleanly.
This type of device preys on drivers' misunderstanding of how modern engines work. A modern engine's fuel injectors are finely tuned to produce an ultrafine mist of fuel in the combustion chamber. The technology behind this system is so advanced that only a tiny fraction of the injected fuel is not burned. Even if these devices did make the fuel vapor burn better (debatable, to say the least), the miniscule amount of extra fuel burned would not account for the significant mileage gains that the products' sellers claim [source: Tony's Guide to Fuel Saving Gadgets].