This category of devices includes vapor injectors, fuel and engine ionizers, fuel-line magnets and metallic catalysts inserted into the fuel tank. All of these devices are marketed on the claim that they cause the fuel to behave differently, essentially making it possible to burn fuel with more efficiency.
Some of these devices use magnets, electromagnetic fields or so-called "ionic corona generators" to allegedly change the molecular structure of the gasoline or diesel fuel. These products' manufacturers claim that this change makes fuel burn faster and more completely in the combustion chamber.
Other products claim to have a similar effect by injecting fuel vapor into the engine upstream of the fuel injectors. The claim behind these devices is that they introduce fuel into the engine in a more burnable state.
Beyond the question of whether these devices actually can change the composition of fuel, the idea of making fuel burn faster or better is a bit of a stretch. The fuel burn in most modern cars is optimized to the point where only a small percentage of the injected fuel leaves the engine without being ignited. Even if these devices did improve fuel burn, the improvement would not produce the mileage boosts of 9 to 13 percent, as some products' Web sites claim [source: Tony's Guide to Fuel Saving Gadgets].
Another factor to consider is that devices such as vapor injectors are essentially overridden by the engine's computer. Adding extra fuel through a vapor injector can cause the engine to reduce fuel flow from its injectors. The engine adapts to the new fuel flow, with little -- if any -- change in its performance [source: Allen].