These devices insert pieces of various metals into the fuel. The claim behind them is that the metals produce a catalytic reaction in the fuel, removing impurities and making it a more efficient energy carrier. Some of these devices even claim to remove harmful bacteria from the fuel.
The merits of these devices are hard to measure without the aid of a mass spectrometer or other equipment that can measure the chemical changes that might take place in the fuel. But this could all be a moot point, given another factor: Modern engines are optimized to run on modern fuel in its current state. Changing that state moves the fuel away from the optimal fuel the engine is designed to burn. This may only be a minimal change, but it still strays from the standard engineered into the engine and its control systems. If the catalyzed fuel does indeed have properties that make it burn cleaner or faster, the engine's controls may not be able to adapt in a way that realizes any mileage improvements [source: Tony's Guide to Fuel Saving Gadgets].
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