Exhaust Emission Controls: These systems are designed to limit harmful gases in an engine's exhaust. Such controls can be grouped into two broad categories: those designed to reduce or eliminate the formation of harmful pollutants in the engine itself (retarded spark settings are one example) and those designed to destroy or otherwise alter the pollutants after they have been formed (air injection and catalytic converters fall into this category). Evaporative emission controls prevent gasoline vapors from escaping into the atmosphere from the fuel tank and the carburetor, and crankcase controls recycle fumes from the crankcase through the engine. Japanese emission requirements are among the most stringent in the world.