Up to now, we've only discussed the creation and causes of tailpipe or exhaust emission output. However, it should be noted that hydrocarbon (HC) emissions come from the tailpipe, as well as other evaporative sources, like the crankcase, fuel tank and evaporative emissions recovery system.
In fact, studies indicate that as much as 20% of all HC emissions from automobiles comes from the fuel tank and carburetor (on carbureted vehicle, of course). Because hydrocarbon emissions are Volatile Organic Compounds (VOCs) which contribute to smog production, it is just as important that evaporative emission controls are in as good a working order as combustion emission controls.
Fuel injected vehicles use an evaporative emissions system to store fuel vapors from the fuel tank and burn them in the engine when it is running. When this system is in good operating order, fuel vapor cannot escape from the vehicle unless the fuel cap is removed. The subject of Evaporative Emissions Systems is addressed in the next section of this program.
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