Emission Control Systems: Description and Operation

VEHICLE EMISSIONS

Hydrocarbons Or HC.
Hydrocarbons (HC) formed by combining Hydrogen and Carbon molecules, make up a major portion of the pollution emitted by automobiles. HC is basically unburned fuel, (gasoline is a Hydrocarbon compound). HC is the only major automotive air pollutant that comes from sources other than engine exhaust. HC pollution comes from fuel system evaporation (20%), engine exhaust (60%), and vapors or blow-by gases from the crankcase (20%).

Carbon Monoxide Or CO.
Carbon Monoxide (CO), a chemical compound of gasoline and oxygen, is a colorless, odorless, poisonous gas. CO is formed when the combustion process is less than complete, usually due to a rich air/fuel mixture. CO is measured primarily at the tailpipe, but may also escape into the crankcase in blow-by gases.

Oxides Of Nitrogen Or NOx
Oxides of Nitrogen (NOx) are chemical compounds of Nitrogen and Oxygen, (both common in atmospheric air). NOx form during the combustion process when engine temperatures exceed 2500° F. NOx mix with hydrocarbons in sunlight to produce Photochemical Smog.

Smog
Smog or "Photochemical-Smog", the main by-product of tailpipe emissions, is produced when HC and NOx mix while being exposed to sunlight.

DEVICES & SYSTEMS: EMISSIONS AFFECTED

EGR / Exhaust Gas Recirculation NOx
EVAP / Evaporative Emission Control HC
HO2S / Heated Oxygen Sensor CO and HC
PCV / Positive Crankcase Ventilation HC
PAIR / Pulse Secondary Air Injection HC and CO
TCCS / Toyota Computer Control System HC, CO, and NOx
TWC / Three-way Catalyst HC, CO, and NOx

SHO2S / Sub Heated Oxygen Sensor (Calif. only) is used by the Engine Control Module (ECM) to monitor the efficiency of the TWC by measuring the exhaust content downstream.