General System Description

PURPOSE:
The Emission Control System used on this engine is designed to reduce the following emissions;

Hydrocarbons
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
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
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.

OPERATION:
These emissions are controlled by the following systems as indicated;

EGR / Exhaust Gas Recirculation NOx
EVAP / Evaporative Emission Control HC
PCV / Positive Crankcase Ventilation HC
TWC / Three-way Catalyst HC, CO, and NOx
TCCS / Toyota Computer Control System HC, CO, and NOx
O2S / The Main Oxygen Sensor provides the Engine Control Module (ECM) with feedback of the oxygen content of the exhaust.
SO2S / The Sub Oxygen Sensor is used by the ECM to monitor the efficiency of the TWC by measuring the exhaust oxygen content downstream.