Emission Control Systems: Description and Operation

SYSTEM PURPOSE
The emission control system is used to control harmful emissions from the vehicle. The system is comprised of several sub-systems as follows.

Positive Crankcase Ventilation (PCV) System
The PCV system scavenges crankcase blow-by cases and directs them back into intake manifold for combustion. The system prevents the release of HC gases into the atmosphere as well as venting the crankcase, protecting bearing surfaces.

Evaporative Emission Control system (EVAP)
The evaporative control system prevents fuel tank vapors from entering the atmosphere. Vapors accumulate in a canister filled with activated charcoal where they remain until proper conditions for purging are present. When engine temperature and speed are correct, the stored vapor is pulled into the intake manifold where it can be burned by the engine.

Exhaust Gas Recirculation (EGR) System
The EGR system reduces the formation of oxides of nitrogen (NOx). Controlled by the engine control module (ECM) and exhaust backpressure, the EGR valve recirculates exhaust gas back into the intake system. The introduction of exhaust cools combustion to a point which reduces the formation of NOx. The system also reduces engine knock (ping) by means of the same cooling effect.

Three-way Catalytic Converter (TWC)
The catalytic converter reduces HC, CO and NOx. Two separate catalyst beds are used to achieve this reduction of emissions. As the exhaust gasses containing HC and CO pass through the converter in the presence of oxygen, the platinum catalyst starts the oxidation (burning) process. The HC and CO then unite with oxygen to form water vapor and carbon dioxide. This oxidation process has no effect on the NOx emissions. Some models incorporate two separate converters.
To reduce the oxides of nitrogen (NOx), a separate reaction is necessary called "reduction." A reduction reaction is the removal of oxygen from a material. In Three Way Catalyst type converters, rhodium is used as a catalyst to break down oxides of nitrogen into nitrogen and oxygen. The effective conversion of pollutants begins at about 250°C (480°F).

Sequential Multiport Fuel injection (SFI)
Regulates all engine functions according to conditions for an overall reduction of emissions.