Emission Control Systems: Description and Operation

PURPOSE
To reduce exhaust emissions, these vehicles use an evaporative emission control system, exhaust gas recirculation system, positive crankcase ventilation system, three-way catalytic converter, computer controlled (spark output) distributor ignition system, intake air temperature control system and (electronic) multiport fuel injection.

COMPUTER CONTROLLED DISTRIBUTOR IGNITION SYSTEM
The ignition system spark timing is controlled by the powertrain control module via the spark output signal to the ignition control module. Precise spark timing results in optimum power and reduced HC and NOx emissions.

EVAPORATIVE EMISSION CONTROL SYSTEM (EVAP)
The evaporative emission control system stores fuel tank vapor in a canister when the engine is not running thus reducing HC emissions. Fuel vapor is drawn into the intake system and burned when engine is running. The Powertrain Control Module (PCM) uses a solenoid valve to regulate vapor volume and maintain proper air/fuel ratio.

EXHAUST GAS RECIRCULATION SYSTEM (EGR)
EGR system reduces NOx emissions by lowering combustion chamber temperatures. To accomplish this, the system introduces a small amount of exhaust gas into the intake manifold. It operates depending on engine load, engine speed, and coolant temperature.

INTAKE AIR TEMPERATURE CONTROL SYSTEM
The intake air temperature control system regulates the air temperature entering the engine. The system helps reduce CO and HC by keeping the intake air temperature fairly constant. This reduces varying air density and improves fuel atomization which aids the computerized engine control system in maintaining the air fuel ratio within the stoichiometric window.

MULTIPORT FUEL INJECTION (MFI)
The MFI system is controlled by the powertrain control module based on inputs form various sensors. This system controls fuel distribution to each engine cylinder at precise amounts to reduce CO and HC pollutants while maintaining optimum driveability conditions and reducing fuel consumption.

POSITIVE CRANKCASE VENTILATION SYSTEM (PCV)
The PCV valve is operated by intake manifold vacuum. When the engine is running at idle, the valve is opened slightly and a small amount of blow-by gas is drawn in. At higher engine speeds, the PCV valve opens further and a larger amount of blow-by gas is drawn in. Blow-by gasses are combustion by products which contain Hydrocarbons (HC). The PCV system helps reduce HC emissions by preventing blow-by gasses from venting to the atmosphere.

THREE-WAY CATALYTIC CONVERTER (TWC)
Catalytic converters promote reactions that change CO, HC, and NOx into carbon dioxide, water vapor, and nitrogen gas respectively. The metals platinum and rhodium are used as catalysts to stimulate the necessary chemical reactions.