Emission Control Systems: Description and Operation

EMISSION TYPES
The three types of emission by-products being controlled on this vehicle are:

HYDROCARBONS (HC)
These are particles, usually vapors, of gasoline that have not burned. They are present in tailpipe emissions and crankcase vapors. Raw gasoline that evaporates from the tank or fuel delivery system is considered an HC.

CARBON MONOXIDE (CO)
This is a poisonous gas formed by incomplete combustion of fuel and air (Carbon, in gasoline, and Oxygen in the atmosphere). It forms in the engine when the burning of air/fuel (combustion) is less than complete. CO is measured at the tailpipe, but traces of CO may also be found in the crankcase.

OXIDES OF NITROGEN (NOx)
The atmosphere is mainly composed of nitrogen and oxygen. Oxides of Nitrogen are formed in the combustion chamber during excessively high engine temperatures, and are part of the tailpipe emissions.

EMISSION CONTROL SYSTEMS
To reduce the levels of HC, CO, and NOx that are released into the atmosphere, a number of emission controls have been employed on this engine. There are three different types, those that effect the fuel charge inside the combustion chamber (INTERNAL ENGINE), those that effect the exhaust gases outside the combustion chamber (EXTERNAL ENGINE), and those that eliminate gasoline vapors from escaping into the atmosphere (FUEL SYSTEM VAPOR CONTROLS).

INTERNAL ENGINE
The internal engine controls incorporate basic engine design (combustion chamber, camshaft, and intake manifold design) as well as controls used to adjust ignition timing, fuel mixture, engine temperature, and other variables. These are designed to offer a more complete buring of the fuel charge within the combustion chamber. Emission systems on this engine that effect combustion is:

Exhaust Gas Recirculation (EGR)
When the temperature of the burning fuel mixture exceed around 2,500°F (1372°C), nitrogen forms NOx. By recirculating a portion of the burned exhaust gas into the intake manifold, the peak flame temperature in the cylinders is lowered thereby lowering the amount of NOx produced. Computer controls regulate the EGR flow into the intake manifold through the EGR valve.

EXTERNAL ENGINE
External engine controls are used to assure continued burning of the exhaust as it is forced out the exhaust valve and passes through the exhaust system. Major Controls here are:

Catalytic Converter
The catalytic converter is used to reduced emission levels of exhaust gases. A properly designed converter, in good condition can produce significant reductions in the levels of HC and CO and sometimes NOx depending on the which catalyst the converter is using.

A catalyst is a substance (platinum, palladium, or rhodium) that will cause an increase in the rate of a chemical reaction (exhaust gases being burned). Yet, it will not be consumed, or permanently altered by the reaction.

The catalyst causes a rapid rise in the temperature of the exhaust gas, reducing the hydrocarbons and carbon monoxide through an oxidizing process which changes them into harmless water vapor and Carbon Dioxide. Oxides of Nitrogen levels are lowered through a reduction process.

FUEL SYSTEM VAPOR CONTROLS
The fuel system vapor controls are designed to eliminate the escape of gasoline fumes (vapor) from the fuel tank, fuel delivery system, and engine crankcase. Systems used on this vehicle are:

Positive Crankcase Ventilation (PCV)
During the compression stroke some of the compressed gas "blow by" the piston rings into the crankcase. These gases contain HC, CO, and NOx. The PCV system keeps these gases from escaping into the atmosphere and utilizes engine vacuum to draw the crankcase fumes back into the cylinders for buring. The system also provides a filtered clean air inlet to the crankcase to replace the polluted air being removed.

Evaporative Control System (EVAP)
The EVAP system works along side the fuel system making sure the engine has all the fuel it needs but that all excess fuel and vapor are either returned to the fuel tank (liquid) or directed to the EVAP canister (vapor). When computer determines the correct operating conditions exist, engine vacuum will be directed to the canister and the fuel vapors will be "purged" into the intake system and burned in the combustion chamber. This lowers the vehicles overall HC pollution.