Emission Control Systems: Description and Operation
Since automobiles are the main producers of air born pollutants, stringent laws have been enacted requiring that automobile engines run "cleaner." Auto manufacturer's have developed components to control these emissions. Along with these components, fuel systems and internal engine designs have changed to aid in the effort. With the advent of electronic fuel injection and computer monitoring systems, not only are these vehicles producing fewer emissions, but they are also running more efficiently.
The three types of emissions being controlled in gasoline engines are:
1. HYDROCARBONS (HC)
Gasoline is a mixture of liquid hydrocarbons and other compounds. When gasoline is not completely burned, some hydrocarbons remain in the exhaust gas. Hydrocarbon vapors can also evaporate from the fuel tank, and anywhere liquid fuel is exposed to the atmosphere.
2. CARBON MONOXIDE (CO)
Under ideal conditions, the combustion products of hydrocarbon fuels are carbon dioxide (CO2) and water vapor (H2O). Conditions in the combustion chamber are less than ideal and complete combustion does not always occur. Carbon monoxide (CO) is a poisonous gas produced by incomplete combustion of petroleum fuel with a less than adequate oxygen supply for the amount of fuel being burned. Carbon monoxide becomes a large part of the tailpipe emissions if not reduced by emission control systems.
3. OXIDES OF NITROGEN (NOx)
The atmosphere consists of mostly nitrogen with a small percentage of oxygen and traces of other gases. Nitrogen and oxygen do not normally combine except at very high temperatures and pressures, conditions which are present in the combustion chamber of gasoline engines. Various compounds of nitrogen and oxygen are formed in the combustion chamber due to the high pressures and temperatures present during the initial stages of combustion, especially when the engine is under heavy load. These pollutants become part of the tailpipe emissions if not reduced by the emission control systems.
These are some of the system components used to decrease emissions:
POSITIVE CRANKCASE VENTILATION (PCV)
Until the early 1960's, automobile engines vented crankcase pressure to the atmosphere through a "road draft" tube, discharging raw oil vapors, fuel and exhaust from piston "blow-by", into the air. The PCV system directs these gases and vapors into the engines intake system so they can be burned along with the regular air/fuel charge.
SECONDARY AIR INJECTION/CATALYST (AIR/CAT)
Air Injection systems originated in the mid 1960's. These systems supply fresh air to the exhaust system to further oxidize HC and CO emissions. This continuation of the burning process in the exhaust system completes the combustion of these pollutants, converting them into water vapor and carbon dioxide. In the 1970's, catalytic converters were introduced to help continue this process. Some AIR systems also supply air to the catalytic converter, to promote oxidation under certain conditions, Three way catalyst will reduce NOx as well as HC and CO.
EVAPORATIVE CONTROL SYSTEM (EVAP)
In the 1960's evaporative control systems were introduced. These systems collect gasoline vapors from the fuel tank (and from the carburetor) and direct them to the air intake when the engine is running, so they can be burned. Newer cars, with more sophisticated vapor recovery systems, have computer and pressure controlled valves that maintain vapor pressure in the fuel tank within a narrow margin and ensure proper purging of the vapor canister.
EXHAUST GAS RECIRCULATION (EGR)
EGR is a control for NOx emissions only. The system reduces NOx by diluting the air/fuel mixture with inert exhaust gases. This reduces peak combustion chamber temperatures and pressures, inhibiting formation of nitrogen/oxygen compounds.
CATALYTIC CONVERTER
In the mid 1970's, catalytic converters were introduced. These devices contain elements which, when heated sufficiently, promote chemical reactions in the exhaust that significantly reduce the amount of harmful pollutants.
ENGINE DESIGN/IMPROVED COMBUSTION SYSTEM (IMCO)
Since the 1960's, automobile engines and fuel systems have undergone design modifications to improve combustion and reduce HC and CO emissions. These include: heated air intake systems; carburetor design improvements; fuel injection; intake manifold, valve and combustion chamber designs to improve fuel atomization; and ignition timing controls, just to name a few.