In order to help reduce emissions and improve fuel economy in the 1980s, manufacturers started equipping vehicles with three-way catalysts and electronic fuel metering systems. By 1988, the California Air Resources Board (CARB) began regulation of the OBD systems sold in California. These OBD systems were designed to monitor fuel, ignition, and emissions system components to determine if they were operating correctly. When a system was found to be operating out of specification, a fault code was stored in the Engine Control Module (ECM). In some cases, a "check engine light" would illuminate. Technicians could connect to the ECM through a Data Link Connector (DLC) and download fault codes.
The new emission control systems were a significant departure from traditional engine systems. Instead of using mechanical systems to control key engine components, such as the carburetor and ignition system, these components are controlled by the on-board computer known as the ECM. Control of emissions is now geared toward the precise tuning of combustion for each set of operating conditions as determined by the input from specific sensors. This minimizes engine emissions while improving performance.
Three-way catalysts are effective in simultaneously reducing emissions of HC, CO and NOX . Proper operation of a three-way catalyst requires precise control of the fuel metering system. If there's too much air, the converter will not reduce NOX emissions. If there's too much fuel, the converter will not reduce HC and CO emissions.
To achieve this precise control, electronic fuel metering systems began incorporating oxygen sensors in the exhaust to provide feedback to the ECM on whether the air/fuel mixture was rich (too much fuel) or lean (too much air). When these electronic fuel metering systems read the input from the oxygen sensor, they are said to be running in "closed loop". "Open loop" describes the mode of operation when these electronic fuel metering systems disregard the oxygen sensor signal.
A closed loop fuel control system precisely controls the air/fuel mixture. The vehicle's ECM maintains the air/fuel mixture at the optimum conditions for minimizing emissions, while maximizing performance.
The fuel system and catalytic converter must have the proper balance of air and fuel in order to maintain low emissions. The stoichiometric 14.7:1 air/fuel ratio is the proper reference point in which catalyst efficiency is greatest in uniformly reducing all emissions. The carbon monoxide emissions will be lower at a fuel mixture leaner than 14.7:1, but a sacrifice is made with an increase in hydrocarbons and oxides of nitrogen.
The fuel program that the engine uses is based on an air/fuel ratio of 14.7:1 for optimum catalyst efficiency. This balance is difficult to maintain under normal circumstances because of the changing variables such as RPM and engine load. To overcome the difficulties of maintaining balance, the fuel management system forces the system rich for approximately 300 milliseconds and then forces the system lean for the same amount of time. If the system stayed rich longer then it stayed lean, the system is correcting for a lean condition and is still considered to be in "closed loop" fuel control. The carbureted fuel control systems of this era were only capable of making approximately 10 changes in a second.
The Clean Air Act Amendments of 1990 recognized the fact that vehicles with malfunctioning emissions control systems could go undetected for extended periods. Annual emissions inspection programs were not enough. The EPA required vehicle manufacturers to produce vehicle OBD systems capable of immediately identifying the vehicle operator of emissions faults, effective from 1996. As part of the OBD II system, all emissions-related components would be monitored for malfunction or deterioration.
On today's new vehicles, HC and CO emissions are reduced by more than 95% when compared to a 1960's vintage vehicle; NOX emissions are reduced by 90%.
The exhaust emissions of automotive engines contain a number of harmful pollutants. In order to minimize the amount of harmful pollutants being produced, manufacturers have developed automotive emissions controls. The following is a list of the harmful exhaust gases manufacturers plan to reduce, which includes how the gases are formed and why they are dangerous.
Consists of carbon & oxygen. This colorless, odorless, poisonous gas is the product of incomplete combustion. By weight, carbon monoxide accounts for the 47% of air pollution.
Hydrocarbons consist of carbon and hydrogen. Hydrocarbons are emitted in an unburned form from equipment which uses a petroleum product as a source of fuel. Hydrocarbons are one of the key elements responsible for the production of photochemical smog.
Oxides of nitrogen consist of nitrogen combined with varying amounts of oxygen. NOX are produced by heat and pressure during the combustion process. NOX are also a main component in smog.
Photochemical smog, commonly referred to simply as smog, is a by-product of the combination of HC and NOX . In the presence of sunlight these two elements form ozone (O3 ), nitrogen dioxide, and nitrogen nitrate; all of which cause respiratory problems. Nitrogen dioxide is a light brown colored gas which can affect visibility in the air corridors around major airport terminals and above highways.
Particulates are tiny particles of liquids and solids which are dispersed into the atmosphere during any burning process. Particulates are composed of carbon, ash, oil, grease, and metal oxides. Smoke, haze, and dust are types of air pollution which are readily visible and are known to complicate respiratory problems cause by smog.
Sulfur oxides consist of various amounts of oxygen and sulfur. Sulfur oxides result from the burning of lower grades of fossil fuels, such as coal or oil.