Gasoline Emissions Reduction Technology

Catalytic Converters

The most common device used to reduce vehicular emissions is the catalytic converter. A catalytic converter is a devise which uses exhaust heat to oxidize (“burn”) pollutive emissions, thereby reducing their output into the environment.

There are two basic types of catalytic converters: Two-Way Converters and Three-Way Converters. Differences between converter categories are a function of technical evolution. Thee-Way Catalysts are now standard features of most new automobiles, and according to CleanAirNet.Org, 85% of all new gasoline vehicles are equipped with Three-Way Catalysts which simultaneously reduce emissions of carbon monoxide, hydrocarbons, and nitrogen oxides (14).

Catalytic Converter Categories:

Two-way Converters began to appear in the United States in the years 1975 – 1980. These converters were called “two-way converters” because they reduced two classes of pollutive emissions: hydrocarbons and carbon monoxide. By employing the heat contained in a vehicle’s exhaust systems, these devices oxidize hydrocarbons and carbon monoxide into water and carbon dioxide.

Three-Way Converters are a significant advancement in the evolution of catalytic technology. Three-Way Converters not only oxidize hydrocarbon and carbon monoxide into water and carbon dioxide; they also convert nitrogen oxides into elemental nitrogen and oxygen.

Functioning and Componentry of Modern Three-way Catalytic Converter

Most modern cars are equipped with Three-Way Catalytic Converters. A Three-Way Catalytic Converter is a devise which uses exhaust heat to oxidize (burn) pollutive emissions, thereby reducing their output into the environment. The term "three-way" refers to the three emissions that Three-Way Converters reduce: 1) Carbon Monoxide, 2) Hydrocarbons and 3) Nitrogen Oxides.

A modern Three-Way Converter typically employs two types of catalysts (a reduction catalyst, and an oxidization catalyst), and an electronic control system. Both catalysts are usually ceramic or metallic honeycomb structures which are coated with precious metal catalysts: platinum and/or palladium, and rhodium. The function of these highly conductive precious metal catalysts is to lower the minimum temperatures required for the converter to achieve oxidation. The final component, the Control System, monitors the exhaust upstream of the converter and uses the information it gathers to optimize combustion by controlling the air / fuel ratio (15).

The Reduction Catalyst is the first stage of a catalytic converter. It uses platinum and rhodium to help reduce nitrogen oxide emissions. When NOx molecules come into contact with the reduction catalyst, the catalyst strips the nitrogen atom from the molecule, thereby freeing oxygen. Freed nitrogen atoms then bond with other nitrogen atoms which are also in contact with the catalyst, therein forming N2.

The Oxidation Catalyst is typically the second-stage, in-line catalyst within the converter. The oxidation catalyst reduces unburned hydrocarbons and carbon monoxide by oxidizing them with the remaining oxygen contained in the exhaust gas. These gases are super heated using platinum and/or palladium (16).

The Control System of a Three-Way Catalytic Converter monitors the exhaust stream and uses the information it collects via its oxygen sensor (sometimes called a “Lambda Sensor”) to control the fuel injection system. The oxygen sensor is mounted upstream of the catalytic converter and it is what tells the Electronic Control Mechanism (“ECM”), the devise which controls the fuel injectors, how much oxygen is in the exhaust. Based on the information provided to it, the ECM increases or decreases the amount of oxygen in the exhaust by adjusting the combustion (“air-to-fuel”) ratio via the automobile’s fuel injection system.

The overall objective of the control system is to maintain stoichiometry, that is, the point at which the air / fuel mixture in the engine’s combustion chamber is modulated, around the ideal ratio of approximately 14.6 to 1 air/fuel. This is the ratio at which the greatest amount of fuel is actually burned during combustion, i.e., it is the point where the most complete combustion is achieved (17). By precisely controlling fuel delivery, the oxygen content of the exhaust stream can be managed within a narrow range that supports the most efficient operation of the converter (18). A stoichiometric air/fuel ratio also provides increases fuel economy (19).

There are two basic types of control systems: 1) the Closed Loop Feedback Control System, and the Open Loop Control System:

  1. A Closed Loop Feedback Control System (CLFCS) is one in which the ECM—the devise which monitors and manages the air-to-fuel ratio by feeding instructions to the fuel injectors—relies solely upon the information provided to it by the oxygen sensor. In CLFCS operation, the oxygen sensor measures the oxygen content in the exhaust gas relative to stoichiometry. The air inlet and fuel injection are then controlled by the ECM to affect a stoichiometric ratio between oxygen (air) and fuel. The objective of the ECM is to keep the air-to-fuel ratio (A/F ratio) within the so-called “Lambda Window”. In this narrow range, high conversions (80% - 90%) of CO, HC, and NOx can be achieved simultaneously. If the A/F ratio is below 14.6, the exhaust gas contains more reducing reactants (CO, HC) than oxidizing reactants (O2, NOx), and the engine operates under rich conditions. If the A/F ratio exceeds 14.6, the engine operates under lean conditions. The reduction reactions of NOx are favored under rich conditions, whereas lean conditions favor the catalytic oxidation reactions of CO and hydrocarbons (20).
  2. An Open Loop Control System is a system that sometimes ignores the signals being sent to it by the oxygen sensor, and instead, controls the air-to-fuel mixture using other sensory information. This “ignoring of the oxygen sensor” occurs at times when operating conditions require that mixtures be richer or leaner than what is normally considered ideal. “Open Loop Operation” typically occurs during engine starts, cold engine operation, high torque conditions, acceleration, deceleration, moderate-to-heavy load conditions and wide-open throttle (21)

Toyota Motor Sales, U.S.A., Inc. has published a number of exceptionally helpful illustrations of both Open and Closed Loop Feedback Control Systems in its paper entitled Emissions Sub Systems - Closed Loop Feedback Control System.