CATALYTIC CONVERTERS
by Kyle McFadden

When an automobile fails an emissions test performed by your local auto repair shop, one of the most common causes is a failed catalytic converter. The catalytic converter lowers emission levels by changing harmful pollutants into relatively harmless gases. The catalytic converter works by using heat, combined with catalyzing agents, to create a chemical process that changes hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide and water. Some catalytic converters are designed to reduce an additional pollutant, oxides of nitrogen (NOx), by breaking it down into base components, nitrogen and oxygen. The catalytic converter resembles a muffler and is usually placed in the exhaust system close to the exhaust manifold. This is done to provide more heat to the catalytic converter for faster ignition of the catalyst. Auto repair mechanics will typically perform an exhaust backflow test in order to determine the catalytic converter’s viability. The catalyst consists of pellets or a honeycomb grid, coated with platinum, rhodium, or palladium. The catalyst is placed in a shell, inside the converter and insulation is usually placed around the shell and covered with an outer shield. Because the heat generated inside the catalytic converter can exceed 1400F, heat shields are generally placed between the converter and the floorpan of the vehicle. Troubleshooting and diagnosing lack of power problems will often involve an auto repair mechanic inspecting a clogged catalytic converter.

Catalytic converters were required equipment starting in 1975. The first catalytic converter to appear and still in use today, was the oxidation catalyst. The oxidation catalyst targets only HC and CO. It contains catalyst material consisting of platinum and palladium, either in pellet or honeycomb form. When exhaust gas flows through the catalyst and minimum temperatures are reached, a combustion process is started causing oxygen to combine with HC and CO molecules. This process converts HC and CO into water vapor and carbon dioxide. The oxidation catalyst requires a higher than normal exhaust oxygen content to operate, so they are generally used in conjunction with an air pump system. Vehicles that do not use an air pump supply added oxygen to the exhaust system using lean air/fuel mixtures.

The dual bed converter first appeared in 1980. It is designed to reduce NOx in addition to converting HC and CO. The dual bed converter is actually two types of catalyst, combined into one unit. The front section targets NOx and the rear section, an oxidation catalyst, is dedicated to HC and CO conversion. An air supply hose is generally connected to the center of the dual bed catalyst to furnish additional oxygen to the rear oxidation catalyst section. The front NOx reducing catalyst is designed to work in a low oxygen atmosphere so it does not receive an air supply. The NOx reducing section of the converter usually contains rhodium as a catalyst. The catalyst breaks down the NOx into separate base components of nitrogen and oxygen. The remaining exhaust flows downstream into the oxidation section for HC and CO conversion. Better equipped auto repair shops have diagnostic equipment to test such conversion.

The three way catalyst works in a similar fashion as the dual bed catalyst, but provides NOx reduction and HC and CO oxidation using a single catalyst. The three way catalyst only operates efficiently when the air/fuel mixture is near 14.7 to 1, so it is generally only used on vehicles with computerized fuel and emission controls. The three way catalyst contains platinum, rhodium or palladium as a catalyzing agent. Some three way catalysts also contain oxygen storage elements such as cerium to aid the catalyst process. The three way catalyst works along with normal rich to lean cycles that occur with the use of oxygen sensor feedback controls. When the exhaust cycle is rich and low in oxygen, the reducing portion of the converter operates to control NOx. When the exhaust cycles back to lean, oxygen levels are high and the oxidation process converts HC and CO to water and carbon dioxide. The three catalysts depend on the constant switching from rich to lean, to operate properly to control all three target emissions.

Catalytic converters do not normally wear out. Rather, they are usually subject to failure from external physical damage such as impacts with road debris or internal damage to the catalyst from overheating. Overheating of the catalyst usually occurs from excessively rich fuel mixtures or an engine misfire. When this occurs, raw fuel enters the catalytic converter and is ignited. The result is extremely high temperatures that exceed the melting point of the catalyst components. Therefore, a clogged catalytic converter can usually be traced back to a fuel system problem; any competent auto repair mechanic will take this into consideration. The typical result is a restricted exhaust system from the melting of catalyst material inside the converter. The catalyst material can also become contaminated by silicon or leaded fuel. Whenever a damaged catalytic converter is replaced, the cause of the catalytic converter failure should also be located and corrected.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)