Catalytic Muffler All exhaust gases pass through a catalyst that is mounted in the catalytic muffler. It filters the harmful gases through a dense honeycomb structure coated with precious metals such as platinum, palladium, and rhodium. Chemical reactions occur on these surfaces to convert the pollutants into less harmful gases. Catalysts store oxygen on lean mixtures (less than optimal amount of fuel) and release oxygen on rich mixtures (more than optimal amount of fuel). The primary pollutant produced on the lean swing is nitrous oxide. Oxygen is removed from nitrous oxide by the converter, resulting in nitrogen gas, a harmless emission.
On the rich cycle, the primary pollutant is carbon monoxide. By adding the oxygen that was stored on the lean cycle to the carbon monoxide, carbon dioxide is produced. Inside the catalytic muffler is a three-way catalyst as well as sound dampening and spark arresting features. The three-way catalyst section consists of a honeycomb coated with a mixture of platinum, palladium and rhodium. As engine exhaust gases flow through the converter passageways, they contact the coated surface, which initiate the catalytic process. The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions.
The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. Cerium is also used to promote oxygen storage and improve oxidation efficiency. As exhaust and catalyst temperatures rise the following reaction occurs: z Oxides of nitrogen (NOx) are reduced into simple nitrogen (N2) and carbon dioxide (CO2). z Hydrocarbons (HC) and carbon monoxide (CO) are oxidized to create water (H2O) and carbon dioxide (CO2). The third stage is the MI-04 control system that monitors the exhaust stream, and uses this information to control the air/fuel mixture.
By using the signal from the HEGO the SECM can increase or decrease the amount of oxygen in the exhaust by modulating the FTV and adjusting the air/fuel ratio. This control scheme allows the SECM to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO. Catalyst Efficiency Operational Range of Converter Air Fuel Ratio Richer Leaner Oxidation and Reduction Process Reduction: NOx, CO > N2 + CO2 Oxidation: HC, CO, O2 > H2O + CO2 3-Way Catalytic Muffler Figure 25 G420E/G424E Tier LP Engine System Operational Overview 26