As engine exhaust gases flow through the converter passageways, they contact the coated surface which initiate the catalytic process. As exhaust and catalyst temperatures rise, the following reaction occurs:
· Oxides of nitrogen ( NOx) are reduced into simple nitrogen (N2) and carbon dioxide (CO2)
· Hydrocarbons (HC) and carbon monoxide (CO) are oxidized to create water (H2O) and carbon dioxide (CO2)
Catalyst operating efficiency is greatly affected by two factors; operating temperature and feed gas composition. The catalyst begins to operate at around 285°C; however, efficient purification does not take place until the catalyst reaches at least 400°C. Also, the converter feed gasses (engine-out exhaust gases) must alternate rapidly between high CO content, to reduce NOx emissions, and high O2 content, to oxidize HC and CO emissions.
Let's look at the effects of a catalytic converter failure. Keep in mind, there are many different factors that can cause its demise.
· Poor engine performance as a result of a restricted converter. Symptoms of a restricted converter include; loss of power at higher engine speeds, hard to start, poor acceleration and fuel economy.
· A red hot converter indicates exposure to raw fuel causing the substrate to overheat. This symptom is usually caused by an excessive rich air/fuel mixture or engine misfire. If the problem is not corrected, the substrate may melt, resulting in a restricted converter.
· Rotten egg odor results from excessive hydrogen sulfide production and is typically caused by high fuel sulfur content or air/fuel mixture imbalance. If the problem is severe and not corrected, converter meltdown and/or restriction may result.
Like the oxygen sensor, the most common cause of catalytic converter failure is contamination. Examples of converter contaminants include:
· Overly rich air/fuel mixtures will cause the converter to overheat causing substrate meltdown.
· Leaded fuels, even as little as one tank full, may coat the catalyst element and render the converter useless.
· Silicone from sealants (RTV, etc.) or engine coolant that has leaked into the exhaust, may also coat the catalyst and render it useless.
A temperature sensor is used as a warning system. The function of the sensor is to warn of catalytic converter temperature above the safe limit of 750°C. More recent catalytic converter designs are not as susceptible to temperature damage and can withstand sustained temperatures of 900°C.
