Catalytic Converter Design
Passenger vehicle manufacturers have made tremendous progress in reducing emissions since the introduction of the first automotive catalytic converter in the mid-1970s. Early converters, called “two-way” converters, burned a percentage of the unused hydrocarbons (HC) and carbon monoxide (CO) produced by the relatively inefficient, low compression engines of the day.
Two-way (oxidizing) converters burn HC and CO molecules with the assistance of a precious-metals catalyst. This process “converts” these harmful gasses into water vapor and carbon dioxide (CO2). It’s important to understand that two-way converters are most effective when used with engines that have a lean air/fuel mix because this condition provides ample oxygen to “burn” the pollutants.
Three-way catalytic converters use two catalyst processes — reduction and oxidation —- and a sophisticated oxygen storage/engine control system to convert three harmful gasses – HC, CO and oxides of nitrogen (NOx). This is not an easy task: the catalyst chemistry required to clean up NOx is most effective with a rich air/fuel mix, whereas HC and CO reduction are most effective with a lean air/fuel bias. To operate properly, therefore, a three-way converter first must convert NOx (with a rich air/fuel bias), then HC and CO (with a lean bias).
Older three-way converters, called “three-way with air” or “three-way plus oxidation,” perform this process by introducing additional oxygen between the reduction and oxidation stages to create a lean condition for the oxidation catalyst. (These converters are easily identified by their air tube.)
Modern three-way units, found on most vehicles manufactured since the late 1980s, rely on an advanced catalyst chemistry that stores and releases oxygen on a single substrate, and an oxygen monitoring and control system (utilizing one or more O2 sensors) that causes the engine to oscillate between lean and rich conditions. This oscillation, combined with the oxygen storage and release on the catalyst surface, enables the unit to convert all three harmful gasses with the same catalyst brick.
Today’s “three-way” OBD II converters are just the last step in a highly sophisticated emissions control process. The chief component of this system is the engine itself, which, when operating properly, is significantly more efficient — and environmentally friendly — than its 1970s and 80s counterparts.
Other leading components and systems playing important roles in reducing emissions in today’s vehicles are engine sensors/controls, improved combustion chamber design and electronic fuel injection technology, each of which enhance the efficiency of the combustion process, resulting in fewer unburned pollutants.
Catalytic Converter Failure
Catalytic converter failures typically fall into one of four categories:
- Thermal failure (overheating)
- Plugged substrate
- Thermal shock
- Physical damage
Thermal failure is most often caused when excessive raw fuel comes into contact with the catalyst, and “burns” in the converter instead of in the engine. The high quantity of fuel generates temperatures well in excess of the capacity of the converter, causing meltdown of the ceramic monolith. The melted ceramic could block the exhaust path, leading to a significant loss of engine power. Visible symptoms include heat-related discoloration of the converter shell.
Potential causes of thermal failure include: misfire, malfunctioning oxygen sensor, fuel delivery issue, improper choke setting/operation, and ECU malfunction.
A plugged or contaminated substrate can be the result of an overly rich air/fuel mixture, radiator sealant, and oil or antifreeze entering the exhaust flow. The resultant carbon deposits restrict the operation – and ultimately the flow characteristics – of the converter by coating the unit’s reactive surface. This degrades the converter’s ability to perform its chemical conversion process, leading to potentially illegal levels of HC, CO, and NOx.
Root causes of this problem are a malfunctioning O2 sensor, plugged or inoperable fuel injectors, piston blow-by, leaking head gasket, broken or frozen choke or carburetor float, excessive cranking time, and repeated incidences of running out of gas.
Thermal shock occurs when a fully heated converter suddenly is “cold-quenched,” such as coming into contact with snqw or ice. This leads to sudden contraction of the converter housing, which can cause cracks and disintegration of the ceramic substrate. Symptoms include a “rattling” sound when the converter is tapped with a fist or mallet (monolith-type converters only).
Physical damage, caused by running over road debris, collisions and other impacts, is usually easy to diagnose. This type of damage can break up the ceramic substrate or cause restriction that changes the flow characteristics of the converter or impacts the efficiency of the catalyst.