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Catalytic Converters: How And Why They Work
At first glance, this may sound a little complex. As is the case with most complex issues, if you understand each piece, the whole makes sense.
A catalytic converter is a smog device that uses a chemical reaction to change engine exhaust to less toxic elements. The converter's catalyst material and heat combine to perform the work. The main materials in a converter are:
- Platinum/Palladium - these are oxidizing catalysts for HC and CO.
- Rhodium - this is a reducing catalyst for NOx.
- Cerium - this helps oxygen storage, which improves oxidation efficiency.
The chemical reactions within the converter
Operation
As the exhaust flows through the converter, it passes through coated passageways. The combination of the coating and the exhaust causes the following reactions:
- Rhodium converts the oxides of nitrogen (NOx) and carbon monoxide (CO) to simple nitrogen (N2) and carbon dioxide (CO2)
- Platinum and palladium oxidizes hydrocarbon (HC) and carbon monoxide, producing water (H20) and carbon dioxide (CO2).
The efficiency of the catalyst is significantly impacted by two factors: the operating temperature and the exhaust composition. The catalytic converter begins to work at about 550° F. Full efficiency doesn't take place until 750° F. This is why an engine should be fully warmed up before having a smog test performed. The exhaust composition must alternate between high CO content (carbon monoxide is used to reduce NOx emissions) and high oxygen content (oxygen is used to oxidize HC and CO). On a properly functioning car, the switching occurs automatically.
Air/Fuel Mixtures
The car must have a functioning closed loop system for the catalyst to work properly. If you have no idea what this means, it's simple. Closed loop is when the ECU (engine control unit, IE the engine computer) uses signals from the oxygen sensors to determine air:fuel ratios. Open loop is when the air:fuel ratios are determined strictly by fuel maps. An engine is in closed loop mode at idle, and at cruise. During acceleration and deceleration, fuel maps are used.
The closed loop system is designed to ensure the exhaust composition is correct. In closed loop mode, the engine rapidly alternates between slight rich, and slightly lean. This causes the exhaust content to alternate between a slightly heavier carbon monoxide content, and a slightly heavier oxygen content. As an example, with the engine at normal operating temperature, at 2500 RPM the voltage supplied by an oxygen sensor should fluctuate above and below .45 volts at least eight times every ten seconds. While rich, the voltage should exceed 550 mv (millivolts). While lean, output should be less than 400 mv.
When the air:fuel ratio is leaner than 14.7:1 (14.7 parts air to 1 part fuel), the exhaust's oxygen content increases, and the carbon monoxide levels falls. This is a better environment for the oxidizing catalysts (platinum and palladium) to operate. It also creates an environment where the cerium can store excess oxygen. This excess oxygen will be released during the rich cycle.
When the air:fuel ratio is richer than 14.7:1, the carbon monoxide content rises and the oxygen level falls. This is a better environment for the reducing catalyst (rhodium).
This is why a dead oxygen sensor can cause a car to fail a smog test. Correct oxygen sensor signals are required for closed loop to function properly, and the catalytic converter requires a functioning closed loop system.
That begs the question - 'So what kills an oxygen sensor?' The answer is "contamination". The two main contaminants are typically carbon and silicon.
Carbon contamination will occur with age. The engine produces HC - hydrocarbons. After a 100,000 mile hydrocarbon bath, the lifespan of an oxygen sensor is usually over. One thing that can shorten the life of an oxygen sensor is "excessive hydrocarbons". What causes "excessive hydrocarbons"? Short trips. Short trips are great for keeping the odometer mileage low, but frequently, the engine doesn't reach normal operating temperature. With a cold engine, the fuel doesn't burn as completely as it would if the engine were up to temperature. This allows the oxygen sensor to get a light carbon coating. If the engine is constantly driven a short distance and rarely gets up to full operating temperature, that light carbon coating becomes a thick coating. That will cause the oxygen sensor to give false readings.
Antifreeze contains silicates. The silicates are there to protect aluminum. If an engine blows a head gasket, antifreeze can be sucked into the combustion chamber. The superheated environment results in the exhaust containing silicon dioxide. The silicon dioxide forms a glaze on the oxygen sensor, which causes it to give false readings.
False oxygen sensor readings will result in either a rich or lean mixture, which can cause a car to fail it's smog test.
How to determine whether the cat is bad
If the converter is clogged, the engine will perform poorly. Some of those symptoms can include; a lack of power in the higher rev range, starting difficulty, poor midrange acceleration, and decreased fuel economy.
Another bad sign is if the converter has a reddish glow. This usually occurs when the engine is running very rich. When unburned fuel makes it's way to the converter, the substrate overheats. Normal temperatures in a converter can reach 1200° F, so you can understand how overheating would cause the metal shell to glow. If the problems isn't corrected, the center substrate can melt, which will clog the converter.
If the exhaust smells like rotten eggs, that can be a bad sign. The smell can be caused by fuels with a high sulfur content. However, if your other cars are using the same fuel, and they smell OK, it may be a sign that the air:fuel mixtures are off.
Failing an emissions test may also be a sign that the cat is bad. In this case the converter may by bad, but the bad converter may be a symptom of other problems.
Other things that can harm a catalytic converter
Two things come to mind that can damage a converter: thermal shock and physical damage.
Thermal shock can occur if a hot converter is immersed in water. Vehicles that are used to launch boats are especially susceptible to this type of damage. If you use you vehicle for this type of work, allow it to cool down in the parking lot before backing the boat (and your vehicle) into the water.
Running over something that hits the catalytic converter is the most common way of physically damaging it. A dent in the catalytic converter's shell is usually a good sign that something has hit it. The dent can mean the center substrate has been damaged.
The converter can also sustain physical damage without bumping into anything. Sometimes the welds that hold the center substrate in place fail. This allows the center substrate to rattle around inside the shell. This typically leads to the substrate breaking up and clogging the exhaust. This can be confirmed with the "rattle test". The rattle test can be performed by slapping a cold converter with the palm of your hand, or tapping it lightly with a rubber mallet. If the inside of the converter rattles, it's bad. In some cases, you can hear the converter rattle by revving the engine a little while in neutral. On some models of Mercedes-Benz, bad converters will rattle just above idle.
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