Catalytic Converters

The use of electronics has enabled engine designers to greatly increase the efficiency of the combustion process. This has led to a significant decrease in exhaust emissions (HC, CO, NOx). But the one piece of technology that has really made the modern automobile engine clean is the catalytic converter (CAT). While altering the combustion process to control one pollutant can be effective, this modification can have an adverse effect on another. The 3-way catalytic converter, however, can control all three emissions.

If combustion was perfect, the only byproducts would be CO2 (carbon dioxide), H20 (water) and N2 (nitrogen, an inert gas). Since combustion can never be perfect, there will always be some undesirable byproducts. This is where the catalytic converter plays a major role. The converter uses exotic materials, such as platinum and palladium, as catalysts to assist in completing the combustion process, bringing it as close to perfect as possible. The net result is minimal emissions. Let’s see how this is done.

SOME EMISSION BASICS

Prior to the creation of the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), emissions from automobile exhaust were uncontrolled. As a result of incomplete combustion, even in the best tuned engine, unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx ) were emitted from the tailpipe.

In rural areas and lightly populated small towns and cities, this was not really a problem. In the big cities, however, the concentration of these pollutants caused major air quality problems. For example, in the city of Los Angeles during the late ‘50s and early ‘60s, from time to time in the late afternoon, the air quality would deteriorate to the point that headlights were required in traffic and pedestrians needed to cover their mouths and noses to minimize the breathing of these contaminants.

The concentration of pollutants were just as much of a problem as the pollutants (this is still true, to some extent, today). Imagine smoking a cigarette in the park at a family picnic. The smoke is dispersed rather quickly and really doesn’t bother anyone. Smoke that cigarette in a small room with your family and the smoke does not disperse and now becomes a problem. A similar scenario comes into play when a summer inversion layer occurs over a city located in a valley (such as Los Angeles).

Regardless of location and weather conditions, though, automobile engines were a large contributor to the problem and something had to be done to clean up tailpipe emissions. With an efficiency of less than 35 percent, a significant amount of unburned combustion products were being emitted into the atmosphere. The net results included smog, related health issues and environmental damage.

THE 3-WAY CATALYTIC CONVERTER

Electronic ignition, which was pioneered by Chrysler, helped produce a more efficient combustion process, but more was needed to control emissions. Fuel injection also played a major role, but nagging problems, especially with NOx, continued to plague automakers. This is where the catalytic converter came into play.

The 3-way catalytic converter simultaneously converts exhaust emissions (HC, CO, NOx) into harmless gases. A typical catalytic converter is shown in Figure 1. Specifically, HC and CO emissions are converted into water (H2O) and carbon dioxide (CO2). Oxides of nitrogen (NOx) are converted into elemental nitrogen and water. It should be noted that this type of converter is most efficient when the air/fuel ratio is stoichiometric (14.7:1).

The modern 3-way catalytic converter is a canister-like device that is in-line with the exhaust system, upstream of the muffler. Internally, the converter consists of a ceramic honeycomb structure, or substrate (similar to a beehive), that is coated with a very thin film of several noble metals, or catalysts.

The converter is divided into two sections: (1) the oxidation section, coated with platinum and palladium, that burns, or oxidizes HC and CO to complete the combustion process. The products of this reaction, as stated above, are water vapor and carbon dioxide; and (2) the reduction section, which is upstream of the oxidation section and is coated with rhodium. In this section, NOx is broken down, or reduced, to nitrogen and oxygen. Also, the converter substrate is coated with an Oxygen Storage Component which stores and releases oxygen within the converter.

Efficient converter operation is dependent on the ability of the catalyst to store and release oxygen. As a catalyst deteriorates, its ability to store oxygen is reduced. It can be seen, then, that oxygen storage can be used as an indicator of catalyst performance (more on this later).

The combustion reaction caused by the catalyst releases additional heat in the exhaust system. Heat shields are needed to protect both the vehicle and the environment from the high temperatures developed near the catalytic converter. A typical heat shield is shown in Figure 2.

The catalytic converter operates most efficiently when the engine is running at the optimum air/fuel ratio (14.7:1) and the engine control system is in a closed loop. When the engine is running at the correct air/fuel ratio, excess oxygen is reduced, preventing the formation of NOx in the exhaust stream. Closed loop operation indicates the engine has reached the correct operating temperature and is no longer running rich, as is the case during warm-up. Rich running conditions promote the formation of HC and CO.

In order to achieve this correct air/fuel ratio, oxygen sensors are utilized. One oxygen sensor is located upstream from the catalytic converter. The other oxygen sensor is located near the outlet of the catalytic converter (Figure 3).

While the engine is in open loop operation, the inputs from the two oxygen sensors are not monitored. Such cases include engine start-up, engine warm-up and wide-open throttle. But once the engine goes into closed loop operation, the Powertrain Control Module (PCM) monitors the inputs from these sensors.

The input from the upstream heated oxygen sensor input tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air/fuel ratio by adjusting the injector pulse width. The goal is the aforementioned ideal air/fuel ratio (14.7:1).

As vehicles accumulate mileage, the catalytic converter deteriorates. This deterioration results in a less efficient catalyst. To monitor this decline in efficiency, the PCM compares the readings from the upstream and downstream oxygen sensors to calculate the oxygen storage capacity and converter efficiency. When the catalytic converter drops below the mandated emission standards, the PCM stores a diagnostic trouble code (DTC) and illuminates the malfunction indicator lamp (MIL).

When the catalytic converter is new, the inputs from the two oxygen sensors are different. The downstream sensor will detect a greater amount of oxygen than the upstream sensor. This is because the exhaust stream has been treated in the converter and the combustion process is more complete, resulting in more oxygen. A significant decrease in catalytic converter efficiency is realized when the input from the downstream oxygen sensor begins to match the input from the upstream oxygen sensor. In other words, the converter is not efficiently treating the exhaust stream, resulting in a higher emission level and less oxygen.

If you suspect a problem with the catalytic converter, a couple of checks can be performed. First, check the converter for a flow restriction. A scan tool and PEP module pressure tester are required. Remove the upstream oxygen sensor and insert the Back Pressure Test Adaptor. Connect the Pressure Transducer to the back pressure fitting and all the required cables to the scan tool and PEP module. Select the menu option on the scan tool for the pressure gauge function. Run the engine at 2000 rpm and record the exhaust back pressure. If the back pressure exceeds limits, replace the catalytic converter.

Internal damage to the converter can be detected by using a borescope. Remove the upstream oxygen sensor and insert the scope. Inspect the honeycomb for a cracked or melted substrate. If such damage is found, replace the catalytic converter.