INTRODUCTION TO EXHAUST AFTERTREATMENT

Improvements in engine design and adjustment of engine parameters carried to control engine emissions were inadequate to meet the first set of stringent emission regulations introduced in the USA from 1975. Devices were developed to treat exhaust gas for conversion of engine emissions to harmless gases. Conversion of pollutants present in the exhaust gas is termed as ‘exhaust aftertreatment’. Two basic types of exhaust treatment systems were considered;

• Thermal reactors
• Catalytic reactors or  converters

Initially, thermal reactors for oxidation of HC and CO to CO₂ and H₂O were developed.  However, as high conversion rates of pollutants could not be obtained in the thermal reactors these did not find widespread application and very soon  the catalytic converters became  a standard exhaust aftertreatment device for the spark ignited engine vehicles.

THERMAL REACTORS

If high exhaust gas temperatures are maintained and sufficient free oxygen is present in the exhaust gases, CO and HC can be oxidized in the engine exhaust system. Oxidation rate of HC can be estimated by an expression given in Module 2.  Thermal conversion efficiency for HC and CO as a function of temperature is presented in Fig 5.9.

• For 50% oxidation of CO and HC temperatures in excess of 500 and 600 C, respectively are required.
• For conversion of 80 percent, temperatures required are about 600 and 750º C for HC and CO, respectively.
• Residence time in reactor is another is another important variable.  At 750º C, conversion of HC up to 90 percent may be obtained in 100 ms while at 850º C only 50 ms are required.  Similarly, for 90 percent oxidation of CO 250 ms and 70 ms would be necessary at 750º C and 850º C, respectively.

The thermal reactors are to be located in the exhaust manifold.
In the conventional spark ignition engines, the gas temperature near the exhaust port may vary from about 300º C at idle to 900º C at full load. Exhaust gas temperature are higher for rich mixtures and increase with retard of spark timing. When the exhaust gas flows down the exhaust pipe, it cools at a rate of about 50º C to 100º C/meter. The design of the thermal reactors has the following features to ensure adequately high gas temperatures;