Automobile engine exhausts

Spray-aluminised coatings are used for exhaust valves in automobile engines, exhaust and silencer systems (double and triple life), tyre moulds, gas ducting, heat-treatment pots, furnace ladles, carburising boxes and fans handling hot gases. Similar applications utilise Calorised and hot-dip aluminised coatings. Hot-dip aluminised steel wire has been used in steel-cored aluminium conductors for overhead transmission lines. For some... 

B.K. Cho, Performance of Pt/Al203 catalyst in automobile engine exhaust with oscillatory air/fuel ratio. lEC. Res. 27 30 (1988). 

Figure 9.2 Schematic plot of the concentration of unburnt hydrocarbons, carbon monoxide and nitric oxide emissions in automobile engine exhaust as a function of the air fuel ratio. The three-way catalyst (TWC) ensures removal of all these emissions only if the air fuel ratio is stoichiometric.

Carbon monoxide is found in high concentrations in automobile engine exhaust and in cigarette smoke. CO is a health hazard because it can form a strong bond witii hemoglobin and tiius reduce the capacity for blood to transfer oxygen from the lungs. 

Metal-containing catalysts are widely used in the chemical industry and in other technological applications. For example, catalytic converters contain platinum or palladium to catalyze the oxidation of unburnt hydrocarbons in automobile engine exhaust. When the catalyst is hot, the oxidation to carbon dioxide is rapid enough that it is essentially complete in a fraction of a second, which is necessary given the short residence time of exhaust gas in the converter. 

Automotive Catalytic Converter Catalysts. California environmental legislation in the early 1960s stimulated the development of automobile engines with reduced emissions by the mid-1960s, led to enactment of the Federal Clean Air Act of 1970, and resulted in a new industry, the design and manufacture of the automotive catalytic converter (50). Between 1974 and 1989, exhaust hydrocarbons were reduced by 87% and nitrogen oxides by 24%. 

The precious metals possess much higher specific catalytic activity than do the base metals. In addition, base metal catalysts sinter upon exposure to the exhaust gas temperatures found in engine exhaust, thereby losing the catalytic performance needed for low temperature operation. Also, the base metals deactivate because of reactions with sulfur compounds at the low temperature end of auto exhaust. As a result, a base metal automobile exhaust... 

Figure 11.10. Gas sensor to monitor oxygen content of exhaust gases from automobile engines...

Sfi f-Test 6.5A An automobile engine does 520. kj of work and loses 220. kj of energy as heat. What is the change in the internal energy of the engine Treat the engine, fuel, and exhaust gases as a closed system. 

Heterogeneous catalysts are the active ingredients in automobile catalytic converters. When combustion occurs in an automobile engine, side reactions generate small amounts of undesired products. Some carbon atoms end up as poisonous CO rather than CO2. Another reaction that takes place at the high temperatures and pressures in automobile engines is the conversion of N2 to NO. Furthermore, the combustion process fails to bum all the hydrocarbons. Hydrocarbons, CO, and NO all are undesirable pollutants that can be removed from exhaust gases... 

Three-way catalysts are used in most 1981 gasoline-fueled automobiles to lower the levels of NO, CO, and hydrocarbons in engine exhaust. These catalysts normally operate under dynamic conditions catalyst temperature increases rapidly after the engine starts (during catalyst "warmup"), and exhaust flowrate and composition fluctuate under most modes of operation. The warmup of automotive catalysts is reasonably well understood (1). The operation of three-way catalysts in the dynamic exhaust environment after warmup is complex and less well understood. 

Miscellaneous. Volatile nitrosamines can escape into the atmosphere from a variety of other sources. Automobile and diesel engine exhausts may contain N-nitroso compounds, including NDMA at trace levels (5). Nitrosodiethanolamine (NDEIA) is a likely air contaminant in machine shops which use cutting and grinding fluids contaminated with high concentrations of NDElA (10). Several herbicides, known to contain appreciable levels of volatile nitrosamines (9), are applied as aqueous sprays it is likely that worker exposure via inhalation may be appreciable. 

This is an extremely important reaction to which we wiU refer throughout this book. It is responsible for all NO, formation in the atmosphere (the brown color of the air over large cities) as well as nitric acid and acid rain. This reaction only occurs in high-temperature combustion processes and in lightning bolts, and it occurs in automobile engines by free-radical chain reaction steps, which will be the subject of Chapter 10. It is removed from the automobile exhaust in the automotive catalytic converter, which wiU be considered in Chapter 7. 

A lo-cm-diameter 25-cm-long extruded monolith catalytic converter consisting of 2-mm square channels is used to remove pollutants from an automobile. Assuming that there is 1% CO in the exhaust and that the surface reaction rate is infinite with excess O2, calculate the conversion of CO to be expected for the automobile engine described above. 

In fact, most of us benefit from the use of catalysis. Automotive catalytic converters have represented the most massive application of environmental catalysis and one of the most challenging and successful cases in catalysis, generally. Automobile catalysts deseive a few more comments. The engine exhaust emission is a complex mixture, whose composition and flow rate change continuously depending on a variety of factors such as driving conditions, acceleration, and speed. Despite the variability of the conditions, three-way catalysts have achieved the reduction of exhaust carbon monoxide, hydrocarbons, and... 

Under excess of the second reactant (in automobile exhaust gas typically H20, C02 and for lean-burn engines exhaust specifically also 02), the effectiveness factor calculation can be simplified by approximating the reaction rate Rj by a pseudo-first-order rate law with respect to the component using new rate constant kiefj (evaluated from the original rate law)... 

Converters now in use contain noble metals on a ceramic substrate (e.g., platinum dispersed on alumina). The convener is typically located in the exhaust system in one of two general locations an underfloor location, or a close-coupled location near (he manifold. The operating temperature range lor noble metal catalyst is from 600 In I200 F (316 lo 649 C). which is similar to the exhaust pipe skin temperature range normally encountered or standard automobile engines. 

FIGURE 12.19 The gases exhausted from an automobile engine pass through a catalytic converter where air pollutants such as unburned hydrocarbons (CjHy), CO, and NO are converted to C02, H20, N2, and 02. The photo shows a cutaway view of a catalytic converter. The beads are impregnated with the heterogeneous catalyst. 

Hydrocarbon Trap System. The concept of a hydrocarbon trap or adsorber system is based on molecular sieve hydrocarbon adsorber systems. The temperatures at which hydrocarbon adsorption takes place exist in the auto engine exhaust system during the period of cold start of an automobile when the catalytic control system has not yet reached functional temperature. Zeolites have been reportedly useful for hydrocarbon adsorption (53,169). Zeolites desorb hydrocarbons at temperatures of 400°C, ie, once the catalytic control system is functional. Therefore, hydrocarbons adsorbed by the zeolite can also be desorbed then oxidized by a catalyst. Methods to accomplish cold start hydrocarbon adsorption, heatup of the main catalyst, and desorption have been identified. Some of these systems use exhaust pipe valves to divert the exhaust gases to the hydrocarbon trap for the low temperature portion, and by-pass the gases around the trap after the main catalyst has heated up. One device that uses a heat exchanger is shown in Figure 15 (44). The Si—Al ratio in the zeolite is important, and by lowering the alumina content, the zeolite is rendered more hydrophobic and more able to adsorb... 

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