SMOG-FORMING AUTOMOTIVE EMISSIONS

16.6 SMOG-FORMING AUTOMOTIVE EMISSIONS

Internal combustion engines used in automobiles and trucks produce reactive hydrocarbons and nitrogen oxides, two of the three key ingredients required for smog to form. Therefore, automotive air emissions are discussed next.

Under the conditions of high-pressure combustion in an automobile engine, elemental nitrogen and elemental oxygen react to produce nitric oxide, NO:

N 2 + O 2 → 2NO (16.6.1) In the absence of suitable air pollution measures, the NO produced is emitted with

the exhaust gas. As discussed later in this chapter, the NO gets converted to NO 2 , which is a key ingredient in the formation of photochemical smog. At the high–temperature and –pressure conditions in an internal combustion engine, products of incompletely burned gasoline undergo chemical reactions that produce several hundred different hydrocarbons. Many of these are highly reactive in forming photochemical smog. As shown in Figure 16.8 , the automobile has several potential sources of hydrocarbon emissions other than the exhaust. The first of these to be controlled was the crankcase mist of hydrocarbons composed of lubri- cating oil and “blowby.” The latter consists of the mixture of exhaust gas and unox- idized fuel/air that enters the crankcase from the combustion chambers around the pistons. This mist is destroyed by recirculating it through the engine intake manifold by way of the positive crankcase ventilation (PCV) valve.

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Fuel tank

Carburetor

(15% of hydrocarbons from evaporation)

Exhaust (65% of Crankcase (20% of

hydrocarbons produced) hydrocarbons produced)

Figure 16.8 Potential sources of pollutant hydrocarbons from an automobile without pollution control devices.

A second major source of automotive hydrocarbon emissions is the fuel system, from which hydrocarbons are emitted through fuel tank and carburetor vents. When the engine is shut off and the engine heat warms up the fuel system, gasoline may be evaporated and emitted to the atmosphere. In addition, heating during the daytime and cooling at night causes the fuel tank to breathe and emit gasoline fumes. Such emissions are reduced by fuel formulated to reduce volatility. Automobiles are equipped with canisters of carbon that collect evaporated fuel from the fuel tank and fuel system, to be purged and burned when the engine is operating.

Control of Exhaust Hydrocarbons

To understand the production and control of automotive hydrocarbon exhaust products, it is helpful to understand the basic principles of the internal combustion engine. As shown in Figure 16.9 , the four steps involved in one complete cycle of the four-cycle engine used in most vehicles are the following:

1. Intake: Air is drawn into the cylinder through the open intake valve. Gasoline is either injected with the intake air or injected separately into the cylinder.

2. Compression: The combustible mixture is compressed at a ratio of about 7:1. Higher compression ratios favor thermal efficiency and complete combustion of hydrocarbons. However, higher temperatures, premature combustion (“pinging”), and high production of nitrogen oxides also result from higher compression ratios.

3. Ignition and power stroke: As the fuel-air mixture normally produced by injecting fuel into the cylinder is ignited by the spark plug near top- dead-center, a temperature of about 2500˚C is reached very rapidly at pressures up to 40 atm. As the gas volume increases with downward movement of the piston, the temperature decreases in a few milliseconds. This rapid cooling “freezes” nitric oxide in the form of NO without

allowing it time to dissociate to N 2 and O 2 , which are thermodynamically favored at the normal temperatures and pressures of the atmosphere.

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4. Exhaust: Exhaust gases consisting largely of N 2 and CO 2 , with traces of CO, NO, hydrocarbons, and O 2 , are pushed out through the open exhaust valve, thus completing the cycle.

Spark plug

Air in

Exhaust gases out

Intake Compression Ignition/Power Exhaust

Figure 16.9 Steps in one complete cycle of a four-cycle internal combustion engine. Fuel is mixed with the intake air or injected separately into each cylinder.

The primary cause of unburned hydrocarbons in the engine cylinder is wall quench, wherein the relatively cool wall in the combustion chamber of the internal combustion engine causes the flame to be extinguished within several thousandths of

a centimeter from the wall. Part of the remaining hydrocarbons may be retained as residual gas in the cylinder, and part may be oxidized in the exhaust system. The remainder is emitted to the atmosphere as pollutant hydrocarbons. Engine misfire due to improper adjustment and deceleration greatly increases the emission of hydrocarbons. Turbine engines are not subject to the wall quench phenomenon because their surfaces are always hot.

Several engine design characteristics favor lower exhaust hydrocarbon emissions. Wall quench, which is mentioned above, is diminished by design that decreases the combustion chamber surface/volume ratio through reduction of compression ratio, more nearly spherical combustion chamber shape, increased displacement per engine cylinder, and increased ratio of stroke relative to bore.

Spark retard also reduces exhaust hydrocarbon emissions. For optimum engine power and economy, the spark should be set to fire appreciably before the piston reaches the top of the compression stroke and begins the power stroke. Retarding the spark to a point closer to top-dead-center reduces the hydrocarbon emissions markedly. One reason for this reduction is that the effective surface to volume ratio of the combustion chamber is reduced, thus cutting down on wall quench. Second, when the spark is retarded, the combustion products are purged from the cylinders sooner after combustion. Therefore, the exhaust gas is hotter, and reactions consuming hydrocarbons are promoted in the exhaust system.

As shown in Figure 16.10 , the air to fuel ratio in the internal combustion engine has a marked effect upon the emission of hydrocarbons. As the air to fuel ratio becomes richer in fuel than the stoichiometric ratio, the emission of hydrocarbons increases significantly. There is a moderate decrease in hydrocarbon emissions when

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ratio somewhat richer in fuel than the stoichiometric ratio, decreasing hydrocarbon

Figure 16.10 Effects of air/fuel ratio on pollutant emissions from an internal combustion piston engine.

concentration in the quench layer with a leaner mixture, increasing oxygen concen- tration in the exhaust with a leaner mixture, and a peak exhaust temperature at a ratio slightly leaner in fuel than the stoichiometric ratio.

Catalytic converters are now used to destroy pollutants in exhaust gases. Currently, the most commonly used automotive catalytic converter is the three-way conversion catalyst, so called because a single catalytic unit destroys all three of the main class of automobile exhaust pollutants—hydrocarbons, carbon monoxide, and nitrogen oxides. This catalyst depends upon accurate sensing of oxygen levels in the exhaust combined with computerized engine control that cycles the air to fuel mixture several times per second back and forth between slightly lean and slightly rich relative to the stoichiometric ratio. Under these conditions carbon monoxide, hydrogen, and hydrocarbons (CcHh) are oxidized.

CO + 1 / 2 O 2 → CO 2 (16.6.2)

CcHh + (c + h/ 4 )O 2 → cCO 2 + h/ 2 H 2 O

(16.6.4) Nitrogen oxides are reduced on the catalyst to N 2 by carbon monoxide, hydrocar-

bons, or hydrogen as shown by the following reduction with CO: CO + NO → 1 / 2 N 2 + CO 2 (16.6.5)

Automotive exhaust catalysts are dispersed on a high surface area substrate, most commonly consisting of cordierite, a ceramic composed of alumina (Al 2 O 3 ),

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Since lead can poison auto exhaust catalysts, automobiles equipped with catalytic exhaust-control devices require lead-free gasoline, which has become the standard motor fuel. Sulfur in gasoline is also detrimental to catalyst performance, and a controversial topic in 1999 was a proposed change in regulations to greatly decrease sulfur levels in fuel.

The internal combustion automobile engine has been developed to a remarkably high degree in terms of its emissions. The ultimate development of such an engine is one claimed to be so clean that when it is operated in a smoggy atmosphere, its exhaust is cleaner than the air that it is taking in!

The 1990 U.S. Clean Air Act called for reformulating gasoline by adding more oxygenated compounds to reduce emissions of hydrocarbons and carbon monoxide. However, this measure has been rather controversial and as noted earlier in this chapter, questions have been raised regarding one of the major oxygenated additives, methyltertiarybutyl ether, MTBE, which has become a common water pollutant in some areas.