Tailpipe

The success of the O2 sensor has made the auto manufacturers, regulators, and environmentalists anxious to extend chemical sensing to a variety of tailpipe gases, notably CO, NO, and short-chain hydrocarbons. Considerable research and development is needed for these molecules to be monitored in the hostile exhaust system environment (36). 

Fig. 1. CO and hydrocarbon tailpipe emissions. Data from a test vehicle during a test cycle where the catalyst was mounted - 1.2 m from the exhaust part...

The oxidation catalyst (OC) operates according to the same principles described for a TWO catalyst except that the catalyst only oxides HC, CO, and H2. It does not reduce NO emissions because it operates in excess O2 environments. One concern regarding oxidation catalysts was the abiUty to oxidize sulfur dioxide to sulfur trioxide, because the latter then reacts with water to form a sulfuric acid mist which is emitted from the tailpipe. The SO2 emitted has the same ultimate fate in that SO2 is oxidized in the atmosphere to SO which then dissolves in water droplets as sulfuric acid. 

The Clean Air Act of 1990 establishes tighter pollution standards for emissions from automobiles and trucks. These standards will reduce tailpipe emissions of hydrocarbons, carbon monoxide, and nitrogen oxides on a phased-in basis beginning in model year 1994. Automobile manufacturers will also be required to reduce vehicle emissions resulting from the evaporation of gasoline during refueling. 

HC/CO converter, which decomposes the oxides of nitrogen to oxygen and nitrogen before the gases are exhausted from the tailpipe. 

Always check for reaction force from the tailpipe. 

Figure 10-2. Two relief valves with interlocked isolation valves. This figure is diagrammatic. If any liquid might be present, the tailpipe should fall, not rise, after it leaves the relief valve. Otherwise, liquid may collect in the dip and produce a back pressure.

Figure 10-4 shows what happened to the tailpipe of a steam relief valve that was not adequately supported. The tailpipe was not provided with a drain hole (or if one was provided, it was too small), and the tailpipe filled with water. When the relief valve lifted, the water hit the curved top of the tailpipe with great force. Absence of a drain hole in a tailpipe also led to the incident described in Section 9.2.1 (g). 

On other occasions, drain holes have been fitted in relief-valve tailpipes even though the relief valve discharged into a flare system. Gas has then escaped into the plant area. 

Figure 10-4. This relief-valve tailpipe was not adequately supported. 10.4.5 Relief-Valve Faults...

Drain holes in relief valve tailpipes. If they choke, rainwater will accumulate in the tailpipe (see Section 10.4). 

Besides cleaner fuels, vehicle makers have developed many emission-reducing technologies—both in cleaner combustion and in catalytic converter technologies—to comply with ever stricter tailpipe emission standards. The U.S. EPA stringent standards proposed in 1999 for model year 2004 vehicles will result in new vehicles emitting less than 1 percent of the VOC and NO, emissions of their 1960s counterparts. 

Tends to burn visually cleaner at the exliaust tailpipe, since it operates in a closed-loop electronic mode (oxygen sensors interacting with the powertrain control module) to maintain an ideal air/fucl ratio of 14.7 1. 

Most urban rail service is electric-powered and most urban bus service is diesel-powered, although diesel rail and electric bus operations do exist, as noted above. The efficiency and environmental impacts of electricity depend gi eatly on the source of electric power. Although electric vehicles produce no tailpipe emissions, generation of electricity can produce significant emissions that can travel long distances, Eor example, coal-powered electricity plants produce particulate emissions that travel halfway across North America, Urban buses also can be powered by a variety of alternative fuels. 

Exhaust system The engine operating mode controls the tailpipe emissions of hydrocarbons (HC) and carbon monoxide (CO). Over 80% of HC and CO emissions are generated during cold-start and warm-up due to incomplete combustion. Fuel vaporization and fuel/ air mixing are important factors in achieving thorough combustion of the hydrocarbons. 

Aromatic levels and carbon content of the gasoline also have a significant effect on the tailpipe emissions of HC and CO. Because of their high heat of vaporization and high boiling point (see Figure 10-1), aromatics do not vaporize readily. This is an incentive to minimize aromatics. 

Tailpipe emissions of HC and CO are affected by the levels of heavy aromatics in gasoline. Like sulfur, the heavy aromatics are in the back end of the boiling range (Figure 10-4). As with sulfur, reduction of end point directly controls the concentration of heavy aromatics in finished gasoline. 

Oxidative catalytic converters are used to reduce CO and HCs originating from imperfect combustion in engines. At certain temperatures, these converters may also oxidize NO to NO2. Original equipment manufacturer (OEM) particle filters (PFs) employ NO2 to oxidize trapped soot at lower temperatures. However, the excess NO2 may escape from the system as tailpipe emissions. NO2 is very toxic to humans, and it also has impacts on atmospheric ozone-forming chemistry. Alvaraz et al. have stated that the primary NO2 emissions of modern diesel cars are increasing [76]. 

Figure 4 shows how variations in the electric heater size affect the relative contributions of the heated element and the main (unheated) converter to the overall HC conversion performance of the EHC system for the case of 20 s heating at 2500 W. As expected, the HC conversion over the heater increases with increasing heater volume. With a large electric heater, however, the conversion performance of the main converter (as given by the difference between the dashed and solid curves in Fig. 4) is predicted to be substantially lower than that with a small-volume heater, so that the best overall conversion performance (i.e., lowest tailpipe emissions) can be obtained in the regime of small heater volumes. 

This suggests that the primary function of the small-volume electric heater in the EHC system is to transfer the supplied electrical energy downstream for rapid lightoff of the main converter rather than to provide additional catalytic conversion. In fact, consistent with this argument, computer simulations for the EHC system with a 0.4-cm-long heater predicted very similar tailpipe HC emissions regardless of whether or not the electric heater is catalyzed (see Fig. 4). 

Fig. 4. Effects of electric heater volume on the post-heater and tailpipe HC emissions (20 s heating at 2500 W), Also shown is the tailpipe HC emission predicted with the 0.4 cm-long inert electric heater.

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