Emission tailpipe

Emissions (tailpipe and station), compared with alternative vehicle types... 

Engine Out Emissions Tailpipe Emissions California ULEV Standards Europe Phase 2 Standards (Gasoline)... 

Composition and size distribution of the emitted particles depend on the contribution of the individual emission sources related with road traffic—in particular combustion and non-tail-pipe emissions. Tailpipe emissions are vehicle exhaust emissions which are produced during fuel combustion (including additives) and released through the vehicle tailpipe (Rogge et al. 1993 Cadle et al. 1999). The particles derived from tail-pipe emissions are mainly composed of EC and OC, thus average total carbon emission rates are usually very close to the PM mass emission rates. Inorganic anions account for some percent of total tail-pipe emissions, the contribution of the elemental fraction is also in the order of few percent. 

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. 

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. 

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.

One of the considerations regarding the use of methanol as a fuel is that it emits higher amounts of formaldehyde, which is a contributor to ozone formation and a suspected carcinogen, compared to gasoline. Proponents of methanol dispute this, saying that one-third of the formaldehyde from vehicle emissions actually comes from the tailpipe, with the other two-thirds forming photochemically, once the emissions have escaped. They state that pure methanol vehicles produce only one tenth as much of the hydrocarbons that are photochemically converted to formaldehyde as do gasoline automobiles. 

The federal Clean Air Act Amendments of 1990 appear to be working. In the 1990s, Tier 1 standards greatly reduced tailpipe emissions of new light-duty vehicles which includes cars and most sport utility vehicles. 

By 2010, Tier 2 standards should further reduce vehicle emissions by extending regulations to larger SUVs and passenger vans. The use of gasoline with a lower sulfur content will also reduce emissions and it also makes it easier to build cars that can achieve further reductions. These standards should allow new U.S. cars to be extremely free of air pollutants. But, the Clean Air Act does not cover vehicle C02 emissions. Many new cars are called near zero emissions by their manufacturers and may have tailpipe emissions cleaner than some urban air. Hydrogen fuel cell vehicles will have almost no emissions besides some water vapor and would be much cleaner. 

Irrespective of the fuel supply chain, alternative fuels have generally lower tailpipe emissions in terms of local pollutants (such as NO, CO, S02, VOC and particle emissions) than conventional gasoline and diesel engines for instance, natural gas completely eliminates particle emissions synfuels are manufactured with very low sulphur and aromatic contents alcohol-based fuels have high octane numbers, which... 

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