.. uction in carbon monoxide, hydrocarbon, and nitrogen oxide emissions. Figure 3.1 illustrates the percentage of these pollutant resulting from automobile emissions. POLLUTANT TOTAL AMOUNT VEHICLE EMISSIONS Amount Percentage NITROGEN OXIDES 36 019 17 012 47 HYDROCARBONS 33 869 13 239 39 CARBON MONOXIDE 119 148 78 227 66 Table 3-1 Pollution Accounted by Automobile Emissions in 1989 (1000 tons) The 1970 amendment requirements were so stringent for that period that they could not be met with available engine technology. New technology has since been developed and the requirements have been met.
However, more rigid standards are continuously being proposed to improve emissions. While significant improvements to fuel economy, power output, and emissions have been made in recent years by modification and control, none of them have resulted in an engine capable of meeting current American standards while maintaining satisfactory driveability, power output, and fuel economy without the use of catalyst units in the exhaust system. 3.2 THE USE OF CATALYSTS FOR EMISSION CONTROL The concept of using a catalyst to convert carbon monoxide, hydrocarbons, and nitrogen oxides to less environmentally threatening compounds such as nitrogen, water and carbon dioxide was a well established practice prior to the need arising from motor vehicle emissions. However, rapid changes in exhaust gas temperature, volume and composition were features not previously encountered in chemical and petroleum industry applications. Other unique requirements were the control of emissions such as ammonia, hydrogen sulfide and nitrous oxide which could result from secondary catalytic reactions and for the catalyst system to maintain its performance after high temperature excursions up to 1000C and in the presence of trace catalyst poisons such as lead and phosphorous.7 The principal reactions on automobile exhaust Catalysts are as follows: Oxidation Reactions: 2CO + O2 2CO2 4HC + 5O2 4CO2 + 2H2O Reduction Reactions: 2CO + 2NO 2CO2 + N2 4HC + 10NO 4CO2 + 2H2O + 5N2 By the nature of the oxidation and reduction reactions which are involved in the removal of carbon monoxide, hydrocarbons and nitrogen oxides and the operating characteristics of the preferred catalyst, several combinations of engine/catalyst systems have been used since catalysts were introduced on American cars in 1975.
3.2.1 The Carbon Monoxide/Hydrocarbon Oxidation Catalyst Concept When emission control is primarily concerned with carbon monoxide and hydrocarbons and not with nitrogen oxide, such as is the case in the European “Euronorms” standards, oxidation catalysts are used. Key features of this system are the use of a secondary air supply to the exhaust gas stream to ensure oxidizing conditions under all engine operating loads and the use of exhaust gas recirculation (EGR) to limit nitrogen oxide emissions from the engine. A schematic of this system is shown in Figure 3.1. Figure 3-1 The Oxidation Catalyst This System was used initially in America to meet interim emission standards and is likely to be adopted to meet similar standards on medium and smaller engine cars (less than 2 litter engines) in Europe. 3.2.2 Dual Bed and Threeway Catalyst Concepts In order to overcome the limitations imposed by the use of EGR and to meet more rigid nitrogen oxide standards, catalysts capable of reducing nitrogen oxide emissions are necessary.
Initially, as a result of the difficulty of controlling air/fuel ratios to the tolerances required by a single catalyst unit, a dual catalyst bed was used. In order to ensure reducing conditions in the first catalyst bed, where nitrogen oxides were reacted, the engine was tuned slightly rich of the stoichiometric ratio. Secondary air was then injected into the exhaust stream ahead of the second catalyst bed (oxidation bed) to complete the removal of carbon monoxide and hydrocarbons. With developments in engine control and catalyst technology involving widening the air/fuel operating window for 90 % removal of hydrocarbons, carbon monoxide and nitrogen oxides, the dual bed system has been replaced with a single threeway catalyst unit. A schematic of this system is shown in Figure 3.2. Figure 3-2 The Three-way Catalyst Key features of this system, in addition to the catalyst unit, are an electronically controlled air/fuel management system incorporating in its most advanced form, the use of an oxygen sensor to monitor and control exhaust gas combustion.
Systems such as this are now universal on American and Japanese cars and in those countries that have adopted similar emission standards. The performance of the Threeway Catalyst system is summarized in Table 3.2 and Table 3.3. Cold ECE 15 HC + NOX NOX CO cycle, g/test Without Catalyst With Catalyst Without Catalyst With Catalyst Without Catalyst With Catalyst PEUGEOT 205 18.3 8.5 7.8 5.8 26.3 8.8 FIAT UNO 45 15.2 4.1 6.2 2.7 26.7 9.8 VW GOLF C 16.1 6.4 5.7 2.0 50.5 42.7 ROVER 213 12.3 5.2 3.6 1.4 46.7 27.5 Table 3-2 Emission Levels from small vehicles Polycyclic Aromatic Emissions, mg/mile Hydrocarbon Without Catalyst With Catalyst phenanthrene 1.85 0.16 anthracene 0.61 0.04 fluoranthrene 2.27 0.23 pyrene 2.91 1.50 perylene 1.21 0.40 benzo(a)pyrene 0.94 0.17 benzo(e)pyrene 2.76 0.41 dibenzopyrenes 0.28 0.23 coronene 0.41 0.27 Table 3-3 Polycyclic Aromatic Hydrocarbon Emissions from a Programmed Combustion Engine 3.2.3 Lean Burn Catalyst Systems Engine operations with air/fuel ratios of 20:1 is a good way of reducing nitrogen emissions and improving fuel economy. However, with current engine technology, in order to achieve nitrogen emissions consistent with US legislation, the engine must operate in a very lean region where, as shown in Figure 3.3, hydrocarbon emissions that increase to levels which may exceed current American standards. In these situations an oxidation catalyst is incorporated into the exhaust system to control hydrocarbon emissions.
Figure 3-3 The Effect of Air/Fuel Ratio on Engine Operation A feature of the ECE15 European test cycle was its low average speed as it is intended to be representative of city driving. The emissions that result are therefore typical of low speed, low acceleration conditions. A more representative cycle incorporating higher speeds and accelerations has been introduced so as to assess emissions under other conditions including urban and highway driving. In order to develop and maintain a higher speed more power is required from the engine which, in the case of the lean burn system, means decreasing the air/fuel ratio. This in turn increases nitrogen oxide emissions to levels where current engine technology is likely to exceed standards (See Figure 3.3).
It is therefore desirable that catalysts used on lean burn engines should in addition to having a hydrocarbon oxidation capability also have a nitrogen oxide reduction capability when fuel enrichment occurs for increased engine power. The effect on the reduction of hydrocarbons and nitrogen oxide emissions which can be achieved on a lean burn engine using a catalyst with oxidation and reduction capabilities is shown in Table 3.4 for a Volkswagen Jetta Series 1, powered by a 1.4 litter Ricardo High Ratio Compact Chamber lean burn engine. ECE 15 Cold Start Cycle g/test Hydrocarbons Carbon Monoxide Nitrogen Oxides Without Catalyst 11.7 15.9 5.9 With Catalyst 1.7 12.4 4.2 Table 3-4 Lean Burn Engine Emissions 3.2.4 Diesel Exhaust Emission Control Although Diesel engines emit relatively low concentrations of carbon monoxide and hydrocarbons and have a better fuel economy compared to gasoline powered vehicles, particulate emissions are of concern. Along with the carbon particulates which are produced during the combustion process are a range of aromatic hydrocarbons, which was one of the main reasons that the EPA established standards to limit particulate emissions.8 The carbon and the associated organics produced during combustion may be collected on a filter and removed by oxidation so that the filter regenerates and is effective for the life of the vehicle. As the particulates are not oxidized at a significant rate below 600C which occurs in the exhaust system only when the engine is running at or near full power, catalysts are introduced into the filter which reduces the oxidation temperature to approximately 300C. Table 3.5 compares emissions from an exhaust system with a catalyst to that of a system without.9 g/mile HC CO NOX Particulate Without catalyst 0.24 1.01 0.90 0.23 With catalyst 0.05 0.16 0.79 0.11 Table 3-5 Catalytic Control of Diesel Exhaust Emissions 3.2.5 Catalytic Combustion Nitrogen oxide emissions result mainly from the reaction between oxygen and nitrogen at temperatures arising from the combustion of fuel whether it is initiated by spark, as in the gasoline engine, or compression as in the diesel engine.
Leanburn operation of a gasoline engine, as described earlier, offers a partial solution to the problem but is limited by hydrocarbon emissions as the non-flammability limit for spark ignition is approached. While the diesel engine does not have these advantages it is limited by high particulate emissions. A solution to this problem is to use a catalyst to ignite the air/fuel mixture thus overcoming the constraining factors of the gasoline and diesel engines. Having removed this constraint, the engine is able to operate at a compression ratio of 12 to 1. Combustion efficiency and mechanical energy is thus optimized which results in a maximized fuel economy.10 The principle of the catalytic engine is that during the engine operating cycle, the fuel is injected into the combustion chamber just before the start of combustion is required. This fuel is then mixed with the air already in the cylinder and then passed through the catalyst, where heat release occurs.
Since the charge is passed through a catalyst, oxidation can occur at low temperatures and very lean mixtures. This results in complete fuel oxidation which enables the engine to run unthrottled and therefore lean, which provides good fuel economy. The formation of nitrogen oxides and carbon monoxide in the combustion chamber is also strongly dependent on the air/fuel ratio and lean operation results in reduced emissions of these pollutants in the exhaust. The catalyst enables oxidation of hydrocarbons at much lower temperatures than normally possible, so the emission is also reduced. 4.
CONCLUSION Since the introduction of legislation in America in 1970 requiring substantial reductions in emissions from motor vehicles, catalyst technology has played a major part in maintaining air quality. With the introduction of similar standards in other countries, the automobile industry represents the largest single use for catalyst systems. However, it must be noted that the internal combustion engine will soon approach its development limit as far as emission technology is concerned. The need for significant reduction in carbon dioxide, hydrocarbon, and nitrogen oxide emissions will ultimately require the use of an alternative energy source to power vehicles. Developments are being pursued in the use of “clean fuels” such as reformulating gasoline and diesel fuel as well as methanol and natural gas in advanced engine design.
Ultimately however, we can expect severe environmental legislation which will be met only by a completely new power source. Efforts are being undertaken by the automotive industry to replace the current power source for automobiles. Electric powered cars, solar powered cars and vehicles which utilize several power sources concurrently (hybrid) are all being intensively researched. While the emission standards for cars set by the 1970 Clean Air Act Amendments were considered adequate at the time, air quality has not significantly improved as projected due to the expanding car population in industrialized countries. By observing the possible ill effects to human health and well being mentioned earlier, it can only be concluded that for the eventual “cleaning” of our atmosphere, a power source with 0 emission will one day need to be implemented in our main means of transportation, the automobile. Bibliography K.C. Taylor, Chem Tech., London, New York: Chapman and Hall, 1990; pp 525-60 8.
H Klingenberg & H. Winneke, Total Environment, Houston: Gulf publishing, 1990; pp 95-106. 9. B.E. Enga, Platinum Metals Review, New York: Chapman and Hall, 1982;pp26-32 10. Ibid., pp 45-54.