MOBILE SOURCE POLLUTION The US EPA and US DOE produce the Fuel Economy Guide to help car buyers choose from the best fuel gallons per mile (mpg) ratings for both city and highway traffic modes A pdf version may be found at http://www.epa.gov/ cgibin/epaprintonly.cgi1 In the present discussion, we shall concentrate on the major moving source of pollutants, the internal combustion engine In the automobile, evaporative losses of pollutants occur from the fuel tank and carburetor (ca 5%), fumes from the crankcase (ca 20%) and the exhaust system (ca 75%) The major offenders are unburned hydrocarbons, carbon monoxide, nitrogen oxides and HC oxidants Positive Crankcase Ventilation (PCV) on all modern cars reduces the emissions by pulling air and fumes into the engine by maintaining a vacuum at the engine Of course a highly efficient combustion process will eliminate the partially oxidized substances One reason for incomplete combustion is that as the “flame front” generated from the spark moves toward the relatively cool cylinder walls, a quenching action takes place preventing further reaction, the kinetics of which are very temperature sensitive Other factors influencing incomplete combustion are improper dilution by poor cycle timing and less than the proper excess of oxygen admitted at the carburetor Standard engine cycles have been developed so that pollutant guidelines might be drawn The Federal Driving Cycle initiated in 1972 in considered the standard of vehicle testing A federal short cycle Table has been used for convenience in some instances Measured emission values averaged for various test sites in representative US cities are presented in Table for steady state at various miles per hour (mph) and for the short cycle Carbon monoxide and NOx levels increase as the mph level increases The federal 1975 proposed standards for CO, hydrocarbons and NOx are 11, 0.5 and 0.9 grams per mile, respectively, and 0.5%, 40 ppm and 225 ppm, respectively Their extent of enforcement is increasing as of the early 1990s The CO standard is based primarily on tests which have shown that the capacity of blood to carry oxygen is hampered by CO absorption.2 Brain function is retarded after an exposure of 10 to 15 ppm CO for several hours The HC and NOx levels are mainly based on their role in photochemical smog using ambient levels Shy et al.3 found in their Chattanooga study that “NO2 alone and exposure to suspended particulate matter alone appear to be the most probable explanation for the observed excess in respiratory illness rates.” The data are not completely convincing as pointed out by automobile Mobile sources, alternatively called transportation or vehicular sources, include cars, trucks, buses, ships and various aircraft Air pollutants emitted will vary, depending on the fuel being combusted or reacted (in the case of fuel cells or batteries) and the engine design of each vehicle THE AUTOMOBILE The automobile’s discovery appears to satisfactorily combine a human desire for rapid transportation with the desire for independence and flexibility However, the increasing vehicle population poses a series of threats to continued physical and psychological well-being and to the future of our environment As the automotive industry expands, other auxiliary industries such as petroleum production and concrete and tire manufacturing also grow, with additional potential for pollution problems Additional roads also have to be built, with a negative impact on both ecology and landscape A balance must be made between the right of an individual to use his own car when and where he drives and the harm brought upon society as a whole by his doing so If it is accepted that society needs to be protected, a number of legislative and economic measures can be initiated to discourage automotive usage Legislation has been enacted to limit the emission of HC (hydrocarbons), NOx (oxides of nitrogen), CO (carbon monoxide) and particulate matter including lead compounds However, other waste materials such as tires and the automobile itself (including the repugnant abandoned cars) must be disposed of The broad approach to automotive pollution control is to encourage alternative means of transportation This would include improvements in mass transit such as high-speed trains, moving sidewalks, increased links and modernization In the city, bicycle riding and walking are low pollution, high exercise alternatives; in rural areas car pools might be formed Cars have been built using different energy concepts (ex battery, turbine engine, sterling engine) and to run on different fuels (ex natural gas, alcohol) Special control devices (ex catalytic reactors, afterburners) placed at the exhaust of internal combustion engines are the primary means of reducing emissions of various materials The total solution to the problem will probably combine technical and strategic methods 701 © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 701 11/18/2005 10:42:26 AM 702 MOBILE SOURCE POLLUTION industry researchers.4 However, the U.S government took the initiative for the welfare of the public in not waiting for all the details to be perfectly established Certainly there are those among us (aged, with heart conditions, etc.) who will be more severely affected than the average by typical ambient pollutant concentrations and will need protection At least two approaches to the removal of internal combustion engine pollutants have come into wide acceptance One is the improvement of the combustion process itself This includes reducing manifold vacuum (and hence dilution of charge) between exhaust and intake steps; increasing the cylinder wall temperature; and designing for less maximum horsepower by minimizing the surface to volume ratio The other approach is controlling exhaust emissions by further reaction, either after-burning or catalytic Afterburning can be accomplished slightly downstream from the exhaust valve by additional oxygen injection converting HC and CO to CO2 and water vapor The after-burner has the disadvantage of not being able to remove NOx In fact, because of its high temperature, still more NOx is formed The catalytic approach is to find a catalyst or set of catalysts and temperature or set of temperatures which will completely oxidize both CO and HC, but also will reduce the NOx These two approaches will be discussed in detail below Meteorological and vehicle persistence factors have been developed for estimation of hour carbon monoxide concentrations defined as worst-case total persistence factors.33 IMPROVEMENT OF THE COMBUSTION PROCESS As mentioned above the explosion of fuel accounts for chemical pollutant formation In the Rankine cycle engines, liquids may continually be vaporized and recondensed, and in the Sterling cycle engines a gas is repeatedly heated and cooled Both cycles are accomplished in a sealed container and the heat used for the process comes from an open flame external to the engine Very little pollution is generated in such a controlled rather than explosive flame Exhaust gas recirculation (EGR) is used on American Motors, Chevrolet and Chrysler automobiles This includes a diaphragm-actuated flow control valve located between the exhaust and intake manifolds The valve is operated by ported vacuum directed through hoses and a coolant temperature override Permitting metered amounts of exhaust gases to enter the intake manifold, which are mixed with incoming fuel mixtures, lowers the combustion temperatures within the cylinders Reducing maximum cylinder combustion temperatures minimizes the creation of Oxide of Nitrogen (NOx) EGR operation does not take place until engine operating temperature has reached a preset level and engine load is sufficient to permit proper EGR operation Almost all autos now contain a PVC system which directs filtered air into the crankcase and channels vapors but toward the manifold leading to the combustion chamber Fuel tank vapors are also concentrated by charcoal canisters in American Motors vehicles for recycled combustion An H.E.W report5 discusses engine modification systems, “Features shared by essentially all versions of the engine modification system include calibrated carburetors that provide (a) relatively lean air–fuel mixtures for idle and cruise operation and (b) higher engine idle speeds Refined control of spark timing is also used, and, in some cases, regarded spark timing at idle is employed In addition, many engines are fitted with special air cleaners and ducting designed to supply heated air at nearly constant temperature to the carburetor, to permit even leaner mixture settings Most versions also incorporate high-temperature radiator thermostats to raise coolant temperatures, and thus improve mixture distribution and promote complete combustion In some cases, higher capacity cooling systems are used to handle the additional cooling load at idle that results from wider throttle openings and retarded ignition timing during this operating condition In addition, combustion chamber design attempts to avoid flame quenching zones where combustion might otherwise be incomplete, and result in high hydrocarbon emissions.” Hydrocarbon and CO emissions are reduced by adjusting the carburetor to a fuel-lean mixture during part throttle TABLE Federal short cycle Mode No Type Speed range Time in mode (sec) Average speed (mph) Average acceleration rate (mph/sec) Accel 0–16 6.0 8.00 2.67 Accel 16–29 23.0 22.50 0.57 Cruise 29 10.0 29.00 0.00 Accel 29–37 18.0 33.00 0.44 Accel 37–42 4.5 39.50 1.11 Decel 42–37 2.5 39.50 2.00 Decel 37–20 32.0 28.50 -0.53 Decel 20–0 7.5 10.00 -2.67 Idle 21.5 0.00 0.00 © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 702 11/18/2005 10:42:27 AM MOBILE SOURCE POLLUTION and idle operation “Lean surge during cruise has been largely overcome through improvement in manifolding (better mixture distribution), better carburetor fuel-metering characteristics, higher coolant temperatures, increased heating of the air–fuel mixture, and, in some cases, provision for heating the incoming air to the carburetor Exhaust emissions of CO and HC are particularly difficult to control during engine idle and closed-throttle operation (deceleration) Considerable effort has gone into designing carburetor idle systems that will provide a lean air–fuel mixture and minimize emissions during these periods To ensure that idle air–fuel mixture cannot be adjusted to be too rich (which would tend to increase CO and HC emissions appreciably), some means of limiting idle-mixture adjustment is used on most carburetors Such devices allow idle mixture to be adjusted leaner than a predetermined value, but not richer.” The effects of charge dilution on the exhaust emission of nitric oxide (NO) from a single-cylinder engine were evaluated over a range of engine design and operating parameters.6 703 Nitric oxide emission decreased as much as 70% as charge dilution fraction was increased from 0.0065 to 0.164 due to increased valve overlap, external exhaust recirculation, and reduced compression ratio NO emission was strongly dependent on charge dilution fraction, but was independent of the specific method used to change charge dilution The combined effects of increased charge dilution and 10 degree spark retard reduced NO emission 90% However, definite limits of operation were observed on the single-cylinder engine with high charge dilution The Ford Motor Company uses a system which reduces the hydrocarbon and carbon monoxide content of exhaust gases by continuing the combustion of unburned gases after they leave the combustion chamber This is achieved by injecting fresh air into the hot exhaust stream leaving the exhaust ports At this point, the fresh air mixes with hot exhaust gases to promote further oxidation of both the hydrocarbons and carbon monoxide, thereby reducing their concentration and converting some of them to carbon dioxide and water TABLE Summary of results, steady state tests and federal short cycle vehicle exhaust emissions in grams per vehicle milea City 30 mph 45 mph 60 mph 1.45 5.28 3.09 2.91 2.60 5.69 17.02 69.11 29.50 24.60 25.51 48.53 CO2 72.32 409.53 333.12 357.90 372.47 438.48 NOx Denver Effluent Idle FSCb 15 mph HC CO Los Angeles 0.08 0.51 1.69 3.78 5.51 3.10 HC 1.47 6.06 3.74 3.94 3.87 7.07 CO 17.20 86.49 52.13 56.43 71.13 94.68 348.85 CO2 345.65 297.92 330.09 368.91 0.12 0.83 1.53 3.07 4.52 2.13 HC 1.30 5.47 3.48 3.59 3.62 5.74 CO 15.49 66.57 29.77 30.53 32.63 59.74 CO2 Chicago 60.54 NOx 63.85 358.70 318.61 357.65 417.11 392.28 NOx 0.10 0.87 2.36 4.37 6.26 3.42 HC 1.50 5.68 3.02 2.83 2.47 5.52 CO 18.74 77.29 35.11 32.38 31.71 63.16 CO2 Houston 76.13 391.06 318.01 356.13 404.79 417.03 NOx St Louis 0.14 0.83 1.49 3.47 5.70 2.83 CC 16.43 67.68 29.54 26.13 26.97 56.43 378.28 CO2 334.24 301.64 315.26 362.26 0.08 0.53 1.57 3.38 5.43 3.10 HC 1.12 4.23 2.59 2.57 2.88 4.46 CO 13.25 56.10 26.19 25.40 25.79 48.11 CO2 Washington 64.28 NOx 65.04 380.44 343.54 390.83 452.50 417.79 NOx All except Denver 0.13 1.04 2.88 6.04 8.90 4.23 HC 1.34 5.11 2.99 2.90 2.85 5.34 b 16.19 67.36 30.02 27.79 28.50 55.15 68.35 374.23 323.03 355.55 401.60 408.96 NOx a CO CO2 0.11 0.75 2.00 4.21 6.35 3.34 Idle results in grams per minute NOx not corrected for humidity FSC—Federal Short Cycle © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 703 11/18/2005 10:42:27 AM 704 MOBILE SOURCE POLLUTION EMISSION CONTROL DEVICES Additional Combustion An afterburner is an additional baffled tubular reactor in which the gases are reignited and burned to completion An air pump provides the necessary oxygen-rich mixture and the heat of reaction maintains a high temperature to speed its completion It is not necessary in all cases to have a separate afterburner The aforementioned report5 states, “An injection systems decrease exhaust CO and HC emissions by injecting air at a controlled rate and at low pressure into each exhaust port Here, the oxygen in the air reacts with the hot exhaust gases, resulting in further combustion of the unburned hydrocarbons and CO that would otherwise be exhausted to the atmosphere Optimum reduction of emissions by this method depends on proper air injection rates over a wide range of engine operating conditions, carefully tailored air-fuel mixture ratios and spark advance characteristics, and in some cases the use of heated carburetor air Some engines also provide for retarded ignition timing during closed-throttle operation All air injection systems use essentially the same basic air pump, a positive displacement rotary-vane type To guard against excessive temperature and back pressure in the exhaust system resulting from high air delivery rates at full throttle and high speeds, a pressure-relief valve is installed in the pump housing The valve opens to bleed off some of the pump flow at a predetermined pressure setting Output from the air pump is directed through hoses and an air distribution manifold (or two manifolds—one for each bank on V-8 engines) to the air injection tubes located in each exhaust port A check valve between the air distribution manifold and the air pump prevents reverse flow of hot exhaust gases in the event that pump output is interrupted A vacuum-controlled antibackfire valve is used to prevent flow of air to the exhaust ports during the initial stage of closed-throttle acceleration The high vacuum that occurs during deceleration causes rapid evaporation of liquid fuel from the intake manifold walls The resulting rich mixture creates a potentially explosive vapor in the exhaust manifold if injected air is present As with engine modification systems, most air injection systems also employ spark retard during idle or idle and deceleration through use of ‘ported’ vacuum sources of dual-diaphragm distributor-vacuum-advance mechanisms.” Besides residual gas dilution and wall quenching, engine variables have the most effect on the amount of hydrocarbons in exhaust gases Over sixty privately owned and operated automobiles fueled with commercial leaded gasoline have been tested and seven main engine variables were found which changed the hydrocarbon concentration in the exhaust gases These results can be briefly summarized as: 1) Air-fuel ratio: Low value for gas mixture results in higher hydrocarbon concentration in exhaust gases 2) Ignition timing 3) Speed: Increase in speed of engine decreases the amount of hydrocarbon 4) Air-flow rate: The effect of air-flow rate on the total hydrocarbon concentration depends on the air-flow ratio, ignition timing combination 5) Compressor-ratio: Increasing the compression ratio, by decreasing head-to-piston distance, increases the total hydrocarbon concentration 6) Exhaust Back Pressure: The amount of hydrocarbon in exhaust decreases with increasing exhaust pressure 7) Coolant Temperature: Increasing the coolant temperature decreases the hydrocarbon concentration Catalytic Reactions The engine exhaust gases may be passed through a cylindrical shaped canister packed with catalytic particles Although this method has great potential, two problems may arise The long term stability of the catalyst (50,000 miles) is difficult to maintain since lead and other chemicals in trace amounts poison the catalyst The catalyst structural stability is difficult to maintain under the influence of varying gas flow rates and fairly high temperature Also, removing three or four pollutants simultaneously can prove difficult for any single type of catalyst The removal of NOx, for instance, requires a reduction catalyst, whereas CO requires an oxidation catalyst For this reason dual stage catalytic reactors have been proposed The technical problems for this method are greater but the potential advantages are even greater Nitric Oxide Removal A review of some chemical reaction data which might be useful in automobile pollution control work has been presented by Shelef and Kummer.7 One approach to solving the stability problems has been to avoid leaded fuel in automobiles containing converters The catalytic approach to conversion for American Motor cars is to use a pellet-type of catalyst with a monolithic-type warm-up feature for California and high altitude cars The warm-up converter is separately mounted ahead of the catalytic converter Chrysler Corporation and Chevrolet use a catalyst support coated with platinum, palladium and rhodium Hydrocarbons, CO and NOx are all reduced by this three component catalyst An extensive literature exists on more economical active phases11,13,16,22 which are not as effective converters The air to exhaust ratio in catalytic converters is computer controlled in American Motor cars For this reason, reactions which involve combination with rather than decomposition of NO2 are being studied very carefully The equilibrium constants in terms of partial pressure are given in Table for NO combination with hydrogen, CO and methane at various exhaust temperatures The thermodynamic conditions (large Kp values) are generally favorable for conversion (reduction) A Monel (nickel– copper alloy) catalyst has been found reasonably successful for removing NO by combining it with residual CO in the exhaust stream The Monel dissociates the oxygen from NO and then oxides CO to the harmless dioxide © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 704 11/18/2005 10:42:27 AM MOBILE SOURCE POLLUTION TABLE The equilibrium constants for some reduction reactions of nitric oxide17 log Kp Reactions 600 700 800 900 NO ϩ 5H2 → 2NH3 ϩ 2H2O 49.0 48.1 32.13 26.44 21.8 NO ϩ 2H2 → N2 ϩ 2H2O 51.7 43.4 37.1 32.2 28.3 NO ϩ CO → ½N2 ϩ CO2 27.3 22.6 19.7 16.46 14.29 NO ϩ CH4 → ½N2 ϩ 2H2 19.59 18.1 17.6 16.2 15.5 NO ϩ Hat → HNO 12.4 NO ϩ ½ H2 → HNO Ϫ3.7 9.77 Ϫ3.6 7.79 6.24 5.0 Ϫ3.52 Ϫ3.46 Ϫ3.42 Temperature 1000K 600 CO % °C Time, FIGURE In this system, when the engine operation is too rich (too little oxygen) nitrogen oxide reduction is found to be excellent, but carbon monoxide conversion poor The nitrogen oxide will readily combine with Monel, forming Monel oxide and nitrogen since there is little oxygen to compete with the nitrogen oxide for active sites on the Monel However, since the CO:NO ratio is so large, there will be insufficient Monel oxide formed to give up its oxygen to the carbon monoxide On the other hand, if the engine operation is too lean, NO reduction will be poor and CO conversion excellent since the reaction becomes significant and oxygen will compete with the nitrogen oxide for sites on the Monel Some typical conversion data1 at space velocities of 50,000 v/vhr indicated that conversion of NO in synthetic gas mixture (2% CO, 1000 ppm NO) was over 95% if the temperature was above 700ЊC, but fell off sharply at temperature below 650ЊC Problems associated with this technique are the formation of NH3 if any residual H2 is present and back pressure buildups with the current catalyst structure In addition, dusts form which are emitted as particulates It is essential for efficient performance of this catalyst that it warms up to operating temperatures very rapidly Lead in the fuel reduces the chemical activity and ultimately increased the rate of deterioration of the catalyst Another catalyst which showed promise for the same reaction but at lower temperature (200 to 350ЊC) has also been mentioned.18 Despite the fact that conversions of better than 90% were reported equal amounts of CO and NO were used Automotive exhaust have about 16 times more CO than 705 NO Activated carbon has been used19 successfully with H2 gas at 600ЊC However, activated carbon lost a considerable portion of its activity after only hours service CO and Hydrocarbon Removal Major automotive and petroleum companies have combined efforts in the development of an inexpensive multi-thousand mile catalytic package for reducing CO and HC exit concentrations Since the input of gasoline and hence the effluent gases are rarely at steady state, any study of a catalytic reactor must consider the dynamic situation To give some idea of magnitude, the exhaust gas flow in standard cubic feet per minute (SCFM) is roughly twice the miles per hour equivalent of an automobile and the temperatures of the exhaust gas change from that of the ambient to well over 600ЊC Figure shows typical CO and temperature levels in the exhaust stream after engine startup in a Federal cycle For balancing pollution problems with no catalytic or afterburner control an air–fuel ratio of about 16 is recommended Above this level the NOx level increases markedly and below it the amount of unburned HC and CO substantially increases With catalytic devices this ratio may no longer be optimum, since the catalyst selectivity may be greater towards removal of one of the pollutants than any of the others Wei20 has noted that the kinetics of CO oxidation over an egg-shell catalyst turn out to be first order for CO and 0.2 order for O2 in the range of to 9% oxygen The curvature in the Arrhenius plot (Figure 2) is believed to be caused by a pore diffusional phenomena As the catalyst ages and activity falls the reaction rate becomes controlling and the Arrhenius plot becomes a straight line The catalyst of the figure is best above 350ЊC, but a lower operating temperature may be preferred for longer catalyst life Wei20 found that, “As the catalyst lost 90% of its activity, the emission rose by only 30%; but the last 10% of activity loss would result in a precipitous rise of carbon monoxide emission A catalyst with 50% reduction in heat capacity performs much better; a reactor with 50% reduction in volume performs better when the catalyst is fresh and worse then it is aged.” His philosophy is, “It is our engineering goal to produce a low-cost and convenient solution However, any solution requires some inconvenience and cost from everyone Quick warm-up is no problem if we are willing to sit and wait in the car for for an auxiliary heater to warm up the catalyst bed before the car moves We have ninety million cars on the road, and a $100 device will cost us nine billion dollars How much are we willing to pay for 90% cleaner air? These decisions belong to the public, not the engineers For the sake of everyone we hope to be able to present to the public an economical and convenient solution in the near future.” Stein et al.21 have evaluated the effectiveness of possible catalysts by a microcatalytic technique based on gas chromatography The technique which is described in detail allows a large variety of hydrocarbons and catalysts to be rapidly tested over a wide range of temperatures (100–600ЊC) In general oxides of cobalt, chromium, iron, manganese, and nickel are the most effective catalysts The higher molecular weight hydrocarbons are more easily oxidized than the lower © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 705 11/18/2005 10:42:27 AM 706 MOBILE SOURCE POLLUTION X X X 100 INTRINSIC 120 SCFM 50 FRESH X ACCELERATION 60 AGED k SEC 30 –1 10 X needed for 80% conversion 10 IDLE 1.5mm 0.5 25mm 500 400 300 X 200 °C I / T °K FIGURE Carbon monoxide activity of an eggshell catalyst and hydrocarbons of a given carbon number increased in reactivity according to the series: aromatic Ͻ branched paraffin or alicyclic Ͻ normal paraffin Ͻ olefinic Ͻ acetylenic The olefinic hydrocarbons, generally considered the most undesirable, are relatively easy to remove Other results could be summarized in the following manner: 1) With most of the catalysts tested, some cracking occurs during the oxidation of hydrocarbons Oxidation without CH4 formation was possible with the oxides of Co, Cr, Cu, Mn and Ag only Zirconium oxides are unique in that they produce only cracking products, mainly methane and smaller amounts of intermediate hydrocarbons 2) Complete removal of all hydrocarbons was not attained with some of the catalyst, even at 600ЊC The oxidation is not a simple function of temperature The potential of copper oxide-alumina catalyst for air pollution control has been studied by Sourirajan and Accomazzo.22 They stated that, “the simultaneous removal of hydrocarbons and carbon monoxide present in the auto exhaust gases has been tested making use of a six-cylinder Chevrolet engine run on leaded gasoline fuel The hydrocarbon and carbon monoxide concentrations encountered in these studies varied in the range 170–17,000 ppm and 1–7%, respectively It was found that the minimum initial temperature of the catalyst bed required for the complete removal of both hydrocarbons and carbon monoxide, simultaneously, was 226ЊC under no load condition, 342ЊC, under an engine load of 2.5hp, 400ЊC, under an engine load of 5.1hp or higher, and 236ЊC under deceleration conditions The catalyst showed no deterioration in performance even after 100 hours of continuous service in conjunction with the above auto exhaust gases The extent of removal of hydrocarbons from the exhaust gases was found to depend on the initial temperature of the catalyst bed and the engine load condition It is realized that a successful 100 hour run does not constitute a life test on the catalyst, but it does indicate the potential applicability of the catalyst in air pollution control devices The engineering design of the suitable converter for any particular practical application of the catalyst should naturally take into account the heat liberated during oxidation Instantaneous catalyst temperatures of the order of 900ЊC have been encountered in this work with no deleterious effect on the subsequent effectiveness of the catalyst the heat liberated during the reaction can be advantageously used to maintain the full effectiveness of the catalyst under all conditions of engine operation encountered in normal practice.” When contaminants are passed through a Hopcalite (unsupported coprecipitate of copper and manganese oxides) catalyst burner, the results vary from almost complete oxidation of some organics to very slight oxidation of the lower molecular weight aliphatic hydrocarbons23 at some 300ЊC Nitrogen compounds form N2O when oxidized and halogenated compounds indicate a strong acid reaction when the reactor effluent is tested with detector paper An interesting example of the use of exhaust gas recycle and catalysts has been presented by the Esso Research Group In Figure 3, typical hot cycle traces of CO, O2 and NO are presented for cases before the catalyst, after Monel catalyst and after a 2nd stage Platinum-alumina catalyst, for instance, with and without recycle The major beneficial effect of recycle is on the NO concentration The combustion of gasoline is more or less incomplete regardless of the quantity of excess air used About 1% of the exhaust gas is composed of harmful products chiefly carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC) A significant variable affecting each of these pollutant concentrations is the air to fuel ratio (ATFR) The stoichiometric value, (ATFR)STO is about 14.7:1.0 on a weight basis Using a catalytic three way converter, more than 90% of the pollutants can be converted to harmless substances To avoid catalyst contamination lead free gasoline must be used In the closed loop electro-mechanical control of (ATFR)STO described by Robert Bosch,34 1985, an oxygen sensor in the exhaust gas transmits a signal which is used to correct ATFR deviations This control method is particularly effective on fuel injection engines because they not have the additional delay times of carburetor engines For catalytic converter operation, the optimum ATFR range is © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 706 11/18/2005 10:42:28 AM MOBILE SOURCE POLLUTION Without Recycle AFTER MONEL AFTER BOTH CATALYSTS Without Recycle Without Recycle With Recycle With Recycle With Recycle % O2 1000 % CO PPM NO BEFORE CATALYSTS 707 2 = CO ~ MPH 60 40 20 30 60 90 120 30 60 90 120 30 60 90 120 TIME, SECONDS FIGURE between 99 and 100% of (ATFR)STO Above this range NOx levels increase markedly as ATFR is increased; below this range CO and HC levels increase as ATFR is decreased Electric vehicles (EV’s) operated by high energy high powered batteries are making great strides toward commercialization The near term goal is to provide over 100 miles per recharge at accelerations capable of matching the internal combustion engine The California Air Resources Board has issued a technical document in December of 1995 supporting the concept of such vehicles The key performance parameter for EV’s is its specific energy (or energy density), measured in watthours per kilogram, with a near term goal of 80–100 wh/kg, suggested by the US Advanced Battery Consortium (USABC).38 Another important measure of performance is the peak specific power (or power density), which gives us an idea of an EV’s acceleration It is measured in units of watts per kg with an USABC near term goal of 150 w/kg that can be sustained for 30 seconds during discharge down to 80% depth of discharge A comparison of different chemical system performance parameters is presented in Figure All of the batteries are expected to achieve the USABC’s midterm goals for these EV parameters, which coincide with lasting about five years (-600 cycles) and costing no more than $150 per kwhr of battery capacity Hybrid power electric-diesel engines became available in 1995 They alternate between diesel power operation at 2000–2600 RPM where its efficiency is best to battery power at other RPM RELATED TRANSPORTATION PROBLEMS Diesel Exhaust Odors Diesel engines are found in buses, trucks, off-road vehicles and power applicators and increasingly in automobiles Public reaction to diesel-engine exhaust odors provides the impetus for controlling effluents of that type of fuel combustion.24 A list of oily kerosene and smokyburnt odor compound identified by A.D Little, Inc is included in Table Exhaust odor and smoke from diesel engines are more objectionable than those from spark ignition CO emissions are generally less serious but NOx is troublesome (4 to 10 g/mile) Improved fuel injection and afterburners are considered to be the most promising of the existing control methods The injection differs from the internal combustion engine in that fuel does not enter the cylinder as a mixture with air, but is injected under high pressure into the chamber in exact quantities through low tolerance nozzles For NOx removal the basic approaches © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 707 11/18/2005 10:42:29 AM 708 MOBILE SOURCE POLLUTION 250 SPECIFIC ENERGY USABC long-term goal Watthours per kilogram 200 2001 2004 150 2001 1998 1998 100 USABC midterm goal 1996 1998 1996 1998 50 Sodiumnickel chloride Sodiumsulfur Lithiumion Nickelmetal hydride Nickelcadmium Lead-acid 500 PEAK SPECIFIC POWER USABC long-term goal 2004 400 2004 300 USABC midterm goal 200 1998 100 Sodiumnickel chloride Sodiumsulfur Lithiumion Nickelmetal hydride Nickelcadmium Lead-acid © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 708 11/18/2005 10:42:29 AM 709 MOBILE SOURCE POLLUTION 2500 CYCLE LIFE 2000 2000 1998 1500 2000 USABC long-term goal 2004 2002 1000 USABC midterm goal 500 Sodiumnickel chloride Sodiumsulfur Lithiumion Nickelmetal hydride Nickelcadmium Lead-acid FIGURE 4: Performance Parameter Comparison for Electric Vehicles as a Function of Advanced Battery Type (After Moore38) are engine refinement, fuel additives, catalytic conversion and reduction of peak combustion temperature Gas Turbines The exhaust of gas turbines contains perhaps less pollutants than that of any other internal combustion process The high airport traffic density makes the problem a real one, however Sulfur dioxide emissions are low, but the smoke and odor producing compound levels are high For automobile use, the total mass of gas turbine exhaust is many times greater than that for the gasoline engine of equivalent power Hydrocarbon and CO mass emissions are known to be low and difficult to reduce A greater deal of the pollution control work underway is in the engine modification area In aircraft the turbofan engine is largely replacing the turbojet Turbofans bypass some of the air past the engine and rejoin it with the burner gases at the exhaust tailpipe Modern dry NOx combustion systems can obtain emissions of 25 ppm at 15% O2.36,37 The airplane is a much more efficient carrier (pollution wise) than the automobile, on a people × miles basis Aircraft engine research has been concerned primarily with smoke reduction Fuel type, fuel additives and combustion chamber design have been the primary variables studied PARTICULATE EMISSIONS The particulate matter emitted from automobiles has been characterized25 as consisting of “lead salts, alkaline earth compounds, iron oxides, soot carbonaceous material, and tars This material ranges in size from large flakes to submicron particles and varies in consistency from hard and brittle particulate to oil mists Some of the particulate material is generated in the engine combustion chamber and nucleated and agglomerated in the vehicle exhaust system before it passes out the tail pipe On the other hand, a large portion of the particulate material generated in the engine subsequently deposits on various surfaces of the exhaust system At some later time, this deposited material flakes off and becomes reentrained in the exhaust gas to be emitted from the vehicle Obviously, opportunities exist for various types of chemical and physical processes to take place and, as a result, the overall particulate emission process for a vehicle is quite complex and difficult to define.” © 2006 by Taylor & Francis Group, LLC C013_005_r03.indd 709 11/18/2005 10:42:30 AM 710 MOBILE SOURCE POLLUTION TABLE Partial list of odorifous compounds General classification Compounds Indans and tetralins Methyl indan Tetralin Dimethyl indan Methyl tetralin Dimethyl tetralin Trimethyl tetralin Alkyl tetralin Alkyl-substituted naphthalenes Methylnaphthalenes Indenes, acenapthenes, and Alkyl-substituted indenes benzothiophenes Dimethylbenzothiophenes Dimethylnaphthalenes Acenaphthene General class Carbon range Alkenone C5 to C11 Furan C6 to C10 Diene/ C9 to C12 Furfural C6 to C7 Methoxy benzene C8 to C9 Phenol C7 to C12 Benzaldehyde C7 to C10 Benzofuran C8 to C9 Indanone C6 to C10 Indenone C9 to C12 Naphthol C10 to C14 Naphthaldehyde C11 TABLE Lead particulate emissions as a function of size and mileage25 Average mileage Average lead salt emissions, g/mile >9 microns