2Pollutant Formation and Control All IC engines produce undesirable emissions as a result of combustion.The emissions of concern are unburned hydrocarbons (UHC), carbonmonoxide (CO), oxides of nitrogen such as nitric oxide and nitrogen dioxide(NOx), sulfur dioxide, and solid carbon particulates.These emissions pollute the environment and contribute to acid rain, smogodors, and respiratory and other health problems.HC emissions from gasoline-powered vehicles include a number of toxicsubstances such as benzene, polycyclic aromatic hydrocarbons (PAHs),1,3-butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein).Carbon dioxide is an emission that is not regulated but is the primarygreenhouse gas responsible for global warming.
3Historical Perspective During the 1940s air pollution as a problem was first recognized in the LosAngeles basin.Two causes of this were the large population density and the natural weatherconditions. Smoke and other pollutants combined with fog to form smog.In 1966 HC and CO emission limits were introduced in California.All of North America usually follows California’s lead (all US in 1968).By making more fuel efficient engines and with the use of exhaust aftertreatment, emissions per vehicle of HC, CO, and NOx were reduced byabout 95% during the 1970s and 1980s.Automobiles are more fuel efficient now (2x compared to 1970) but there aremore of them and the trend is to larger SUVs, as a result fuel usage is unchanged over this period.
4Ontario Drive Clean Program In Ontario every vehicle must undergo a tail pipe emission test every otheryear to check compliance with regulation:Nitrogen Oxide – rpmCarbon Monoxide – 3000 rpm and 800 rpmUnburned hydrocarbons – rpm and rpmParticulates (diesels only at present) – 30% opacityEvaporative Emissions (SI only at present)
5Ontario Drive Clean Program Stats Test results between 1999 and March 2004Light-Duty Program*: % failed testHeavy-Duty Diesel**: % failed testHeavy-Duty Non-Diesel**: 27.3% failed test* 6 million vehicles (automobiles, vans, SUVs, pick-ups) in program** 200,000 vehicles in program
6Nitrogen OxidesNOx includes nitric oxide (NO) and nitrogen dioxide (NO2)In SI engines the dominant component of NOx is NOForms as a result of dissociation of molecular nitrogen and oxygen.Since the activation energy of the critical elementary reaction O+N2→NO+Nis high the reaction rate is very temperature dependent, w''′ ~ exp (-E/RT)Therefore NO is only formed at high temperatures and the reaction rate isrelatively slow.At temperatures below 2000K the reaction rate is extremely slow, so NOformation not important.
7SI Engine In-cylinder NO Formation Since the cylinder temperature changes throughout the cycle the NO reaction rate also changes.Each fluid element burns to its AFT based on its initial temperature, elements that burn first near the spark plug achieve a higher temperature.Since the chemistry is not fast enough the actual NO concentration tends toward but never achieves the equilibrium value.If NO concentration is lower than equilibrium value – NO formsIf NO concentration is higher than equilibrium value – NO decomposesOnce the element temperature reaches 2000K the reaction rate becomes so slow that the NO concentration effectively freezes at a value greater than the equilibrium value.The total amount of NO that appears in the exhaust is calculated by summing the frozen mass fractions for all the fluid elements:
8(assuming no mixing of fluid elements) -15o (x = 0)25o (x = 1)(assuming no mixing of fluid elements)Equilibrium concentration:based on the local temperature, pressure,equivalence ratio, residual fractionActual NO concentration:based on kineticsx = 0x = 1
9Effect of Equivalence Ratio on NO Concentration One would expect the peak NO concentrations to coincide with highest AFT.Typically peak NO concentrations occur for slightly lean mixtures – thatcorresponds to lower AFT but higher oxygen concentration.
10Effect of Various Parameters on NO Concentration Increased spark advance and intake manifold pressure both result in highercylinder temperatures and thus higher NO concentrations in the exhaust gasPi= 658 mm HgPi= 354 mm Hg= 0.97= 0.96= 1.31= 1.27
11Exhaust NO Concentration Reduction Since the formation of NO is highly dependent on cylinder gas temperature anymeasures taken to reduce the AFT are effective:increased residual gasexhaust gas recirculation (EGR)moisture in the inlet airIn CI engines the cylinder gas temperature is governed by the load andinjection timingIDI/NA – indirect injectionnaturally aspiratedDI/NA – direct injection
12HydrocarbonsHydrocarbon emissions result from the presence of unburned fuel in theengine exhaust.However, some of the exhaust hydrocarbons are not found in the fuel, but arehydrocarbons derived from the fuel whose structure was altered do tochemical reaction that did not go to completion. For example: acetaldehyde,formaldehyde, 1,3 butadiene, and benzene all classified as toxic emissions.About 9% of the fuel supplied to the engine is not burned during the normalcombustion phase of the expansion stroke.Only 2% ends up in the exhaust the rest is consumed during the otherthree strokes.As a consequence hydrocarbon emissions cause a decrease in the thermalefficiency, as well as being an air pollutant.
13Hydrocarbon Emission Sources for SI Engines There are six principal mechanisms believed to be responsible forhydrocarbon emissions:% fuel escapingSource normal combustion % HC emissionsCrevicesOil layersDepositsLiquid fuelFlame quenchExhaust valve leakageTotal
14Hydrocarbon Emission Sources Crevices – these are narrow regions in the combustion chamber into whichthe flame cannot propagate because it is smaller than the quenching distance.Crevices are located around the piston, head gasket, spark plug and valveseats and represent about 1 to 2% of the clearance volume.The crevice around the piston is by far the largest, during compression the fuelair mixture is forced into the crevice (density higher than cylinder gas since gasis cooler near walls) and released during expansion.CrevicePiston ring
15Hydrocarbon Emission Sources Oil layers - Since the piston ring is not 100% effective in preventing oilmigration into the cylinder above the piston, oil layers exist within thecombustion chamber. This oil layer traps fuel and releases it later duringexpansion.Deposits – With continued use carbon deposits build up on the valves, cylinderand piston head. These deposits are porous with pore sizes smaller than thequenching distance so trapped fuel cannot burn. The fuel is released laterduring expansion.Liquid fuel – For some fuel injection systems there is a possibility that liquidfuel is introduced into the cylinder past an open intake valve. The less volatilefuel constituents may not vaporize (especially during engine warm-up) and beabsorbed by the crevices or carbon deposits.Flame quenching – It has been shown that the flame does not burn completelyto the internal surfaces, the flame extinguishes at a small but finite distancefrom the wall. Most of this gas eventually diffuses into the burned gas duringexpansion stroke.
16Hydrocarbon Exhaust Process When the exhaust valve opens the large rush of gas escaping the cylinderdrags with it some of the hydrocarbons released from the crevices, oil layerand deposits.During the exhaust stroke the piston rolls the hydrocarbons distributed along thewalls into a large vortex that ultimately becomes large enough that a portion ofit is exhausted.BlowdownExhaustStroke
17Hydrocarbon Exhaust Process The first peak is due to blowdown and the second peak is due to vortex roll upand exhaust (vortex reaches exhaust valve at roughly 290o)ExhaustvalveopensExhaustvalveclosesBCTC
18Hydrocarbon Emission Sources for CI Engines Crevices - Fuel trapped along the wall by crevices, deposits, or oil due toimpingement by the fuel spray (not as important as in SI engines).Undermixing of fuel and air - Fuel leaving the injector nozzle at low velocity,at the end of the injection process cannot completely mix with air and burn.Overmixing of fuel and air - During the ignition delay period evaporated fuelmixes with the air, regions of fuel-air mixture are produced that are too lean toburn. Some of this fuel makes its way out the exhaust.Longer ignition delay more fuel becomes overmixed.Exhaust HC, ppm C
19Note for the direct injection diesel the hydrocarbon emission are the worst at light load (long ignition delay)
20ParticulatesA high concentration of particulate matter (PM) is manifested as visiblesmoke in the exhaust gases.Particulates are any substance other than water that can be collected byfiltering the exhaust, classified as:1) solid carbon material or soot2) condensed hydrocarbons and their partial oxidation productsDiesel particulates consist of solid carbon (soot) at exhaust gas temperaturesbelow 500oC HC compounds become absorbed on the surface.In a properly adjusted SI engines soot is not usually a problemParticulate can arise if leaded fuel or overly rich fuel-air mixture are used.burning crankcase oil will also produce smoke especially during engine warmup where the HC condense in the exhaust gas.
21Particulates (soot)Most particulate material results from incomplete combustion of fuel HC whichoccurs in fuel rich mixtures.Based on equilibrium the composition of the fuel-oxidizer mixture at the onsetof soot formation occurs when x ≥ 2a (or x/2a ≥ 1) in the following reaction:i.e. when the (C/O) ratio exceeds 1. Experimentally it is found that the criticalC/O ratio for onset of soot formation is between 0.5 and 0.8The CO, H2, and C(s) are subsequently oxidized in the diffusion flame toCO2 and H2O via the following second stageAny carbon not oxidized in the cylinder ends up as soot in the exhaust!
22Particulates and CI Engines Particulates are a major emissions problem for CI engines.Exhaust smoke limits the full load overall equivalence ratio to about 0.7f = 0.7One technique for measuring particulateinvolves diluting the exhaust gas withcool air to freeze the chemistry beforemeasurementsf = 0.5f = 0.3An outstanding problem for diesel engine designers is that in order to reduceNOx one wants to reduce the AFT but this has the adverse effect of decreasingthe amount of soot oxidized, or increases the amount of soot in the exhaust.
23Particulates and CI Engines An example of this dilemma is changing the start of injection, e.g., increasingthe advance increases the AFTCrank angle bTC forstart of injection
24Carbon monoxide appears in the exhaust of fuel rich running engines. For fuel rich mixtures there is insufficient oxygen to convert all the carbon inthe fuel to carbon dioxide.C8H18-air
25Carbon MonoxideThe C-O-H system is more or less at equilibrium during combustion andexpansion.Late in the expansion stroke when the cylinder temperature gets down toaround 1700K the chemistry in the C-O-H system becomes rate limited andstarts to deviate from equilibrium.In practice it is often assumed that the C-O-H system is in equilibrium untilthe exhaust valve opens at which time it freezes instantaneously.The highest CO emission occurs during engine start up (warm up) when theengine is run fuel rich to compensate for poor fuel evaporation.Since CI engines run lean overall, emission of CO is generally low and notconsidered a problem.
26Emission ControlThe current emission limits for HC, CO and NOx have been reduced to 4%,4% and 10% of the uncontrolled pre-1968 values, respectively.Three basic methods used to control engine emissions:Engineering of combustion process - advances in fuel injectors, oxygensensors, and on-board computers.2) Optimizing the choice of operating parameters - two NOx control measuresthat have been used in automobile engines since 1970s are spark retard andEGR.3) After treatment devices in the exhaust system - catalytic converter
27Catalytic ConverterAll catalytic converters are built in a honeycomb or pellet geometry to exposethe exhaust gases to a large surface made of one or more noble metals:platinum, palladium and rhodium.Rhodium used to remove NO and platinum used to remove HC and CO.Lead and sulfur in the exhaust gas severely inhibit the operation of a catalyticconverter (poison).
28Three-way Catalytic Converter A catalyst forces a reaction at a temperature lower than normally occurs.As the exhaust gases flow through the catalyst, the NO reacts with the CO,HC and H2 via a reduction reaction on the catalyst surface.e.g., NO+CO→½N2+CO2 , NO+H2 → ½N2+H2O, and othersThe remaining CO and HC are removed through an oxidation reaction formingCO2 and H2O products (air added to exhaust after exhaust valve).A three-way catalysts will function correctly only if the exhaust gas compositioncorresponds to nearly (±1%) stoichiometric combustion.If the exhaust is too lean – NO are not destroyedIf the exhaust is too rich – CO and HC are not destroyedA closed-loop control system with an oxygen sensor in the exhaust is used todetermine the actual A/F ratio and used to adjust the fuel injector so that theA/F ratio is near stoichiometric.
29Effect of Mixture Composition Since thermal efficiency is highest for slightly lean conditions it may seem thatthe use of a catalytic converter is a rather severe constraint.The same high efficiency can be achieved using a near stoichiometric mixtureand diluting by EGR
30Effect of TemperatureThe temperature at which the converter becomes 50% efficient is referred toas the light-off temperature.The converter is not very effective during the warm up period of the engine
31Catalytic Converter for Diesels For Diesel engines catalytic converters are used to control HC and CO, butreduction of NO emissions is poor because the engine runs lean in order toavoid excess smoke.The NO is controlled by retarding the fuel injection from 20o to 5o before TC inorder to reduce the peak combustion temperature.This has a slight negative impact, increases the fuel consumption by about 15%.
32IC Engine FuelsCrude oil contains a large number of hydrocarbon compounds (25,000).The purpose of refining is to separate crude oil into various fractions via adistillation process, and then chemically process the fractions into fuels andother products.A still is used to heat a sample, preferentially boiling off lighter componentswhich are then condensed and recovered.The group of compounds that boil off between two temperatures are referredto as fractions.The order of the fractions as they leave the still are naptha, distillate, gas oil,and residual oil. These are further subdivided using adjectives light, middle,and heavy.The adjectives virgin or straight run are often used to signify that no chemicalprocessing has been done to a fraction.
34GasolineLight virgin (or straight run) naptha can be used as gasoline.Gasoline fuel is a blend of hydrocarbon distillates with a range of boiling pointsbetween 25 and 225oC (for diesel fuel between 180 and 360oC)Chemical processing is used to:Produce gasoline from a fraction other than light virgin, orUpgrade a given fraction (e.g., Alkylation increases the MW and octanenumber of fuel: produce isooctane by reacting butene with isobutane in thepresence of a catalyst.
35Reformulated Gasoline In order to reduce CO and HC the oxygen content of gasoline is increased toabout 3% by weight (U.S. oxygenated fuels program, winter only).The U.S. reformulated gasoline program is a year-round program usedto reduce ozone by requiring a minimum oxygen content of 2% by weight andmaximum benzene content of 1%.The primary oxygenates are MTBE (CH3)OC(CH3)3 and ethanol (C2H5OH)Also as part of the reformulated gasoline program sulfur is restricted to 31 ppmNote: gasoline with 10% ethanol by volume also marketed as “gasohol”