Engine Exhaust Emissions

Transcription

1 Engine Exhaust Emissions 1

2 Exhaust Emission Control Particulates (very challenging) Chamber symmetry and shape Injection characteristics (mixing rates) Oil control Catalyst (soluble fraction) Particulate trap Odor (oxidation catalyst) CO 2 (global warming) 2

3 Exhaust Aftertreatment ECU EGR VALVE THROTTLE AIR FLOW METER DOC DOC NOx DEVICE PARTICLE FILTER VAPORIZER TEMPERATURE SENSORS PRESSURE DROP SENSOR 3

4 Advantages: Diesel Engines Efficiency (most efficient prime mover) Emissions (low CO, CO 2, good durability) Very high torque and performance Disadvantages: Emissions -- more challenging to control NOx, particulates Higher cost Heavier Noisier 4

5 Pollutant Formation and Control All IC engines produce undesirable emissions as a result of combustion. The emissions are unburned hydrocarbons (UHC), carbon monoxide (CO), oxides of nitrogen NO and nitrogen dioxide (NO 2 ), sulfur dioxide, and solid carbon particulates. HC emissions from gasoline-powered vehicles include a number of toxic substances such as benzene, polycyclic aromatic hydrocarbons (PAHs), 1,3- butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein). These emissions pollute the environment and contribute to acid rain, smog odour and create other health problems. Carbon dioxide is an emission that is the primary greenhouse gas responsible for global warming. 5

6 Ontario Drive Clean Program In Ontario every vehicle must undergo a tail pipe emission test every other year to check compliance with regulation: Nitrogen Oxide rpm Carbon Monoxide 3000 rpm and 800 rpm Unburned hydrocarbons rpm and rpm Particulates for diesels at present 30% opacity Evaporative Emissions for SI at present 6

7 Nitrogen Oxides NO x includes nitric oxide (NO) and nitrogen dioxide (NO 2 ). In SI engines the dominant component is NO, which forms as a result of dissociation of molecular nitrogen and oxygen. Since the activation energy of the elementary reaction O+N 2 NO+Nis high the reaction rate is very temperature dependent - recall W ~ exp (-E/RT) Therefore NO is only formed at high temperatures and the reaction rate is relatively slow. At temperatures below 2000K the reaction rate is extremely slow, so that NO formation not important. Recall Ya.B. Zel dovich or thermal mechanism (Ch.12) named is "thermal" because the reaction has very high activation energy due to the strong bond in the N 2 -molecule. 7

8 SI Engine: In-cylinder NO Formation Once 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 x = 0 x = 1-15 o (x = 0) 25 o (x = 1) (no mixing of fluid elements) x = 0 Equilibrium concentration: based on the local temperature, pressure, equivalence ratio, residual fraction Actual NO concentration: based on kinetics x = 1 8

9 Effect of Equivalence Ratio on NO Concentration The peak NO concentrations almost coincides with highest adiabatic flame temperature (AFT). Typically peak NO concentrations occur for slightly lean mixtures that corresponds to lower AFT but higher oxygen concentration. 9

10 Effect of Other Parameters on NO Concentration Increased spark advance and intake manifold pressure both result in higher cylinder temperatures and thus higher NO concentrations in the exhaust gas P i = 658 mm Hg P i = 354 mm Hg φ= 0.97 φ= 0.96 φ= 1.31 φ=

11 Methods to Reduce Exhaust NO Concentration The formation of NO is highly dependent on cylinder gas temperature therefore any measures taken to reduce the AFT are effective. These are: increased residual gas exhaust gas recirculation (EGR) moisture in the inlet air In CI engines the cylinder gas temperature is governed by the load and injection timing IDI/NA indirect injection naturally aspirated DI/NA direct injection naturally aspirated 11

12 Hydrocarbon emissions Hydrocarbon emissions caused by the presence of unburned fuel in the engine exhaust. Some of the exhaust hydrocarbons are not found in the fuel, but derived from the fuel whose structure was altered due to chemical reaction that did not go to completion. These are: acetaldehyde, formaldehyde, 1,3 butadiene etc - all toxic emissions. Typically, about 9% of the fuel supplied to the engine is not burned during the normal combustion phase of the expansion stroke. Only 2% ends up in the exhaust the rest is consumed during the other three strokes. The hydrocarbon emissions cause a decrease in the thermal efficiency, as well as an air pollutant. 12

13 Mechanisms believed to be responsible for hydrocarbon emissions: % fuel escaping Source normal combustion % HC emissions Crevices Oil layers Deposits Liquid fuel Flame quench Exhaust valve leakage Total

14 Hydrocarbon Emission Sources Crevices are narrow regions in the combustion chamber into which the flame cannot propagate because it is smaller than the quenching distance. Crevices are located around the piston, head gasket, spark plug and valve seats and represent about 1 to 2% of the clearance volume. The crevice around the piston is by far the largest, during compression the fuel air mixture is forced into the crevice (density higher than cylinder gas since gas is cooler near walls) and released during expansion. Crevice Piston ring 14

15 Hydrocarbon Emission Sources Oil layers - Since the piston ring is not 100% effective in preventing oil migration into the cylinder above the piston, oil layers exist within the combustion chamber. This oil layer traps fuel and releases it later during expansion. Deposits With continued use carbon deposits build up on the valves, cylinder and piston head. These deposits are porous with pore sizes smaller than the quenching distance so trapped fuel cannot burn. The fuel is released later during expansion. Liquid fuel For some fuel injection systems there is a possibility that liquid fuel is introduced into the cylinder past an open intake valve. The less volatile fuel constituents may not vaporize (especially during engine warm-up) and be absorbed by the crevices or carbon deposits. Flame quenching It has been shown that the flame does not burn completely to the internal surfaces, the flame extinguishes at a small but finite distance from the wall. Most of this gas eventually diffuses into the burned gas during expansion stroke. 15

16 Hydrocarbon Emission Sources for CI Engines Crevices - Fuel trapped along the wall by crevices, deposits, or oil due to impingement by the fuel spray. 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 fuel mixes with the air, regions of fuel-air mixture are produced that are too lean to burn. Some of this fuel makes its way out the exhaust. As longer ignition delay - more fuel becomes overmixed. 16

17 Particulates A high concentration of particulate matter (PM) is manifested as visible smoke in the exhaust gases. Particulates are any substance other than water that can be collected by filtering the exhaust: 1) solid carbon material or soot 2) condensed hydrocarbons and their partial oxidation products Diesel particulates consist of solid carbon (soot) at exhaust gas temperatures below 500 o C HC compounds become absorbed on the surface. In a properly adjusted SI engines soot is not usually a problem Particulate can arise if leaded fuel or overly rich fuel-air mixture are used. Smoke especially produced during engine warm up where the HC condense in the exhaust gas. 17

18 Particulates (soot) Most particulates result from incomplete combustion which occurs in fuel rich mixtures. Any carbon not oxidized in the cylinder ends up as soot in the exhaust! Based on equilibrium the composition of the fuel-oxidizer mixture at the onset of soot formation occurs when x 2a (or x/2a 1) in the following reaction: y C H + ao 2aCO + H + ( x 2a) C( s) x y i.e. when the (C/O) ratio exceeds 1. Experimentally it is found that the critical C/O ratio for onset of soot formation is between 0.5 and 0.8 The CO, H 2, and C(s) are subsequently oxidized in the diffusion flame to CO 2 and H 2 O via the following second stage 1 1 CO + O CO C() s + O CO H + O H O

19 Particulates and CI Engines Particulates are a major emissions problem for CI engines. Exhaust smoke limits the full load overall equivalence ratio to about 0.7 φ = 0.7 φ = 0.5 φ = 0.3 One technique for measuring particulate involves diluting the exhaust gas with cool air to freeze the chemistry before measurements The problem for diesel design is that in order to reduce NOx one wants to reduce the AFT but this has the adverse effect of decreasing the amount of soot oxidized, or increases the amount of soot in the exhaust. 19

20 Particulates and CI Engines An example of this dilemma is changing the start of injection, e.g., increasing the advance increases the AFT Crank angle BTC for start of injection 20

21 Carbon Monoxide Carbon monoxide appears in the exhaust of fuel rich running engines, when there is insufficient oxygen to convert all the carbon in the fuel to carbon dioxide. The C-O-H system is more or less at equilibrium during combustion and expansion. Late in the expansion stroke when the cylinder temperature gets down to around 1700K the chemistry in the C-O-H system becomes rate limited and starts to deviate from equilibrium. In practice it is often assumed that the C-O-H system is in equilibrium until the exhaust valve opens at which time it freezes instantaneously. The highest CO emission occurs during engine start up (warm up) when the engine is run fuel rich to compensate for poor fuel evaporation. Since CI engines run lean overall, emission of CO is generally low and not considered a problem. 21

22 Emission Control The current emission limits for HC, CO and NOx have been reduced to 3%, 2%, >10%. Three basic methods used to control engine emissions: 1) Engineering of combustion process - advances in fuel injectors, oxygen sensors, and on-board computers. 2) Optimizing the choice of operating parameters - two NOx control measures that have been used in automobile engines since 1970s are spark retard and EGR. 3) After treatment devices in the exhaust system - catalytic converter 22

23 Catalytic Converter Catalytic converters are built in a honeycomb or pellet geometry to expose the 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 catalytic converter (poison). 23

24 Three-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 H 2 via a reduction reaction on the catalyst surface. e.g., NO+CO ½N 2 +CO 2, NO+H 2 ½N 2 +H 2 O, and others The remaining CO and HC are removed through an oxidation reaction forming CO 2 and H 2 O products (air added to exhaust after exhaust valve). A three-way catalysts will function correctly only if the exhaust gas composition corresponds to nearly (±1%) stoichiometric combustion. If the exhaust is too lean NO are not destroyed If the exhaust is too rich CO and HC are not destroyed A closed-loop control system with an oxygen sensor in the exhaust is used to determine the actual A/F ratio and used to adjust the fuel injector so that the A/F ratio is near stoichiometric. 24

25 Effect of Temperature The temperature at which the converter becomes 50% efficient is referred to as the light-off temperature. The converter is not very effective during the warm up period of the engine 25

26 Catalytic Converter for Diesels For Diesel engines catalytic converters are used to control HC and CO, but reduction of NO emissions is poor because the engine runs lean in order to avoid excess smoke. The NO is controlled by retarding the fuel injection from 20 o to 5 o before TC in order to reduce the peak combustion temperature. This has a slight negative impact, increases the fuel consumption by about 15%. 26

27 IC Engine Fuels Crude oil contains a large number of hydrocarbon compounds (25,000). The purpose of refining is to separate crude oil into various fractions via a distillation process, and then chemically process the fractions into fuels and other products. A still is used to heat a sample, preferentially boiling off lighter components which are then condensed and recovered. The group of compounds that boil off between two temperatures are referred to as fractions. The order of the fractions as they leave the still are naptha, distillate, gas oil, 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 chemical processing has been done to a fraction. 27

28 Distillation Process Refining Process 28

29 Gasoline Light virgin (or straight run) naptha can be used as gasoline. Gasoline fuel is a blend of hydrocarbon distillates with a range of boiling points between 25 and 225 o C (for diesel fuel between 180 and 360 o C) Chemical processing is used to: Produce gasoline from a fraction other than light virgin, or Upgrade a given fraction (e.g. increases octane number of fuel: produce isooctane by reacting butene with isobutane in the presence of a catalyst. 29

30 Reformulated Gasoline In order to reduce CO and HC the oxygen content of gasoline is increased to about 3% by weight The U.S. reformulated gasoline program is a year-round program used to reduce ozone by requiring a minimum oxygen content of 2% by weight and maximum benzene content of 1%. The primary oxygenates are MTBE (CH 3 )OC(CH 3 ) 3 and ethanol (C 2 H 5 OH) As part of the reformulated gasoline program sulfur is restricted to 31 ppm 30