Presentation on theme: "Combustion engines main principles and definitions"— Presentation transcript:
1 Combustion engines main principles and definitions Reciprocating combustion engines architectureReciprocating engines dynamic propertiesEngine components and systemsThe engine management system for gasoline and Diesel enginesThe emission Requirements & TechnologyEngine vehicle integration7.1 Engine layout and mounting7.2 Engine-vehicle cooling system7.3 Intake system7.4 Exhaust system
2 Emission Requirements & Technology Short overview on: Pollutants Test Procedure (European and USA)Test LimitsAftertreatment TechnologyEngineering Fundamentals of the Internal Combustion Engine / Willard W. Pulkrabek (University of Wisconsin-Platteville – Prentice Hall-New Jersey)Internal Combustion Engines Fundamentals / John B. Heywood (McGraw-Hill, Inc)Engine Testing – Theory and Practice / A.J. Martyr, M.A. Plint (BH Elsevier)
3 The Combustion Process In the internal combustion engines the combustion is the fundamental process which converts the chemical energy of the fuel in thermal energy inside the combustion chamber. This thermal energy is then converted into mechanical energy through the piston-con rod- crankshaft system. The combustion is a chemical reaction between the air and the fuel generally composed by a mixture of different hydrocarbons molecules. The reaction can be simplified as follows:The ideal combustion process should produce only carbon dioxide (CO2) and water vapor (H2O). Instead the real combustion process produces also some exhaust gases which are the products of an uncompleted oxidation as carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and per particulate matter (PM).gasolinem (g/km)Cl (l/100 km)Diesel
4 Hydrocarbons (HC)Exhaust gases leaving the combustion chamber of an SI engine contain up to 6000 ppm of hydrocarbon components, the equivalent of % of the fuel. About 40% of this is unburned gasoline fuel components. The other 60% consists of partially reacted components that were not present in the original fuel. These consist of small non-equilibrium molecules which are formed when large fuel molecules break up (thermal cracking) during the combustion reaction. The makeup of HC emissions will be different for each gasoline blend, depending on the original fuel components. Combustion chamber geometry and engine operating parameters also influence the HC component spectrum.When hydrocarbon emissions get into the atmosphere, they act as irritants and odorants; some are carcinogenic. All components except CH4 react with atmospheric gases to form photochemical smog.Causes of HC Emissions - HC emission levels are a strong function of A/F ratio. With a fuel-rich mixture there is not enough oxygen to react with all the carbon, resulting in high levels of HC and CO in the exhaust products. This is particularly true in engine startup, when the air-fuel mixture is purposely made very rich. It is also true to a lesser extent during rapid acceleration under load. If A/F ratio is too lean, poorer combustion occurs, again resulting in HC emissions. The extreme of poor combustion for a cycle is total misfire. This occurs more often as AF is made more lean. One misfire out of 1000 cycles gives exhaust emissions of 1 gm/kg of fuel used.Even when the fuel and air entering an engine are at the ideal stoichiometric mixture, perfect combustion does not occur and some HC ends up in the exhaust. There are several causes of this. Incomplete mixing of the air and fuel results in some fuel particles not finding oxygen to react with. Flame quenching at the walls leaves a small volume of unreacted air-and-fuel mixture.
5 Hydrocarbons (HC)The thickness of this unburned layer is on the order of tenths of a mm. Some of this mixture, near the wall that does not originally get burned as the flame front passes, will burn later in the combustion process as additional mixing occurs due to swirl and turbulence. Another cause of flame quenching is the expansion which occurs during combustion and power stroke. As the piston moves away from TDC, expansion of the gases lowers both temperature and pressure within the cylinder. This slows combustion and finally quenches the flame somewhere late in the power stroke. This leaves some fuel particles unreacted. High exhaust residual causes poor combustion and a greater likelihood of expansion quenching. This is experienced at low load and idle conditions. High levels of EGR will also cause this. It has been found that HC emissions can be reduced if a second spark plug is added to an engine combustion chamber. By starting combustion at two points, the flame travel distance and total reaction time are both reduced, and less expansion quenching results.During the compression stroke and early part of the combustion process, air and fuel are compressed into the crevice volume of the combustion chamber at high pressure. As much as 3% of the fuel in the chamber can be forced into this crevice volume. Later in the cycle during the expansion stroke, pressure in the cylinder is reduced below crevice volume pressure, and reverseblow-by occurs. Fuel and air flow back into the combustion chamber, where most of the mixture is consumed in the flame reaction. However, by the time the last elements of reverse blow-by flow occur, flame reaction has been quenched and unreacted fuel particles remain in the exhaust. Location of the spark plug relative to the top compression ring gap will affect the amount of HC in engine exhaust, the ring gap being a large percent of crevice volume. The farther the spark plug is from the ring gap, the greater is the HC in the exhaust. This is because more fuel will be forced into the gap before the flame front passes. Crevice volume around the piston rings is greatest when the engine is cold, due to the differences in thermal expansion of the various materials. Up to 80% of all HC emissions can come from this source.
6 Carbon monoxide (CO)Carbon monoxide, a colorless, odorless, poisonous gas, is generated in an engine when it is operated with a fuel rich equivalence ratio. When there is not enough oxygen to convert all carbon to CO2, some fuel does not get burned and some carbon ends up as CO. Typically the exhaust of an SI engine will be about 0.2% to 5% carbon monoxide. Not only CO is considered undesirable emission, but it also represents lost chemical energy that was not fully utilized in the engine. CO is a fuel can be combusted to supply additional thermal energy:CO+1/2 O CO2 + heatMaximum CO is generated when an engine runs rich, such as when starting or when accelerating under load. Even when the intake air fuel mixture is stoichiometric or lean, some CO will be generated in the engine. Poor mixing, local rich regions, and incomplete combustion will create some CO.A well designed SI engine operating under ideal conditions can have an exhaust mole fraction of CO as low as Compression Engines (CI) that operate overall lean generally have a very low CO emissions.
7 Oxides of Nitrogen (NOx) Exhaust gases of an engine can have up to 2000 ppm of oxides of nitrogen. Most of this will be nitrogen oxide (NO), with a small amount of nitrogen dioxide (NO2), and traces of other nitrogen-oxygen combinations. These are all grouped together as NOx with x representing some suitable number. NOx is a very undesirable emission, and regulations that restrict the allowable amount continue to become more stringent. Released NOx reacts in the atmosphere to form ozone and is one of the major causes of photochemical smog. NOx is created mostly from nitrogen in the air. Nitrogen can also be found in fuel blends, which may contain trace amounts ofNH3, NC, and HCN, but this would contribute only to a minor degree. There are a number of possible reactions that form NO, all of which are probably occurring during the combustion process and immediately after. These include but are not limited to:
8 Oxides of Nitrogen (NOx) The higher the combustion reaction temperature, the more diatomic nitrogen, N2, will dissociate to monatomic nitrogen, N, and the more NOx will be formed. At low temperatures very little NOx is created.Although maximum flame temperature will occur at a stoichiometric air-fuel ratio (A/F = 1), maximum NOx is formed at a slightly lean equivalence ratio of about A/F = At this condition flame temperature is still very high, and in addition, there is an excess of oxygen that can combine with the nitrogen to form various oxides. In addition to temperature, the formation of NOx depends on pressure, air-fuel ratio, and combustion time within the cylinder, chemical reactions not being instantaneous. Therefore generally NOx is reduced in modern engines with fast-burn combustion chambers.The amount of NOx generated also depends on the location within the combustion chamber. The highest concentration is formed around the spark plug, where the highest temperatures occur. Because they generally have higher compression ratios and higher temperatures and pressure, CI engines with divided combustion chambers and indirect injection (IDI) tend to generate higher levels of NOx.In SI engines NOx can also be correlated with ignition timing. If ignition spark is advanced, the cylinder temperature will be increased and more NOx will be created.NOx is one of the primary causes of photochemical smog, which has become a major problem in many large cities of the world. Smog is formed by the photochemical reaction of automobile exhaust and atmospheric air in the presence of sunlight. NO2 decomposes into NO and monatomic oxygen:NO2 + energy from sunlight NO + O + smogMonatomic oxygen is highly reactive and initiates a number of different reactions, one of which is the formation of ozone:O + O O3
9 Emissions from an SI engine as a function of equivalence ratio Emissions from an SI engine as a function of equivalence ratio. A fuel rich air-fuel ratio does not have enough oxygen to react with all the carbon and hydrogen, and both HC and CO emissions increase. HC emissions also increase at very lean mixtures due to poor combustion and misfires. The generation of nitrogen oxide emissions is a function of the combustion temperature, being greatest nearstoichiometric conditions when temperaturesare the highest. Peak NOx emissions occur at slightly lean conditions, where the combustion temperature is high and there is an excess of oxygen to react with the nitrogen.
10 Engine Exhaust Gases and Normalized A/F Ratio Combustion process in the engine:CmHn + (m+n/4) O2 -> m CO2 + n/2 H2OExhaust gas emissions:CO, HC, Nox, CO2FuelconsumptionpowerNOxHCO2COH2Leancombustionsystem3-waycatalystCO2NOx / 1000 ppm HC / ppmNormalized A/F ratio lH2, CO, CO2, O2 / vol %76543211412108Current Air-/Fuel ratio=lStoichiometric Air- /Fuel ratiol < 1: rich mixture,fuel excessl = 1: stoichiometriccombustion14,7kg air for 1kg gasolinel > 1: lean mixure,air excess
11 Particulates (PM)The exhaust of CI engines contains solid carbon soot particles that are generated in the fuel-rich zones within the cylinder during combustion. These are seen as exhaust smoke and are an undesirable odorous pollution. Maximum density of particulate emissions occurs when the engine is under load at WOT. At this condition maximum fuel is injected to supply maximum power, resulting in a rich mixture and poor fuel economy. This can be seen in the heavy exhaust smoke emitted when a truck o railroad locomotive accelerates up a hill or from a stop. Soot particles are clusters of solid carbon spheres. These spheres have diameters from 10 nm to 80 nm (1 nm = 10-9 m), with most within the range of nm. The spheres are solid carbon with HC and traces of other components absorbed on the surface. A single soot particle will contain up to 4000 carbon spheres. Carbon spheres are generated in the combustion chamber in the fuel-rich zones where there is not enough oxygen to convert all carbon to CO2:CxHy + z 02 ~a CO2 + bH20 + cCO + dC(s)Then, as turbulence and mass motion continue to mix the components in the combustion chamber, most of these carbon particles find sufficient oxygen to further react and are consumed to CO2:C(s) + O2 ~ CO2Over 90% of carbon particles originally generated within an engine are thus consumed and never get exhausted. If CI engines would operate with an overall stoichiometric air-fuel mixture, instead of overall lean as they do, particulate emissions in the exhaust would far exceed acceptable levels.
12 Particulates (PM)Up to about 25% of the carbon in soot comes from lubricating oil components which vaporize and then react during combustion. The rest comes from the fuel and amounts to % of the fuel. Because of the high compression ratios of CI engines, a large expansion occurs during the power stroke, and the gases within the cylinder are cooled by expansion cooling to a relatively low temperature. This causes the remaining high-boiling-point components found in the fuel and lubricating oil to condense on the surface of the carbon soot particles. This absorbed portion of the soot particles is called the soluble organic fraction (SOF), and the amount is highly dependent on cylinder temperature. At light loads, cylinder temperatures are reduced and can drop to as low as 200°C during final expansion and exhaust blowdown. Under these conditions, SOF can be as high as 50% of the total mass of soot. Under other operating conditions when temperatures are not so low, very little condensing occurs and SOF can be as low as 3% of total soot mass. SOF consists mostly of hydrocarbon components with some hydrogen, S02, NO, N02, and trace amounts of sulfur, zinc, phosphorus, calcium, iron, silicon, and chromium. Diesel fuel contains sulfur, calcium, iron, silicon, and chromium, while lubricating oil additives contain zinc, phosphorus, and calcium.Particulate generation can be reduced by engine design and control of operating conditions, but quite often this will create other adverse results. If the combustion time is extended by combustion chamber design and timing control, particulate amounts in the exhaust can be reduced. Soot particles originally generated will have a greater time to be mixed with oxygen and combusted to CO2. However, a longer combustion time means a high cylinder temperature and more NOx generated. Dilution with EGR lowers NOx emissions but increases particulates and HC emissions. Higher injection pressure gives a finer droplet size, which reduces HC and particulate emissions but increases cylinder temperature and NOx emissions. Engine management systems are programmed to minimize NOx, HC, CO, and particulate emissions by controlling ignition timing, injection pressure, injection timing, and/or valve timing. Obviously, compromising is necessary. In most engines, exhaust particulate amounts cannot be reduced to acceptable levels solely by engine design and control.
13 A short emission history Vehicle emission legislation dates from 1966 when the California Air Resources Board (CARB) produced tailpipe emission standards for hydrocarbons (HC) and carbon monoxide (CO) within the State of California. The Environmental Protection Agency (EPA) and the Clean Air Act were introduced by the US Government in the 1960s. European and Japanese legislation followed from 1970.The increasing public awareness of the environmental and human health problems resulting from vehicular emissions plus legislative barriers to important automotive markets began a developmentimpetus that produced, and continues to produce, new emission-reducing technologies and requiring improved fuels. Catalytic converters required lead-free gasoline, thus the whole support infrastructure and pattern of vehicle registration has increasingly become dominated by emission legislation; the process whereby automotive technology, test technology and legislative requirements ‘leapfrog’ each other continues to this day.The huge improvements in individual engine emissions between 1965 and 2005 have meant that now the refining and formulation of lubricating oils and fuels have to be improved worldwide in order to benefit from the modern combustion control and after-treatment technologies.In spite of improved instrumentation and much reduced levels of pollutants produced by vehicles and engines, the basic form of much legislative testing has remained very similar to the original EPA methodology of the 1970s.
14 In general, all engine emission legislation consists of three component parts. 1. Test cycle describes the operation of the tested vehicle or engine. For light duty vehicles, it simulates the actual driving on the road in that it defines a vehicle velocity profile over the test time. For heavy duty and off-road engines where only the engine is tested on an engine dynamometer, the test cycle defines a speed and torque profile over the test time.2. Test procedure defines in detail how the test is executed, which measurement method and which test systems have to be used. It defines the test conditions and result calculations to apply. Probably the best known example of a legislative procedure is the use of sample bags in which an accumulation of diluted sample of exhaust gas resulting from a drive cycle is stored for analysis of content and concentration.3. Test limits, which define the maximum allowed emission of the regulated components in the engine exhaust. For light duty vehicles, the limit is expressed in mass per driving distance (g/km); for heavy duty vehicles, the limits are expressed in mass per unit of work (g/kWh).The three pillars of emission legislation
15 The European Emission Test Procedure The new NECE urban driving cycle (UDC) was devised to represent city driving conditions while the vehicle is on a chassis dynamometer. It is characterized by low vehicle speed, low engine load, and low exhaust gas temperature.Before the test, the vehicle is allowed to soak for at least 6 h at a test temperature of 20–30C. The entire cycle includes four ECE segments repeated without interruption, followed by one EUDC segment.The EUDC (extra urban driving cycle) segment has been added after the fourth ECE cycle to account for more aggressive high speed driving modes. The maximum speed of the EUDC cycle is 120 km/h.The UDC plus EUDC, called NEDC (New European Driving Cycle), is used for emission certification of light duty vehicles in Europe.There have been criticisms of these test procedures because of their lack of severity, in particular of the modest acceleration rates and it has been claimed that they underestimate emissions by 15–25 per cent compared with more realistic driving at the same speed, specifically test procedure does not include the air conditioning operation, greatly diffused also in Europe, that significantly increases real emission and consumption because of a higher engine load.
16 EUROPEAN TEST CYCLE NEW EUROPEAN EMISSION TEST PROCEDURE (NEDC) 1954001,0132,0263,0394,05211,007time (s)distance(km)Extra Urban Driving CycleVEL. MEDIA = 62 km/hVEL. MAX = 120 km/hLUNGHEZZA = 6,955 kmUrban Driving CycleVEL. MEDIA = 19 km/hVEL. MAX = 50 km/hLUNGHEZZA = 4,052 kmNew European Dri. CycleVEL. MEDIA = 33,6 km/hLUNGHEZZA = 11,007 kmvelocity’ (km/h)12010080604020ECE/UDCEUDCNEW EUROPEAN EMISSION TEST PROCEDURE (NEDC)Speed tolerance: ± 2 km/hTime tolerance: ± 1,0 sOne digit tolerance allowed for gear shiftingUDC cycle only at –7°C
17 The US Federal light duty exhaust emission Test Procedure (FTP-75) The US Federal Test Procedure (FTP-75) is a more complex procedure than the European test and is claimed to more realistically represent actual road conditions. The cycle is illustrated in the next page and, in contrast to the European test, embodies a very large number of speed changes.The cycle has three separate phases:1. a cold-start (505-s) phase known as bag 1;2. a hot-transient (870-s) phase known as bag 2;3. a hot-start (505-s) phase known as bag 3.The three test phases are referred to as bag 1, bag 2 and bag 3 because exhaustsamples are collected in separate sample bags during each phase. During a 10-mincool down between the second and third phase, the engine is switched off. The 505-s driving sequences of the first and third phase are identical. The total test time for the FTP 75 is 2457 seconds (40.95 min), the top speed is 56.7 mph, the average speed is 21.4 mph and the total distance covered is 11 miles.An higher average speed test cycle (highway cycle) is used in combination with the FPT one to measure the certified fuel economy/CO2 values.
18 USA CVS’75 TEST CYCLE Fase transitoria 0 505 s Fase stabilizzata 506 1371 sCiclo di guida USACVS ’75 (trans. + stab.)Velocità media 34,0 km/hVelocità max. 91,0 km/hPercorrenza 12.1 km/h
19 USA HIGHWAY TEST CYCLE Velocità media 78,0 km/h Velocità max. 96,0 km/hPercorrenza 16,5 km
20 Constant volume sampling (CVS) systems In both diesel and gasoline testing the vehicle exhaust gases are diluted with ambient air to prevent condensation of water in the collection system. It is necessary to measure or control the total volume of exhaust plus dilution air and collect a continuously proportioned volume of the sample for analysis. The use of a full flow CVS system is mandatory in some legislation, particularly that produced by the EPA and European Community. It may be assumed that this will change over time with developing technology and ‘mini-dilution systems’ will become widely allowed.CVS systems consist of the following major component parts:A tunnel inlet air filter in the case of diesel testing or a filter/mixing tee in the case of gasoline testing, which mixes the exhaust gas and dilution air in a ratio usually about 4:1. There is also a sample point to draw off some of the (ambient) dilution air for later analysis from a sample bag.The dilution tunnel, made of polished stainless steel and of sufficient size to encourage thorough mixing and to reduce the sample temperature to about 52° C. It is important to prevent condensation of water in the tunnel so in the case of systems designed for use in climatic cells, the air will be heated before mixing with gas.A proportion of the diluted exhaust is extracted by the bag sampling unit for storage in sample bags.The critical flow venturi controls and measures the flow of gas that the turbo-blowerdraws through the system.
21 The gas sample storage bag array. The analyzer and control system by which the mass of HC, CO, NOx, CO2 and CH4 is calculated from gas concentration in the bags, the gas density and total volume, taking into account the composition of the dilution air component.In the case of diesel testing, part of the flow is taken off to the dilution sampler containing filter papers for determining the mass of particulates over the test cycle.A typical layout of a CVS system based on a chassis dynamometer is shown in the next figure. The arrangement shown is for a cell certifying vehicles, either diesel or gasoline, to Euro 5 regulation. The system shown includes critical flow venturi (marked CFV) through which the gas/air is sucked by a fixed speed ‘blower’ fitted with an outlet silencer. Some systems control flow with a variable speed blower and a different type of ‘subsonic’ venturi.US Federal regulations require that the flow and dilution rates can be tested by incorporation of a critical flow orifice (CFO) check. This requires a subsidiary stainless steel circuit that precisely injects per cent propane as a test gas upstream of the mixing point, the CVS system dilutes this gas according to the flow rate setting allowing the performance to be checked at a stabilized temperature.The full flow particulate tunnel for heavy vehicle engines is a very bulky device. The modules listed above may be dispersed within a test facility because of the constraints of the building. The tunnel has to be in the cell near the engine and there are some legislative requirements concerning the distance from the tail pipe and the dilution point, ranging from 6.1m for light duty to about 10m for heavy duty systems.
22 Constant volume sampling (CVS) Test Stand scheme Aria ambienteGas di scarico“Sample”di gas di scarico diluiti“Sample” di ara ambiente (background)AspiratoreGas di scarico al caminoMixing PointAria ambiente di diluizioneFiltri aria ambienteBanco dinamometricoa rulliTraccia del ciclo di guidaVentilatore di raffreddamentoProgrammatore del ciclo di guida “Driver’s Aid”Banco di analisiConstant Volume Sampler (CVS)Controllo del banco dinamo-metricoUnità di controlloComputer per analisi in tempo reale (Emissioni e ciclo di guida)Critical Flow VenturiScambiatore di caloreSeparatore a cicloneSacchi di raccolta gasVentilatore raffreddamento: portata circa 9000 m3/hBanco a rulli:volano e potenza frenatura proprie del veicoloLaboratorio:temperatura e umidità controllate20 ÷ 30°C 5,5 – 12,0 g H2O/kg aria (31 – 67%)
23 20 years evolution of the European Emission Standards CO Emission Standards (g/Km)0.50.40.30.20.11970 Baseline = 35g/KmEuro 1Euro 2-81,6%Euro 3*Euro 419901995200020052010* Test cycle change
24 20 years evolution of the European Emission Standards Limiti emissioni HC + NOx (g/Km)1,2510.750.50.251970 Baseline = 9.5g/KmEuro 1Euro 2Euro 3*- 69%Euro 419901995200020052010* Test cycle changeAnni
25 20 years evolution of the European Emission Standards 19700.55PM Emission Standards (g/Km)0.50.40.30.20.11970 Baseline = 0.55g/KmEuro 1*Euro 2- 93%-95,5%Euro 3*Euro 419901995200020052010* Test cycle change
26 EURO 5 & EURO 6 Exhaust Emission Standards EMISSIONIGasoline / Gas (mg/km)DIESEL (mg/km)EURO 4EURO 5EURO 6CO1000500THC100-NMHC68NOx8060250180THC+NOx300230170PM5,0 (*)25,05,0(*) Only for direct injection gasoline engines27 marzo 2017
27 (Euro 6 – no difference between gasoline and Diesel engines) Diesel Vehicle European Emission Standards - towards “Fuel Neutral Standards”(Euro 6 – no difference between gasoline and Diesel engines)NOx(mg / Km)250EURO 4(2005/6)180EURO 5(2009/10)8060EURO 6(2014/15)525P.M.EURO 5/6 Benzina(mg / km)
28 Catalytic converterThe catalytic converters used in spark-ignition engines consist of an active catalytic material in a specially designed metal casing which directs the exhaust gas flow through the catalyst bed. The active material employed for CO and HC oxidation or NO reduction (normally noble metals, though base metals oxides can be used) must be distributed over a large surface area so that the mass transfer characteristics between the gas phase and the active catalyst surface are sufficient to allow close to 100 percent conversion with high catalytic activity. The most diffused configuration is shown in the figure.The system employs a ceramic honeycomb structure or monolith held in a metal can in the exhaust stream. The active (noble metal) catalyst material is impregnated into a highly porous alumina washcoat about 20 pm thick that is applied to the passageway walls. The typical monolith has square-cross-section passageways with inside dimensions of about 1 mm separated by thin (0.15 to 0.3 mm) porous walls.The number of passageways per square centimeter varies between about 30 and 60. The washcoat, 5 to 15 percent of the weight of the monolith, has a surface area of 100 to 200 m2/g.
30 Oxidation catalystsThe function of an oxidation catalyst is to oxidize CO and hydrocarbons to CO2 and water in an exhaust gas stream which typically contains about 12 percent CO2 and H2O, 100 to 2000 ppm NO, about 20 ppm SO2, 1 to 5 percent O2 , 0.2 to 5 percent CO, and 1000 to 6000 ppm HC, often with small amounts of lead and phosphorus. About half the hydrocarbons emitted by the SI engine are unburned fuel compounds. The saturated hydrocarbons (which comprise some 20 to 30 percent) are the most difficult to oxidize. The ease of oxidation increases with increasing molecular weight. Sufficient oxygen must be present to oxidize the CO and HC. This may be supplied by the engine itself running lean of stoichiometric or with a pump that introduces air into the exhaust ports just downstream of the valve.Because of their high intrinsic activity, noble metals are most suitable as the catalytic material. They show higher specific activity for HC oxidation, are more thermally resistant to loss of low-temperature activity, and are much less deactivated by the sulfur in the fuel than base metal oxides. A mixture of platinum (Pt) and palladium (Pd) is most commonly used. For the oxidation of CO, olefins, and methane, the specific activity of Pd is higher than that of Pt. For the oxidation of aromatic compounds, Pt and Pd have similar activity. For oxidation of paraffin hydrocarbons (with molecular size greater than C3), Pt is more active than Pd. Pure noble metals sinter rapidly in the 500 to 900 °C temperature range experienced by exhaust catalysts. Since catalytic behavior is manifested exclusively by surface atoms, the noble metals are dispersed as finely as possible on an inert support such as y- A2 O3 which prevents particle-to-particle metal contact and suppresses sintering. The particle size of the noble metal particles in a fresh catalyst is less than 50 nm. This can increase to about 100 nm when the catalyst is exposed to the high temperatures of the exhaust in vehicle operation. Typical noble metal concentrations in a commercial honeycomb catalyst are between 1 and 2 g/dm3 of honeycomb volume, with Pt/Pd = 2 on a weight basis. As a rough rule of thumb, the ceramic honeycomb volume required is about half the engine displaced volume. This gives a space velocity through the converter (volume flow rate of exhaust divided by converter volume) over the normal engine operating range of 5 to 30 per second.
31 Oxidation catalystsThe conversion efficiency of a catalyst is the ratio of the rate of mass removal in the catalyst of the particular constituent of interest to the mass flow rate of that constituent into the catalyst .The variation of conversion efficiency of a typical oxidizing catalytic converter with temperature is shown in the next figure. At high enough temperatures, the steady-state conversion efficiencies of a new oxidation catalyst are typically 98 to 99 percent for CO and 95 percent or above for HC. However, the catalyst is ineffective until its temperature has risen above 250 to 300°C. The term light-off temperature is often used to describe the temperature at which the catalyst becomes more than 50 percent effective.The above numbers apply to fresh noble metal oxidation catalysts; as catalysts spend time in service their effectiveness deteriorates. Catalysis involves the adsorption of the reactants onto surface sites of high activity, followed by chemical reaction, then desorption of the products. Catalyst degradation involves both the deactivation of these sites by catalyst poisons and a reduction in the effective area of these sites through sintering. Poisoning affects both the warm-up and steady-state performance of the catalyst. When poisoning occurs, catalytic activity is impeded through prolonged contact with interfering elements that either physically block the active sites or interact chemically with the active material.
32 Three-way catalystsIf an engine is operated at all times with an air/fuel ratio at or close to stoichiometric, then both NO reduction and CO and HC oxidation can be done in a single catalyst bed. The catalyst effectively brings the exhaust gas composition to a near-equilibrium state at these exhaust conditions; i.e., a composition of CO2 , H 2O and N2. Enough reducing gases will be present to reduce NO and enough O 2 to oxidize the CO and hydrocarbons. Such a catalyst is called a three-way catalyst since it removes all three pollutants simultaneously. Figure shows the conversion efficiency for NO, CO, and HC as a function of the air/fuel ratio. There is a narrow range of air/fuel ratios near stoichiometric in which high conversion efficiencies for all three pollutants are achieved. The width of this window is narrow, about 0.1 air/fuel ratios for catalyst with high mileage use, and depends on catalyst formulation and engine operating conditions.This window is sufficiently narrow to be beyond the control capabilities of an ordinary carburetor, though it can sometimes be achieved with sophisticated carburetors and fuel-injection systems. Thus closed-loop control of equivalence ratio has been introduced. An oxygen sensor in the exhaust is used to indicate whether the engine is operating on the rich or lean side of stoichiometric, and provide a signal for adjusting the fuel system to achieve the desired air-fuel mixture. Commercial three-way catalysts contain platinum and rhodium (the ratio Pt/Rh varying substantially in the range 2 to 17) with some A2O3, NiO, and CeO2 . Alumina is the preferred support material.Chemical reactions are shown in the slide 34 figure.
33 l Three-way catalysts NOx CO HC RICHLEANThree way catalyst conversion efficiency vs equivalence ratio
37 Diesel Particulate Filter A Diesel particulate filter, called DPF, is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot, and can at times (heavily loaded condition) attain soot removal efficiencies of close to 100%. A diesel-powered vehicle equipped with functioning filter will emit no visible smoke from its exhaust pipe.These filters have a cellular structure with individual channels open and plugged at opposite ends. Exhaust gases enter the open end, flow through the pores of the cell walls, and exit the filter through the adjacent channel. Soot particles are too large to flow through the pores, and they collect on the channel walls. Periodically the filter has to be regenerated burning off the accumulated particulate, either through the use of a catalyst (passive), or through an active technology, such as a fuel burner which heats the filter to soot combustion temperatures, through engine management dedicated strategies (the engine is set to run a certain specific way when the filter load reaches a pre-determined level, either to heat the exhaust gases, or to produce high amounts of NO2, which will oxidize the particulates at relatively low temperatures), or through other methods. This is known as "filter regeneration". Sulfur in the fuel interferes with many "regeneration" strategies, so almost all jurisdictions that are interested in the reduction of particulate emissions, are also passing regulations governing fuel sulfur levels.
38 Diesel Particulate Filter The most common filter is made of cordierite (a ceramic material also used as catalytic converter supports). Cordierite filters provide excellent filtration efficiency, are relatively inexpensive, and have thermal properties that make packaging them for installation in the vehicle simple. The major drawback is that cordierite has a relatively low melting point (about 1200 °C) and cordierite substrates could melt down during filter regeneration. This is mostly an issue if the filter has become loaded more heavily than usual, and is more of an issue with passive systems than with active systems, unless there is a system break down. New material technology has strongly improved the cordierite used for DPF due to the high interest for cost advantage.For the first application the most diffused filter material is silicon carbide, or SiC. It has a higher (2700 °C) melting point than cordierite, however it is not as stable thermally, making packaging an issue. Small SiC cores are made of single pieces, while larger cores are made in segments, which are separated by a special cement so that heat expansion of the core will be taken up by the cement, and not the package. SiC cores are usually more expensive than cordierite cores, however they are manufactured in similar sizes.
39 Diesel Particulate Filters (DPF) Channels alternatively open and closedExhaust flow and particulateCake filtration of the particulatePorous wall of the channel
40 SiC Morphology and Coating Filter wall surface is coated with a mixture of metal oxide containing noble metals like Platinum (Pt)This “active” layer :catalyzes soot combustion during regeneration fase and minimize residual sootreduces CO and HC emission during regeneration
44 Audi - Catalyzed DPF TDI 3.0 6-cylinder (A8 Quattro Vehicle) Main featureSmall close coupled DOC (about 1.0 dm3)Under-floor DOC (2.0 dm3) and catalyzed DPF (4.5 dm3)Catalyzed SiC DPF ~4.5 dm³DOC ~2.0 dm³
45 Diesel NOx catalystsThe conventional three-way catalyst technology used on petrol engines needs a 'richer' environment with less oxygen in the exhaust than is available on the Diesel engines (or directed injected stratified gasoline engines) to be able to reduce NOx, so new catalyst technology is required. Selective Catalytic Reduction, and NOx adsorbers are technologies that can be used in these lean applications.Selective Catalytic Reduction (SCR) was originally introduced on stationary power plants and stationary engines and then large engines such as those on ships, but it is now fitted to most new heavy-duty (i.e. truck and bus) diesel engines in Europe. It will equip most trucks in the US by Systems are also being introduced on light-duty diesel vehicles. The efficiency of SCR for NOx reduction also offers a benefit for fuel consumption. It allows diesel engine developers to take advantage of the trade-off between NOx, PM and fuel consumption and calibrate the engine in a lower area of fuel consumption than if they had to reduce NOx by engine measures alone. Particulate emissions are also lowered and SCR catalytic converters can be used alone or in combination with a particulate filter.In the SCR system, ammonia is used as a selective reductant, in the presence of excess oxygen, to convert over 70% (up to 95%) of NO and NO2 to nitrogen over a special catalyst system. Different precusors of ammonia can be used; one of the most common option is a solution of urea i water (e.g. AdBlue®) carefully metered from a separate tank and sprayed into the exhaust system where it hydrolyses into ammonia ahead of the SCR catalyst.AdBlue® is a stable, non-flammable, colorless fluid containing 32.5% urea which is not classified as hazardous to health and does not require any special handling precautions. It is made to internationally-recognised standards. Urea is used as an artificial fertilizer and is found in products such as cosmetics. The consumption of AdBlue® is typically 3-4% of fuel consumption for a Euro IV engine, and 5-7% for a Euro V engine, depending on driving, load and road conditions. A truck can have an AdBlue® tank which will hold enough urea solution to last for up to km. On-board systems alert the driver when it is time to fill up the urea tank.
46 SCR catalyst reactions reductantNO2NON2NOxNitrogen4 NH3 + 4 NO + O2 4 N2 + 6 H2O Standard reaction (... NO only)4 NH3 + 2 NO + 2 NO2 4 N2 + 6 H2O Fast reaction (...NO and NO2)4 NH3 + 3 NO2 3.5 N2 + 6 H2O Slow reaction (...if NOx contains > 50% NO2)AD Blue metering –tank system
47 Diesel NOx catalystsNOx adsorbers (NOx traps) adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation catalyst so that NO2 can be rapidly stored as nitrate on alkaline earth oxides. A brief return to stoichiometric* or rich operation for one or two seconds is enough to desorb (remove) the stored NOx and provide the conditions for a conventional three-way catalyst mounted downstream to reduce (destroy) NOx.Unfortunately, NOx adsorbers also adsorb sulfur oxides resulting from the fuel sulfur content. For that reason fuels with a very low sulfur content (European 'zero' sulfur fuel contains less than 10ppm sulfur) are required. The sulfur compounds are more difficult to desorb, so periodically the system has to automatically run a short 'desulfation' cycle to remove them.