Exhaust aftertreatment
Exhaust gas aftertreatment is the term used for processes in which the combustion gases are cleaned mechanically, catalytically or chemically after they have left the combustion chamber or combustion chamber . According to this definition, other measures to reduce emissions that influence mixture formation or combustion (especially exhaust gas recirculation ) do not belong to exhaust gas aftertreatment. Certain emissions (e.g. lead, sulfur) have been reduced by limiting the fuel.
Although exhaust gas aftertreatment is also used in other areas, for example flue gas desulphurisation in thermal power plants , only exhaust gas cleaning specifically in motor vehicles will be discussed here due to the paramount importance of motorized individual transport for air quality .
General
In addition to the unchanged air components nitrogen and argon and the main combustion products water and carbon dioxide, the exhaust gases of an internal combustion engine contain smaller proportions of other substances (slightly more than 1% by volume). Some of these components are not limited (e.g. hydrogen , methane in certain legislations ), but the majority is classified and limited as a pollutant.
For the composition of the exhaust gas, molar or volume percentages are predominantly given, but representations in mass fractions are also common. Since the exhaust gas components have very different molecular weights, the numerical values especially for the light components (H 2 , H 2 O) vary greatly depending on the volume or mass.
Otto engines are currently (2019) mainly operated with a fixed air-to-fuel ratio (λ = 1) and therefore have a constant exhaust gas composition in the main components (see diagrams). In the case of lean-burn gasoline or diesel engines, a variable amount of air is added, so that the amount of exhaust gas increases and both the main combustion products water and carbon dioxide and the pollutants are diluted, i.e. are contained in the exhaust gas in a lower concentration for the same mass.
In exhaust gas legislation, therefore, it is not the proportion of pollutants in the exhaust gas that is limited, but the emission related to the work done by the engine or to a distance covered by the vehicle.
Without exhaust aftertreatment, the globally applicable pollutant limits cannot be met.
introduction
In Germany, development began in 1985 with the legally required introduction of the regulated three-way catalytic converter for vehicles with gasoline engines. Vehicle buyers who bought a new vehicle with a regulated catalytic converter before the due date were rewarded with tax discounts. For a while, there were also tax incentives for retrofitting an unregulated catalytic converter .
For vehicles with diesel engines, on the other hand, for a long time there was only exhaust gas aftertreatment without reducing nitrogen oxide emissions, which are, however, significantly lower in diesel engines. The oxidation catalytic converter was only able to oxidize the insufficiently burned exhaust gas components - carbon monoxide (CO) and hydrocarbons (C x H y ) - and render them harmless. With the emission standard EU3 in 2000, limit values for NO x were set for diesel engines and the SCR catalytic converter and the NO x storage catalytic converter were introduced. Particle filters, on the other hand, were first introduced in diesel engines, and particles were only partially limited in gasoline engines with the EU6 emissions standard from 2015.
Because of the principle-related differences between gasoline engine and diesel engine work processes, the options for exhaust gas aftertreatment must also be considered separately.
Exhaust aftertreatment in gasoline engines
In the case of a gasoline engine, the air ratio must be within the ignition limits, i.e. between approx. 0.6 and approx. 1.5. Beyond the ignition limits, uneven engine operation, misfires and an exorbitant increase in emissions and / or fuel consumption must be expected. It is not the operating behavior of the engine that requires an air ratio regulated exactly to 1. On the contrary: in the slightly rich area, the engine's willingness to perform would be greater, in the slightly lean area the specific (performance-related) consumption would be lower.
Regulated system
It is the catalytic converter that requires the air ratio to be regulated to λ = 1 . Only then can all components in the exhaust gas be reduced, both in the oxidative range and in the reductive range. This is where the name regulated catalyst comes from .
In the case of a gasoline engine, the power is adjusted by the amount of mixture used in the combustion chamber ( quantity control ). The amount is set by throttling , but the composition of the mixture remains basically unchanged at λ = 1.
The graph of the pollutant concentrations as a function of the air ratio shows that, calculated at λ = 1, there is no optimal condition for the raw emissions. If you follow the curves from left to right, i.e. in the direction of increasing λ, CO and HC (unburned hydrocarbons) have not yet subsided very far compared to the rich range. Nitrogen oxides (NO x ) have their maximum at around λ = 1.1 . However, λ = 1.0 is the optimum value for catalytic exhaust gas cleaning. Because in this narrow range there is a sufficiently high oxygen content for the oxidation of CO and HC. For the reduction of the nitrogen oxide molecules, however, it is necessary to always provide some potential reactants in the exhaust gas that can absorb the oxygen that is released. This is not possible in an atmosphere with a constant excess of oxygen.
Because of these relationships, the motor must be operated in a very narrow range λ = 1 ± 0.005 . One speaks of a catalytic converter window . This accuracy can only be achieved through precise regulation of the mixture with a λ probe as a sensor in front of the catalytic converter.
Unregulated system
In the first few years after the statutory introduction of the 3-way catalytic converter (regulated system) in 1985, existing engine concepts did not use a λ-1 control. The legislature has opened up this possibility, also for retrofitting vehicles that have already been registered. The tax exemption granted was lower than with the more complex solution of the regulated system. In this case, the engine was operated slightly lean in order to be able to oxidize the exhaust gas components CO and HC. A regulation was dispensed with, a control of the mixture composition was sufficient, a λ probe was not required. However, one had to forego a reduction in nitrogen oxides.
NO x storage catalytic converter
The abbreviations LNT (Lean NOx Trap) and NSC (NOx Storage Catalyst) are also used for the NO x storage catalyst.
The noble metal catalysts described above also fulfill their tasks of oxidative aftertreatment of CO and HC in lean-burn direct injection gasoline engines. The reductive reaction path relating to nitrogen oxide emissions is, however, ineffective in the lean exhaust gas. This engine concept therefore requires a new aftertreatment concept for compliance with worldwide NO x standards. In the meantime, NO x storage technology has established itself for the lean-burn gasoline engine , but currently (2019) almost no gasoline engines with lean operating modes are available.
With this technology, the nitrogen oxides emitted during lean operation are stored in the catalyst coating, the washcoat, and are thus temporarily stored. For regeneration, operating phases with a sub-stoichiometric - i.e. rich - mixture must be interposed cyclically. The CO and HC components then present in the raw exhaust gas are then used to absorb the oxygen in the nitrogen oxides. The oxidized and harmless exhaust gas components CO 2 , H 2 O are then present at the outlet of the catalytic converter , a more or less large residual amount of oxygen (O 2 ) and the reduced proportion of harmless nitrogen (N 2 ). The CO and HC components that are also present due to the rich engine operation are also oxidized and rendered harmless by the precious metal coating.
The rich engine operation for regeneration, which is inserted cyclically over a few seconds, is started by the engine control unit. The increased fuel injection during these phases causes a certain increase in fuel consumption, which is around 1–2% in the European driving cycle.
The problem with this technology is the temperature window to be maintained. If the exhaust gas temperatures are too low, a partial function, namely NO 2 formation (from NO), does not work. If the temperatures are too high, the stability of the nitrate formed decreases. In general, temperatures that are too high should be avoided as this will cause the catalyst to age faster.
Particle filter
Gasoline engines with direct injection are increasingly being equipped with gasoline particle filters, since without them the maximum number of particles with the latest Euro 6d-TEMP emissions standard can often no longer be complied with.
Exhaust aftertreatment in diesel engines
In contrast to the Otto engine, the diesel engine works with much higher air-fuel ratios (air ratios). In addition, the load on the engine is not set via the mixture volume, as is the case with a gasoline engine, but rather via the amount of fuel injected. Since the amount of air always remains the same, the fuel ratio changes depending on the set load. In the case of diesel engines, one speaks of quality control . The "air ratio" parameter is therefore in principle ruled out as a possibility for influencing the exhaust gas. A slight influence is only possible at full load by limiting the fuel supply. This allows the smoke number or particle emissions to be limited. In the past, that was the only point of view with regard to the emissions problem with diesel engines. However, that has changed since the mid-1980s. As with the gasoline engine, the emissions legislation has also been tightened significantly for the diesel engine.
Thermal reactors
Thermal reactors have been used in both gasoline and diesel engines. The mode of operation is the same: CO and HC should be "post-burned" through correspondingly high exhaust gas temperatures behind the engine outlet. However, the necessary temperatures are seldom reached or only briefly in diesel engines, especially diesel with direct injection . The effect of thermal reactors is therefore very limited. The situation is similar with the post-reaction of soot, with both the temperature and the residence time not being sufficient. Thermal reactors are therefore not very suitable for diesel engines.
Oxidation catalyst
Until recently, only oxidation catalysts were used in passenger car diesel engines. Hydrocarbons HC, carbon monoxides CO and soluble particles above a temperature of around 170 ° C can be oxidized. For a consistently high conversion rate it is important that
- the catalyst is not "poisoned" by sulfur oxidation products,
- no contamination of the catalytically active surfaces by z. B. Soot deposition occurs.
Both have an influence on the conversion rate of CO and HC. However, the danger is no longer high, since in the highly developed countries of the triad - i.e. Europe, the EU (25 states), USA (largely) and Japan - the sulfur content of diesel fuels has already been reduced to 10ppm in the past.
Nitrogen oxides remain unchanged in the oxidation catalytic converter, since a reduction is not possible with excess air or oxygen.
Selective Catalytic Reduction (SCR)
For the reduction of nitrogen oxides in SCR, catalysts and reducing agents - z. B. NH 3 (ammonia) - used. An aqueous urea solution is then injected , from which ammonia is formed by hydrolysis in the further course of the transport through the exhaust pipe . In the meantime, SCR has largely established itself in commercial vehicles. The standardized urea solution AUS 32 is available at many filling stations around the world. For a few years now there have also been a few models in the passenger car sector that reduce nitrogen oxides using SCR.
In order to reduce the consumption of urea solution, the SCR exhaust gas aftertreatment in vehicles is often illegally switched off or manipulated.
NO x storage catalytic converter
It works in the same way as the NO x storage catalytic converter described above in gasoline engines. The operation with a rich engine mixture, which is periodically necessary for regeneration, is more difficult to achieve with diesel engines and is not possible in all operating ranges.
Particle filter
The strict limitation of particle emissions is countered with regenerative particle filters . So far, filters made of porous ceramics have prevailed, which have to be regenerated with a certain load of soot particles. The soot particles are burned during regeneration, and this process is initiated by the engine control unit. Due to the increasing soot load in the filter, the exhaust gas back pressure increases steadily, so that the engine has to use more and more power to expel its exhaust gas. The differential pressure across the particle filter is sensed as a measure of the need for a regeneration to be carried out. Above approx. 600 ° C the particles burn off to form CO 2 . Such high temperatures do not occur in the diesel engine during normal driving, so that separate measures are necessary for this. The engine spreader initiates the start of the combustion by delaying the injection so that part of the injected fuel continues to burn in the exhaust pipe at correspondingly high temperatures. Once the soot combustion has started, the heat generated in the filter ensures that the rest of the particle load is also captured. The injection time can then assume the normal map values again.
Operating temperatures of catalysts
The effectiveness of a catalyst, the conversion or conversion rate, depends, among other factors, on the operating temperature. There are practically no reactions below approx. 250 ° C. This is the reason why, within the first few seconds of a driving cycle carried out for the purpose of type testing of vehicles, it is decided whether the test will be passed or not. Because within these first seconds, the engine is not yet warm and emits a lot of CO and HC. However, the catalytic converter is also not yet at operating temperature and does not sufficiently convert the emitted pollutants.
There are several strategies for raising the exhaust gas temperature quickly: you work with secondary air , or you place the catalytic converter near the engine. With the latter measure, however, there is - at least in the case of gasoline engines - the risk that the temperatures will become too high in other operating ranges, for example in the vicinity of the rated output. Because temperatures above 1000 ° C destroy the catalyst. Good conversion rates and a long service life are given at 400 ° C to 800 ° C.
literature
- Richard van Basshuysen , Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6
- Hans-Hermann Braess, Ulrich Seiffert (Hrsg.): Handbook Automotive Technology 3rd Edition, Friedr. Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2003, ISBN 3-528-23114-9
- F. Schäfer, R. van Basshuysen: Pollution reduction and fuel consumption of internal combustion engines in passenger cars , Springer-Verlag, Vienna, New York, 1993, ISBN 3-211-82485-5
See also
- Exhaust gas cleaning
- Vehicle catalytic converter
- Three way catalytic converter
- Selective catalytic reduction
- Secondary air system
- Lambda control
- Lambda probe
Individual evidence
- ↑ Horst Bauer: Exhaust technology for gasoline engines . Ed .: Robert Bosch GmbH. 6th edition. Stuttgart 2002, ISBN 3-7782-2020-9 , pp. 43 .
- ↑ Sibylle Wilke: Emission-reducing requirements in traffic. In: Umweltbundesamt.de. Federal Environment Agency, accessed on February 5, 2016 .
- ^ Robert Bosch GmbH (ed.): Kraftfahrtechnisches Taschenbuch . 25th edition. Vieweg, 2003, ISBN 3-528-23876-3 , pp. 719 ff .
- ^ Bosch, Kraftfahrtechnisches Taschenbuch, edition 24, p. 566.
- ↑ Braess, Seiffert: Vieweg Handbook Motor Vehicle Technology, 3rd edition, p. 199.
- ^ Rainer Klose: The cold start dilemma. Preheat catalysts. In: empa.ch . February 27, 2020, accessed March 2, 2020 .