Exhaust gas recirculation
The exhaust gas recirculation ( EGR , engl. EGR Exhaust Gas Recirculation) is to reduce the emission of nitrogen oxides (NO x is used), which in the combustion of fuel in gasoline engines , diesel engines , gas turbines , boilers , etc. will. It makes sense to reduce the formation of nitrogen oxides already during combustion, because the prescribed emission limit values are not or only high with measures of exhaust gas aftertreatment ( selective catalytic reduction , NO x storage catalytic converter ), which lead to a chemical reduction of the nitrogen oxides Effort achievable. This has particularly been the case since the introduction of the Euro VI limit values for cars in September 2014. In diesel engines, exhaust gas recirculation is one of the most important measures to reduce nitrogen oxide emissions. In gasoline engines, exhaust gas recirculation also helps to lower gas exchange losses and thus also reduces fuel consumption in the partial load range. With stratified EGR, the recirculated exhaust gas is deliberately distributed inhomogeneously - outside of the spark plug or injection valve area - in order to have only a small proportion of exhaust gas in the core area of the combustion.
In connection with large systems, the process is referred to as flue gas recirculation .
Goal setting
At high combustion temperatures, environmentally harmful nitrogen oxides are produced in the engine. The higher the combustion temperatures in the cylinder and the longer the period above 2300 K combustion temperature, the higher the proportion of nitrogen oxides in the exhaust gas. This relationship is described in the equations of the Zeldovich mechanism . A decisive criterion is the combustion air ratio , as can be seen in the adjacent graphic. Below a stoichiometric air ratio λ = 1 (i.e. in the case of rich mixtures) the NO x proportion is very low because the oxygen molecules are required for the oxidation of the carbon and hydrocarbon molecules. With air conditions above λ = 1 (i.e. with increasingly lean mixtures), the nitrogen oxide emissions initially rise significantly, as the supply of free oxygen increases. The greater the air ratio λ is set, the greater the oxygen supply, but the combustion temperature drops as the air ratio increases further. The maximum of nitrogen oxide formation is reached in the graph at λ = 1.1. If, in this example, the engine is operated even leaner, the nitrogen oxide components fall again due to the falling combustion temperature.
When describing the objective, a distinction must be made between gasoline and diesel engines. In principle, however, the following relationship applies: When a fuel-air mixture is burned in the combustion chamber of an internal combustion engine, the hydrocarbon molecules of the fuel used are oxidized with atmospheric oxygen. The oxygen used is completely used up in the gasoline engine ( stoichiometric combustion ), so that there are almost no oxygen molecules in the exhaust gas.
According to the Zeldovich mechanism , nitrogen oxide formation increases exponentially with the combustion temperature. If exhaust gas is added to the clean air used, the oxygen concentration of the mixture drops. Because of the lower oxygen concentration that this causes, less fuel is injected in order to still burn it completely. This reduces the heating energy of the mixture as well as the reaction speed and combustion temperature (and consequently the engine output). Therefore, fewer nitrogen oxides are formed; Cooling the recirculated exhaust gas increases the effect.
Systematics
- Internal EGR: The exhaust valve - regulated by the camshaft adjustment - remains open temporarily during the intake stroke , which means that exhaust gas is sucked back into the cylinder.
- External EGR: The exhaust gas is taken from the exhaust gas tract and fed to the intake tract via a line, a cooler and a valve.
- Low pressure EGR: The extraction takes place after the exhaust gas treatment, the introduction before the turbo compressor .
- High-pressure EGR: The extraction takes place before the turbine of the turbocharger and the exhaust gas aftertreatment, the introduction after the intercooler and the throttle valve .
- Reusable EGR: This is a combination of low and high pressure EGR. It is used, for example, in the Mercedes-Benz OM651 engine for the A 220 CDI vehicle series.
Diesel engines
The use of EGR in diesel engines has always been characterized by the conflicting objectives of ensuring low nitrogen oxide emissions while minimizing particle emissions. High exhaust gas recirculation rates result in low nitrogen oxide emissions, but promote the formation of soot particles during combustion. Since both are limited by the existing exhaust gas standards - for example the Euro 6 standard currently valid in Europe - it is important to carefully consider how much exhaust gas can be returned to combustion. As the engine load increases, the tendency of the diesel engine to emit soot particles increases. In such operating states, care must be taken to reduce the high return rate, which also promotes soot, in order to avoid visible soot emissions - for example when the vehicle is accelerating. The engine management is responsible for this. Rapid detection of the respective engine operating point and rapid activation of the feedback valve enable the feedback rate to be adjusted in good time. This is only possible if the design of the feedback valve allows for such quick responses. Pneumatic control, as was common in the past, or electromagnetic actuation may not be fast enough. If you want to build on such a concept anyway, for reasons of cost, you generally have to limit the level of the return rates and try to limit the nitrogen oxides with other measures.
Petrol engines
With gasoline engines, the objective for using an exhaust gas recirculation system is different. The focus here is not on minimizing pollutant emissions, but on reducing fuel consumption. This is influenced by the gas exchange losses, especially at part load.
In the partial load operation of a conventional Otto engine, the throttle valve generates a negative pressure in the intake duct, which reduces the intake air mass and consequently the engine output is reduced. At the same time, the gas exchange work increases. This has always been the principle of power regulation in gasoline engines. By admixture of exhaust gas throttling of the clean air is reduced for a given amount of fuel, hence the associated losses (to pumping losses ) is reduced.
If it is a gasoline engine with direct injection into the combustion chamber and charge stratification , in partial load operation the fuel burns from the outset with excess air, i. H. with throttle valve wide open. This means that the throttling losses are smaller and consumption is already reduced. An ignitable mixture is only around the spark plug. In these operating states, the EGR has an effect similar to that of a diesel engine: the combustion temperature is lowered and nitrogen oxide emissions are reduced.
functionality
The production of nitrogen oxides is reduced by supplying an inert gas. Such an inert gas is, for example, exhaust gas, a small part of which is fed back into the combustion chamber. The rapid oxidation of fuel molecules is hindered by the presence of exhaust gas molecules. The temperature peaks and the NO x emissions are thus reduced. This effect is supported by the higher heat capacity of the main components of the exhaust gas, carbon dioxide and water (in gaseous state).
The exhaust gas is returned to the intake chamber by mixing part of the intake fresh air through a pipe . The proportion of the recirculated exhaust gas must not become too high, otherwise the particle emissions ( soot ) will increase too much. The limit depends on the load and engine speed. The recirculation is regulated by an exhaust gas recirculation valve installed outside of the engine (external exhaust gas recirculation). Depending on the system, with all 4-stroke engines, the exhaust gas can also be regulated within certain limits during the intake stroke through an open exhaust valve (internal exhaust gas recirculation).
High pressure exhaust gas recirculation (HP EGR)
Since the recirculated exhaust gases from the so-called hot or HP EGR have high temperatures (up to 400 ° C), the addition of the inert exhaust gas to the fresh air in the intake manifold would result in reduced air masses. As a result, the filling decreases, the engine works with a lower air ratio and the average temperature of the fresh charge also increases. This would be counterproductive. Euro 3 concepts still had a non-cooled EGR. Vehicles from Euro 4 usually have a cooled EGR line. This is especially true for heavier vehicles. However, there are now much more complex EGR concepts.
Low pressure exhaust gas recirculation (LP EGR)
LP-EGR is taken further back, after the diesel particulate filter (DPF), from the exhaust system and fed back in before the turbocharger , which with its cooler and low-particle EGR offers several advantages over HP-EGR:
- The intake air is not heated by hot exhaust gas, which improves the degree of delivery .
- The exhaust gas mass flow is only reduced after the turbine of the exhaust gas turbocharger, which works more effectively and responds better to load changes
A technical problem with LP-EGR that needs to be solved is that if the exhaust gases fall below the dew point, condensate can form and cause corrosion, especially on the compressor.
Euro 6 engine concepts are also being considered that have a cooled LP EGR section and, due to dynamic and cold start aspects, an uncooled HP EGR section.
Exhaust gas recirculation in gasoline engines
In gasoline engines, exhaust gas recirculation is consciously used to lower specific fuel consumption in the partial load range. The addition of non-flammable gas makes it possible to open the throttle valve further with the same desired engine power and to reduce the flow losses at this point. With the targeted use of exhaust gas recirculation, the consumption of a gasoline engine under partial load can be reduced with only minor disadvantages in terms of drivability.
Control of exhaust gas recirculation
Exhaust gas recirculation mainly takes place in the partial and medium load range. The heavier the vehicle, the higher the load spectrum; heavy commercial vehicles use EGR even at full load. New gasoline engine concepts also use full-load EGR as an alternative to enrichment. The maximum exhaust gas recirculation rates are around 60% for diesel engines, around 50% for direct-injection gasoline engines and around 20% for conventional intake manifold gasoline engines.
The internal EGR (gasoline engines only) is now controlled via adjustable camshafts . In older camshaft drives without adjustment, the internal EGR - if it exists at all - is structurally defined.
Appropriate facilities are required for EGR control. The position feedback is used purely for pre-controlling the EGR rate, which is a function of pressure and temperature at the inlet. For the actual regulation of the air mass, an air volume or air mass meter ( HFM ) and / or good air mass models that allow control are always required .
In vehicles with on-board diagnosis (OBD), the exhaust gas recirculation is monitored (depending on the vehicle manufacturer) via HFM (air volume deviation), via the position feedback from the EGR valve and, in some cases, via other pressure and temperature sensors in the intake manifold, if available. The sensors report to the control unit whether the system is regulating and actuating. If there is no feedback, the engine management system or the engine control unit activates the MIL = ( Malfunction Indicator Light ) in the instrument cluster.
distribution
Many cars with Euro 3, most with Euro 4 and absolutely all cars with Euro 5 emissions standard have an exhaust gas recirculation system.
In the commercial vehicle sector, cooled EGR is the standard up to the Euro 3 norm. From Euro 4, which became mandatory in October 2005 for newly developed vehicle types, the cooled EGR is still partly available from MAN and Scania . Other manufacturers rely on the more complex principle of selective catalytic reduction (SCR), in which the aqueous urea solution AUS 32 also has to be refueled.
Conflicting goals and problem solving
In general, there is a trade-off between low soot and nitrogen oxide emissions. The former occur increasingly at low combustion chamber temperatures, while significantly more NO x is formed at high temperatures . The performance is also negatively influenced by EGR, the highest possible performance cannot be achieved with regard to the nitrogen oxide emissions.
The use of low combustion temperatures also results in an increase in carbon monoxide , soot and unburned hydrocarbons in the exhaust gas. A lambda probe , which measures the oxygen concentration in the exhaust gas and regulates the fuel supply accordingly, can minimize this undesirable effect as far as possible in conjunction with a vehicle catalytic converter . In order to completely avoid this conflict of objectives and to meet future emissions standards, NO x storage catalytic converters are increasingly being installed in modern cars . Most commercial vehicle manufacturers, and increasingly also car manufacturers, are relying on selective catalytic reduction (SCR). The engine is optimized for maximum performance and minimum soot formation, both of which are only possible at high combustion temperatures. The resulting nitrogen oxide is then reduced to nitrogen and water in a second step in the SCR catalytic converter .
Mechanical problems
Increased soot accumulation in the exhaust gas recirculation valve often leads to a defect. Poor throttle response and ultimately a significant loss of power or increased consumption can then be the result, depending on the position in which the valve remains. Most vehicles that are equipped with an EGR system indicate the defect by illuminating the engine control or workshop lamp and by saving an error code. In order to continue to meet the emission limits of the corresponding Euro standard, a repair is definitely necessary. In the case of newer vehicles with the Euro 4 and Euro 5 standard, it is only possible to shut down the EGR valve to a limited extent with conventional means. The engine control unit usually detects an error due to an excessively high air mass, since the specified travel of the exhaust gas recirculation valve means that an exhaust gas component can be expected, but this is missing due to the shutdown.
Individual evidence
- ↑ Exhaust stratification on a gasoline engine with BDE and spray-guided combustion process. The stratification of recirculated exhaust gas offers great potential for reducing nitrogen oxide emissions, especially in gasoline engines with direct injection and spray-guided combustion processes. Institute for Piston Machines - KIT , archived from the original on March 14, 2014 ; accessed on October 14, 2016 .
- ↑ K. Borgeest: Electronics in automotive engineering. Vieweg, 2nd ed., 2010.
swell
- Kolbenschmidt Pierburg AG ( Memento from March 15, 2014 in the Internet Archive ) Manufacturer's brochure with information on how EGR valves work (PDF, 563 kB)
- F. Schäfer, R. Basshuysen : Reduction of pollutants and fuel consumption of car internal combustion engines. Springer-Verlag Vienna New York, 1993.
- F. Schäfer, R. Basshuysen: Handbook Internal Combustion Engine: Fundamentals, Components, Systems, Perspectives. Vieweg + Teubner Verlag, 2007.
- Reif Wallentowitz: Handbook of automotive electronics: Basics, components, systems, applications. Springer-Verlag, 2006.
