Automotive Engineer looks at the developments in exhaust gas recirculation (EGR). EGR is an increasingly important measure for both types of engines, providing a cost-effective method of reducing engine-out emissions and fuel use. But to get the best performance and cost benefit, it's essential to integrate the EGR system at the earliest possible stage of the engine design process. Doing so allows you to reduce catalyst loading, and some aftertreatment can be avoided altogether. Diesel engines operate with excess air. It makes them efficient but means they produce relatively high NOx emissions. Euro II systems comprised a simple on/off EGR valve, which evolved to a modulated valve for Euro III. For Euro IV a lot of engines adopted cooled EGR. At the same time, EGR rates have increased from 10-15% in the 1990s to as much as 70% today, enabling more advanced combustion modes that can reduce NOx even further, such as premixed low-temperature combustion. If EGR levels are too high, you can get smoke, higher hydrocarbon and carbon monoxide emissions. Variable geometry turbos (VGTs) can help here. Closing the turbine vanes gives good boost and low-speed transient response but also produces more driving pressure for EGR. Effective cooling increases the EGR density, maintaining the oxygen levels needed for complete combustion. EGR coolers with bypass valves optimise the temperature. Meeting future emissions legislating means engines must run EGR just after start-up. Hot EGR is used initially. Cool EGR would cause misfiring and emissions spikes. It's a technology that tends to feature in the clean diesels destined for the US. They also have lean NOx traps (LNTs) or complex catalytic reductionsystem with a urea additive, making aftertreatment costlier still. Many OEMs will adopt the same strategy for Euro VI. EGR can also provide significant reductions in combustion noise by reducing the burn rate and giving lower rates of pressure rise within the cylinder. Gasoline ER has evolved too. EGR valves were introduced to naturally aspirated engines to reduce engine-out NOx levels by up to 50% through reduction of peak combustion temperatures. The trend now is for internal EGR through cam phasers, says Ricardo's gasoline product group. Depending on the number and how they are used, phasers enable tuning of the torque curve, improved NOx emissions and fuel economy and cost savings by deleting the external EGR system. The EGR layout ist under development. Mahle ist developing a very fast acting (3ms) air pulse valve which, although not an essential part of the circuit, enables full use of the dynamic pressure ratios across the engine. Gasoline exhaust gas is much hotter than diesel's - development of cooled EGR is challenging. Stratified charge direct injectin offers fuel economy gains but system costs are high. Mercedes and BMW offer the technology mainly on premium six-cylinder engines. EGR is still used to limit NOx but is supplied via EGR valve, not by cam phasing. Like diesel, gasoline EGR rates are limited by combustion stability. The real advantage comes with control. Cylinder pressure sensors will appear in some of VW's V6 diesels later this year; and in GM's in 2009. Advanced gasoline engines will get them too. Suppliers are matching eingine development with new EGR technologies. Valeo, Pierburg and Magneti Marelli agree that EGR, not NOx aftertreatment, is the most efficient means of meeting future emissions legislation. Exhaust suppliers have developed catalysts for the EGR system to prevent fouling of the coolers. Pierburg and Valeo both say that this is feasible but low-pressure systems should mitigate the need. Bosch says: EGR would constitute around 10 or even 20% of the further development for Euro VI. Most suppliers are working on greater EGR modules integration, saving packaging space and improving performance and cost. Diesel experience helps when applying cooled EGR to turbocharged gasoline direct injection engines.
Diminishing emissions. Coming around again
Emissionsminderung - kommt immer wieder zurück
Automotive Engineer ; Feb ; 37-39
2008
3 Seiten, 1 Bild
Article (Journal)
English
Abgasemission , Abgasrückführung , Kraftfahrzeugmotor , Dieselmotor , Benzinmotor , Emissionsminderung , Drehmoment , Drehmomentenverlauf , Abgastemperatur , Verbrennungsgeräusch , Stickstoffoxid , Abgasturboaufladung , Abgasturbolader , Übergangsantwort , OEM (Autohersteller) , Automobilzulieferer , Stickoxidentfernen , geregelter Katalysator , Kraftstoffverbrauch , Direkteinspritzung
Automotive engineering | 2008
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EDITORIAL - Coming up short, again.
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