Cooled EGR - A Key Technology for Future Efficient HD Diesels 980190
The paper reports on achievements obtained in an ongoing development program which is part of a european EUREKA joint research project named EFFLED (EFFicient Low Emission Diesel) being performed at AVL in cooperation with the companies DAF Trucks, Serck Heat Transfer, Robert Bosch and the Community of the City of Rotterdam. The main objective of this project is the development and refinement of a venturi supported exhaust gas recirculation (EGR) system for a turbocharged and intercooled heavy-duty (HD) diesel engine enabling map controlled cooled EGR rates which are high enough to achieve future low NOx emission standards at acceptable fuel consumption level. In addition to EGR, further technologies have been investigated, which may be required to meet future exhaust emission standards. As a result of these investigations it was found that for EURO 3 standards (NOx / PM = 5/0.1 g/kWh in the new european steady state cycle - ESC) cooled EGR alone leads to the required NOx reduction without worsening fuel consumption from EURO 2 levels. However, for meeting indicated EURO 4 standards (NOx / PM = 3/0.1 g/kWh in new european transient cycle - ETC), the use of a particulate trap will most probably be required with today's diesel fuel qualities in spite of only modest increase of soot due to EGR. Therefore, two different particulate filter systems as potential candidates have been tested, and their pros and cons compared.
INCREASINGLY STRINGENT exhaust emission regulations, which are expected to come into force around the turn of the century and thereafter worldwide in industrialized countries will require NOx and particulate emisson standards to be reduced by up to 60% from today's (1998) levels, see Fig. 1. To meet these future emission standards, the application of new technologies including exhaust gas aftertreatment systems will be indispensable, especially in view of maintaining or even improving the thermal efficiency of HD diesel engines.
Two different routes for meeting the future european standards are currently under discussion, Fig. 2. The application of SCR systems using urea-water solution as a NOx reducing agent (SCR = selective catalytic reduction) is assessed as the most effective NOx reduction technology in the long range /1,2,3,4/, with the benefit of allowing combustion to be optimized for best fuel efficiency and lowest particulates.
However, logistics for urea-water solution is a prerequisite, and on vehicles space for dosage system, urea tank, SCR catalyst and a modified exhaust muffler must be foreseen. Furthermore, durability and on-board-diagnostic (OBD) issues of the system need to be solved.
Alternatively, fig. 2 also suggests that cooled Exhaust Gas Recirculation (EGR) is offering a similar NOx reduction potential as does the SCR approach. Only moderate EGR rates would be necessary to meet EURO 3 standards with fuel consumption being equal to EURO 2 level and with minor effects on particulates /5,6,7/, whereas for EURO 4 at current status of development the use of a soot trap for reducing particulates seems to be a must due to the higher EGR rate requirements. In either case, a variety of potential problems have to be investigated and solved before EGR can be accepted in the heavy-duty diesel engine environment. More specific, major concerns are related to the durability of EGR components, EGR control requirements for transients, engine fouling (EGR cooler, intake ducts), and increased wear due to increased soot-in-oil and/or accelerated oil degradation.
In general, brake specific fuel consumption (BSFC) is of utmost importance to the truck user, and in the future it is expected to be improved continuously. Conducive measures, already in use today on a variety of engines, are the application of multi valve (3V & 4V) cylinder heads instead of two-valve heads, the use of fuel injection systems with increased fuel pressures in conjunction with reduced swirl levels, improved lube oil control, and the use of electronic engine management, especially for injection timing. Thus, fuel consumption for a given NOx level could be improved by up to 5 %. However, the magnitude of improvement depends also on the type of the test cycle.
The change of the european certification test cycle /8/ from the current 13-Mode ECE R49 to the new 13-Mode ACEA/OICA (or new european steady state cycle - ESC) for EURO 3 and finally to the FIGE transient test (or european transient cycle - ETC) for EURO 4, as shown in Fig. 3, makes the assessment of test cycle fuel consumption and emission results more complicated due to the different load/speed operating modes and weightings factors as well as due to transient effects.
The influence of the various test cycles on emissions and fuel consumption of a given EURO 2 engine is shown in Fig. 4. For NOx emissions being nearly at the same level (approx. 6.5 g/kWh) for all three test cycles, particulates and cycle fuel consumption decrease in the ESC due to the higher load factor and operation shifted to the mid speed range in this cycle, whereas in the ETC both fuel consumption and particulates increase, mainly as a result of transient operation.
In view of this background and the development directions discussed previously, a joint research project named EFFLED (EFFicient Low Emission Diesel, EUREKA No. 1593) was initiated by AVL List GmbH in cooperation with companies DAF Trucks N.V., Serck Heat Transfer, Robert Bosch GmbH, and the Community of the City of Rotterdam. The main objective of this project is the development and refinement of a venturi supported EGR system for a turbocharged and air/air intercooled HD diesel engine enabling map controlled cooled EGR with EGR rates high enough to achieve the NOx emission levels required beyond 2000 at competitive fuel consumption level. Since with current diesel fuel qualities particulates inevitably tend to increase with the required high EGR rates, also the use of particulate trap oxidizers is considered within the EFFLED project.
In the following sections, achievements of the still ongoing project are presented, with emphasis on both, the EGR system and its performance, and the behaviour of two different particulate trap systems being envisaged for future field operations of the engine, which finally is to be built into a DAF container vehicle and operated within the Community Vehicle Fleet of the City of Rotterdam.