The high efficiency of the diesel engine has made it the primary propulsion source for truck applications worldwide. In Europe, the penetration of diesel engines in passenger cars is growing rapidly in an effort to meet the stringent CO2 future emission targets. However, future regulations call for increasingly lower NOx emission levels. Extremely low engine out NOx emission levels, if at all possible, have significant negative effects on fuel economy and engine life-cycle cost. As such, a viable strategy is to tune the engine for optimum fuel consumption, while relying on an exhaust aftertreatment system to reduce NOx emissions.
The Selective Catalytic NOx Reduction (SCR) enables the engine to run in the overall speed/load range at best fuel economy, while the exhaust aftertreatment system reduces the NOx emissions significantly. The NOx reduction efficiency of an “engine map controlled”, open-loop SCR system is about 65 percent. The efficiency can be improved significantly by using closed-loop control and a highly efficient oxidation catalyst.
This highly effective SCR technique requires low sulfur fuel for particulate emission control and an infrastructure for the reducing agent. In Europe, urea will be the reducing agent and the low sulfur fuel will be available in 2004. A comparison of total life-cycle cost for different EURO-4 techniques based on a Class 8 long-distance truck demonstrates the advantage of the SCR technology.
The most important disadvantage of all alternatives to SCR other than cost is the increase of CO2 emission. Results of some fundamental research indicate significant disadvantages for the NOx adsorber technology. Therefore, DaimlerChrysler will introduce SCR in an initial version to fulfill EURO-4 emission limits. A system with further improved efficiency will be available for EURO-5.