Evaluation of Valve Train Variability in Diesel Engines

The continuously decreasing emission limits lead to a growing importance of exhaust aftertreatment in Diesel engines. Hence, methods for achieving a rapid catalyst light-off after engine cold start and for maintaining the catalyst temperature during low load operation will become more and more necessary. The present work evaluates several valve timing strategies concerning their ability for doing so. For this purpose, simulations as well as experimental investigations were conducted. A special focus of simulation was on pointing out the relevance of exhaust temperature, mass flow and enthalpy for these thermomanagement tasks. An increase of exhaust temperature is beneficial for both catalyst heat-up and maintaining catalyst temperature. In case of the exhaust mass flow, high values are advantageous only in case of a catalyst heat-up process, while maintaining catalyst temperature is supported by a low mass flow. Another focus of simulation was on analyzing the exhaust temperature gaining effects relevant for the considered alternative valve timings. Simulation results have shown that an early exhaust valve opening, a late intake valve closing and the deactivation of cylinders is of particular interest for exhaust thermomanagement. Besides the validation of simulation results, the main focus of measurements was on analyzing effects which are not covered by simulation (transient operation, cold engine conditions 298 K, emissions, test cycles), which is exemplarily shown here for early exhaust valve opening (EEVO). In addition to the methods based on alternative valve timings, conventional exhaust thermomanagement methods like a retarded combustion were also considered in simulations. Moreover, an electrical heating device was considered experimentally and compared to EEVO in the FTP75 with regard to SULEV30 for passenger cars.