Exhaust Temperature Management for Diesel Engines Assessment of Engine Concepts and Calibration Strategies with Regard to Fuel Penalty

Both, the continuous strengthening of the exhaust emission legislation and the striving for a substantial reduction of carbon dioxide output in the traffic sector depict substantial requirements for the development of future diesel engines. These engines will comprise not only the mandatory diesel oxidation catalyst (DOC) and particulate filter DPF but a NO_x aftertreatment system as well - at least for heavier vehicles. The oxidation catalysts as well as currently available NO_x aftertreatment technologies, i.e., LNT and SCR, rely on sufficient exhaust gas temperatures to achieve a proper conversion. This is getting more and more critical due to the fact that today's and future measures for CO₂ reduction will result in further decrease of engine-out temperatures. Additionally this development has to be considered in the light of further engine electrification and hybridization scenarios. To maintain the high NO_x conversion level in the aftertreatment system adequate temperature management strategies will be beneficial. This includes not only conventional calibration measures such as throttling, split-main or post injection but also further evolution of the engine hardware such as cam phasing.

Split-cooling and other thermal management measures have the potential to reduce CO₂ emissions and increase exhaust temperature during cold start at the same time. But also highly variable valve trains open up a wide spread of potential thermo management measures. In this paper different concepts for exhaust gas temperature management will be analyzed and compared. The assessment will focus on the effectiveness regarding the exhaust temperature increase and the related fuel economy penalty. Further factors such as robustness, effects on operation strategy and required software functions and cost are discussed as well. The engine used in this study was an optimized in-line 4-cylinder research engine to achieve best combustion behavior for lowest engine-out emissions and highest fuel efficiency. The investigations were carried out with pilot injection and simulated closed loop combustion control. The engine used in this study is capable to meet Euro 6 emissions limits.

With all accomplished variations a significant increase in temperature downstream low pressure turbine can be achieved. The quantity of pilot and post injection plays an important role for emission formation under warm and under cold conditions. By using an exhaust cam-phaser CO-, HC- and NO_x emissions can be significantly reduced distinguishing exhaust cam-phasing from the other investigated strategies.