In view of the current political debate, it can be assumed that the nitrogen oxide limits for commercial vehicles will be further reduced. This is also demonstrated by the currently voluntary certification of the CARB Optional Low NOX legislation, which requires nitrogen oxide emissions of 0.027 g/kWh. This corresponds to a reduction of 93 % compared to the current EU VI standard. Therefore, the optimization of EAT systems represents an essential research focus for future commercial vehicle applications.
One way to optimize the EAT system may be the usage of variable valve actuation. Existing investigations show an exhaust gas temperature increase with intake valve timing adjustment, also known as Miller timing. But the authors conclude that it cannot accelerate the warm up process. With regard to the effects on the exhaust aftertreatment system and the resulting tailpipe emissions, only improved HC and CO oxidation could be identified so far. In addition, a potential for improved NOX reduction is expected in the exhaust gas temperature increase.
Therefore, further investigations have been carried out. The investigations presented in this paper are based on test bench measurements of characteristic points in the WHTC on a single-cylinder commercial vehicle engine, which are then evaluated by a detailed pressure analysis. In combination with validated simulation models of a commercial vehicle series engine and its exhaust aftertreatment system, the effects of the Miller timing on the exhaust aftertreatment are investigated. The results show an improved SCR efficiency, DPF regeneration and also a decreased EAT cool down in the partial load range. Additionally, a change in the NO2/NOX ratio before the SCR system can be detected. This leads to lower nitrous oxide emissions, given by a suitable choice of valve timing. Therefore, variable valve timing can contribute to fulfil future emission legislations.