Current exhaust gas emission regulations can only be well adhered to through optimal interplay of combustion engine and exhaust gas after-treatment systems. Combining a modern diesel engine with several exhaust gas after-treatment components (DPF, catalytic converters) leads to extremely complex drive systems, with very complex and technically demanding control systems. Current engine ECUs (Electronic Control Unit) have hundreds of functions with thousands of parameters that can be adapted to keep the exhaust gas emissions within the given limits. Each of these functions has to be calibrated and tested in accordance with the rest of the ECU software. To date this task has been performed mostly on engine test benches or in Hardware-in-the-Loop (HiL) setups.In this paper, a Software-in-the-Loop (SiL) approach, consisting of an engine model and an exhaust gas treatment (EGT) model, coupled with software from a real diesel engine ECU, will be described in detail. A virtual (SiL) test bench is realized with which the diesel engine software functions can be calibrated without any special hardware, using industry- standard calibration tools like INCA from ETAS. Furthermore, this SiL concept is very versatile and can be used in the design of future technologies or new powertrain concepts not currently available in hardware form.The SiL methodology and its potential for increasing the efficiency of the development and calibration process, together with the tools and models used to set up and run the virtual test bench, are the focus of this paper.