Future diesel engine legislation Tier 4 / Stage V and EU6d demand further improvements to reduce CO2 while keeping the already low NOx emissions levels. For US trucks a more strict limit of 0.2 g/bhp-hr NOx emissions need to be achieved. In this trade-off, system costs and complexity of the after-treatment are defining the constraint in which the common rail fuel injection system layout has to be defined. The increase of rail pressure was in the past the major step to control the soot emissions in view of low engine-out NOx emissions by applying massive EGR. With the on-going development of NOx-aftertreatment by Selective Catalytic Reduction (SCR), conversion efficiencies of up to 97% allow to reduce the EGR usage and rail pressure usage. In that context, the steepness of injection rate, the nozzle flow rate and the injection pressure are remaining parameters to control the NOx emissions. A shallow injection rate in combination with larger nozzle flow rates is beneficial to reduce the NOx emissions thanks to a reduced premixing of fuel with air. To study this effect, the latest solenoid injector with improved magnetic actuation is utilized. The influence of the steepness of injection rate is studied on a 6 cylinder heavy-duty Diesel engine on 3 representative part- load points of the WLTC cycle. Depending on the engine-out NOx emissions requirements, two scenarios are considered. In case of a strong NOx aftertreatment, the shallow injection rate steepness is beneficial for the fuel consumption. In case of less NOx aftertreatment, high EGR rates are required and soot emissions can be controlled through the steepness of injection rate.