Tailoring fuel injection to control NOx

The next big step to help heavy-duty diesel engines meet stricter emissions regulations involves adapting the fuel-injection system to the combustion needs.

Future diesel engine legislation, such as the EU's Stage V and possible Tier 5 in the U.S. for off-highway, demand further improvements to reduce CO₂ while keeping the already-low NOx emissions levels. For on-highway trucks in the U.S., a stricter limit of 0.2 g/bhp-hr NOx emissions needs to be achieved, with even tougher ultra-low NOx limits on the horizon. In this trade-off, system costs and complexity of the aftertreatment are defining the constraint in which the common-rail fuel injection system layout is defined.

In the past, the increase of rail pressure was the major step to control the soot emissions in view of low engine-out NOx emissions by applying massive EGR (exhaust gas recirculation). With the on-going development of NOx-aftertreatment by selective catalytic reduction (SCR), conversion efficiencies of up to 97% allow a reduction in the EGR 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.