Assessment of Water Injection in a Heavy Duty Diesel Engine for NO _x Reduction Potential

Diesel engine pollutants include Oxides of Nitrogen (NO_x) and Particulate Matter (PM) which are traditionally known for their trade-off characteristics. It’s been a challenge to reduce both pollutants at the source simultaneously, except by efforts through low temperature combustion concepts. NO_x formation is dependent on the combustion temperature and thus the in-cylinder reduction of NO_x formation remains of utmost importance. In this regard, water injection into the intake of a heavy-duty diesel engine to reduce peak combustion temperature and thereby reducing NO_x is found to be a promising technology.

Current work involves the use of 1-D thermodynamic simulation using AVL BOOST for modeling the engine performance with water injection. Mixing Controlled Combustion (MCC) model was used which can model the emissions. Initially, the model validation without the water injector was carried out with experimental data. Further to this, the water injector module was integrated to inject decided quantity of water from the experiment to get optimum emission reduction. 3D CFD analysis using CONVERGE was carried out in-order to assess the water spray characteristics. A selection of water injector based on the required injection quantity was carried out and engine experiments were carried out based on the recommendations of the simulation results. European Steady-state Cycle (ESC) and European Transient Cycle (ETC) emissions were carried out using water injection to assess the potential emission reduction. Water to fuel ratio was varied from 3 to 16% in-order to study the impact on NO_x reduction. Results of CFD study showed placement of water injector in the intake air pipe down-stream the EGR mixing point helps to improve the evaporation characteristics of water spray. Engine experiment results show reduction of up to 13% NO_x by use of 16% water to fuel ratio in ESC cycle and up to 7% reduction in ETC cycle. Use of higher quantity of water is limited by droplet size of water spray and thus tends to increase the soot emissions. It is recommended to use water to fuel ratio in the range of 3-16% in-order to avoid the NO_x-PM trade off.