Influence of H2 enrichment for improving low load combustion stability of a Dual Fuel lightduty Diesel engine

Dual Fuel (DF) combustion can help to reduce the environmental impact of internal combustion engines, since it may provide excellent Brake Thermal Efficiency (BTE) combined with ultra-low emissions. This technique is particularly attractive when using biofuels, or fuels with a low Carbon content, such as Natural Gas (NG). Unfortunately, as engine load decreases and the homogeneous NG-air mixture tends to become very lean, the high chemical stability of NG can be a serious obstacle to the completion of combustion. As a result, BTE drops and UHC and CO emissions become very high. A possible way to address this problem could be the addition of hydrogen (H2) to the NG-air mixture. In this paper, a numerical study has been carried out on an automotive Diesel engine, modified by the authors in order to operate in both conventional Diesel combustion and DF NG-diesel mode. A previous experimental characterization of the engine is the basis for the CFD-3D modeling and calibration of the DF combustion process, using a commercial software. The effects on combustion stability and emissions of different NG-H2 mixtures (six blends with 5%, 10%, 15%, 20%, 25%, and 30% by volume of hydrogen) are numerically investigated at a low load (BMEP = 2 bar, engine speed 3000 rpm). The results of the CFD-3D simulations demonstrate that NG-H2 blends are able to decrease strongly CO, UHC, and CO2 emissions at low loads. Advantages are also found in terms of thermal efficiency and NOx emissions.