Numerical Investigation of Diesel-Spray-Orientated Piston Bowls on Natural Gas and Diesel Dual Fuel Combustion Engine

Low combustion efficiency and high hydrocarbon emissions at low loads are key issues of natural gas and diesel (NG-diesel) dual fuel engines. For better engine performance, two diesel-spray-orientated (DSO) bowls were developed based on the existing diesel injector of a heavy-duty diesel engine with the purpose of placing more combustible natural gas/air mixture around the diesel spray jets. A protrusion-ring was designed at the rim of the piston bowl to enhance the in-cylinder flame propagation. Numerical simulations were conducted for a whole engine cycle at engine speed of 1200 r/min and indicated mean effective pressure (IMEP) of 0.6 MPa. Extended coherent flame model 3 zones (ECFM-3Z) combustion model with built-in soot emissions model was employed. Simulation results of the original piston bowl agreed well with the experimental data, including in-cylinder pressure and heat released rate (HRR), as well as soot and methane emissions. Turbulence kinetic energy, IMEP and methane emissions of the DSO piston geometries were compared with that of the original piston geometry. The results showed that both in-cylinder pressure and heat release rate of both DSO piston geometries increased due to higher turbulence kinetic energy comparing to the original piston geometry while methane emissions significantly decreased. Methane combustion with the DSO piston geometries tended to occur far away from the piston center compared to that with the original piston design at the same crank angle due to the protrusion-ring at the rim of piston bowl. Overall, the methane combustion with the DSO pistons was enhanced in the whole combustion chamber, which led to improved combustion efficiency and lower methane emissions.