To address the poor matching between piston bowl and spray due to in-cylinder charge density being increased

a simulation model of a heavy-duty diesel engine with stepped-lip piston bowl was established with CONVERGR

a computational fluid dynamics software

and was calibrated with the experimental data. The effects of the lip diameter

lip depth

and bottom arc radius of piston bowl on the combustion process and heat transfer loss at low and medium speeds with full load and high charge density were studied

and the shape of piston bowl was optimized to further improve the indicated thermal efficiency. Results show that a relatively flat piston bowl with small lip depth and moderate concave arc radius should be applied when the charge density in the cylinder was higher. The optimization brought higher heat release rate in the later stage of combustion and lower heat transfer loss

making the indicated thermal efficiency increase from 49.00% to 49.47%. At the same time

the NOx emissions were basically unchanged while the soot emissions were reduced by 50%.