ABSTRACT Many pre- and post-treatment strategies have been instigated in compression ignition engines to meet the stringent vehicular emissions norms. The partially pre-mixed charged compression ignition (PCCI) combustion technique has shown a significant reduction in emissions without much affecting the efficiency of the conventional diesel engine by emitting low nitrogen oxides and smoke emissions simultaneously. However, the PCCI engine has also shown some limitations by producing higher hydrocarbon (HC) and carbon monoxide (CO) emissions from the engines. The toroidal combustion chamber has proven its potential to reduce the HC and CO emissions from the conventional engine. Therefore, in the current experimental study, PCCI engine characteristics were investigated with toroidal piston geometry and compared with conventional engine characteristics having hemispherical combustion chamber (HCC). The experiments were performed on single-cylinder, four strokes, direct injection, diesel engine modified to operate on PCCI mode. The homogeneous mixture for PCCI combustion was obtained with an external mixture formation technique with a fuel vaporizer (retrofit device). The tests were conducted with 15% and 30% diesel vapor without and with 10% exhaust gas recirculation and then compared with the conventional engine. The investigation results show that better air–fuel mixing results in low engine emissions. NOx emissions were reduced by 29% and 36%, and smoke opacity was decreased by 33% and 38% with 15% and 30% diesel vapor with EGR with a marginal decrement in thermal efficiency compared to conventional engine.