A novel 2-zone combustion chamber concept (patent pending) was developed using multi-dimensional modeling. At minimum volume, an axial projection in the piston divides the volume into distinct zones joined by a communication channel. The projection provides a means to control the mixture formation and combustion phasing within each zone. The novel combustion system was applied to reactivity controlled compression ignition (RCCI) combustion in both light-duty and heavy-duty diesel engines.Results from the study of an 8.8 bar BMEP, 2600 RPM operating condition are presented for the light-duty engine. The results from the heavy-duty engine are at an 18.1 bar BMEP, 1200 RPM operating condition.The effect of several major design features were investigated including the volume split between the inner and outer combustion chamber volumes, the clearance (squish) height, and the top ring land (crevice) volume.The results show significant improvements in the peak rate of pressure rise (ROPR), unburned hydrocarbon (UHC) emissions, and carbon monoxide (CO) emissions relative to open chamber RCCI combustion. Ultra-low soot and NOx emissions as well as a significant fuel consumption improvement, relative to conventional diesel combustion, were obtained with the novel 2-zone combustion chamber.The novel 2-zone combustion system allows later, ignition controlling, diesel injection timings to be used while maintaining low UHC and CO emissions. Therefore control over combustion phasing and the peak ROPR are enhanced.