With introductions of stringent diesel engine emission regulations, the DOC and DPF systems have become the mainstream technology to eliminate soot particles through diesel combustion under various operation conditions. Urea-based SCR has been the mainstream technical direction to reduce NOx emissions. For both technologies, low-temperature conditions or cold start conditions pose challenges to activate DOC or SCR emission-reduction performance. To address this issue, mini or full flow burner systems may be used to increase exhaust temperature to reach DOC light-off or SCR initiation temperature by combustion of diesel fuel. In essence, the burner systems incorporate a fuel injector, spray atomization, proper fuel / air mixing mechanisms, and combustion control as independent heat sources.To effectively integrate burners, Computational Fluid Dynamics (CFD) combustion models have been developed to investigate the influence of temperature distribution, fuel droplet transport and breakup, fuel vapor combustion, heat radiation and other relevant phenomena. Development work starts with a baseline mini-burner design, and measures have been taken to improve CFD modeling approaches and optimize the performance of downstream subsystems including connecting pipe, DOC, and mixers.Since burners work as heating sources, one performance factor of interest is the light-off delay time for substrates to reach catalyst activation temperature or active regeneration temperature. 0-D analytical formula and CFD models were developed to predict lightoff time history. The dependence of time history on substrate thermal property was investigated in multiple scenarios.