The implementation of aftertreatment systems in passenger car diesel engines, such as a lean NOx trap (LNT) and a diesel particulate filter (DPF), requires an in-cylinder postinjection (POI) for a periodic regeneration of those aftertreatment systems to consistently reduce tail-pipe emissions. Although the combustion and emission characteristics are changed from the normal engine operating conditions due to the POI, POI is generally applied with a look-up table (LUT) based feedforward control because of its cost effectiveness and easy implementation into the engine management system (EMS). However, the LUT-based POI control necessities tremendous calibration work to find the optimal timing to supply high exhaust gas temperature or enough reductants such as carbon monoxide (CO) and hydrocarbon to regenerate the aftertreatment systems while maintaining low engine-out smoke emissions. To solve this problem, we propose a novel combustion analysis method based on the cylinder pressure information. This method investigates the relation between the POI timing with the exhaust emissions and compensates the combustion phase shift occurred by the engine operating condition changes, such as the engine speed and injection quantity. A burning rate of fuel after a location of the rate of heat release maximum (BRaLoROHRmax) was derived from the combustion analysis. A mass fraction burned X% after a location of the rate of heat release maximum (MFBXaLoROHRmax) was determined using the BRaLoROHRmax and main injection (MI) quantity. Nonlinear characteristics of the exhaust emissions according to POI timing variations and the combustion phase shift due to the engine operating condition changes can be easily analyzed and compensated in terms of the proposed MFBXaLoROHRmax domain. The proposed method successfully evaluated its utility through the engine experiments for the LNT and DPF regeneration.