The present work investigates the performance and emission characteristics of pongamia oil and diesel fuelled direct injection compression ignition (DICI) and homogeneous charge compression ignition (HCCI) engine. The primary objective of the work is to investigate the feasibility of application of unmodified pongamia oil in Diesel engine and to estimate the maximum fraction of diesel fuel replaced by the neat pongamia oil. This investigation also deals with the HCCI operation using unmodified pongamia oil. In DICI mode the neat pongamia oil is admitted into the engine in the form of pongamia oil and diesel blends. The blend that offers highest diesel replacement is considered as the test blend and it is tested further to find its maximum possible brake thermal efficiency by changing the engine operating parameters. The selected maximum blend is then tested in the new setting of the engine to determine the maximum possible performance and emission characteristics. The conventional emissions of DICI engine such as NO and smoke are disappeared in the homogeneous charge compression ignition mode of operation. The HCCI engine tested in the present work is fuelled by 40% neat pongamia oil and 60% diesel fuel through direct injection and vapour induction, respectively. The ignition or combustion phasing of the HCCI operation is carried out by the exhaust gas recirculation method. The amount of exhaust gas re-circulation governs the timing of combustion. The results of the experiments show that the neat pongamia oil performed well in HCCI mode and offered approximately ten times lower NO and smoke emission. Finally, the results of the DICI mode and HCCI mode are compared with each other to reveal the truths of neat pongamia oil in heterogeneous and homogeneous combustion. © 2022 Elsevier B.V., All rights reserved.