This study delves into the ignition delay and in-cylinder pressure behavior of waste plastic-derived biodiesel mixed with octanal, both individually and blended with neat diesel, within a diesel engine environment. Investigations were conducted using a Kirloskar TV1 single-cylinder diesel engine test rig under various engine loading conditions. The focus was on evaluating the ignition delay, a crucial parameter influencing combustion efficiency and emissions, and in-cylinder pressure characteristics, offering insights into combustion processes. The biodiesel blends, comprising waste plastic-derived biodiesel at 10, 20, and 30 along with octanal, each blend at a 10% concentration referred to as WPOCT10, WPOCT20, WPOCT30, respectively, were compared against neat diesel. Results reveal significant variations in ignition delay and in-cylinder pressure profiles among the tested fuel blends. The waste plastic-derived biodiesel exhibits distinct combustion characteristics owing to its low calorific value, while the addition of octanal further influences overall cetane index of fuel enhancing ignition behavior. Out of all the mixes, WPOCT10 outperformed in the shortest ignition delay in 0, 50, 100% engine load as 0.26 oCA, 0.30 oCA, and 0.23 oCA than neat diesel. Similarly, in peak cylinder pressure at 0, 50, 100% engine load, the WPOCT10 peaked at 0.17, 0.18, and 4.3 bar than neat diesel. This research contributes to a deeper understanding of the combustion properties of waste plastic-derived biodiesel and its blends with octanal and neat diesel. The findings offer valuable insights for optimizing engine performance, emissions, and fuel efficiency in diesel engines utilizing alternative fuel formulations. Furthermore, this study underscores the potential of waste plastic-derived biodiesel as a viable and sustainable alternative fuel source, while highlighting the importance of tailored fuel blends for optimal engine operation in the transition toward a greener future.