An experimental investigation was carried out to study two process parameters of thermocatalytic pyrolysis of high density polyethylene (HDPE) plastic waste, and the liquid yield of pyrolysis (plastic oil) was analyzed over its working characteristics in a direct injection compression ignition (DICI) engine. A series of pyrolysis experiments with four different heating rates (5, 10, 15, and 20 degrees C/min) were performed in identical environments with the objective of analyzing the heating rate and residence time on end products. It was found that the liquid yield was maximum at the heating rate of 10 degrees C/min and the residence time decreases with an increase in the heating rate due to variation of reaction kinetics at different heating rates. The characterization of the plastic oil revealed that the major constituents belong to alkene, alkane, and alcohol functional groups; totally 90 six fuel range hydrocarbons were present ranging from C-5 to C-28, and the other thermophysical properties were comparable with that of petrodiesel. The plastic oil was tested for its working characteristics as a main fuel in a DICI engine with the aim of analyzing the effect of chemical constituents present in it, and the experimental results showed that no hardware modifications were required for employing the plastic oil in place of petrodiesel in DICI engine. The brake thermal efficiency and specific fuel consumption were higher by 4.53% and 7.16%, respectively, for plastic oil than petrodiesel due to the presence of alcohols, alkenes, and constituents with higher molecular weight hydrocarbons in the plastic oil. A reduction of 25.9% in ignition delay was evidenced with plastic oil than with petrodiesel at a rated load which might possibly be due to the structural influence, i.e., the absence of aromatics with ring type molecular structure in the plastic oil.