In this work, an experimental investigation was conducted to analyze dual fuel (DF) engine operation using synthetic biogas fuel under high load at constant percentage energy substitution rate (PES). The performance, ignition delay, and other combustion characteristics of engines operating in dual fuel mode (biogas/diesel), are compared to the conventional mode. The synthetic biogas, composed of 60% methane and 40% carbon dioxide, is the primary fuel which is blended with the air in the engine inlet manifold, whereas the pilot fuel is diesel. The equivalence ratio (fuel-air) (phi) was varied by changing air flow rate while the energy introduced into the engine remained constant for all the examined cases. Combustion analysis showed that with increasing 41, the ignition delay tends to become longer and the peak of heat release rate was increased. Furthermore, as phi increased from 0.35 to 0.7, HC and CO emissions were reduced by 77% and 58% respectively. The NOx emissions decreased at 60% PES by 24% while the BTE was improved by 13%.