Single cylinder diesel engines are widely used as a power source of gen-sets, three wheelers, agricultural machines in small household applications in Indian automotive sector. Due to its simple structure and mechanism, the operation, maintenance and repairing for this kind of engines is also easily possible. Now, in order to satisfy stringent emission norms there is a need for the development of the above engines.In this paper, software-predicted performance at different load conditions for a constant speed diesel engine (Gen-set application) are being determined by using GT-Power software. This performance is being validated with the actual Experimented Performance Results. In the above-mentioned simulation we basically used "EngCylCombDIJet Simulation" model. The validated model is used for further work. Critical parameters were listed based on the sensitivity analysis on the base model. Critical parameters were optimized for achieving the desired fuel injector nozzle hole diameter.A very close validation with less than 1% difference is achieved at full load. While a less than 10% difference is achieved at part loads. The simulation model predicts the Power, Torque, BSFC, A/F Ratio, Cylinder pressure, Exhaust Temperature and NOx with a very good accuracy.The optimized nozzle hole diameter of the above engine is validating with the experimental results and compared with the baseline model. Thus, an improvement in Performance and Emission results is achieved.