Fuel atomization and dispersion are complicated physical phenomena which play crucial roles in diesel engine combustion. The models adopted for simulation of these processes strongly affect the accuracy of the calculation results. The purpose of present study is to numerically investigate the effects of various droplet collision and spray breakup models on in-cylinder fuel spray characteristics as well as in-cylinder pressure. The numerical simulation procedures are based on a Lagrangian drop and an Eulerian gas formulation. This paper tries to indicate the differences between the effects of these various models on spray characteristics as well as in-cylinder pressure in a caterpillar heavy-duty diesel engine and present some practical reasons for probable distinctions. The simulation results showed the significant influence of different spray breakup models on fuel spray penetration and in-cylinder pressure. Experimental data have been used to validate the numerical results.