Three-dimensional simulations were conducted to study combustion inside a direct injection diesel engine. A complex physical phenomenon of spray and fuel combustion interaction experienced in IC engine. This spray-combustion interaction was evaluated by Spray penetration length and lift-off length of flame. To study this interaction, constant volume chamber simulations with engine-like conditions were conducted. These simulations were conducted to mimic the experimental results of ECN at Sandia national laboratory for n-heptane as a surrogate fuel. After this validation, in-cylinder combustion model was validated against in-house experimental results. Only the closed part of the engine cycle (inlet valve closing to exhaust valve opening) is modeled. A flamelet based model with 106 species skeleton mechanism was used to validate lift-off length and in-cylinder combustion. The numerical results for spray penetration and lift-off length showed good agreement with experimental results. For diesel engine simulation was validated for pressure trace and NOX. The results are in good agreement with experimental results with very less error percentage. A parametric study was then done for different SOI and the pressure trace and NOX results are compared with in-house experimental results. Turbulence modeling was performed using the standard k-ε model.