Homogeneous Charge Compression Ignition (HCCI) offers many benefits over conventional engines. However, the start of combustion (SOC) is the key challenge for a practical HCCI engine. Numerical simulations are cheap and reliable way to study the HCCI engines. In this work, HCCI engine is modeled as a zero-dimensional reactor using MATLAB and CANTERA. The fuel considered is n-heptane, a widely used surrogate for commercial diesel fuel. SOC is also found to be very sensitive to intake conditions. So the effect of different parameters such as Air Fuel ratio, intake air temperature, inlet pressure, compression ratio and engine speed on HCCI combustion are investigated with heat transfer between wall and environment considered. The different mechanisms available for n-heptane are also investigated and found to have significant effect on the SOC. Therefore, caution has to be exercised while choosing mechanisms for HCCI studies. The simulations are successfully validated with an experimental work and previous computational work.