A new combustion model is presented that is capable of simulating the diesel combustion process. This combustion process is broken into three phases: low-temperature ignition kinetics, kinetically limited reactions, and mixing limited combustion. Low-temperature ignition kinetics are modeled using the Shell model. For combustion limited by mixing, a probability density function combustion model is used, which utilizes a commercial flamelet solver. For kinetically limited (premixed) combustion, an Arrhenius rate is used. To account for temperature fluctuations, this rate is weighted with a temperature probability density function. To transition between the premixed and diffusion burning modes, a transport equation for premixed fuel is solved. The ratio of fuel in a computational cell that is premixed is used to determine the combustion mode. This combustion model has been implemented into the KIVA-3V code. Results show that this combustion model accurately simulates the diesel combustion process.