In this study, a new combustion model for simulating the diesel combustion process is introduced. This model was verified by comparing numerical simulations to experimental data for a six-mode test cycle using a Caterpillar 3400 series engine. Additional comparisons are made for baseline cases for both a Caterpillar 3500 series engine and a Sandia optical access engine. In the combustion model, reactions limited by diffusion are modeled using a probability density function (PDF) model. For kinetically limited (premixed) combustion, an Arrhenius rate is used. To include effects of temperature fluctuations, this reaction rate is weighted by a temperature probability density function. A transport equation for premixed fuel was implemented to transition between the premixed and diffusion burning modes. The ratio of fuel in a computational cell that is premixed is used to determine the combustion mode.Results using this new combustion model closely match the experimental data for pressure, heat release and engine out emission levels. As this model accurately captures the chemistry-turbulence interaction, there are very few adjustable constants. This new model can be readily implemented into existing computational fluid dynamics (CFD) codes to simulate the diesel combustion process with low computational overhead.