A four-mode low-dimensional model for the in-cylinder combustion process in an internal combustion engine is developed. The lumped parameter ordinary differential equation model is based on two mixing times that capture the reactant mixing limitations inside the cylinder and mixing limitations caused by the input and exit stream distribution. For a given inlet and operating conditions, the model predicts the exhaust composition of regulated gases (total unburned HCs, CO, and NOx) as well as the in-cylinder pressure and temperature. The model is able to capture the qualitative trends observed with change in fuel composition (gasoline and ethanol blending), air/fuel ratio spark timing, engine load, and speed. The results show good qualitative and fair quantitative agreement with the experimental results published in the literature and demonstrate the possibility of such low-dimensional model for real-time control. Improvements and extensions to the model are discussed. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2472-2492, 2011