Abstract
In this paper, two approaches are used to formulate more accurate nonlinear engine models; one applies the Newtonian method; the other uses the Lagrangian method to derive the nonlinear governing equations for internal combustion engine kinematics and dynamic forces. Also included is the discussion of the discrepancy between the nonlinear time-varying inertia model and previous constant inertia model in their shaft velocity and acceleration fluctuations, and in the reconstructed cylinder pressure using the Sliding Observer. (A)