A thermodynamics-based homogeneous charge compression ignition engine model for adaptive nonlinear controller development

Abstract

Low-temperature combustion modes, such as homogeneous charge compression ignition, represent a promising means to increase the efficiency and to reduce significantly the emissions of internal combustion engines. Implementation and control are difficult, however, owing to the dependence of the combustion event on the chemical kinetics rather than an external trigger. This work describes a nonlinear control-oriented model developed for a single-cylinder homogeneous charge compression ignition engine, which is physically based on a five-state thermodynamic cycle. This model is aimed at capturing the behavior of an engine which utilizes fully vaporized gasoline-type fuels, exhaust gas recirculation, and intake air heating in order to achieve homogeneous charge compression ignition operation. The onset of combustion, which is vital for control, is modeled using an Arrhenius reaction rate expression which relates the combustion timing to both the charge dilution and the temperature. Despite the fact that homogeneous charge compression ignition combustion is indeed fast, it is not perfectly instantaneous and therefore requires some finite amount of time to occur. To account for this phenomenon within the model, a Δθ term is added which shifts the point of instantaneous combustion from the start of combustion to a point of very-high-energy release based on experimental heat release data. The model is validated against experimental data from a single-cylinder compression ignition engine operating under homogeneous charge compression ignition conditions at two different fueling rates. Parameters relevant to control such as the combustion timing, peak in-cylinder pressure, and pressure rise rates from the simulation agree very well with the experiment in both operating conditions. The extension of the model to other fuels is also investigated via the octane index of several different gasoline-type fuels. Since this nonlinear model is developed from a controls perspective, both the output and the state update equations are formulated such that they are functions of only the control inputs and the state variables, therefore making them directly applicable to state-space methods for control. The result is a discrete-time nonlinear control model which provides a platform for developing and validating various nonlinear control strategies.