Abstract
HCCI combustion phenomenon involves rapid reaction kinetics of the order of few milliseconds and depends on several parameters such as temperature, pressure, thermo-chemistry of the cylinder gases, valve timings, etc. To render the dynamic operation of an HCCI engine feasible, the engine control system must effectively regulate the conditions that determine combustion. In order to tackle this problem, a mathematical model which is valid for a wide operating range is needed. This paper outlines the development of a variable valve actuation based control model of HCCI combustion. The outlined model is based on propane; however, it could be easily extended to other conventional fuels. The exhaust gas recirculation is modeled by a literature-based analytical model of residual gas fraction. The manifold dynamics are modeled in discrete time to effectively capture the mass flow rates into the cylinder. A mean burn duration model for propane was adopted using literature-based empirical correlations and integrated into the overall engine model to analyze and capture combustion-control issues from a system-level perspective. The model effectively captures the mass flow rates through the valves and is applicable for variations in speeds and fueling rates. The effects of change in speed on ignition timing, burn duration, in cylinder temperatures and pressures have been subsequently outlined in detail. The model is able to regulate the amount of fresh charge inducted per cycle, consequently enabling simultaneous control of both SOC and peak pressure even with change in engine speed. The indicated torque is estimated by the use of a deterministic equation which is a function of cylinder pressure and engine geometry. The model has been implemented in MATLAB to facilitate simulation studies. The results corresponding to different operating or control scenarios of an HCCI engine are subsequently discussed in detail. Experimental data obtained from literature was used to validate the model; the model validation shows that a fair agreement between the simulation and experimental data for any given engine can be obtained by proper tuning of the parameters.
Keywords: Controlled Auto ignition, HCCI, Mathematical modelling, Ignition timing control, IC Engines