The thermal conditions of an engine structure, in particular the wall temperatures, have been shown to have a great effect on the HCCI engine combustion timing and burn rates through wall heat transfer, especially during transient operations. This study addresses the effects of thermal inertia on combustion in an HCCI engine.
In this study, the control of combustion timing in an HCCI engine is achieved by modulating the residual gas fraction (RGF) while considering the wall temperatures. A multi-cylinder engine simulation with detailed geometry is carried out using a 1-D system model (GT-Power®) that is linked with Simulink®. The model includes a finite element wall temperature solver and is enhanced with original HCCI combustion and heat transfer models. Initially, the required residual gas fraction for optimal BSFC is determined for steady-state operation. The model is then used to derive a map of the sensitivity of optimal residual gas fraction to wall temperature excursions. Finally, the model is used to determine the necessary changes in the control strategy to compensate for the thermal inertia effects during transients.
The results show that, when the non-equilibrium transient wall temperature difference from the steady-state value is moderate, load and speed transitions in the HCCI operating regime can be managed by using controllers based on steady-state RGF maps. However, with large non-equilibrium transient wall temperature excursions from the steady-state, the results indicate the need for wall temperature dependent calibration of combustion parameters for best fuel economy and knock-free performance. Compensating for the wall temperature effects results in improved fuel economy while satisfying knock and misfire constraints.