Modeling and control of exhaust recompression HCCI using split injection

Homogeneous charge compression ignition (HCCI) is currently being pursued as a cleaner and more efficient alternative to conventional engine strategies. This paper presents an approach for modeling the effect of a small pilot injection during recompression on combustion in an HCCI engine with exhaust trapping, and a controller that uses the timing of this pilot injection to control the phasing of combustion. The model is incorporated into a nonlinear physical model presented in previous work that captures the effect of fuel quantity and intake and exhaust valve timings on work output and combustion phasing. It is seen that around the operating points considered, the effect of a pilot injection can be modeled as a change in the Arrhenius threshold, an analytical construct used to model the phasing of combustion as a function of the thermodynamic state of the reactant mixture. The relationship between injection timing and combustion phasing can be separated into a linear, analytical component and a nonlinear, empirical component. A feedback controller based on this model is seen to be effective in tracking a desired load-phasing trajectory and enables steady operation at low load conditions.