Deterministic characteristics of cyclic combustion dynamics of conventional and advanced reciprocating engines

Abstract

This work investigates the characteristics of cyclic combustion dynamics of lean/diluted SI, HCCI, and RCCI engines. Nonlinear dynamical and chaotic methods such as bifurcation diagrams, phase space reconstruction, recurrence plots, and symbolization, along with statistical methods, are employed to analyze the cyclic dynamics represented by cycle-resolved combustion data. At first, methods are used to test their ability to reveal the inherent determinism in cyclic combustion dynamics of SI engines as represented by a discrete noisy nonlinear dynamic model. Later these methods are used to investigate the deterministic characteristic underlying cyclic combustion dynamics of HCCI and RCCI engines. More than four periods are observed in bifurcation diagrams corresponding to a lean and diluted charge operation of the SI engine which is hidden by the noisy components. At highly lean/diluted operation, the complex chaotic behavior is observed in bifurcation diagrams and phase space trajectories. Recurrence plot-based analysis successfully captures the determinism underlying the cyclic combustion dynamics of SI engines as represented by the nonlinear model. Effects of changing intake charge temperature, relative air-fuel ratio, and engine speed are observed on the dynamical transitions in the HCCI engine, while for the RCCI engine, it is done for changing diesel injection timings and mass of the port-injected fuel at a constant engine speed. Deterministic features in combustion dynamics are improved whenever combustion phasing is retarded. Retarded combustion phasing can be due to reduced intake temperature or increased relative air-fuel ratio in HCCI engine or advanced diesel injection timing, or increased low reactivity port-injected fuel mass. Deterministic periodic-2 and 4 behaviours are observed in HCCI and RCCI engines when the charge becomes overall or locally lean. The shift from conventional dual-fuel to RCCI mode was coupled with the onset of period-2 bifurcations. Deterministic features of the location of maximum in-cylinder pressure are also investigated and compared with combustion phasing. Also, the changes in interactions between combustion phasing and location of maximum pressure are investigated for changing operating conditions to understand the applicability of the latter as a feedback signal in model-predictive control due to its low computation cost. Although deterministic features dominate, the correlation between combustion phasing and location of maximum pressure is reduced when the charge is made highly leaner. HCCI operation with a moderately lean charge and RCCI operation with a moderately advanced diesel injection timing is suitable for getting the benefits of using a computationally cheap parameter, i.e., location of maximum pressure as a feedback signal for model predictive control applications.