Nonlinear cycle dynamics in lean spark ignition combustion

Abstract

The goal of this study was to experimentally investigate the onset of lean combustion instability in spark ignition engines. New data analysis techniques from nonlinear dynamics and chaos theory were employed to reveal previously unrecognized patterns in cycle-resolved measurements of combustion heat release. The results indicated a transition from stochastic behavior to noisy nonlinear determinism as equivalence ratio was decreased from stoichiometric to very lean conditions. The transition to nonlinear deterministic behavior appeared to occur via a period-doubling bifurcation sequence. Experimentally observed patterns were compared with patterns predicted by a recently proposed engine model. The comparison supported the hypothesis that the combustion instability develops as a noisy period-doubling bifurcation. Experimental results from a single-cylinder research engine and a production eight-cylinder engine showed similar trends in cycle dynamics under lean conditions. The similar behavior of these very different engines suggests that the basic phenomena involved may be common in spark ignition engines. The observation of nonlinear determinism under lean conditions may have important implications for engine diagnostics and control because cyclic dispersion under very lean conditions is not a purely random process.