Time Irreversibility and Comparison of Cyclic-Variability Models

We describe a method for detecting and quantifying time irreversibility in experimental engine data. We apply this method to experimental heat-release measurements from four sparkignited engines under leaning fueling conditions. We demonstrate that the observed behavior is inconsistent with a linear Gaussian random process and is more appropriately described as a noisy nonlinear dynamical process. INTRODUCTION Cyclic combustion variability (CV) in spark-ignited (SI) engines has been observed and debated for over 100 years. Proposed explanations for the causes have ranged from turbulent, in-cylinder mixing fluctuations to deterministic effects of residual gas (the so-called prior-cycle effect). The CV issue continues to be important because these combustion instabilities are responsible for higher emissions and limit the practical levels of lean-fueling and EGR which can be achieved. Better models for simulating the dynamic character of CV could lead to new design and control options f..