Cyclic Variability and Dynamical Instabilities in Autoignition Engines With High Residuals

Abstract

The dynamical phenomenon of cyclic variability in combustion governed by autoignition in homogeneous charge compression ignition engines with large amounts of residual gases is investigated. A novel model is derived with two states that capture the coupling between engine cycles due to the thermal energy in the recycled residual gases and the recycled chemical energy in the unburned fuel. With the parameters tuned to data from a single cylinder engine at one level of residuals, the model predictions agree well with the experimental observations of decreasing residuals, which are associated with later phasing and increasing cyclic variability approaching misfire. A stability analysis of the model with respect to the amount of recycled residual gases shows how instabilities develop and that the dynamic behavior of the combustion phasing from cycle to cycle is stable for a range of residual gas fractions. The model and analysis offers an explanation for the experimentally observed cycle-tocycle variability, and provides a foundation for further analysis as well as development of controls mitigating the variability.

Real-time, adaptive machine learning for non-stationary, near chaotic gasoline engine combustion time series

“…While HCCI is already a significant challenge given the above complexity, the combustion mode also exhibits a period doubling bifurcation cascade to chaos [6,9,10], similar to what is seen in high residual spark ignition engines [11]. When nearly chaotic, HCCI is still deterministic, but becomes oscillatory and very sensitive to parameter variations (e.g.…”

“…2. This structure shows a deterministic transition to oscillatory high CV behavior as combustion moves towards later combustion timing that can be viewed as at least a single period doubling bifurcation with sensitive dependence on the engine set point [6,9,10]. While mathematically interesting, this oscillatory high CV structure undesirably constrains practical HCCI engine operation.…”

“…At the other computational complexity extreme, loworder approximation models of HCCI for control have been developed since at least the early 2000s [16], based on spark ignition engine knock models developed in the 1950s [17]. Recently, efforts have been made to extend this type of model to the high CV regions of HCCI by injecting random residual gas fraction noise to capture uncertainties in the mixture state and composition [10]. This model was tuned for a limited set of steady-state conditions and only "predictive" in the sense that when injected with random (i.e.…”

“…When nearly chaotic, HCCI is still deterministic, but becomes oscillatory and very sensitive to parameter variations (e.g. residual gas fraction fluctuations [9,10]). This oscillatory "stability limit" behavior is commonly referred to as high Cyclic Variability (CV) and it severely constrains the available load limits of HCCI.…”

Mode switches among SI, SACI, and HCCI combustion and their influence on drive cycle fuel economy

“…Therefore, Diesel-like efficiencies can be obtained but with significantly lower emissions [4]. The load range in recompression HCCI engines is limited to low and intermediate engine speed and loads [5], [6] due to high cyclic variability and very high pressure rise rates (ringing), respectively.…”