Quantifying Cyclic Variability in a Multicylinder HCCI Engine With High Residuals

Abstract

Cyclic variability (CV) in lean homogeneous charge compression ignition (HCCI) combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multicylinder engine. Experiments are peiformed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combtistion analysis method that estimates the utiburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as air fiow and wall temperature, between cylinders strongly infiuetice the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.

Cyclic Variability and Dynamical Instabilities in Autoignition Engines With High Residuals

“…Moreover, analogous experiments investigating high CV were performed on a four-cylinder 2 L engine in [24] and compared to data from the single-cylinder used here. The analysis in [24] shows that the dynamical evolution of the combustion phasing θ 50 is qualitatively similar between the single-cylinder and the individual cylinders on the multicylinder engine. The agreement between different engine platforms supports that the dynamical evolution of θ 50 is indeed a general characteristic of lean HCCI operating with high residuals.…”

“…where x r (k) is given by (24), f is defined by (14) and (23), and g is defined by (4). The outputs are…”

“…Model predictions for varying nvo are generated by feeding (25) with x r (k) according to (24) by generating samples from a normal distribution with differentx r . The mean valuex r is set equal to the mean value estimated for each experimental condition.…”

“…For conditions with higher CV at late phasing, the relationship between cycles is clearly nonlinear and a linear correlation between cycles does not capture the experimentally observed patterns. This behavior is mainly due to the sharp falloff in the combustion efficiency for late phasing, as shown in [4] and [24]. As a consequence, the unburned fuel mass creates an important coupling between cycles in addition to the residual gas temperature.…”

“…The approach followed here includes only the dominating mechanisms of temperature and unburned fuel as identified through the experiments in [4], [6], and [24]. The two-state model is still capable of predicting misfires, and is amendable for analysis as well as suitable for future control of the nvo and cycle-to-cycle fuel injection for reducing CV.…”