James A. Drallmeier

Characterization of lean combustion instability in premixed charge spark ignition engines

Abstract The focus of this study was to identify and characterize the development of lean combustion instability in spark ignition engines. Statistical techniques from non-linear dynamics were used to process experimental combustion observations to reveal previously unrecognized patterns in cycle-to-cycle combustion variations. The presence of non-linear deterministic structure was confirmed in lean combustion variations from a single cylinder research engine and a four-cylinder production engine. The transition to non-linear deterministic behaviour appeared to occur via a period-doubling bifurcation sequence. Over the bifurcation region, engine dynamics appeared to pass through distinct dynamic stages including stochastic, periodic and possibly chaotic behaviour. The level of dynamic complexity and corresponding cycle-to-cycle communication were found to be a strong function of the residual gas fraction. Experimental observations were also compared with patterns predicted by a recently proposed low-order engine model. Further analysis of the time-series results indicated that the engine frequently exhibited complicated repeating combustion patterns 15 to 20 cycles in length under certain lean operating conditions. Similar dynamics were seen for the two very different engine designs. The work suggests that the underlying cyclic dynamics may not be dependent upon the details of such processes as mixing and combustion but are characteristic of all lean premixed spark ignition engines.

Developing Laminar Gravity-Driven Thin Liquid Film Flow Down an Inclined Plane

Three-dimensional (3D)—steady—developing—laminar—isothermal—and gravity-driven thin liquid film flow adjacent to an inclined plane is examined and the effects of film flow rate, surface tension, and surface inclination angle on the film thickness and film width are presented. The film flow was numerically simulated using the volume of fluid model and experimental verification was conducted by measuring film thickness and width using a laser focus displacement instrument. The steady film flow that is considered in this study does not have a leading contact line, however, it has two steady side contact lines with the substrate surface at the outer edge of its width. Results reveal that the film width decreases and the average film thickness increases as the film flows down the inclined plane. The film thickness and width decrease but its streamwise velocity increases as surface inclination angle (as measured from the horizontal plane) increases. A higher film flow rate is associated with a higher film thickness, a higher film width, and a higher average film velocity. Films with higher surface tension are associated with a smaller width and a higher average thickness. A ripple develops near the side contact line, i.e., the spanwise distribution of the film thickness exhibits peaks at the outer edges of the film width and the height of this ripple increases as the surface tension or the film flow rate increases. The width of the film decreases at a faster rate along the streamwise direction if liquid film has higher surface tension. Measurements of the film thickness and the film width compare favorably with the numerically simulated results.

Hydrocarbon vapor measurements in fuel sprays: a simplification of the infrared extinction technique

An equal optical thickness approximation that greatly simplifies the application and data reduction of the infrared extinction technique for measuring fuel vapor concentrations in sprays is investigated. A general approach is given, with specific results for a 3.39- and a 0.6328-μm-wavelength system. It is shown that the infrared (3.39-μm) drop optical thickness can be approximated by the use of the visible (0.6328-μm) optical thickness for drop size distributions with area mean diameters greater than 20 μm for hydrocarbon fuel sprays.

Hydrocarbon-vapor measurements in pulsed fuel sprays.

The feasibility of the use of a nonintrusive, line-of-sight averaging, infrared extinction technique for the quantitative measurement of fuel-vapor concentration and flux in transient sprays has been examined. A collinear visible and infrared system with a compact sandwich detector design allows for the detection of radiation at both wavelengths simultaneously. With a controlled simulated fuel spray that consists of styrene spheres and methane vapor, vapor concentrations were measured within 10% of the known input value for a large range of particle loadings. Quantitative measurements were also made in a transient isooctane spray with an automotive-type injector. The time-resolved vapor mole fraction, velocity, and mass flux were compared with the transient liquid-phase characteristics obtained with a phase/Doppler anemometer system. The combined use of both instruments for discerning differences in liquid and vapor transport is discussed.

Neural Network Controller Development and Implementation for Spark Ignition Engines With High EGR Levels

Past research has shown substantial reductions in the oxides of nitrogen (NOx) concentrations by using 10% -25% exhaust gas recirculation (EGR) in spark ignition (SI) engines (see Dudek and Sain, 1989). However, under high EGR levels, the engine exhibits strong cyclic dispersion in heat release which may lead to instability and unsatisfactory performance preventing commercial engines to operate with high EGR levels. A neural network (NN)-based output feedback controller is developed to reduce cyclic variation in the heat release under high levels of EGR even when the engine dynamics are unknown by using fuel as the control input. A separate control loop was designed for controlling EGR levels. The stability analysis of the closed-loop system is given and the boundedness of the control input is demonstrated by relaxing separation principle, persistency of excitation condition, certainty equivalence principle, and linear in the unknown parameter assumptions. Online training is used for the adaptive NN and no offline training phase is needed. This online learning feature and model-free approach is used to demonstrate the applicability of the controller on a different engine with minimal effort. Simulation results demonstrate that the cyclic dispersion is reduced significantly using the proposed controller when implemented on an engine model that has been validated experimentally. For a single cylinder research engine fitted with a modern four-valve head (Ricardo engine), experimental results at 15% EGR indicate that cyclic dispersion was reduced 33% by the controller, an improvement of fuel efficiency by 2%, and a 90% drop in NOx from stoichiometric operation without EGR was observed. Moreover, unburned hydrocarbons (uHC) drop by 6% due to NN control as compared to the uncontrolled scenario due to the drop in cyclic dispersion. Similar performance was observed with the controller on a different engine.

A Separation Criterion With Experimental Validation for Shear-Driven Films in Separated Flows

The behavior of a shear-driven thin liquid film at a sharp expanding corner is of interest in many engineering applications. However, details of the interaction between inertial, surface tension, and gravitational forces at the corner that result in partial or complete separation of the film from the surface are not clear. A criterion is proposed to predict the onset of shear-driven film separation from the surface at an expanding corner. The criterion is validated with experimental measurements of the percent of film mass separated as well as comparisons to other observations from the literature. The results show that the proposed force ratio correlates well to the onset of film separation over a wide range of experimental test conditions. The correlation suggests that the gas phase impacts the separation process only through its effect on the liquid film momentum.

Hydrocarbon absorption coefficients at the 3.39-μm He-Ne laser transition

In view of the application of light-extinction techniques for fuel-specie-concentration measurements in combustion systems, the vapor-absorption coefficient of several hydrocarbon species at the 3.39-μm He-Ne laser transition has been measured. The hydrocarbon species include paraffins, olefins, and aromatics. Results are included for total pressure over the range of approximately 200–650 Torr at 295 K with air as the buffer gas. Observations are made regarding the difference in the absorption coefficient within and between hydrocarbon classifications.

A Method for Predicting Performance Improvements with Effective Cycle-To-Cycle Control of Highly Dilute Spark Ignition Engine Combustion

Abstract Dilute spark ignition engine combustion offers a promising method of improving fuel efficiency and reducing engine-out emissions and yet is not currently feasible owing to high levels of cyclic variability under highly dilute homogeneous operation. The deterministic nature of the undesirable cycle-to-cycle variations in combustion heat release implies that appropriate control schemes should yield significant reductions in cyclic variability, making it possible to use higher levels of charge dilution in production engines. A novel analysis approach is used to predict the improvement in fuel conversion efficiency that could be expected with effective control. Additionally, this analysis gives some insight into the effect of spark timing on the dynamics and controllability of the system.

Nonlinear cycle dynamics in lean spark ignition combustion

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.

Near Optimal Output-Feedback Control of Nonlinear Discrete-time Systems in Nonstrict Feedback Form with Application to Engines

A novel reinforcement-learning based output-adaptive neural network (NN) controller, also referred as the adaptive-critic NN controller, is developed to track a desired trajectory for a class of complex nonlinear discrete-time systems in the presence of bounded and unknown disturbances. The controller includes an observer for estimating states and the outputs, critic, and two action NNs for generating virtual, and actual control inputs. The critic approximates certain strategic utility function and the action NNs are used to minimize both the strategic utility function and their outputs. All NN weights adapt online towards minimization of a performance index, utilizing gradient-descent based rule. A Lyapunov function proves the uniformly ultimate boundedness (UUB) of the closed-loop tracking error, weight, and observer estimation. Separation principle and certainty equivalence principles are relaxed; persistency of excitation condition and linear in the unknown parameter assumption is not needed. The performance of this controller is evaluated on a spark ignition (SI) engine operating with high exhaust gas recirculation (EGR) levels and experimental results are demonstrated.