Aristotelis Babajimopoulos

A fully coupled computational fluid dynamics and multi-zone model with detailed chemical kinetics for the simulation of premixed charge compression ignition engines

Abstract Modelling the premixed charge compression ignition (PCCI) engine requires a balanced approach that captures both fluid motion as well as low- and high-temperature fuel oxidation. A fully integrated computational fluid dynamics (CFD) and chemistry scheme (i.e. detailed chemical kinetics solved in every cell of the CFD grid) would be the ideal PCCI modelling approach, but is computationally very expensive. As a result, modelling assumptions are required in order to develop tools that are computationally efficient, yet maintain an acceptable degree of accuracy. Multi-zone models have been previously shown accurately to capture geometry-dependent processes in homogeneous charge compression ignition (HCCI) engines. In the presented work, KIVA-3V is fully coupled with a multi-zone model with detailed chemical kinetics. Computational efficiency is achieved by utilizing a low-resolution discretization to solve detailed chemical kinetics in the multi-zone model compared with a relatively high-resolution CFD solution. The multi-zone model communicates with KIVA-3V at each computational timestep, as in the ideal fully integrated case. The composition of the cells, however, is mapped back and forth between KTVA-3V and the multi-zone model, introducing significant computational time savings. The methodology uses a novel re-mapping technique that can account for both temperature and composition non-uniformities in the cylinder. Validation cases were developed by solving the detailed chemistry in every cell of a KIVA-3V grid. The new methodology shows very good agreement with the detailed solutions in terms of ignition timing, burn duration, and emissions.

Comparing enhanced natural thermal stratification against retarded combustion phasing for smoothing of HCCI heat-release rates

Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions. To further exploit the effect, the heat-transfer rates were further enhanced by increasing the in-cylinder air swirl. This led to even longer combustion durations. Unfortunately, the higher heat losses associated with high air swirl decreased the IMEP g . When the fueling rate was increased to compensate, most of the improvements on the heat-release rates were lost. Overall, combustion phasing retard was found to have better potential for smoothing heat-release rates than enhancing the thermal stratification by the means considered in this work. However, operation with highly retarded combustion requires precise control of the ignition timing. Furthermore, it is found that the acceptable intake temperature range narrows rapidly with increasing equivalence ratio. Above a certain fueling rate a steady state operating point cannot be established by setting the intake temperature to a fixed value. This problem is caused by wall heating and the coupling between wall temperature and combustion phasing.

Understanding the Dynamic Evolution of Cyclic Variability at the Operating Limits of HCCI Engines with Negative Valve Overlap

An experimental study is performed for homogeneous charge compression ignition (HCCI) combustion focusing on late phasing conditions with high cyclic variability (CV) approaching misfire. High CV limits the feasible operating range and the objective is to understand and quantify the dominating effects of the CV in order to enable controls for widening the operating range of HCCI. A combustion analysis method is developed for explaining the dynamic coupling in sequences of combustion cycles where important variables are residual gas temperature, combustion efficiency, heat release during re-compression, and unburned fuel mass. The results show that the unburned fuel mass carries over to the re-compression and to the next cycle creating a coupling between cycles, in addition to the well known temperature coupling, that is essential for understanding and predicting the HCCI behavior at lean conditions with high CV.

Thermodynamic sweet spot for high-efficiency, dilute, boosted gasoline engines:

Recent developments in ignition, boosting, and control systems have opened up new opportunities for highly dilute, high-pressure combustion regimes for gasoline engines. This study analytically explores the fundamental thermodynamics of operation in these regimes under realistic burn duration, heat loss, boosting, and friction constraints. The intent is to identify the benefits of this approach and the path to achieving optimum engine and vehicle-level fuel economy. A simple engine/turbocharger model in GT-Power is used to perform a parametric study exploring the conditions for best engine efficiency. These conditions are found in the mid-dilution range, a result of the tradeoff between fluid property benefits of lean mixtures and friction benefits of higher loads. Dilution with exhaust gas is nearly as effective as air dilution when compared using a ‘fuel-to-charge’ equivalence ratio defined as Φ′≡Φ (1-RGF) where RGF is the total residual gas fraction. Optimal brake efficiencies are obtained over a range 0.45 ≤ Φ′ ≤ 0.65 for operation up to 3 bar manifold pressure, yielding peak temperatures under 2100 K and peak pressures under 150 bar. These conditions are intermediate between homogeneous charge compression ignition and spark-ignition regimes, and are the subject of much current research on advanced combustion modes. An engine–vehicle drive train simulation shows that accessing this thermodynamic sweet spot has the potential for vehicle fuel economy gains between 23% and 58%.

Modeling Iso-octane HCCI Using CFD with Multi-Zone Detailed Chemistry; Comparison to Detailed Speciation Data Over a Range of Lean Equivalence Ratios

Multi-zone CFD simulations with detailed kinetics were used to model iso-octane HCCI experiments performed on a single-cylinder research engine. The modeling goals were to validate the method (multi-zone combustion modeling) and the reaction mechanism (LLNL 857 species iso-octane) by comparing model results to detailed exhaust speciation data, which was obtained with gas chromatography. The model is compared to experiments run at 1200 RPM and 1.35 bar boost pressure over an equivalence ratio range from 0.08 to 0.28. Fuel was introduced far upstream to ensure fuel and air homogeneity prior to entering the 13.8:1 compression ratio, shallow-bowl combustion chamber of this 4-stroke engine. The CFD grid incorporated a very detailed representation of the crevices, including the top-land ring crevice and headgasket crevice. The ring crevice is resolved all the way into the ring pocket volume. The detailed grid was required to capture regions where emission species are formed and retained. Results show that combustion is well characterized, as demonstrated by good agreement between calculated and measured pressure traces. In addition, excellent quantitative agreement between the model and experiment is achieved for specific exhaust species components, such as unburned fuel, formaldehyde, and many other intermediate hydrocarbon species. Some calculated trace intermediate hydrocarbon species do not agree as well with measurements, highlighting areas needing further investigation for understanding fundamental chemistry processes in HCCI engines.

Analysis of Load and Speed Transitions in an HCCI Engine Using 1-D Cycle Simulation and Thermal Networks

Exhaust gas rebreathing is considered to be a practical enabler that could be used in HCCI production engines. Recent experimental work at the University of Michigan demonstrates that the combustion characteristics of an HCCI engine using large amounts of hot residual gas by rebreathing are very sensitive to engine thermal conditions. This computational study addresses HCCI engine operation with rebreathing, with emphasis on the effects of engine thermal conditions during transient periods. A 1-D cycle simulation with thermal networks is carried out under load and speed transitions. A knock integral autoignition model, a modified Woschni heat transfer model for HCCI engines and empirical correlations to define burn rate and combustion efficiency are incorporated into the engine cycle simulation model. The simulation results show very different engine behavior during the thermal transient periods compared with steady state. Hot walls advance the ignition timing, while cold walls may result in misfire. Realizable operating regions during the thermal transitions are very dependent on the wall temperatures and are quite different from the steady state. This implies that thermal inertia must be considered in order to fully optimize HCCI engine operation.

An extended multi-zone combustion model for PCI simulation

Novel combustion modes are becoming an important area of research with emission regulations more stringent than ever before, and with fuel economy being assigned greater importance every day. Homogeneous Charge Compression Ignition (HCCI) and Premixed Compression Ignition (PCI) modes in particular promise better fuel economy and lower emissions in internal combustion engines. Multi-zone combustion models have been popular in modelling HCCI combustion. In this work, an improved multi-zone model is suggested for PCI combustion modelling. A new zoning scheme is suggested based on incorporating the internal energy of formation into an earlier conventional HCCI multi-zone approach, which considers a two-dimensional reaction space defined by equivalence ratio and temperature. It is shown that the added dimension improves zoning by creating more representative zones, and thus reducing errors compared to the conventional zoning approach, when applied to PCI simulation.

ON THE ROLE OF TOP DEAD CENTER CONDITIONS IN THE COMBUSTION PHASING OF HOMOGENEOUS CHARGE COMPRESSION IGNITION ENGINES

Abstract There is an inherent difficulty in trying to distinguish between the thermodynamic and the chemical effects in a homogeneous charge compression ignition (HCCI) engine. This article attempts to isolate the chemical kinetics effects in the framework of a zero-dimensional, thermo-kinetic model combined with a detailed chemical mechanism for iso-octane and an auto-ignition correlation. The study focuses on the behavior of single-stage ignition fuels near top dead center (TDC), as a means of relating conditions at TDC to predicted ignition timings. It is found that a unique relationship exists between combustion phasing, and the constant volume ignition delay at TDC conditions expressed in crank angle degrees (CAD). This relationship holds for given engine parameters and composition over a wide range of RPM. For ignition near TDC, the ignition delay at TDC conditions expressed as a crank angle interval must be between 8 and 12 CAD, depending on engine design parameters.

A comprehensive engine to drive-cycle modelling framework for the fuel economy assessment of advanced engine and combustion technologies

A comprehensive engine to drive-cycle modelling framework has been developed for evaluating fuel economy improvements of new engine technologies. The framework comprises three key components: (a) full engine system models and routines for the generation of engine performance and fuel consumption maps; (b) an improved experimental heat release analysis and model calibration tool, which was created by coupling an in-house heat release analysis program to an engine cycle simulation, and which can be used for model calibration and validation when experimental data are available; and (c) an integrated vehicle modelling and drive-cycle simulation platform for fuel economy assessment. The framework implementation has been demonstrated through a fuel economy study of three engine and combustion technologies: a conventional spark-ignition (SI) engine, a high compression ratio SI engine with early intake valve closing (EIVC), and a homogeneous-charge compression ignition engine (HCCI) employing a recompression valve strategy, used in dual-mode SI-HCCI operation. Simulation results for three drive cycles indicate potential fuel economy gains of the order of 7–11% for the high-compression EIVC SI engine and 7–21% for the dual-mode HCCI engine configurations. Further analysis of the latter, however, reveals frequent mode switching and short excursions in the HCCI region, which could be a challenge during practical implementation, and potentially result in reduced fuel economy gains.

Assessment of Residual Mass Estimation Methods for Cylinder Pressure Heat Release Analysis of HCCI Engines With Negative Valve Overlap

Increased residual levels in Homogeneous Charge Compression Ignition (HCCI) engines employing valve strategies such as recompression or negative valve overlap (NVO) imply that accurate estimation of residual gas fraction (RGF) is critical for cylinder pressure heat release analysis. The objective of the present work was to evaluate three residual estimation methods and assess their suitability under naturally aspirated and boosted HCCI operating conditions: i) the Simple State Equation method employs the Ideal Gas Law at exhaust valve closing (EVC); ii) the Mirsky method assumes isentropic exhaust process; and iii) the Fitzgerald method models in-cylinder temperature from exhaust valve opening (EVO) to EVC by accounting for heat loss during the exhaust process and uses measured exhaust temperature for calibration. Simulations with a calibrated and validated “virtual engine” were performed for representative HCCI operating conditions of engine speed, fuel-air equivalence ratio, NVO and intake pressure (boosting). The State Equation method always overestimated RGF by more than 10%. The Mirsky method was most robust, with average errors between 3–5%. The Fitzgerald method performed consistently better, ranging from no error to 5%, where increased boosting caused the largest discrepancies. A sensitivity study was also performed and determined that the Mirsky method was most robust to possible pressure and temperature measurement errors.© 2011 ASME