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Large Eddy Simulations (LES) and tracer-based Laser-Induced Fluorescence (LIF) measurements were performed to study the dynamics of fuel wall-films on the piston top of an optically accessible, four-valve pent-roof GDI research engine for a total of eight operating conditions. Starting from a reference point, the systematic variations include changes in engine speed (600; 1,200 and 2,000 RPM) and load (1000 and 500 mbar intake pressure); concerning the fuel path the Start Of Injection (SOI=360°, 390° and 420° CA after gas exchange TDC) as well as the injection pressure (10, 20 and 35 MPa) were varied. For each condition, 40 experimental images were acquired phase-locked at 10° CA intervals after SOI, showing the wall-film dynamics in terms of spatial extent, thickness and temperature.

The current trend in automobiles is towards electrical vehicles, but for the most part these vehicles still require an internal combustion engine to provide additional range and flexibility. These engines are under stringent emissions regulations, in particular, for the reduction of CO2. Gas engines which run lean burn combustion systems provide a viable route to these emission reductions, however designing these engines to provide sustainable and controlled combustion under lean conditions at λ=2.0 is challenging. To address this challenge, it is possible to use a scavenged Pre-Chamber Ignition (PCI) system which can deliver favorable conditions for ignition close to the spark plug. The lean charge in the main combustion chamber is then ignited by flame jets emanating from the pre-chamber nozzles. Accurate prediction of flame kernel development and propagation is essential for the analysis of PCI systems.

In the present work numerical simulations were carried out investigating the effect of fuel type, nozzle-geometry variations and back-pressure changes on high-pressure gas injections under application-relevant conditions. Methane, hydrogen and nitrogen with a total pressure of 500 bar served as high-pressure fuels and were injected into air at rest at 200 bar and 100 bar. Different nozzle shapes were simulated and the analysis of the results lead to a recommendation for the most advantageous geometry regarding jet penetration, volumetric growth, mixing enhancement and discharge coefficient. Additionally an artificial inlet boundary conditions was tested for the use with real-gas thermodynamics and was shown to be capable of reducing the simulation time significantly.

The interaction of turbulent premixed methane combustion with the surrounding flow field can be studied using optically accessible test rigs such as a rapid compression expansion machine (RCEM). The high flexibility offered by such a test rig allows its operation at various thermochemical conditions at ignition. However, limitations inherent to such test rigs due to the absence of an intake stroke do not allow turbulence production as found in IC-engines. Hence, means to introduce turbulence need to be implemented and the relevant turbulence quantities have to be identified in order to enable comparability with engine relevant conditions. A dedicated high-pressure direct injection of air at the beginning of the compression phase is considered as a measure to generate adjustable turbulence intensities at spark timing and during the early flame propagation.

Three-dimensional reactive computational fluid dynamics (CFD) plays a crucial role in IC engine development tasks complementing experimental efforts by providing improved understanding of the combustion process. A widely adopted combustion model in the engine community for (partially) premixed combustion is the G-Equation where the flame front is represented by an iso-level of an arbitrary scalar G. A convective-reactive equation for this iso-surface is solved, for which the turbulent flame speed ST must be provided. In this study, the commonly used and well-established Damköhler approach is compared to a novel correlation, derived from an algebraic closure for the scalar dissipation of reaction progress as proposed by Kolla et al. [1].

An existing three-stage ignition delay model which has seen successful application to Primary Reference Fuels (PRFs) has been extended to six surrogate fuels which constitute potential candidates for future Homogeneous Charge Compression Ignition (HCCI) engines. The fuels include petroleum-derived and oxygenated components and can be divided into low, intermediate and high cetane number groups. A new methodology to obtain the model parameters is presented which relies jointly on simulation and experimental data: in a first step, constant volume adiabatic reactor simulations using chemical kinetic mechanisms are performed to generate ignition delays for a very wide range of conditions, namely variations in equivalence ratio, Exhaust Gas Recirculation (EGR), pressure and temperature.

In this study, the ignition and combustion process in a lean burn, spark-ignited stationary gas engine was investigated using a level-set (G-equation) combustion model in the context of Reynolds-averaged Navier-Stokes (RANS) simulations. An ignition model based on Herweg and Maly [1] and its coupling with the G-equation combustion model was implemented in the framework of the STARCD/es-ICE solver. A first validation was performed by means of spherically expanding methane/air flame measurements in an optically accessible spherical combustion bomb at elevated pressure by Lawes et al. [2]: predictions of the kernel size and the flame expansion are in good agreement with the experimental data at both stoichiometric and lean conditions. The model was subsequently applied to study combustion in a premixed lean burn stationary gas engine with a displacement volume of roughly two liters, ignited by means of a centrally located “G-type” spark plug.

Large Eddy Simulations (LES) provide instantaneous values indispensable to conduct statistical studies of relevant fluctuating quantities for diesel sprays. However, numerous realizations are generally necessary for LES to derive statistically averaged quantities necessary for validation of the numerical framework by means of measurements and for conducting sensitivity studies, leading to extremely high computational efforts. In this context, the aim of this work is to explore and validate alternatives to the simulation of 20-50 single realizations at considerably lower computational costs, by taking advantage of the axisymmetric geometry and the Quasi-Steady-State (QSS) condition of the near nozzle flow at a certain time after start-of-injection (SOI).

Knock is often the main limiting factor for brake efficiency in spark ignition engines and is mostly attributed to auto-ignition of the unburned mixture in front of the flame. In order to study knock in a systematic way, spark angle sweeps with ethanol and iso-octane have been carried out on single cylinder spark ignition engine with variable intake temperatures at wide open throttle and stoichiometric premixed fuel/air mixtures. Much earlier and stronger knock can be observed for iso-octane compared to ethanol at otherwise same engine operating conditions due to the cooling effect and higher octane number of ethanol, leading to different cycle-to-cycle variation behavior. Detailed chemical kinetic mechanisms are used to compute ignition delay times at conditions relevant to the measurements and are compared to empirical correlations available in literature. The different correlations are used in a knock model approach and are tested against the measurement data.

Numerical simulations of a heavy-duty diesel engine fuelled with n-heptane have been performed with the conditional moment closure (CMC) combustion model and an embedded two-equation soot model. The influence of exhaust gas recirculation on the interaction between post- and main- injection has been investigated. Four different levels of EGR corresponding to intake ambient oxygen volume fractions of 12.6, 15, 18 and 21% have been considered for a constant intake pressure and temperature and unchanged injection configuration. Simulation results have been compared to the experimental data by means of pressure and apparent heat-release rate (AHRR) traces and in-cylinder high-speed imaging of natural soot luminosity and planar laser-induced incandescence (PLII). The simulation was found to reproduce the effect of EGR on AHRR evolutions very well, for both single- and post-injection cases.

This paper presents numerical simulations of in-cylinder soot evolution in the optically accessible heavy-duty diesel engine of Sandia Laboratories performed with the conditional moment closure (CMC) model employing a reduced n-heptane chemical mechanism coupled with a two-equation soot model. The influence of exhaust gas recirculation (EGR) on in-cylinder processes is studied considering different ambient oxygen volume fractions (8 - 21 percent), while maintaining intake pressure and temperature as well as the injection configuration unchanged. This corresponds to EGR rates between 0 and 65 percent. Simulation results are first compared with experimental data by means of apparent heat release rate (AHRR) and temporally resolved in-cylinder soot mass, where a quantitative comparison is presented. The model was found to fairly well reproduce ignition delays as well as AHRR traces along the EGR variation with a slight underestimation of the diffusion burn portion.

Large eddy simulations (LES) of a port-injected 4-valve spark ignited (SI) engine have been carried out with the emphasis on the combustion process. The considered operating point is close to full load at 3,500 RPM and exhibits considerable cyclic variation in terms of the in-cylinder pressure traces, which can be related to fluctuations in the combustion process. In order to characterize these fluctuations, a statistically relevant number of subsequent cycles, namely up to 40, have been computed in the multi-cycle analysis. In contrast to other LES studies of SI engines, here the G-equation (a level set approach) has been adopted to model the premixed combustion in the framework of the STAR-CD/es-ICE flow field solver. Tuning parameters are identified and their impact on the result is addressed.

Knock is studied in a single cylinder direct injection spark ignition engine with variable intake temperatures at wide open throttle and stoichiometric premixed ethanol-air mixtures. At different speeds and intake temperatures spark angle sweeps have been performed at non-knocking conditions and varying knock intensities. Heat release rates and two zone temperatures are computed for both the mean and single cycle data. The in-cylinder pressure traces are analyzed during knocking combustion and have led to a definition of knocking conditions both for every single cycle as well as the mean engine cycle of a single operating point. The timing for the onset of knock as a function of degree crank angle and the mass fraction burned is determined using the “knocking” heat release and the pressure oscillations typical for knocking combustion.

The heat release of the low temperature reactions (LTR or cool-flame) under Homogeneous Charge Compression Ignition (HCCI) combustion has been quantified for five candidate fuels in an optically accessible Rapid Compression Expansion Machine (RCEM). Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented. In our previous work, an empirical, three-stage, Arrhenius-type ignition delay model, parameterized on shock tube data, was found to be applicable also in a transient, engine-relevant environment.

An optically accessible Rapid Compression Expansion Machine (RCEM) has been used to investigate the homogeneous auto-ignition of five candidate fuels for Homogenous Charge Compression Ignition (HCCI) combustion. Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. The PRF25 and PRF50 were selected in order to approximately match the CN of the two Naphthas. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented.

Future engine emission legislation regulates soot from Diesel engines strictly and requires improvements in engine calibration, fast response sensor equipment and exhaust gas aftertreatment systems. The in-cylinder phenomena of soot formation and oxidation can be analysed using a pyrometer with optical access to the combustion chamber. The pyrometer collects the radiation of soot particles during diffusion combustion, and allows the calculation of soot temperature and a proportional value for the in-cylinder soot density (KL). A four-cylinder heavy-duty Diesel engine was equipped in all cylinders with prototype pyrometers and state of the art pressure transducers. The cylinder specific data was recorded crank angle-resolved for a set of steady-state and transient operating conditions, as well as exhaust gas recirculation (EGR) addition and over a wide range of soot emissions.

Numerical simulations of in-cylinder soot evolution in the optically accessible heavy-duty diesel engine of Sandia National Laboratories have been performed with the multidimensional conditional moment closure (CMC) model using a reduced n-heptane chemical mechanism coupled with a two-equation soot model. Simulation results are compared to the high-fidelity experimental data by means of pressure traces, apparent heat release rate (AHRR) and time-resolved in-cylinder soot mass derived from optical soot luminosity and multiple wavelength pyrometry in conjunction with high speed soot cloud imaging. In addition, spatial distributions of soot relevant quantities are given for several operating conditions.

The combustion of gaseous fuels like methane in internal combustion engines is an interesting alternative to the conventional gasoline and diesel fuels. Reasons are the availability of the resource and the significant advantage in terms of CO2 emissions due to the beneficial C/H ratio. One difficulty of gaseous fuels is the preparation of the gas/air mixtures for all operation points, since the volumetric energy density of the fuel is lower compared to conventional liquid fuels. Low-pressure port-injected systems suffer from substantially reduced volumetric efficiencies. Direct injection systems avoid such losses; in order to deliver enough fuel into the cylinder, high pressures are however needed for the gas injection which forces the fuel to enter the cylinder at supersonic speed followed by a Mach disk. The detailed modeling of these physical effects is very challenging, since the fluid velocities and pressure and velocity gradients at the Mach disc are very high.

In the field of heavy-duty applications almost all engines apply the compression ignition principle, spark ignition is used only in the niche of CNG engines. The main reason for this is the high efficiency advantage of diesel engines over SI engines. Beside this drawback SI engines have some favorable properties like lower weight, simple exhaust gas aftertreatment in case of stoichiometric operation, high robustness, simple packaging and lower costs. The main objective of this fundamental research was to evaluate the limits of a SI engine for heavy-duty applications. Considering heavy-duty SI engines fuel consumption under full load conditions has a high impact on CO₂ emissions. Therefore, downsizing is not a promising approach to improve fuel consumption and consequently the focus of this work lies on the enhancement of thermal efficiency in the complete engine map, intensively considering knocking issues.

The behavior of spray auto-ignition and combustion of a diesel spray in a lean premixed methane/air charge was investigated. A rapid compression expansion machine with a free-floating piston was employed to reach engine-relevant conditions at start of injection of the micro diesel pilot. The methane content in the lean ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber, the ambient equivalence ratio for the methane content ranged from 0.0 (pure air) to 0.65. Two different nozzle tips with three and six orifices were employed. The amount of pilot fuel injected ranged between 0.8 and 1.8 percent of the total energy in the combustion chamber. Filtered OH chemiluminescence images of the combustion were taken with a UV-intensified high-speed camera through the optical access in the piston.