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Particle image velocimetry measurements of the in-cylinder flow in an optically accessible single-cylinder research engine were taken to better understand the effects of intake pressure variations on the flow field. At a speed of 1500 rpm, the engine was run at six different intake pressure loads from 0.4 to 0.95 bar under motored operation. The average velocity fields show that the tumble center position is located closer to the piston and velocity magnitudes decrease with increasing pressure load. A closer investigation of the intake flow near the valves reveals sharp temporal gradients and differences in maximum and minimum velocity with varying intake pressure load which are attributed to intake pressure oscillations. Despite measures to eliminate acoustic oscillations in the intake system, high-frequency pressure oscillations are shown to be caused by the backflow of air from the exhaust to the intake pipe when the valves open, exciting acoustic modes in the fluid volume.

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 this work, numerical simulations of an automotive-sized scavenged pre-chamber mounted in an optically-accessible rapid compression-expansion machine (RCEM) have been carried out using two different turbulence models: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). The RANS approach is combined with the G-equation combustion model, whereas the LES approach is coupled with the flamelet generated manifold (FGM) model for partially-premixed combustion. Simulation results are compared with experimental data in terms of OH* chemiluminescence in the main chamber. Both RANS and LES results were found to qualitatively reproduce the main features observed experimentally in terms of spatial flame development. Simulation results are further analysed by means of early flame propagation within the pre-chamber (related to the fuel and turbulence intensity distributions) and the ignition process in the main chamber.

Most of the phenomena that occur during the high pressure cycle of a spark ignition engine are highly influenced by the gas temperature, turbulence intensity and turbulence length scale inside the cylinder. For a pre chamber gas engine, the small volume and the high surface-to-volume ratio of the pre chamber increases the relative significance of the gas-to-wall heat losses, the early flame kernel development and the wall induced quenching; all of these phenomena are associated up to a certain extent with the turbulence and temperature field inside the pre chamber. While three-dimensional (3D) computational fluid dynamics (CFD) simulations can capture complex phenomena inside the pre chamber with high accuracy, they have high computational cost. Quasi dimensional models, on the contrary, provide a computationally inexpensive alternative for simulating multiple operating conditions as well as different geometries.

Formation of soot in an auto-igniting n-dodecane spray under diesel engine relevant conditions has been investigated numerically. As opposed to research addressing turbulence-chemistry interaction (TCI) by coupling diffusive turbulence models with more sophisticated models in the context of Reynolds-Averaged Navier-Stokes equations (RANS), this study employs the advanced sub-grid scale k-equation model in the framework of a Large Eddy Simulation (LES) together with the uninvolved Direct Integration (DI) approach. A reduced n-heptane chemical mechanism has been employed and artificially accelerated in order to predict the ignition for n-dodecane accurately. Soot processes have been modelled with an extended version of the semi-empirical, two-equation model of Leung, which considers C2H2 as the soot precursor and accounts for particle inception, surface growth by C2H2 addition, oxidation by O2, oxidation by OH and particle coagulation.

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.

Large-bore natural gas engines may use pre-chamber ignition. Despite extensive research in engine environments, the exact nature of the jet, as it exits the pre-chamber orifice, is not thoroughly understood and this leads to uncertainty in the design of such systems. In this work, a specially-designed rig comprising a quartz pre-chamber fit with an orifice and a turbulent flowing mixture outside the pre-chamber was used to study the pre-chamber flame, the jet, and the subsequent premixed flame initiation mechanism by OH* and CH* chemiluminescence. Ethylene and methane were used. The experimental results are supplemented by LES and 0D modelling, providing insights into the mass flow rate evolution at the orifice and into the nature of the fluid there. Both LES and experiment suggest that for large orifice diameters, the flow that exits the orifice is composed of a column of hot products surrounded by an annulus of unburnt pre-chamber fluid.

Despite significant benefits in terms of the ignition enhancement, the strength and timing of the turbulent flame jets subsequently issuing into the main chamber strongly depend on the pre-chamber combustion process and, thus, are sensitive to the specific engine operating conditions it experienced. This poses considerable difficulties in optimizing engine operating conditions as well as controlling engine performance. This paper investigates the influence of engine operating conditions on the pre-chamber combustion event using both experimental and numerical methods. A miniaturized piezo-electric pressure transducer was designed to be placed inside the engine cylinder head to record the pre-chamber inner volume pressure, in addition to conventional pressure indication inside the main chamber.

Cycle-to-cycle variations in internal combustion engines are known to lead to limitations in engine load and efficiency, as well as increases in emissions. Recent research has led to the identification of the source of cyclic variations of pressure, soot and NO emissions in direct injection common rail diesel engines, when employing a single block injection and operating under long ignition delay conditions. The variations in peak pressure arise from changes in the diffusion combustion rate, caused by randomly occurring in-cylinder pressure fluctuations. These fluctuations result from the excitation of the first radial mode of vibration of the cylinder gases which arises from the rapid premixed combustion after the long ignition delay period. Cycles with high-intensity fluctuations present faster diffusion combustion, resulting in higher cycle peak pressure, as well as higher measured exhaust NO concentrations.

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.

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.

New emission legislations applicable in the near future to sea-going vessels, off-road and off-highway vehicles require drastic nitric oxides emission reduction. A promising approach to achieve part of this decrease is charge air temperature reduction using Miller timing. However, it has been shown in literature that the reduction potential is limited, achieving a minimum in NOx emissions at a certain end-of-compression temperature. Further temperature reduction has shown to increase NOx emissions again. Some studies have shown that this increase is correlated to an increased amount of premixed combustion. In this work, the effects of pilot injection on engine out NOx emissions for very early intake valve closure (i.e. extreme Miller), high boost pressures and cold end-of-compression in-cylinder conditions are investigated. The experiments are carried out on a 3.96L single cylinder heavy-duty common-rail Diesel engine operating at 1000 rpm and at constant global air-to-fuel ratio.

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 study presents an application of the conditional moment closure (CMC) combustion model to marine diesel sprays. In particular, the influence of fuel evaporation terms has been investigated for the CMC modeling framework. This is motivated by the fact that substantial overlap between the dense fuel spray and flame area is encountered for sprays in typical large two-stroke marine diesel engines which employ fuel injectors with orifice diameters of the order of one millimeter. Simulation results are first validated by means of experimental data from the Wärtsilä optically accessible marine spray combustion chamber in terms of non-reactive macroscopic spray development. Subsequently, reactive calculations are carried out and validated in terms of ignition delay time, ignition location, flame lift-off length and temporal evolution of the flame region. Finally, the influence of droplet terms on spray combustion is analyzed in detail.

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.

In this study, the influence of injector diameter on the combustion of diesel sprays in an optically accessible combustion chamber of marine engine dimensions and conditions has been investigated experimentally as well as numerically. Five different orifice diameters ranging between 0.2 and 1.2 mm have been considered at two different ambient temperatures: a “cold” case with 800 K and a “warm” case with 900 K, resulting in a total of ten different test conditions. In the experiment, the reactive spray flames were characterized by means of high-speed OH* chemiluminescence imaging. The measurements revealed a weak impact of the injector diameter on ignition delay (ID) time and flame lift-off length (LOL) whereas the influence of ambient temperature was found to be more pronounced, consistent with former studies in the literature for smaller orifice diameters.

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.