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In this study, a novel 3D-CFD combustion model employing Flamelet Generated Manifolds (FGM) for dual fuel combustion was developed. Validation of the platform was carried out using recent experimental results from an optically accessible Rapid Compression Expansion Machine (RCEM). Methane and n-dodecane were used as model fuels to remove any uncertainties in terms of fuel composition. The model used a tabulated chemistry approach employing a reaction mechanism of 130 species and 2399 reactions and was able to capture non-premixed auto ignition of the pilot fuel as well as premixed flame propagation of the background mixture. The CFD model was found to predict well all phases of the dual fuel combustion process: I) the pilot fuel ignition delay, II) the Heat Release Rate of the partially premixed conversion of the micro pilot spray with entrained methane/air and III) the sustained background mixture combustion following the consumption of the spray plume.

Dual-fuel natural gas engines are seen as an attractive solution for simultaneous reduction of pollutant and CO2 emissions while maintaining high engine thermal efficiency. However, engines of this type exhibit a tradeoff between misfire as well as high UHC emissions for small pilot injection amounts and higher emissions of soot and NOX for operation strategies with higher pilot fuel proportion. The aim of this study was to investigate POMDME as an alternative pilot fuel having the potential to mitigate the emissions tradeoff, enabling smokeless combustion due to high degree of oxygenation, and being less prone to misfire due to its higher cetane number. Furthermore, POMDME can be synthetized carbon neutrally. First, characteristics of POMDME ignition in methane/air mixture and the transition into premixed flame propagation were investigated optically in a rapid compression-expansion machine (RCEM) by employing Schlieren and OH* chemiluminescence imaging.

The sooting propensity of dual-fuel combustion with n-dodecane pilot injection in a lean-premixed methane-air charge has been investigated using an optically accessible Rapid Compression-Expansion Machine (RCEM) to achieve engine-relevant pressure and temperature conditions at the start of pilot injection. A Diesel injector with a 100 μm single-hole coaxial nozzle, mounted at the cylinder periphery, has been employed to admit the pilot fuel. The aim of this study was to enhance the fundamental understanding of soot formation and oxidation processes of n-dodecane in the presence of methane in the air charge by parametric variation of methane equivalence ratio, charge temperature, and pilot fuel injection duration. The influence of methane on ignition delay and flame extent of the pilot fuel jet has been determined by simultaneous excited-state hydroxyl radical (OH*) chemiluminescence and Schlieren imaging.

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.

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.

The operation of dual fuel engines, operated with natural gas as main fuel, offers the potential of substantial savings in CO2. Nevertheless, the operating map area where low pollutant emissions are produced is very narrow. Especially at low load, the raw exhaust gas contains high concentrations of unburned methane and, with high pilot fuel portions due to ignition limitations, also soot. The analysis of the combustion in those conditions in particular is not trivial, since multiple combustion modes are present concurrently. The present work focuses on the evaluation of the individual combustion modes of a dual fuel engine, operated with natural gas as main and diesel as pilot fuel, using a combustion model. The combustion has been split in two partwise concurrent combustion phases: the auto-ignition phase and the premixed flame propagation phase.

This study investigates n-dodecane split injections of “Spray A” from the Engine Combustion Network (ECN) using two different turbulence treatments (RANS and LES) in conjunction with a Conditional Moment Closure combustion model (CMC). The two modeling approaches are first assessed in terms of vapor spray penetration evolutions of non-reacting split injections showing a clearly superior performance of the LES compared to RANS: while the former successfully reproduces the experimental results for both first and second injection events, the slipstream effect in the wake of the first injection jet is not accurately captured by RANS leading to an over-predicted spray tip penetration of the second pulse. In a second step, two reactive operating conditions with the same ambient density were investigated, namely one at a diesel-like condition (900K, 60bar) and one at a lower temperature (750K, 50bar).

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].

Fuel spray propagation and its morphology are important aspects for the in-cylinder mixture preparation in Diesel engines. Since there is still a lack of suitable measurements with regard to large 2-stroke marine Diesel engines combustion systems, a comprehensive data set of spray characteristics has been investigated using a test facility reflecting the specific features of such combustion systems. The spray penetration, area and cone angle were analysed for a variation of gas density (including the behaviour at evaporation and non-evaporating conditions), injection pressure and nozzle diameter. Moreover, spray and swirl flow interaction as well as fuel quality influences have been studied. To analyse the impacts and effects of each measured parameter, an empirical correlation for the spray penetration has been derived and discussed for all measurements presented.

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.

Direct injection of natural gas in engines is considered a promising approach toward reducing engine out emissions and fuel consumption. As a consequence, new gas injection strategies have to be developed for easing direct injection of natural gas and its mixing processes with the surrounding air. In this study, the behavior of a hollow cone gas jet generated by a piezoelectric injector was experimentally investigated by means of tracer-based planar laser-induced fluorescence (PLIF). Pressurized acetone-doped nitrogen was injected in a constant pressure and temperature measurement chamber with optical access. The jet was imaged at different timings after start of injection and its time evolution was analyzed as a function of injection pressure and needle lift.

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.

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 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.

Addition of hydrogen-rich gas to gasoline in internal combustion engines is gaining increasing interest, as it seems suitable to reach near-zero emission combustion, able to easily meet future stringent regulations. Bottled gas was used to simulate the output of an on-board reformer (21%H2, 24%CO, 55%N2). Measurements were carried out on a 4-stroke, 2-cylinder, 0.5-liter engine, with EGR, in order to calculate the heat release rate through a detailed two-zone model. A quasi-dimensional model of the flame was developed: it consists of a geometrical estimate of the flame surface, which is then coupled with the heat release rate. The turbulent flame speed can then be inferred. The model was then applied to blends of gasoline with hydrogen-rich gas, showing the effect on the flame speed and transition from laminar to turbulent combustion.

The goal of the Clean Engine Vehicle project (CEV) was the conversion of a gasoline engine to dedicated natural gas operation in order to achieve a significant reduction in CO2 emissions. The targeted reduction was 30% compared with a gasoline vehicle with similar performance. Along with the reduction in emissions, the second major requirement of the project, however, was compliance of the results with Euro-4 and SULEV emission limits. The project entailed modifications to the engine and the pre-existing model-based engine control system, the introduction of an enhanced catalytic converter and downsizing and turbocharging of the engine. As required by the initiators of the project, all components used were commonly available, some of them just being optimized or modified for natural gas operation.

In order to meet future requirements for emission reduction and fuel economy a variety of concepts are available for gasoline engines. In the recent past new pathways have been found using alternative fuels and fuel combinations to establish cost optimized solutions. The presented concept for a SI-engine consists of combined injection of gasoline and hydrogen. A hydrogen enriched gas mixture is being injected additionally to gasoline into the engine manifold. The gas composition represents the output of an onboard gasoline reformer. The simulations and measurements show substantial benefits to improve the combustion process resulting in reduced cold start and warm up emissions and optimized part load operation. The replacement of gasoline by hydrogen-rich gas during engine start leads to zero hydrocarbons in the exhaust gas.