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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.

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.

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).

Large Eddy Simulation (LES) of premixed turbulent combustion in a confined cylinder setup at engine relevant conditions has been carried out for three different initial turbulence intensities, mimicking different flame propagation regimes. Direct Numerical Simulation (DNS) of the setup under investigation provides the reference data to be compared against. The DNS fields have been filtered on the LES grid and are used as initial conditions for the LES at onset of combustion, guaranteeing a direct comparability of the single realizations between the modeled and reference data. Two different combustion models, the G-Equation and CMC-premixed (Conditional Moment Closure) are compared with respect to their predictive capabilities as well as their usability and computational cost. While the G-Equation is a widely adopted approach for industrial applications and usually relies on a tunable turbulent flame speed closure, the novel LES-CMC comes as a tuning parameter free model.

A phenomenological model for heat release rate predictions taking into account the characteristic processes inside a direct injection gasoline engine is presented. Fuel evaporation and preparation as well as the specifics of premixed and mixing controlled combustion phase are regarded. Only a few model constants need to be set which have been fit empirically for the application in a one-cylinder research engine. This jet guided direct injection gasoline engine employs a modern common-rail injection system and runs predominantly in stratified mode. The model allows the prediction of the influence of numerous parameter variations, e.g. injection-ignition phasing, load, engine speed, swirl, etc. on the combustion process. Furthermore efficient simulations can be carried out without using expensive three-dimensional CFD (computational fluid dynamics) calculations.

The physical behavior of the combustion process in a jet-guided direct injection spark ignition engine has been investigated with three different measurement techniques. These are flame visualization by use of endoscopy, ion-current sensing at 16 different locations in the combustion chamber and the estimation of the flame temperature as well as soot concentration based on multi-wavelength-pyrometry. The results of all these measurement techniques are in good agreement between each other and give a coherent picture of the physical behavior of the combustion process and make it possible to characterize the main influence parameters on combustion. This serves as a basis for validation and improvement of simulation tools for the engine thermodynamics and combustion.

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].

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.

Modern Diesel engines have become ever more complex systems with many degrees of freedom. Simultaneously, with increasing computational power, simulations of engines have become more popular, and can be used to find the optimum set up of engine operation parameters which result in the desired point in the emission-efficiency trade off. With increasing number of engine operation parameter combinations, the number of calculations increase exponentially. Therefore, adequate models for combustion and emissions with limited calculation costs are required. For obvious reasons, the accuracy of the ignition timing is a key point for the following combustion and emission model quality. Furthermore, the combination of mixing and chemical processes during the ignition delay is very challenging to model in a fast way for a wide range of operation conditions.

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.

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.

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.

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.

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.

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.

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.

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.

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).

A recently developed auto-ignition model based on a single transport equation in combination with a reduced kinetic scheme has been validated and tested in combination with a cascade jet and droplet breakup model. The validation has been performed by comparing ignition locations and delays for various thermodynamic conditions with experimental data from a high-pressure combustion cell. Also for medium-size diesel engine applications, predictions of ignition delay are in good agreement with experimental observations. In addition, a new approach to the modeling of the early flame development in diesel engine combustion is introduced. The reaction rate in the transition phase from the premixed to the mixing-controlled combustion mode is determined by means of a sub-grid scale model, which describes the evolution of a turbulent diffusion flame. The model has been tested during the early combustion phase of a medium-size, medium-speed DI diesel engine.

A computational study of the near-wall premixed flame propagation in homogeneous charge spark ignited engines is presented on the basis of a spectral concept accounting for flow-chemistry interaction in the flamelet regime. Flame surface enhancement due to wrinkling and modification of the local laminar flame speed due to flame stretch are the main phenomena described by the model. A high pass filter in the turbulent kinetic energy spectrum associated with the distance between the ensemble-averaged flame front location and the solid surface has been also introduced. In addition a probability density function of instantaneous flamelet positions around the above mean flame front location allows to consider statistical effects in a simplified way. Issues of temperature distribution within the boundary layer and associated heat losses, except for the concept of a thermal quenching distance, are thereby not explicitly taken into account.