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Measurements of ignition behavior, homogeneous reactor simulations employing detailed kinetics, and quantitative in-cylinder imaging of fuel-air distributions are used to delineate the impact of temperature, dilution, pilot injection mass, and injection pressure on the pilot ignition process. For dilute, low-temperature conditions characterized by a lengthy ignition delay, pilot ignition is impeded by the formation of excessively lean mixture. Under these conditions, smaller pilot mass or higher injection pressures further lengthen the pilot ignition delay. Similarly, excessively rich mixtures formed under relatively short ignition delay conditions typical of conventional diesel combustion will also prolong the ignition delay. In this latter case, smaller pilot mass or higher injection pressures will shorten the ignition delay. The minimum charge temperature required to effect a robust pilot ignition event is strongly dependent on charge O2 concentration.

Fuel tracer-based planar laser-induced fluorescence is used to investigate the vaporization and mixing behavior of pilot injections for variations in pilot mass of 1-4 mg, and for two injection pressures, two near-TDC ambient temperatures, and two swirl ratios. The fluorescent tracer employed, 1-methylnaphthalene, permits a mixture of the diesel primary reference fuels, n-hexadecane and heptamethylnonane, to be used as the base fuel. With a near-TDC injection timing of −15°CA, pilot injection fuel is found to penetrate to the bowl rim wall for even the smallest injection quantity, where it rapidly forms fuel-lean mixture. With increased pilot mass, there is greater penetration and fuel-rich mixtures persist well beyond the expected pilot ignition delay period. Significant jet-to-jet variations in fuel distribution due to differences in the individual jet trajectories (included angle) are also observed.

In spark-ignition engines, high exhaust gas recirculation (EGR) rates have demonstrated their potential in reducing fuel consumption and emissions. However, irregular combustion at high residual gas concentrations limits the EGR rates. The following study presents a strategy that has been developed to investigate the influence of complex charge motion on mixture formation and combustion for high residual gas concentrations with the aim of extending these limits. An optically accessible single-cylinder SI Engine with direct injection was used to measure the charge distribution by means of laser induced fluorescence (LIF). A special device inside the inlet pipe gave the possibility to generate a defined swirl motion overlaying a tumble motion given by the design of the inlet ports.

Biofuels and alternative fuels are increasingly being blended with conventional gasoline fuel to decrease overall CO₂ emissions. A promising way to achieve this is the use of DISI (direct-injection spark-ignition) technology. However, depending on temperature, pressure, chemical composition and the spark timing, unwanted pre-ignition may occur. Despite higher compression ratios, this engine knock can be decreased by lowering the mixing temperature. This results from the larger fuel evaporation enthalpy of certain biofuels which provides a non-homogeneous mixture throughout the combustion chamber. This work focuses on estimating the biofuel evaporation rate from absolute local vapor temperature and concentration. Measurements conducted in a high temperature/pressure cell using a multi-hole injector are carried out by applying planar, 2-line, laser-induced fluorescence and phase doppler interferometry.

This paper describes a computer simulation of Diesel spray formation and the locations of self-ignition nuclei. The spray is divided into small elementary volumes in which the amounts of fuel and fuel vapours, air, mean, maximum and minimum fuel droplet diameter are calculated, as well as their number. The total air-fuel and air-fuel vapour ratios are calculated for each elementary volume. The paper introduces a new criterion for determining self-ignition nuclei, based on assumptions that the strongest self-ignition probability lies in those elementary volumes containing the stoichiometric air ratio, where the fuel is evaporated or the fuel droplet diameter is equal to or lower than 0.0065 mm. The most efficient combustion in regard to consumption and emission will be in those elementary volumes containing stoichiometric air ratio, and fuel droplets with the lowest mean diameters. Measurements of injection and combustion were carried out in a transparent research engine.

The influence of fuel composition on sprays was studied in an injection chamber at DISI conditions with late injection timing. Fuels with high, mid and low volatility (n-hexane, n-heptane, n-decane) and a 3-component mixture with similar fuel properties like gasoline were investigated. The injection conditions were chosen to model suppressed or rapid evaporation. Mie scattering imaging and phase Doppler anemometry were used to investigate the liquid spray structure. A spray model was set up applying the CFD-Code OpenFOAM. The atomization was found to be different for n-decane that showed a smaller average droplet size due to viscosity dependence of injected mass. And for evaporating conditions, a stratification of the vapor components in the 3-component fuel spray was observed.

Multi-hole DI injectors are being adopted in the advanced downsized DISI ICE powertrain in the automotive industry worldwide because of their robustness and cost-performance. Although their injector design and spray resembles those of DI diesel injectors, there are many basic but distinct differences due to different injection pressure and fuel properties, the sac design, lower L/D aspect ratios in the nozzle hole, closer spray-to-spray angle and hense interactions. This paper used Phase-Contrast X ray techniques to visualize the spray near a 3-hole DI gasoline research model injector exit and compared to the visible light visualization and the internal flow predictions using with multi-dimensional multi-phase CFD simulations. The results show that strong interactions of the vortex strings, cavitation, and turbulence in and near the nozzles make the multi-phase turbulent flow very complicated and dominate the near nozzle breakup mechanisms quite unlike those of diesel injections.

Flexible and multiple injections are an important strategy to fulfill today's exhaust emission regulations. To optimize injection processes with an increasing number of adjustable parameters knowledge about the basic mechanisms of spray breakup, propagation, evaporation and ignition is mandatory. In the present investigation the focus is set on spray formation and ignition. In order to simulate current diesel-engine conditions measurements were carried out in a high-temperature (1000 K) and high-pressure (10 MPa) vessel with optical accesses. A piezo servo-hydraulic injector pressurized up to 200 MPa was used to compare four single injection durations and four multi-injection patterns in the ignition phase. All measurements were performed with CEC RF-03-06, a legislative reference fuel. For the spray measurements, a program of 16 to 18 different operating points was chosen to simulate engine conditions from cold start to full load.

Combustion processes with partially or fully premixed cylinder load combined with self-ignition provide high combustion efficiency and low emissions of Nitrogen Oxides (NOx) and particulate matter at the same time. Since the number of diesel operated passenger cars is still rising, it would be interesting, if such a combustion concept can be realized in an ordinary DI-Diesel engine which is operated with conventional diesel fuel. In this study, the influence of nozzle geometry, Tintake, pTDC and injection timing on the functioning chain of combustion was analyzed in a transparent single-cylinder diesel engine equipped with a common rail injection system by means of optical measurement techniques. Simultaneously, different optical diagnostics (laser-based and non laser-based) were used to study the fuel distribution, ignition and combustion in the combustion chamber of the optically accessible diesel engine. The liquid fuel was visualized by Mie scattering at 532nm.

Laser-based measurements of charge temperature and exhaust gas recirculation (EGR) ratio in an homogeneous charge compression ignition (HCCI) engine are demonstrated. For this purpose, the rotational coherent anti-Stokes Raman spectroscopy technique (CARS) was used. This technique allows temporally and locally resolved measurements in combustion environments through only two small line-of-sight optical accesses and the use of standard gasoline as a fuel. The investigated engine is a production-line four-cylinder direct-injection gasoline engine with the valve strategy modified to realize HCCI-operation. CARS-measurements were performed in motored and fired operation and the results are compared to polytropic calculations. Studies of engine speed, load, valve timing, and injection pressure were conducted showing the strong influence of charge temperature on the combustion timing.

Planar laser-induced fluorescence (PLIF) has been successfully used for the investigation of the mixture formation process in hydrogen engines. Detailed information has been obtained about the process development (qualitative measurements) and on the fuel/air-ratio (quantitative measurements) in the combustion chamber. These results can be used for further optimization of the mixture formation and the combustion process concerning emissions and fuel consumption. The measurement technique used here is not limited to hydrogen and can also be applied to other fuel gases like natural gas. The main topic of this paper is the experimental verification of the PLIF data by simultaneous Raman scattering measurements. By Raman scattering the fuel/air-ratio can directly be determined from the direct concentration measurements of the different gas species.

Detailed experimental investigation of fuel sprays are of utmost importance for the development of appropriate injection systems for gasoline direct injection (GDI) engines. A number of laser based techniques have been developed to study the spray formation. The temperature of the gas phase surrounding the fuel droplets was not accessible up to now. In this work for the first time, to the best of our knowledge, gas-phase temperatures were measured within the vaporizing spray of a high pressure GDI injector using pure rotational coherent anti-Stokes Raman spectroscopy (CARS). Results from an isooctane fuel spray of a multi-hole injector in a heated injection chamber are presented with the probe volume located at a distance of 70mm downstream the injector nozzle in the centre of the spray cone.

The influence of the injector temperature on the spray distribution and fuel volatility of a high-pressure swirl injector of the type used in direct-injection gasoline engines and thus of flash boiling effect was investigated in a pressure chamber with optical access. Laser-induced (exciplex) fluorescence was used to visualize the liquid phase and the vapour phase of the spray. The experiments were conducted at a chamber pressure of 50 kPa and a chamber temperature of 323 K by varying the injector temperature (323 K, 343 K, 363 K and 381 K) at a constant rail pressure of 8 MPa. Three single-component fuels with different boiling points (n-hexane: Tb = 339 K, iso-octane: Tb = 372 K and n-octane: Tb = 398 K) and a non-aromatic multi-component fuel (mcf) (Tb = 303 K - 473 K) were chosen for the investigations. The dopant was a combination of 2% by mass TEA (triethylamine) and 3.4% by mass benzene in the non-fluorescing substitutional fuels.

The optimization of the vaporization and mixture formation process is of great importance for the development of modern gasoline direct injection (GDI) engines, because it influences the subsequent processes of the ignition, combustion and pollutant formation significantly. In consequence, the subject of this work was the development of a measurement technique based on the laser induced exciplex fluorescence (LIF), which allows the two dimensional visualization and quantification of the in-cylinder air/fuel ratio. A tracer concept consisting of benzene and triethylamine dissolved in a non-fluorescent base fuel has been used. The calibration of the equivalence ratio proportional LIF-signal was performed directly inside the engine, at a well known mixture composition, immediately before the direct injection measurements were started.

Mie-Scattering and laser induced exciplex fluorescence (LIEF) were used to visualize the distribution of liquid fuel and fuel vapor inside an optical accessible one-cylinder research engine with gasoline direct injection (GDI). Using a tracer which was developed especially for the environments of gasoline combustion engines, LIEF enables an extensive separation between liquid and vapor phase and delivers a signal proportional to the equivalence ratio. Simultaneous images of LIEF and Mie scattering proof the high quality of the phase separation using this tracer concept. The mixture formation process will be shown exemplary at one operation point with homogeneous load and another with stratified load. First results of determining the air/fuel ratio by means of linear Raman spectroscopy will be presented and compared with the two-dimensional qualitative distribution of the fuel vapor (LIEF).

Two-dimensional Mie and LIEF techniques were applied to investigate the spray propagation, mixture formation and charge distribution at ignition time inside the combustion chamber of a direct injection SI engine. The results obtained provide the propagation of liquid fuel relative to the piston motion and visualize the charge distribution (liquid fuel and fuel vapor) throughout the engine process. Special emphasis was laid on the charge distribution at ignition time for stratified charge operation. By means of a LIEF technique it was possible to measure cyclic fluctuations in the fuel vapor distributions which explain the occurrence of misfiring.

Two-dimensional Mie and LIEF techniques were applied to investigate the spray formation of a high pressure gasoline swirl injector in a constant volume chamber. The results obtained provide information on the propagation of liquid fuel and fuel vapor for different fuel pressures and ambient conditions. Spray parameters like tip penetration, cone angles and two new defined parameters describing the radial fuel distribution were used to quantify the fuel distributions measured. Simultaneous detection of liquid and vapor fuel was applied to study the influence of ambient temperature, injector temperature and ambient pressure on the evaporating spray.

In small high speed direct injection diesel engines the injected fuel spray impinges on the walls of the piston bowl. The mixture formation process is influenced considerably by the spray-wall interaction. Stringent exhaust gas emission regulations and growing demands for fuel economy are leading to the application of high-pressure fuel injection systems, e.g. common-rail. The trend towards downsized engines with smaller piston displacements leads to reduced distances between nozzle and wall. Higher injection pressures and smaller nozzle-wall distances both increase the significance of spray-wall interaction and near-wall mixture formation. In the present study the influence of governing parameters like injection pressure and wall temperature on the characteristics of the impinged spray was investigated.

The vapor phase of an evaporating spray from a heavy-duty Diesel common-rail injection system has been investigated with an optical diagnostic technique based on linear Raman scattering, which has been extended to the application in fuel sprays. One-dimensional spatially resolved Raman measurements of the air/fuel-ratio have been performed in the spray region with high local and temporal resolution in an injection chamber at an air pressure of 4.5 MPa and at a temperature of 450°C. The influence of different parameters, such as rail pressure, nozzle geometry and injection duration on the temporal evolution of the local air/fuel-ratio in the vapor phase has been studied quantitatively, and results from a selected spatial location are compared. Furthermore, the effect of physical/chemical fuel properties on the evaporation dynamics has been investigated by performing measurements with two different fuels.

Nitric oxides are the key pollutants emitted from SI engines today. In the work presented, the effect of different fuel-components on the NOx-emission of a four stroke SI engine and cross connections between different fuel properties were investigated in front of and behind the catalyst and compared to investigations described in literature. For the investigation presented a variety of different fuels has been produced. The content of aromatics, olefins, oxygen and the mid-range volatility has been changed systematically while only fuels with a good driveability were included in the investigation. The NOx-emission of 17 fuels tested was measured in front of and behind the catalyst. The tests were carried out with a single cylinder test engine using a constant air/fuel ratio.