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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.

In most optimization strategies of combustion processes in gasoline IC engines, spatial inhomogeneities of the temperature, of the residual exhaust gas and of the fuel-air distribution play a major role. Hence, the development of experimental methods for the simultaneous quantification of both, concentration and temperature fields, is highly desirable. One method which is in particular suitable for measuring these quantities is the technique of two-laser excitation of the fluorescence of ketones like 3-pentanone and acetone. Different groups have used 3-pentanone for the measurement of fuel concentration. In this work we present the determination of the exhaust gas concentration field simultaneously with the temperature field using acetone as an intake air tracer. Acetone is a more suitable gas tracer than 3-pentanone due to its higher vapor pressure and its better stability regarding thermal decomposition.

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.

This paper presents calibration results of the two excitation laser wavelength fluorescence of acetone for the determination of temperature fields. The calibration was performed under engine relevant conditions and comprises the simultaneous variation of temperature and pressure in the range of 293 K to 750 K and of 0.1 MPa to 2 MPa, respectively. The influence of different gas compositions, e.g., resulting from exhaust gas recirculation, was checked by calibrating for pure nitrogen, synthetic air and carbon dioxide and for a certain extend for water as bath gas. The two excitation wavelengths used were 248 nm and 308 nm.

The direct view into the combustion chamber of a direct injection (DI) Diesel engine allows a fast, comprehensive analysis of the influence of different engine parameters on the combustion process. Therefore and in order to acquire a maximum amount of information from one engine cycle, a combination of four non-intrusive temporally and spatially highly resolving optical measurement techniques were applied simultaneously to a passenger car DI Diesel transparent engine. These measurement techniques include the detection of the flame luminosity in the UV-range as well as the detection of thermally excited soot radiation in the visible range, the visualization of the distribution of the liquid fuel phase by the Mie scattering technique and the laser-induced incandescence (LII) technique for the characterization of the two-dimensional soot distribution inside a selected plane of the combustion chamber.

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 application of exhaust gas aftertreatment systems is currently discussed to be the most suitable solution to significantly reduce soot and nitrogen oxide emissions of modern diesel engines. Nevertheless, an improvement of the engine combustion process reducing the raw emissions must be seen in combination with such systems or as a replacement. In this study, the influence of nozzle geometry, rail pressure and pre-injection on injection, vaporisation and combustion was analysed in a transparent single-cylinder diesel engine equipped with a common rail injection system by means of optical measurement techniques. The results show that a high-speed fuel intake into the combustion bowl, in combination with high rail pressures, forces the injection jets to break-up close to the wall of the combustion bowl. The engine swirl and the influence of the wall improve the mixture formation.

Increasing decentralization of production combined with just-in-time delivery of products and components calls for a flexible and reliable transportation system. So far, trucks offer the most versatile and efficient solution to those problems. In consideration of increasingly strict emission standards and customer demands for more engine power and less fuel consumption, further selective developments and optimization of DI-diesel engines are necessary. One step in this direction is the application of 4 valves per cylinder in heavy-duty diesel engines to improve mixture formation of fuel and air to get a cleaner combustion and a higher power output. For visualizing the combustion processes inside the engine, an optically accessible heavy-duty DI-diesel engine was used. This engine is a slightly modified conventional heavy-duty MAN engine based on the D0824 LFL 06 series.

Future exhaust gas limits for diesel-driven passenger cars will force the automotive industry to significantly improve the design of the vehicles respectively of the drive assemblies. Especially the contributions of soot and nitrogen oxide will be the main problems in the future. One possible solution could be the application of suitable exhaust gas aftertreatment systems, but since modern common rail injection systems deliver more degrees of freedom referring to the injection process, again the optimisation of the injection process could offer a possibility to meet the exhaust gas limits. In this study, a passenger car common rail system, applied to an optically accessible transparent engine based on the AUDI V6 TDI engine, was investigated using a solenoid-driven and a piezo-driven injector, both equipped with the same injection nozzle.

A central point for the further development of direct injection engines is the optimization of the mixture formation process, because all subsequent processes as ignition, combustion and pollutant formation are mainly influenced by the local air/fuel-ratio inside the cylinder. Especially for passenger car engines the interaction between the spray and the combustion chamber walls is an important issue for mixture formation. For that reason this interaction was object of the investigation described. The investigations were carried out in a heatable high pressure high temperature chamber under typical diesel engines conditions of 450°C temperature and 50 bar pressure. A passenger car common rail system was used as injection system equipped with a 6 hole nozzle with common rail specific seat geometry, mini-sac hole geometry and double needle guide.

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.

The development of new generations of internal combustion engines requires appropriate measurement techniques for all relevant limited exhaust gas species and particulates. However, because of stricter future emission limits, there is a severe lack especially with respect to soot particles. Conventional methods, like gravimetric sampling, have substantial deficiencies in sensitivity and temporal resolution, which is strongly required for transient tests. Furthermore, artifacts arise from other exhaust components, like sulfuric acid, water vapor and volatile hydrocarbons. In contrast to the state-of-the-art techniques, laser induced incandescence (LII) has been proved to be a favorable technique, which overcomes these deficiencies and offers additional information, which allows new insight into combustion phenomena. Besides soot mass concentration, also the soot primary particle size is accessible by this technique.

The linear Raman scattering has been applied for the investigation of the influence of fuel properties on the mixture formation of high pressure swirl injectors. The measurements have been performed in an optically accessible high pressure high temperature chamber with a multi-component fuel consisting of benzene and n-decane. The local air/fuel-ratio and the composition of the fuel vapor phase were detected simultaneously with high spatial and local resolution along a line inside the spray region of the injector. In addition to the measurements performed with this two-component fuel mixture formation of the injector has also been studied by the separate use of the fuel components alone. Furthermore the influence of injector cooling on mixture formation has been investigated.