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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.

Measurements of crank angle resolved air velocity and fuel droplet velocity inside the intake port of a six cylinder four valve production engine were performed using two component Laser Doppler Velocimetry (LDV). Prior to the engine measurements the fuel injector was characterized by determining time resolved droplet sizes and velocities with Phase Doppler Velocimetry (PDV) at an injector test rig with complete optical access. PDV results indicate that during spray penetration into quiescent air at atmospheric pressure (test rig conditions) large droplets move at the tip of the spray while small droplets due to their low force of inertia are slowed down by aerodynamic pressure and pile up at the end of the spray. Mean values of the droplet diameter rise with the distance from the injector because the smallest droplets do not reach the downstream measurement locations.

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.

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.

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 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.