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In an optical accessible 4-stroke engine laser-induced fluorescence (LIF) imaging measurements of fuel tracer (3-pentanone) and formaldehyde were performed during the compression stroke and combustion. Formaldehyde (HCHO) is intermediately present at high concentrations within the cool flame and is burned later on when the “hot” combustion proceeds. It can be used as an internally generated tracer to observe the boundaries of the hot combustion zones. Despite the fact that a frequency-tripled Nd:YAG laser excites only weak transitions in the HCHO molecule, the high concentration (several thousands ppm) provide for sufficient signal intensity when detecting fluorescence above 395 nm. Using formaldehyde LIF, auto-ignition (occurring close to 356°ca) and the further development of combustion was observed.

A database of information concerning the spray development and pollutant formation in common-rail, direct-injection Diesel engine is constructed using a transparent model Diesel engine. Spray development is investigated using optical diagnostics: Mie scattering and Laser Induced Exciplex Fluorescence (LIEF) make possible qualitative visualization of liquid and vapor phases. The injection pressure/nozzle hole diameter is found to be the most important parameter (in the parameter range used for the study): it reduces the liquid penetration length and improves the mixing of vapor fuel. Direct imaging of combustion development shows the influence of different engine parameters on flame location. Comparison with measured vapor distributions shows the effect of thermal expansion on the vapor plume before any light from combustion is visible. Soot formation is investigated using Laser Induced Incandescence imaging.

Using a combination of imaging techniques, we have produced a database of the macroscopic properties of sprays produced by a common-rail injection system in a diesel simulation cell. The parameters of the data base include injection pressure (40, 80 and 150 MPa), gas-side temperature (387, 800 and 1100 K), gas density (12, 25 and 30 kg/m3), injector nozzle hole size (0.17 and 0.20 mm) and injection programs (with and without pilot injection). Single component fuels (heptane and dodecane) were used in order to simplify data interpretation and modeling. The spray characteristics which were measured include the initial “dispersion” angle of the nozzle, initial spray tip speeds, and spray tip penetration vs. time for both the liquid and vapor parts of the spray. The sites of initial self ignition and combustion propagation within the sprays were visualized, and a luminous delay was measured for several of the operating conditions.

An indirect method to map the burned gases in SI engine has been developed. It is based on visualisation by Laser Induced Fluorescence of the unburned mixture seeded with biacetyl. Both internally and externally recirculated burned gases are monitored. This diagnostic is complementary to the LIF technique applied to measure the gasoline distribution. These LIF gasoline and burned gases measurements are applied in a 4-valve optical access SI engine for a large range of operating conditions. These include variations of both fuel injection and burned gas recirculation modes causing different types of stratification leading to very distinct heat release and exhaust emissions characteristics. Tumble level and spark location are also modified. The observation of the actual stratification in the engine forms a sound basis explanation of the engine performance.

In a 4-valve SI engine, various levels of gasoline stratification and bulk flows were achieved by modifying the intake configuration and injection mode. Mean flow and turbulence were evaluated with Laser Doppler Velocimetry. Planar Laser Induced Fluorescence on a gasoline doped with a tracer was used to characterise the equivalence ratio field while the pressure traces were simultaneously acquired. Very positive effects of the elevation of tumble levels on combustion speed were observed. The possibility of stratifying the mixture was assessed. An excellent correlation was observed between local equivalence ratio, and early heat release and total burn duration. With highly stratified gasoline distributions, an improvement of combustion behaviour was obtained when ignition was in the richer region.