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High-speed (12 kHz) imaging of combustion luminosity (enhanced by using a sodium fuel additive) has been analyzed and compared to crank angle resolved heat release rates and mass fraction burn profiles in a spray-guided spark-ignited direct-injection (SG-SIDI) optical single-cylinder engine. The addition of a sodium-containing additive to gasoline greatly increases the combustion luminosity, which allows unintensified high-speed (12 kHz) imaging of early partially premixed flame kernel growth and overall flame propagation with excellent signal-to-noise ratio for hundreds of consecutive engine cycles. Ignition and early flame kernel growth are known to be key to understanding and eliminating poor burn cycles in SG-SIDI engines.

Unburned hydrocarbon (HC) emissions and processes leading thereto are quantified in a single-cylinder version of an experimental V6 direct-injection (DI) two-stroke engine. Fast-response HC sampling at the exhaust port of the engine is integrated with simultaneous acquisition of individual-cycle cylinder-pressure data and with high-speed imaging of the fuel spray and spectrally resolved combustion luminosity. For every engine cycle, both the total HC mass and the fractions thereof that leave the cylinder during the cylinder-blowdown, main-scavenging, and port-closing phases are determined using a pressure-based calculation of the individual-cycle exhaust mass flow rate. At light load, HCs exhausted during the main-scavenging phase (when the transfer ports are open) account for 60-70% of the total HC mass and are strongly correlated with the amount of unburned fuel in each cycle.

Crevice flows of hydrocarbon fuel (both liquid and vapor) have been observed directly from fuel-injector mounting and nozzle-exit crevices in an optically-accessible single-cylinder direct-injection two-stroke engine burning commercial gasoline. Fuel trapped in crevices escapes combustion during the high-pressure portions of the engine cycle, exits the crevice as the cylinder pressure decreases, partially reacts when mixed with hot combustion gases in the cylinder, and contributes to unburned hydrocarbon emissions. High-speed laser Mie-scattering imaging reveals substantial liquid crevice flow in a cold engine at light load, decreasing as the engine warms up and as load is increased. Single-shot laser induced fluorescence imaging of fuel (both vapor and liquid) shows that substantial fuel vapor emanates from fuel injector crevices during every engine cycle and for all operating conditions.

Particle-image velocimetry (PIV) has been used in an engine to produce a virtually continuous two-dimensional velocity-vector map over a 12 × 32 mm area. The particle-seeded flow field in the clearance volume of a motored engine (600 r/min, 8:1 compression) was illuminated by a double-pulsed sheet of laser light (20-40μs pulse separation) oriented parallel to the piston. The illuminated particles (<1μm) were photographed at 78deg BTDC compression and 12deg ATDC with 1 × magnification, resulting in paired particle images separated by distances ∼200μm. The two-dimensional velocity distribution was determined by interrogating 0.9-mm square spots on a 0.5-mm grid spacing. The average particle image-pair displacement within each interrogation spot was determined by performing a spatial correlation, and thus the magnitude and direction of the average velocity within the interrogation spot was inferred from the light-pulse separation.

Laser-Doppler velocimetry has been used to investigate the effects of piston-bowl geometry (cylindrical and reentrant) and intake-swirl ratio (4.5 and 6.5) on the structure and evolution of the turbulent flow field in a motored engine (compression ratio: 10.6, speed: 600 r/min). High-shear regions and associated turbulence production are observed just inside the bowl entrance around TDC of compression. Before TDC, these regions are created in both geometries by the opposing effects of swirl and squish. As the piston passes through TDC and the bulk squish flow reverses, the high-shear, turbulence-producing region inside the rim of the cylindrical bowl disappears, but it persists within the reentrant bowl as a direct consequence of the geometry.