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Recently, high-speed optical imaging data for a single operating point of a spray-guided gasoline engine has, along with the flamelet model and the G-equation theory, enabled the development of the new spark-ignition model SparkCIMM. Within its framework, detailed chemistry flamelet models capture the experimental feature of multiple localized ignition events along the excessively stretched and restriking spark channel, as well as the observations of non-spherical highly corrugated early turbulent flame fronts. The developed flamelet models account for the substantial turbulent fluctuations in equivalence ratio and enthalpy present under spray-guided conditions. A non-unity Lewis number formulation captures the deficient species diffusion into the highly curved flame reaction zone.

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.

Two- and three-dimensional images of fuel distributions in a continuously firing direct-injection stratified-charge engine have been recorded under moderate-load conditions using planar laser-induced fluorescence (LIF) from commercial gasoline. Cyclic variations in the fuel concentration at the spark gap (deduced from individual-cycle two-dimensional images) appear sufficient to account for the observed incidence of misfires and partial burns. Tomographic three-dimensional LIF images of the average fuel distribution at the time of spark indicate that ignitable mixture is present only in a thin shell around the periphery of the fuel cloud. Differences in power output and combustion stability during engine warm-up observed with two injectors of the same type are reflected in systematic differences in the fuel concentration near the spark gap as inferred from LIF data.

In order to generate turbulence and thereby improve fuel-air mixing and combustion, direct-injection engines often incorporate high-squish piston bowls and intake-generated air swirl. Here, laser-velocimetry measurements of turbulent air motion in a motored direct-injection engine are examined with power-spectral analysis and with conventional and filtered ensemble-averaging techniques. Results from cylindrical and square piston bowls are interpreted in the context of conventional eddy-cascade concepts of turbulence. In particular, the results show that after intake, as the velocity fluctuations decrease in intensity, their power spectrum (frequency distribution) E(f ) relaxes toward the canonical ƒ-5/3 form associated with stationary, homogeneous turbulence in the inertial subrange. During the turbulence-production period around compression TDC, however, the power spectrum exhibits increased high-frequency content and (in the square bowl) anisotropy.

Computer simulations are compared with measurements of the three-dimensional, unsteady scavenging flows of a motored, crankcase-scavenged, two-stroke engine. Laser Doppler velocimetry measurements were made on a modified Suzuki DT-85 ported engine. Calculations were performed using KIVA-3, a computer program that efficiently solves the transfer and exhaust port flows along with those in the cylinder. Measured and computed cylinder pressures and velocities are compared. Pressures agree well over the cycle as do the velocities at the transfer port/cylinder interface. In-cylinder velocities differ in detail, but the tumbling motion in the cylinder is well replicated in a vertical plane passing through the cylinder axis.

Air-assist fuel sprays have been investigated experimentally with exciplex laser-induced fluorescence visualization and computationally with the KIVA-3 code. The exciplex-fluorescence technique provided simultaneous but distinct cross-sectional images of the liquid and vapor fuel distributions under simulated light-load conditions in both an atmospheric-pressure test rig and in a motored two-stroke engine. The computations resolved the flow through the injector passages upstream of and around the poppet, and included the effects of aerodynamic drop breakup, drop collisions and vaporization. Both the measurements and the calculations show that the fuel initially emerges from the injector as a hollow-cone jet. This two-phase jet collapses downstream as entrainment of air produces a low-pressure region beneath the poppet.

Transfer-port and in-cylinder flow fields have been mapped in a crankcase-compression, loop-scavenged two-stroke engine under motored conditions (1600 r/min; delivery ratio: 0.5). The impulsive, high-velocity flow (initially ≳2200 m/s) issuing from the transfer ports is fairly uniform and symmetric in space. The resulting in-cylinder flow field displays a classic scavenging loop pattern, but is complex and asymmetric. The data also characterize backflow from the cylinder into the transfer ports and the spin-up and breakdown of the scavenging-loop vortex during compression. The detailed LDV results provide some quantitative support for the widely used Jante scavenging test. FOR THE GREATER PART OF A CENTURY, the scavenging process has been recognized as critical to the performance of two-stroke-cycle engines.

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.

Fluid-motion and turbulence phenomena important to fuel-spray dynamics, mixing, and combustion have been examined in a motored direct-injection engine using photon-correlation laser-Doppler velocimetry. Operating at 11:1 compression ratio, the engine incorporated a centrally located, cylindrical piston bowl and an off-axis, directed intake port. This study concentrates on the tangential (swirl) and radial (squish) velocity components during the compression stroke and the early portion of the expansion stroke. Measurements were made at low engine speeds (primarily 600 r/min) along two orthogonal radii at four depths within the piston-bowl's TDC volume. The mean flow pattern in diametral planes can be described as off-axis swirl with a moving center of rotation that varies with both depth and engine speed. Peak swirl velocities occur about 20 deg BTDC of compression.