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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.

Effects of fuel/air equivalence ratio variations (Φ = 1.0±0.02) on engine-out and catalyst-out hydrocarbon (HC) mass and speciated emissions were measured under simulated cold-start conditions in order to suggest ways to optimize the engine-controls-catalyst system for minimum HC mass emissions and specific reactivity. A single-cylinder engine (installed in a temperature-controlled room and using commercial-grade gasoline) is run under controlled steady-state conditions (at 24 °C or -7 °C) which simulate cold starting. Speciated and total hydrocarbon emissions are measured from engine-out exhaust samples and from samples taken after an oven-temperature-controlled catalyst (either a fresh platinum/rhodium production catalyst, a 50,000 mile vehicle-aged catalyst, or a ceramic brick with standard washcoat containing no noble metal). Changes in engine fuel/air equivalence ratio (Φ = 1.0±0.02) have a small effect on engine-out HC mass emissions (± 10 %) and specific reactivity (0 - 2%).

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.