Advanced diagnostics for minimizing hydrocarbon emissions from a direct-injection gasoline engine

Minimizing unburned-hydrocarbon (HC) emissions at light load is essential for realizing the potential fuel-economy, cold-start, and transient-HC advantages of direct-injection (DI) stratified-charge engines. This paper summarizes the application of several advanced diagnostics to understand and quantify HC sources in an experimental DI two-stroke engine. Single-cycle (two-dimensional) and multicycle-averaged (two-dimensional and reconstructed three-dimensional) laser-induced-fluorescence (LIF) imaging of gasoline (1) characterizes the highly stratified fuel distribution at the time of ignition, (2) identifies cyclic variations in the fuel concentration near the spark gap as a principal cause of misfires and partial burns, (3) reveals regions of fuel-air mixture around the periphery of the fuel cloud that are too lean to burn, and (4) detects the outgassing of unburned fuel from the fuel injector nozzle-exit crevice late in the engine cycle. Cyclic variations are investigated further by collecting continuous, time-resolved data on liquid fuel distributions, combustion, and exhaust hydrocarbon emissions over many consecutive engine cycles. Specifically, high-speed (4000 frames/s) video imaging of the fuel spray and of spectrally resolved combustion luminosity is combined with simultaneous exhaust-HC sampling using a close-coupled fast-response (similar to 2 ms) flame-ionization detector. Cylinder pressure is also digitized simultaneously, so that the imaging results can be correlated with the heat released and the exhaust HC mass for each engine cycle. The results (1) show that combustion begins as partially premised flame propagation and ends as slower mixing-limited or diffusion burning, (2) reveal quantitatively the fate of unburned fuel in misfire and partial-built cycles, and (3) provide strong evidence that the dominant HC sources are incomplete combustion of the injected fuel cloud and late release of fuel trapped in the injector nozzle-exit crevice (rather than fuel trapped in the piston top-ring-land crevice, which is the dominant HC source in conventional homogeneous-charge four-stroke engines).