Donald T. French

Cycle-Resolved Laser-Velocimetry Measurements in a Reentrant-Bowl-in-Piston Engine

Cycle-resolved measurements of in-cylinder air velocity have been made in a motored engine (600-r/min engine speed, 10.6:1 compression) having a high-squish reentrant-bowl piston. The velocity fluctuations have been analyzed both by low-pass/highpass frequency filtering and by evaluation of non-stationary velocity autocorrelation functions. These two complementary analyses and general issues pertaining to their interpretation are carefully examined

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 (∼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 premixed flame propagation and ends as slower mixing-limited or diffusion burning, (2) reveal quantitatively the fate of unburned fuel in misfire and partial-burn 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).

Swirl, squish and turbulence in stratified-charge engines: laser-velocimetry measurements and implications for combustion

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.

High-speed flow measurements in a two-stroke engine by photon-correlation laser velocimetry.

Three-dimensional mapping of the spatial structure and the temporal evolution of the air flow field in a commercially available two-stroke engine requires the ability to resolve a wide range of flow velocities (±250 m/s) at measurement locations near the cylinder wall and the piston crown. Laser-Doppler velocimetry that uses photon-correlation signal processing and Fourier-transform analysis permits ensemble-averaged velocity-distribution functions to be extracted from scattered-light signals that are sampled at nearly the Nyquist limit and whose strength is typically less than 1 photon per Doppler cycle. Selected results from a systematic study illustrate the impulsive jet-like flow through the transfer ports into the engine cylinder, the complexity of the resultant in-cylinder flow field, and the likelihood of large-scale flow-field variations from one engine cycle to another.