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For practical, extensive 3-D computations for engine improvements, each physical submodel needs to be the simplest that is compatible with the accuracy of all other physical submodels and of the numerics. The addition of one progress variable controlled by one Arrhenius term is shown to be adequate to reproduce Diesel ignition delay in 2-D and 3-D computations. The rest of the model is that used for years by the authors to optimize combustion in reciprocating and rotary engines with premixed and non-premixed charges, including all of its model constants. This minimal Diesel autoignition submodel reproduces well trends and magnitudes of ignition delay versus chamber temperature and pressure. As in experiments, it is found that multiple ignition sources develop in rapid succession at various locations around the fuel spray after the first ignition event.

The exciplex fluorescence technique has been used to separately visualize liquid and vapor phase fuel in engines since its development by Melton. However, as a fluorescence technique it has the potential to be quantitative and the underlying assumptions have been outlined by Melton. An initial quantitative application of the TMPD/naphthalene system, based on these assumptions, applied to a hollow-cone spray in a two-stroke engine, indicated that it substantially over-estimates the concentration of fuel vapor about TDC. The reasons for the discrepancy were investigated and it was concluded that a major factor is the effect of temperature on the photophysics of the species involved. Thus the absorption spectra of the exciplex dopants were determined at temperatures up to 700 K. These experiments showed that the increase in absorption with temperature above 500 K is responsible for the failure of the earlier calibration.

A two-color Particle Image Velocimetry (PIV) technique has been applied for the first time to a firing, production, three-cylinder, two-stroke, direct-injection stratified-charge engine operated under realistic conditions. In comparison to single color PIV, two-color PIV can resolve the directional ambiguity of the velocity by cross-correlating two digitized photographic images of a particle-seeded flow field, acquired sequentially at two different light wavelengths. Such an approach is essential in complex, a priori unknown, flow fields, such as those of most I.C. engines. To gain optical access to the combustion chamber, the engine head was equipped with two optical windows in such a way that its original geometry was practically undisturbed. Although the field of view was relatively small, it covered a critical area of the combustion chamber. The measurements were made in the plane perpendicular to the engine longitudinal axis, within the crank angle range of 70 to 10 degrees BTDC.