Conditional Analysis of Enhanced Combustion Luminosity Imaging in a Spray-Guided Gasoline Engine with High Residual Fraction

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. Ignition is difficult in these engines especially at low speeds and light loads because of the very lean overall operation, high levels of residuals or EGR to minimize NOx emissions, and adverse conditions near the spark plug (e.g., high velocities, intense turbulence, and large fuel-air stratification). In this paper, automated combustion image analysis is used to identify the flame outline, to determine its area and to evaluate contours of flame probability (progress variable) with sub crank angle resolution. Ignition and early flame growth are compared for good cycles (mass fraction burned = 0.8-1.0) and partial-burn cycles (in mass fraction burned intervals) by using conditional analysis. The good burn cycles show fast early flame kernel growth, fast subsequent flame growth and nearly spherical flame propagation (as viewed through the bottom piston window). Poor burn cycles show slower early flame kernel growth, somewhat slower later flame growth and much slower and more asymmetric flame propagation. For these operating conditions, acceptable combustion occurs after the flame kernel has reached a critical radius of 5 mm or 1% mass fraction burned.