2-Color Thermometry Experiments and High-Speed Imaging of Multi-Mode Diesel Engine Combustion

Abstract

Although in-cylinder optical diagnostics have provided significant understanding of conventional diesel combustion, most alternative combustion strategies have not yet been explored to the same extent. In an effort to build the knowledge base for alternative low-temperature combustion strategies, this paper presents a comparison of three alternative low-temperature combustion strategies to two high-temperature conventional diesel combustion conditions. The baseline conditions, representative of conventional high-temperature diesel combustion, have either a short or a long ignition delay. The other three conditions are representative of some alternative combustion strategies, employing significant charge-gas dilution along with either early or late fuel injection, or a combination of both (double-injection). These operating conditions are investigated for soot volume fraction, soot temperatures, calculated adiabatic flame temperatures, and soot radiation heat loss through 2-color soot thermometry experiments. The spatial location of in-cylinder soot is imaged using a high-speed CMOS camera, and exhaust-gas NO x is also measured. The soot thermometry and high-speed soot luminosity imaging show that the low-temperature operating conditions have lower in-cylinder soot than the high-temperature conditions. Also, soot is formed upstream in the jet for high-temperature operating conditions, but for low-temperature operating conditions, the soot is formed farther downstream, closer to the bowl edge. For all conditions, the onset of in-cylinder soot occurs after the premixed bum, during the mixing-controlled combustion phase. As the amount of soot decreases, the radiation heat loss also decreases drastically. For conventional diesel diffusion combustion operating condition, radiation from soot is about 1.1 percent of the total fuel energy, but for low-temperature combustion operating conditions, the soot radiative heat loss is almost negligible (≈ 0.01 percent). The condition with high soot radiation had peak soot temperatures as much as 300 K lower than the peak adiabatic flame temperatures near 2700 K, and exhaust NO x emissions were near 600 ppm. For the low-temperature conditions, the peak soot temperatures were only about 200 K lower than the peak adiabatic temperatures near 2200 K, and the exhaust NO x concentrations were less than 10 ppm.