Quantitative high-speed imaging of burned gas temperature and equivalence ratio in internal combustion engines using alkali metal fluorescence

Abstract

Alkali metal atoms show an intense natural fluorescence in the burned gas region of internal combustion engines. This fluorescence offers great opportunity for spectroscopic combustion analysis in internal combustion engines without the requirement of laser excitation or image intensifiers. To quantify this fluorescence intensity, spectroscopic and thermodynamic properties of the alkali metals lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs) and their oxidation products and ions were analyzed. Collisional energy transfer and reabsorption effects (including temperature- and pressure-dependent lineshapes) were calculated over the range of engine environments. Three compounds containing Li, Na and K, respectively, were selected as fuel additives for engine experiments. The experiments were conducted on an optical, single-cylinder, spark-ignited, direct-injection research engine, and the fluorescence of the three alkali components was recorded simultaneously using three complimentary metal-oxide semiconductor high-speed cameras. The two-component fluorescence intensity ratios (Na/K, Li/K and Na/Li) are shown to depend on temperature, pressure and equivalence ratio. However, the three-component ratio Na·Li/K2 is nearly independent of pressure and equivalence ratio in the tested range of operating conditions and can serve as a direct marker for burned gas temperature. Subsequently, equivalence ratio can be determined from any of the bicomponent fluorescence ratios.

Invited Review: Combustion instability in spray-guided stratified-charge engines: A review

“…Second—in marked contrast to the usual picture of an initially spherical flame kernel that gradually becomes more wrinkled by turbulence as it grows—the flame kernel here is non-spherical and highly wrinkled (<∼1 mm) at even the earliest times (Figure 10(c)). The images in Figure 10(c) were obtained using sodium-enhanced combustion luminosity imaging, 38,77 in which thermal emission from a sodium-containing additive in the fuel provides strong, visible-wavelength emission from combustion that otherwise emits predominantly in the ultraviolet and is therefore invisible to a non-intensified camera. The results in Figure 10 are completely consistent with high-speed planar Mie-scattering imaging of ignition and early flame development 57,73,76,78 as well as with a study in the UM engine using a combination of toluene LIF for fuel distribution imaging and high speed OH* imaging to locate the spark plasma and the developing flame.…”

Thermocouple temperature measurements in diesel spray flame for validation of in-flame soot formation dynamics

“…Application of other relatively advanced temperature measurement techniques such as spontaneous Raman scattering, 20 coherent anti-Stokes Raman scattering (CARS), 21 tracer-based laser-induced fluorescence (LIF), 13,22 phosphor thermography 23 and natural fluorescence from fuel-doped alkali metals 24 to the soot formation region of diesel spray flame is also extremely challenging due to high reactivity of the soot formation region, spectral broadening under engine-equivalent high-pressure conditions and strong background from luminous soot surrounding the spray core.…”

Experimental study of the relationship between injection rate shape and Diesel ignition using a novel piezo-actuated direct-acting injector

“…Fuel-air mixing process, combustion and emission phenomena are necessarily linked together in direct injection Diesel engines [1][2][3][4][5]. In a scenario where the global emission standards require higher engine performances in terms of combustion efficiency and emissions reduction, all the efforts by the engine community to improve the understanding of the fuel atomization, spray development and combustion are largely justified.…”

“…Experimental facilities, such as optically accessible engines [13] and test rig cells [11,14] combined with imaging techniques [5,15], have become the most common tools used in spray research. Throughout the years, the efforts put in place in these studies generated an accurate and deep understanding of the injection combustion event in Diesel engines.…”