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This study presents diagnostic development of diffuse backilluminated extinction imaging of soot. The method provides high temporal and spatial resolution of the line-of-sight optical density of soot (KL) in compression-ignited fuel sprays relevant to automotive applications. The method is subjected to two major sources of error, beam steering effects and broadband flame luminosity effects. These were investigated in detail in a direct injection combustion chamber with diesel fuel, under high and low sooting conditions. A new method for correcting flame luminosity effects is presented and involves measuring the flame luminosity using a separate high-speed camera via a beam splitter. The new method and existing methods are applied and the resulting flame luminosity correction errors are compared. The new method yields 50% lower errors than the most promising method (optical flow method). The impact on KL was investigated, showing that the KL uncertainty when using the optical flow method is unbounded for KL values above 2.7, while the new method has an uncertainty of 0.5 for the maximum KL value of 3.8. The new method yields overall lower uncertainties and is more suited to measuring KL in optical thick conditions. Large refractive index gradients in the path of the incident light cause false attenuation, resulting in ambiguity of the measured KL, referred to as beam steering. A detailed investigation of the beam steering effects caused by the non-uniformities in the diffused light source was performed. A beam steering model was made and qualitatively validated from experiments. The results from the beam steering model showed the importance of having a large collection angle, in order to average out small-scale non-uniformities in the light source. The model also showed that large-scale non-uniformities in the light source could affect the measurement even if the collection angle is large.

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This study investigates in-ame sooting characteristics of biodiesel surrogates in compressionignited spray ames. The aim of the study is to produce reliable experimental data on in-ame soot for validation of kinetic mechanisms and soot models. A rapeseed oil biodiesel (RME) was compared to neat methyl oleate (MO) and methyl decanoate (MD). In addition, neat nheptane was chosen as a baseline fuel and the eect of blending n-heptane and MD on soot production was investigated. The study was performed in a single cylinder research engine with optical access to a single spray for varying ambient gas temperatures (825 K to 990 K).The in-ame soot was measured by a high speed diuse back-illuminated extinction imaging system, and the ame lift-o length (FLOL) and the corresponding estimated equivalence ratio at FLOL was measured from high speed OH* chemiluminescence imaging. Results show that RME has the highest tendency to soot, closely followed by MO, independent of the FLOL equivalence ratio. Having almost identical fuel properties, this is likely due to the higher degree of unsaturation in RME compared to MO.When comparing MD to n-heptane, a much lower in-ame soot production rate is observed despite the fact that MD has a higher number of carbon-carbon bonds, concluding that the high fuel oxygen ratio in MD is eectively reducing in-ame soot production. Generally, for FLOL equivalence ratio leaner than ∼2, no in-ame soot was produced for all ambient gas temperatures. The in-ame soot production rate also showed a clear ambient gas temperature dependence for constant FLOL equivalence ratios, where the soot production rate increased with increased gas temperatures.

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