Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.4.3.1
 
Direct High Speed Imaging
The simplest technique to follow when studying combustion optically is to use high 
speed imaging to capture the flame luminosity directly. The method relies upon the 
flame or combustion to produce enough luminosity for the aperture to capture and has 
been shown to work well with both gasoline and diesel combustions [177, 178]. When 
luminosity has been found problematic, a sufficient increase in luminosity levels has 
been achieved through an addition of selected salts to the fuel that are thought not to 
affect flame propagation [179]. Winklhofer and Fuchs [180] state that with shutter 
speeds of 10 µs or faster allow for good imaging of the flame structure under the 

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2.4 Combustion Analysis 
65 
influence of turbulent flow field but make note of the high requirements of the optical 
aperture because of short exposure times.
2.4.3.2
 
Shadowgraphy 
Shadowgraphy implies that a light source faces the camera directly or often 
through a diffuser to generate equal light distribution. A beam of light is directed 
through the working section and is refracted due to density changes that affect the 
refractive index of the working environment. Schematics of a simple shadowgraphy 
set-up can be seen in Figure 2.9 [181]. The method does not produce a focused optical 
image of the object but instead produces a shadow of the investigated phenomena. Its 
response to the second spatial derivative of the refractive index makes it well suited 
for experiments with large density gradients. Although sometimes applied to 
combustion analysis [182], as demonstrated by several researchers, shadowgraphy is 
mostly used for fuel spray analysis [183, 184, 185]. 
Figure 2.9: Diagram of Shadowgraph method [181]