Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.4.3.3
 
Schlieren Imaging 
Schematics for the Schlieren system can be seen in Figure 2.10 [186] where 
ε 
represents the refraction angle of light rays. 
Schlieren system works on the same 
principal of variation in refractive index as shadowgraphy but uses twin concave 
mirrors or lenses to form a collimated beam of light that travels through the working 
section. Before being reaching the camera again, the light rays are re-focused. 
A light source is directed through a condenser lens to create a collated light or 
alternatively a collated light source is used, is sent to the first concave mirror that 
reflects it as a parallel beam through the working environment. The second concave 

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2.4 Combustion Analysis 
66 
Figure 2.10: Schlieren system aperture schematics [186] 
mirror focuses the beam at which point part of the light is cut off by a knife edge or 
graded filter. This effectively acts as means to control the contrast in light intensity. 
The remaining light is allowed to reach the Schlieren camera. Images respond to the 
first derivative of refractive index and as a result possess much higher sensitivity to 
changes in density than shadowgraph images. Furthermore, the Schlieren system 
produces a 1:1 scale of the studied object and according to Kostiuk and Cheng [187], 
unlike in shadowgraph images where detail is often reduced, it is emphasised.
 
2.4.3.4
 
Other Techniques 
Direct, shadowgraph and Schlieren imaging of flames allows for overall 
characterisation of combustion reactions but often understanding of specific reactions 
and species present during those reactions is sought after.
In such cases numerous laser techniques can be employed [151]. Laser induced 
fluorescence (LIF) uses a laser source to excite radicals that then can be captured by 
an intensified charge couple device (ICCD) camera. If enough energy from the laser 
is available this can further be converted to a planar laser induced fluorescence 
technique (PLIF), where a point source is converted to a laser sheet, enabling view of 
the cross section of the flame. Radicals such as CH, CH
2
O and OH have been 
associated with inner flame front, preheat and oxidation/post flame zones, respectively 
[188]. Dependent upon the wavelength of the laser, different radicals can be excited 
which enables visualisation of the reaction zones and as a result characterisation of the 
flame structures under various conditions. Moreover, simultaneous OH and CH
2
O 
fluorescence imaging can be used to evaluate the local heat release rate [189].
A series of experiments on diesel combustion in a constant volume combustion 
vessel were carried out at University of Valladolid [190, 191]. Using 306 nm and 430 

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2.4 Combustion Analysis 
67 
nm band-pass filters on two Hamamatsu 9536 photomultiplier tubes (PMTs) to detect 
the OH* and CH* radical chemiluminescence, respectively, to measure the auto-
ignition time of diesel sprays. The time-resolved signal traces of the OH* and CH* 
radical chemiluminescence were compared to that of pressure and mechanical 
vibrations and good agreement was found. 
Several other combustion characterisation methods are used. Hentschell [192] 
applied the use of optical fibres within a modified head gasket. About 100 optical 
fibres were used to form an optical grid covering the cross section of the engine. The 
flame position could be resolved spatially by using a tomographic reconstruction 
algorithm. Fibres were also used by Spicher and Velji [193] for flame detection but 
were built into the walls of the combustion chamber of a single cylinder SI engine. 
They bring out the possibility of a three-dimensional resolution to the flame 
propagation analysis compared to other optical techniques.