Effect of Gasoline Fuel Additives on Combustion and Engine Performance

Figure 2.7: PDA optical arrangement [155] 
2.3.3.3
 
Laser Diffraction Granulometry 
Laser diffraction (LD) systems calculate particle size and distribution from the 
light forward scatter pattern. It is a line of sight technique that averages a volume 

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2.3 Spray Characterisation 
59 
defined by the intersection of the spray and the laser beam. A typical laser diffraction 
set-up can be seen in Figure 2.8.  
Figure 2.8: Basic principle of Laser Diffraction Granulometer 
A diode-produced cylindrical laser beam is directed trough the working 
section. As the light hits a particle it is deflected at an angle proportional to its surface 
radius of curvature and is collected by the scatter detector. The light scatter is analysed 
based on the Mie Theory to predict a droplet size and size distribution. Smaller 
droplets with larger curvature will deflect light more and will be detected by the outer 
rings while larger droplets will be caught by inner. 
LD systems are very easy to set up and quick for obtaining experimental 
results. Also they can offer very high acquisition rates and offer particle size ranges 
from 0.1-2,000 µm [156]. Further, unlike PDA and PDIA, LD systems, due to their 
relatively large laser diameters, enable analysis of relatively large volumes that can 
provide for better representation of spray characteristics. However, this also reduces 
their spatial resolution compared to other techniques. 
Comparative studies between PDIA, PDA and LD techniques were carried out 
by Fdida et al. [110, 153] and Dodge [157]. Good to excellent correlation between 
results was seen between the aforementioned techniques. It has been noted that droplet 
sizes found with the LD method can be of smaller diameter than those of PDA or PDIA 
[110, 116]. 
Limitations of the LD method arise with the presence of the beam steering 
effect [158]. This is especially prominent phenomena with volatile fuels. Beam 
steering is caused by the evaporation of droplets that effects the local gaseous phases 
and changes the refractive index. The resultant small deflections of the beam result in 

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2.3 Spray Characterisation 
60 
the system having exaggerated readings on the inner most detection rings, thus, 
skewing the results. Unlike background noise, this effect cannot be eliminated. Fdida 
et al. [110] found beam steering to produce a bimodal droplet size distribution with 
higher peak occurring from droplets in the range 90 < D < 130 µm. PDA 
measurements, however, showed the droplets to be in 4 < D < 45 µm range. This 
knowledge allowed for some of the out most detection rings to be discounted from 
further analysis.
As mentioned in Section 2.2.2, however, PFI injectors produce droplets within 
this beam steering region. It could be argued that fuel evaporation as a result of large 
droplets would not be a prominent issue but if possible droplet sizing of PFI sprays 
would as a result be advised to be analysed by alternative drop sizing methods. 
A further limitation of the LD technique is the multiple scatter effect resulting 
from measuring dense sprays [159]. Multiple scatter refers to situation where a 
deflected light beam is further deflected off another droplet before reaching the light 
detector, thus giving a false reading [160]. According to Triballier et al. [161] this 
phenomena comes into effect when the transmission of light from the laser to the 
detector below 40 %. Commercial software packages include a correction algorithm 
for such cases. It is claimed this allows for droplet size analysis at transmission levels 
as low as 2 % whilst remaining within a 1 % margin of the true droplet diameter value 
[162].