Effect of Gasoline Fuel Additives on Combustion and Engine Performance

Figure 3.5: Spray tip velocity at increasing fuel injection pressure as measured to 60 mm 
from the nozzle 
3.1.3
 
Droplet Sizing 
As was discussed in Section 2.3.2, droplet size is the primary microscopic 
characteristic of spray atomisation quality. Droplet size analysis characterisation was 
carried out using a Malvern Instruments Spraytec laser diffraction system. The system 
uses a 660nm wavelength 10 mm diameter laser beam directed through the working 
section and sampled on a 32 ring receiver. Maximum sampling rate of the system is 
2,500 Hz. Initial testing showed great difficulty in aligning the laser through the 88 
mm thick quartz windows due to diffraction of light at different laser to rig alignment 
angles. If normal positioning between the two was not achieved, small deflections in 
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40
50
60
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90
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110
120
Sp
ra
y
 T
ip 
Velo
cit
y
, 
m
/s
Pressure, bar

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3.1 Spray Investigation 
79 
the laser beam caused the inner most rings of the receiver to misread the input and 
give false readings in the form of large droplets. As a result, two of the windows were 
removed and the testing carried out under ambient pressure and temperature 
conditions. 
3.1.4
 
Viscosity and Surface Tension Measurement 
In order to better understand drop size behaviour between different additives 
and fuels, measurements were carried out on viscosity and surface tension. As 
explained in Section 2.3.2 these are the two prominent physical properties of liquids 
that affect the droplet size and can change in a non-linear manner to their constituent 
composition. Viscosity measurements were taken with a 
Brookfield DV – III Ultra
programmable shear rheometer combined with a heated bath for temperature control. 
The principle of operation is using a calibrated spring to drive a spindle [218]. The 
liquid is placed within a cylinder and a rotating spindle is lowered into it. As the 
spindle rotates, the liquid exerts a measurable torque on it which is converted into 
shear rate and viscosity of the liquid. The rheometer gave accuracies to within two 
decimal places.
Surface tension of the fuels was measured with a 
Kimble & Chase Surface 
Tension Analyzer
. The analyser works on the principle of capillary action, whereby a 
liquid fuel is forced vertically up a tube and then let fall down due to gravitational 
forces [219]. In this process wetting of the capillary walls occurs and the surface 
tension induced tensile stress tends to pull the liquid free surface towards the solid 
surface. This encourages formation of a curved meniscus and, if the capillary diameter 
is small, creates a capillary rise. The surface tension could be calculated using an 
equation proposed by the manufacturer and followed the form: 
𝜎 =
1
2
ρ𝑔ℎ𝑟
3.1 
where: σ – surface tension ρ – density of sample, g – acceleration due to gravity, h – 
distance between menisci and r – radius of the capillary. It was claimed that accuracies 
to within 20 % of true values were possible with the analyser. 

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3.2 Combustion Experiments 
80