Effect of Gasoline Fuel Additives on Combustion and Engine Performance



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Thesis Mart Magi

2.3.2.1
 
Fuel Viscosity 
Viscosity is among the main physical properties of fuels and other liquids to 
determine the droplet size. As can be seen in Figure 2.4, an increase in viscosity of the 
Figure 2.4: Effect of viscosity on Sauter Mean Diameter with varying injection pressure 
[136] 


2.3 Spray Characterisation 
54 
fuel is accompanied by an increase in the droplet size. This, according to Lefebvre 
[137], originates from the dampening effect viscosity has on either short or long 
surface waves that precede atomisation phenomena. According to Lefebvre and Ballal 
[138], liquid jet break-up occurs when the magnitude of disruptive forces, originating 
from aerodynamic interactions, overcome the consolidating surface tension and 
viscosity forces.
For a given experimental set up, such aerodynamic forces are solely affected 
by the pressure drop across the injector nozzle. Everything else remaining equal, for a 
given pressure drop, an increase in viscosity is likely to increase the viscous forces. 
This means an increase in the level of disruptive forces required for liquid jet break-
up and as such increased droplet size, as demonstrated in Figure 2.4. Furthermore, Yao
et al. [126] note a decreased spray cone angle with increased liquid viscosity that 
inhibits air entrainment and the subsequent atomisation processes. 
2.3.2.2
 
Fuel Surface Tension 
Although early research would suggest that droplet radius of curvature has an 
effect of surface tension [139] it is accepted that droplet size is affected by the surface 
tension and that similarly to viscosity, an increase in surface tension provides for larger 
droplets [140]. Lu et al. [141] believe this to be derived from the increased cohesion 
of fuel molecules on the surface of the fuel.
Lefebvre [137] explains this by the resistance to distortion or disturbance, such 
as surface waves, on the liquid surface. As shown in Figure 2.5, this results in a 
delayed ligament formation and subsequent atomisation, causing larger diameter 
droplets to occur for a given pressure drop. 

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