Effect of Gasoline Fuel Additives on Combustion and Engine Performance

t
2
the 
large droplets have already passed the laser beam and a mist of small droplets is seen. 
These droplets originate from partial break-up of the early large droplets. 
Smallest droplets in early stages of spray are seen just before 
t
3
when the 
maximum amount of fuel passes the laser. It is evident from the graph that droplet size 
starts to increase from this point to just after 
t
4
where it peaks. One of the causes for 
the increase in SMD is believed to be the dense fuel portions entraining less air which 
hinders fuel evaporation. Although current experimental work was carried out at 
ambient temperatures, the high volatility of gasoline means that significant 
evaporation of droplets can still take place [110, 230]. Furthermore, any air that is 
entrained would be increasingly affected by momentum exchange with fuel droplets, 
thus reducing relative velocities between air and fuel. Pribicevic and Sattelmayer [231] 
note that this results in diminished aerodynamic forces that would otherwise promote 
primary and secondary droplet break-up. A further cause for the increase in SMD is 
likely to be coalescence. This is a phenomena where, upon collision, two droplets 
merge into one. Coalescence is dependent on the We number and an impact parameter, 
B, that depends on droplet radius and separation distance of the colliding droplet 
centres. It can be seen from Appendix B that the We numbers experienced in this study 
varied between 26 – 47, which, dependent on parameter B, Qian and Law [232] have 
shown to be a region where possible coalescence occurs. 
At time instants 
t
5
and 
t
6
it can be seen that fuel quantity decreases 
(corresponding to increasing transmission) and, at least initially, as a result of 
increased aerodynamic forces and improved evaporation characteristics, a decrease in 
SMD is experienced. Due to the slowing fuel spray and reduced aerodynamic effects, 
the SMD values remain relatively constant around t
6
as We number is reduced and the 
rates of secondary break-up and coalescence converge. Because of the number of 
conditions involved, it was deemed useful to compare the SMD results based on a 
single value for each injection pressure. However, as previously explained, temporal 
change in SMD occurs and it was agreed that finding a mean SMD for the total 
duration of the spray would not be representative of actual spray conditions. 
Comparison of Figure 4.4 and Figure 4.5 revealed that at 50 bar and 110 bar 
injection pressures, fuel sprays went through same characteristic phases but occurred 

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4.1 Spray Features 
100 
at different time bases. Analysing further, Figures 4.2 and 4.3 showed that maximum 
fuel quantity for 50 bar injection pressure passed the laser 0.4 ms later than for the 110 
bar case (2.6 ms vs 3.0 ms). This suggested that comparing SMD values on temporal 
basis at different pressure conditions would not produce comparable results, as 
different phases of the spray formation would be considered. 
A better method, therefore, was thought to be comparing SMD values for a 
given transmission value. Most relevant SMD value was considered to when 
transmission was at its lowest, thus representing a time instant when maximum 
quantity of fuel per pressure passes the laser beam. A further point was selected where 
transmission was at 50 % towards the end of the spray. Although this instant marks a 
different stage in the spray formation for different pressures, it was thought it could 
provide information on the effects of fuel pressure on equal quantities of fuel. Due to 
the large distance from nozzle to the measuring location, the time of measurement for 
both chosen conditions occurred after the end of injection. 
Figure 4.8 displays the change in minimum transmission levels as the injection 
pressure is increased. A transmission value 10.25 % can initially be seen which 
decreases to 4.21 
%
for 120 bar injection pressure. It is apparent that the decrease in 
transmission levels at higher pressures is smaller than that of lower injection pressures. 
This could be as a result of limitations in the injector fuel flow rate at pressures close 
to and above the rated 110 bar.