Effect of Gasoline Fuel Additives on Combustion and Engine Performance

Figure 4.1: Fuel spray plume and measurement location definition 
will be explained in the following sections, capturing first spray measurement while 
injector was still activated. It is also representative of conditions in GDI engines [116, 
224]. 
As described in Section 2.3.2, for large injection pressures the effect of 
pressure drop across injector nozzle on droplet size diminishes. As such, additional 
tests at lower pressures, where possible additive effects would be more prominent, 
were carried out. Injection pressure was varied between 50-110 bar in 10 bar steps. 
Pressures below 50 bar were considered too low for practical engine applications and 
as such were not considered for the analysis. Although at different injection pressures 
the quantity of fuel varies, it was thought that under identical experimental conditions 
between different additives no additional benefit could be gained from adjusting 
injection duration to injection pressure. 
4.1.1
 
Effect of Location and Injection Pressure on SMD 
For the Malvern laser diffraction system to work reliably, transmission of light 
to the detector/receiver needs to be above 2 %. This proved difficult along the central 
axis of the spray with the chosen injector and location off the centre axis had to be 
used instead. Although recommended practice for GDI spray characterisation involves 
measurement along the central axis [225], for the purpose of the current comparative 
study where changes in fuels are investigated, an off axis location was still considered 
suitable. It was desired that the injector was still active at the time of acquisition of the 
z 
x 

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4.1 Spray Features 
94 
first SMD data and a location as close as possible to the central axis was used. A 
distance of 60 mm from the injector nozzle and 10 mm from the central axis was found 
to be the closest possible location to satisfy the aforementioned criterion. Similar 
measuring locations were used by Suh et al. [226] and Chen et al. [227] although 
neither author has given reasoning for the selection. The following analysis will show 
the effect varying x and injection pressure at z = 60 mm, where x and z denote the 
spray jet radial and axial directions, respectively. The point (0, 0) was defined as the 
injector nozzle tip. 
Figures 4.2 and 4.3 display the effect of location on the transmission of light 
at 50 bar and 110bar injection pressures, respectively. Shadowgraph images of spray 
development at 50 bar and 110 bar can be seen in Appendix A. It can be detected for 
both pressures, that initial rapid decrease in transmission to a minimum value is 
followed by a less rapid increase in transmission back towards 100 %. Also, it can be 
seen that transmission is higher for locations further from the central axis, suggesting 
the presence of less fuel in these regions. Furthermore, it can be seen that higher 
pressure accounts for extra fuel which is represented by the lower transmission values 
for a given location.