Effect of Gasoline Fuel Additives on Combustion and Engine Performance

SM
D
, µ
m
Pressure, bar
Toluene
50/50 Mixture
Heptane

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5.2 Droplet Sizing with Alternative Fuels 
131 
changes in chemical properties in fuel where radical absorption by toluene was 
suspected. 
5.2.3
 
Experimental Error Assessment for Droplet Sizing 
As explained in Section 4.1, over 50 injections, the cumulative average of 
measured SMD was reduced to below 0.3 % of mean value. Thus, the effect of an 
additional spray event would have not had a significant effect on the measured SMD 
value. However, the base fuel standard deviation over 19 measured base fuel SMDs 
was on average ± 0.43 μm and ± 0.47 μm at minimum and 50 % transmission cases, 
respectively. Total peak to peak variability around ± 1.2 μm across the measured 
pressure range was noted. Although these figures are based on gasoline SMD 
measurements, it was assumed similar repeatability characteristics would be seen with 
alternative fuels as identical equipment and experimental conditions were employed 
in the study. 
For additive analysis, the base fuel mean value with its standard deviation was 
chosen as the basis for identifying additive effects. Anything beyond standard 
deviation was thought to represent a significant change. However, as demonstrated in 
Section 5.1.3, the variability in base fuel SMD values meant that additive readings 
could be outside the identified window while changes from the respective base fuel 
measurements were still within ± one standard deviation. As such, it was not possible 
to identify with confidence any changes in gasoline direct injection atomisation quality 
with additive bearing fuels.
The variability in the study can be assumed to result from the injector 
characteristics. Injector needle movement variability due to control timing and voltage 
characteristics changes are likely to be the primary causes although small changes in 
ambient conditions could also have had an effect on the readings. However, what is 
clear, is that the effect that additives have on the base fuel properties is very small and 
any further characterisation of the system would not have provided additional benefits. 
This also applied to the alternative fuel study, where general trends in SMD sizes could 
be identified but further system characterisation was not thought to be valuable. 

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5.3 Viscosity and Surface Tension Measurement of Experimental Fuels 
132