Effect of Gasoline Fuel Additives on Combustion and Engine Performance

 
 
 
 
Chapter 7 
7
 
Conclusion and Suggestions for 
Further Studies 
The previous chapters have offered an in-depth description of the work carried 
out throughout the present study. The work has focused on investigating the effect 
additives have on base fuel physical and chemical characteristics with the primary 
objective being to involve experimental methods to improve on the existing 
understanding of the de-coupled effects of additives on the combustion processes. 
Chapter 7 seeks to provide concluding notes on the findings as well as suggest 
directions in which future investigations could improve on the existing body of 
knowledge on fuel additives.
7.1
 
Spray Investigations 
Fundamental DI spray characteristics were analysed for a range of additives 
and fuels at varying conditions. A laser diffraction system was utilised to produce 
SMD data that enabled quantification of spray quality differences and a novel fuel 
quantity based comparison method was developed to take into account of temporal 
variation of spray characteristics at various injection pressures. 

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7.1 Spray Investigations 
164 
The investigations were carried out on 17 fuel additives from 6 functional 
groups in two concentrations. Additionally, a non-fuel bound viscosity modifier was 
studied along with a selection of single component fuels and their binary mixtures. 
Due to the proprietary nature of the additives in present study, only restricted evidence 
regarding their chemistry was available. The limited information offered was 
summarised in Section 5.1.1 (Page 117). Additionally fuel viscosity and surface 
tension was measured for the base fuel and its additive bearing derivatives at 
concentrations ten times higher than the commercially suggested rate. Similarly to 
previously published studies on diesel sprays with additives, it was concluded that the 
effect of additives on gasoline atomisation characteristics is too small to be identified 
within the experimental error. The key findings can be summarised as follows: 

Fuel atomisation characteristics remained unaffected by fuel additives even in 
concentrations ten times higher than the commercially suggested rates under 
high fuel injection pressure and ambient temperature conditions. 

Other than drag reducing agents, fuel viscosity remained unaffected by the 
addition of fuel additives in concentrations ten times higher than the 
commercially suggested rate. Drag reducing agents increased the base fuel 
viscosity by nearly 200 % due to their high molecular weight molecules and 
resulting molecule agglomerates. Due to their nature, however, the high 
pressure DI environment meant the molecules went through shear degradation 
resulting in a more than 45 % loss in viscosity. This drop in viscosity was 
enough to make capturing changes to SMD indistinguishable with the chosen 
methodology. 

No fuel additive seemed to affect the surface tension of the base fuel. 

Studies of the binary mixtures of single component fuels showed that fuel 
physical properties (surface tension and viscosity) change proportionately to 
their constituent components and have a similar effect on the fuel atomisation 
characteristics. This result was important from additives point of view. 
Applying the proportionality to additive quantities in fuel explains why no 
change in spray characteristics could be identified. The exceptions in this case 
were the drag reducing additives that affected physical properties of fuels 
beyond the proportionality due to their high molecular weight molecules and 

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7.2 Combustion with Additives 
165 
resulting molecule agglomerates. However, as mentioned, these agglomerates 
break up during high pressure DI processes. 
Although, as shown in Chapter 4, the 50 spray events over which the averaging 
was carried out reduced moving average changes to below 0.3 %, each of the base fuel 
measurements demonstrated limitations in overall experimental repeatability levels. 
The variability could have been induced by small fluctuations in ambient temperature 
conditions or injector operational characteristics. Although measures such as 
controlling temperature beyond ambient conditions and characterising injector coil 
current and voltage, injector needle lift and mass flow rate of the nozzle would enable 
reduced experimental error and improved analysis of results, the effects additives 
impose on the physical properties of the base fuel are very small and the potential 
benefits of refining analysis further are limited. 
In conclusion, the study into the effect of additives on gasoline high pressure 
sprays demonstrated that fuel additives, in commercially used quantities, impose no 
undesirable properties on fuel atomisation characteristics that could hinder 
combustion efficiency and their only effect can be assumed to occur within their 
intended functionality.