Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.5
 
Summary 
The research published on fuel additives and their interactions with fuel spray 
and combustion development is limited. In order to be able to successfully develop 
and employ new additives, an improved understanding of the phenomena is needed. 
Research on sprays with additives in the fuel has shown very little effect from 
additives. However, the only additives found investigated in the literature include 2-
EHN, which is a combustion improver and two deposit control additives of an 
unknown chemistry. Effect of additives on spray formation from other functional 
groups seems not to have been studied. Moreover, all published research thus far on 
fuel additive effects on sprays has been carried out using diesel fuels. The extent to 

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2.5 Summary 
72 
which the physical properties of gasoline with additives would be altered is currently 
unknown. It is likely that the higher volatility and lower viscosity of gasoline 
compared to diesel fuel would result in more significant changes to atomisation 
characteristics. This could especially be true for additive groups such as friction 
modifiers and drag reducing agents where long chain polymers are used. 
Although an extensive amount of combustion investigations with additives in 
fuels have been carried out, majority of these have studied the additive effectiveness 
against their function. Anti-knocks were the first additives to commercially find their 
way into fuels. Similarly, ignition promoters in diesel fuels have been extensively 
investigated. Some interesting developments have been reported with combustion 
improvers where diesel ignition promoters have successfully been used in gasoline 
fuels to eliminate cold start misfires. Intrinsically, adding ignition promoters to fuels 
increases their cetane rating while the octane rating reduces. The relationship between 
the two can be represented by the Wilke equation [216]: 
𝐶𝑁 = 60 − 0.5 𝑥 𝑀𝑂𝑁
2.26 
where CN and MON stand for cetane number and motor octane number, respectively. 
It can be seen that an increase in CN would be accompanied by a decrease in MON 
and a subsequent knocking behaviour of the fuel would be expected. A study into the 
effects of ignition promoters in gasoline would help understand the fundamental 
processes responsible for the improved characteristics of gasoline combustion. 
Anti-knock additives have been studied extensively since their introduction, 
although majority of published work remains on metallic compounds, the use of which 
is now forbidden in developed countries. The mechanisms of auto ignition suppression 
are thought to be well understood in both metallic and organic compounds, while the 
effects on other combustion properties such as fuel efficiency, heat release and 
emissions characteristics seems to have often been neglected.
It is evident that gaps exist in understanding the full effects of fuel additives. 
Modern direct injection fuel systems and emissions regulations mean an improved 
understanding of additives is vital. Current project, therefore, has aimed at improving 
the understanding of additive effects through the investigations of changes to the 

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2.5 Summary 
73 
physical and chemical properties of base fuel under varying conditions and additive 
quantities. 

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