Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.1.1
 
Deposit Control Additives 
The presence of deposits within an internal combustion engine has been known 
since the beginning of 20
th
century when most effective way of removing deposits was 
thought to be ‘burning out cylinders’ with pure oxygen [13]. Since then deposit control 
additives (DCA) have been developed. They are designed to remove deposits from 

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2.1 Fuel Additive Review 
28 
vehicle fuel systems and the combustion chamber. By removing different oxides from 
fuel they also provide corrosion resistance. DCA are petrol soluble chemicals that in 
most cases are also preferred to be soluble in lubricating oil. DCA are the most 
researched additive group with approximately 57 % share of all patents and patent 
applications in 2002 [14]. 
Although most deposits are likely be derived from the fuel and combustion, 
some also originate from crankcase lubricant.
Gasoline DCA are mostly detergents and dispersants often combined with 
carrier fluids. Typical additives come from chemical groups like amides, amines, 
amine carboxylates, polybutene succinimides, polyether amines and polyolefin 
amines [4, 15, 16, 17]. The molecule consists of a long fuel soluble hydrocarbon tail 
and a polar head group [4, 18]. The two parts of the molecule have different solubility 
within the fuel and as a result often occur as an inverse micelle. Both detergents and 
dispersants function similarly in that they trap deposit precursors (which are due to 
insolubility in fuel attracted to the DCA molecules) within the micelles. For smaller 
particles (20-50 nm) DCA create a thin film around the deposits in fuel preventing 
deposit coagulation. Particles of size >50 nm often carry a surface charge that attracts 
detergent and dispersant additive molecules. By doing so, the further coagulation of 
deposits is inhibited by charge repulsion [18]. A further mode of action is that DCA 
coat the surfaces of the fuel system, reducing the available surface area for the deposits 
to be attracted towards. Concentration of additives in fuel can vary between 30-20,000 
ppm [19, 20, 21]. Dispersants are often referred to as ashless due to the lack of metals 
to form metal oxides during a combustion event, while detergents often use 
magnesium and calcium in their composition. A typical dispersant molecule has a 
longer hydrocarbon tail than a detergent and as a result can hold up to 10 times the 
amount of deposit particles. 
Deposit control additives for gasoline fuels are divided into 3 main categories 
based on the area of action:
i.
 
Fuel Metering System 
Fuel metering system includes carburettors and injectors. Deposits in these 
areas if large enough can obstruct fuel flow into the induction system or in case of 
direct injection to the combustion chamber, resulting in higher air-fuel ratios, reduced 
power output and in more extreme cases, injector failure [22, 23]. The additives in 
these areas work as dispersants or detergents. Also, coating surfaces is possible 

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2.1 Fuel Additive Review 
29 
whereby a layer is created on the surfaces of the metering device that prevents 
accumulation of deposits. Additives used for these areas include low molecular weight 
amines and amine carboxylates. 
ii.
 
Induction System
Induction system comprises any areas between the injector/carburettor and the 
combustion chamber, i.e. intake manifold, intake ports and intake valves. It has been 
reported poorly formulated additives can cause build up around intake valve seat area 
and become viscous at low temperatures preventing valve movement [24]. Further, 
deposit build up in this area can inhibit closing the valve in other IC engine strokes 
than the intake, resulting in loss of compression. This can affect the running of the 
engine or further promote formation of deposits within the combustion chamber, as 
shown by Fukui et al. [25] who found that lower pressure conditions exhibit higher 
tendency of deposit build up. This could be explained by lower combustion efficiency 
at lower compression ratio conditions. Additives designed for the induction system 
operate on the principle of detergents but also coat the induction system surfaces, thus 
preventing new build-up of deposits. Longer chain higher molecular weight additives 
such as polybutene amines are used [14].