Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.1
 
Fuel Additive Review 
Fuel additives are chemicals added to fuels in small quantities in order to 
enhance their performance or add properties that were not present in the base fuel. 
Improving combustion characteristics would enable reduction of emissions through 
for example reduced burning temperatures or reduction of deposits. The latter, if 
present, could increase combustion temperatures and/or contribute towards an increase 
of unburnt hydrocarbons. 
The first fuel additives to find their way into wider use were antiknock 
additives in gasoline, first found in 1921. Combustion in gasoline engines is initiated 
by an external energy source. However, given certain conditions, it is possible for a 
gasoline fuel to self-combust as a result of high pressures and temperatures. Such event 
is called ‘knocking’. Gasoline engines in general are designed for pressure levels far 

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2.1 Fuel Additive Review 
27 
lesser than the diesel engines and self-combustion in gasoline engines often results in 
sudden pressure hikes and can damage the engine. In order to prevent this from 
happening, antiknock additives are used in gasoline fuels. 
Fuel additives can be grouped based on different functional criteria. The most 
common separation is made between diesel and gasoline fuel additives. There are 
some species of additives that only contribute towards one group of fuels but many, 
such as lubricity or deposit control additives, have properties that work well with either 
of the fuels. Fuel additives can also be classified into ‘distribution’ and ‘vehicle fuel 
system’ additives. The former group are the ones that are used in refineries to produce 
fuels of specific specification at optimum cost levels. The latter group are used in the 
vehicle’s fuel system and only provide benefits when the fuel enters the engine inlet 
system or the combustion chamber.
Typical fuel system additives include deposit control additives, combustion 
improvers and lubricity additives whilst distribution system additives include drag 
reducing agents and often anti-static additives. In some cases oxygenates such as 
methyl t-butyl ether (MTBE) are added to fuels to increase their octane rating [12] but 
are, due to the volumes in use, not considered additives but fuel compounds. Typical 
performance additives treat rates fall within the 1-10,000 mg/L (often referred to as 
ppm) range whereas some oxygenates and bio-fuel blends can be treated at rates of 
more than 50 % by volume.
Additives are usually sold to oil companies as a package satisfying the 
specified requirements. Due to financial gains, most oil companies share refineries 
and only add additives into fuels before fuel retail stations. This allows for great cost 
efficiencies and product differentiation.
The following sections will aim at giving a better understanding of the 
additives that this project is concerned with. The additive groups under investigation 
are deposit control additives, combustion improvers, anti-statics, drag reducing agents, 
friction modifiers and carrier fluids.