Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.1.4
 
Drag Reducing Agents 
Drag reducing additives (DRA) are high molecular weight (typically 
exceeding 1 x 10
5
) polymers added in low concentrations to single or multiphase fluids 
to reduce frictional pressure losses in pipes and ducts [78]. Energy is needed to 
overcome such pressure losses and is taken directly from the fluid pressure. In water, 
the pressure loss over a meter of pipe with an increase in mass flow rate from 10 g/s 
to 20 g/s, increases approximately 300 % [79]. Pressure drops in pipes increase with 
increasing flow rate until they equal to the fluid supply pressure. Using DRA has 
enabled a reduction in frictional losses and an increase in production rate of crude oil 
by increasing the flow rate of oil for a given pressure drop or lowering the pressure 
drop for a given flow rate. The additives, other than in petroleum based products, are 
known to be used in flood water disposal, biomedical systems, field irrigation, 
firefighting and other applications [80]. In general, DRA can, due to their large 
molecular weight, be considered viscosity modifiers if used in large enough quantities, 
although in concentrations used in industry a notable effect is unlikely. 
Although effect of drag reducing additives is known, the drag reducing 
mechanism is not fully understood [81]. Losses in pipes and ducts are thought to result 
from friction created by turbulent processes in the flow. Warholic et al. [82] believed 
these to be caused by Reynolds shear stresses and velocity fluctuations normal to the 
pipe wall resulting from small scale flow disturbances along the walls that develop to 
form large scale turbulent structures. Drag reduction increases as the Reynolds shear 
stress tends towards zero. They conclude from their experiments that drag reduction 
results from aggregates of additive molecules being broken up. This means entangled 
additive molecules are separated in turbulent flow, dissipating energy from turbulent 
flow sections. Further, this explains the degrading nature of some drag reducing 
additives. Their experiments using a polyacrylamide and sodium-acrylate co-polymer 
showed up to 10 % reduction in drag with additive concentrations as little as 0.25 ppm 
whereas maximum reduction in drag was seen at concentrations of 13 ppm and 50 
ppm.
Similarly to drag reducing mechanism, the effect of concentration or molecule 
size is yet to be agreed upon. Al-Yaari et al. [83] showed, however, that with a 3 x 10
5
molecular weight a small negative effect can be seen due to greater rate of dispersion 
of different liquid phases, whilst molecular weights of 4 x 10
6
and 8 x 10
6
kept 

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2.1 Fuel Additive Review 
37 
different liquid phases stratified and displayed an increase in drag reduction with 
increasing molecular weight as well as increasing concentration. These results are in 
good agreement with Deshmukh et al. [84] who show an increase in drag reduction of 
up to 65 % with increasing concentration and Shanshool and Al-Qumaje [85] who 
showed an increase of 6.3 % in drag reduction with an increase in molecular weight 
from 2.6 x 10
6
to 5.2 x 10
6
. The decrease in frictional losses can be assumed to result 
from enhanced entanglement of additive molecules with larger molecular weight or 
quantity of molecules. 
Deshmukh et al. [84] further investigated the bio- and shear degrading of DRA. 
Due to degradation, the effect on frictional pressure losses disappears whilst the heavy 
molecules remain in fuel and as Al-Arji [86] has shown, can contribute towards 
deposit build up in the internal combustion engine intake valves and combustion 
chamber. Deshmukh et al. [84] conclude that up to 32 time increase in bio- and shear 
degradation resistance can be achieved if graft copolymer derivatives of guargum and 
polyacrylamide are used instead of their commercially available versions.
According to Hoyt et al. [87] DRA additionally work as anti-misting AM 
agents. AM agents are mainly used in jet fuels to inhibit formation of fine fuel mists 
that can occur after aircraft crash landings [88]. The high molecular mass of the 
additives increases the fuel viscosity and as such also affects the morphology of the 
fuel droplets generated in such conditions. Little et al. [89] bring out the quantities 
used as the difference between DRA and AM agents. DRA usage remains within the 
ppm range while AM agents can be used in concentrations of several tenths of a per 
cent by volume.