Effect of Gasoline Fuel Additives on Combustion and Engine Performance

iii.
 
Combustion Chamber 
Deposits in this region accumulate on the piston tops, cylinder head area, spark 
plugs and exhaust valves. It has been widely reported that combustion chamber 
deposits (CCD) contribute towards
ignition through deposit induced hot spots [26, 27, 
28], the subsequent fuel octane requirement increase (ORI) [29, 30] and emissions 
increase [31, 32]. ORI results from thermal insulation of metal parts by deposits and 
possible heat storage which can ignite the air-fuel mixture before the spark event 
causing pressures hikes and raised temperatures leading to engine pre-ignition and 
knock [33, 34]. Additives aimed at reducing CCD neutralise acidic compounds 
derived from combustion that can act as deposit precursors [35]. Bert et al. [30] claim 
an ORI of up to 15 % could be experienced in an engine due to deposit build up within 
the combustion chamber. Raised temperatures within the combustion chamber can 
increase the engine’s NO
x 
output as nitrogen from the air breaks down to monatomic 
nitrogen. This in turn could further contribute towards CO emissions if some of the 
carbon is unable to form CO
2 
because oxygen assumed available for combustion is 
used up previously by nitrogen. The importance of lubricating oil and additive 
composition in formation of CCD was emphasised by Bartleson and Hughes [36]. 

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2.1 Fuel Additive Review 
30 
However, that was nearly 60 years ago and in recent times no significant study has 
been found except for that of [25], where a 2-stroke engine was used for the 
experimental work. It could be assumed that increased engine manufacturing 
tolerances have prevented significant leakage of lubricant into the combustion 
chamber.
Zerda et al. [26, 37] carried out two experiments using derivatives of polyether 
amine and polybutene amine on the effect of DCA on the microstructure of CCD. 
They showed that the highly porous nature of deposits results in the total inner surface 
area of approximately 300 m
2
/g of deposit. They argued that the volume available 
within the deposits promotes incomplete combustion and HC emissions whereby part 
of the air-fuel mixture is locked in the available space and remains unburned. 
Reduction in available surface area with the use of an additive was noted. Further, 
polyether amine additive showed a decrease of intake valve deposits compared to base 
fuel of 73 % while a 21 % increase in CCD was experienced. No data on intake valve 
deposits from polybutene amine was enclosed but a 10 % increase in CCD was 
reported. HC emissions can increase further if CCD build-up hinders full closure of 
exhaust valves letting some of the air fuel mixture to leak into the exhaust manifold. 
It has been shown that DCA used with conventional PFI injection systems is 
ineffective in reducing CCD while DI equipped cars can see a significant reduction in 
CCD when same additives are used [4]. As such, in PFI engines, often the benchmark 
for performance is neutrality whereby no new CCD occurs. Development of GDI 
engines has caused a shift in research such that in 2001 – 2005 nearly half of all 
detergent patents aimed at removing CCD were designed for GDI engines [38]. 
Typical additives against CCD are the same as for previous two groups and include 
different amines and alkenyl succinimides. 
Development since the introduction of DCA in the 1950’s has been significant 
but several unknowns and discrepancies in research exist. Lewis and Honnen [39] 
describe an additive formulation that is effective in the fuel metering and induction 
systems but does not contribute towards CCD, whilst the need for additives that are 
simultaneously suitable for carburettor, PFI and GDI fuel metering technologies has 
also been recognized [21]. Although it is thought that ashless dispersants, due to their 
lack of metals, discourage ash formation on combustion [18], Fukui et al. [25] found 
that at combustion pressures of 1.5 MPa an ashless dispersant produces up to 50 % 
more CCD than a metal based additive. They mention, however, that under regular 

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2.1 Fuel Additive Review 
31 
engine operating conditions such pressures are improbable. Zerda et al. [26] found that 
additives with head groups of lesser polarity are more effective at reducing CCD than 
those with higher polarity but are unable to provide a mechanism to explain the 
phenomenon. Similar results were reported by Martin et.al [40] who found that 
polarised sulphur compounds in fuels promote deposit formation.
It is evident that DCA is a very important and widely investigated group of 
additives. It is also clear that testing of effectiveness only applies to suitability against 
the designed criteria. Effects on combustion and spray formation are unknown and 
advances to DI systems mean whole set of additives designed to be used in the 
induction system are made redundant.