|
T
im
e
o
f
P
ea
k
P
re
ss
ure,
CAD
Ignition Timing, CAD BTDC
CI-Additional Base
CI-Additional 1X
6.5 Conclusion
161
The combustion study in the research engine was carried out over a 300 cycle
average which for the IMEP values meant that a maximum error of 1.7 % was noted.
Since the emissions study was carried out continuously during engine running, only a
single reading was taken per test and since each test was carried out under slightly
different conditions it was not possible to estimate the variability in the measurements.
From Section 6.3 it could be seen that erratic emissions reading were taken, especially
for the anti-knock additives. This was thought to result from misfiring cycles where
increased oxygen levels reduced CO, CO
2
and NO
x
emissions. Although this effect
could be taken out of analysis of pressure and HRR, it was not possible for emissions
measurements. However, the large number of repeat cycles over which the analysis
was carried out meant that the emissions readings were taken over about a 60 second
period which was thought to be sufficient to enable identification of general trends in
measured parameters.
6.5
Conclusion
Chapter 6 has presented the results of an investigation into the effects of fuel
additives on the chemical properties of base fuel in combusting environments.
Initially, gasoline vapour combustion with homogeneous mixtures in the constant
volume combustion vessel was analysed. The study included heat release, burning
velocity, ignition energy and emissions analysis. Subsequently, tests were carried out
in a research engine and analysed based on heat release and emissions analysis.
Several noteworthy results were observed.
The effectiveness of CI-A additive was most noticeable at 1X concentration.
In both, the combustion vessel and engine experiments, the CO level reduced by up to
40 % compared to the base fuel. Also, in both cases at 10X treat rate, the emission
characteristics were closer to base fuel. Based on results observed in current study, it
was assumed the drop in CO originated from reduced flame quench layer thickness
near combustion vessel and combustion chamber walls. It is unclear as to why the
benefits of the additive could only be seen at 1X and not 10X treat rates. Higgins et al.
[10] observed improvements in combustion characteristics when the base treat rate
used was 60 % higher than the 10X used in the current study. The CO characteristics
are especially remarkable since engine testing revealed increased low temperature
reactions at 10X concentration compared to 1X through increased knock intensity.
6.5 Conclusion
162
Although it has been suggested that additive bound nitrogen can contribute towards
increased NO
x
emissions [48, 49], current investigation cannot support the claim due
to emissions at 10X treat rate being significantly lower than at 1X. It is worth
mentioning, however, that comparisons are made to research carried out on diesel fuels
and changes in chemical reaction pathways compared to gasoline could exist.
CI-I antiknock additive results in the combustion vessel did not produce
conclusive results. Highest peak heat release rate and pressure were seen. However, it
is unclear if this was caused by limitations in experimental repeatability, as based on
literature the opposite should have been observed. This was not the case in the engine
tests. Reductions in knock intensity with increasing additive concentrations could be
observed as well as reductions in IMEP and peak in-cylinder pressures. Furthermore,
improvements in emissions characteristics were seen which was thought to result from
reduced reaction rates that enabled improved air-fuel mixing in the combustion
chamber due to longer time scales available. Similar results to CI-I were observed for
CI-Additional, although improved antiknock properties at similar RON changes were
experienced.
The results found from present study have shown the suitability of the methods
employed for the analysis of fuel additive effects on fundamental combustion
characteristics although some improvements could be implemented. Vapour
combustion displayed that changes in emissions and flame speed to an extent could be
observed, although low experimental repeatability suggested a larger number of
repeats should be carried out. In the engine, a good coefficient of variance was
achieved through the use of a 300 cycle average. However, as was the case with CI-I
and CI-Additional, slightly lean air-fuel ratios affected the emissions readings. It is
possible that this was due to misfires that caused increases in the oxygen levels and
subsequent changes to all measured emissions.
163
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