Effect of Gasoline Fuel Additives on Combustion and Engine Performance

7.2
 
Combustion with Additives 
Investigations into the effects of additives on combustion characteristics were 
carried out in a constant volume combustion vessel with pre-mixed gasoline vapour 
and in a single cylinder research engine. The study concentrated on investigating 
combustion improvers and their effects on gasoline heat release rate, flame speed, 
knock intensity and emissions. Although initially spray combustion in the combustion 
vessel was planned, preliminary results showed small pressure rises and low 
experimental repeatability and consequently vaporised gasoline combustion was 
carried out. 
The additives investigated included an organic and an organometallic anti-
knock additive and an organic ignition improver. A detailed description of the 
additives used was given in Section 6.1 (Page 138). Similarly to spray investigations, 
two additive concentrations were studied except for the organometallic anti-knock 

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7.2 Combustion with Additives 
166 
additive that was only investigated at commercially used treat rate. The main 
observations of the study on CI additive effects on combustion characteristics are: 

CO emissions from pre-mixed gasoline vapour combustion for CI-A additive 
at 1X treat rate reduced by 37.7 %. In the engine testing a maximum of 12.8% 
reduction was experienced at spark timing of 35 CAD BTDC. At 10X treat 
rate, no improvements in the characteristics over base fuel were noted. 
Although it is unclear why improvements in the 10X concentration were not 
experienced, at 1X it is believed the reductions originate from reduced flame 
quenching near the combustion chamber walls. The lower level of CO 
reduction in the engine experiments can be explained by a smaller wall surface 
area of the combustion chamber compared to the combustion vessel. 

Improved CO to CO
2
conversion was seen with antiknock additives. This was 
thought to result from increased combustion duration that allowed for longer 
periods for the combustion reactions to take place. On average, compared to 
base fuel peak HRR was reached 1.3 CAD later with CI-I additive at both 1X 
and 10X concentrations and 1.4 CAD later for peak in-cylinder pressure. An 
increase with both anti-knock additives was observed for NO
x
emissions which 
was thought to be caused by longer residence of hot gases within the 
combustion chamber as a result of increased combustion durations. 

Burning velocity for premixed gasoline vapour combustion with CI-A additive 
at 1X treat rate increased by 5.4 % when compared to the base fuel. At 10X 
treat rate no change could be noted. However, the results are based on a small 
sample and no definite conclusions can be made on the additive effects on 
flame speed. 

Investigation into the effect of ignition energy revealed that no effect could be 
determined on premixed gasoline vapour combustion. This compares 
favourably to several researcher who showed that as long as minimum ignition 
energy (MIE) is supplied by the spark discharge, no changes to the subsequent 
reactions can be seen. MIE for a gasoline-air mixture is said to be 0.8 mJ while 
minimum recorded energy in the present study was 8.7 mJ. No measurements 
of spark energy were completed in the engine experiments. 

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7.3 Further Work 
167 

Measured knock intensity in the engine for CI-A 1X was very similar to base 
fuel except for advanced spark timings (>55 CAD BTDC). Greatly increased 
KI for CI-A 10X could be seen with ignition timing of 35 CAD BTDC or 
earlier. CI-I 1X KI was very similar to that of the base fuel. CI-I 10X and CI-
Additional 1X behaved similarly with greatly suppressed knocking observed 
throughout the tested range of ignition timings. This is in line with estimated 
RON increases described in Table 6.1 (Page 139). 
The combustion investigation demonstrated, mainly due to the number of 
samples involved, engine testing to be a better suited method for studying the effect 
of additives on combustion characteristics than the combustion vessel. It was evident 
from results that initial conditions in the combustion vessel had a large effect on the 
combustion characteristics and a significantly larger sample needs to be utilised to 
confidently draw conclusions from the study.
The most significant result noted from the study was the effect of ignition 
promoter CI-A (2-EHN) on gasoline combustion. Engine and combustion vessel 
investigations demonstrated significant reductions in CO levels throughout the tested 
range with an increase in engine KI levels only occurring at ignition advance of 55 
CAD BTDC or earlier at concentrations usually utilised in diesel fuels. As a result, 
there is basis to suggest that in low concentrations, ignition promoters can offer 
potential emissions benefits in spark ignited combustion systems without 
compromising fuel’s resistance to auto-ignition.