Effect of Gasoline Fuel Additives on Combustion and Engine Performance

4.2.2
 
Gasoline Vapour Combustion 
In order to reach significant pressure rises and reduce experimental variability, 
gasoline vapour combustion investigation was performed in the combustion vessel. A 
fully pre-mixed gaseous mixture was prepared and in addition to pressure, heat release 
and emissions analysis, flame propagation speed was computed from direct high speed 
imaging. In current study the spherically expanding flame was analysed in terms of 
turbulent flame speed. According to Ferguson and Kirkpatrick [236] the relationship 
between the laminar and turbulent flame speed can be expressed as: 
𝑆
𝑡
𝑆
𝑙
= 𝑎 (
𝑈
𝑡
𝑆
𝑙
)
𝑏
4.1 

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4.2 Combustion Investigations 
109 
where S
t
is the turbulent flame speed, S
l
the laminar flame speed, U
t
turbulent intensity 
and a & b are constant that depend on the geometry and specific conditions in the 
combustion chamber. The difference between the laminar and turbulent burning 
velocities can be 3-30 times [236]. Although laminar flame speed could be derived 
from the measurements, it was considered not to offer additional benefits as 
comparative fuel properties were under investigation and not each fuel separately.
Flame images were used to calculate the cross sectional area and the 
circumference of the propagating flame and on the assumption that the flame 
propagates spherically, radius increase per time step could be found. All raw images 
were background corrected and the threshold used for flame front detection was set 
based on the intensity of the specific combustion event. Electrode size was used for 
calibrating the pixels/mm value.
A sample of the image processing and time series of a combustion event can 
be seen in Figure 4.20. Figure 4.21 displays the radius change in time based on the 
two methods described. It can be seen that the circumference based measurement 
method produced calculated radius values twice as large as the area based method. 
This was likely to have been caused by difficulties in flame edge detection. It was 
especially prominent feature at the beginning of the combustion event and at time 
instants after 35-40 ms, where high spark and flame intensities, respectively, caused 
misinterpretation of images due to limitations of the threshold values used.
Furthermore, a much steadier change in radius was found with area based calculations.
As such, it was decided that only the area based calculation of flame speed 
would be used, where the typical averaging time period was between 15-35 ms. Figure 
4.22 displays typical pressure and heat release rate traces for gasoline vapour 
combustion. It can be observed that the peak pressure reached was two orders of 
magnitude larger than that for spray combustion. Additionally, repeatability in peak 
pressure and heat release rate over five combustions was found to be less than 2% 
which was thought to be low enough for the current investigations. Furthermore, using 
a Lambda sensor, exhaust gas analysis showed that around 1.3% error in pre- 
combustion gas mixture composition was experienced over the same number of 
combustion events.

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4.2 Combustion Investigations 
110