Optical Analysis of Ignition Sparks and Inflammation Using Background-Oriented Schlieren Technique

To determine the timing of inflammation in gas and gasoline combustion engines, the
point of 10% mass fraction conversion of fuel (MFB10) is commonly used. The MFB10 can be
determined from the heating curve, which in turn is calculated from the in-cylinder pressure curve.
However, the cylinder pressure is an indirect parameter with regard to inflammation, as it is the
result of the combustion that follows the inflammation. An attempt is made to derive a new, direct
parameter of inflammation based on optical measurements in order to detect inflammation more
rapidly and accurately. The background-oriented Schlieren technique (BOS) in combination with
high-magnification optics and a high-speed camera is used to detect local density changes coming
from the particle wave around the ignition kernel of a hydrogen combustion inside a combustion
chamber. Via BOS and regular high-magnification high-speed imaging, the influence of ignition coil
dwell time and in-cylinder pressure on the spark phases and the inflammation itself are evaluated.
As a potential direct parameter for inflammation, the size of the particle wave resulting from the
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expanding ignition kernel is evaluated. It was found that a higher coil energy supports a faster
propagation of the particle wave at ambient pressure. At higher pressures, general combustion effects
override the effect of the influence of the coil energy on the propagation speed of the particle wave. In
addition, the presence of successful inflammation was found to influence the spark phases. A directly
measurable parameter for ignition could be found at a basic level, which will serve as a starting point
for further detailed investigations.