Investigations on gas-air mixture formation in the ignition chamber of two-stage combustion chamber using high-speed Schlieren imaging

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Schlieren-the nlightened are lead to the nd change in in ms of tempera variations in is study, Sch he differences NG pressure chlieren-signal ed by the expa compared to th he chamber.J Fig. 6.Series located on the left side (series A) has been registered in terms of chamber overpressure of 2 bars and 7 bars absolute pressure in the CNG rail.Series presented on the right side (series B) shows jets evolution under CNG-rail pressure increased to 9 bar abs.The image on the beginning of injectors control signal has been shown in the previous chapter and represents the outline of ignition chamber with sparkplug elements, which consists an obstacle on the way of expanding fuel.The first occurrence of the signal with luminosity differences has been marked as a timestamp 0. In this moment, a wider area of the chamber has been covered with fuel for the series B, where the CNG rail pressure was higher.From the analysis of pictures registered 0.8 ms later it can be stated, that in the case of series B fuel expands deeper in the horizontal direction, while comparable vertical penetration.It means intensified expansion of fuel in the area of the spark plug and the faster beginning of mixture formation in the spark gap volume under increased pressure in fuel supply.This character of horizontal penetration causes fuel-enriched regions occurring close to the wall of the chamber, which can be also observed in further steps.Comparing the total series of registered images, faster expansion in the volume of the chamber can be observed for the operating point executed under increased rail pressure.The bottom has been achieved in shorter time period.As it was mentioned, the intensity of Schlierensignal is influenced by the pressure gradients.The regions of highly intensified luminosity have been identified, mainly in the top of chambers volume.The occurrence of these spots is combined with the constructional feature of the ignition chamberorganization of fuel supply.Additionally, this effect can be caused by the turbulent movement around the mass electrode of the spark plug.Next regions with identified higher luminosity are located in the middle and lower part of ignition chamber, where the area of horizontal chambers cross section becomes smaller.It leads to the rise of pressure combined with rise of flow velocity.The dynamics of gas jet expansion will be assessed in details in the next chapter.

Dynamic behaviour of gas jet in the chamber
The dynamics of gas jets motion has been assessed based on the position of the furthest point, which has been calculated from registered Schlieren-signal and named jets-tip.Vertical positions of this point, resolved over the time, have been presented in the Fig. 7a.Analyzing the diagram of injection into the chamber with atmospheric pressure under p inj = 5 bar abs (blue line), the calculated time to reach the bottom of the chamber equals t b = 2 ms.In the case of operating point with increased p inj (green line), the time period to reach the bottom of the chamber was significantly shorter, while the early stage of injection shows comparable character.This situation finds the justification for intensified horizontal expansion in terms of increased CNG-rail pressure.
The rise of p ch represents a real engine conditions in retarded moment of injection.Analyzing the data, significant changes in jets evolution (case marked with a red line on Fig. 7a) are observed -approximately 4.5fold longer time period is required to cover the chamber with expanding fuel.Similar to the atmospheric case, increased injection pressure (orange line) leads to shorter expansion time combined with a comparable vertical course jets development in the early phase.Locationresolved values of velocity have been presented in the Fig. 7b.The rapid rise of the V y has been found in the initial phase of fuel expansion in the chamber.In the lower part of the chamber, lower V y has been noted.The source of this drop can be the rise of chamber pressure in terms rapid expansion of additional mass into the volume of the chamber.Alternatively, the drop of supply pressure combined with wave-effect, but to confirm this phenomena research including fast analysis of pressure in fuel supply is required.Higher velocities of the jets tip have been calculated for the injection into the lower p ch due to the smaller density of ambient charge and therefore smaller static mass breaking the expanding gas.Independently from chamber back-pressure, increased p inj resulted in shifted (for later) point of maximal velocity.In the lower part of the chamber (17 mm of distance from the fuel supply point), the slower drop of the velocity has been noted.This has a connection with the issue identified on optical data and finds its justification in the convergent construction of lower part of the chamber causing the increase in pressure and expansion velocity.

Summary and conclusions
Within the study, the optical research method has been applied to the mixture formation analysis in the ignition chamber of the spark-jet ignition system.The applicability of the Schlieren method in "Z"configuration to the research on injection of gaseous fuel into the ambient air has been confirmed.Based on the optical data, the parameters of jets tip have been calculated and dynamics of gas expansion in the chambers internal volume has been assessed.The impact of both chamber back-pressure and pressure in fuel supply system on the mixture formation has been evaluated.Shorter time to cover the chamber volume with fuel, promoting better homogenization of mixture has been found in terms of increased pressure in fuel supply.The regions of intensified motion of the charge have been identified.The research can be continued and complemented by the fast measurement of pressure in the fuel supply system, and its analysis to understand better the impact of gas-dynamic phenomena on mixture formation.
The study presented in this article was performed within the statutory research (contract No. 05/52/DSPB/0246).

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Fig. 7 .
Fig. 7. Time-resolved vertical length of the gas jet (a) and distribution of vertical component of velocity over the position in the chamber.