Shock-Tube Investigation of Ignition Delay Times of Model Fuels

Back to Table of Contents THIRD EUROPEAN COMBUSTION MEETING ECM 2007

Shock-Tube Investigation of Ignition Delay Times of Model Fuels M. Hartmann*, M. Fikri, R. Starke, C. Schulz IVG, Universität Duisburg-Essen, Germany Abstract We present an experimental investigation of ignition delay times of model fuels of pure iso-octane and n-heptane as well as of a mixture consisting of iso-octane (80%) and n-heptane (20%) by liquid volume for different fuel qualities. The purpose of this survey is to advance the understanding of ignition time chemistry using shock-tube experiments. For the mixture, to our knowledge, only the data of Fieweger et al. are available so far. Thus, it is helpful to have a new supporting data for comparison and to study the influence of fuels on different purity levels on ignition delay times. The experiments are performed in a high-pressure shock-tube at a pressure p = 40 bar and a constant equivalence ratio of φ = 1.0. The comparison of ignition delay data of the pure substances with literature [1-3] under similar conditions shows good agreement. Introduction The understanding of the auto-ignition of fuels is of great interest for the development of new kinetic models but also for industrial purposes. Optimizing fuels for internal combustion engines in terms of high efficiency and low emission has been and continues to be a major focus of engine development. The description of the properties of practical fuels requires the development of detailed kinetic models for mixtures in order to understand the interaction between hydrocarbon compounds in respect to their influence on the auto-ignition behavior. Due to the complexities of real fuels, which exhibit a near-continuous spectrum of hydrocarbons, fuel surrogates consisting of binary or ternary mixtures are usually employed to approximate practical fuels in their physical and chemical characteristics. Shock-tubes are a suitable tool for investigations of auto-ignition, as they offer the possibility to achieve fast heating and homogenous conditions after the reflected shock wave. The auto-ignition of pure model fuels has been investigated by several groups. For mixtures, the spectrum is not as wide as for pure fuels. To our knowledge, the data of Fieweger et al. [1] are the only one for a stoichiometric mixture of iso-octane (80%) and n-heptane (20%) by liquid volume at a pressure of 40 bar. They also conducted experiments with a blend of 10% n-heptane and 90% iso-octane as well as with a proportion of 40/60%. A similar study was performed by Voinov et al. [4]. Auto-ignition delay times were determined for a variety of fuel qualities, in order to figure out the potential influence of impurities. All presented measurements were performed for fuels and mixtures in HPLC as well as in ASTM quality. Experimental The presented experiments are carried out in a highpressure shock-tube with an inner diameter of 90 mm. The driven section has a length of 6.1 m, the driver section is 6.4 m long. They are separated by an aluminium diaphragm. * Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2007

The driver gases, Helium and Argon, are mixed in situ using two high-pressure mass-flow controllers (Bronkhorst Hi-Tec flow meter F-136AI-FZD-55-V and F-123MIFZD-55-V). Helium was used as the main component. Argon was added to match the acoustic impedance of the test gas. So, tailored driver-gas conditions are achieved and observation times are extended up to 16 ms. A detailed description of this setup can be found in [5]. The ignition was observed by measuring pressure profiles with a piezoelectric gauge (PCB HM 112 A 05) located 15 mm upstream of the end flange. Also, the observation of CH* chemiluminescence emission onset at 431.5 nm is selected by coupling the emitted light into a fiber after passing an interference filter at 431 nm with a bandwith of 5 nm. The signal is measured by a photomultiplier. For ignition-delay measurements the liquid fuel is injected into the evacuated driven section and after subsequent complete evaporation, synthetic air is added manometrically to ensure the desired equivalence ratio. All presented experiments were performed under stoichiometric conditions. Figure 1 shows the schematic setup of the highpressure shock-tube facility.

Fig. 1: Schematic setup of the high-pressure shocktube


normalized ignition delay / µs

Results and Discussion Figure 2 shows the results for the ignition delay time measurements of pure n-heptane conducted in the temperature range 716 K ≤ T ≤ 1249 K at φ = 1.0 and at p = 40 bar. The data-points are normalized to p = 40 bar. A comparison of the results of our measurements (squares for ASTM quality, circles for HPLC quality) with measurements by Fieweger (stars) [2] and Davidson (triangles) [3] is also depicted. The measured ignition delay from Fieweger is slightly longer than in our measurements. The NTC behavior seems to be more pronounced in the case of our measurements. For lower temperatures, the discrepancy is unequivocally strong in terms of ignition time between the data set. Here, the ignition delay in our measurements is apparently longer. A reliable comparison with the data of Fieweger is not possible because of the lack of the data in low temperature range.


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Fig. 3: Normalized ignition delay time for iso-octane (squares for ASTM and circles for HPLC) at p5= 40 bar and φ = 1.0 in comparison to literature values form Fieweger (triangles). investigation of mixtures. They are also useful targets for the validation and refinement of reaction mechanisms. There are many studies of ignition delay times for PFR mixtures with different mixing proportion, e.g. [6]. However, to our knowledge, the only literature data for the mixture with n-heptane/isooctane in the following proportion 20%/80% exists in [1]. We performed similar experiments to those with the pure fuels with a mixture of iso-octane (80%) and n-heptane (20%) by liquid volume in air at stoichiometric conditions. The ignition delay times were measured in the temperature range 673 K ≤ T ≤ 1184 K at pressure p = 40 bar. Figure 4 shows the normalized ignition delay data for the mixture, illustrated as squares (ASTM) and circles (HPLC) compared to the normalized results for the ignition delay times of the pure fuels depicted as – stars (n-heptane) and triangles (iso-octane). As expected, the normalized ignition delay times for the mixture are in between the values of the pure substances and show a less pronounced NTC behavior similarly to the data of Fieweger (fig. 5). However, the ignition delay times differ from those of Fieweger. The consensus is, indeed, rather good in the hightemperature range. The investigations of ignition delay of the mixture iso-octane (80%) and n-heptane (20%) were carried out for two different qualities, namely ASTM and HPLC. Figure 4 clearly shows no differences in ignition delay times for different fuel qualities. This effect is not really surprising as both pure fuels do not show any differences in ignition delay times, too. Assuming a power-law pressure dependence of the ignition delay times, the experimental values of the ignition delay times of the mixture can be expressed as

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Fig. 2: Normalized ignition delay time for n-heptane at p5= 40 bar and φ = 1.0. The measured data (squares for ASTM and circles for HPLC) are compared to literature values taken under similar conditions (Fieweger et al. [2] – stars, Davidson et al. [3] triangles). A comparison of the ignition delay time using different fuel qualities does not show any significant difference. These results are also shown in fig. 2. Similarly, ignition delay investigations of isooctane/air mixtures were conducted in the temperature range 778 K ≤ T ≤ 1193 K with an equivalence ratio of φ = 1.0, and at the target pressure of p = 40 bar. Figure 3 shows the normalized ignition delay times for pure isooctane in both qualities, ASTM and HPLC. The squares and circles show our results, the ignition delay times illustrated in triangles are taken from [1]. Again, we observe an excellent agreement, especially in the high-temperature range, between the ignition delay time values from this survey with literature. Only for low temperatures, small differences in the ignition delay times are seen. As previously, no difference in terms of ignition delay for the different fuel qualities is observed. It turns out that the fuel qualities chosen here have no influence on the ignition delay times in the measured conditions. Recently, ignition delay time study extended to the

τ / µs = 1.19937·exp(2465 K / T) (p / bar)–1.43 for temperatures T > 1000 K.

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• The used fuel qualities do not show any influence on the auto-ignition behavior for the pure fuels as well as for the mixture. • Basically, the literature data from Fieweger et al. for a mixture of iso-octane (80%) and n-heptane (20%) at φ = 1.0 and p = 40 bar could be reproduced using our shock-tube experiments. • The results show also a discrepancy in terms of ignition delay times with the data of Fieweger in the NTC range. At temperatures above 950 K, however, the experimental data of this work show good agreement with the literature. More investigations are needed for this mixture in that NTC region due to the discrepancy between literature and our measurements.


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Fig. 4: Normalized ignition delay times for the mixture of iso-octane (80%) and n-heptane (20%) by liquid volume in different fuel qualities (squares: ASTM, circles: HPLC) in comparison to the pure fuels (isooctane: triangles, n-heptane: stars)

Acknowledgements Financial support from BASF is gratefully acknowledged. The authors are also very grateful to. N. Schlösser for performing experiments and L. Jerig for technical support.

A comparison of our measurements with literature data [1] (see figure 5) shows the same characteristics. For high temperatures, both data coincide. The same behavior can be seen for temperatures T < 850 K. Also, the curves show a pronounced NTC behavior for temperatures 850 K < T < 1000 K. Nevertheless, the data of Fieweger show longer ignition delay times than the data of this work in the NTC range.

References 1. Fieweger, K., Blumenthal, R. and Adomeit, G., Combust. Flame 109, 599-619 (1997). 2. Fieweger, K., Blumenthal, R. and Adomeit, G., Proc. Combust. Inst. 25, 1579-1585 (1994). 3. Davidson, D.F., Gauthier, B.M. and Hanson, R.K., Combust. Flame 139, 300-311 (2004). 4. Voinov, A.N., Skorodelov, D.I., Borisov, A.A. and Lyubimov, A.V. Russ. J. Phys. Chem. 41, 605-607 (1967). 5. Herzler, J., Jerig, L. and Roth, P. Combust. Sci. and Tech., 176:1627-1637 (2004). 6. Herzler, J., Fikri, M., Hitzbleck, K., Starke, R., Schulz, C., Roth, P., Kalghatgi, G.T., Combust. Flame (2007) in press.


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Fig. 5: Normalized ignition delay times for the mixture of iso-octane (80%) and n-heptane (20%) by liquid volume (squares: ASTM, circles: HPLC) rom Fieweger [1]. Both curves show the same characteristics. Conclusions • Ignition delay measurements for n-heptane and isooctane (equivalence ratio φ = 1) were performed at intermediate temperatures and at high pressures. • A direct comparison to literature data, which is available from many groups especially for pure fuels, shows good agreements.

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