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Influence of temperature inhomogeneities on knocking combustion
Combustion and Flame 153 (2008) 562–573 www.elsevier.com/locate/combustflame
Influence of temperature inhomogeneities on knocking combustion M. Pöschl ∗ , T. Sattelmayer Lehrstuhl für Thermodynamik, Technische Universität München, Boltzmannstr. 15, 85747 Garching, Germany Received 19 June 2007; received in revised form 7 November 2007; accepted 14 November 2007 Available online 5 February 2008
Abstract The use of a rapid compression and expansion machine (RCEM) provides the possibility of investigating the fundamental kinetic behavior of combustion. The aim of this work is the investigation of the influence of temperature inhomogeneities on the reaction kinetics of knocking combustion under well-known conditions without having random fluctuations. In the RCEM a clearly defined temperature distribution was generated that covers a large area and facilitates the investigation of the influence of a temperature gradient on knocking combustion. The paper deals with the analysis and characterization of the different phases of low-temperature kinetics during knocking as well as nonknocking combustion. Combustion was controlled by ignition timing, variation of the spark plug position, compression ratio, and temperature distribution. The results show the formation of a fast-propagating flame due to local and sequentially progressing autoignitions, which initiate a detonation as a consequence of pressure waves, which progress into an already reacting gas mixture enriched with intermediate species and radicals. © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Knocking combustion; Rapid compression and expansion machine; Temperature inhomogeneities; Temperature stratification
1. Introduction Numerous studies have been carried out for many decades to explain the mechanism of knocking combustion, to identify the influencing factors, and to develop countermeasures against engine knock. Already in the 1930s and 1940s, several publications indicated that inhomogeneities of the gas mixture may lead to this undesired combustion mode. Sokolik and Voinov [1,2] observed that autoignition at hot spots in the * Corresponding author.
E-mail address: [email protected] (M. Pöschl).
end-gas will not always lead to knocking combustion. During knocking combustion a fast-propagating flame was observed. The detected flame propagation velocities were in the range of up to v = 2000 m/s. The authors emphasize the key role of the reaction in the gas mixture away from the primary flame region, where the knocking combustion has its origin. Similar results were attained by the investigations of the National Advisory Committee for Aeronautics (NACA) [3–5]. It is assumed that knocking combustion develops in gases that have undergone some extent of reaction. It was found that it does not matter whether the reaction was initiated by the primary flame or the autoignition.
0010-2180/$ – see front matter © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2007.11.009
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Both groups assume that the fast-propagating flame is a detonation, since the detected pressure amplitudes and the propagation velocities can be explained by the physics of detonation but not by homogeneous autoignition. However, it was hard to explain in what manner the coupling of the energy release and the pressure wave develops in small volumes such as the combustion chamber of an engine. On the basis of the publications of Oppenheim [6] and Zeldovich et al. [7,8], numerous later investigations were concerned with the influence of exothermic centers and the inhomogeneous distribution of temperature and concentration on knocking combustion [9–13]. The fundamental idea of these approaches is a gas mixture that exhibits spatial differences of the ignition time as a consequence of inhomogeneity and permits flame propagation due to local and sequentially proceeding autoignition. The propagation velocity is a function of the gradient of the ignition time, which in turn is a function of the temperature or concentration inhomogeneity. If the propagation velocity has a velocity similar to that of the pressure wave, which is produced by the autoignition, the coupling of heat release and pressure wave, and hence the formation of a detonation, is possible. Among other things, the papers confirm that detonations may also develop in small volumes. In this context, it is particularly important to mention the investigation of Lee et al. [14]: An explosive gas mixture with an inhomogeneity of the ignition time was created by heating the gas mixture with a xenon flash tube. By changing the heating input, different characteristics of the concentration distribution were created. A pressure wave was formed by local autoignition, which proceeded into the adjacent area. Since this area already exhibits thermal conditions near the limit, autoignition is initiated there, and the pressure wave is amplified and proceeds into the next adjacent area. Due to the fact that the heat release and the pressure wave are propagating in-phase, the explosive wave is continuously amplified until a detonation is formed. Lee et al. named this mechanism SWACER (shock wave amplification by coherent energy release). Further published investigations focus on the behavior of the end-gas during the development of knocking combustion in order to derive the chemical behavior of hydrocarbons [15–17] or the temperature development [18–20]. It was shown that lowtemperature kinetics within the end-gas region plays a key role concerning in the development of knocking combustion. Moreover, Griffiths et al. [21,22] emphasize the impact of the negative temperature coefficient (NTC) by investigating the influence of temperature on the autoignition of n-pentane. In the Griffiths et al. study [21], a rapid compression machine was used in which the air–fuel mixture exhibited a tem-
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perature distribution across the vertical cross section, with a maximum temperature difference of T = 8 K at starting condition. At this time the average temperature was given by T = 310 K. At a compressed gas temperature of Tc = 690 K the first emissions were seen in the upper and hence hottest part of the combustion chamber, followed by a development to the entire space with a relatively slow velocity for about 2 ms. Nonknocking combustion resulted whereas the higher gas temperature of Tc = 770 K provoked knocking combustion. The first light emissions were detected in the lowest and therefore coldest part of the combustion chamber in case of knocking combustion. The very fast covering of the whole space required less than 50 µs. Griffiths et al. concluded that autoignition starts in the hottest area, when the gas mixture reaches compression temperatures within the range of positive temperature coefficients. At the same time, a higher reactivity is found in the coldest area for temperatures within the range of negative temperature coefficients, which smooths out temperature inhomogeneities. The increased species concentration leads to knocking combustion, which starts in the area with coldest gas temperature at starting condition. The transition from nonknocking toward knocking combustion takes place at temperatures that exhibit the minimum ignition time. Despite very many investigations, knocking combustion is still a crucial topic concerning engine design and development, with remaining uncertainties and unsolved questions. Especially, the differentiation between detonations and fast propagating flames, which are formed by local and sequentially proceeding autoignitions, is difficult. Furthermore, there is no experimental evidence as to how, in the case of detonation, bidirectional coupling between the pressure wave and the energy release develops. In particular, the small scale of hot and cold spots in engines makes the analysis difficult and demanding. 2. Experimental apparatus and measuring technique The experiments were performed with a rapid compression and expansion machine (RCEM) that was designed at the Lehrstuhl für Thermodynamik of the Technische Universität München. A sketch of the apparatus with the piston position at the start of the experiment is shown in Fig. 1. With the objective of minimizing the vibrations for optical measurements during the experiments, the kinematic principle is based on two masses that are moving in opposite directions due to hydraulic coupling. A detailed description of the apparatus and its kinematics was given by Eisen [23].
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Fig. 1. Sketch of the rapid compression and expansion machine at starting position.
At the beginning of the experiment, the driver tube (1), which is connected with the piston (2), ends at the orifice (3) on the right side, and the balance weight (4) is located near the air supply valves (5). By means of compressed air, which is supplied by a compressedair storage with a variable volume, the balance weight is accelerated toward the orifice. Due to hydraulic coupling between the balance weight and the driver tube, including the piston, simultaneous movement in opposite directions is achieved. As soon as the balance weight approaches the orifice, a shear flow of the oil is created in the gap between the balance weight and the orifice. The resulting dissipation of the kinetic energy decelerates the movement of the masses. Additionally, the pressure generated inside the combustion chamber (6) by compression and combustion leads to a deceleration and to the reversal of the direction of motion of the piston as well as the balance weight. The initial pressure at the beginning of the experiment can be freely selected to simulate the pressure charging of a real engine. Since the air supply valves (7) and the corresponding exhaust valves (7) are tangentially arranged in the upper part of the cylinder liner, the entire valve train in the cylinder head (8) of a real engine is not required. By means of gas injection through the air supply valves, swirl and turbulence can be created in the combustion chamber during the compression stroke. The position of the piston at the beginning of the experiment and hence the compression stroke is selected by means of the hydraulically adjustable position of the back plate (9). In contrast to a real engine, the volume at the top dead center is not defined by the kinematics of the crank driving mechanism, but is a function of the volume of the compressed-air storage, the driving pressure, the shear flow within the orifice, and the pressure within the combustion chamber. The desired operating points are adjusted by varying these parameters. As, unlike a real engine, the RCEM is not operated continuously, all processes of interest must be captured during one single compression stroke and the following expansion stroke. The combustion chamber has a diameter of dc = 78.3 mm. The optically accessible area, with a diameter of do = 49 mm (Fig. 2), is obtained via a glass insert (10) in the piston. The
Fig. 2. Optically accessible area of the combustion chamber.
light emissions are transferred from the test rig to the camera via a tilted mirror (11). The presented experiments were performed with a stoichiometric air–fuel mixture. The fuel used was made out of the primary reference fuels n-heptane and iso-octane, with an octane number ON of 69. To avoid inhomogeneities of the incoming air–fuel mixture, the fuel was entirely evaporated and homogenized in an external mixing chamber. Prior to compression, a vacuum was generated in the combustion chamber before it was purged with the air–fuel mixture. A strip heater was used in the upper section of the combustion chamber for proper thermal conditioning of the test rig during the experiments (see Fig. 3). The temperature of the cylinder head was set to TW,CH = 373 K by means of a thermocouple. In the horizontally positioned cylinder liner, an inhomogeneous temperature distribution in the axial direction develops with a lower temperature of TW,BDC ≈ 353 K at the piston surface before the experiment is started, and the piston is located at the bottom dead center. The time between the charging of the combustion chamber with air–fuel mixture and the experiment itself is selected so that natural convection leads to a horizontally inhomogeneous temperature distribution of the charge due to stratification by gravity. A temperature difference of approximately T = 10–20 K between the upper and the lower part of the combustion chamber during each shot was determined with resistance thermometers. The following presented experiments indicate that after charging of the machine, turbulence decays quickly and the motion of the piston during compression does not produce substantial turbulence. For this reason the temperature stratification of the air–fuel mixture caused by natural convection is stable during the compression stroke.
3. Measuring technique The light emissions of the combustion intermediates were detected either by a Photron Ultima APX
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Fig. 4. Pressure traces of pressure transducer P1 for three different ignition times.
Fig. 5. Sequence of flame luminescence for the experiment with ignition time t2 . Fig. 3. Heating of the combustion chamber and temperature stratification of the air–fuel mixture before and during the compression stroke.
I2 intensified high-speed camera or by a LaVision Flamestar II intensified single-frame camera. Additionally, an optical measuring technique with faster dynamics than the high-speed camera was developed. The purpose was the acquisition of high-velocity data on the spatial and temporal distribution of the heat release and the dynamic processes during knocking combustion. Fifty optical fibers were incorporated all over the cross section of the cylinder head, including the outer parts of the combustion chamber, which are not optically accessible via the piston. The light intensity of each fiber was transformed by a photodiode or optionally by a photomultiplier tube, was amplified and then saved with a sampling frequency of 250 kHz per measurement point. Pressures during compression and combustion were measured by a Kistler 7061B piezoelectric transducer. The piston position was de-
tected by scanning a ruler that is integrated into the test rig with an AMO PMK-02-25 optical sensor.
4. Results 4.1. Variation of ignition time Fig. 4 shows the pressure traces of three experiments with different ignition times, while all further parameters remained identical. It can be seen that a shift of the spark ignition toward the expansion stroke reduces the knock intensity and even leads to nonknocking combustion. The corresponding sequences of the light emissions from combustion at ignition times t2 and t3 are shown in Figs. 5 and 6, which were recorded by an intensified high-speed camera APX I2 at a frame-rate of f = 15,000 fps in combination with a narrow-band interference filter with transmission of the CH band (λm = 431.4 nm). Up to the first
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M. Pöschl, T. Sattelmayer / Combustion and Flame 153 (2008) 562–573 Table 1 Characteristics of two-stage ignition with two distinctive pressure rises Ignition Time of 1st time increase of pressure rise t1 t2 t3
−0.5 ms +1.1 ms +1.8 ms
Time of 2nd increase of pressure rise
Time interval
+0.4 ms +2.8 ms +4.1 ms
0.9 ms 1.7 ms 2.3 ms
Fig. 6. Sequence of flame luminescence for the experiment with ignition time t3 .
Fig. 8. Pressure traces and corresponding chemiluminescence during cool flame and main heat release of nonknocking combustion; ε = 8.7.
Fig. 7. 3-dimensional scheme of the different flames in the combustion chamber.
frames of both sequences, the spark-ignited flame already filled the entire optically accessible area, which is smaller than the full cross section of the combustion chamber. In the upper area a reacting zone is visible which moves downwards. It represents an autoignition which starts in the hottest area and continues in direction of the temperature gradient. The visible reaction zone disappears near the center due to the advanced propagation and reaction progress of the centrally located spark ignited flame. Fig. 7 shows a three-dimensional scheme of the different flames in the combustion chamber. The distance between the cylinder head and the piston crown was in the range of l = 9–16 mm for the presented frames. Subsequent to the downward propagating reacting zone, a strongly radiating flame follows in the case of combustion at ignition time t2 , which moves upward. This flame corresponds to the beginning of strong pressure oscillations. Despite autoignition, nonknocking combustion occurs in the experiment with the ig-
nition time t3 . This shows clearly that autoignition does not necessarily provoke knocking combustion but leads to an augmented heat release which results in increased pressure rise and increased light intensity. The earlier the ignition occurs, the higher is the contribution of the expanding flame to pressure rise. In the case of the earliest ignition time t1 the sparkignited flame causes a substantial increase of the pressure. Nevertheless, the shape of the pressure trace shows characteristics of two-stage ignition with two distinctive pressure rises, which can be also seen during the experiments with ignition times t2 and t3 (see Table 1). Qualitatively similar characteristics of the pressure traces are reached for experiments without spark ignition (see the first part of Fig. 8). As will be discussed in Section 4.2, the pressure increases can be clearly correlated with the events of autoignition by means of optical fibers, which are incorporated all over the cylinder head, including the outer range near the cylinder liner. The analysis of the high-speed cinematography reveals further that the spark-ignited flame shows an almost laminar shape without significant wrinkles. This was given for all performed experiments without swirl generation. The detected expansion velocities are in the range of vsf = 3–6 m/s, which indicates that there may be small-scale but not large-scale turbulence. Compared to autoignition, the
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Fig. 9. Sequence of the cool flame. The time interval between the first frame and the last frame is about t = 0.6 ms.
spark-ignited flame shows a low propagation velocity with a low reaction rate. Furthermore, the contribution to the overall pressure rise is smaller than that in a real engine. The pressure traces at ignition times t2 and t3 show only small differences until they reach top dead center. However, the mode of combustion changes significantly. This fact highlights how sensitive the heat release is near the knocking limit. Small changes of temperature or pressure may essentially influence the following reactions, as is very well known from spark ignition studies in engines. 4.2. Analysis of autoignition in the end-gas region Instead of using spark ignition, during the experiments presented below the compression ratio ε was increased to isolate the autoignition from effects created by the spark initiation. That is, the propagating autoignition can be detected and investigated without the interference of the flame luminescence originating from the spark ignited flame. The traces of the pressure transducers P1 and P2 (see Fig. 2) in the first part of Fig. 8 show a moderate increase of the rate of pressure rise at t = −1.2 ms. The simultaneous use of photomultiplier tubes and narrow-band interference filters with transmission in the OH band (λm = 305.9 nm) and CH band (λm = 431.4 nm) is presented in the second part of Fig. 8. At this time there are no signals within the OH band, although light emissions within the CH band were detected. This behavior is characteristic of the cool flame of lowtemperature kinetics, in which a high concentration of formaldehyde (CH2 O∗ ) is produced. Formaldehyde emits light signals within the CH band but not in the OH band and at longer wavelengths [24,25]. Fig. 9 shows a sequence of single frames taken with the LaVision Flamestar II intensified singleframe camera without any spectral filter during a series of experiments made under identical conditions.
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The cool flame starts in the upper and hence hottest area. Afterward the reaction zone propagates downward along the temperature gradient. The time interval between the first frame and the last frame is about t = 0.6 ms. A couple of milliseconds later, a visible reaction front with light emissions in the OH-band, CHband, and λ > 580 nm follows. It corresponds to the downward-moving reaction front in Figs. 5 and 6, which is termed as “autoignition.” It represents the main heat release of a two-stage reaction that is governed by high-temperature kinetics. The light emissions are transferred to photomultiplier tubes by means of optical fibers that are incorporated into the upper part of the combustion chamber, near the cylinder liner. This area is not optically accessible via the piston. Both characteristic pressure rises can be clearly correlated with the beginning of light emissions, and hence with both phases of autoignition which was already described in Section 4.1. The detected mean propagation velocities of the cool flame in the range of vcf,m = 50–200 m/s and of the main heat release up to vmh,m = 500 m/s cannot be explained by laminar or turbulent flame propagation. It is a sequential autoignition process of areas that ignite successively due to the temperature stratification that has already been described by Zeldovich (see Section 1). These forms of propagation were modeled for all test conditions by means of kinetic calculations with a multizone model. The reaction mechanism used was particularly designed for mixtures of the primary reference fuels n-heptane and iso-octane and contains 4238 elementary reactions with 1034 species [26]. The combustion volume was split into four reactors, R1 to R4, linked with walls to each other, which move without friction and permit pressure equilibrium (first part of Fig. 10). The starting temperatures Ti,0 , with a temperature difference of T = 10 K, represent the existing temperature stratification in the RCEM. The entire volume of the combustion chamber, V (t), is given as a function of time and corresponds to the compression stroke of the RCEM. The calculated ignition delay between the hottest reactor R1 and the coldest reactor R4 is tcf = 0.7 ms in case of the cool flame and tmh = 0.15 ms in case of the main heat release. The respective propagation velocities can be derived from the calculated ignition delays by using the diameter of the combustion chamber dc = 78.3 mm. In the case of the cool flame the propagation velocity is vcf,m = 112 m/s and in the case of the main heat release vmh,m = 522 m/s. The calculated values agree very well with the experimentally observed values. This leads to the very important finding that the propagation of the cool flame and the onset of the main
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Fig. 10. Kinetic calculation by means of four-zone model.
heat release are totally independent of laminar or turbulent transport. The propagating reaction zone of the main heat release causes pressure waves and a compression of the gases in the lower part of the combustion chamber. The propagating intensity and the propagating speed depend on the initial thermal conditions and test parameters. Fig. 8 shows nonknocking combustion. The propagation of the main heat release is weak and slow enough to permit pressure equalization. This can be seen by means of the signals of both pressure transducers P1 in the upper part of the combustion chamber and P2 in the lower part of the combustion chamber. A different behavior was observed during knocking combustion in Fig. 11, which shows a sequence of the propagating reaction zone of main heat release and the corresponding pressure traces. The frames show a narrow vertical section of the optically accessible combustion chamber (see Fig. 2). The downscaling of the detected area made it possible to increase the frame rate up to f = 87,600 fps. There is a pressure rise in the upper area at time tP1 = 1.97 ms, which correlates with the onset of the main heat release. The downward-propagating pressure wave is detected in the lower part of the combustion chamber at tP2 = 2.06 ms. This period correlates with a sound wave that propagates between the two pressure transducers at the present gas temperature of approximately T = 800 K. The propagating main heat release for t < 2.295 ms is faster than in the previously described case. In the lower part, the first light emissions can be seen at time t = 2.306 ms. At the same time, the pressure signal shows a steep increase,
Fig. 11. Sequence and corresponding pressure traces during main heat release of knocking combustion.
with pressure differences of more than p = 60 bar between the pressure transducers P1 and P2. In contrast to nonknocking combustion with a main heat release that reacts almost homogeneously and without pressure oscillations, there is an abrupt change to a bright flame, which travels upward with supersonic speed. The detected velocities, which were measured by means of the fiber technique, are in the range of up to v = 1400 m/s. It is interesting to note that this reaction starts in the originally coldest region and proceeds in the direction opposite to the direction of the preceding reactions, the cool flame and the onset of the main heat release. The subsequently beginning pressure oscillations correspond to the start of this third combustion mode. The measured pressures and propagating velocities indicate that a detonative reaction occurs. The onset of the detonation might be initiated by two mechanisms. The first one is direct initiation due to inhomogeneities, according to the description of Zeldovich et al. or the description of Lee et al. Hence, the detonation would be initiated during the downward propagating reaction zone of main heat release. The second possibility is initiation as a consequence of a pressure wave that is formed because of the propagating main heat release. After being reflected and focused by the wall in the lower part of the combustion chamber, the pressure wave passes upward
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Fig. 12. Variation of the ignition position.
through the reacting gas mixture, which exhibits high radical concentrations. The bidirectional coupling of heat release and pressure wave is established once the pressure wave is strong enough and the gas mixture is reactive enough. The experiments presented below will investigate this hypothesis in greater detail in order to clarify this point.
Fig. 13. Pressure traces for different ignition positions; εC = εB = εT .
4.3. Variation of the ignition position The experiments presented above showed a stable and reproducible progress of the reaction, which develops from the initial temperature distribution. The essential parameters are the local differences of induction time leading to distributed sequential autoignition. The following experiments investigate with which measures and to what extent the temperature distribution may be influenced in localized areas in order to avoid knocking combustion. For this purpose, these experiments were performed with spark ignition. The spark-ignited flame provides a local heat release zone near the ignited gas, with radicals and intermediate products that change the spatial distribution of the induction time in the area near the spark plug. Three different ignition positions were used in the experiments presented below (Fig. 12): • Center (C): The hottest and coldest gases in the end-gas region are reached by the spark-ignited flame at the same time. • Bottom (B): The ignition is forced in the coldest gas. • Top (T): The ignition takes place in the hottest gas, where autoignition always occurs first. The three pressure traces in Fig. 13 match totally until the onset of the cool flame.1 Following central spark ignition (Fig. 14), the spark-ignited flame already filled the entire optically accessible area up to the first frame at t = 2.816 ms. The integral view of 1 The description of cool flame and main heat release refers exclusively to the autoignited reactions.
Fig. 14. Sequence of flame luminescence with ignition at the center.
the light emissions shows a superposition of the main heat release, which propagates downward and causes an increase of the pressure rise rate, and the sparkignited flame. The contour of the propagating main heat release can only be seen at the outer area on the left side after reaching the area near the center position at t = 2.883 ms. It disappears completely at t = 3.083 ms because of the spark-ignited flame in this area: due to the advanced chemical progress near the center position, the propagating main heat release of autoignition occurs only in the outer area of the combustion chamber. The velocity of the propagating main heat release and the intensity of the following pressure waves are smaller than for the experiments without spark ignition. The weak pressure oscillations indicate that this operating point is near the knocking limit. Interestingly, the ignition at the bottom leads to knocking combustion with strong pressure oscilla-
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Fig. 15. Sequence of flame luminescence with ignition at the bottom.
tions. The spark-ignited flame has the longest way to reach the area with the hottest gases under starting conditions in the upper part of the combustion chamber. This means that autoignition may propagate undisturbed for a long distance until it reaches the area of the spark-ignited flame. The main heat release starts t = 0.7 ms earlier and propagates with faster dynamics and higher intensity compared to the case with centrally located ignition. Fig. 15 shows that the spark-ignited flame is pushed back and compressed further. The following development is identical to the case with no spark ignition: A bright flame starts at the bottom and moves upward. The ignition in the top position (Fig. 16) leads to smooth combustion with a substantial shift of the heat release toward the expansion stroke. As previously mentioned in case of the centrally located spark ignition, the propagating main heat release and hence the resulting pressure waves cannot evolve with high intensity in areas with an advanced reaction progress. Due to the spark-ignited primary flame in the area of the hottest gases, the main heat release of autoignition can only be detected within a small area located in the lower part of the combustion chamber. The propagation is rather slow, with a weak intensity, similarly to the experiment with ignition at the center. Subsequently, a moderate pressure rise without any oscillations follows. In this experiment it can be clearly seen that the propagating spark-ignited flame is not able to trigger knocking combustion, since both the reaction rate and the propagation velocity are too low. The propagation velocity with approximately vsf = 5–6 m/s is much lower than the downward propagation of the main heat release zone driven by autoignition.
Fig. 16. Sequence of flame luminescence with ignition at the top.
Fig. 17. Pressure traces for different ignition positions; εB1 < εT2 < εT3 .
The spark ignition at the bottom shows a stronger tendency to knock compared to the ignition at the center and at the top. This was confirmed by the experiments in Fig. 17, which show a comparison between the ignition positions at the bottom (B1) and at the top (T2 and T3) positions. Strongly knocking combustion results from the spark ignition at the bottom, although the lowest compression ratio εB1 was chosen. In contrast, only small pressure oscillations are being observed in case of ignition at the top position (T2), even though the pressure is much higher. The higher pressure is reached by an increased compression ratio εT2 > εB1 and a greater contribution of the expanding spark ignited flame to pressure rise. A further increase of the compression ratio εT3 causes a shift of the heat release toward the top dead center and provokes strong pressure oscillations. The corresponding sequence is shown in Fig. 18. Only the spark-ignited flame is visible before t = 0.987 ms.
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Fig. 19. Pressure traces for different temperature fields.
of the gas mixture in the lower part is not sufficient for a bidirectional interaction with the passing pressure wave. 4.4. Variation of the temperature field
Fig. 18. Sequence of flame luminescence with ignition at the top; εT3 .
The propagating main heat release appears near the spark-ignited flame in the following frame. At the same time, the light emissions in the upper part of the combustion chamber increase. A much brighter luminescence from combustion is visible in this area at t = 1.187 ms, which correlates with the beginning of knock. This fact is remarkable, since the beginning of knocking combustion was in the lower part of the combustion chamber for all other discussed operating points. The propagating main heat release generates pressure waves that are reflected at the lower wall of the combustion chamber. Subsequently, the upwardpropagating pressure waves are passing a gas mixture that exhibits a sufficient rate of progress for the interaction between pressure wave and heat release only in the upper part of the combustion chamber. Later, only the upper part of the combustion chamber shows bright light emissions. Further on, the rate of progress
All experiments presented above consistently point out that the propagating main heat release has a great impact on the formation of knock. The crucial parameters are the propagating velocity and the propagating distance. Both are mainly determined by the temperature distribution of the gas mixture. The following section will discuss to what extent the knock may be influenced by disturbing the temperature stratification. It is possible to vary the temperature stratification in the RCEM by gas injection via the tangentially arranged supply valves (see Fig. 19). A small amount of air–fuel mixture is injected during the very first phase of compression. This small mass flow is sufficient for the destruction of the temperature stratification, but the impulse is not strong enough for the generation of a swirl flow. The additionally injected mass was compensated in the experiments without gas injection by an initial charging of p = 0.06 bar. Spark ignition was not used in order to provide the best conditions for the detection and investigation of autoignition. A comparison of the undisturbed and the disturbed case is shown in Figs. 20 and 21. Fig. 20 demonstrates the case of a stratified temperature field over the entire section of the combustion chamber, which causes knock with strong pressure oscillations (Fig. 19). In contrast to this, it can be seen in Fig. 21 that the distribution of the main heat release is more homogeneous. The resulting pressure traces exhibit significantly reduced oscillations for the latter case, although the compression ratio was increased from ε = 12.7 to ε = 13.4. Additionally, a pressure trace is shown, which was recorded with an undisturbed temperature field and a compression ratio of ε = 13.3. Again knock is observed with strong pressure oscillations which exceed the measurement range of p =
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Fig. 20. Sequence of flame luminescence with an undisturbed temperature field; ε = 12.7.
100 bar. The onset of the cool flame and the main heat release begin earlier than for the disturbed temperature field. The gas injection causes some mixing of the charge during compression and there is a higher heat exchange between cold and hot spots of the charge adjacent to each other. Both effects cause a reduction of the maximum charge temperature and hence a delayed autoignition. However, the shift of the reaction toward the expansion stroke plays a minor role. This can be seen by reviewing the case with ε = 12.7. The key role for the significant reduction of the knock intensity and the tendency for knock as such, which can be seen in Fig. 22, is the prevention of sequential autoignition across a long distance. The knock intensity KI, which describes the intensity of the pressure oscillations, was calculated on the basis of the measured pressure pc and the low-pass filtered pressure signal pfilt according to the equation KI =
1 nsum
·
n sum
pc (n) − pfilt (n)
2
(bar2 ),
Fig. 21. Sequence of flame luminescence with a disturbed temperature field; ε = 13.4.
Fig. 22. Influence of the compression ratio ε and the temperature field on the knock intensity KI.
(1)
n=1
where nsum is the number of samples. Fig. 23 shows the power density spectrum Pxx for the pressure traces shown in Fig. 19. All traces exhibit their maximum at fmax = 7 kHz, which corresponds to the 1. acoustic eigenfrequency. This means that pressure waves are transversally passing the combustion chamber. The maximum of the power density spectrum is significantly smaller in the case of the disturbed temperature field. 4.5. Summary and conclusions The experiments show the formation of knock due to temperature inhomogeneities. Natural convection causes a temperature stratification of the gas mixture in the combustion chamber of the employed rapid
Fig. 23. Power density spectrum for different temperature fields (analysis of the pressure traces shown in Fig. 19).
compression and expansion machine. The study reveals a previously unknown key role for knock in charges with nonhomogeneous temperature distribution. The observed mechanism that triggers knock is clearly different from those presented by Griffiths et al. In contrast to the findings of Griffiths et al., it was found that the negative temperature coefficient (NTC)
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does not play any role in the evolution of knock. Instead, the experiments reveal the crucial impact of dynamic processes on the initiation of a detonative combustion: • Sequential autoignition leads to a fast-propagating reaction front along the temperature gradient. With a spectral analysis the first occurring reaction front was identified as the first phase of lowtemperature kinetics, the cool flame, which travels with average speeds of vcf,m = 50–200 m/s. No difference in speed or intensity during this phase between knocking and nonknocking combustion was observed. • Subsequently a second reaction front follows, the onset of the main heat release with average speeds up to vmh,m = 500 m/s. There is a clear relation between the propagation velocity and the knock intensity: The faster the reaction front of main heat release propagates, the higher the knock intensity and the tendency to the formation of knock as such become. • Pressure waves are generated by the fast-propagating reaction front. After being reflected and focused at the convex wall in the lower part of the combustion volume, the pressure wave passes the reactive gas mixture with high concentrations of radicals and intermediate species. A bidirectional coupling of pressure wave and heat release develops if the pressure wave is strong and the gas mixture is reactive enough. This detonation exhibits an intensely illuminating flame that travels upward in the direction opposite to the original temperature gradient at speeds up to v = 1400 m/s.
References [1] A.S. Sokolik, A.N. Voinov, NACA Report No. 928 (1940). [2] A.S. Sokolik, Self-ignition, Flame and Detonation in Gases, Israel Program for Scientific Translations Ltd., Jerusalem, 1963. [3] C.D. Miller, NACA Report No. 855 (1946).
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[4] C.D. Miller, H.L. Olsen, W.O. Logan, G.E. Osterstrom, NACA Report No. 857 (1946). [5] A.M. Rothrock, R.C. Spencer, C.D. Miller, NACA Report No. 704 (1941). [6] A.K. Oppenheim, Society of Automotive Engineers 841339 (1984). [7] Ya.B. Zeldovich, V.B. Librovich, G.M. Makhviladze, G.I. Sivashinsky, Acta Astronaut. 15 (1970) 313–321. [8] Ya.B. Zeldovich, Combust. Flame 39 (1980) 211–214. [9] A. Dreizler, U. Maas, COMODIA 98 (1998) 197–202. [10] X.J. Gu, D.R. Emerson, D. Bradley, Combust. Flame 133 (2003) 63–74. [11] G. Goyal, U. Maas, J. Warnatz, Society of Automotive Engineers 900026 (1990). [12] L. He, C.K. Law, Phys. Fluids 8 (1) (1996) 248–257. [13] V. Smiljanovski, Ph.D. thesis, Ein numerisches Verfahren zur Berechnung schneller Vormischflammen und der Deflagrations-Detonations-Transition, Institut für Verbrennung, RWTH Aachen, 1996. [14] J.H. Lee, R. Knystautas, N. Yoshikawa, Acta Astronaut. 5 (1978) 971–982. [15] T. Itoh, T. Nakada, Y. Takagi, Soc. Automotive Eng. Jpn. Ser. B 38 (2) (1995) 230–237. [16] H. Shoji, K. Shiino, H. Watanabe, A. Saima, S. Automotive Eng. Jpn. 15 (1994) 109–116. [17] H. Shoji, K. Yoshida, A. Saima, ASME/JSME Thermal Eng. Conf., vol. 3, 1995, pp. 207–214. [18] D. Bradley, G.T. Kalghatgi, M. Golombok, Society of Automotive Engineers 962105 (1996) 2224–2237. [19] B. Grandin, I. Denbratt, J. Bood, C. Brackmann, P. Bengtsson, Society of Automotive Engineers 2000-012831 (2000) 2003–2016. [20] R.M. Green, R.P. Lucht, COMODIA 85 (1985) 259– 266. [21] J.F. Griffiths, J.P. MacNamara, C.G.W. Sheppard, D.A. Turton, B.J. Whitaker, Fuel 81 (2002) 2219–2225. [22] J.F. Griffiths, B.J. Whitaker, Combust. Flame 131 (2002) 386–399. [23] S. Eisen, Ph.D. thesis, Visualisierung der dieselmotorischen Verbrennung in einer schnellen Kompressionsmaschine, Lehrstuhl A für Thermodynamik, Technische Universität München, Germany, 2003, p. 165. [24] R.S. Sheinson, F.W. Williams, Combust. Flame 21 (1973) 221–230. [25] F.W. Williams, D. Indritz, R.S. Sheinson, Combust. Sci. Technol. 11 (1975) 67–73. [26] H.J. Curran, P. Gaffuri, W.J. Pitz, C.K. Westbrook, Combust. Flame 129 (2002) 253–280.
Influence of temperature inhomogeneities on knocking combustion M. Pöschl ∗ , T. Sattelmayer Lehrstuhl für Thermodynamik, Technische Universität München, Boltzmannstr. 15, 85747 Garching, Germany Received 19 June 2007; received in revised form 7 November 2007; accepted 14 November 2007 Available online 5 February 2008
Abstract The use of a rapid compression and expansion machine (RCEM) provides the possibility of investigating the fundamental kinetic behavior of combustion. The aim of this work is the investigation of the influence of temperature inhomogeneities on the reaction kinetics of knocking combustion under well-known conditions without having random fluctuations. In the RCEM a clearly defined temperature distribution was generated that covers a large area and facilitates the investigation of the influence of a temperature gradient on knocking combustion. The paper deals with the analysis and characterization of the different phases of low-temperature kinetics during knocking as well as nonknocking combustion. Combustion was controlled by ignition timing, variation of the spark plug position, compression ratio, and temperature distribution. The results show the formation of a fast-propagating flame due to local and sequentially progressing autoignitions, which initiate a detonation as a consequence of pressure waves, which progress into an already reacting gas mixture enriched with intermediate species and radicals. © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Knocking combustion; Rapid compression and expansion machine; Temperature inhomogeneities; Temperature stratification
1. Introduction Numerous studies have been carried out for many decades to explain the mechanism of knocking combustion, to identify the influencing factors, and to develop countermeasures against engine knock. Already in the 1930s and 1940s, several publications indicated that inhomogeneities of the gas mixture may lead to this undesired combustion mode. Sokolik and Voinov [1,2] observed that autoignition at hot spots in the * Corresponding author.
E-mail address: [email protected] (M. Pöschl).
end-gas will not always lead to knocking combustion. During knocking combustion a fast-propagating flame was observed. The detected flame propagation velocities were in the range of up to v = 2000 m/s. The authors emphasize the key role of the reaction in the gas mixture away from the primary flame region, where the knocking combustion has its origin. Similar results were attained by the investigations of the National Advisory Committee for Aeronautics (NACA) [3–5]. It is assumed that knocking combustion develops in gases that have undergone some extent of reaction. It was found that it does not matter whether the reaction was initiated by the primary flame or the autoignition.
0010-2180/$ – see front matter © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2007.11.009
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Both groups assume that the fast-propagating flame is a detonation, since the detected pressure amplitudes and the propagation velocities can be explained by the physics of detonation but not by homogeneous autoignition. However, it was hard to explain in what manner the coupling of the energy release and the pressure wave develops in small volumes such as the combustion chamber of an engine. On the basis of the publications of Oppenheim [6] and Zeldovich et al. [7,8], numerous later investigations were concerned with the influence of exothermic centers and the inhomogeneous distribution of temperature and concentration on knocking combustion [9–13]. The fundamental idea of these approaches is a gas mixture that exhibits spatial differences of the ignition time as a consequence of inhomogeneity and permits flame propagation due to local and sequentially proceeding autoignition. The propagation velocity is a function of the gradient of the ignition time, which in turn is a function of the temperature or concentration inhomogeneity. If the propagation velocity has a velocity similar to that of the pressure wave, which is produced by the autoignition, the coupling of heat release and pressure wave, and hence the formation of a detonation, is possible. Among other things, the papers confirm that detonations may also develop in small volumes. In this context, it is particularly important to mention the investigation of Lee et al. [14]: An explosive gas mixture with an inhomogeneity of the ignition time was created by heating the gas mixture with a xenon flash tube. By changing the heating input, different characteristics of the concentration distribution were created. A pressure wave was formed by local autoignition, which proceeded into the adjacent area. Since this area already exhibits thermal conditions near the limit, autoignition is initiated there, and the pressure wave is amplified and proceeds into the next adjacent area. Due to the fact that the heat release and the pressure wave are propagating in-phase, the explosive wave is continuously amplified until a detonation is formed. Lee et al. named this mechanism SWACER (shock wave amplification by coherent energy release). Further published investigations focus on the behavior of the end-gas during the development of knocking combustion in order to derive the chemical behavior of hydrocarbons [15–17] or the temperature development [18–20]. It was shown that lowtemperature kinetics within the end-gas region plays a key role concerning in the development of knocking combustion. Moreover, Griffiths et al. [21,22] emphasize the impact of the negative temperature coefficient (NTC) by investigating the influence of temperature on the autoignition of n-pentane. In the Griffiths et al. study [21], a rapid compression machine was used in which the air–fuel mixture exhibited a tem-
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perature distribution across the vertical cross section, with a maximum temperature difference of T = 8 K at starting condition. At this time the average temperature was given by T = 310 K. At a compressed gas temperature of Tc = 690 K the first emissions were seen in the upper and hence hottest part of the combustion chamber, followed by a development to the entire space with a relatively slow velocity for about 2 ms. Nonknocking combustion resulted whereas the higher gas temperature of Tc = 770 K provoked knocking combustion. The first light emissions were detected in the lowest and therefore coldest part of the combustion chamber in case of knocking combustion. The very fast covering of the whole space required less than 50 µs. Griffiths et al. concluded that autoignition starts in the hottest area, when the gas mixture reaches compression temperatures within the range of positive temperature coefficients. At the same time, a higher reactivity is found in the coldest area for temperatures within the range of negative temperature coefficients, which smooths out temperature inhomogeneities. The increased species concentration leads to knocking combustion, which starts in the area with coldest gas temperature at starting condition. The transition from nonknocking toward knocking combustion takes place at temperatures that exhibit the minimum ignition time. Despite very many investigations, knocking combustion is still a crucial topic concerning engine design and development, with remaining uncertainties and unsolved questions. Especially, the differentiation between detonations and fast propagating flames, which are formed by local and sequentially proceeding autoignitions, is difficult. Furthermore, there is no experimental evidence as to how, in the case of detonation, bidirectional coupling between the pressure wave and the energy release develops. In particular, the small scale of hot and cold spots in engines makes the analysis difficult and demanding. 2. Experimental apparatus and measuring technique The experiments were performed with a rapid compression and expansion machine (RCEM) that was designed at the Lehrstuhl für Thermodynamik of the Technische Universität München. A sketch of the apparatus with the piston position at the start of the experiment is shown in Fig. 1. With the objective of minimizing the vibrations for optical measurements during the experiments, the kinematic principle is based on two masses that are moving in opposite directions due to hydraulic coupling. A detailed description of the apparatus and its kinematics was given by Eisen [23].
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Fig. 1. Sketch of the rapid compression and expansion machine at starting position.
At the beginning of the experiment, the driver tube (1), which is connected with the piston (2), ends at the orifice (3) on the right side, and the balance weight (4) is located near the air supply valves (5). By means of compressed air, which is supplied by a compressedair storage with a variable volume, the balance weight is accelerated toward the orifice. Due to hydraulic coupling between the balance weight and the driver tube, including the piston, simultaneous movement in opposite directions is achieved. As soon as the balance weight approaches the orifice, a shear flow of the oil is created in the gap between the balance weight and the orifice. The resulting dissipation of the kinetic energy decelerates the movement of the masses. Additionally, the pressure generated inside the combustion chamber (6) by compression and combustion leads to a deceleration and to the reversal of the direction of motion of the piston as well as the balance weight. The initial pressure at the beginning of the experiment can be freely selected to simulate the pressure charging of a real engine. Since the air supply valves (7) and the corresponding exhaust valves (7) are tangentially arranged in the upper part of the cylinder liner, the entire valve train in the cylinder head (8) of a real engine is not required. By means of gas injection through the air supply valves, swirl and turbulence can be created in the combustion chamber during the compression stroke. The position of the piston at the beginning of the experiment and hence the compression stroke is selected by means of the hydraulically adjustable position of the back plate (9). In contrast to a real engine, the volume at the top dead center is not defined by the kinematics of the crank driving mechanism, but is a function of the volume of the compressed-air storage, the driving pressure, the shear flow within the orifice, and the pressure within the combustion chamber. The desired operating points are adjusted by varying these parameters. As, unlike a real engine, the RCEM is not operated continuously, all processes of interest must be captured during one single compression stroke and the following expansion stroke. The combustion chamber has a diameter of dc = 78.3 mm. The optically accessible area, with a diameter of do = 49 mm (Fig. 2), is obtained via a glass insert (10) in the piston. The
Fig. 2. Optically accessible area of the combustion chamber.
light emissions are transferred from the test rig to the camera via a tilted mirror (11). The presented experiments were performed with a stoichiometric air–fuel mixture. The fuel used was made out of the primary reference fuels n-heptane and iso-octane, with an octane number ON of 69. To avoid inhomogeneities of the incoming air–fuel mixture, the fuel was entirely evaporated and homogenized in an external mixing chamber. Prior to compression, a vacuum was generated in the combustion chamber before it was purged with the air–fuel mixture. A strip heater was used in the upper section of the combustion chamber for proper thermal conditioning of the test rig during the experiments (see Fig. 3). The temperature of the cylinder head was set to TW,CH = 373 K by means of a thermocouple. In the horizontally positioned cylinder liner, an inhomogeneous temperature distribution in the axial direction develops with a lower temperature of TW,BDC ≈ 353 K at the piston surface before the experiment is started, and the piston is located at the bottom dead center. The time between the charging of the combustion chamber with air–fuel mixture and the experiment itself is selected so that natural convection leads to a horizontally inhomogeneous temperature distribution of the charge due to stratification by gravity. A temperature difference of approximately T = 10–20 K between the upper and the lower part of the combustion chamber during each shot was determined with resistance thermometers. The following presented experiments indicate that after charging of the machine, turbulence decays quickly and the motion of the piston during compression does not produce substantial turbulence. For this reason the temperature stratification of the air–fuel mixture caused by natural convection is stable during the compression stroke.
3. Measuring technique The light emissions of the combustion intermediates were detected either by a Photron Ultima APX
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Fig. 4. Pressure traces of pressure transducer P1 for three different ignition times.
Fig. 5. Sequence of flame luminescence for the experiment with ignition time t2 . Fig. 3. Heating of the combustion chamber and temperature stratification of the air–fuel mixture before and during the compression stroke.
I2 intensified high-speed camera or by a LaVision Flamestar II intensified single-frame camera. Additionally, an optical measuring technique with faster dynamics than the high-speed camera was developed. The purpose was the acquisition of high-velocity data on the spatial and temporal distribution of the heat release and the dynamic processes during knocking combustion. Fifty optical fibers were incorporated all over the cross section of the cylinder head, including the outer parts of the combustion chamber, which are not optically accessible via the piston. The light intensity of each fiber was transformed by a photodiode or optionally by a photomultiplier tube, was amplified and then saved with a sampling frequency of 250 kHz per measurement point. Pressures during compression and combustion were measured by a Kistler 7061B piezoelectric transducer. The piston position was de-
tected by scanning a ruler that is integrated into the test rig with an AMO PMK-02-25 optical sensor.
4. Results 4.1. Variation of ignition time Fig. 4 shows the pressure traces of three experiments with different ignition times, while all further parameters remained identical. It can be seen that a shift of the spark ignition toward the expansion stroke reduces the knock intensity and even leads to nonknocking combustion. The corresponding sequences of the light emissions from combustion at ignition times t2 and t3 are shown in Figs. 5 and 6, which were recorded by an intensified high-speed camera APX I2 at a frame-rate of f = 15,000 fps in combination with a narrow-band interference filter with transmission of the CH band (λm = 431.4 nm). Up to the first
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M. Pöschl, T. Sattelmayer / Combustion and Flame 153 (2008) 562–573 Table 1 Characteristics of two-stage ignition with two distinctive pressure rises Ignition Time of 1st time increase of pressure rise t1 t2 t3
−0.5 ms +1.1 ms +1.8 ms
Time of 2nd increase of pressure rise
Time interval
+0.4 ms +2.8 ms +4.1 ms
0.9 ms 1.7 ms 2.3 ms
Fig. 6. Sequence of flame luminescence for the experiment with ignition time t3 .
Fig. 8. Pressure traces and corresponding chemiluminescence during cool flame and main heat release of nonknocking combustion; ε = 8.7.
Fig. 7. 3-dimensional scheme of the different flames in the combustion chamber.
frames of both sequences, the spark-ignited flame already filled the entire optically accessible area, which is smaller than the full cross section of the combustion chamber. In the upper area a reacting zone is visible which moves downwards. It represents an autoignition which starts in the hottest area and continues in direction of the temperature gradient. The visible reaction zone disappears near the center due to the advanced propagation and reaction progress of the centrally located spark ignited flame. Fig. 7 shows a three-dimensional scheme of the different flames in the combustion chamber. The distance between the cylinder head and the piston crown was in the range of l = 9–16 mm for the presented frames. Subsequent to the downward propagating reacting zone, a strongly radiating flame follows in the case of combustion at ignition time t2 , which moves upward. This flame corresponds to the beginning of strong pressure oscillations. Despite autoignition, nonknocking combustion occurs in the experiment with the ig-
nition time t3 . This shows clearly that autoignition does not necessarily provoke knocking combustion but leads to an augmented heat release which results in increased pressure rise and increased light intensity. The earlier the ignition occurs, the higher is the contribution of the expanding flame to pressure rise. In the case of the earliest ignition time t1 the sparkignited flame causes a substantial increase of the pressure. Nevertheless, the shape of the pressure trace shows characteristics of two-stage ignition with two distinctive pressure rises, which can be also seen during the experiments with ignition times t2 and t3 (see Table 1). Qualitatively similar characteristics of the pressure traces are reached for experiments without spark ignition (see the first part of Fig. 8). As will be discussed in Section 4.2, the pressure increases can be clearly correlated with the events of autoignition by means of optical fibers, which are incorporated all over the cylinder head, including the outer range near the cylinder liner. The analysis of the high-speed cinematography reveals further that the spark-ignited flame shows an almost laminar shape without significant wrinkles. This was given for all performed experiments without swirl generation. The detected expansion velocities are in the range of vsf = 3–6 m/s, which indicates that there may be small-scale but not large-scale turbulence. Compared to autoignition, the
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Fig. 9. Sequence of the cool flame. The time interval between the first frame and the last frame is about t = 0.6 ms.
spark-ignited flame shows a low propagation velocity with a low reaction rate. Furthermore, the contribution to the overall pressure rise is smaller than that in a real engine. The pressure traces at ignition times t2 and t3 show only small differences until they reach top dead center. However, the mode of combustion changes significantly. This fact highlights how sensitive the heat release is near the knocking limit. Small changes of temperature or pressure may essentially influence the following reactions, as is very well known from spark ignition studies in engines. 4.2. Analysis of autoignition in the end-gas region Instead of using spark ignition, during the experiments presented below the compression ratio ε was increased to isolate the autoignition from effects created by the spark initiation. That is, the propagating autoignition can be detected and investigated without the interference of the flame luminescence originating from the spark ignited flame. The traces of the pressure transducers P1 and P2 (see Fig. 2) in the first part of Fig. 8 show a moderate increase of the rate of pressure rise at t = −1.2 ms. The simultaneous use of photomultiplier tubes and narrow-band interference filters with transmission in the OH band (λm = 305.9 nm) and CH band (λm = 431.4 nm) is presented in the second part of Fig. 8. At this time there are no signals within the OH band, although light emissions within the CH band were detected. This behavior is characteristic of the cool flame of lowtemperature kinetics, in which a high concentration of formaldehyde (CH2 O∗ ) is produced. Formaldehyde emits light signals within the CH band but not in the OH band and at longer wavelengths [24,25]. Fig. 9 shows a sequence of single frames taken with the LaVision Flamestar II intensified singleframe camera without any spectral filter during a series of experiments made under identical conditions.
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The cool flame starts in the upper and hence hottest area. Afterward the reaction zone propagates downward along the temperature gradient. The time interval between the first frame and the last frame is about t = 0.6 ms. A couple of milliseconds later, a visible reaction front with light emissions in the OH-band, CHband, and λ > 580 nm follows. It corresponds to the downward-moving reaction front in Figs. 5 and 6, which is termed as “autoignition.” It represents the main heat release of a two-stage reaction that is governed by high-temperature kinetics. The light emissions are transferred to photomultiplier tubes by means of optical fibers that are incorporated into the upper part of the combustion chamber, near the cylinder liner. This area is not optically accessible via the piston. Both characteristic pressure rises can be clearly correlated with the beginning of light emissions, and hence with both phases of autoignition which was already described in Section 4.1. The detected mean propagation velocities of the cool flame in the range of vcf,m = 50–200 m/s and of the main heat release up to vmh,m = 500 m/s cannot be explained by laminar or turbulent flame propagation. It is a sequential autoignition process of areas that ignite successively due to the temperature stratification that has already been described by Zeldovich (see Section 1). These forms of propagation were modeled for all test conditions by means of kinetic calculations with a multizone model. The reaction mechanism used was particularly designed for mixtures of the primary reference fuels n-heptane and iso-octane and contains 4238 elementary reactions with 1034 species [26]. The combustion volume was split into four reactors, R1 to R4, linked with walls to each other, which move without friction and permit pressure equilibrium (first part of Fig. 10). The starting temperatures Ti,0 , with a temperature difference of T = 10 K, represent the existing temperature stratification in the RCEM. The entire volume of the combustion chamber, V (t), is given as a function of time and corresponds to the compression stroke of the RCEM. The calculated ignition delay between the hottest reactor R1 and the coldest reactor R4 is tcf = 0.7 ms in case of the cool flame and tmh = 0.15 ms in case of the main heat release. The respective propagation velocities can be derived from the calculated ignition delays by using the diameter of the combustion chamber dc = 78.3 mm. In the case of the cool flame the propagation velocity is vcf,m = 112 m/s and in the case of the main heat release vmh,m = 522 m/s. The calculated values agree very well with the experimentally observed values. This leads to the very important finding that the propagation of the cool flame and the onset of the main
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Fig. 10. Kinetic calculation by means of four-zone model.
heat release are totally independent of laminar or turbulent transport. The propagating reaction zone of the main heat release causes pressure waves and a compression of the gases in the lower part of the combustion chamber. The propagating intensity and the propagating speed depend on the initial thermal conditions and test parameters. Fig. 8 shows nonknocking combustion. The propagation of the main heat release is weak and slow enough to permit pressure equalization. This can be seen by means of the signals of both pressure transducers P1 in the upper part of the combustion chamber and P2 in the lower part of the combustion chamber. A different behavior was observed during knocking combustion in Fig. 11, which shows a sequence of the propagating reaction zone of main heat release and the corresponding pressure traces. The frames show a narrow vertical section of the optically accessible combustion chamber (see Fig. 2). The downscaling of the detected area made it possible to increase the frame rate up to f = 87,600 fps. There is a pressure rise in the upper area at time tP1 = 1.97 ms, which correlates with the onset of the main heat release. The downward-propagating pressure wave is detected in the lower part of the combustion chamber at tP2 = 2.06 ms. This period correlates with a sound wave that propagates between the two pressure transducers at the present gas temperature of approximately T = 800 K. The propagating main heat release for t < 2.295 ms is faster than in the previously described case. In the lower part, the first light emissions can be seen at time t = 2.306 ms. At the same time, the pressure signal shows a steep increase,
Fig. 11. Sequence and corresponding pressure traces during main heat release of knocking combustion.
with pressure differences of more than p = 60 bar between the pressure transducers P1 and P2. In contrast to nonknocking combustion with a main heat release that reacts almost homogeneously and without pressure oscillations, there is an abrupt change to a bright flame, which travels upward with supersonic speed. The detected velocities, which were measured by means of the fiber technique, are in the range of up to v = 1400 m/s. It is interesting to note that this reaction starts in the originally coldest region and proceeds in the direction opposite to the direction of the preceding reactions, the cool flame and the onset of the main heat release. The subsequently beginning pressure oscillations correspond to the start of this third combustion mode. The measured pressures and propagating velocities indicate that a detonative reaction occurs. The onset of the detonation might be initiated by two mechanisms. The first one is direct initiation due to inhomogeneities, according to the description of Zeldovich et al. or the description of Lee et al. Hence, the detonation would be initiated during the downward propagating reaction zone of main heat release. The second possibility is initiation as a consequence of a pressure wave that is formed because of the propagating main heat release. After being reflected and focused by the wall in the lower part of the combustion chamber, the pressure wave passes upward
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Fig. 12. Variation of the ignition position.
through the reacting gas mixture, which exhibits high radical concentrations. The bidirectional coupling of heat release and pressure wave is established once the pressure wave is strong enough and the gas mixture is reactive enough. The experiments presented below will investigate this hypothesis in greater detail in order to clarify this point.
Fig. 13. Pressure traces for different ignition positions; εC = εB = εT .
4.3. Variation of the ignition position The experiments presented above showed a stable and reproducible progress of the reaction, which develops from the initial temperature distribution. The essential parameters are the local differences of induction time leading to distributed sequential autoignition. The following experiments investigate with which measures and to what extent the temperature distribution may be influenced in localized areas in order to avoid knocking combustion. For this purpose, these experiments were performed with spark ignition. The spark-ignited flame provides a local heat release zone near the ignited gas, with radicals and intermediate products that change the spatial distribution of the induction time in the area near the spark plug. Three different ignition positions were used in the experiments presented below (Fig. 12): • Center (C): The hottest and coldest gases in the end-gas region are reached by the spark-ignited flame at the same time. • Bottom (B): The ignition is forced in the coldest gas. • Top (T): The ignition takes place in the hottest gas, where autoignition always occurs first. The three pressure traces in Fig. 13 match totally until the onset of the cool flame.1 Following central spark ignition (Fig. 14), the spark-ignited flame already filled the entire optically accessible area up to the first frame at t = 2.816 ms. The integral view of 1 The description of cool flame and main heat release refers exclusively to the autoignited reactions.
Fig. 14. Sequence of flame luminescence with ignition at the center.
the light emissions shows a superposition of the main heat release, which propagates downward and causes an increase of the pressure rise rate, and the sparkignited flame. The contour of the propagating main heat release can only be seen at the outer area on the left side after reaching the area near the center position at t = 2.883 ms. It disappears completely at t = 3.083 ms because of the spark-ignited flame in this area: due to the advanced chemical progress near the center position, the propagating main heat release of autoignition occurs only in the outer area of the combustion chamber. The velocity of the propagating main heat release and the intensity of the following pressure waves are smaller than for the experiments without spark ignition. The weak pressure oscillations indicate that this operating point is near the knocking limit. Interestingly, the ignition at the bottom leads to knocking combustion with strong pressure oscilla-
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Fig. 15. Sequence of flame luminescence with ignition at the bottom.
tions. The spark-ignited flame has the longest way to reach the area with the hottest gases under starting conditions in the upper part of the combustion chamber. This means that autoignition may propagate undisturbed for a long distance until it reaches the area of the spark-ignited flame. The main heat release starts t = 0.7 ms earlier and propagates with faster dynamics and higher intensity compared to the case with centrally located ignition. Fig. 15 shows that the spark-ignited flame is pushed back and compressed further. The following development is identical to the case with no spark ignition: A bright flame starts at the bottom and moves upward. The ignition in the top position (Fig. 16) leads to smooth combustion with a substantial shift of the heat release toward the expansion stroke. As previously mentioned in case of the centrally located spark ignition, the propagating main heat release and hence the resulting pressure waves cannot evolve with high intensity in areas with an advanced reaction progress. Due to the spark-ignited primary flame in the area of the hottest gases, the main heat release of autoignition can only be detected within a small area located in the lower part of the combustion chamber. The propagation is rather slow, with a weak intensity, similarly to the experiment with ignition at the center. Subsequently, a moderate pressure rise without any oscillations follows. In this experiment it can be clearly seen that the propagating spark-ignited flame is not able to trigger knocking combustion, since both the reaction rate and the propagation velocity are too low. The propagation velocity with approximately vsf = 5–6 m/s is much lower than the downward propagation of the main heat release zone driven by autoignition.
Fig. 16. Sequence of flame luminescence with ignition at the top.
Fig. 17. Pressure traces for different ignition positions; εB1 < εT2 < εT3 .
The spark ignition at the bottom shows a stronger tendency to knock compared to the ignition at the center and at the top. This was confirmed by the experiments in Fig. 17, which show a comparison between the ignition positions at the bottom (B1) and at the top (T2 and T3) positions. Strongly knocking combustion results from the spark ignition at the bottom, although the lowest compression ratio εB1 was chosen. In contrast, only small pressure oscillations are being observed in case of ignition at the top position (T2), even though the pressure is much higher. The higher pressure is reached by an increased compression ratio εT2 > εB1 and a greater contribution of the expanding spark ignited flame to pressure rise. A further increase of the compression ratio εT3 causes a shift of the heat release toward the top dead center and provokes strong pressure oscillations. The corresponding sequence is shown in Fig. 18. Only the spark-ignited flame is visible before t = 0.987 ms.
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Fig. 19. Pressure traces for different temperature fields.
of the gas mixture in the lower part is not sufficient for a bidirectional interaction with the passing pressure wave. 4.4. Variation of the temperature field
Fig. 18. Sequence of flame luminescence with ignition at the top; εT3 .
The propagating main heat release appears near the spark-ignited flame in the following frame. At the same time, the light emissions in the upper part of the combustion chamber increase. A much brighter luminescence from combustion is visible in this area at t = 1.187 ms, which correlates with the beginning of knock. This fact is remarkable, since the beginning of knocking combustion was in the lower part of the combustion chamber for all other discussed operating points. The propagating main heat release generates pressure waves that are reflected at the lower wall of the combustion chamber. Subsequently, the upwardpropagating pressure waves are passing a gas mixture that exhibits a sufficient rate of progress for the interaction between pressure wave and heat release only in the upper part of the combustion chamber. Later, only the upper part of the combustion chamber shows bright light emissions. Further on, the rate of progress
All experiments presented above consistently point out that the propagating main heat release has a great impact on the formation of knock. The crucial parameters are the propagating velocity and the propagating distance. Both are mainly determined by the temperature distribution of the gas mixture. The following section will discuss to what extent the knock may be influenced by disturbing the temperature stratification. It is possible to vary the temperature stratification in the RCEM by gas injection via the tangentially arranged supply valves (see Fig. 19). A small amount of air–fuel mixture is injected during the very first phase of compression. This small mass flow is sufficient for the destruction of the temperature stratification, but the impulse is not strong enough for the generation of a swirl flow. The additionally injected mass was compensated in the experiments without gas injection by an initial charging of p = 0.06 bar. Spark ignition was not used in order to provide the best conditions for the detection and investigation of autoignition. A comparison of the undisturbed and the disturbed case is shown in Figs. 20 and 21. Fig. 20 demonstrates the case of a stratified temperature field over the entire section of the combustion chamber, which causes knock with strong pressure oscillations (Fig. 19). In contrast to this, it can be seen in Fig. 21 that the distribution of the main heat release is more homogeneous. The resulting pressure traces exhibit significantly reduced oscillations for the latter case, although the compression ratio was increased from ε = 12.7 to ε = 13.4. Additionally, a pressure trace is shown, which was recorded with an undisturbed temperature field and a compression ratio of ε = 13.3. Again knock is observed with strong pressure oscillations which exceed the measurement range of p =
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Fig. 20. Sequence of flame luminescence with an undisturbed temperature field; ε = 12.7.
100 bar. The onset of the cool flame and the main heat release begin earlier than for the disturbed temperature field. The gas injection causes some mixing of the charge during compression and there is a higher heat exchange between cold and hot spots of the charge adjacent to each other. Both effects cause a reduction of the maximum charge temperature and hence a delayed autoignition. However, the shift of the reaction toward the expansion stroke plays a minor role. This can be seen by reviewing the case with ε = 12.7. The key role for the significant reduction of the knock intensity and the tendency for knock as such, which can be seen in Fig. 22, is the prevention of sequential autoignition across a long distance. The knock intensity KI, which describes the intensity of the pressure oscillations, was calculated on the basis of the measured pressure pc and the low-pass filtered pressure signal pfilt according to the equation KI =
1 nsum
·
n sum
pc (n) − pfilt (n)
2
(bar2 ),
Fig. 21. Sequence of flame luminescence with a disturbed temperature field; ε = 13.4.
Fig. 22. Influence of the compression ratio ε and the temperature field on the knock intensity KI.
(1)
n=1
where nsum is the number of samples. Fig. 23 shows the power density spectrum Pxx for the pressure traces shown in Fig. 19. All traces exhibit their maximum at fmax = 7 kHz, which corresponds to the 1. acoustic eigenfrequency. This means that pressure waves are transversally passing the combustion chamber. The maximum of the power density spectrum is significantly smaller in the case of the disturbed temperature field. 4.5. Summary and conclusions The experiments show the formation of knock due to temperature inhomogeneities. Natural convection causes a temperature stratification of the gas mixture in the combustion chamber of the employed rapid
Fig. 23. Power density spectrum for different temperature fields (analysis of the pressure traces shown in Fig. 19).
compression and expansion machine. The study reveals a previously unknown key role for knock in charges with nonhomogeneous temperature distribution. The observed mechanism that triggers knock is clearly different from those presented by Griffiths et al. In contrast to the findings of Griffiths et al., it was found that the negative temperature coefficient (NTC)
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does not play any role in the evolution of knock. Instead, the experiments reveal the crucial impact of dynamic processes on the initiation of a detonative combustion: • Sequential autoignition leads to a fast-propagating reaction front along the temperature gradient. With a spectral analysis the first occurring reaction front was identified as the first phase of lowtemperature kinetics, the cool flame, which travels with average speeds of vcf,m = 50–200 m/s. No difference in speed or intensity during this phase between knocking and nonknocking combustion was observed. • Subsequently a second reaction front follows, the onset of the main heat release with average speeds up to vmh,m = 500 m/s. There is a clear relation between the propagation velocity and the knock intensity: The faster the reaction front of main heat release propagates, the higher the knock intensity and the tendency to the formation of knock as such become. • Pressure waves are generated by the fast-propagating reaction front. After being reflected and focused at the convex wall in the lower part of the combustion volume, the pressure wave passes the reactive gas mixture with high concentrations of radicals and intermediate species. A bidirectional coupling of pressure wave and heat release develops if the pressure wave is strong and the gas mixture is reactive enough. This detonation exhibits an intensely illuminating flame that travels upward in the direction opposite to the original temperature gradient at speeds up to v = 1400 m/s.
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