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Automotive Innovation

, Volume 2, Issue 2, pp 137–145 | Cite as

Impact Resistance of Spark Plug’s Ceramic Insulator During Ultra-high-Pressure Combustion Under Deto-Knock Conditions

  • Yunliang Qi
  • Boyuan Wang
  • Zhi WangEmail author
Article
  • 370 Downloads

Abstract

The ceramic insulators of spark plugs in gasoline engines are especially prone to damage when deto-knock occurs. To understand the damage process and mechanism, the present work investigated the impact resistance of ceramic insulators using detonation waves as impact sources. A test device that generates detonation waves was developed, representing a novel means of evaluating the knock resistance of ceramic insulators. Various impact types and detonation intensities were employed, and detonation initiation and propagation at peak pressures greater than 100 MPa were assessed using synchronous high-speed direct photography and pressure measurements. The test results demonstrate that ceramic insulators tend to break at the base of the breathing chamber when damaged by a single high peak pressure detonation wave impact. In contrast, multiple low pressure impacts eventually break the insulator into multiple fragments. The data also show that the positioning of a ground electrode upstream of the ceramic insulator greatly increases the resistance of the ceramic to the detonation impact. A two-dimensional computational fluid dynamics simulation coupled with a chemical kinetics analysis demonstrated that this improved resistance can be ascribed to a reduced peak pressure that appears after the detonation wave diffracts from the electrode prior to contacting the ceramic insulator.

Keywords

Spark plug Ceramic insulator Detonation wave Constant volume combustion chamber 

1 Introduction

High boost and direct injection have become the main approaches to obtain high-power densities and high efficiencies in gasoline engines in recent years. However, increasing the boost ratio has been found to produce a new engine knock mode in gasoline engines, especially in conjunction with a high brake mean effective pressure (BMEP) and low speeds, as shown in Fig. 1a. This new knock mode [1] has been termed deto-knock or chaoji knock by Tsinghua, super-knock by Shell, mega knock by AVL company and low-speed pre-ignition (LSPI) by Southwest Research Institute. The peak pressure during deto-knock is much higher than during conventional knock and can exceed 50 MPa [2] such that it can potentially damage engine parts, as shown in Fig. 1b. At present, deto-knock is a major obstacle to achieve further improvements in the power density and thermal efficiency of turbocharged gasoline engines.
Fig. 1

Operating regime and combustion characteristics of deto-knock

Many studies have determined that deto-knock can damage engine components [1, 3, 4, 5, 6], including the cylinder head, piston, connecting rod and spark plug. Among these components, spark plugs are the most prone to damage, and the most frequent damage mode is breakage of the insulator, as shown in Fig. 1b. The insulator is a necessary part of the spark plug as it provides a dielectric barrier that isolates the high voltage required to initiate combustion of the air–fuel mixture. The insulator of a modern spark plug is typically made of an Al2O3 ceramic because of the excellent dielectric performance of these materials [7]. These ceramics have high static mechanical strength but are fragile and easily broken when subjected to cyclical high load impact or vibration. Therefore, the failure of spark plugs under deto-knock conditions is generally considered to result from the impact of the strong detonation waves on the ceramic insulator. This is in contrast to the high-temperature thermal erosion of the electrode that occurs due to conventional knock. Eliminating deto-knock should decrease the rate of spark plug failure, but this remains challenging while also attempting to improve engine performance. An alternative approach is to improve the mechanical strength of the ceramic material or optimize the structural design of the spark plug. Regardless, it is crucial to understand the failure mechanism of the spark plug under deto-knock conditions, although there have been few studies focusing on this mechanism to date. Moreover, due to the random occurrence of deto-knock and the difficulty associated with visualization of the combustion process in real engines, the failure mechanism of spark plug ceramic insulators is still not clear.

Deto-knock is the result of the propagation of a detonation wave in the combustion chamber [8]. The effects of this detonation wave on the spark plug can be examined by subjecting plugs to such waves while controlling the impact time and intensity. This paper presents such a study aimed at investigating the resistance of spark plugs to impact with detonation waves in a constant volume combustion chamber (CVCC) with optical accessibility.

2 Experimental Setup

The experimental setup is schematically shown in Fig. 2a. The CVCC was made of stainless steel, and a drawing of the vertical cross section of the apparatus is provided in Fig. 2b. The cavity of the CVCC was 100 mm (L) × 20 mm (W) × 16 mm (H) in size and was connected to eight mounting holes. Each hole was the same size and could be used to mount charge inlet and outlet ports, spark plugs and pressure sensors interchangeably according to the experimental requirements, through replacing various adaptors. In this study, holes A and G were employed to mount the charge inlet and outlet ports, respectively, while the igniting spark plug and test spark plug were mounted at holes H and B, respectively. Hole F was used to mount the pressure sensor, and the other holes were not used and were plugged. The CVCC also had two side windows that allowed visualization of the entire cavity. Each window consisted of two layers of quartz glass to ensure appropriate sealing of the cavity.
Fig. 2

Schematic diagram of the experimental setup

The impact of the detonation wave on the spark plug was assessed by high-speed visualization of the detonation initiation and propagation processes at specific initial pressure values. A DG535 pulse generator (Stanford Research Systems, Sunnyvale, CA, USA) was used to simultaneously trigger the high-speed camera (Photron SA-X2, Photron Limited, Tokyo, Japan), the spark ignition and the pressure data acquisition system. Both the igniting and test spark plugs used in these experiments were the same commercially available aftermarket J-type M12 models, having a 26 mm thread length and an Al2O3 ceramic insulator. The spark plugs were mounted with their shell tip surfaces flush with the cavity wall. A PCB 119B11 (PCB Piezotronics, Inc., Depew, NY, USA) pressure sensor was employed, which is capable of measuring the dynamic pressure up to 520 MPa with the resolution less than 7 kPa, resonant frequency greater than 400 kHz and rise time less than 2 μs [9]. The pressure signal was amplified by a Kistler 5018A amplifier (Kistler Group, Winterthur Switzerland) and then recorded with a LeCroy Waverunner 4000 oscilloscope (Teledyne LeCroy, Chestnut Ridge, NY, USA).

Detonation was initiated using a stoichiometric mixture of pure propane (> 99%) and pure oxygen (> 99.999%). Propane was selected as the fuel because of its high vapor pressure, which facilitated the preparation of a high-pressure gaseous mixture. To ensure safe operation of the apparatus, the gas mixture was prepared within the test equipment just prior to each trial based on the partial pressures of the two gases, rather than pre-prepared and stored in a tank. Prior to preparing the detonation mixture, the CVCC was evacuated to a pressure of less than 100 Pa, after which propane and oxygen were separately and consecutively added into the CVCC using two HORIBA S48-300/HMT mass flow controllers (HORIBA METRON, Beijing, China), while monitoring the pressure using an OMEGA DPG409 pressure gauge (OMEGA Engineering, Inc., Norwalk, CT, USA) with a resolution of 10 Pa.

3 Results and Discussion

3.1 Detonation Initiation and Propagation

High initial pressures were used to initiate detonation and generate an ultra-high peak pressure upstream of the test spark plug. Figure 3 shows photographic images of the detonation initiation and propagation at an initial pressure of p0 = 1.08 MPa. These images were captured at a rate of 360 kfps and a shutter speed of 293 ns with a 431-nm filter. The mixture was ignited at t = 30.6 μs, after which the flame propagated at an average speed of 44.8 m/s up to t = 52.8 μs. At this point, localized explosions occurred at both the upper and the bottom walls of the combustion chamber, meaning that detonation was initiated. The explosion sites were very near the inlet and outlet charge ports mounted on the chamber through adaptors. Although the adaptors were mounted with their tip surfaces flush with the combustion chamber wall, the orifices of the ports could possibly facilitate the transition of the combustion process to detonation. After the two detonation waves merged, a strong curved detonation wave was formed and subsequently propagated through the test chamber at 2357 m/s. Due to the impact force applied by this high-pressure wave, cracks appeared on the surfaces of the quartz windows immediately following the appearance of the wave.
Fig. 3

Detonation initiation and propagation at p0 = 1.08 MPa

Figure 4 shows the pressure history of the combustion process from which the images in Fig. 3 were obtained. Due to the immediate cracking of the quartz windows in response to the detonation wave, no useful information could be observed in the post-detonation region. It should also be noted that an real detonation wave differs from an ideal plane wave as it will have a complicated structure due to triple point movement [10]. For this reason, it is difficult to interpret the pressure peaks in a trace in the absence of enough optical evident. According to the travel times of the detonation wave and its reflection that were obtained experimentally and from thermodynamic calculations, Peak 1 in Fig. 4 corresponds to the first pass of the incident detonation wave, while Peak 4 represents the reflected shock wave from the right end wall of the combustion chamber. Peaks 2 and 3 may result from the reflection of transverse waves. In most cases, detonation is initiated by a localized explosion [10] and transverse waves are generated after the circular explosion wave impinges on the wall, after which they move following the detonation wave. These transverse waves would impinge on the pressure sensor to generate a pressure peak. Another possible explanation for the appearance of Peaks 2 and 3 may be the reflection of the shock wave from the base of the breathing chamber. It should be noted that, although the breathing chamber is narrow, a flammable gas mixture will be present in this area such that the detonation wave can propagate through the region [11]. When the detonation wave reaches the base of the breathing chamber, the flammable gas mixture will have been consumed and the detonation wave will be reflected downward by the wall as a shock wave. This shock wave impinges on the pressure sensor in a head-on manner to produce a localized increase in pressure. Based on the above, although Peak 2 and Peak 3 have very high pressures, these pressures are not applied directly to the ceramic insulator. Therefore, the first peak pressure (Peak 1) in the pressure history was used to evaluate the detonation wave intensity in this study.
Fig. 4

Pressure as a function of time during a trial at an initial pressure of 1.08 MPa

Figure 5 presents photographic images tracking detonation initiation and propagation at an initial pressure of p0 = 1.80 MPa. In contrast to the results from the trial at p0 = 1.08 MPa, detonation was initiated closer to the igniting spark plug. It appears that the detonation was directly initiated by the spark discharge because it appeared only 2.8 μs after the spark was observed. At that point in time, the combustion wave front was just beyond the ground electrode of the igniting spark plug and had not reached the wall. Figures 3 and 5 demonstrate that as the initial pressure increases, the run-up distance for the detonation wave formation gets shorter. Besides, when the initial pressure is higher than 1.08 MPa, detonation wave can be formed and fully developed upstream the test spark plug. Therefore, these detonation waves can be used as the impact sources.
Fig. 5

Detonation initiation and propagation at p0 = 1.80 MPa

3.2 Single-Impact Tests

During the single-impact tests, each spark plug was only impacted once by the detonation wave and then replaced by a new one. All the spark plugs used in these trials were mounted with the ground electrode oriented toward the direction from which the detonation wave approached (i.e., toward the igniting spark plug). This position is denoted as Orientation A, as shown in Fig. 6.
Fig. 6

Mounting orientations of the ground electrode

In previous experiments carried out in a rapid compression machine, the highest recorded deto-knock peak pressure was over 50 MPa when a stoichiometric isooctane/air mixture was used under real engine operation conditions [2]. To obtain similarly high peak pressures in the present trials, four initial pressures were used: p0 = 1.08, 1.50, 1.80 and 2.10 MPa. Five tests were performed at each initial pressure, except for p0 = 2.10 MPa, and the resulting data are summarized in Table 1. At an initial pressure of 1.08 MPa, the first peak pressure was approximately 75 MPa [Table 1(a)]. Each of the five spark plugs tested was able to discharge and form a spark normally after a single impact, and no damage of the insulator/spark plug was evident on visual inspection. When the initial pressure was increased to 1.50 MPa, the first peak pressure was also increased, to over 100 MPa in some trials, the test results did not change compared to the cases of p0 = 1.08 MPa. After the initial pressure was further increased to 1.80 MPa, three trials produced insulator failure, while the other two spark plugs exhibited normal operation after the impact. An example of a ceramic insulator failure is shown in Fig. 7. In each failure, the insulator was fractured at the base of the breathing chamber, at which point the diameter changes abruptly such that there is a stress concentration. It should be noted that, although the ceramic insulator was broken into two parts, the lower part (shown in Fig. 7) was not further broken into small pieces but remained in one piece. Table 1(c) demonstrates that a peak pressure over 115 MPa typically resulted in insulator failure. The discrepant result in test 5, in which almost the same peak pressure was obtained as in test 1, but the insulator was not broken, can possibly be attributed to variations in the strength of the plugs, since the mechanical properties of the ceramic insulator can be affected by many factors during the production process [7]. The different results obtained from tests 1 and 5 also indicate that the critical impact pressure for the test spark plug was approximately 115 MPa. At the highest initial pressure of 2.10 MPa, only two tests were performed for reasons related to safety. As shown in Table 1(d), the first peak pressures obtained from these trials were the highest recorded in this study, and both led to failure of the spark plug. In each case, the failure mode was the same as that shown in Fig. 7.
Table 1

Results from single-impact trials

No.

First peak pressure (MPa)

State after impact

(a) p0 = 1.08 MPa

 1

79.5

Normal

 2

65.2

Normal

 3

80.1

Normal

 4

67.3

Normal

 5

67.1

Normal

(b) p0 = 1.50 MPa

 1

94

Normal

 2

100.2

Normal

 3

104.7

Normal

 4

95.4

Normal

 5

101.8

Normal

(c) p0 = 1. 80 MPa

 1

115.1

Failure

 2

127.2

Failure

 3

107.9

Normal

 4

129.9

Failure

 5

115.7

Normal

(d) p0 = 2.10 MPa

 1

129.4

Failure

 2

132.4

Failure

Fig. 7

Fracturing of a ceramic insulator at the base of the breathing chamber

3.3 Multiple-Impact Tests

In real engines, spark plug failure is more likely to occur after a long-time running, during which multiple deto-knock events may appear. It should also be noted that not all such events generate the very high peak pressures as shown in Table 1. In fact, the literature shows that, during the majority of pre-ignition and deto-knock cycles, the peak pressures are less than 20 MPa [12]. Therefore, the failure of the insulator may also result from multiple impacts by low-intensity detonation waves.

The data in Table 1 demonstrate that a single impact does not always break the insulator at p0 = 1.80 MPa and that no breaks occur at lower initial pressures. In subsequent trials, repeated impacts were imparted to the spark plugs to determine the ability of the ceramic to withstand such multiple impacts. During these tests, the state of the insulator was monitored after each impact by removing the side window of the CVCC rather than unmounting the spark plug. The results of these multiple-impact experiments are provided in Table 2. At an initial pressure of 1.80 MPa, the insulator failed randomly after either one or two impacts. In the case that the failure appeared after one impact, the failure mode was the same as that shown in Fig. 7. However, in the case that the failure required two impacts, a different failure mode was evident in which the insulator was broken into many small fragments. Figure 8 shows a spark plug after failure, along with two relatively large fragments of the broken ceramic insulator. It should be noted that, in most trials during which the insulator fragmented, the ceramic was not broken at the base of the breathing chamber but rather between the base and the tip. At p0 = 1.50 and 1.08 MPa, the insulator also failed after multiple impacts and the lower initial pressure required more impacts to produce failures.
Table 2

Results from multiple-impact trials

No.

p0 (MPa)

Averaged first peak pressure (MPa)

Impact times to cause failure

Failure mode

1

1.80

117

1–2

Break/fragment

2

1.50

99

2–3

Fragment

3

1.08

73

3–4

Fragment

Fig. 8

Fragments of a broken ceramic insulator

Multiple-impact tests were also performed even under lower pressure conditions, beginning at p0 = 0.12 MPa and increasing the pressure in intervals of 0.12 MPa, with the results shown in Table 3. After 11 impacts at increasing initial pressures up to 0.84 MPa, the insulator exhibited normal operation. However, when the initial pressure was further increased to 0.96 MPa, the insulator was damaged after only one impact and the failure mode was fragmentation. The fragments obtained after multiple impacts (see data in Tables 2, 3) suggest that impacts that did not immediately break the insulator might have generated cracks in the ceramic. Repeated impacts would develop the cracks and eventually break the insulator into small pieces.
Table 3

Results from multiple-impacts trials at lower initial pressures

p0 (MPa)

Impact times

State after impact

0.12

2

Normal

0.24

2

Normal

0.36

2

Normal

0.48

1

Normal

0.60

1

Normal

0.72

2

Normal

0.84

1

Normal

0.96

1

Fragment

3.4 Effect of Ground Electrode Orientation

Because detonations would not be initiated at the same place in real engines, the spark plug could conceivably be impacted by detonation waves traveling in different directions. To study the effect of the impact direction on the insulator, two additional spark plug orientations, denoted as Orientations B and C in Fig. 6, were assessed in single-impact trials. The results are shown in Table 4. Orientation C resulted in similar outcomes to those obtained using Orientation A. However, when the spark plug was mounted using Orientation B, the insulator continued to exhibit normal performance even at p0 = 2.10 MPa, thus demonstrating better impact resistance than either Orientations A or C. These results indicate that the ground electrode provided some degree of protection to the ceramic insulator when it was situated in the upstream direction. To further assess this protection effect associated with the electrode orientation, two-dimensional computational fluid dynamics (2D CFD) simulations were carried out using the large eddy simulation (LES) approach coupled with chemical kinetics [13].
Table 4

Results of trials with different ground electrode orientations

Orientation of ground electrode

State after impact

p0 = 1.50 MPa

p0 = 1.80 MPa

p0 = 2.10 MPa

B

Normal

Normal

Normal

C

Normal

Failure

Failure

Figure 9 provides the simulated detonation wave propagation results and the resulting impacts on ceramic insulators for Orientations A and B at p0 = 1.80 MPa. The plane presented in Fig. 9 models the upper wall of the combustion cavity, which is flush with the outer shell of the spark plug, as shown in Fig. 2b. Pressure histories were examined at four characteristic points for the two ground electrode orientations, as shown in Fig. 10. Points 1 and 2 represent the left and right end points along the horizontal centerline of the insulator, respectively, while Point 3 is the upper end point along the vertical centerline, and Point 4 is situated on the wall on the same centerline as Point 3.
Fig. 9

Simulated detonation propagations and impacts on a ceramic insulator in the CVCC

Fig. 10

Pressure traces at different sites as obtained from simulations

In the case of Orientation A, prior to impact, the pressure at the wave front of the propagating detonation was 104.6 MPa, which is close to the value determined experimentally. Following impact, the pressure at Point 1 increased to 486.7 MPa due to the shock wave reflection and then the detonation wave diffracted around the ceramic insulator. When the diffracted detonation waves reunited behind the insulator at Point 2, the maximum local pressure decreased to 166.9 MPa. Conversely, the peak pressures at Points 3 and 4 were much lower than at Points 1 and 2, because the shock wave was not reflected at these locations when the detonation wave diffracted around the spark plug.

When applying Orientation B, the detonation wave first diffracted at the ground electrode and then impacted the insulator. The diffraction at the ground electrode reduced the local pressure in the area between the ground electrode and the insulator. This in turn reduce the peak pressure at Point 2 when the reunited diffracted detonation waves were reflected after impacting the ceramic insulator. The peak pressure at Point 2 was 271.5 MPa, equal to only 55.8% of the peak pressure at Point 1 in the case of Orientation A. This outcome indicates that the ground electrode can provide protection to the ceramic insulator when it is positioned upstream. As a consequence, the probability of failure of the insulator is dramatically reduced when Orientation C is employed.

Because these experiments show that the ceramic insulator does not break at its tip but rather at its base under these conditions, it was considered that the high pressure associated with the impact might apply a flexural load to the insulator. The effective load would depend on the pressure difference between the front and the back of the insulator, which can simply be regarded as the pressure difference between Points 1 and 2, and Fig. 11 plots the absolute value of this difference as a function of time. The high pressure difference only lasts for 2.2 μs. However, longitudinal and shear waves in Al2O3 propagate at 9900 and 4100 m/s, respectively [14] (that is, much faster than the detonation speed of 2680 m/s in the simulation), and so, these waves resulting from the detonation wave impact likely reached the base of the breathing chamber and impart a flexural stress at that location before the detonation wave leaves the ceramic insulator. Based on this possibility, the ceramic insulator can be treated as a cantilever beam with a variable cross section, as shown in Fig. 12.
Fig. 11

Absolute values of the pressure differences between Points 1 and 2 as functions of time

Fig. 12

Schematic showing the model used for flexural stress calculations

Because the pressure difference changes as the detonation wave passes the ceramic insulator, the average pressure value within this period, \(\bar{p}\), is taken as the effective load, as shown in Fig. 11. According to the material mechanics theory [15], the flexural strength in a cantilever beam with a variable annular cross section can be calculated by the following equations.
$$\sigma \left( x \right) = \frac{{\bar{p}h\left( {r_{1} + r_{2} } \right)x}}{{\frac{{\uppi R^{3} }}{4}\left( {1 - \alpha^{4} } \right)}}$$
(1)
$$\begin{aligned} & R = r_{2} + x\left( {\frac{{r_{2} - r_{1} }}{h}} \right)\;(x \le x_{c} ) \\ & R = R_{c} + r_{4} \left( {\cos \left( {\theta_{1} } \right) - \cos \left( {\theta_{x} } \right)} \right)\;\left( {x > x_{c} } \right) \\ \end{aligned}$$
(2)
$$\alpha = \frac{{r_{3} }}{R}$$
(3)

The parameters in the above equations are depicted schematically in Fig. 12. It should be noted that Point C is the turning point at which the diameter of the ceramic insulator changes abruptly.

The calculated flexural stress values along the axis of the ceramic insulator are plotted in Fig. 13, in which the two squares indicate Point C in Fig. 12. It is evident that, due to the stress concentration caused by the abrupt change in the diameter, the maximum flexural stress appears at Point C, with values of 578.7 and 435.9 MPa for Orientations A and B, respectively. These values are much higher than the flexural strength of Al2O3 (330 MPa for 94% Al2O3 and 379 MPa for 99.5% Al2O3 [16]). Therefore, it is very likely that the ceramic insulator will break at Point C when the spark plug is impacted by a strong detonation wave.
Fig. 13

Calculated flexural stresses at different positions along the axis of the ceramic insulator

4 Conclusions

This study demonstrated a novel method of evaluating the impact resistance of ceramic insulators of spark plugs. Experimental investigations were carried out under deto-knock conditions in a CVCC, using stoichiometric propane–oxygen mixtures to generate detonation and hence to represent deto-knock. The detonation initiation and propagation processes were visualized and recorded using high-speed photography and pressure data acquisition. Both high-pressure single-impact and low-pressure multiple-impact conditions were applied. The results of this work can be summarized as follows:
  1. 1.

    A CVCC that allowed visual monitoring was designed so as to study detonation initiation and propagation. Ultra-high-pressure combustion processes were tracked using synchronous high-speed photography and high-speed pressure data measurements. The highest detonation wave peak pressure recorded was 132.4 MPa.

     
  2. 2.

    Under high initial pressure conditions, a spark-ignited flame readily transitioned to detonate as a result of small disturbances close to the spark plug in a stoichiometric propane–oxygen mixture. The quartz window was immediately cracked during propagation of the detonation wave due to the associated high-pressure impact.

     
  3. 3.

    The ceramic insulator could be damaged by a single impact from a detonation wave having a very high peak pressure or by multiple impacts with relatively low peak pressures. In the former case, the ceramic insulator was fractured into two parts at the base of the breathing chamber, due to a stress concentration at the location at which the diameter of the insulator changed abruptly. However, the broken portion of the insulator remained as one piece. In the latter case, the ceramic insulator was broken at a point between the base and the tip of the breathing chamber and was shattered into many fragments.

     
  4. 4.

    The ground electrode provided some protection from the detonation wave to the ceramic insulator when located in the upstream direction. This occurred because the peak pressure was reduced after the detonation wave underwent diffraction at the electrode.

     

Notes

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant Nos. 91541206 and 51706121) and China Postdoctoral Science Foundation (Grant No. 2017T100076).

Compliance with Ethical Standards

Conflict of interest

On behalf of all authors, the corresponding authors state that there is no conflict of interest.

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Copyright information

© China Society of Automotive Engineers (China SAE) 2019

Authors and Affiliations

  1. 1.State Key Laboratory of Automotive Safety and EnergyTsinghua UniversityBeijingChina
  2. 2.Graduate Aerospace LaboratoriesCalifornia Institute of TechnologyPasadenaUSA

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