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ARInstitut für Verbrennungskraftmaschinen und Thermodynamik, Technische Universität Graz
15. Tagung "DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS"
24./25. September 2015
Arc Travel Ignition Technology
Luigi Tozzi, Emmanuella Sotiropoulou Shengrong Zhu
Prometheus Applied Technologies, LLC
David Lepley – Altronic, LLC. Hoerbiger Engine Solutions
Shinji Yasueda – GDEC, Inc.
Abstract
The use of spark plugs with precious metal electrode materials is used for extending maintenance
intervals with high power density gas engines. Furthermore, the demand for lower engine emissions
has increased the level of ignition spark energy requirements. Under these conditions, the electrode
firing surfaces can suffer major damages, such as beading, whiskering, flaking or high rate of
erosion resulting in early spark plug failures. Moreover, the incidence of these undesired
phenomena is increasing as spark plug electrodes are located in either quiescent or in high flow
velocity zones. An accurate combustion CFD model is used to determine the movement and the heat
transfer characteristic of the spark discharge. This paper describes the use of a novel arc travel
ignition technology, as a tool to resolve field problems and advance the state of the art of ignition
systems. Analytical results are confirmed by experimental observation.
1.
INTRODUCTION
Fine wire Platinum metal alloy electrodes are widely used in modern gas engines. Combustion
computational fluid dynamics (CFD) simulations and thermal finite element analysis (FEA) can be
very effective tools to analyze the thermal conditions resulting from the spark discharge and the
combustion process in high Brake Mean Effective Pressure (BMEP) engines fueled by Natural Gas
fuel. Under these conditions, the electrode beading, defined as small spherical beads of molten
material, is typically seen on the surface of the spark plug electrodes. These beads can grow in time
as a result of the additional sparks. In some cases the growth of the beads can lead to a reduction in
the electrode gap size resulting in misfire. According to the literature, the general mechanism for a
bead formation is the particle ejection, especially for precious metal electrodes [1]. Figure 1
illustrates the particle ejection mechanism. After the abrupt cessation of the arc, an unbalanced recoil
force is directed away from the electrode. If the opposing surface tension force is sufficiently low,
a droplet will be ejected from the surface resulting in beading [2]. Another aspect of bead formation
[3, 4] is related to the flow behavior of the liquid in the molten pool during ion bombardment, which
is determined by the Reynolds number (Re). If Re > 1, the flow will be turbulent and material
sputtering will occur. At the end of the spark event, the surface tension and viscosity of the pool of
molten material formed during spark (especially for Platinum alloys) can cause the molten material
to solidify as a bead. In cases of low surface tension and high viscosity, the ejection of molten
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 112 droplets and bead formation are expected to be less. On the other hand, in cases of high surface
tension and low viscosity, the formation of spherical beads has a higher likelihood of occurrence.
Figure 1: Schematic representation of the particle ejection model: The recoil force balances the
surface tension force. If the recoil force is larger, a droplet will be ejected [4].
1.1. Use of Analytical Tools
The fundamentals of spark discharge have been covered by the authors in previous publications [5,
6]. The combustion CFD is coupled with a thermal FEA to determine the effect of the spark discharge
on the temperature of the electrode and the size of the molten pool of material formed during the
spark. This is a critical parameter responsible for the formation of beads. Figure 2 shows the results
from the thermal FEA discussed by the authors in previous publications [5]. It can be seen that, in
the case of spark discharge occurring in a quiescent environment, the temperature distribution on the
electrode surface at the area of impact with the plasma discharge column, consists of a relatively
large region where temperatures are above the melting point of the electrode material.
Figure 2: Axisymmetric model of a round Iridium electrode: Left – complete model, right – zoomed
in results
As the first step, a number of CFD simulations are performed at 100% and 10% engine load
conditions to determine the lambda distribution and flow fields in the spark gap. Then combustion
CFD calculations are performed to determine the arc travel and flame growth. This information
combined to the thermal FEA is used to determine the conditions that result in electrode beading. In
order to confirm the mechanism of bead formation, a bench top experiment is conducted at
representative engine ignition and combustion conditions.
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 113 1.2. Engine Information
The basic engine information is summarized in Table 1, while the CFD simulated conditions are
summarized in Table 2.
Engine Output 402 kW
BMEP 1.83 Mpa
Engine Speed 1500 RPM
Fuel Natural Gas
Table 1: Engine information
Engine Speed
Lambda
100% Load
10% Load
1500 RPM
1500 RPM
1.75
1.0
Table 2: Simulated engine conditions
1.3. Observations on Electrode Beading
Figure 3 illustrates several definitions relating to the geometry of the electrodes that will also be
helpful for the beading discussion. In general, the electrode beading occurs as a collection of molten
material at the trailing edge of the electrode, in the direction of the flow.
Figure 3: Definition of leading and trailing edge for the electrodes
An analysis of the electrode gap of Figure 4 shows that the majority of the beading is located on the
downstream side of the electrode, where the arc will anchor (i.e., at the trailing edge) and the flow
velocity is lower due to the electrode blocking the flow.
Figure 4: Evidence of electrode beading along the length of the electrode gap after approximately
1000 hours of engine operation (picture on the right)
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- 114 2.
CFD SIMULATIONS
Figure 5 shows the lambda distribution in the electrode gap for the two load conditions. The
comparison illustrates that the low load case has the richest lambda at =1.0 and the full load
condition is the leanest at =1.75.
Figure 5: Lambda distribution in the electrode gap for the two different load conditions
The spark gap is characterized by high velocity (approximately 27m/s) swirling flow, as shown in
Figure 6. There is lower velocity on the downstream side (as indicated by the white arrows), where
the flow is blocked by the electrode. Although the velocity is similar at 10% load, the flow
momentum is lower due to the lower density of the gas.
Figure 6: Flow fields in the electrode gap for all three load conditions
The combustion calculations were performed with the spark waveform shown in Figure 7. In this
case the spark duration is approximately 2 CAD (222 µs) and the spark current is approximately
130mA. The precise location of spark occurrence on the electrode surface is calculated from the
electric field, the flow velocity and the fuel distribution in the region of the gap, and the electrode
temperature. The results of this analysis indicate that the spark initiates at the leading corner of the
electrode. After initiation and due to the flow velocity, the spark travels the width of the electrode
in the direction of the flow.
Figure 7: Spark waveform used for the combustion simulations
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- 115 2.1. Full Load Case
After spark initiation, the plasma column (or arc) is shown by a temperature contour in Figure 8. It
can be seen that, due to the flow velocity, the arc travels in the direction of the flow until it reaches
the trailing (downstream) corner of the electrode. At this point, the arc becomes a stationary plasma
column which is anchored to the corner of the electrode where it transfers energy to the electrode
and to the incoming flow stream. This arc travel occurs in less than 0.2 CAD, which is a relatively
short interval considering that the spark duration is 2 CAD. When the arc reaches the trailing edge
of the electrode and stops moving, it is cooled slightly by the high velocity flow in the electrode gap.
This is seen by a reduction in the diameter of the temperature contour. The smaller diameter arc
remains anchored to the trailing edge until the spark event is complete. Melting and beading is
expected to occur on the trailing corner while the arc is stationary.
Figure 8: Evolution of the arc in the electrode gap at full load
In contrast, as discussed by the authors in a previous publication [6], in the case of low flow velocity
at the gap the arc travel may be insufficient causing high concentration of localized spark energy
resulting in electrode beading. Under these conditions, tailoring of the spark waveform has proven
to reduce incidents of electrode beading.
2.2. Low Load Case
Also in this case, the spark initiates on the leading (upstream) corner. Figure 9 contains a comparison
of the plasma column (or arc) in the electrode gap for the two load conditions as shown by a
temperature contour. As the engine load decreases, the lower gas density results in a progressively
larger diameter plasma column. Due to the high flow velocity, the arc travels in the direction of the
flow until it reaches the trailing edge. Again, as the engine load decreases the flow momentum
decreases causing the arc motion to become slower. Therefore, the low load condition has the largest
diameter plasma column and requires the longest time for the arc to become stationary as shown in
Figure 9.
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Low Load
Figure 9: Comparison of the arc travel at full load (top) and at low load (bottom)
Figure 10 shows the arc travel until the end of the spark. It can be seen that the arc dwells on the
trailing edge for 1.7 CAD compared to 1.8 CAD obtained at full load.
When the spark event is complete, the flame is no longer anchored to the electrode by the arc and
begins to move away from the electrode as shown. As with the previous two cases, melting and
beading is expected to occur on the trailing corner when the arc is stationary.
Figure 10: Evolution of the arc in the electrode gap at low load (from -24.00 CAD to -23.00
CAD)
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- 117 2.3. CFD Simulation Results in Relation to Electrode Beading
With reference to Figure 3, the following information has been determined from the CFD simulation
results and from observations of the particular beading phenomenon:





When the spark occurs, the high flow momentum causes the arc to move in the direction of
the flow. As the engine load decreases, the lower gas density results in lower flow
momentum causing the arc to move at slower speeds.
If the spark occurs on the trailing (or downstream) edge of the electrode, then the arc will
anchor at that location causing the maximum melting and beading for a particular load
condition. As the load decreases, the diameter of the plasma column increases and, therefore,
transfers energy to a larger area of the electrode. Beading is expected to be lower compared
to higher loads, because the heat transfer is less concentrated (i.e., the heat flux [W/mm2] is
less).
If the spark occurs on the leading (or upstream) edge of the electrode, then the arc moves in
the direction of the flow until it reaches the trailing edge. This occurs quite quickly (fractions
of a CAD) compared to the 2 CAD total spark duration. Melting and beading is expected to
be more distributed compared to when the spark occurs on the trailing edge, but will still
occur largely on the trailing edge. As load decreases, the flow momentum is lower requiring
more time for the arc to travel the width of the electrode due to its lower velocity. Melting
and beading at the trailing edge is expected to be less compared to higher loads because the
larger diameter plasma column results in less concentrated heat transfer and the arc is
stationary for a shorter duration.
For a new spark plug with sharp edges on the front of the electrode, the arc will remain
mostly on the front edge of the electrode while sweeping through the gap because there is a
higher electrical field along the front edge than on the top of the electrode.
Over time, the sharp edges will wear and the arc will travel more over the top of the electrode
following the direction of the flow.
It is believed that the fundamental cause of the bead formation is the high flow velocity, combined
with the small cross-section Platinum alloy electrode used in this application. The high flow velocity
causes the spark energy discharge to take place mostly in one location (i.e., the trailing edge of the
electrode). The small cross-section of the electrode results in a short travel path for the arc which
also causes the discharge to take place mostly in one location (i.e., the trailing edge of the electrode).
Lastly, precious metal alloys in general, have a more pronounced tendency to bead due to the higher
melting point and higher surface tension in the molten state. Improving any one of these factors may
mitigate the beading phenomenon. However, the high flow velocity does help prevent the beads
from collecting in between the two electrodes, causing the gap to bridge and resulting in engine
misfire.
Figure 11 contains an illustration of the probable mechanism causing the electrode beading. As the
arc moves across the electrode, it creates a small pool of molten material where the plasma column
impacts the electrode. As the arc dwells at the trailing edge of the electrode it causes a larger pool
of molten material. When the spark is complete, surface tension of the Platinum alloy causes the
molten material to solidify into a ball (or bead) shape in the direction of the flow. Successive arcs
will dwell in a similar location (i.e., at the bead) due to the shape causing a high electric field and
anchoring the arc. Repeated arc dwelling at the same location causes multiple beads to be formed
on top of each other, which results in the observed molten metal growth pictured in Figure 12Error!
Reference source not found..
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
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Figure 11: Illustration of the probable mechanism causing the electrode beading observed in this
case
Figure 12: Examples of observed molten metal growth
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 119 3.
THERMAL FEA
In order to further confirm the probable mechanism causing electrode beading, thermal FE) is
performed on the spark plug electrode. The combustion CFD results at full load and at low load are
used to set the boundary conditions. This thermal FEA includes the step of determining the electrode
bulk temperature and the step of determining the electrode local temperature. The electrode bulk
temperature relates to the engine cycle average temperature and is the steady state electrode overall
average temperature. The electrode local temperature refers to the instantaneous maximum local
temperature during spark event and is responsible for the electrode melting and beading.
3.1. Electrode Bulk Temperature
The first step of the thermal FEA analysis is to determine the electrode bulk temperature. The
boundary conditions used to determine the bulk temperature at full load and low load conditions are
based on the combustion CFD results.
The electrode bulk temperatures at full and low load conditions are shown in Figure 13. It clearly
shows that the temperature distributions for both full and low load conditions are very similar, but
the full load condition has approximately 70K higher electrode temperature than that of the low load
condition.
Figure 13: Comparison bulk temperature of an electrode at full load and low load conditions
3.2. Electrode Local Temperature
The second step in the thermal FEA analysis is to determine the electrode local temperature. The
results from the previous step were used to initialize the bulk electrode temperature. Then the effect
of the arc dynamic (i.e., the travel and dwell), based on the results of the CFD, were applied to the
electrodes to determine the local and instantaneous temperatures. The local maximum temperature
(Tmax) is responsible for the electrode melting and beading. In order to provide for a better
understanding of the arc dynamic on electrode beading, the local electrode temperature distributions
are shown only when the electrode reaches its local Tmax. Figure 14 contains a comparison of the
local electrode temperature for full load and low load conditions. The temperature contour at the
start of the spark event is also provided as a reference to indicate the arc dynamic (i.e., the direction
of the arc travel and location where the arc dwells). At both full load and low load conditions, the
maximum electrode temperature is located at the trailing edge, which is consistent with the CFD
results that the spark discharge takes place mostly on the trailing edge of the electrode due to the
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 120 high velocity flow field in the gap. Therefore, the trailing edge of the electrodes has much higher
local temperature because the spark dwells at that location for the majority of the spark duration.
Figure 14: Comparison of the electrode local temperature on the firing surface of the electrode by
the end of spark for full load and low load conditions
Error! Reference source not found. contains a comparison of the local electrode temperature
penetration into the electrode for full load and low load conditions. This new view shows the
penetration of temperature into the electrode as viewed from the downstream (or trailing) side of the
electrode. As expected, the highest local temperature is on the surface of the electrode where the
spark discharge occurs.
Figure 15: Comparison of the electrode local temperature penetration into the electrode by the end
of spark for full load and low load conditions
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- 121 3.3. Thermal FEA Results and Conclusions
Table 3 contains a summary of the thermal FEA results for the full load and low load conditions
compared to the melting temperature for Platinum. The thermal FEA results for the full load and
low load conditions show that there is a small portion of the electrode, in the order of 7x10-5 mm3 for
full load and roughly half of that for the low load that is above the melting point and, therefore, can
form a molten pool. Surface tension and electrostatic forces can then cause this molten material to
solidify as a bead. The volume of material that is above the melting point is larger for the full load
case indicating that the beading is expected to be more severe for this condition. This is in agreement
with the CFD calculation results that indicate that the beading is expected to be more severe for the
full load condition.
Table 3: Thermal FEA results (top) for full load and low load conditions compared to the melting
point of Platinum (bottom)
4.
BENCH TEST
The primary objective of the bench test is to prove that under the pressure and temperature
conditions approaching those of the engine, beads will form on the electrode surface. The results
of the combustion CFD, in combination with the thermal FEA results, are used to guide the bench
test. Various conditions of pressure and temperature are simulated to determine the effect on the
formation of beads.
A picture of the bench test setup is provided in Figure 16. The main components of the setup include:
the test chamber, the spark plug, the ignition system and the air supply. The test chamber features
controllable gas pressure and temperature up to 10000 kPa and 1000 oK respectively. Close-up
pictures of the test chamber are shown in Figure 17.
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Figure 16: Picture of the bench test setup and equipment
Figure 17: Pictures of the simulation test chamber
The test chamber simulation rig was used to create beads on the electrode of the spark plug. The
conditions of simulated pressure and temperature were varied from ambient to approximately 4000
kPa and 700 K. Comparatively, the estimated in-cylinder pressure and temperature conditions were
calculated to be approximately 2100 kPa/ 770 K and 5500 kPa/ 820 K, at 10% load and at 100% load
respectively. This means that the in-cylinder density conditions simulated in the test chamber (~20
kg/m3), correspond to approximately to 80% load.
At ambient pressure and temperature conditions, and after approximately 400,000 sparks, no
evidence of beading was observed. As the test chamber conditions of pressure and temperature
approached the conditions of the engine, clear signs of bead formation were observed.
A comparison of the beads obtained in the test chamber, with approximately 400,000 sparks, compared
to the beads from the engine, with more than 4,000,000 sparks is depicted in Figure 18Error! Reference
source not found.. It can be seen that, as expected, the extent of the beading in the engine case is greater
due to the much larger number of sparks. However, the general morphology of the beading between the
simulated test chamber and the engine is quite similar. A more detailed imaging, comparing the surface
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 123 of the new electrode to the beads formed in the test chamber, is provided in Figure 19. These results
clearly confirm the validity of the beading formation mechanism discussed in this report.
Figure 18: Test results depicting the bead formation over time in the bench test compared to the
engine.
Figure 19: Comparison between the new electrode surface and the beads formed in the test
chamber.
5.
SUMMARY AND CONCLUSIONS
The probable mechanism causing the beading on the surface of Platinum alloy electrodes, operating
in a gas engine, has been analyzed by means of combustion CFD and thermal FEA. The high flow
velocities, combined with the small cross-section of the Platinum alloy electrode, are the main factors
causing the beading. Under these conditions, the spark discharges take place mostly in one location
(i.e., the trailing edge of the electrode). Furthermore, the precious metal alloys (in this case a
Platinum alloy) have a distinct tendency to bead which is due to the higher melting point and higher
surface tension in the molten state. However, the high flow velocity does help prevent the beads
from collecting in the gap, causing it to bridge and resulting in engine misfire.
Furthermore, the experimental investigation confirmed that, at ambient pressure and temperature
conditions, there is no significant evidence of bead formation. On the other hand, as the test chamber
pressure and temperature approach the engine conditions, then a certain bead formation is achieved
on the electrodes and appears to be related to the cumulative number of spark events acting on the
electrode surface. The morphology presented by the beads from the test chamber is comparable to
that exhibited by the beads formed in the engine. This experimental evidence confirms the validity
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015
- 124 of the bead formation mechanism illustrated in this paper by means of the combustion CFD and the
thermal FEA analysis results. Therefore, the arc travel ignition technology is an effective
methodology for the study of the effects that electrode material, flow velocity at the gap and spark
discharge waveform have on the formation of molten beads.
7.
LITERATURE
[1]
Young, C. T., et al, “Erosion Mechanisms of Automotive Spark Plug Electrodes,” SAE
Technical Paper 780330, 1978
[2]
Augis, J. A., et al, “Plasma and Electrode Interactions in Short Gap Discharges in Air. II.
Electrode Effects,” Int. J. Electron, 1971, 30, 315
[3]
Wang, B. J., et al, “Spark Erosion behavior of silver-based particulate composites,” Wear, 1996,
195, 133
[4]
Jeanvoine, N., “Plasma-Material Interaction and Electrode Degradation in High Voltage Ignition
Discharges,” Dissertation, 2009
[5]
Sotiropoulou (a.k.a. McCoole), M.-E., et al., “Solutions for improving Spark Plug Life in High
Efficiency, High Power Density, Natural Gas Engines,” Proceedings of ASME Internal
Combustion Engine Division 2006 Spring Technical Conference, ICES2006-1417
[6]
Lepley, D.T., et al., “Optimizing High-Energy Tunable Ignition Technology: Preventing
Electrode Damage while Extending the Lean Flammability Limit of Gas Engines,” Proceeding
of Gas Machinery Conference, 2014
15. Tagung “DER ARBEITSPROZESS DES VERBRENNUNGSMOTORS”, Graz, 2015