PREHEATING A SPARK PLUG
BACKGROUND OF THE INVENTION
The invention relates to spark plugs for turbine
5 engines. In particular, the present invention relates to
the reliability of spark plugs that have semiconductor
bodies between their electrodes. Such plugs are, among
others, plugs for igniting combustion chambers of turbine
engines.
10 One type of known spark plug has a first electrode
and a second electrode separated by a semiconductor body.
This type of spark plug offers good reliability aHd also
makes it possible to reduce the size of the ignition
boxes that power the plugs. The high-voltage
15 semiconductors that are used may be referred to as
"cermet'° or "pellet" semiconductors, and are made of a
ceramic insulator and of grains of a conductive material.
By lowering the breakdown voltage, this technology makes
it possible to avoid electricity leakage from the
20 harnesses to which the plugs are connected, and to reduce
the size of the power supply transformers,
Unfortunately, that type of spark plug suffers from
certain drawbacks. In particular, the semiconductors
that are used are sensitive to the conditions of the
25 surrounding environment. In particular, under freezing
or wet conditions, rapid deterioration of the plugs is
observed. For example, a spark plug having a life cycle
under normal conditions of greater than 10,000 ignition
cycles may be almost destroyed after 1200 cycles in the
30 presence of persistent moisture. In addition, under wet
or freezing conditions, it is observed that many of the
sparks ordered are not generated by the plug. Such
deficiency can delay ignition of the turbine engine and
thus accelerate deterioration of the plug because the
35 cycle is lengthened. Finally, under certain conditions,
ignition of the turbine engine does not occur.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is to propose an ignition
method that does not suffer from at least some of the
drawbacks suffered by the above=mentioned prior art. A
5 particular object of the invention is to avoid rapid
deterioration of a spark plug.
To this end, the invention provides a method of
igniting a turbine engine using a spark plug comprising a
first electrode, a second electrode and a semiconductor
10 body between the first electrode and the second
electrode, the semiconductor body having an exposed
surface, the ignition method comprising a step of
generating a spark adjacent to said exposed surface by
applying a voltage difference greater than a first
15 predetermined threshold between the first electrode and
the second electrode, said method being characterized by
the fact that, prior to said step of generating a spark,
it further comprises a preheating step consisting in
applying a voltage difference less than a second
20 predetermined threshold between the first electrode and
the second electrode, said second predetermined threshold
being less than said first predetermined threshold.
Generating a spark involves ionizing the gas
adjacent to the exposed surface of the semiconductor
25 body, However, in freezing or wet conditions, ice or
water can cover the exposed surface of the semiconductor
body and thereby limit the quantity of gas that can be
ionized. In the event an attempt is made to generate a
spark in this situation, this results in an increase in
30 the breakdown voltage and in a concentration of the
discharge at the surface of the semiconductor, thereby
leading to rapid erosion of the semiconductor body and to
cracks being formed in the semiconductor body, which
cracks accelerate degradation of said body.
35 The preheating step makes it possible to avoid such
rapid degradation. Application of a low voltage between
the two electrodes does not generate any spark but rather
it generates a leakage current that flows through the
semiconductor body. The heat generated makes it possible
to dry the plug. Thus, after the preheating step, a
spark can be generated without ice or water covering the
5 exposed surface of the semiconductor body.
In an implementation, said preheating step has a
predetermined duration greater_ than 5 seconds, For
example, the predetermined duration may lie in the range
30 seconds to 10 minutes,
10 The first predetermined threshold may be greater
than 900 volts (V)o The second predetermined threshold
may be less than 900 V. For example, the second
predetermined threshold is less than or equal to 100 V.
In a variant, during the preheating step, the
15 voltage difference applied between the first electrode
and the second electrode is constant.
In another variant, during the preheating step, the
voltage difference applied between the first electrode
and the second electrode is controlled by a current
20 regulator.
The invention also provides a method of starting a
turbine engine comprising a step of causing a starter
motor to start rotating the turbine engine, and a step of
igniting the turbine engine by implementing the above
25 ignition method, wherein the preheating step starts when
the speed of rotation of the turbine engine reaches a
predetermined threshold,
The invention also provides an ignition system for a
turbine engine, which system comprises a spark plug and a
30 power supply device connected to said spark plug, the
spark plug comprising a first electr_ode,' a second
electrode and a semiconductor body between the first
electrode and the second electrode, the semiconductor
body having an exposed surface, the power supply device
35 comprising generation means for generating a spark
adjacent to said exposed surface, which means are
suitable for applying a voltage difference greater than a
first predetermined threshold between the first electrode
and the second electrode, said ignition system being
characterized by the fact that the power supply device
further comprises preheating means suitable for applying
5 a voltage difference less than a second predetermined
threshold between the first electrode and the second
electrode, said second predetermined threshold being less
than said first predetermined threshold.
The power supply device may further comprise an
10 input interface for receiving a control signal, and
activation means suitable for activating said generation
means or said preheating means as a function of the
control signal.
15 BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be better understood on reading
the following description, given merely by way of non---
limiting indication and with reference to the
accompanying drawings, in which:
20 ° Figure 1 is a diagram of an embodiment of an
ignition system of the invention;
Figure 2 is a section view of a spark plug of the
ignition system shown in Figure 1;
Figure 3 shows the end of the spark plug of
25 Figure 2, as covered with ice or with water;
Figure 4 is a graph that, for a plurality of
Figure 2 plugs tested, shows the current flowing through
the plug as a function of the voltage that is applied;
and
30 a Figure 5 is a graph showing a control signal and
the voltage difference applied to the electrodes of the
Figure 2 plug, as a function of time.
DETAILED DESCRIPTION OF AN EMBODIMENT
35 Figure 1 shows an ignition system 10 for a turbine
engine 11. The -ignition system 10 generally comprises a
plurality of spark plugs designed to generate sparks for
5
igniting the turbine engine 11. The spark plugs are
connected to a power supply box 9. The power supply box
9 has an input interface 12 for receiving a control
signal. Figure 1 shows a single spark plug 10
5 Figure 2 is a section view of the spark plug 1. The
spark plug 1 has an electrode 2 and an electrode 3.
The electrode 2 has an orifice 7 that is
substantially circularly cylindrical, and the electrode 3
is received in the orifice 7. On the right side of
10 Figure 1, the end of the electrode 3 comes flush with the
end of the electrode 2 and a semiconductor body 4
separates the electrodes 2 and 3. The semic=condu_ic:t.or
body 4 has an exposed surface 5.
Inside the orifice 7, the electrodes 2 and 3 are
15 separated by insulating material 6. Finally, on the left
side of the Figure 1., the orifice 7 is flared and the end
of the electrode 3 is unobstructed, so as to form a
connector 8 making it possible to connect the spark plug
1 to the power supply box 9.
20 Thus, the power supply box 9 can apply a large
voltage difference between the electrodes 2 and 3,
thereby generating a spark 14 in front of the exposed
surface 5 of the semiconductor body 4, as shown in
Figure 2. However, in freezing or wet conditions, a
25 buildup 13 of ice or of water can cover the expo ed
surface 5 of the semiconductor body 4, as shown in
Figure 3. The buildup 13 can hinder or prevent
generation of a spark.
Figure 4 shows the current I flowing through the
30 spark plug 1 as a function of the voltage difference T
applied between the electrodes 2 and 3. The curves 15,
16 and 17 correspond respectively to semiconductor bodies
4 having different compositions.
For a large voltage difference T, typically greater
35 than 900 V, the current I is also high. In the absence
of the build-up 13, a spark 14 is generated. The
6
encircled zone 19 corresponds to the zone in which a
spark is generated 14.
Conversely, for a small voltage difference T,
typically less than 900 V, no spark is generated.
5 However, the spark plug 1 allows a low leakage current I
to flow through it, with the value of the current
depending on the voltage difference that is applied.
Figure 4 shows that the leakage current is relatively
stable in a zone 18.
10 In order to avoid the problems of deterioration of
the spark plug 1 caused by the presence of the build-up
13, the spark plug 1 is preheated before a spark =14 is
generated.
More precisely, during a preheating step preceding
15 the step of generating a spark 14, the power supply box 9
applies a small voltage difference between the electrodes
2 and 3, typically approximately in the range 20 V to
100 V. In a variant, a voltage difference of up to 900 V
could be applied because no spark is generated. As shown
20 in Figure 4, a leakage current flows through the
semiconductor body 4, which heats up by the Joule effect.
The heat generated dries the spark plug 1, thereby
removing the build-up 13.
The preheating step is, for example, of duration
25 that is predetermined as a function of the voltage:
difference applied and of the spark plug 1. Typically,
the predetermined duration may lie in the range 30
seconds to 10 minutes.
For example, during tests conducted on a spark plug
30 1 covered with ice at -l5°C, the following drying times
were measured:
6 minutes for a voltage difference of 28 V
(current of 10 milliamps (mA));
2,5 minutes for a voltage difference of 50 V
35 (current of 20 mA); and
35 seconds for a voltage difference of 100 V
(current of 500 mA).
During the preheating step, a voltage difference of
constant value is applied. In a variant, the voltage
difference may be determined by a current regulator that
keeps the current constant.
5 After the preheating step, the step of generating a
spark can take place in conventional manner. More
precisely, during a charging stage, the power supply box
9 accumulates energy in a storage element. Then, the
stored energy is transferred to the spark plug 1 in order
10 to generate a spark.
The power supply box 9 has an input interface 12
making it possible to receive a control signal. ''he
control. signal indicates to the power supply box 9 to
switch between a state in which it applies a low voltage
15 for the preheating step and a state in which it applies a
high voltage for the step of generating a spark.
For example, the control signal comprises a pulse of
short duration for requesting the preheating step and a
pulse of longer duration for requesting the step of
20 generating a spark. This example is shown in Figure 5.
In Figure 5, curve 20 shows the applied voltage
difference T as a function of time t, and curve 21 shows
the control signal S as a function of time.
In known manner, starting the turbine engine 11
25 begins with causing the turbine engine 11 to start.
rotating by means of a starter motor. The speed of
rotation of the turbine engine 11 increases
progressively. When the speed of rotation reaches a
determined level, sparks are generated in order to ignite
30 the turbine engine 11. Given the variation in the speed
of rotation of the turbine engine 11 and the
predetermined duration for the step of preheating the
spark plug 1, it is possible to choose a rotation speed
threshold for starting the preheating step.
35
8

CLAIMS
1. A method of igniting a turbine engine (11) using a
spark plug (1) comprising a first electrode (2), a second
electrode (3) and a semiconductor body (4) between the
5 first electrode and the second electrode, the
semiconductor body having an exposed surface (5), the
ignition method comprising a step of generating a spark
(14) adjacent to said exposed surface by applying a
voltage difference greater than a first predetermined
10 threshold between the first electrode and the second
electrode, said method being characterized by the fact
that, prior to said step of generating a spark, i_1_;
further. comprises a preheating step consisting in
applying a voltage difference less than a second
15 predetermined threshold between the first electrode and
the second electrode, said second predetermined threshold
being less than said first predetermined threshold.
2. An ignition method according to claim 1., wherein said
20 preheating step has a predetermined duration greater than
5 seconds.
3o An ignition method according to claim 1 or claim 2,
wherein said first predetermined threshold is greater
25 than 900 V.
4. An ignition method according to any one of claims 1 to
3, wherein said second predetermined threshold is less
than 900 V.
30
5. An ignition method according to claim 4, wherein said
second predetermined threshold is less than or equal to
100 V.
35 6. An ignition method according to any one of claims 1 to
5, wherein, during the preheating step, the voltage
9
difference applied between the first electrode and the
second electrode is constant.
7. An ignition method according to any one of claims 1 to
5, wherein, during the preheating step, the voltage
difference applied between the first electrode and the
second electrode is controlled by a current regulator.
8. A method of starting a turbine engine (11) comprising
10 a step of causing a starter motor to start rotating the
turbine engine, and a step of igniting the turbine engine
by implementing the ignition method according to any one
of claims 1 to 7, wherein the preheating step starts when
the speed of rotation of the turbine engine reaches a
15 predetermined threshold,
9. An ignition system for a turbine engine (11), which
system comprises a spark plug (1) and a power supply
device (9) connected to said spark plug, the spark plug
20 comprising a first electrode (2), a second electrode (3)
and a semiconductor body (4) between the first electrode
and the second electrode, the semiconductor body having
an exposed surface (5), the power supply device
comprising generation means for generating a spark (14)
25 adjacent to said exposed surface, which means are
suitable for applying a voltage difference greater than a
first predetermined threshold between the first electrode
and the second electrode, said ignition system being
characterized by the fact that the power supply device
30 further comprises preheating means suitable for applying
a voltage difference less than a secondpredetermined
threshold between the first electrode and the second
electrode, said second predetermined threshold being less
than said. first predetermined threshold.
35
10. An ignition system according to claim 9, wherein said
power supply device further comprises an input interface
1 0
(12) for receiving a control signal, and activation means
suitable for activating said generation means or said
preheating means as a function of the control signal.