U.S. patent application number 15/932360 was filed with the patent office on 2018-12-13 for method and apparatus for operating traveling spark igniter at high pressure.
This patent application is currently assigned to Knite, Inc.. The applicant listed for this patent is Knite, Inc.. Invention is credited to Frederick H. Selmon, III, Artur P. Suckewer, Szymon Suckewer.
Application Number | 20180359844 15/932360 |
Document ID | / |
Family ID | 36747126 |
Filed Date | 2018-12-13 |
United States Patent
Application |
20180359844 |
Kind Code |
A1 |
Suckewer; Artur P. ; et
al. |
December 13, 2018 |
Method and apparatus for operating traveling spark igniter at high
pressure
Abstract
An ignition circuit and a method of operating an igniter
(preferably a traveling spark igniter) in an internal combustion
engine, including a high pressure engine. A high voltage is applied
to electrodes of the igniter, sufficient to cause breakdown to
occur between the electrodes, resulting in a high current
electrical discharge in the igniter, over a surface of an isolator
between the electrodes, and formation of a plasma kernel in a
fuel-air mixture adjacent said surface. Following breakdown, a
sequence of one or more lower voltage and lower current pulses is
applied to said electrodes, with a low "simmer" current being
sustained through the plasma between pulses, preventing total
plasma recombination and allowing the plasma kernel to move toward
a free end of the electrodes with each pulse.
Inventors: |
Suckewer; Artur P.;
(Franklin Park, NJ) ; Suckewer; Szymon;
(Princeton, NJ) ; Selmon, III; Frederick H.;
(Lawrenceville, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Knite, Inc. |
Ewing |
NJ |
US |
|
|
Assignee: |
Knite, Inc.
Ewing
NJ
|
Family ID: |
36747126 |
Appl. No.: |
15/932360 |
Filed: |
February 16, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15186319 |
Jun 17, 2016 |
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15932360 |
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14933938 |
Nov 5, 2015 |
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15186319 |
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14094922 |
Dec 3, 2013 |
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14933938 |
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13222298 |
Aug 31, 2011 |
8622041 |
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14094922 |
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12313927 |
Nov 26, 2008 |
8186321 |
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13222298 |
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11407850 |
Apr 19, 2006 |
7467612 |
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12313927 |
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60672892 |
Apr 19, 2005 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F02P 3/0807 20130101;
F02P 3/0815 20130101; F02P 23/04 20130101; F02P 3/08 20130101; H05H
1/48 20130101; H01T 13/50 20130101; F02P 9/007 20130101 |
International
Class: |
H05H 1/48 20060101
H05H001/48; F02P 3/08 20060101 F02P003/08; F02P 23/04 20060101
F02P023/04; H01T 13/50 20060101 H01T013/50; F02P 9/00 20060101
F02P009/00 |
Claims
1-6. (canceled)
7. A method of plasma generation, comprising: a. applying to an
igniter having at least a pair of electrodes a voltage of amplitude
sufficient to cause breakdown to occur between the electrodes,
resulting in a high current electrical discharge in the igniter in
an initiation region, and formation of a plasma kernel adjacent
said initiation region; and b. following breakdown, applying to
said electrodes at least one follow-on pulse by discharging a
plasma-sustaining capacitor via a current-controlling discharge
path, wherein the discharge path includes a switching element
between the plasma-sustaining capacitor and the electrodes.
8. The method of claim 7, wherein the switching element is of a
type that can be switched while current therethrough is not
zero.
9. The method of claim 8, wherein the switching of the switching
element includes turning off the switching element while the
current therethrough is not zero.
10. The method of claim 8, wherein-the at least one follow-on pulse
are voltage pulses generated by the capacitor pulling its discharge
current through an inductance.
11. The method of claim 10, wherein the inductance is in series
with the capacitor and one of the electrodes.
12. The method of claim 7, wherein the discharge path modulates a
discharge current of the capacitor.
13. The method of claim 7, wherein the discharge path comprises a
path of discharge of energy that formed the plasma kernel.
14. The method of claim 7, further comprising providing at least
one limit of current drain off of the plasma-sustaining capacitor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/222,298 filed Aug. 31, 2011, now allowed,
which in turn is a continuation of U.S. patent application Ser. No.
12/313,927 filed Nov. 26, 2008, now pending, which in turn is a
continuation of U.S. patent application Ser. No. 11/407,850 filed
Apr. 19, 2006, now U.S. Pat. No. 7,467,612, issued Dec. 23, 2008,
which claims the benefit, under 35 USC 119(e) of prior U.S.
provisional patent application Ser. No. 60/672892, filed Apr. 19,
2005, the entire contents of all of which are incorporated herein
by reference in their entireties.
BACKGROUND
Field
[0002] This invention relates to the fields of plasma generation,
ignitions, and internal combustion (IC) engines. In particular, it
relates, but is not limited, to ignition methods and ignition
apparatus for use therein; and, specifically, to ignition methods
and apparatus for various applications, including but not limited
to, high pressure engines. More particularly, some aspects relate
to the delivery of discharge current to traveling spark igniters in
order to maximize their performance and longevity, especially in
internal combustion engines operating at high pressures.
Discussion of Related Art
[0003] For a variety of reasons, there is interest today in
increasing the pressures in internal combustion engines and similar
combustion environments, with a concomitant need for ignition
sources capable of operating in these environments. For example,
automobile companies and manufacturers of internal combustion
engines would like to be able to provide vehicles which have IC
engines which operate at much higher pressures than conventional
internal combustion engines. To date, however, there has not been
an effective and practical ignition system for such engines. Among
other concerns are longevity of igniters (spark plugs) and
reliability of igniter firing.
[0004] The traveling spark igniter (TSI) is a device that has been
discussed as a promising spark plug replacement for internal
combustion engines, but previously not for high pressure engines.
TSIs have, for example, been shown in a number of prior patents
including, for example, U.S. Pat. Nos. 6,321,733 and 6,474,321,
both assigned to the same assignee as this invention and
incorporated by reference in their entireties for their
explanations of TSI devices and ignition systems.
[0005] Briefly, a TSI-based ignition system provides a large plasma
kernel which is propagated along the igniter's electrodes by
Lorentz force (along with thermal forces, to lesser degrees) and
propelled into a combustion chamber. The Lorentz force acting on
the ignition kernel (i.e., plasma) is created by way of the
discharge current in the plasma interacting with a magnetic field
caused by that same current in the electrodes of the igniter. The
magnitude of the Lorentz force is proportional to the square of
that current. In engines operating at normal pressures (i.e., a
maximum of about 120 psi), traveling spark igniters provide
significant advantages over conventional spark plugs due to the
large plasma volume they generate, typically some 100-200 times
larger than in a conventional spark plug, for comparable discharge
energy. Increased efficiency and reduced emissions are
attainable.
[0006] For higher engine operating pressures, however, the
breakdown voltage required for initiating the discharge between the
electrodes of the igniter is significantly higher than in engines
operating at conventional pressures. This creates problems for
TSIs, as for any spark plug. The electrodes in a TSI, as in a
conventional spark plug, are maintained in a spaced apart
relationship by a member called an isolator, which is formed of an
insulating material such as a ceramic. The higher breakdown voltage
causes problems for both the isolator and the electrodes.
[0007] Along the surface of the isolator running between the
electrodes, the breakdown voltage is lower than it is further along
the electrodes in a TSI, or in any conventional spark plug with a
similar gap between the electrodes. Indeed, this difference in
breakdown voltages varies directly with increasing pressure in the
combustion chamber. Consequently, although the breakdown voltage
along the isolator surface increases with pressure, that increase
is less than the increase in the breakdown voltage between the
exposed part of the electrodes away from the isolator surface. When
breakdown occurs (as a result of which the resistance through the
plasma rapidly drops), the current rises rapidly and a very large
current is conducted in the forming plasma along the isolator
surface, thus giving rise to the Lorentz force acting on the
plasma. Such rapidly rising current, though, creates not only a
very high temperature plasma, but also a powerful shock wave in the
vicinity of the surface of the isolator. The larger the current,
the more rapid the plasma expansion and the resulting shock wave.
These combined effects can cause deformation and/or breakage of the
isolator.
[0008] Additionally, the high current produces very rapid erosion
of the electrodes in the vicinity of the isolator surface, where
they are attacked by the high current, thermal heating and
thermionic emission that results therefrom.
[0009] Similar problems have been manifest with igniters based on
the University of Texas "railplug" design which generates a Lorentz
force in a plasma traveling along a high aspect ratio discharge gap
(as contrasted with a TSI, which has a low aspect ratio discharge
gap).
[0010] Although both the railplug and the TSI generate significant
plasma motion at relatively low pressures, when the combustion
chamber pressure is increased to a high pressure, the plasma
behaves differently and it is this difference in behavior that
leads to unsatisfactory results. In a low pressure environment, the
force exerted on the plasma by the pressure is relatively small.
The plasma moves easily along the electrodes in response to the
Lorentz force. As the ignition chamber pressure is increased,
however, that pressure provides a force of significant magnitude
that resists the Lorentz force and, thus, plasma motion.
Consequently, the plasma tends to become more concentrated, and to
collapse on itself; instead of having a diffused plasma cloud, a
very localized plasma--an arc--is formed between the electrodes
below a certain current threshold. This arc, though occupying a
much smaller volume than the plasma cloud of the low-pressure case,
receives similar energy. As a result, the current density is higher
and at the electrodes, where the arc exists, there is a higher
localized temperature and more power density at the arc-electrode
interfaces. That is, the current density is quite high at those
interfaces, producing more localized heating of the electrodes than
in the low pressure environment. The localized heating of the
electrodes, in turn, produces thermionic emission of electrons and
ions. The observed effect is that the arc appears to "attach"
itself at relatively fixed locations on the electrodes, producing
erosion of the electrodes as the entire discharge energy is
deposited at the "attachment point;" this is to be contrasted with
the low pressure environment where a lower density, diffused area
of plasma contact moves along the electrodes without significantly
damaging them.
[0011] Concurrently, the plasma, affected by the Lorentz and
thermal forces, bows out from the arc attachment points. This
causes the magnetic field lines to no longer be orthogonal to the
current flow between the electrodes, reducing the magnitude of the
Lorentz force produced by a given current. So, in addition to the
other problems, there is a loss in motive force applied to the
plasma.
[0012] Overall, there is a reduction in plasma motion as compared
with the lower pressure environments, and dramatically increased
electrode wear at the arc attachment points.
[0013] Accordingly, a variety of needs exist, including needs for
plasma generators, in general, needs for improved ignition systems,
needs for ignition systems for use in internal combustion engines
and needs for an ignition system and method which generates a large
ignition kernel and which is usable with high pressure engines, and
is commercially practical.
[0014] If a traveling spark igniter is to be used in a high
pressure combustion environment, a need further exists to overcome
the above negative effects on the isolator material and electrodes
of the igniter. See U.S. Pat. Nos. 5,704,321, 6,131,542, 6,321,733,
6,474,321, 6,662,793, and 6,553,981, for example, incorporated by
reference herein. That is, a need exists for an igniter and
ignition system for use in high pressure combustion engines,
wherein the isolator and electrodes exhibit substantial lifetimes
(preferably comparable to that of conventional spark plugs in low
pressure engines) without being destroyed by the discharge process.
Desirably, such a traveling spark igniter and ignition system will
be usable and useful in internal combustion engines operating not
only at high and very high pressures (i.e., several hundred psi),
but also at lower, conventional pressures.
SUMMARY
[0015] The above and other needs are addressed, and advantages
provided, with a new method, and corresponding apparatus, for
generating and sustaining a plasma, operating a traveling spark
igniter and providing an ignition for internal combustion and other
engines, particularly high pressure internal combustion engines.
Typically, a high initial breakdown voltage is applied to the
igniter to initiate a plasma kernel in a plasma initiation region
of the igniter, but preferably at a current lower than that
previously employed with TSI ignitions, as the breakdown current
need not produce a large Lorentz force. After the breakdown current
pulse, various mechanisms may be employed to prolong the plasma
while recombination is occurring and to allow the plasma to become
easily detached (or detachable) from the initiation region
(typically, on or adjacent the surface of an isolator between the
igniter electrodes. Before the plasma has a chance to recombine
completely, the current is turned on again to provide a short
follow-on pulse of energy (preferably at a current substantially
less than that of the breakdown pulse). The follow-on current pulse
generates a corresponding pulse of Lorentz force to move the plasma
away from its previous location, further along the electrodes of
the igniter. A number of such follow-on pulses may be provided,
with an "off" interval between successive pulses, during which
interval one or more mechanisms prolong the plasma and allow only
partial recombination of the plasma. This is called "simmering."
Prior to total recombination of the plasma, the next follow-on
pulse of current "kicks" the plasma even further along the
electrodes; and the final follow-on pulse ejects the plasma from
the electrodes. One mechanism for producing simmering is to reduce
the current through the igniter to a relatively low (but non-zero)
level, called a "simmer current." Alternatively, if a summer
current is not applied, similar effects may be obtained by using
any of a number of other techniques for prolonging recombination
and preventing "total" recombination of the plasma kernel by the
time the next follow-on pulse arrives. For example, the follow-on
pulses may be timed and possibly even waveform-shaped to more
closely follow each other so that only partial recombination occurs
between pulses; or each follow-on pulse may be preceded by a high
sub-breakdown voltage; or the plasma may be excited by RF or laser
energy. That is, numerous ways are contemplated of preventing total
plasma recombination. By "total" in reference to recombination is
meant that the plasma effectively has been extinguished and high
energy is needed to reignite it.
[0016] The invention is manifested in several ways, or aspects, and
example implementations are presented below. Other ways of
practicing the invention will become apparent to those skilled in
the art. The various aspects may be practiced alone or in any of
many combinations, all of which cannot be reasonably enumerated
here. It is intended that features of various embodiments be
practiced in combinations other than those illustrated, not all
features being shown in connection with all embodiments, for
brevity.
[0017] Aspects of the invention include the following, at
least:
[0018] A method of plasma generation, comprising applying a high
voltage to an igniter, said high voltage being of amplitude
sufficient to cause breakdown to occur between the electrodes,
resulting in a high current electrical discharge in the igniter in
an initiation region, and formation of a plasma kernel adjacent
said initiation region; and following breakdown, applying to said
electrodes a sequence of at least two relatively lower voltage
follow-on pulses, whereby the plasma kernel is forced to move
toward a free end of said electrodes by said follow-on pulses.
[0019] A method of plasma generation, comprising applying a high
voltage to an igniter, said high voltage being of amplitude
sufficient to cause breakdown to occur between the electrodes,
resulting in a high current electrical discharge in the igniter in
an initiation region, and formation of a plasma kernel adjacent
said initiation region; and following breakdown, applying to said
electrodes a sequence of one or more relatively lower voltage
follow-on pulses of current sufficiently low as to maintain a
diffuse attachment of the current arc to the electrodes, whereby
the plasma kernel is forced to, and can, move toward a free end of
said electrodes under the influence of said follow-on pulses.
[0020] The initiation region may be on or adjacent the surface of
an isolator disposed between said electrodes. A current of the
follow-on pulses, for an internal combustion engine, may be between
about 3 and 450 Amperes. The method may include preventing total
kernel recombination of the plasma prior to at least one follow-on
pulse. This may be done in various was, including between pulses of
the sequence, maintaining a simmer current between the igniter
electrodes sufficient to prevent total recombination of the plasma
kernel. It also may include, in an interval between follow-on
pulses, for at least part of said interval maintaining a voltage
across electrodes of the igniter below a breakdown voltage but
sufficient to sustain enough current to prevent total recombination
before the end of the interval. The igniter may be a traveling
spark igniter. Successive pulses in said sequence are separated by
intervals of about 2-600 microseconds and preferably about 20-250
microseconds, most preferably 50-100 microseconds. Each of said
follow-on pulses may have a maximum amplitude of about 3-450
Amperes. The amplitudes may not be uniform. The follow-on pulses
may have a maximum amplitude of about 20-120 Amperes, which may not
be uniform. Each of said follow-on current pulses preferably may
have an average duration of less than about 200 microseconds, which
may not be uniform. The follow-on pulses may have an amplitude of
about 10-5000 V and preferably about 20-275 V. The follow-on pulses
need not all have the same polarity of voltage and current and the
currents of the follow-on pulses need not be constant.
[0021] A fuel ignition method, comprising applying a high voltage
to an igniter in the presence of a combustible fuel, said high
voltage being of amplitude sufficient to cause breakdown to occur
between the electrodes of the igniter, resulting in a high current
electrical discharge in the igniter in an initiation region, and
formation of a plasma kernel adjacent said initiation region; and
following breakdown, applying to said electrodes a sequence of two
or more relatively lower voltage follow-on pulses, whereby the
plasma kernel is forced to move toward a free end of said
electrodes by said follow-on pulses. The initiation region may be
on or adjacent the surface of an isolator disposed between said
electrodes. The igniter may be in an internal combustion engine. A
current of the follow-on pulses for a gasoline-fueled internal
combustion engine, may be between about 3 and 450 Amperes.
Preferably, said method includes preventing total kernel
recombination of the plasma prior to a follow-on pulse.
[0022] Preventing total recombination may include, between pulses
of the sequence, comprises maintaining a current (termed a simmer
current) through the plasma kernel sufficient to prevent total
recombination of the plasma kernel. Preventing total recombination
of the plasma kernel also may include, in an interval between
follow-on pulses, for at least part of said interval maintaining a
voltage across electrodes of the igniter below a breakdown voltage
but sufficient to sustain enough current through the plasma to
prevent total recombination before the end of the interval.
[0023] Follow-on pulses need not all have the same polarity of
voltage and current, which need not be constant.
[0024] The igniter may be in an internal combustion engine in which
there is a relatively high pressure at the time of ignition.
[0025] The methods may further include, after a follow-on pulse,
re-triggering or re-striking the plasma kernel at a time an
ionization level of the plasma kernel has fallen below a desired
level, with a current and at a relatively low voltage sufficient to
cause the plasma kernel to grow before total recombination occurs,
followed by a next follow-on pulse.
[0026] The methods also may include simmering the plasma kernel
between at least some follow-on pulse pairs.
[0027] An ignition circuit for powering an igniter in an internal
combustion engine, comprising means for providing a high voltage
capable causing an electrical breakdown discharge, at a high
current, between electrodes of an igniter, in an initiation region
between said electrodes, when said igniter is disposed in a
fuel-air mixture of an engine, whereby a plasma kernel is formed in
said region by said discharge; and means for providing a sequence
of one or more relatively lower voltage and lower current pulses
having voltage and current amplitude and timing sufficient to force
the plasma kernel to move toward a free end of said electrodes by
said lower voltage, lower current pulses. The means for providing a
high voltage capable of causing electrical breakdown discharge may
include a high voltage, low inductance ignition coil having a
primary winding and a secondary winding, the secondary winding
having a lead for connection to one electrode of an igniter, and a
circuit for triggering a signal in the primary winding to induce a
high voltage pulse in the secondary winding. The means for
providing a sequence of relatively low voltage pulses may comprise
a relatively low voltage source and, for each said pulse, a
capacitor charged by the relatively low voltage source and a pulse
transformer having a secondary winding connected to said lead and a
primary winding through which the capacitor is discharged in
response to a trigger signal, inducing said pulse in said lead. The
ignition circuit may further include means for providing to the
igniter, in an interval between the breakdown discharge and a first
follow-on pulse a simmer current sufficient to prevent total
recombination of the plasma kernel in said interval. It also may
include means for providing to the igniter, in an interval between
each successive pair of follow-on pulses a simmer current
sufficient to prevent total recombination of the plasma kernel in
said interval. The ignition coil preferably includes a saturable
core on which the primary and secondary windings are formed and the
core substantially saturates when said electrical breakdown occurs,
whereby the secondary winding thereafter has substantially reduced
inductance.
[0028] An ignition circuit for powering an igniter in an internal
combustion engine, comprising a high voltage pulse generator which
generates on an output for connection to an igniter a pulse whose
maximum voltage, when delivered to the igniter, is capable causing
a breakdown discharge and consequent high current between
electrodes of the igniter, in an initiation region between the
electrodes, when said igniter is disposed in a fuel-air mixture,
whereby a plasma kernel is formed adjacent said surface by said
discharge; and a lower voltage pulse generator which generates on
the output a sequence of one or more relatively lower voltage and
lower current follow-on pulses having voltage and current amplitude
and timing sufficient to force the plasma kernel to move toward a
free end of said electrodes by said lower voltage, lower current
pulses. There may also be included a simmer current source which
supplies on the output line, in an interval between the breakdown
discharge and a first follow-on pulse a simmer current sufficient
to prevent total recombination of the plasma kernel in said
interval. As well, there may be a voltage source which maintains
between follow-on pulses, for at least a portion of an interval
between said follow-on pulses, a voltage on the igniter electrodes
below a breakdown voltage but sufficient to prevent total
recombination of the plasma kernel during said interval.
[0029] An ignition circuit substantially as shown and described in
the drawing figures, particularly any of FIGS. 8-10.
[0030] The ignition circuit also may include means operable after a
follow-on pulse, for re-triggering or re-striking the plasma kernel
at a time an ionization level of the plasma kernel has fallen below
a desired level, with a current and at a relatively low voltage
sufficient to cause the plasma kernel to grow before total
recombination occurs, followed by a next follow-on pulse.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The accompanying drawings are not intended to be drawn to
scale. In the drawings, each identical or nearly identical
component that is illustrated in various figures is represented by
a like numeral. For purposes of clarity, not every component may be
labeled in every drawing. In the drawings:
[0032] FIG. 1 is a schematic illustration, in cross section, of a
prior art traveling spark igniter, illustrating the principle of
its operation;
[0033] FIG. 2 is a part-schematic, part-block diagram of a typical
prior art ignition circuit for the TSI of FIG. 1;
[0034] FIG. 3 is a generalized representation of the voltage
between the electrodes of an igniter as shown in FIG. 1, using an
ignition circuit of the type shown in FIG. 2;
[0035] FIG. 4 is a diagrammatic illustration of the creation of a
plasma cloud by a current pulse in a TSI, and the subsequent
collapse of the plasma, in a TSI operating in a high pressure
environment;
[0036] FIG. 5 is a waveform of an example of a drive current
applied to a TSI in accordance with the teachings of the present
invention;
[0037] FIGS. 6 and 7 are diagrammatic illustrations of the motion
of the plasma cloud of FIG. 4 in a TSI which is operated in
accordance with the principles exemplified in the waveform of FIG.
5;
[0038] FIG. 8 is a simplified schematic circuit diagram for an
example of an ignition drive circuit usable to generate a current
drive waveform for a TSI as taught herein, including, for example,
the waveform or drive signal of FIG. 5;
[0039] FIG. 9 is a simplified part-block, part schematic circuit
diagram of another embodiment of an ignition circuit for generating
an ignition drive to a TSI as taught herein;
[0040] FIG. 10 is a simplified part-block, part schematic circuit
diagram of yet another embodiment of an ignition circuit for
generating an ignition drive to a TSI as taught herein; and
[0041] FIG. 11 is a simplified part-block, part schematic circuit
diagram of a still further embodiment of an ignition circuit for
generating an ignition drive to a TSI as taught herein.
DETAILED DESCRIPTION
[0042] Herein are explained in greater detail numerous aspects of
the invention; the problems addressed by the invention, in greater
detail than above; and a single embodiment of an example of an
ignition circuit for practicing aspects of the invention.
[0043] According to a first aspect, there will be shown a method of
operating an igniter in an internal combustion engine, comprising:
applying a high voltage to electrodes of the igniter, said high
voltage being of amplitude sufficient to cause electrical discharge
breakdown to occur between the electrodes, in an initiation region
(e.g., over a surface of an isolator) between the electrodes,
resulting in a high current electrical discharge in the igniter,
and formation of a plasma kernel in an air or fuel-air mixture
adjacent said surface; and following breakdown, applying to said
electrodes (preferably a simmer current) and a sequence of one or
more lower voltage and lower current pulses, whereby the plasma
kernel is forced to move toward a free end of said electrodes by
said lower voltage, lower current pulses.
[0044] Between breakdown and a first pulse of the sequence, and
between pulses of the sequence, a current desirably is maintained
through the plasma kernel sufficient to prevent total recombination
of the plasma. Alternatively, such a current need not be
maintained, if the intervals between breakdown and the first pulse
of the sequence, and between additional follow-on pulses of the
sequence, are sufficiently short, such that total recombination
does not occur prior to the start of such pulses. (If total
recombination occurs, then a high breakdown voltage is needed to
restart the plasma formation process.) If total recombination is
avoided (no matter how) before the start of a follow-on pulse, the
follow-on pulse can be a relatively low current pulse (compared to
a number of previous approaches, but still appreciable) and it will
still provide a suitable Lorentz force to advance the plasma, and
it will, itself, create a current arc that can move along the
electrodes. As another alternative, recombination can be slowed by
imposing a relatively high (but less than breakdown) voltage across
the electrodes prior to the start of a follow-on pulse. All three
mechanisms facilitate the establishment of a moving plasma kernel
without requiring re-generation of a high energy breakdown
condition, reducing the tendency of the current path to "re-attach"
to the electrodes at fixed locations. The number of follow-on
pulses varying according to design requirements and/or operating
conditions.
[0045] The igniter is preferably a traveling spark igniter.
[0046] Desirably, a first pulse of the sequence follows the
breakdown discharge by an interval of from about 2 to about 100
microseconds, preferably from about 10 to about 20 microseconds,
but this will depend on the recombination time for a plasma in the
particular kind of fuel mixture being employed. Desirably, each of
said follow-on pulses has a maximum amplitude of about 5-200
Amperes. But the amplitudes need not be uniform. Preferably, said
lower voltage, lower current pulses have a maximum amplitude of
about 25-105 Amperes, and more preferably about 40-80 Amperes. The
pulses may have a duration of from about 2 to about 200
microseconds. Successive pulses in said sequence preferably are
separated by intervals of about 10-500 microseconds and even more
preferably, 40-120 microseconds, but the intervals may not be
uniform. In terms of voltage, each of said pulses typically may
have an amplitude of about 50-5000 V and, more preferably, about
300-500 V. All pulses need not have the same polarity of voltage or
current; and neither the voltage nor the current in a pulse need be
constant. The foregoing numbers are all representative only and are
not intended to reflect any inherent limits on the invention. Other
ranges may be employed in appropriate embodiments. These numbers
may be useful, though, as an aid to identifying differences with
other ignition systems and methods.
[0047] The invention is intended for use in high pressure engines,
but is not so limited.
[0048] According to a related aspect, an ignition circuit is
provided for powering an igniter in an internal combustion engine,
the circuit comprising means for providing a high voltage capable
of causing a breakdown discharge, at a relatively high current (but
preferably lower than prior TSI ignitions have used), between
electrodes of an igniter, and in an initiation region (e.g., on or
over a surface of an isolator which separates the electrodes), when
said igniter is disposed in a fuel-air mixture, whereby a plasma
kernel is formed adjacent said surface by said discharge; and means
for providing a sequence of one or more relatively lower voltage
and lower current follow-on pulses having voltage and current
amplitude and timing sufficient to create Lorentz force pulses
causing the plasma kernel to move toward a free end of said
electrodes by said follow-on pulses. The means for providing a high
voltage capable of causing breakdown may include a high voltage,
low inductance ignition coil having a primary winding and a
secondary winding, the secondary winding having a lead for
connection to one electrode of an igniter, and a circuit for
triggering a signal in the primary winding to induce a high voltage
pulse in the secondary winding.
[0049] The means for providing a sequence of relatively lower
voltage (i.e., sub-breakdown voltage) pulses may comprise a low
voltage source and, for each said pulse, a capacitor charged by the
low voltage source and a pulse transformer having a first winding
connected to said lead and a second winding through which the
capacitor is discharged in response to a trigger signal, inducing
said pulse in said lead. The ignition circuit may further include
means for providing to the igniter, in an interval between the
breakdown discharge and a first lower voltage pulse a simmer
current sufficient to prevent total recombination of the plasma
kernel in said interval. It also may include means for providing to
the igniter, in an interval between successive follow-on pulses a
simmer current sufficient to prevent total recombination of the
plasma kernel in said interval. Alternatively the means for
providing a sequence of relatively low voltage pulses includes
means for providing pulses separated in time by an interval
sufficiently short that total recombination of the plasma kernel
does not occur in said interval. As another alternative, the means
for providing a sequence of relatively low voltage pulses may
comprise a means for preceding each such follow-on pulse by a high,
sub-breakdown voltage.
[0050] According to a further aspect, an ignition circuit is shown
for powering an igniter in an internal combustion engine, the
circuit comprising a high voltage pulse generator which generates
on an output for connection to an igniter a pulse whose maximum
voltage, when delivered to the igniter, is capable causing a
breakdown discharge, at a high current, in an initiation region
between electrodes of the igniter (e.g., adjacent a surface of an
isolator which separates the electrodes), when said igniter is
disposed in a fuel-air mixture, whereby a plasma kernel is formed
adjacent said surface by said discharge; and a low voltage pulse
generator which generates on the output a sequence of one or more
lower voltage and lower current pulses having voltage and current
amplitude and timing sufficient to force the plasma kernel to move
toward a free end of said electrodes by said lower voltage, lower
current pulses. The ignition circuit may further include a simmer
current source which supplies on the output, in an interval between
the breakdown discharge and a first lower voltage pulse, a simmer
current sufficient to prevent total recombination of the plasma
kernel in said interval. Alternatively, the circuit may include a
follow-on pulse generator that supplies, on the output, follow-on
pulses which follow each other so closely (i.e., are separated by a
sufficiently short interval) that total recombination of the plasma
does not occur in the interval between such pulses. As another
alternative, the circuit may include a pulse source providing a
sequence of relatively low voltage pulses and a high voltage source
which provides, preceding each such follow-on pulse, a
sub-breakdown high voltage sufficient to delay total recombination
such that total recombination has not occurred when the relatively
low voltage pulse starts.
[0051] Thus, this invention is not limited in its application to
the details of construction and the arrangement of components set
forth in the following description or illustrated in the drawings.
The invention is capable of other embodiments and of being
practiced or of being carried out in various ways. Any embodiments
are presented by way of example only. Also, the phraseology and
terminology used herein is for the purpose of description and
should not be regarded as limiting. The use of "including,"
"comprising," or "having," "containing," "involving," and
variations thereof herein, is meant to encompass the items listed
thereafter and equivalents thereof as well as additional items.
[0052] It is useful, now, to attempt to better understand the
problems encountered when one attempts to operate an igniter in a
high pressure engine. A traveling spark igniter (TSI) is an
ignition device which is in the nature of a small plasma gun. A
typical TSI is illustrated in FIG. 1, taken from U.S. Pat. No.
6,321,733. An isolator (e.g., ceramic) material 14 maintains
electrode spacing. A plasma 16 is created along the surface of the
isolator, due to a high voltage breakdown process occurring there.
As the discharge current passes through the plasma, the temperature
and volume of the plasma increase, leading to a further decrease in
plasma resistivity and resistance. This increases the current in
the plasma, which is limited primarily by the impedance of the
electrical discharge circuit that produces the current supplied to
the igniter.
[0053] A typical ignition circuit for operating a TSI is shown in
FIG. 2, which is also taken from U.S. Pat. No. 6,321,733. The
circuit consists of two main parts: (1) a conventional ignition
system 42 and (2) a follow-on current generator comprising
capacitors such as 46 and 48, a low voltage power supply 44 and
diode 50. The conventional ignition system 42 provides a high
voltage for creating a breakdown (at a high current) in the spark
gap along the isolator surface 56 between the electrodes 18 and 20,
to form an initial plasma in the gaseous combustion mixture near
that surface. The follow-on current generator provides a current
through the initial plasma, in the spark gap, after breakdown
discharge, forming a much larger plasma volume. Resistor 54 may
(but need not) be used to limit the maximum current from capacitor
48. A typical voltage discharge profile (not to scale) is shown in
FIG. 3, taken from U.S. Pat. No. 6,474,321.
[0054] The conventional ignition system 42 initiates discharge in
the discharge gap at time t=t.sub.0. As a result, the voltage in a
secondary coil in the high voltage (HV) ignition transformer
therein rises until it reaches the breakdown voltage in the spark
gap at t=t.sub.1. After breakdown occurs at t=t.sub.1 the voltage
across the discharge gap drops rapidly to value of about 500 volts
or less at t=t.sub.2, corresponding to low plasma resistivity. The
voltage is substantially constant until a time t=t.sub.3, when just
about all the energy from capacitors 46 and 48 has been
transferred, following which the voltage and current rapidly
diminish to a near-zero value at time t=t.sub.4. For simplicity, we
shall assume that the interval from t.sub.3 to t.sub.4 is
negligibly short. The interval .DELTA.t=t.sub.3-t.sub.2 is related
to the energy stored in capacitors 46 and 48 as well as the voltage
of the follow-on current through the discharge gap after breakdown
occurred. The following energy balance equation relates the
variables:
1 2 C ( V t 2 2 - V t 4 2 ) = .intg. t 2 t 4 V ( t ) i ( t ) dt
##EQU00001##
where V(t) is the voltage as a function of time, between the
electrodes defining the discharge gap, such voltage having an
initial value V.sub.t.sub.2 at time t.sub.2 and a final value
V.sub.t.sub.4.apprxeq.0 at t>t.sub.4, i(t) is the current in the
spark gap as a function of time and C is the sum of the discharging
capacitance (here, the sum of capacitances of capacitors 46 and
48). In the time interval .DELTA.t=t.sub.3-t.sub.2, one can assume,
as a first approximation, that V(t).apprxeq.V.sub.0 and is roughly
constant, therefore,
V.sub.t.sub.2.sup.2-V.sub.t.sub.4.sup.2.apprxeq.V.sub.0.sup.2. If
one further assumes that the plasma resistivity is constant, one
can make the assumption i(t).apprxeq.i.sub.0. One can use these
simplifying assumptions to obtain a basic relationship between
.DELTA.t (.DELTA.t.apprxeq.t.sub.4-t.sub.2 because
t.sub.4-t.sub.3>>.DELTA.t) and the circuit parameters
described by C, V.sub.0, and i.sub.0:
.DELTA.t=CV.sub.0/2i.sub.0
This simple relationship provides information about pulse duration
as a function of capacitance and average current i.sub.0 during
discharge, for a given operating (relatively low) voltage V.sub.0
on the capacitors. For a given energy provided to the igniter
(hence, given V.sub.0 and C), this relationship teaches that for
current i.sub.0 to increase, the pulse duration .DELTA.t has to
decrease. However, increasing current i.sub.0 also increases the
Lorentz force F.sub.L. Increasing the Lorentz force moves the
plasma away from the isolator surface faster, toward the end of the
electrodes, into the combustion chamber of the engine. Pressure in
the combustion chamber, however, provides a countervailing pressure
force F.sub.p in the igniter. Force F.sub.p works against the
Lorentz force preventing the speed of the plasma from increasing
above some limiting value, independent of the length .lamda. of the
electrodes (i.e., .lamda. is the distance between the surface of
the isolator and the free end of electrodes facing into the
combustion chamber).
[0055] The net force available to move the plasma is the difference
between the Lorentz force F.sub.L and the pressure force F.sub.p
(assuming one can ignore the thermal force on the plasma as it is
significant only at the earlier stages of plasma propagation and
diminishes quickly as the plasma moves away from the isolator
surface). It is useful to develop a model of the forces in order to
understand how to overcome the pressure force. The Lorentz force
F.sub.L can be represented as a magnetic pressure p.sub.B on the
plasma, given by the well-known relationship p.sub.B=B.sup.2/8.pi.,
multiplied by the effective plasma surface area,
S.sub.p.lamda.:
F L = B 2 8 .pi. S p .lamda. ##EQU00002##
The gas pressure force Fp can be presented in the form
F.sub.p=pS.sub.p, where p is the effective gas pressure from the
combustion mixture (facing the plasma during its movement). Hence,
one can write the equation for the net force governing plasma
movement can be presented as:
(F.sub.L-F.sub.p)=m.sub.p. d.nu..sub.p/dt,
where .nu..sub.p.sub. is plasma velocity and m.sub.p.sub. is plasma
mass. In turn, plasma mass can be presented as the product of
plasma mass density .rho..sub.p.sub. and plasma volume
V.sub.p=S.sub.p.DELTA..sub.p, where .DELTA..sub.p.sub. is a
fraction representing the portion of the electrode length occupied
momentarily by the plasma.
[0056] The net force equation can be simplified, and useful
relationships derived from it, by making some rough assumptions.
One can assume that the plasma volume, after its formation, is
constant as the plasma propagates along the electrodes; thus,
S.sub.p, .DELTA..sub.p.sub. and .rho..sub.p.sub. are constant and
forces F.sub.L and F.sub.p are also constant. Then, by integrating
one obtains:
(F.sub.L-F.sub.p).DELTA.t.apprxeq..rho..sub.p.DELTA..sub.pS.sub.p.nu..su-
b.p,
where it was assumed that the initial plasma velocity .nu..sub.t2
was much smaller than its final velocity, .nu..sub.p.
[0057] Replacing F.sub.L by B.sup.2 where B= {square root over
(8.pi..alpha.)} i and .alpha. is a constant coefficient, and
F.sub.P as above, we obtain
(.alpha.i.sub.0.sup.2-p).DELTA.t=.rho..sub.p.lamda..DELTA..lamda..sub.p.-
lamda..nu..sub.p.lamda..
Because 1/2 .DELTA.t .nu..sub.p.apprxeq., we can write
.DELTA. t = 1 i 0 ( 2 .lamda. .rho. p .lamda. .DELTA..lamda.
.lamda. p / .alpha. 1 - p / .alpha. i 0 2 ) 1 / 2 ##EQU00003##
From this equation, one observes that for relatively small pressure
(i.e., p<<.alpha.i.sub.0.sup.2), .DELTA.t
i.sub.0.apprxeq.constant; and in this range of parameters,
increasing i.sub.0 leads to decreasing .DELTA.t. Then from the
above relationships, one can see that the plasma can be moved
faster with increasing i.sub.0 without really increasing the
discharge energy (of course, this is only true for
.rho..sub.p.DELTA..sub.p.apprxeq.const.; with increasing i.sub.0,
.rho..sub.p.DELTA..sub.p.sub. may also increase, so some additional
energy may be required).
[0058] However, when it is not true that
p<<.alpha.i.sub.0.sup.2 (i.e., the assumption fails), then
increasing pressure p could lead to p/.alpha.i.sup.2.gtoreq.1 and
the plasma could stop moving altogether. In such a case, it will be
necessary to increase i>i.sub.0 to the point that
p/.alpha.i.sup.2<1. This requires a significant increase in
energy, though, due to increased .DELTA.t and i.
[0059] Recombination processes in the plasma pose a further hurdle.
The front portion of the hot plasma that is in contact with a
relatively cold combustion mixture cools rapidly. The plasma
recombination rate at high pressure is a function of plasma
temperature, T, that varies as 1/T.sup.3/2. Hence, at low
temperature, plasma recombination occurs very fast at its
propagation front where it interacts with the cold gaseous mixture.
At high pressures, such recombination rate could be as fast as the
plasma propagation velocity, meaning that the Lorentz
force--induced movement would be entirely negated by the speed of
recombination, effectively causing the plasma to stand still. In
such a situation, the net plasma velocity along the electrodes is
substantially zero and the plasma will seem to stay near the
surface of the isolator during the entire discharge. The plasma, of
course, recombines near the surface of the isolator, as well,
though at a much slower rate because the gas there is much hotter
than at the plasma's front edge. Consequently, plasma resistivity
near the isolator surface is lower than at the front edge of the
plasma and most of the discharge current will be concentrated in
that region, preventing further plasma recombination near the
isolator.
[0060] As shown above, increasing operating combustion chamber
pressure lowers the net motive force on the plasma so it moves more
slowly and the time it takes for the plasma to move to the
combustion chamber thus increases. Therefore, for sufficiently
large pressures, the plasma may never succeed in reaching the end
of the igniter.
[0061] To prevent the plasma from slowing down so much, the
discharge current has to be raised, in order to increase the energy
being fed into the plasma. The increased energy input, though, is
concentrated near the isolator. That is quite problematic. There
are thermal stresses imposed on the isolator and shock waves are
generated that can damage the isolator. There are also large
thermal effects on the portions of the electrodes near the
isolator. Assuming the ignition circuit supplies sufficient energy
to create a net force that will effectively move the plasma, then
the higher the pressure in the combustion chamber, the worse the
negative effects on the isolator and electrodes. These conditions
decrease isolator and electrode longevity in high pressure
environments, unless something is done to prevent those negative
impacts.
[0062] The problem of decreasing longevity of traveling spark
igniters with increasing gas (i.e., combustion mixture) pressure is
significantly decreased, or even eliminated, at least in part by
decreasing the difference between the speed of recombination at the
front of the plasma (facing the combustion chamber) and the back of
the plasma (facing the isolator). By making plasma recombination
more symmetrical, a significant net force on the plasma is directed
into the combustion chamber.
[0063] FIG. 4 diagrammatically illustrates the problem. A
relatively short first current pulse forms a volume of plasma 42,
as indicated by the dashed line. During that first pulse, the
center of the plasma moves to the right, away from isolator 14,
under the influence of the Lorentz force. As the pulse is of
relatively short duration, neither the isolator surface nor the gas
near the surface is heated significantly. Therefore, after the
first current pulse ends, the plasma recombines at its back (left)
side and its front (right) side fairly symmetrically, leaving a
relatively narrow plasma kernel 44. The narrow plasma kernel still
can support an arc, as explained above.
[0064] The present invention improves the symmetry of plasma
recombination by using a different approach to energizing the
igniter. Several short current discharge bursts (follow-on pulses)
are applied after the breakdown pulse, between times t.sub.2 and
t.sub.3. The follow-on pulses have moderately high peak current
amplitude, but significantly less than the breakdown pulse. Between
the breakdown pulse and the first follow-on pulse, and between
follow-on pulses, the (simmer) current preferably is maintained at
a low, non-zero value, to prevent total recombination.
[0065] In FIG. 5, in which the waveform is shown for one example of
an igniter current that may be used to excite a TSI as explained
above, breakdown occurs at time t.sub.1 (peak voltage, followed by
maximum current) and is complete at time t.sub.1*. Beginning at
time t.sub.2, a series of (one or more) lower amplitude current
pulses 52A-52E (i.e., five pulses, in this example, though the
number of pulses is variable) are provided between the electrodes
of the igniter. The discharge interval ends at time t.sub.3, when
the plasma reaches the end of the electrodes. The plasma started at
the isolator at time t.sub.1. The durations .tau..sub.1,
.tau..sub.2 . . . .tau..sub.n of the respective pulses 52 and their
peak current magnitude, i.sub.0, should be chosen according to
igniter design and gas pressure p. In a traveling spark igniter,
the pulse durations and magnitudes are selected, preferably, in
accordance with the length of the electrodes and the gap between
them. Experimentation is a satisfactory way, and for the moment
probably the best way, of setting the values of those parameters
for a given igniter design and maximum pressure of its operation.
The time between pulses also depends on igniter design and
pressure. The time between the breakdown current, when it reaches
near-zero level at t.sub.1* and the first follow-on pulse 52A,
indicated as .DELTA.t.sub.b,1, depends on the breakdown voltage and
the specifics of the isolator between the electrodes. The simmer
current i.sub.S is non-zero and, as such, helps avoid total plasma
recombination; otherwise, a large voltage (comparable to the
breakdown voltage) would be needed for initiating the next pulse.
So, the current i.sub.S facilitates each subsequence pulse and
allows its formation without the need for an additional breakdown
pulse. The following table provides parameter values which have
been found useful with TSI igniters operating in a simulated
combustion chamber at 400 psi pressure:
Electrode length: .lamda.=2.5 mm Peak pulse current:
i.sub.0.apprxeq.20-40Amperes, Duration of the k-pulse:
.tau..sub.k.apprxeq.10-20 microseconds, Time between two
consecutive pulses k and k+1: .DELTA.t.sub.k,k+1.apprxeq.50-100
microseconds, n (i.e., number of pulses).apprxeq.3 to 4, Simmer
current: i.sub.s.apprxeq.1-3 Amperes, Time between end of breakdown
and the first follow-on pulse: .DELTA.t.sub.b,1.apprxeq.5-20
microseconds.
[0066] These parameters can be significantly different for
different design of spark plugs or values of pressure p. For
example, for a TSI similar to the one in the previous example and
operating at pressure p=900 psi, suitable parameters that have been
found useful are:
[0067] i.sub.0.apprxeq.60-80Amperes,
[0068] .tau..sub.k.apprxeq.20-40 microseconds,
[0069] .DELTA.t.sub.k,k+1.apprxeq.30-40 microseconds,
[0070] n.apprxeq.7 to 10 pulses,
[0071] i.sub.s.apprxeq.3-5 Amperes, and
[0072] .DELTA.t.sub.b,1.apprxeq.3-10 microseconds.
[0073] Though the peak pulse values i.sub.0 and pulse durations
.tau..sub.k and the times between individual pulses
.DELTA.t.sub.k,k+1 have been shown as constant, they need not be
uniform or constant. For example, they could actually increase or
decrease as a function of time.
[0074] FIGS. 6 and 7 diagrammatically illustrate the operation
produced by this pulsed drive scheme. It is assumed the breakdown
pulse has already occurred and the first follow-on pulse is in a
position .DELTA..sub.1 away from the surface of the isolator, as in
FIG. 4. After a time interval .DELTA.t.sub.1, 2 following the first
pulse, the next pulse .tau..sub.2 occurs, after which the plasma is
in a new position .DELTA..sub.2 away from the surface of the
isolator. With each successive pulse, the plasma kernel is moved to
the right and then at the end of the pulse, allowed to recombine
(FIG. 6, showing the plasma position after two pulses), until
eventually (FIG. 7) the plasma reaches the end of the electrodes
after n current pulses, and is ejected into the combustion chamber.
The number of follow-on pulses, n, will depend on the pressure p in
chamber, igniter parameters (e.g., the length of the electrodes,
the gap between the electrodes, and the shape of the electrodes)
and current discharge parameters (e.g., peak values of pulses,
their durations, the inter-pulse intervals, and minimum current
value between pulses). Some experimentation may be required to find
suitable values.
[0075] Although the current pulses are shown as positive pulses in
FIG. 5, it should be realized that negative pulses can also be
used, or alternating pulses or some other pattern of pluralities.
The Lorentz force F.sub.L is proportional to the square of the
current and is, therefore, independent of current polarity.
Additionally, the discharge current pulses, shown as rectangular in
FIG. 5, could have any suitable waveform, such as triangular shape
or sinusoidal shape.
[0076] As stated above, with increased operating pressure, the
breakdown of voltage along the surface of the isolator also
increases. Increase in breakdown voltage has a negative impact on
the lifetimes of the isolator and electrodes. Such negative effects
can be avoided or significantly reduced by limiting the breakdown
current. For example, introducing a resistor into the high voltage
circuit, as described below, limits breakdown current without
wasting significantly energy when the breakdown discharge is of
short duration in comparison with the total interval of follow-on
discharge pulses. Limiting the current causes the mode of operation
to differ substantially from that of prior TSI systems. In prior
TSI systems, such as those shown in U.S. Pat. Nos. 6,321,733 and
6,474,321, it was desired that a high breakdown current be followed
immediately by high current from capacitors to create maximum
acceleration and plasma speed. The goal was to get the plasma to
reach the end of the electrodes and move into the combustion
chamber in a single discharge pulse. In contrast, in a high
pressure environment, plasma motion is small following breakdown.
Thus, it is acceptable to limit the breakdown current since the
breakdown current is only used to create the plasma near the
isolator surface, rather than to actually produce significant
plasma motion.
[0077] The interval between the end of the breakdown current pulse
and the first follow-on current pulse, .DELTA.t.sub.b,t1 depends on
the peak value of the discharge current. Assuming that a resistor
R.sub.b is used to achieve this current limiting effect, than the
delay time depends on the value of that resistor, which depends on
the applied breakdown voltage which, in turn, depends upon the
pressure p. Thus, the value of resistor R.sub.b can be chosen to
minimize stress on the isolator and electrode wear.
[0078] FIG. 8 shows a partial schematic circuit diagram for an
example of an electronic circuit for producing the breakdown pulse
and follow-on pulses as depicted in FIG. 5. In FIG. 8, circuitry is
shown for generating only the breakdown pulse and one follow-on
pulse. For each additional follow-on pulse that is desired, the
circuitry 110 enclosed in a dashed line can be replicated and all
such circuits can be connected with the secondary windings of their
boost transformers 102 in series, so that each such circuit will,
in turn, deliver one of the sequenced pulses to the igniter. (Note
that a parallel arrangement is also possible.)
[0079] A high voltage, for providing breakdown discharge is
generated by a high energy ignition coil 100, triggered by a signal
applied at 104 to cause switching of SCR 104A. Coil 100 may be any
suitable ignition coil such as, but not limited to, coil model 8261
sold by Autotronic Controls Corporation of El Paso, Tex., d/b/a MSD
Ignition. Though usually referred to in the industry as an
"ignition coil," element 100 actually is a transformer. The
aforementioned model 8261 ignition coil has a low inductance
primary and provides a 42-43 kV output from its secondary coil when
the primary coil is energized. The secondary coil of transformer
100 is directly connected (through secondary coil 102B of boost
transformer 102) to one or more electrodes of igniter 101, another
electrode of which is grounded.
[0080] The string 106 of diodes, each paralleled by a high
resistance, limits the output voltage of the ignition coil 100 to a
single polarity and prevents ringing.
[0081] After the breakdown pulse, a trigger signal is applied at
105 to cause a follow-on pulse to be generated. The boost
transformer 102 feeds the high voltage line (HVL) to igniter 101
with a pulse of current induced by discharging capacitor 103.
Capacitor 103 is charged to a relatively low voltage such as, for
example, about 500V and then discharged through the primary coil
102A of transformer 102 to ground through the SCR 105A.
[0082] The trigger signals can be generated by any suitable circuit
that may provide either fixed or programmable parameters.
[0083] The igniter electrode(s) connected to the high voltage line
are also connected, through a string of diodes 107, and an RC
network 111, to a low voltage supply, such as the indicated 500V
supply. The resistor values in network 111 are set to deliver the
simmer current, i.sub.s
[0084] The ignition circuit of FIG. 8, it will be appreciated,
represents just one way to generate the breakdown voltage and to
deliver the initial current and the follow-on pulses of current
that are desired. Any other suitable mechanism may be employed that
generates comparable pulsing. For example, a resonant current
circuit that could provide oscillating current pulses, such as
sinusoidal current pulses, could be used instead of the indicated
plurality of sub-circuits, each of which generates a single pulse.
Moreover, by proper inversion of polarities of voltage and diodes,
the circuit of FIG. 8 could be used to generate negative pulses
instead of positive pulses.
[0085] Another example of an ignition circuit architecture (in
simplified form) is shown in FIG. 9 at 130. Only the basic circuit
components are shown, it being understood that a practical
implementation may require other customary components. Power supply
132 supplies a voltage (termed the "high" voltage for purposes of
distinguishing it, only). The voltage is high enough so that it can
generate, when stepped up by transformer 134, a breakdown voltage
sufficient to create a plasma at the igniter (not shown). Power
supply is connected to a first end of primary winding 134A through
a diode 136, to charge a capacitor 138, connected between the other
end of the primary winding and ground. A pulse generator 142
supplies a train or sequence of pulses. On a first pulse, an output
signal from pulse generator 142 closes electronically controlled
switch 144. This action grounds the anode of diode 136, effectively
disconnecting supply 132 so that it is not short-circuited, and
allows capacitor 138 to discharge through the primary winding,
Transformer 134 is a saturable-core step-up transformer. The HV
supply 132 typically has an output voltage of a few hundred volts.
The closing of switch 144 generates a large voltage swing across
the transformer primary. Typically, a turns ratio of about
1:35-1:40 may be used in the transformer, and this will step up the
several hundred volt swing on the primary up to the range of tens
of thousands of volts across the secondary winding, 134B. This
latter voltage is sufficient to produce breakdown when applied to
an igniter (connected to one end of the secondary winding, but not
shown).
[0086] The aforesaid pulse preferably also saturates the core of
transformer 134.
[0087] Due to the core saturation, if a next pulse is supplied by
the pulse generator 142 before the saturation ebbs totally, such
pulse will not generate a breakdown-level output voltage on output
line 152.
[0088] The other end of primary winding 134B, at 154, and one end
of a capacitor, 156, are tied to ground via a diode 158. Capacitor
156 is charged by a "low voltage" (LV) supply through a protective
diode 164. When a pulse from pulse generator 142 is received by
electronic switch 166, node 168 is grounded and capacitor 156 is
grounded through series-connected diode 172, resistor 174 and
switch 168.
[0089] Low Voltage supply 162 may typically supply a voltage in the
range of 0-1000 volts. Capacitor 156 is a large capacitance in a
typical ignition system and resistor 174 may be sized to limit the
discharge current (pulled through the secondary winding 134 of the
transformer) to about 50 Amperes (less if a lower current will
suffice in the follow-on pulses).
[0090] Diodes 182 and 184 merely protect their respective switches
from reverse polarity spikes that could be destructive to them.
[0091] Supplies 132 and 162 are shown as separate but a single
supply may be used in some applications. Also, the terms low
voltage and high voltage are not intended to require that the
output of supply 132 be at a higher voltage than the output of
supply 162, though that is most typical.
[0092] Diode 164 is included for the same reason as diode 136, to
protect its associated power supply from having a short-circuited
output when the associated switch is closed.
[0093] Depending on the exact construction of the supplies 132,
162, it also may be desirable to place a resistance in series
between the one or both of the supplies and corresponding switch
144 or 166, as applicable, to limit the output current of the
supply and the charging time of the corresponding capacitor.
[0094] Switches 144, 166 may be implemented using various
semiconductors, such as SCRs, IGBTs (especially for switch 144),
MCTs and other high voltage switching elements as now or in the
future may exist.
[0095] A small capacitor, 159, may bypass diode 158, providing a
low impedance path to ground for rapid voltage changes and
protecting diode 158 against large reverse spikes.
[0096] Other variations are possible. For example, instead of a
single pulse generator actuating switches 144 and 166, each switch
may be actuated by a different pulse generator, or one pulse
generator may be employed with different outputs or differently
conditioned output signals (possibly derived from a common signal)
driving the switches. Or, one switch may be used, instead of two
switches, as shown in FIG. 10, referring to switching element
(e.g., MCT) 186. (In FIG. 10, the resistors R are expressly shown
though they may not be needed, depending on power supply details.)
If different pulse generators drive each of the switches, they can
be controlled independently and this will permit a variety of modes
of operation to be accommodated.
[0097] In FIG. 9, resistor 174 is shown in a dashed-line box, to
indicate it is optional. Irrespective of the fact that supply 162
may be set in conjunction with capacitor 156 to control the desired
amplitude of follow-on current pulses, all of the energy stored in
capacitor 156 cannot be transferred to the arc. To sustain a
current in the follow-on pulses over the interval of each pulse,
the capacitor 156 must be discharged at a controlled rate. One way
to do this is to discharge the capacitor through a resistor, such
as resistor 174. Unfortunately, the use of resistor 174 results in
the dissipation of a lot of the stored energy as heat. Indeed, more
energy may be lost as heat in resistor 156 than is expended in the
movement of the plasma. Hence this circuit suffers from inefficient
use of energy.
[0098] It is possible to improve the efficiency of the circuit and
to reduce the heat dissipation by making the switch element 166 a
controlled current drainage path. Then, instead of using resistance
174 to limit the current drain off of capacitor 156, the switch
transistor (or like element) takes care of that need, providing
controlled discharge. More specifically, as shown in FIG. 11, an
active switching element (here indicated as a MOSFET 166'), is
connected from node 168 to ground through a resistor 192. The
voltage across that resistor is sensed as a proxy for measuring the
actual current through transistor 166'. Gate drive logic 194
interposed between the pulse generator and the gate of transistor
166', responsive to the voltage on resistor 192, operates the
transistor as a switching regulator, with variable duty cycle and a
resulting lower power dissipation than that arising from the use of
resistor 174. Drive logic 194 may be implemented in various ways
and may include fixed logic or it may include programmable logic,
possibly including a microcontroller to operate the logic. An
advantage of using a microcontroller is that the logic can then be
configured to operate the circuit to perform in the various modes
discussed herein--e.g., with or without simmer current.
[0099] Note that although the generation of pulses of positive
polarity will result from the illustrated examples of ignition
circuits, those skilled in the art of electronics will readily be
able to derive therefrom ignition circuits that will produce
negative polarity pulses and even pulses of varied polarities,
should it be desired to have same. It may also be desirable that
some or all trigger pulses be o polarity differing from the output
pulses.
[0100] The detailed design of the drive logic and the parameters
for the breakdown voltage, follow-on pulses, igniter, etc. will all
depend on the particular engine specifications which the ignition
system is required to meet. Those requirements, and considerations
such as cost, component availability, and so forth will influence
component selection, as well. Determination of some of these
parameters may require a degree of experimentation on a model of
the engine(s) for which the ignition system or circuit is
intended.
[0101] Although the problems and their solution have been discussed
using just one form of TSI, both apply equally to other TSI
designs, using both parallel and coaxial electrodes.
[0102] While certain methods and apparatus have been discussed
herein for use with internal combustion engines operating at high
and very high pressures, it will be understood that this technology
also can be used with traveling spark igniters in internal
combustion engines operating at lower, conventional pressures, or
even with conventional spark plugs. The advantages, however,
probably will be greatest with traveling spark igniters.
[0103] Also, it should be understood that although a theory of
operation has been presented, there are number of simplifying
assumptions which may very much limit application of this theory.
Nevertheless, the invention, as claimed, does produce a working
ignition system in a simulated high pressure engine environment,
and any simplifications or errors in analysis will be understood
not to detract from the value of the invention.
[0104] Having thus described several aspects of at least one
embodiment of this invention, it is to be appreciated various
alterations, modifications, and improvements will readily occur to
those skilled in the art. Such alterations, modifications, and
improvements are intended to be part of this disclosure, and are
intended to be within the spirit and scope of the invention.
Accordingly, the foregoing description and drawings are by way of
example only.
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