Note: Descriptions are shown in the official language in which they were submitted.
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G-3,094 C-4,149
COMBUSTION PRESSURE SENSOR
Background of the Invention
It is helpful in the operation of internal
combustion engines to know the pressure in a combustion
chamber, and more specifically in an engine cylinder.
For example, this pressure can be used to control
ignition timing, thus allowing the engine to obtain
better fuel consumption results. Combustion chamber
pressure can also be used to detect engine knock.
It is known in the art that a pressure sensing
device, such as a piezoelectric force ring, may be used
to indicate combustion chamber pressure. A description
illustrating such an arrangement is found in U.S. Pat.
No. 4,153,019 to Laubenstein et al, issued May 8, 1979.
In the disclosure of that patent a cylinder head bolt
secures a force ring to the engine. This enables the
force ring to detect loads that are transferred to the
cylinder head bolt through the cylinder head as a
result of combustion chamber pressure.
Although force rings secured under head bolts
can detect loads acting on the cylinder head as a
result of combustion chamber pressure, they also detect
extraneous forces, such as thermal expansion and
external shock or inertia loads. This event often
occurs when the extraneous forces are of comparable
magnitude to the load being measured from combustion
chamber pressure. Consequently, these extraneous
forces must be filtered away so that the sensor
provides a clean output signal. Thermal stress, which
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has low frequency characteristics, is easily filtered
away without affecting the signal. Filtering high
frequency shock noise, however, introduces a phase lag
that may cause inaccuracies in the timing of engine
S control operations. In addition, such an arrangement
provides a signal that is non-linear due to the lack of
elasticity of the head gasket, which is variably loaded
as combustion chamber pressure moves the engine head
relative to the engine block. It is therefore
desirable to provide a linear pressure sensing
arrangement which, compared to a headbolt sensor, is
comparably more sensitive to combustion chamber
pressure than to extraneous forces, is more linear, and
introduces no phase or time lag.
An advancement toward that goal is shown in
the sensor of U.S. Pat. No. 4,601,196 to Frelund,
issued July 22, 1986. The sensor of the Frelund patent
utilizes a pressure sensing device that has a main body
and a probe. The probe engages a wall that flexes in
direct response to varying combustion chamber pressure.
The body engages a different wall that is relatively
fixed. The result is that when combustion chamber
pressure changes, the probe will move relative to the
body and thus generate a signal. This type of
arrangement may be implemented through a coolant
passage of the engine cylinder head, with the body
retained in an upper wall and the probe extending
through the coolant passage to a lower wall forming
part of the combustion chamber. If the sensor is
properly placed, the relative motion between the upper
and lower walls due to varying combustion chamber
pressure places a load on the sensor that is much
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greater than any loads caused by extraneous forces.
This greatly increases the signal to noise ratio of the
sensor output.
It is preferable, however, that the sensor is
placed near the center of the cylinder relative to the
other cylinders. This location produces a stronger
signal than other locations because the cylinder head
wall flexes more near the center than at the sides.
Additionally, this location also produces a linear
signal that is free from interference from adjacent
cylinders. In some situations, however, the sensor
described in the Frelund patent cannot be placed near
the center of the cylinder. A common example
illustrating this point is an engine having four valves
lS for each cylinder with a spark plug opening centrally
located between the valves. This would require using
an alternate location which would provide a weaker
signal and might subject the sensor to interference
from adjacent cylinders. Thus, it is advantageous to
have a sensor that can be placed near the center of the
cylinder in these situations.
one arrangement in the prior art that can be
used to locate a sensor near the center of a cylinder
is described in U.S. Pat No. 4,602,506 to Sawamoto et
al, issued July 29, 1986. The Sawamoto et al patent
shows an arrangement where an annular pressure sensor
is clamped to a spark plug seat by the spark plug
itself. However, although the spark spark plug may be
centrally located at the top of the combustion chamber,
the Sawamoto et al sensor measures the strain that is
placed on the spark plug threads rather than the
movement between a wall subject to combustion chamber
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pressure and a fixed wall. ThiS arrangement, then,
does not provide the advantages of the aforementioned
Frelund et al sensor. In particular, its output is
lower and tends to be non-linear. In addition, the
annular ~ensing element is loaded by the spark plug
itself. Therefore it must work with the same axial
preload and is further subject to torsional loads by
the rotation of the spark plug. It is also a burden to
have the sensing arrangement disturbed when a spark
plug has to be replaced.
Therefore, in conclusion, it is desirable to
have a combustion chamber pressure sensing arrangement
located where combustion chamber pressure loads are of
much greater magnitudes than the surrounding extraneous
forces. Ideally, the sensing arrangement should be
placed near the center of the cylinder to insure that
the sensor produces a strong linear signal and is
minimally influenced by adjacent cylinders.
Furthermore, the sensing element should be easy to
install, separated from other engine components for
independent operation and servicing, and free from
torsional loads that may cause the failure of the
sensor.
Summary of the Invention
The present invention involves the application
of a combustion pressure sensing device of the type
shown in Frelund et al in an engine component, or more
particularly, in an engine cylinder head. Structurally
the cylinder head has a first wall defining a portion
of a combustion chamber. The first wall has a mounting
boss and is capable of flexing in response to varying
combustion chamber pressure. Thus, when combustion
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chamber pressure varies, a load is transferred through
the first wall. A distance away from the first wall is
a ~econd wall. The second wall has an opening; but, in
contrast to the first wall, the second wall remains
relatively rigid when combustion chamber pressure
varies. The first wall mounting boss and second wall
opening are connected by a annular wall which defines
an access well for the mounting boss.
The sensor is mounted in the access well, and
is comprised of two main elements, an annular insert
and an annular load sensing element. The annular
insert secures the sensor in the access well and
retains the annular load sensing element for axial
loading between the first and second walls in parallel
with at least an axially compressible portion of the
annular wall. The load produced by combustion chamber
pressure is thus transferred through the first wall and
split between the annular wall and the sensor. The
part of the load transferred through the sensor is
applied by the annular insert axially to the annular
load sensing element. Consequently, the annular load
sensing element receives a predetermined portion of the
combustion chamber pressure load and thus provides an
output signal of combustion chamber pressure.
In a preferred example, the engine has a spark
plug access well centrally located at the top of each
cylinder. Thus, some advantages over the prior are
evident. First, the output signal produced is strong
and linear because the sensor is located near the
center of the cylinder, where the flexing of the first
wall is greatest. The annular load sensing element
also generates a signal that is free from adjacent
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cylinder interference. The sensor, although located in
the spark plug access well, is not physically in
contact with the spark plug. Thus, its operation and
service are independent of the spark plug. This
invention is easily implemented in existing engine
structures with only minimal extra machining operations
required. Finally, although this invention can be
considered in many ways a special case of the sensor
arrangement shown in the Frelund patent, it has an
advantage over the embodiment shown in that patent in
that it does not require breaching a coolant chamber
wall.
These and other features and advantages of the
invention will be more fully understood from the
following description of certain preferred embodiments,
taken together with the accompanying drawings.
Brief Description of the Drawings
In the drawings:
Fig. 1 is a transverse cross-sectional view of
a portion of an engine illustrating a combustion
chamber pressure sensor in accordance with the
invention;
Fig. 2 is a cross-sectional view of a lower
portion of the combustion chamber pressure sensor
showing an embodiment of the present invention;
Fig. 3 is a cross-sectional view similar to
Fig. 2 and showing an alternative embodiment; and
Fig. 4 is a perspective view of the combustion
chamber pressure sensor in accordance with the
invention.
Detailed Description of a Preferred Embodiment
Referring first to Fig. 1 of the drawing,
there is shown an internal combustion engine generally
indicated by numeral 10. Engine 10 includes a cylinder
block 11 having one or more cylinders 12. Cylinder
block 11 has an upper wall 13 having openings 14 at the
top of cylinders 12.
A cylinder head 15 is attached to block 11 by
headbolts, not shown, in the usual manner. Cylinder
head 15 has a lower wall 16 seated on block upper wall
13 with a standard engine headgasket 9 located
therebetween. Located in each cylinder 12 is a piston
17. Cylinder head lower wall 16 extends over and
closes opening 14 of each cylinder 12, while a piston
upper portion 18 closes an opening at the lower end of
each cylinder 12. Together, cylinder head lower wall
16 and piston upper portion 18 define a combustion
chamber 19 for each respective cylinder 12. Cylinder
head 15 also houses intake and exhaust valves 8 which
allow fuel mixture to enter and exhaust gases to exit
combustion chamber 19. Cylinder head 15 may be made of
aluminum or steel in the normal manner.
Cylinder head 15 further has an upper wall 20.
Cylinder head lower wall 16 includes a mounting boss
21. Mounting boss 21 has an opening 22 threaded to
receive a spark plug. Mounting boss 21 further has an
upper portion that forms a spark plug seat 23. A spark
plug 24 has a lower portion 25 threadably retained in
opening 22 and a radially wider portion 28 which
engages seat 23 in the normal manner through a standard
gasket 27. Cylinder head upper wall 20 and mounting
boss 21 are connected by an annular wall 29 to define a
spark plug access well or opening 30. Annular wall 29
is comprised of three portions: an upper portion 31, a
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lower portion 32, and a shoulder therebetween defining
an annular seat 33. Annular wall upper portion 31 is a
radially thinner portion than annular wall lower
portion 32. Located to the outside of annular wall 29
S is a coolant passage 35.
As previously mentioned, spark plug 24 ic
secured in mounting boss opening 22 at the bottom of
access well 30. A sensor 36 that measures combustion
chamber pressure is also located in access well 30 but
does not engage spark plug 24. This is an advantage
over the prior art because a different torque may be
applied to each component during installation. Sensor
36 comprises an annular insert 37. In a preferred
embodiment, annular insert 37 is comprised of a stiff
material such as stainless steel. A high stiffness to
mass ratio is required because that characteristic
provides the best output signal and also aids in high
frequency respon~e. If annular insert 37 is comprised
of the same material as cylinder head 15, an advantage
is obtained in that annular insert 37 and cylinder head
15 may respond similarly to changes in thermal
conditions and thus reduce the need to filter out
thermal effects in the sensor output signal. However,
this is not a requirement of the invention; the
stiffness to mass ratio mentioned above is more
important. Thus, annular insert 37 is made of steel
whether cylinder head 15 is steel or aluminum. With
regard to the stiffness to mass ratio, a spark plug
boss and access well, particularly if centrally located
at the top of the cylinder, appears to provide a highly
favorable such ratio for strong high frequency response
with minimal phase lag due to its low equivalent mass.
Annular insert 37 has an upper portion 38 that
threadably engages cylinder head upper wall 20 to
secure sensor 36. A torque of 25 to 50 ft.-lbs. should
be applied to annular insert 37 for full engagement to
essentially eliminate non-linearities in the output
signal due to thread compliance. Comparing this to a
typical 17 ft.-lbs. applied to a spark plug, one may
see the advantage in the separation of the sensor from
the spark plug in allowing separate torques for
optimization of each. Annular insert 37 also has a
lower portion 39 that engages annular seat 33. Annular
insert 37 holds a known annular load sensing element
40: in particular, a piezoelectric device. Annular
load sensing element 40, as shown in Figures 2 and 3,
has electrically conductive electrodes 41 and 42 on its
upper and lower surfaces between which a load related
voltage is generated. Annular load sensing element 40
is retained between annular insert upper portion 38 and
annular insert lower portion 39, with electrically
insulating annular ceramic spacers 56 and 57, so as to
be axially loaded by combustion chamber pressure. In
situations when torque is applied to sensor 36, such as
when it is being installed in or removed from access
well 30, annular insert 37 reduces the torque on
annular load sensing element 40 by transferring at
least a portion of the torque axially past element 40,
in a manner to be described. Variations of this
invention, however, may provide that annular load
sensing element 40 engage annular seat 33.
Furthermore, it should be understood that other axially
loaded sensing devices may be adapted for use in this
application.-
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Annular load sensing element 40 also has wires53 and 54 attached to electrodes 41 and 42 on its upper
and lower surfaces, such as by soldering, for
transmitting the output voltage signal. Part of wires
53 and 54 are housed in an axial channel 45 inside
annular insert 37. rig. 4 shows the remaining portion
of wires 53 and 54 wrapped around an indented annular
channel 46 of annular insert 37. Wires 53 and 54 lead
up to a connector, not shown, which is housed in a
chamber 59 at the upper end of channel 45. In
assembling the unit, the connector may be affixed to
the ends of wires 53 and 54 outside the unit; and wires
53 and 54 may then be pulled downward through channel
45 so as to pull the connector into chamber 59.
Annular load sensing element 40 may then be rotated so
that the excess lengths of wires 53 and 54 are wrapped
around annular insert 37 within annular channel 46.
Annular channel 46 may then be filled with a plastic
molding material 60 to protect the portion of wires 53
and 54 not in channel 45. It is contemplated that,
although the sensor and spark plug 24 remain separately
mounted and loaded, a common connector could be
designed and used for both.
In the operation of internal combustion engine
10, cylinder head lower wall 16 flexes in response to
the varying load of combustion chamber pressure, with
maximum movement at mounting boss 21. Cylinder head
upper wall 20, however, remains relatively rigid. This
load is thus transmitted to lower portion 32 of annular
wall 29. Annular wall lower portion 32 is not
significantly axially compressible, due to its
comparative radial thickness. Thus the load is
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transferred through annular wall lower portion 32 to
annular seat 33. From annular seat 33 the load is
split between annular wall upper portion 31 and annular
insert 37, both of which engage at their upper ends
the rigid cylinder head upper wall 20. Annular load
sensing element 40 is included in the axial load path
through annular insert 37; and the proportion of the
total axial load applied thereto depends on the
relative compressibility of annular insert 37 and
annular wall 29. Thus, annular load sensing element 40
provides a combustion chamber pressure signal that can
be used to control engine functions. This arrangement
provides an advantage over prior arrangements because
the invention uses existing structures in internal
combustion engines, and its operation does not depend
on other parts also occupying those structures. It
should also be noted that, since sensor 36 is located
further from combustion chamber 12 than spark plug 24
and is in contact with annular wall 29, which may
itself be in contact with coolant fluid, sensor 36
remains somewhat cooler than the lower portion of spark
plug 24.
Referring now to Fig. 2, there is shown an
embodiment of annular insert lower portion 39. Annular
insert lower portion 39 starts with a thin annular
extension 47 of annular insert upper portion 38 which
passes radially inside annular load sensing element 40.
Annular extension 47 ends in a radially outwardly
extending flange 48 to define a groove 49 that retains
annular load sensing element 40, ceramic insulators 56
and 57 and an annular retainer 61 made of
incompressible steel. Flange 48 may be bent around the
11
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lower surface of retainer 61 by known cold forming
processes after elements 56, 40, 57, and 61 are slipped
over extension 47. In addition, annular extension 47
is relatively axially compressible compared with the
remainder of annular insert 37 so that axial loads on
annular insert 37 are applied to annular load ~ensing
element 40; but it remains somewhat torsionally rigid
to reduce torsional loads on annular load sensing
element 40 during installation and removal of the
sensor.
Fig. 3 illustrates an alternative embodiment
of annular insert lower portion 39. Annular insert
lower portion 39 has a thin annular extension 50 of
annular insert upper portion 38 which passes radially
inside annular load sensing element 40. Annular insert
lower portion 39 in Fig. 3 also has an annular retainer
51. Annular extension 50 and annular retainer 51 have
keys 52 engaged as shown in Fig. 4 to allow axial
sliding motion but prevent relative rotation. Annular
load sensing element 40 is thus retained for axial
loading between annular retainer 51 and annular insert
upper portion 38 with annular extension 50 and annular
retainer 51 reducing torsional load on annular load
sensing element 40.
By properly machining cylinder head 15, the
time and cost of implementing sensor 36 is reduced.
Normally, the spark plug well and boss are machined in
two steps. In the first step, a tool machines the
inner diameter of mounting boss opening 22, the top
surface of spark plug seat 23, and, if required, the
inner diameter of wall 29. In a second step, the
threads of mounting boss opening 22 are tapped. For
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the manufacture of an engine equipped with a sensor
according to this invention, the first step is amended
so that the machining tool simultaneously machines the
inner diameters of upper wall portion 31 and lower wall
portion 32 as it machines the inner diameter of
mounting boss opening 22 and further machines annular
seat 33 of the shoulder between portions 31 and 32
simultaneously with spark plug seat 23. If the threads
of annular insert 37 have the same pitch as those of
spark plug 24, the second step may be modified by
simultaneously tapping mounting boss opening 22 and
upper wall portion 31 of annular wall 29. If the
threads of annular insert 37 are given a finer pitch to
decrease the torque required for fuIl engagement of
annular insert 37 with annular wall 29 for a more
linear sensor output, the tapping will probably be
accomplished in two operations. Otherwise, however,
the preparation of engine 10 for the sensor of this
invention requires only a change in tooling with no new
machining steps.
It is intended that the invention not be
limited to the embodiments described but that it have
the full scope permitted by the language of the
following claims.
