Note: Descriptions are shown in the official language in which they were submitted.
Field of the Invention
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This invention relates to a control device
for energizing dies01 engine glow plugs, and more
specifically, is directed to a thermally
25 controlled switching device for controlling the
glow plugs.
Bacl~round of the Invention
Diesel engines depend on the heating of a
combustible mixture of fuel and air by compres-
sion~ During starting of a diesel engine whenthe engine is cold, the compressive heating may
not be sufficient to produce ignition. It has
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1 therefore been the practice to provide diesel engines with
glow plugs to provide auxiliary heating during starting to
produee ignition of the air-fuel mixture. The lower the
ambient temperature conditions during the starting operation,
the greater the amount of heat that must be supplied by the
glow plugs to initiate combustion. Heretofore it has been the
practice to preheat the glow plugs by connecting the glow plugs
across the battery of the vehicle or sufficient time to allow
the glow plugs to come up to the necessary operating tempera-
ture before engaging the starter. When the engine is staxtedat relatively low ambient temperatures, such as may be
experienced in the winter time in frigid areas, the heating
time may be annoyingly long. While the heating time can be
reducea by increasing the current to the glow plugs, this may
~5 result in damage to the glow plugs due to temperature overshoot.
While automatic control circuits for diesel glow plugs have
heretofore been proposed, such known circuits have merely
controlled the ti~ing of the turning on and turning off of the
~ glow plugs, but have not controlled the temperature level of
the glow plugs as a function of ambient temperature conditions.
Summary of the Invention
The present invention provides an improved controller
for the glow plugs of the diesel engine. The controller is a
~5 thermally-operated device which operates as a thermal analog
of the glow plugs by making the thermal responsive device
subject to the same ambien~ conditions as the glow plugs. By
making the temperature profile of the glow plugs proportional
to that of the thermal responsive control device, the power
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to the glow plugs can be controlled t:o match the
desired heating characteristics. The thermal switch
device is mounted in the block of the engine so as to
be subject to the same ambient temperature conditions
as the glow plugs. ~hen the engine is cold and the
ambient temperature low, the initial time during which
the thermal switch remains closed is substanti~lly
longer than when the engine is hotter and the ambient
temperature is higher. As a result, the glow plugs are
10 brought to a higher temperature when the engine is cold
than when the ambient temperature is higher. The ther-
mal switch allows the glow plugs to be heated more
rapidly, since the thermal switch limits the maximum
temperature which the glow plug can reach during the
initial heating. The temperature of the glow plugs is
then held substantially a-t a constant level by the
cycling of the thermal relay switch. A thermal timing
switch, also subject to the same ambient temperature
conditions, controls the length of time the glow plugs
remain on after the engine is started. Because the
thermal timer is subject to ambient temperature con-
ditions, the time the glow plugs is on is increased as
the ambient temperature decreases. In addition, a
thermal responsive circuit breaker switch mounted with
the other two switches and subject to the same ambient
temperature conditions operates to shut off the primary
controller if the circuit draws excessive current, or
if the primary controller does not function within a
predetermined time interval.
These and other features and ad~antages
of the present invention are achieved by providing
a thermal control device for controlling the
heating of diesel engine glow plugs. The
thermal control device includes a base, with a
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first bimetal ele~ment supported at one end on the base,
and having a first electrical heater means adjacent the
surface o~ the Eirst bimetal element for heating the
bimetal element. A second bimetal element is also
5 supported at one end on the base and has a second
electrical heater means adjacent the surface of the
second bimetal element. Swit:ch means are operated by
each of the bimetal elements" The switch means have
normally closed contacts, and are connected in series~
lO The thermal control device also includes means respon-
sive -to a current through the normally closed contacts
for turning on the glow plugs, so that either bimetal
element, when heated, interrupts current to the glow
plugs. The thermal responsive time of the first
bimetal element and its first heater means and switch
means is substantially faster than the responsive time
of the second bimetal element and its second heater
means and switch means.
- Description of the_D awings
For a more complete understanding of the inven-
tion reference should be made to the accompanying
drawings, wherein:
FIG. l is a schematic wiring diagram of the glow
plug control circuit of the present invention;
~ 25 FIG. 2 is a perspective view of the control
-~ assembly with the housing removed;
FIG. 3 is an exposed view showing the assembly
frame removed from -the base receptacle;
7 ~ r)
FIG. 4 is a partial view oE the outer encl
of the bimetal rame assembly;
FIG. 5 is an exploded view of the main
bimetal assembly;
FIG. 6 is an exploded view showing the
manner in which the after glow timer is assembled
and mounted on the base xeceptacle;
FIG~ 7 is a performance dlagram used in
explaining the operation of the present invention;
and
FIG, 8 is a partial view oE the contact of
the primary switch element.
Detailed Desc~~ption
~ eferring to the circuit diagram of FIG. 1,
the numeral 10 indicates generally the glow plug
controller unit of the present invenkion which
controls a group of glow plugs, two of which are
indicated at 12, from a battery 14 through a relay
16. The relay includes a solenoid coil 18 and a
normally open switch 20~ When the coil 18 is
energi~ed, the switch 20 is closed, completing a
circuit between the battery 1~ and the glow plugs
12. The relay coil 18 is operated from the bat-
tery 14 by the controller unit 10 when an ignition
switch 22 is closed. The controller uni-t 10 is
connected electrically to the rest of the circuit
through a terminal 24 preferably having six
~ contacts.
; The controller unit 10 includes three ther-
30 mally-operated switch elements, the primary switch
element 26j the timer switch element 28, and the
circuit breaker switch element 30~ The design of
the controller unit 10 with each of these three
swi-tch assemblies 26, 28r and 30 is described
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below :in detail. The p:rima:ry switch element 26
includes a bimeta:L element 32 which ope:rates nor-
mally closed switch contacts 34. A :resi.stance
heater element 36 heats the bimetal element 32 to
5 open the normally closed switch contacts 34.
Similarly the timer switch element 28 includes a
bimetal switch membe.~ 38 which operates normally
closed contacts 40. The bimetal member 38 is con-
trolled by a heater 42 whlch heats the bimetal
10 member 38 to open the normally closed contacts 40.
The circui-t breaker switching element 30 includes
a bimetal membe:r 4~ which operates normally closed
contacts 46. A resistance heate:r element 48
applies heat to the bimetal membe:r 44 to open nor-
15 mally closed contacts 46. An auxiliary resistance
heating element 50 in parallel with the switch
contacts 46 ope:rates to hold the switch contacts
in the open position once the current path through
the switch is interrupted.
The normally closed contacts 34, 40, and 46
are connected in series with the relay coil 18 and
the circuit breaker heater 48 across the battery
14 when the switch 22 is closed. Thus when the
ignition switch 22 is closed, the relay 16 is
25 operated, closing the switch 20 and providing CUF~
rent to the glow plugsO Closing of the switch 20
also causes the heater 36 of the primary switch
~: element 26 to be connected across the battery.
This causes both the glow plugs 12 and the bimetal
30 member 32 heated by the heater 3h to begin heat~
- ingO A plot of temperature as a function of time
is shown in FIC. l~A, with to representing -the
time the switch 22 is closedO It will be seen
that the temperature of the glow plugs and the
35 temperature of the heater rise exponentially, the
temperature of the glow plugs increasing more
rapidly than that oE the temperature of the
bimetal member 32. At a time tl, the
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heat applied to the b.imeta] membe:r 32 of the pri
ma:ry switch element 26 i.s sufficient to cause the
normally closed contacts 3'1 to open, thus inte:r-
rupting the flow of cur-rent th:rough the relay coil
18, opening the switch 20 and inter:rupting cu:rrent
flow to the glow plugs 12 and the heater 36.
Thus, as shown in FIG. l-.A, the tempe:rature of th0
glow plugs and the bimetal member 32 fall togethe:r
exponentially. At time t2 the temperatu:re of the
bimetal membe:r 32 of the primary switch 26 has
cooled sufficiently so that the contacts 34 are
again closed, re-energizing the relay 16 and
causing the glow plugs 12 and bimetal membe:r 32 to
again :rise in tempe:rature until, at time t3, the
contacts 34 again open. The cycl.es of alte:rnate
heating and cooling continue in this manner until
the time out period tt at which the thermal timer
switch element 28 operates to open the normally
. closed contacts 40, releasing the relay lfi and
20 turni.ng off the glow plugs 12.
The timer element 28 is operated by con-
- necting the heater 42 across an alternator 52
driven by the diesel engine (not shown) in which
- -the glow plugs are mounted. Thus the timer ele-
.. 25 ment 28 begins to Eunction only when the diesel
engine begins to drive the alternator 52 to pro-
. duce a voltage which is applied across the heater
42. The timer element 28 has a built-in thermal
` delay which causes the contacts 40 to be opened by
30 the bimetal member 38 after a predetermined time
interval after power is applied to the heater 42
The circuit breake:r switch element
30 responds to the flow of current through
the relay coil 18. If, due to some malfunc-
35 tion, the relay coil remains energized duringthe initial period fo:r an inordinate length
of time rather than cutting off at time tl,
.. the bimetal member 44 of the breaker
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element 30 will be heated by the heater 48 suf-
ficiently to cause the contacts 46 to open. This
prevents the glow plugs 12 from becoming damaged
by overhea-ting. Also, if the level of current
through the relay coil is too high for any reason,
the breaker will also interrupt the current
through the relay. After the contacts 46 open, a
small current through the higher resistance auxil-
iary heater 50 ho]ds the contacts open until the
ignition switch 22 is turned off. A suitable
warnin~ light (not shown) may be provided which is
- activated when the contacl:s 46 are open to indi-
cate that the breaker has responded to a malfunc-
tion.
The construction of the controller unit 10
is shown in detail in FIGS. 2~6. The controller
unit 10 includes a base receptacle 54 of insu-
lating material which is adapted to receive a
standard cable connector. A housing 56 fits over
the base receptacle 54 to Eorm a sealed chamber in
which the switching element assembly 58 is
mounted. The outer end of the housing 56 is pro-
vided with a tapered external thread 60 by which
it can be mounted in the engine block. The
housing 56 is made of metal to provide yood ther-
mal conductivity with the engine block. As b~st
seen in FIGS. 3 and 5, the thermal switch assembly
58 includes a substantially channel-shaped metal
frame 62 having a pair of parallel side members
64 and 66 joined at the outer end by a bridging
member 68 and at the other end by a support member
70 connected to the side members 64 and 66 by
integral connecting tabs 72 and 74. A molded
plastic contact support block 76 fits between the
side members 64 and 66 and is held in place by
- projecting lugs 78 which engage openings 80 in the
side members of the frame. The block 76 supports
two electrical contacts 82 and 84. Both
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contacts extend th.rough the block, the contact 82
being threaded into the block 76 so tha~ it can be
adjusted :i.n and out :relative to the block.
A bifu:rcated bimetal blade 86 provides two
switch arms 88 and 90. The blade 8Z is welded or
otherwise secured to the support plate ~0 of the
frame 62 so that the two switch arms 88 and 90 a:re
supported in cantilever fclshion. '.rhe a.rm 88 is
provided with a sp:ring contact leaf 94 which is
spot-welded or otherwise secured at one end to the
bimetal arm 88, as indica~.ed at 96. The othe-r end
of the contact leaf 94 has a p:rojecting lug 98
which engages a slot ].00 in the bimetal arrn 88.
The contact leaE suppo:rts an electrical contact
102 which is positioned to engage the contact 84
carried by the block 76. At the ambient tem-
peratures encountered when the engine is not
warmed up, the bimetal arm 88 moves the contact
leaf 94 and con-tact 102 against the contact 84.
20 The contacts 84 and 102 form the normally closed
contacts 34 of the primary switch element 26 of
FIG. 1, while the bimetal arm 88 ~orms the bimetal
32 of the p:rimary switch element 26, shown in FIG.
. ~
A heater element in the form of a
serpentine-shaped printed circuit conductor 104
terminating at either end in flat output leads 106
is applied to the surface of the arm 88. A very
thin layer of electrically insulating mate:rial is
interposed between the printed circuit heater ele-
ment 104 and the metallic surface o~ the bimetal
arm 88 to provide electrical isolation between the
heater and the bimetal arm while p:roviding good
thermal transfer. The printed circuit element
104 forms the heater 36 of the primary switch ele-
ment 26, shown in FIG. 1.
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As the bimetal arm 88 is heated by passing
current through the heater 104, the bimetal moves the
contact 102 away Erom the contact 8~. A U-shaped
spring 107 is interposed between the outer end of the
bimetal arm 88 and the cross member 68 of the frame
58. The U-shaped spring :is held in place by slots
109 which engage projections 110 and 112 on the end
of the arm 88 and cross member 68, respectively. The
spring 107 provides an overcenter snap action to the
10 movement oE the bimetal arm 88 as it bends back and
forth by heating and cooling. A pair of notches 11~
in each of the side members 64 and 66 of the frame 62
allows the cross membee 68 to be bent slightly with
respect to the rest of the frame, providing an
adjustment to the temperature at which the snap
action takes place and thereby controlling the time
; period of the On-OfE cycle. The contact leaf 94
insures that closed contact pressure is maintained
bet~1een contacts 102 and 84 until the moment the
bimetal arm 88 moves sufficiently to initiate the
snap action. Thus the contact Eorce remains high
until the snap action is in process. This action
greatly extends the contact life of the primary
switch element. A stop 115 on the frame 62 limits
the movement of the bimetal arm 88 in a direction
away from the contact 84,
The bimetal arm 90 forms the bimetal element
44 of the circuit breaker switch element 30 shown
in FIG. 1. The heater 48 of the circuit breaker
28 is provided by a thin metal conductive strip
116 applied to the outex surface of the bimetal arm
90, a thin electrically insulative layer being
superimposed between the conductive strip 116
and the bimetal surEace. A contac-t 117 is secured
adjacent the outer end of the bimetal arm 90 on
the opposite side from the heater in position
to engage the contact 82 of the contact support
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7 7 ~1 5
1 assembly. The contacts 82 and 117 form the normally closed
contac.s 46 of tne circuit breaker switch element 30. A
conductlve strap 118 connects the heater to the back o~ the
contact 82, thus providing a current path through the heater
strip 116, normally closed contacts 82 and 117, and through
the bimetal arm 90 to,the bimetal arm 88. A wire wound coil
122 is wrapped around an insulating sleeve 124 on the bimetal
arm 90. The wire coil 122 operates as the holding heater 50,
. shown in the wiring diagram of FIG. l. One end of the heater
coil 122 is connected to the conductor strap 118, while the
other end is connected to the bimetal arm 90.
.. . As shown in FIG. 3, the entire bimetal switch and frame
assembly is supported on the base recept.acle 54. To this end
the base receptacle has an integrally molded support 126 which
:~ 15 fits between ~he sidè members 64 and 66 of the fra~e 62.
: Rivets 128 are inserted through aligned holes extending thxough
. the frame and the support 126 to secure the frame 62 securely
to the base receptacle 54. The heater leads 106, 108 in the
end of the heater strip 116 are then spot-welded or otherwise
secured to electrical terminals extending from the base
receptacle 54. An adjustment screw 130 is threaded through
.. the support 126 and engages the back of the support plate 70
of the ~rame 62. This screw provides an adjustment for
contxolling the length of the initial On time (to to tl in
FIG. l-A) r
The afterglow timer switch element 28 is mounted as a
separate assembly on the support 126 of the base receptacle 54,
as best seen in FIGS. 3 and 6. The timer element 28 includes
a himetal member 136 which is riveted or otherwise secured to
the support 126 adjacent one end. The other ~nd is bent in a
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U-shape portion 13~ and has a contact 140 secured to the
folded end. The contact 140 normally engages the contact 84
mounted in -the contact support block 76. Thus the fixed
contact 84 provides a direct electrical connection between
S the moving contact 10~ of the primary swltch element and the
moving contact 140 of the timer switch element. A sleeve 142
is positioned on the bimetal member 136 to increase the
thermal mass of the timer element. A wire-wound resistance
heater 144 is wrapped around the sleeve 142 and the ends of
the heater are connected to appropriate electrical terminals
of the base receptacle 54.
The effect of ambient tempexature changes on the
operation of the primary thermal switch element 26 is shown
in FIG. 7. Curve A shows the change in the cycle period as a
function of ambient temperature. It will be ~seen that the
cycle period becomes substantially longer as the ambient
temperature approaches the operating temperature or the engine
block. Curves B and B' show the duty cycle (percentage o
each period that the glow tubes are turned on) as a function
of ambient temperature. It will be seen that as the ambient
temperature approaches the operating temperat~lre o~ the engine
the duty cycle decr~ases tc>wards zeroO Finally,-curves C and
C! show the range over which the time to first cut-off varies
as a function of ambient temperature. Again it will be seen
that as the ambient temperature approaches the o~erati~g
temperature of the ensine, the initial heating time of the
glow tubes is reduced to zero. The desired characteristics
are achieved by controlling the thermal mass and heat
dissipation of the bimetal element, the resistance of the
heater, the areas of the bimetal element that are heated, the
.
1~67i':15
1 temperature differential for the snap-action, and the screw
adjustment for controlling the initial timp for the switch to
open. The time period is controlled by bending the position
of the bridging member 68 to change the snap~action of the
spring 107. The duty cycle is automatically achieved by the
choice of bimetal heat capac:ity,and thermal dissipation
properties. Because both the thermal relay and the glow plugs
operate in unison and are subject to the same voltage changes
o the source, the temperature of the thermal switching
element and the glow plugs are proportional, as shown by
.~ FIG. l-A.
While the described control device is paxticularly well
. suited to thermal control of glow plugs ~7hich operate in
: rapidly changiny am~ient temperature conditions due to engine
heating, the control device may be used to control other types
of electrical loads, such as electric window defrosters, and
the like. The control de~ice, unlike conventional thermostatic
= . controls, does not directly sense the temperature change of
the load, but operates as a'thermal analog which heats and cools
~0 in proportion to the heating and cooling of the load, and yet
responds to changes in the ambient conditions of the loadO
MRS:bjn
, 30
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