CA1040288A - Catalytic heater control - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An electronic control for a catalytic heater device for preheating liquid coolant of an internal combustion engine. The control has a preheat circuit for energizing a resistance wire element for a period of time to heat a catalytic material of the heater de-vice to an operating temperature, a pump circuit for cycling an electric pump to supply liquid fuel to the heater device, and an inhibiting circuit to prevent operation of the electric pump if the heater device is functioning improperly.

Description

1C)40~

This invention relates to a catalytic heat-er device for preheating the liquid coolant of an in-ternal combustion engine and more particularly to an electronic control for such a device.
Objects of this invention are to provide a control for a catalytic heater device which protects the device from becoming damaged if the device func-tions improperly and is compact and of economical manufacture and assembly, and has a long maintenance-10 free service life.
These and other objects, features, and ad- ~ ~ -vantages of this invention will be apparent from the following description, appended claims, and accompany-ing drawings in which:
FIG. 1 is a side view partially in section of a catalytic heater which is cycled by a control device embodying this invention.
FIG. 2 is a semi-diagrammatic view of the catalytic heater of FIG. 1 connected to both a con-trol device embodying this invention and a fuel pumpsupplying gasoline to the catalytic heater.
FIG. 3 is a sectional view of the fuel pump ~ ~
of FIG. 2. -FIG. 4 is a schematic diagram of an elec- ~ -tronic control device embodying this invention. ~
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~eferring in more detail to the drawings, FIG. 1 illustrates a catalytic heater 10 having a heat exchanger jac~et 12 encircling a catalytic heat-er assembly 14. Heat exchanger 12 has a cylindrical chamber 16 defined by an inner metallic tube 18, an outer metallic tube 20 encircling the inner tube, and metallic end rings 22 interposed between the tubes ~ -and fixed thereto, such as by brazing. Engine liquid coolant enters chamber 16 through an inlet conduit 28 fixed to outer tube 20 adjacent the lower end of cham-ber 16 and flows from chamber 16 through an outlet con- -duit 30 fixed to outer tube 20 adjacent the upper end of chamber 16. A check valve 32 in inlet conduit 28 ~ -prevents liquid coolant from flowing out of chamber 16 through the inlet conduit. A plurality of slots or windows 34 and 36 adjacent opposed ends of inner tube 18 allows atmospheric air to flow through the tube ad- ;
jacent the outer periphery of heater assembly 14.
Heater assembly 14 has a porous ceramic tube 38 with upper and lower ceramic end caps 40 and 42 fixed thereto. A layer of a fibrous material 44 is ~ -wrapped around ceramic tube 38 and impregnated with a catalytic agent, such as platinum. A suitable fibrous material is the ceramic felt T-3 fiber produced by Re-fractory Products Company of Carpentersville, Illinois.
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-11~4~Z~8 A nichrome wire 46 extends axially along the out-side of tube 38 beneath the layer of fibrous mate-rial 44 to provide an electrical resistance heating element for preheating the catalytic heater assembly.
One end.of nichrome wire 46 is connected to an insu-lated binding post 48 mounted on tube 18 and the other end of wire 46 is connected to tube 18 and a ground lead wire 50 by a machine screw 52.
A temperature responsive or thermostatic switch 56 with insulated lead wires 58 and 60 connect-ed thereto is potted on upper end cap 40 by a suitable potting material 56, such as Acid-Alk Mortar No. 33 available from Sauereisen Cements Company of Pitts- ~ -burgh, Pennsylvania. A fuel injector 62 with a union 64 and a fuel screen 65 at the upper end of an evap-orator tube 66 extends through and is fixed to the upper end of heater assembly 14 to discharge vaporized fuel into the interior of ceramic tube 38. To prevent high winds from extinquishing the catalytic reaction of fuel with heater assembly 14, a tubular shield 68 on the upper end of heat exchanger 12 extends substan- .
tially above exhaust or outlet ports 36. Shield 68 i8 received over the upper end of outer tube 20 of heat exchanger 12 and retained thereon by a band or hose -clamp 70. .
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-~ OZ~8 To receive and vaporize any liquid fuel discharged from fuel injector 132, a secondary evaporator assembly 72 is coaxially received in the ~
lower end of heater assembly 14. Evaporator assembly :
72 has an evaporator tube 74 into which droplets of fuel discharged from tube 66 are funneled by a flared .
upper end 76. Evaporator tube 74 is fixed to the low- :
er end of heater assembly 14, extends through lower ~- --end cup 42, and is closed at the lower end thereof by a plug 78 received in a union 80 fixed thereto. : -~
Heater assembly 14 is mounted substantially coaxially in heat exchanger 12 by an upper locator ..
disc 82 and a lower locator disc 84. Upper locator ~ . -disc 82 both bears on the upper end of heater assembly 14 and abuts on three circumferentially spaced, inward-ly struck tabs 86 in the upper end of inner tube 18. : .
Heater assembly 14 is yieldably urged into engagement with upper locator disc 82 by a spring 88 interposed between lower locator disc 84 and end cap 42 of the ; .
heater assembly. Lower locator disc 84 is releasably . ...
retained in tube 18 by three circumferentially spacedtabs 90 received in three circumferentially spaced re-entrant slots 92 in the lower end of tube 18.

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104028~3 As shown in FIG. 2, when catalytic heater 10 is installed in a motor vehicle, gasoline may be supplied to injector 62 under pressure by a separate electric fuel pump 94 with an inlet connected by a line 96 through a tee 98 to the fuel line 100 of the vehicle (not shown) between the gasoline tank and the '' fuel pump of the internal combustion engine. The out-let of pump 94 is connected to injector 62 through a fuel line 102. As indicated by arrow 104 (FIGS. 1 and 2), catalytic heater 10 should be mounted so that the longitudinal axis of heater assembly 14 extends generally vertically with fuel injector 62 at the up-per end thereof. m e catalytic heater 10 is mounted at a vertical height between the vertically highest and lowest points in the cooling system of the engine -and preferably closer to the highest point thereof so that the liquid coolant will be circulated by a thermo-siphon action through the heater and engine when the heater is operating. m e inlet conduit 28 of heat ex-- 20 changer 12 is connected by a hose 106 to the engine -cooling system at a low point thereof such as the en- -~
gine drain opening or through a core plug in the side -~
of the block of the engine. m e outlet conduit 30 of heat exchanger 12 is connected by a hose 108 to the engine cooling system at a higher point than the , _5_ .: .

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.,''', .' ' ~ ':; .~ ' ' ' ,, ' ~ ' ' ', ' . ' . ` . ' 1t~4~)Zt~8 inlet 28 such as at the connection of the heater in-let hose to the cooling system of the vehicle.
The useful life of the catalytic agent of ~ -catalytic heater 10 is believed to be substantially decreased by contact of the catalytic agent with liquid fuel, and hence, no fuel should be supplied to the catalytic heater when it is not being operated.
However, it has been discovered that the main fuel pump of at least some internal combustion engines creates sufficient pressure surges or pulses in fuel line 90 to force fuel through conventional electric fuel pumps utilized with catalytic heater 10 and thence into the catalytic heater when it is not oper-ating, thereby decreasing the useful life of the cata-lytic agent thereof. This may be prevented from hap-pening by using conventional valves with a convention-al electric pump supplying fuel to the catalytic heat- - ~
er by arranging the valves to prevent fuel from flowing ~ -through the conventional pump when the catalytic heater '-is not operating. However, it is preferred to use a ~
specially designed pump, such as pump 94, with a suit- ~ -.
able valve arrangement incorporated directly therein.
As shown in FIG. 3, pump 94 has a housing 110 with a flexible diaphragm 112 received therein and under-lying a pump chamber 114 in a carrier plate 116 fixed in '' .: '' .
', 1~40Z~8 the housing. Gasoline is admitted to pump chamber 114 through inlet conduit 118, inlet valve assembly 120, and passageway 122 through carrier plate 116 and discharged from the pump chamber through outlet pass-S ageway 124 in carrier plate 116, outlet valve assembly 126 and outlet conduit 128. The gasoline is moved through pump chamber 114 by the flexing of diaphragm 112 which is actuated by an armature 130 connected thereto, yieldably biased in one direction by a spring 132, and moved in the opposite direction by energiza-tion of solenoid coil 134. The length of the stroke of armature 130 and hence the quantity of fuel deliv-ered on each discharge stroke of pump 94 is controlled by adjustment of a threaded stop screw 136 received in -- 15 a nut 138 fixed to housing 110.
Gasoline is prevented from flowing through pump 94 when solenoid coil 134 is de-energized by a valve assembly 140 mounted on carrier plate 116 and shown in FIG. 3 in the open position with coil 134 energized. Valve assembly 140 has a valve 142 with a bulbous stem 144 received for reciprocation in a coun-terbore 146 in carrier plate 116 to close and open (as shown in FIG. 3) fuel outlet passage 124. Valve 142 has a metallic wear cap 148 on the lower surface there-of and is yieldably biased by a spring 150 received in :: . . :................. - .

1~40Zl38 an annular pocket 152 into engagement with armature 130 for reciprocation therewith.
In operation of pump 94 energization of coil 134 moves armature 130 to the position shown in FIG. 3, thereby opening valve 142 and flexing dia-phragm 112 to pull gasoline into pump chamber 114 through inlet conduit 118, inlet valve 120 and pass-ageway 122. When coil 134 is de-energized, spring 132 moves armature 130 upwardly from the position shown in FIG. 3, thereby moving diaphragm 112 upward- ~:
ly to discharge fuel from pump chamber 114 through passageway 124, outlet valve 126 and outlet conduit 128, and, upon completion of the discharge stroke of diaphragm 112, closing valve 142 against the bias of spring 150. m us, valve 142 remains closed so long as coil 134 is de-energized, thereby preventing gaso-line from being forced through pump 94 when the pump .-is not operating by pressure surges or pulses in the fuel line 100 to which inlet conduit 118 is connected.
In accordance with this invention, a control :
box 156 with an electronic circuit 158 shown schematic-ally in FIG. 10 cycles catalytic heater 10 and fuel pump 94. Power is supplied to a DC power bus 160 of .. -control circuit 158 from a battery 161 of the vehicle -8~

l~E40Z~8 through a fuse 162 and a master power switch 164 with an indicator light 166 connected by lead lines 168, 169 and 170.
Circuit 158 has a preheat control section 172 which is responsive to application of battery power to bus 160 to apply power to heating element 46 for a predetermined time interval to preheat the cata-lyst of heater assembly 14. Control section 172 com-. .
prises a comparator 174 having a reference input 176 connected through a resistor 178 to bus 160, and hav-ing a threshold input 180 connected through a resistor 182 to bus 160 and through a capacitor 184 to ground 186 of the vehicle frame. A latch 188 receives a ~
first input 190 from the output 192 of comparator 174, -and a second input 194 from bus 160 directly. Latch 188 has an output 196 connected through the anode-cathode junction of a silicon diode 198 and thence through a coil 200 of a relay 202 to ground 186. A
second diode 204 is connected in reverse polarity across coil 200 to suppress inductive ringing in the -coil when relay 202 turns off. Relay 202 also has a . ~ . .
pair of normally open contacts 206, 208 connected to bus 160 and preheat element 46, and responsive to energization of coil 200 to apply battery power di- ~ ~ -,: . .
rectly to heating element 46.

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10402l38 In the operation of preheat control sec-tion 172, the output of latch 188 is initially set to a high voltage state by application of battery power to bus 160. This relatively high voltage for- -ward biase~ diode 198 so that relay 202 is energized and preheat current is supplied to element ~6. At the same time current flows through resistor 182 and into capacitor 184 so that a charge is gradually built up thereupon. After an interval of time deter-mined by the values of resistor 182 and capacitor 184, the voltage on the capacitor at threshold input 180 of comparator 174 exceeds the reference voltage at input 176 so that the comparator provides a reset signal at input 190 of latch 188. The latch is thus reset so that the output thereof goes low and relay 202 is de-energized until master switch 164 is turned off and then on again to reset latch 188 to a high voltage state. The preheat time interval determined by re-sistor 182 and capacitor 184 may be in the range of ten to fifteen minutes and is preferably in the range of twelve to fourteen minutes.
Control circuit 158 further comprises a pump control section 210 which includes a first comparator -; -212 having its reference input 214 connected through a ~
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resistor 216 to bus 160, and through series connected , -10-. , , .. . .. ; . . ,............ ~ .

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104()Z~8 resistors 218, 220 to ground 186 of the vehicle frame.
A filter capacitor 230 is connected across resistor 220. The threshold input 232 of comparator 212 is connected through series connected resistors 234, 236 to bus 160, and throug~ a capacitor 238 to ground 186. ~
A second comparator 240 has its threshold input 242 :
connected to threshold input 232 of comparator 212, and has its reference input 244 connected to the junction of resistors 218, 220. Resistors 216, 218 and 220 thus form a voltage divider which places a ~ ~ .
first reference voltage at reference input 214 of comparator 212, and a second lower reference voltage at the reference input 244 of comparator 240. A re-sistor 222 and a normally open pushbutton switch 228 are connected in series across resistor 236. switch 228 i8 mounted on control box 156 and may be manually activated to prime pump 94 as explained in detail here- -~
inafter. ~;
A latch 246 has a first input 248 connected ~-to the output 250 of comparator 212, and has a second ~input 252 connected to the output 254 of comparator . -- 240. The output 256 of latch 246 is connected through a resistor 258 to the base of a PNP transistor 260 .

1040Z~38 which has its emitter connected to bus 160 and its collector connected through the solenoid coil 134 of pump 94 to ground 186. A diode 262 is connected in reverse polarity across coil 126 to suppress in-ductive ringing in the coil when transistor 260 turnsoff. Output 256 of latch 246 is also connected through a resistor 272 to input 176 of comparator 174, and to a first input 264 of a discharge switch 266 which has a second input 268 connected to the junc-tion of resistors 234, 236 and an output 270 connect-ed to ground 186.
In the operation of pump control section 210, output 256 of latch 246 is set high when battery power is initially applied to bus 160 so that the emitter-base junction of transistor 260 is reverse biased and current is blocked from coil 134 of pump 94.
In the meantime current passes through resistors 236, 234 into capacitor 238 so that the voltage at threshold input 232 of comparator 212 gradually rises. When this rising voltage exceeds the reference voltage supplied at input 214 by voltage divider 216, 218, 220, compa- -rator 212 supplies a reset input to latch 246 so that output 256 goes low, transistor 260 conducts and cur-rent is supplied to coil 134 of pump 94. At the same 1()4U2~8 time, input 264 of discharge switch 266 goes low so that switch 266 discharges capacitor 238 through re-sistor 234 and discharge switch input 268 to ground 186. When the decreasing voltage on capacitor 238 at threshold input 242 of comparator 240 drops below the level of the second lower reference voltage sup-plied at reference input 244 by voltage divider 216, 218, 220, comparator 240 provides a set signal at in-put 252 of latch 246 so that the output thereof goes high thus turning off transistor 260 and blocking cur-rent flow to coil 134 of pump 94. Discharge switch 266 is also turned off at this time so that capacitor 238 may recharge. Thus, pump control circuit 210 operates as an oscillator which has a period and duty -15 cycle dependent upon the relative values of resistors 216, 218, 220, 234, 236 and capacitor 238. In the preferred embodiment of catalytic heater 10 the charge time of capacitor 238 through resistors 236, 234 may ~ ...... .....
be in the range of 5 to 15 seconds and preferably is about 10 seconds while the discharge time thereof thro~gh resistor 234 and switch 266 may be in the range of 30 to 80 milliseconds and preferably is about 50 to 60 milliseconds. Thus, current is supplied to coil 126 of pump 94 preferably for 50 to 60 milli-seconds at 10-second intervals. -.''' ,, ~. ~.'.

1(~402~8 To prime fuel pump 94 a switch 228 is closed to place resistor 222 in parallel with re-sistor 236, thereby decreasing the charge time for capacitor 238 and increasing the rate at which pump 94 is cycled. m e decreased charge time for capaci-tor 238 when control section 210 is operating in the pump-priming mode may be in the range of 200 to 700 milliseconds and is preferably about 400 to 500 milli-seconds.
Control circuit 158 also includes a pump inhibit circuit 274 which is responsive to the temp-erature of catalytic heater assembly 14 to selectively -inhibit activation of pump control section 210 if heat-er device 10 is not functioning properly. Circuit 274 includes a first NPN transistor 276 which has its base connected through a resistor 278 to the cathode of diode 198, and its emitter connected directly to ground 186. The collector of transistor 276 is connected through a resistor 280 to bus 160, and to the base of a second NPN transistor 282. The base of transistor 282 is also connected to ground 186 through normally open thermal switch 56 which is mounted on heater assembly 14 and responsive to the temperature thereof to connect the base of transistor 282 directly to ground.

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1~46~21~8 The emitter of transistor 282 is conneced to ground and the collector thereof is connected to the junc~
tion of resistor 234 and capacitor 238. Thus, when latch 188 supplies preheat current to element 46, transistor 276 is turned on and transistor 282 is turned off so that capacitor 238 is allowed to alter-nately charge and discharge as described above. If heater 10 is operating properly, catalytic heater assembly 14 will be p-eheated to a temperature at which thermal switch 56 closes thereby connecting the base of transistor 282 to ground independently of pre-heat section 172 before the preheat section 172 turns off. However, if the temperature of heater assembly 14 is below the temperature at which switch 56 closes when preheat section 172 turns off, or if the heater assembly temperature should thereafter fall below such predetermined temperature, switch 56 will be open ~ ~-and current will be supplied to the base-emitter junc- -tion of transistor 282 through resistor 280. Transistor 282 will then be turned on to drain substantially all of the charge from capacitor 238, and to thereby inhi- .
bit further operation of pump control section 210 and ~: ;
hence pump 94.

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lU4~2~8 Since the useful life of the catalytic agent is decreased by contact with liquid fuel, the ' r quantity of liquid fuel supplied to catalytic heater 10 during operation thereof should be controlled within close tolerances to assure that an excess -quantity of liquid fuel is not supplied to the cata-lytic heater. The quantity of liquid fuel supplied to heater 10 is controlled by adjustment of stop screw 136 of pump 94 and the rate of cycling of fuel pump 94 by pump control section 210 of control cir-cuit 158.
In using catalytic heater 10 to preheat the engine coolant of an internal combustion engine, mas-ter switch 162 is manually actuated to energize con- ~ -trol circuit 158 to supply current to resistance heat- .
ing element 46. Heating element 46 preheats at least portions of the catalytic agent of heater assembly 14 to an operating temperature which would produce a cata-lytic heat reaction with fuel discharged into the heat-~20 er assembly. 1he preheat control section 172 of circuit lS8 controls the length of time current is supplied to heating element 46 and turns the heating element off -after sufficient time has elapsed for at least portions :~
of the catalytic agent to be heated to an operating ~. .

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l~UZ~8 temperature. Prior to the heating element being turned off by control section 172, fuel pump 94 is energized to discharge fuel through injector 132 into heating element 14 by the pump control section ~:
210 of control circuit 158 and, if catalytic heater 10 is functioning properly, temperature responsive ~ :
switch 56 closes to assure continued cycling of pump 94 to supply fuel to the catalytic heater. m e cyclic or pulsating flow of liquid fuel supplied by pump 94 .
to injector 62 is smoothed or evened out by fuel ~
screen 65, at least partially vaporized by evaporator ~ -tube 66, and discharged therefrom into heater assembly ~-14. m e vaporized portion of the fuel under the in-fluence of the catalytic agent of heater assembly 14 undergoes a catalytic reaction producing heat which warms liquid coolant ln heat exchanger 12. If the fuel is not completely vaporized by evaporator tube 66, the liquid portion thereof will drop by gravity into secondary evaporator assembly 72 from which it will be subsequently vaporized and discharged into - ~
the interior of heater assembly 14 to undergo a cata- ~ -lytic reaction producing heat to warm the liquid cool-ant in heat exchanger 12~ Liquid fuel is vaporized in , . . .; . , ~ - j, - . . : . ~ , - . : - -- , , i, . .. , '. .. .

16~40;~8 secondary evaporator assembly 72 because evaporator tube 74 is maintained at an elevated temperature by the heat produced by the catalytic reaction. Heating of the liquid coolant in heat exchanger 12 produces a thermosiphon action to circulate the liquid coolant of the engine through heat exchanger 12 and thus pre-heat the engine to facilitate starting thereof in cold weather.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1.
In combination with a catalytic converter of the type in which a hydrocarbon fuel is flamelessly oxidized in the presence of a catalytic material for preheating liquid coolant of an internal combustion en-gine, a fuel pump to provide said hydrocarbon fuel to said converter, said fuel pump including pump activat-ing means responsive to the application of electrical power to supply said fuel to said converter, and oscil-lator circuit means responsive to connection to a source of electric power to cyclically apply electric power to said activating means to cycle said fuel pump.

2.

The combination set forth in claim 1 further comprising means disposed in heat-transfer proximity to said catalytic material and responsive to the tempera-ture of said material to disable said oscillator cir-cuit means if after a predetermined period of time said temperature is below a predetermined temperature.

3.
The combination set forth in claim 1 where-in said oscillator circuit means comprises an elec-tronic switch having first, second and control elec-trodes, said first and second electrodes connecting said pump activating means to a source of electric power in response to a control signal at said control electrode, and an oscillator connected to a source of electric power to provide said control signal, said oscillator having a first predetermined duty cycle.

4.

The combination set forth in claim 3 wherein said first predetermined duty cycle of said oscillator is effective to apply electric power to said energiz-ing means at intervals in the range of five to fifteen seconds for an amount of time in the range of 30 to 80 milliseconds.

5.
The combination set forth in claim 3 wherein said oscillator circuit means further comprises means electrically connected to said oscillator to change the duty cycle thereof from said first duty cycle to a sec-ond higher duty cycle, said last named means including a switch mechanically activated when priming of said fuel pump is required.

6.

The combination set forth in claim 5 where-in said second higher duty cycle is effective to con-nect said electrical power to said energizing means at intervals in the range of 300 to 600 milliseconds for an amount of time in the range of 30 to 80 milliseconds.

7.
The combination set forth in claim 1 further comprising a control circuit for preheating the cata-lytic material to a temperature at which the catalytic material will react with said hydrocarbon fuel, said control circuit comprising in combination, an elec-trical heater element disposed in heat-transfer rela-tionship with said catalytic material, and energizing means to apply electric power to said heater element for a first predetermined period of time which is in-dependent of the temperature of said material.

8.

The combination set forth in claim 7 wherein said energizing means comprises first switch means hav-ing an opened and a closed position, said switch means applying power to the remainder of said control circuit when in said closed position, and timing means respon-sive to application of said power through said first switch means to energize said heater element for said first predetermined period of time.

9.
The combination set forth in claim 8 wherein said timing means comprises means responsive to clo-sure of said switch means to provide a voltage signal which increases with time from a first to a second level, comparator means having a reference input con-nected to a source of electric power to provide a ref-erence voltage at said reference input, a threshold input electrically connected to said closure responsive means, and an output responsive to reference and thres-hold inputs, and electronic switch means responsive to said comparator means to connect said heater element to a source of electric power when said threshold input is below said reference input and to disconnect said heater element from the source of power when said threshold input exceeds said reference input.

10.
The combination set forth in claim 9 where-in said closure responsive means comprises a capacitor electrically connected to a source of power and to said threshold input.

11.
The combination set forth in claim 8 further comprising means disposed in heat-transfer relationship with said catalytic material and responsive to the temperature of said catalytic material to disable said oscillator circuit means if after said first predeter-mined period of time said temperature is below a pre-determined temperature.

12.
The combination set forth in claim 11 further comprising means responsive to activation of said pre-heating control circuit to allow operation of said oscillator circuit means independently of said tempera-ture responsive means for a second predetermined period of time.

13.
The combination set forth in claim 12 where-in said second predetermined period of time is not greater than said first predetermined period of time.

14.
The combination set forth in claim 12 wherein said first predetermined period of time is in the range of ten to fifteen minutes.

15.
In a catalytic converter of the type in which a hydrocarbon fuel is flamelessly oxidized in the pre-sence of a catalytic material for preheating liquid coolant of an internal combustion engine, a control cir-cuit for preheating the catalytic material to a tempera-ture at which the material will react with the hydrocar-bon fuel comprising in combination, an electrical heater element disposed in heat-transfer relationship with said catalytic material, first switch means having an opened and closed position, said first switch means applying power to the remainder of the control circuit when in said closed position, means responsive to closure of said first switch means to provide a voltage signal which increases with time from a first to a second level, comparator means having a reference input electrically connected to a source of power to provide a reference voltage at said reference input, a threshold input electrically connected to said closure responsive means, and an output responsive to reference and threshold inputs, and electronic switch means respon-sive to said comparator means to connect said heater element to the source of power when said threshold input is below said reference input and to disconnect said heater element from said power source when said threshold input exceeds said reference input.

16.
The combination set forth in claim 15 where-in said closure responsive means comprises a capacitor electrically connected to said power source and to said threshold input.