Abstract:
In the device disclosed, a catalytic converter purifies exhaust gas from an engine. A sensor measures the temperature of the converter and an amplifier amplifies the measurement to produce a temperature signal. A differentiator produces a time-differential of the signal multiplied by a constant representing the time constant of the sensor. An operating circuit adds the multiplied time-differential to the signal to produce a voltage that is compensated for the response delay of the temperature sensor. Electromagnetic relays respond to the voltage by diverting the gases and the secondary air around the converter.

Description:
This is a continuation of application Ser. No. 319,682, filed Dec. 29, 1972. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a control system for exhaust gas purifying devices, and more particularly to a control system which prevents overheating of the exhaust gas purifying device. 
     Temperature sensors have been successfully installed on exhaust gas purifying devices such as intake-type thermal reactors, catalytic converters, afterburners, and similar devices. These sensors prevent the purifying devices from overheating. Heat generation resulting from the purifying operation is suppressed by stopping the operation before the sensor indicates that the heat-resisting critical temperature of the purifying device has been reached. However, in practice the responses of temperature sensors are subject to delays. Thus, if the temperature of the purifying device rises sharply, the sensor may not indicate quickly enough that the heat-resisting critical temperature of the purifying device has been reached. As a result, heat may destroy the purifying device. 
     An object of this invention is to improve gas purifying systems. 
     Another object of this invention is to insure safe operation of gas purifying devices. 
     Still another object of this invention is to compensate for the aforementioned delay in the response of the temperature sensor. 
     SUMMARY OF THE INVENTION 
     According to a feature of this invention, the output signal of a temperature sensor which senses the temperature of an exhaust gas purifying device is added to a signal obtained by multiplying a constant corresponding to the time constant of the temperature sensor by a time-differential of the output signal. This produces a corrected signal corresponding to the actual temperature of the exhaust gas purifying device. 
     According to another feature of the invention, the operation of the exhaust gas purifying device is interrupted when the corrected signal, which corresponds to the actual temperature of the purifying device, exceeds a predetermined value. As a result the purifying device is reliably protected from destruction. 
     According to another feature of the invention, electronic circuits perform the mathematical manipulations so that the overall dimensions of the system can be substantially reduced. 
     These and other features of the inventions are pointed out in the claims. Other objects and advantages of the invention will become evident from the following detailed description when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 schematically illustrates an internal combustion engine exhaust gas purifying system embodying features of the invention, and including a temperature control system embodying features of the invention; 
     FIG. 2 is a graph illustrating the relationship between the temperature rise of exhaust gases in the exhaust gas purifying device of FIG. 1, and the response of the exhaust gas temperature sensor; and 
     FIG. 3 is a graph showing the difference between the temperature indicated by the sensor and the actual temperature, and the time-differentiated values of the indicated temperatures; and 
     FIG. 4 is a schematic diagram illustrating a circuit diagram to be substituted for the electrical circuitry in FIG. 1 according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 exhaust gases from an internal combustion engine 1 pass through a front exhaust pipe 2. They then branch through a catalytic converter 3 and bypass 4. An electromagnetic change-over or 3-way valve 5, which is operated by an exciting-winding W1, at the outlet of the converter 3 and the bypass 4 determines whether the gases will pass through the catalytic converter 3 or the bypass 4. That is to say, the valve 5 preferentially passes the gases through the catalytic converter 3 or through the bypass 4. The gases ultimately pass through a rear exhaust pipe 6 and a muffler 7 in series with the rear exhaust pipe. 
     An air cleaner 8 supplies fresh air into the internal combustion engine 1. A pump 10 for secondary purifying air serves to supply part of the air fed into the air cleaner 8 into a pipe 11. An electromgnetic valve 12 having an exciting winding W2 selectively connects the pipe 11 to either a pipe 13 or a pipe 14. The valve 12 thus selectively passes the air to the inlet of the catalytic converter 3 through the pipe 13 or to the outlet of the catalytic converter 3 through the pipe 14. In the position shown, the valves 5 and 12 respectively pass the exhaust gases through the catalytic converter 3 and the secondary purifying air to the inlet of the converter. Under these circumstances the converter 3 is performing its exhaust gas purifying operation. 
     A temperature sensor 15 in the catalytic converter 3 is connected to the input terminal of a preamplifying stage 16. There an operational amplifier OA1 having an input resistor R 1  and a feedback resistor R 2  multiplies the signal from the sensor by a value R 2  /R 1 . The output of the amplifying stage 16 is connected to a phase-inverting stage 17. There an operational amplifier OA2 having an input resistor R 3  and a feedback resistor R 4  equal to the input resistor R 3 , inverts the phase of the signal from the amplifying stage 16. The output terminal of the amplifying stage 16 is also connected to a differentiating stage 18. There an operational differentiator OA3 having an input condenser C 1  and a feedback resistor R 5  differentiates the output of the stage 16. Differentiator stage 18 is constructed so that the product of the values of the feedback resistance R 5  and the input capacitance C 1  correspond to the time constant of the temperature sensor 15. 
     The output terminals of the phase-inverting stage 17 and the differentiating stage 18 are respectively connected to the input terminal of an adding stage 19. There an operational amplifier OA4 having a pair of input resistors R 6  and R 7  and a feedback resistor R 8 , all equal to each other, adds the differentiated value of the output of stage 16 to the inverted value. An operating unit 20 receives the output of the adding stage 19. In the operating unit an operational amplifier OA5 receives one input at its non-inverting input terminal from the output terminal of the adding stage 19. The inverting input terminal of the amplifier OA5 receives a voltage, formed by voltage dividing resistors R 9  and R 10 , which corresponds to the heat-resisting critical temperature of a catalyst in the catalytic converter 3. A feedback resistor R 11  forms part of the operational amplifier OA5. In the stage 20, a transistor Tr receives the output of the operational amplifier OA5 at its base. The transistor Tr operates a relay Ry when the output of the stage 19 exceeds the value set by the resistors R 9  and R 10 , thereby indicating that the temperature in the converter 3 exceeds or is about to reach the heat-resisting critical temperature of the catalyst as set by the resistors R 9  and R 10 . 
     A diode D1 connected in parallel with the relay Ry prevents the development of an overvoltage in the coil of the relay when the transistor is conductive. The collector circuit of the transistor Tr is connected to a DC power supply 21 through a switch 22 and a fuse 23. A voltage stabilizer or regulator 24 connected to the power supply 21 serves to apply a stabilized voltage to the stages 16, 17, 18, 19, and 20. A contact K1 of the relay Ry connects the power supply 21 to the exciting winding W1 and W2 of the valves 5 and 12, thereby operating these valves when the relay Ry is energized. 
     When a temperature in the catalytic converter 3 is less than a predetermined value, the electromagnetic valve 5 is situated as shown by the broken line. Thus, the exhaust gas from the internal combustion engine 1 flows through members 2, 3, 5, 6, and 7. At the same time, the valve 12 is in the position shown by the broken line. Consequently, secondary air is supplied through the pipe 13 to the inlet of the catalytic converter 3. Thus, noxious components in the exhaust gases are removed by an oxidation reaction within the converter. 
     When excessive unreacted noxious components pass into the catalytic converter due to trouble in the internal combustion engine 1, such as miss fire or excessive concentration of the gas mixture, the amount of heat generated from the reaction increases. This raises the temperature in the converter. The sensor 15 detects the temperature in the catalytic converter. 
     It is assumed that the temperature θs indicated by the electric output signal of the sensor 15 responds to the actual temperature θo of the converter 3 with a primary delay. The Laplace transformation of θs is as follows: ##EQU1## The Laplace transformation of θo is expressed as follows: t,91 ##EQU2## The expression of the system with the primary delay using Laplace transformation is as follows: ##EQU3## Although k and T are constants, k = 1, because under steady state conditions θs agrees with θo. The value T is the time constant of the sensor 15. In view of the above equation (3) can be expressed as follows: ##EQU4## The equation (3)&#39; can be rewritten as follows: 
     
          θs × (1 + sT) =  θo                     (4) 
    
     The inverse Laplace transformation of both sides of equation (4) will produce the following results: ##EQU5## Finally the following formula is established. ##EQU6## 
     Equation (5) expresses the general form of the relation between the indicator temperature of the sensor 15 and the actual temperature of the converter 3. In other words, the actual temperature θo can be obtained by adding the indicator temperatures θs of the sensor 15 to the product of the time gradient dθs/dt of the temperature θs and the time constant T of the sensor. 
     In FIG. 2 the abscissa is time and the ordinate is temperature θ. Here θo increases at a specific gradient from the time t = t o . It illustrates the time variation of θo and θs with respect to time. 
     In FIG. 3 the abscissa is time and the ordinate is the temperature θ as well as the time differential dθ/dt of temperature. The Figure illustrates the time variation of the difference Δθ between θo and θs, that is Δθ = θo - θs, as well as the time variation in the temperature gradient dθs/dt. The temperature θs with respect to the temperature θo is determined as follows by using Laplace transformation: ##EQU7## (wherein C and K are the constants). Using equation (3)&#39; we obtain: ##EQU8## Modifying equation (7) we obtain: ##EQU9## The inverse Laplace transformation of both sides of equation (7)&#39; produces the following: ##EQU10## Since ##EQU11## we obtain: ##EQU12## From equations (8), (10), and (11) ##EQU13## From equation (12) ##EQU14## Moreover, ##EQU15## From equations (13) and (14), it is apparent that the following formula is established: ##EQU16## 
     In FIG. 1, the signal e 1  of the sensor 15 is amplified R 2  /R 1  times by the preamplifying stage 16. The phase of the signal is inverted at this stage. The signal is returned to its initial phase by means of the phase-inverting stage 17. The output of the differentiating stage 18 to which the input ##EQU17## is applied is ##EQU18## Consequently ##EQU19## appear at the non-inverting input terminal of the adding stage 19. The output thereof is ##EQU20## If e i  varies linearly with θs, it is apparent from the above that the output of the adding stage 19 is proportional to the temperature θo. When the temperature θo exceeds the heat-resisting critical temperature θc it is desirable that the operation of the converter be stopped. The heat-resisting critical temperature θc of the catalyst in the converter 3 is established at the dividing resistors R 9  and R 10 . Thus, when θo enters the region B shown in FIG. 2, the value at the output of the stage 19 exceeds the value established by the resistors R 9  and R 10  and the transistor Tr is rendered conductive. This energizes the relay Ry. 
     As a result of the energization of relay Ry, the valves 5 and 12 are energized and pass the exhaust gases from the internal combustion engine 1 through the bypass 4. Moreover, these valves pass the secondary air from the pump 10 through the pipe 14 to bypass the converter 3. This prevents the converter from being overheated. Accordingly, as long as the system is operating normally, the temperature of the converter is always maintained within the safety region as shown in FIG. 2. 
     the embodiment of the invention illustrated in FIG. 1 compensates for the response delay of the temperature sensor. It is also possible to compensate for the time delay of the temperature rise due to th thermal capacity of the catalytic converter 3 itself. This involves utilizing the changes in the temperature θo at the time of trouble in a malfunction of the engine 1. In other words, where it is expected that the temperature of the converter 3 may exceed the heat-resisting critical temperature θc, the aforementioned and described control can be accomplished before θo reaches θc. Under these circumstances the electromagnetic valves 5 and 12 are controlled and shifted from the positions shown by means of a signal obtained by adding the output of the adding stage 19 to the value obtained by multiplying the differentiated value of the output by the time constant of the temperature rise in the catalytic converter 3. In this way, it is possible to eliminate problems arising from response delays of the control signal when the flow of the actual secondary air or exhaust gas changes. In this way, the response of the whole control mechanism is improved. 
     The electrical portion of the apparatus embodying this principle is illustrated in FIG. 4 where the reference characters of FIG. 1 are used to designate like parts in FIG. 4. The circuit of FIG. 4 connects to the mechanical portion of the purifying system in FIG. 1 and together forms an overall system embodying features of the invention. 
     The invention has a number of advantages. For example, it offers great flexibility and insures exceedingly desirable response characteristics. This is due to the fact that compensation of the response characteristic is carried out on a theoretical basis in the portion of the system involving temperature calculations, namely, the differentiating stage and the adding stage. Moreover, not only the response delay of the sensor but also the time for the temperature rise in the catalytic converter, as well as the response to the whole system, including the operating units such as 5 and 12, can be improved. 
     The sensor itself, especially its characteristics such as its response, can be improved only up to a point due to manufacturing restrictions, heat resistance, durability, etc. However, according to the present invention, the effects of defects in the sensor itself can be eliminated. This allows for wide flexibility in the construction of the sensor itself. It allows the whole system to be constructed for optimum operation. This affords the advantage of improving the quality and reducing the cost. 
     While embodiments of the invention have been described in detail it will be obvious to those skilled in the art that the invention may be embodied otherwise without departing from its spirit and scope.