Temperature circuit for oxygen sensor during warm-up

Abstract

 A composite electronic circuit accurately controls air-fuel ratio in an engine having an electronically regulated fuel supply, such as an electronically regulated fuel injection system. The air-fuel ratio is measured by an oxygen sensor connected to the exhaust manifold. A circuit associated with the oxygen sensor compensates for variations in sensor output produced by temperature change during warm-up. After engine warm-up is completed, the circuit provides a voltage having a value independent of engine temperature.

Description

[0001] In order to reduce pollutants in the exhaust from a vehicle engine, it is necessary to achieve close control of the air-fuel ratio (A/F) provided to the engine. The degree of control required depends on the particular devices with which the engine is equipped. For example, it has been determined that for three-way catalyst devices presently used to reduce exhaust emissions of hydrocarbons, carbon monoxide and oxides of nitrogen, air-fuel should be controlled within about one percent. Less accurate control of air-fuel ratio decreases catalyst efficiency, and may even destroy the catalyst. Achievement of accuracies on the order of magnitude of one percent is a formidable task, but appears possible with electronic regulation of fuel flow rate, using an oxygen sensor in the exhaust stream. Such a sensor may be constructed from zirconium oxide. Its output is a strong function of air-fuel, provided the air-fuel mixture used is nearly stoichiometric.

[0002] A major difficulty arising with the use of such an oxygen sensor is that its output voltage also depends on temperature. The difficulty is most severe during warm-up of the engine, when the exhaust temperature may vary from about 600°F (316°C) to about l100°F (593°C). Accurate control of exhaust emissions is especially important during the warm-up process, when emissions of hydrocarbons and carbon monoxide tend to be much higher than usual.

Summary of the Invention

[0003] The present invention provides a composite circuit means associated with an electronic fuel injection system for a vehicle engine having an electronic circuit that determines fuel flow rate. Such composite circuit is adapted to provide a reference voltage with which the output voltage of an oxygen sensor is compared during periods of time required for engine warm-up and engine operation after warm-up has been completed. The composite circuit includes a first means having a resistance element carrying a temperature dependent'resistance-value and positioned relative in a location to be exposed to temperatures analogous to the operating temperatures of the oxygen sensor, for providing a reference voltage during the warm-up period of the engine. A second means including a constant voltage source provides the reference voltage after the engine warm-up period has been completed. The output voltage of the oxygen sensor is contrasted with the reference voltage by a comparison means, which generates a correction signal proportional to the difference between the reference and oxygen sensor voltages. A correction means applies the correction signal to the electronic circuit determining fuel flow rate.

Brief Description of the Drawings

[0004] The invention will be more fully understood and further details will become apparent when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings in which:

Figure 1 is a graph illustrating the relationships between the oxygen sensor output voltage and the temperature of the sensor for a given air-fuel ratio;

Figure 2 is a graph illustrating the relationship between the oxygen sensor output voltage and the air-fuel ratio for three different sensor temperatures;

Figure 3 is a schematic electrical diagram of a composite circuit representing one form of the present inventions;

Figure 4 is a schematic diagram of a circuit for changing fuel flow in response to a correction signal from the composite circuit; and

Figure 5 is a table setting forth resistance values at typical engine operating temperatures for an engine temperature sensing resistance element of said composite circuit.

Description of the Preferred Embodiments

[0005] Referring to Figures 3 and 4, the composite circuit means, shown generally at 10 is associated with controller 12 of an electronic circuit that determines fuel flow rate to a vehicle engine (not shown). Composite circuit 10 provides a reference voltage V_r with which the output voltage V_o of an exhaust sensor is compared during periods of time required for (1) engine warm-up and (2) engine operation after warm-up has been completed. The composite circuit 10 includes a first means having a resistance element T carrying a temperature dependent resistance value and positioned in a location to be exposed to temperatures analogous to the operating temperature of the oxygen sensor. Resistance element T provides a reference voltage V_r during warm-up of the engine. A second means including a constant voltage source Z provides the reference voltage V after the engine warm-up period has been completed. The output voltage V_o of the oxygen sensor is contrasted with the reference voltage V by a comparison means, which generates a correction signal 14 proportional to the difference between the reference and oxygen sensor voltages V and V_o2. A correction means 16 applies the correction signal 14 to the controller 12, determining fuel flow rate.

[0006] Resistance element T is located in the engine coolant, or in any other location representative of engine temperature. Alternatively, the resistance element T may be located directly on or in the oxygen sensor, or in any position representative of the temperature of the exhaust gas. The voltage V across Zener'diode Z is constant to a very high accuracy, even if there are fluctuations in the supply voltage A+. A voltage divider R_1/R₂ allows setting the circuit output voltage V_oR₁/(R₁+R₂) equal to the reference value V required for a warm engine.

[0007] During warm-up, the reference signal V is provided by the resistance element T. The resistance Rt of the thermistor, and hence the voltage V_t, varries in accordance with the temperature t of the oxygen sensor. A shunt resistance R₄ may be used to change the dependence of voltage V_t on oxygen sensor temperature t. By placing the diode D between the thermistor circuit and the Zener diode circuit, and by a suitable choice of resistances R₁, _R2, _R3, _R4 and _R5, the voltage signal V_t will override the signal from the Zener diode circuit during warm-up. The main requirement for this to occur is that resistance R₂ be large compared with the resistance R_t of resistance element T. Given R_t as a function of temperature t, the resistances R₄ and R₅ must be chosen to provide the proper reference signal V _r as a function of temperature t.

[0008] Resistance R₃ serves to limit the current to the Zener diode. The resistance R₁ is adjusted to provide the reference signal V_r required for a warm engine. The operation of the composite circuit can be summarized as follows. When the engine is warm, V_r= V_oR₁/ (R₁+R₂) and V_t <^Vr; while during warm-up V_r=V_t > V_oR₂/(R₁+R₂). The reference signal V_r is compared with the output signal _Vo2 from the oxygen sensor, and the error signal is used to correct the fuel flow rate, as shown schematically in Figure 4.

[0009] The oxygen sensor is a zirconium oxide sensor having an output voltage of the type shown in Figures 1 and 2. In the embodiment described above, the Zener diode is a silicon diode type SZ 4.7, having a breakdown voltage V of 4.7 volt, nominal. The A+ voltage is 14 volt nominal such as that supplied by a vehicle generator and battery. The resistance R₃ is typically about 180 ohm. The voltage divider consists of resistances R₁= 6.8 kiloohm and R₂ = 1.0 kiloohm. This arrangement provides a signal V_r= 600 2 millivolt when the engine is warm. The resistance element T is preferably a FENWALL #JA4IWI having a resistance R_t that varies as a functin of temperatures t in the manner indicated by Table I in Figure 5. The resistance R₄ can be omitted; the resistance R₅ are 10 kiloohm, nominal. The rectifier D is a germanium diode, type 1N277.

[0010] Having thus described the invention in rather full detail it will be understood that these detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the present invention as defined by the subjoined.claims.

Claims

1. A composite circuit means associated with an electronic fuel injection system for a vehicle engine having an electronic circuit that determines fuel flow rate, said composite circuit being adopted to provide a reference voltage with which the output voltage of an exhaust sensor is compared during periods of time required for engine warm-up and engine operation after warm-up has been completed, comprising:

(a) first means having a resistance element carrying a temperature dependent resistance value and positioned relative to said sensor to be exposed to temperatures analogous to the temperature of said sensor, for providing said reference voltage during the warm-up period of said engine;

(b) second means including a constant voltage source for providing said reference voltage after said engine warm-up period has been completed;

(c) comparison means for contrasting the output voltage of said exhaust sensor with said reference voltage to generage a correction signal proportional to the difference between said reference and exhaust sensor voltages; and

(d) correction means for applying said correction signal to said electronic circuit determining fuel flow rate.

2. A composite circuit as recited in claim 1, wherein said resistance element carrying a temperature dependent resistance value is a thermistor.

3. A composite circuit as recited in claim 2, wherein said correction signal assumes one of a pair of limiting values.

4. A composite circuit as recited in claim 2, wherein said first means and said second means are electrically connected by means of a diode.

5. A composite circuit as recited in claim 1, wherein said temperature dependent resistance element is located so as to be exposed to temperatures that change in a manner analogous to the temperature change experienced by said sensor.

Drawing