Secondary Air Supply for Internal Combustion Engine

In internal combustion engines having a three-way catalytic converter in the exhaust system to purify the exhaust gas, the three-way catalytic converter is the most effective when the air-fuel ratio of the intake mixture is at its stoichiometric ratio of 14.7:1. The current practice is to provide feedback control of the air-fuel ratio to the vicinity of the stoichiometric ratio in accordance with the concentration of the exhaust gas and the running state of the engine.

Typically the air-fuel ratio is controlled by providing a secondary air intake passage communicating with the downstream of the throttle valve of a carburetor. The air-fuel ratio is controlled by providing the secondary air passage with an electromagnetic air control valve so that the valve may be opened or closed in response to the output signal of an oxygen concentration sensor positioned in the exhaust system. Whether the actual air-fuel ratio is lean or rich is determined from the output signal of the oxygen concentration sensor. The secondary air passage is opened to supply the secondary air to the downstream of the throttle valve when it is determined that the air-fuel ratio is at a rich value. The secondary air passage is closed to interrupt the supply of the secondary air when it is determined that the air-fuel ratio is at a lean value.

The air-fuel ratio is subject to a proportional (P) control by opening and closing the secondary air passage with the electromagnetic valve. The control rate for the air-fuel ratio is determined from the effective area of the secondary air passage. The difference in the flow rate of the secondary air between when the electro-magnetic valve is open and closed is large because of the large effective area of the secondary air passage. This makes it possible to control the air control valve over a wide range. But when the effective area of the secondary air passage is large the fluctuating range of the air-fuel ratio with respect to the desired stoichiometric value is also large, being increased by the difference in the flow rate of the secondary air between when the electromagnetic valve is opened and closed. This results in a surging running of the engine as exemplified by fluctuations in the engine r.p.m.

It is, therefore, an object of the present invention to provide a secondary air supply system to control the air-fuel ratio over the wide engine speed range while improving the running of the engine.

A secondary air supply system according to the present invention includes: means for determining an air-fuel ratio of the engine; first and second secondary air passages communicating with the downstream of the throttle valve of a carburetor; a first vacuum operated air control valve positioned in the first secondary air passage for changing the effective area of the first secondary air passage in accordance with the vacuum in a first vacuum receiving chamber; an electric-operated air control valve positioned in the second secondary air passage in accordance with the determined air-fuel ratio; a second vacuum operated air control valve positioned in the second secondary air passage for changing the effective area of the second secondary air passage in accordance with the vacuum in a second pressure receiving chamber; vacuum control means for applying a control vacuum to the first and second vacuum receiving chambers so that the effective areas of both of the first and second secondary air passages may be gradually increased or decreased to maintain a desired engine air fuel ratio.

The present invention will be described in connection with a preferred embodiment shown in the accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing the preferred embodiment of the present invention;

FIG. 2 is a flow-chart illustrating the operation of the control circuit used in the embodiment of FIG. 1;

FIGS. 3a, b and c are waveform charts illustrating the secondary air flow of the embodiment of FIG. 1 under different control.

In the secondary air supply system according to one embodiment of the present invention shown in FIG. 1, primary intake air is supplied from an atmosphere suction port 1 through an air cleaner 2 and a carburetor 3 to an engine 4. The carburetor 3 is provided with a throttle valve 5, a venturi 6 upstream of the throttle valve 5, and a choke valve 7 upstream of the venturi 6. In the vicinity of the throttle valve 5 there is formed a throttle vacuum detection port 8 which is positioned upstream of the throttle valve 5 when the throttle valve 5 is closed and downstream of the throttle valve 5 when the throttle valve is open.

A secondary air passage 11 communicates between a point in the intake manifold 9 downstream of the throttle valve 5 and a point in the vicinity of the air discharge port of the air filter 2. The secondary air passage 11 is divided into two air control passages 11a and 11b to provide control of the secondary air. The air control passage 11a is provided with a vacuum operated first air control valve 12. This air control valve 12 includes a vacuum chamber 12a, a valve chamber 12b forming a part of the air control passage 11a, a diaphragm 12c forming a part of the vacuum chamber 12a, a valve spring 12d positioned in the vacuum chamber 12a, and a tapered valve member 12e positioned in the valve chamber 12b and biased through the diaphragm 12c by the valve spring 12d to close the air control passage 11a. The air control valve 12 changes the effective area of the air control passage 11a in accordance with the level of the vacuum prevailing in the vacuum chamber 12a. The effective area of the control passage 11a is increased with the increase in the vacuum and decreased with a decrease in the vacuum on the vacuum chamber 12a.

The air control passage 11b is provided with a second air control valve 13. This second air control valve 13 is constructed like the first air control valve 12 such that it includes a vacuum chamber 13a, a valve chamber 13b, a diaphragm 13c, a valve spring 13d and a tapered valve member 13e. The second air control valve 13 changes the effective area of the air control passage 11b in accordance with the level of the vacuum prevailing in the vacuum chamber 13a such that the effective area is increased in accordance with the increase in the vacuum.

An air correcting passage 15 having an orifice 14 for idle correction bypasses the second air control valve 13. In addition, the air control passage 11b is provided with an electromagnetic valve 16 downstream of the second air control valve 13. The electromagnetic valve 16 includes a solenoid 16a, a valve chamber 16b forming a part of the air control passage 11b, and a valve member 16c positioned in the valve chamber 16b and operated by the solenoid 16a. The electromagnetic valve 16 closes the air control passage 11b when the solenoid 16a is deenergized.

The first and second air control valves 12 and 13 have their vacuum chambers 12a and 13a communicating with the intake manifold 9 by way of a vacuum supply passage 17. This vacuum supply passage 17 is provided with an electromagnetic valve 18. The valve 18 includes a solenoid 18a, a valve chamber 18b forming a part of the vacuum supply passage 17, and a valve member 18c positioned in the valve chamber 18b and operated by the solenoid 18a. The valve chamber 18b is vented to the atmosphere by way of an atmosphere supply passage 19 so that the vacuum supply passage 17 is closed when the solenoid 18a is deenergized and so that the vacuum supply passage 17 from the vacuum chamber 12a is made to communicate with the atmosphere supply passage 19 through the valve chamber 18b.

The vacuum supply passage 17 connecting between the vacuum chamber 12a and the electromagnetic valve 18 includes a surge tank 20. The vacuum supply passage 17 downstream of the throttle valve 5 from the electromagnetic valve 18 includes a constant vacuum control valve 21, a storage tank 22 and a check valve 23.

The constant vacuum control valve 21 stabilizes the vacuum downstream of the throttle valve 5 to a predetermined level P.sub.r when the vacuum exceeds this predetermined level. The check valve 23 only allows communication from a point downstream of the throttle valve 5 to the constant vacuum control valve 21. The vacuum supply passages 17 at both the sides of the electromagnetic valve 18 are provided with orifices 24 and 25. The atmosphere supply passage 19 is provided with an orifice 26.

A control circuit 33 is connected to the solenoids 16a and 18a with drive circuits 31 and 32. The control circuit 33 is connected to an oxygen concentration sensor 35 positioned in the exhaust manifold 34. The oxygen concentration sensor 35 generates an output voltage V.sub.02 at a level corresponding to the oxygen concentration in the exhaust gas. The output voltage V.sub.02 increases in accordance with the increase in the oxygen concentration.

An intake air temperature sensor 36 and a vacuum switch 37 are also connected with the control circuit 33. The air temperature sensor 36 is positioned adjacent the air cleaner 2. When the intake air temperature exceeds a predetermined temperature t.sub.1 the air temperature sensor 36 generates a high level signal at a voltage V.sub.H. The vacuum switch 37 also detects the load upon the engine. The vacuum switch 37 is turned on when the level of the vacuum in a pressure receiving chamber 37a is lower than a predetermined level to generate a high-level signal at the voltage V.sub.H. The pressure receiving chamber 37a communicates with the vacuum detection port 8 by a vacuum supply passage 38 which supplies a vacuum P.sub.C.

The control circuit 33 shown in FIG. 2 includes a comparator 41 for comparing the output voltage V.sub.02 of the oxygen concentration sensor 35 with a predetermined voltage V.sub.1 corresponding to the stoichiometric air-fuel ratio; an inverter 42 connected with the output terminal of the vacuum switch 37; and an AND circuit 43 for taking the logic product of the comparator 41, the inverter 42 and the intake air temperature sensor 36. The output signal from the AND circuit 43 is fed to the drive circuits 31 and 32.

The intake secondary air supply system having the construction previously described operates as follows. First in the operation of the control circuit 33, when the ouput voltage V.sub.02 of the oxygen concentration sensor 35 is higher than the predetermined voltage V.sub.1 (V.sub.02 is greater than or equal to V.sub.1), the air-fuel ratio is in the rich range and the output level of the comparator 41 is a high level. When the output voltage V.sub.02 is lower than the predetermined voltage V.sub.1 (V.sub.02 is less than V.sub.1), the air-fuel ratio is in the lean range and the output of the comparator 41 is a low level.

This assumes that the air temperature sensor 36 and the vacuum switch 37 inputs of the AND circuits 43 (other than the input coming from the comparator 41) are at the high level. This occurs when (1) the intake temperature is higher than the predetermined temperature level t.sub.1 so that the intake temperature sensor 36 is at the high level, and (2) the vacuum switch 37 is turned off because the level of the supply vacuum P.sub.C to the vacuum switch 37 is higher than the predetermined pressure so that the inverter 42 also is at the high level. Therefore the output level of the AND circuit becomes identical to the output level of the comparator 41.

When it is determined from the output level of the oxygen sensor 35 that the air-fuel ratio is in the rich range, the output level of the AND circuit 43 is high and it is supplied as a rich signal to the drive circuits 31 and 32. When it is determined that the air-fuel ratio is in the lean range, the output level of the AND circuit 43 is low and it is supplied as a lean signal to the drive circuits 31 and 32.

The drive circuits 31 and 32 deenergize the solenoids 16a and 18a to render the electromagnetic valves 16 and 18 inoperative in response to the lean signal and energize the solenoids 16a and 18a to render the electromagnetic valves 16 and 18 operative in response to the rich signal.

When the electromagnetic valves 16 and 18 are inoperative the electromagnetic valve 16 closes the air control passage 11b and the electromagnetic valve 18 closes the vacuum supply passage 17. The electromagnetic valve 18 on closing establishes communication between the vacuum chamber 12a and the atmosphere supply passage 19 to gradually reduce the vacuum in the vacuum chamber 12a. As the vacuum in the vacuum chamber 12a is reduced the first air control valve 12 also closes the air control passage 11a. Since no secondary air is supplied from the secondary air passage 11 to the engine 4 when the two air control passages 11a and 11b are closed the air-fuel ratio is controlled to the rich range.

Next, when the electromagnetic valves 16 and 18 are changed from their inoperative states to their operative states, the electromagnetic valve 16 is instantly opened, and the electromagnetic valve 18 restores the communications of the vacuum supply passage 17 but interrupts the communication of the atmosphere supply passage 19 so that the vacuum P.sub.r is supplied to the vacuum chambers 12a and 13a. As a result, the pressure in the vacuum chambers 12a and 13a gradually approach the vacuum P.sub.r so that the openings of the first and second air control valves 12 and 13 are gradually opened to gradually increase the respective flow rates of the secondary air through the intake secondary air passage 11 (air control passages 11a and 11b). When the pressure in the vacuum chambers 12a and 13a, immediately before the supply of the vacuum P.sub.r, is substantially equal to the atmospheric pressure the first and second air control valves 12 and 13 are opened by the supply of the vacuum P.sub.r so that the secondary air begins to gradually flow into the intake secondary air passage 11 until its flow rate is a maximum.

Next, when the electromagnetic valves 16 and 18 are changed from their operative states to their inoperative states, the electromagnetic valve 16 is closed to instantly close the air control passage 11b. The electromagnetic valve 18 similarly closes the vacuum supply passage 17. The electromagnetic valve 18 also establishes communication between the vacuum chambers 12a and 13a and the atmosphere supply passage 19. Therefore, the vacuum chambers 12a and 13a are connected to the atmosphere so that their pressure gradually approach the atmosphere. Consequently, the first and second air control valves 12 and 13 are gradually closed so that the effective area of the air control passage 11a is gradually decreased to reduce the flow rate of the secondary air. As a result, even with the air control passage 11b closed by the closing of the electromagnetic valve 16, secondary air continues to flow through the secondary air passage 11 from the air control passage 11a, since its flow rate is gradually reduced with time.

When the air-fuel ratio is feedback-controlled to its stoichiometric value, the rich and lean signals are generated alternately but continuously. In the air control passage 11a, the secondary air flow rate is increased in the presence of the rich signal and is decreased in the presence of the lean signal (shown in FIG. 3(a)) so that an integral (I) control is operated. In the air control passage 11b, the secondary air intermittently flows (shown in FIG. 3(b)) and its flow rate is gradually changed so that an intermittent proportional integral (PI) control is operated. This results in the flow rate of the secondary air through the intake secondary air passage 11 to the engine 4 being the summation of the secondary air flows through the air control passages 11a and 11b (shown in FIG. 3(c)). In addition, the changes in the secondary air flow rates vary because the respective time periods for the lean signal and the rich signal vary depending on the running state of the engine.

In a cruising state, the lean signal and the rich signal are generated alternately but promptly so that the flow rate of the secondary air through the air control passage 11a becomes substantially constant. This results in the air-fuel ratio being controlled to the vicinity of the stoichiometric value mainly by the flow rate of the secondary air which intermittently flows through the air control passage 11b.

In an accelerating state, the air-fuel ratio is controlled to the stoichiometric value mainly by the increasing flow rate of the secondary air through the air control passage 11a.

Next, if the air intake temperature is below the predetermined temperature t.sub.1, the output level of the intake temperature sensor 36 becomes low so that the output level of the AND circuit 43 becomes low no matter how the output level of the oxygen concentration sensor 35 might change. For a light-load running engine at a slow or idle speed, the throttle valve 5 is substantially closed so that the level of the vacuum P.sub.C becomes lower than the predetermined level. This results in the vacuum switch 37 being turned on to drop the output of the inverter 42 to the low level so that the output of the AND circuit 43 takes the low level. Therefore, even if the air-fuel ratio is in the rich range, the lean signal is fed to the drive circuits 31 and 32 so that the control state of the air-fuel ratio is shifted from the feedback control to an open-loop control. In this open-loop control, both the electromagnetic valves 16 and 18 are inoperative to close the intake secondary air passage 11 so that the air-fuel ratio is controlled to the rich range from the stoichiometric value.

In the present invention two secondary air passages communicating with the downstream of the throttle valve provide control of the secondary intake air. The secondary air through one of the secondary air passages is gradually increased or decreased by the integral control in accordance with the air-fuel ratio signal. Also in accordance with the air-fuel ratio signal, the secondary air through the other intake secondary air passage is caused to intermittently flow by the action of the first electromagnetic air control valve. This intermittently subjects the secondary air flow rate to the PI control. This permits increasing the integrally controlled flow rate of the secondary air through one of the secondary air passages and decreasing the intermittently controlled flow rate of the secondary air flowing through the other secondary air passage. This results in a reduction in the fluctuating range of the air-fuel ratio by the first electromagnetic air control valve while permitting control of the air-fuel ratio over a wide range. This reduces the engine surging phenomenon, and improves control of the exhaust emission resulting in improved running of the engine. In addition, the simplified construction achieves a lower manufacturing cost.

In a internal combustion engine provided with the secondary air supply system of the present invention, which may be used on a motorcycle or other vehicle, the vacuum downstream of the throttle valve can be high and constant at the start of the vehicle. This permits applying the high constant vacuum to the vacuum chambers of the first and second air control valves as soon as the rich signal is generated. This results in an immediate control of the air-fuel ratio to the lean range. The shortened time before the air-fuel ratio takes the stoichiometric value reduces the content of the hydrocarbons and carbon monoxide in the exhaust gas at the start of the vehicle.