Control system for promoting catalytic removal of noxious components from ...

4,056,

CONTROL SYSTEM FOR PROMOTING
CATALYTIC REMOVAL OF NOXIOUS
COMPONENTS FROM EXHAUST GAS OF

INTERNAL COMBUSTION ENGINE 5

BACKGROUND OF THE INVENTION

This invention relates to a system for promoting removal of noxious components from the exhaust gas of an internal combustion engine which is equipped with a 10 carburetor and in its exhaust line a catalytic converter.

With respect to an internal combustion engine, it is one of the fundamental requisites to success in removing, or at least reducing for the most part, noxious components from the exhaust gas that the air-fuel ratio of a 15 combustible mixture fed to the engine is maintained at a predetermined value with high precision. This requisite is critical when removal of the noxious components is accomplished by catalytic conversion in the exhaust system of the engine. 20

There has been proposed an excellent catalyst which comprises a plurality of platinum group metals and catalyzes both the oxidation of carbon monoxide and unburned hydrocarbons and the reduction of oxides of nitrogen. This catalyst exhibits its full ability in the 25 exhaust gas from a conventional gasoline engine only when the engine is run with an air/fuel mixture prepared at approximately, if not exactly, the stoichiometric mixing ratio. When the air-fuel ratio of the mixture exceeds the stoichiometric ratio (about 14.8 by weight 30 for air/gasoline mixture), a sharp drop occurs in the conversion efficiency of removing oxides of nitrogen. On the other hand, the efficiency of oxidizing carbon monoxide and unburned hydrocarbons drops sharply when the air-fuel ratio is lowered from the stoichiomet- 35 ric ratio. It is necessary, therefore, to maintain the air/fuel ratio of the combustible mixture at the stoichiometric ratio with accuracy of better than ±1%. It was impossible, however, with conventional carburetors to accomplish such a precise control of the air/fuel ratio 40 since the air/fuel ratio depends on physical properties such as density and viscosity of air and fuel which are variables depending on the atmospheric pressure, ambient temperature, and fuel temperature.

In connection with control of the air/fuel ratio, it is 45 known that an actual air/fuel ratio in the running engine can be estimated by measuring the concentration of a certain component of the exhaust gas by the use of an electrical sensor. Useful sensors are known for almost every of major components of the exhaust gas such as 50 oxygen, carbon monoxide, carbon dioxide, hydrocarbons and oxides of nitrogen. For example, an oxygen sensor of the concentration cell type having an ion-conducting solid electrolyte is particularly suitable for detecting slight deviations of the air/fuel ratio from the 55 stoichiometric ratio because the relationship between the output voltage of this sensor exposed to the exhaust gas and the air/fuel ratio of the combustible mixture fed to the engine exhibits a very sharp and great change at the stoichiometric air/fuel ratio. 60

With respect to an internal combustion engine which is equipped with a carburetor having an air bleed passage opening into a fuel discharge passage and, in the exhaust system, a catalytic converter containing therein a catalyst which catalyzes oxidation of carbon monox- 65 ide and hydrocarbons and reduction of oxides of nitrogen, it is an object of the present invention to provide a system for promoting removal of noxious components

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from the exhaust gas, which system controls the air/fuel ratio of the combustible mixture fed to the engine to the stoichiometric ratio with high precision based on the concentration of a particular component of the exhaust gas measured in the exhaust system at a location upstream of the catalytic converter.

SUMMARY OF THE INVENTION

A system according to the invention comprises:

A system according to the invention comprises: an auxiliary air admitting passage connected to a fuel discharge passage of the carburetor; a sensor which is disposed in the exhaust system at a location upstream of the catalytic converter and produces an electrical signal representing the concentration of a particular component of the exhaust gas having dependence on the air/fuel ratio of the air/fuel mixture fed to the engine; a control circuit which produces: continual pulses of a variable width at a frequency between 5 and 100 Hz in response to the signal from the sensor; and an electromagnetic valve arranged to cause admission of auxiliary air to the auxiliary air admitting passage only when the individual pulses are applied thereto. The width of the pulses is increased individually when the air/fuel ratio indicated by the signal from the sensor is below a predetermined ratio which is equal to or close to the stoichiometric ratio and decreased individually when the indicated air/fuel ratio is above the predetermined ratio, so that the fuel discharge rate to the induction passage of the carburetor is varied in response to deviations of the indicated air/fuel ratio from the predetermined ratio.

The auxiliary air admitting passage is preferably connected to the air bleed passage at a section downstream of the orifice of the air bleed passage and has preferably such a cross-sectional area at the narrowest section that the air feed rate therethrough when the valve causes the admission of auxiliary air takes a value 1 to 5 times as large as the air feed rate through the air bleed passage.

Another auxiliary air admitting passage under a similar control of a similar electromagnetic valve is preferably provided to a slow-speed fuel discharge passage of the carburetor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the accompanying drawings, wherein:

FIG. 1 is a diagram showing the fundamental constitution of a system according to the invention;

FIG. 2 is a diagram showing more in detail a portion of the same system in association with a carburetor;

FIG. 3 is a graph showing the relationship between the air/fuel ratio of air/gasoline mixture fed to an internal combustion engine and the output voltage of an oxygen sensor exposed to the exhaust gas of the engine;

FIG. 4 is a block diagram of the control circuit in the system of FIG. 1;

FIG. 5 is a fragmentary sectional view of the carburetor of FIG. 2, showing the arrangement of the auxiliary air admitting passage in a system according to the invention;

FIG. 6 is a schematic representation of a modified arrangement of the auxiliary air admitting passage of FIG. 5;

FIG. 7 is a diagram fundamentally similar to FIG. 2 but with a carburetor of the two-barrel, two-stage type; 3

FIG. 8 is a sectional view generally similar to FIG. 5 but shows a slight modification of the auxiliary air admitting passage and an emulsion tube in the carburetor;

FIG. 9 is an enlarged and sectional view of the emulsion tubes, showing modified arrangements of auxiliary 5 air inlets in FIG. 8;

FIG. 10 is a sectional view of a conventional electromagnetic valve for use in the system of FIG. 1;

FIG. 11 is a similar view of an improved electromagnetic valve for the same use; and 10

FIG. 12 is a perspective view of the valve member of the electromagnetic valve of FIG. 11, showing a modification of the support member for the valve member and FIG. 13 is a sectional view of a variant of the electromagnetic valve in FIG. 11. 15

DESCRIPTION OF THE PREFERRED
EMBODIMENTS

Referring to FIG. 1, an internal combustion engine 10 is equipped with an air cleaner 12 and a carburetor 14 in 20 combination with its induction passage 16 and, as the exhaust system, an exhaust manifold 18, an exhaust pipe 20 and a catalytic converter 22 which is arranged to occupy an intermediate section of the exhaust pipe 20. The catalytic converter 22 contains therein a conven- 25 tional catalyst which catalyzes oxidation of carbon monoxide and hydrocarbons and reduction of oxides of nitrogen. When this catalyst is exposed to the exhaust gas containing oxygen, carbon monoxide, unburned hydrocarbons and oxides of nitrogen, the efficiencies in 30 the catalytic actions of the catalyst on these oxidation and reduction reactions depend greatly on the composition of the exhaust gas and, hence, air/fuel ratio of the air/fuel mixture fed to the engine 10.

To the carburetor 14, an auxiliary air admitting pas- 35 sage 24 is provided for admitting air into a fuel discharge passage (not shown) in a manner as will be described hereinafter, and an on-off functioning electromagnetic valve 26 is arranged to control the admission of air into this air admitting passage 24. An exhaust 40 sensor 28 is installed in the exhaust system at a section upstream of the catalytic converter 22, for example, in the exhaust manifold 18. A control circuit 30 receives an electrical signal from the sensor 28 and produces an output for operating the electromagnetic valve 26 in 45 compliance with the amplitude of the signal from the sensor 28.

The exhaust sensor 28 is preferably an oxygen sensor of the well known concentration cell type having an ion-conducting solid electrolyte exemplified by stabi- 50 lized zirconia (Zr02—CaO). The graph of FIG. 3 shows a typical relationship between the air/fuel ratio (by weight) of air/gasoline mixture fed to the engine 10 and the output voltage of the sensor 28 of this type when the sensor 28 is exposed to the exhaust gas of the 55 engine 10. The oxygen sensor 28 may be replaced by any known sensor of a different type which is sensitive to a particular substance contained in the exhaust gas in a variable concentration depending on the air/fuel ratio of the air/fuel mixture fed to the engine 10, for example, 60 carbon monoxide sensor, carbon dioxide sensor, hydrocarbon sensor or nitrogen oxide sensor.

As is known, the air/fuel ratio of the combustible mixture prepared in the carburetor 14 can be varied by controlling the discharge rate of the fuel from the main 65 nozzle and accordingly can be controlled by controlling the feed rate of air to the fuel in the main fuel discharge passage. The auxiliary air admitting passage 24 in a

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system of FIG. 1 is provided for the accomplishment of an air/fuel ratio control in such a manner.

Referring to FIG. 2, the carburetor 14 has a float chamber 32 and a main fuel discharge passage 34 which is formed between a main fuel jet 36 and a main nozzle 38. As usual, an intermediate section of the main fuel passage 34 forms a main well 40, and a main air bleed passage 42 is provided to the main well 40 in the form of a perforated tube 44 having a main air bleed orifice 46 at its exposed end.

The auxiliary air admitting passage 24 is arranged such that auxiliary air is supplied to the fuel in the main well 40 in addition to usual air supply through the main air bleed passage 42. Alternatively, the auxiliary air admitting passage 24 may be connected to the main fuel discharge passage 34 at a section upstream of the main well 40. It has been proposed to control the fuel discharge rate from the nozzle 38 by intermittently interrupting the admission of air through the air bleed orifice 46. When, however, the air feed rate to the fuel is controlled in such a manner, there will arise unfavorable problems such as irregular shifts of the fundamental setting of the carburetor 14 and/or a significant hunting in the fuel discharge rate. The auxiliary air admitting passage 24 is provided to preclude such problems and control the fuel discharge rate smoothly and accurately.

The auxiliary air admitting passage 24 is preferably arranged to open into the main well 40 at a section above the fuel level therein. When the auxiliary air admitting passage 24 is connected to the fuel discharge passage 34 at a section upstream of the main well 40, it is rather difficult to control the air/fuel ratio precisely because blowing of air (gas) into the fuel (liquid) causes turbulence and even pulsation of the fuel flow by reason of the electromagnetic valve 26 being of the on-off functioning type. The arrangement of the auxiliary air admitting passage 24 will be described hereinafter more in detail.

The electromagnetic valve 26 interrupts completely the admission of air from the atmosphere into the auxiliary air admitting passage 24 through its metering orifice 48 when the valve 26 is in the off-state or closed state. In this state, the air feed rate to the fuel in the main fuel passage 34 is dependent solely on the air velocity at the main air bleed 46. When the valve 26 is opened, the fuel discharge rate lowers since the air feed rate to the fuel in the fuel passage 34 is augmented by the opening of the auxiliary air admitting passage 34.

Preferably, the fuel discharge rate through a slowspeed fuel passage 50 of the carburetor 14 also is controlled by the provision of another auxiliary air admitting passage 52 other than a usual air bleed passage 54. The auxiliary air admitting passage 52 is arranged generally as described hereinbefore and will be described hereinafter with respect to the auxiliary air admitting passage 24 for the main fuel discharge passage 34. The admission of air into the auxiliary air admitting passage 52 for the slow-speed circuit is controlled by another set of electromagnetic valve 26' which is separate from but identical with the valve 26 for the main fuel circuit. Alternatively, the auxiliary air passage 52 for the slowspeed circuit is arranged to join the auxiliary air admitting passage 24 for the main circuit at a section upstream of the respective metering orifices 48 and 56, so that the admission of air into both of the two passages 24 and 52 can be controlled by a single electromagnetic valve 26 as shown in FIG. 7 (FIG. 7 shows a carburetor 14A of the two-barrel type as will hereinafter be described, but