US3890946A

US3890946A - Method and system to reduce noxious components in the exhaust emission from internal combustion engines with carburetor supply

Info

Publication number: US3890946A
Application number: US478520A
Authority: US
Inventors: Josef Wahl
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1973-07-21
Filing date: 1974-06-12
Publication date: 1975-06-24
Anticipated expiration: 1992-06-24
Also published as: JPS5859941U; JPS5042227A; FR2238049B1; FR2238049A1; GB1479010A; DE2337198A1; DE2337198C2

Abstract

The carburetor for the engine is set to provide a slightly lean mixture, for example at an air number of lambda 1.1. A fuel injection valve is located in the induction tube, or inlet manifold to the engine. The opening time of the valve is controlled in dependence on inlet manifold pressure, or other operating parameters (and may be non-linear to match nonlinearities of the carburetor) and, additionally, by sensed composition of the exhaust gases from the engine to provide additional fuel so that the overall fuel-air mixture, supplied by the carburetor as well as the injected fuel will be just under the stoichiometric level, for example at an air number of approximately 0.98, so that the exhaust gases will be reducing and permit reduction of NOx compounds in a catalytic converter with presence of minimum amounts of CO and CH compounds in the exhaust.

Description

United States Patent Wahl 1 June 24, 1975 1 1 METHOD AND SYSTEM TO REDUCE 3,738,341 6/1973 Loos 123/119 R ox ous COMPONENTS [N THE EXHAUST 3,759.232 9/1973 Wahl et a1. 123/32 EA 3,782,347 111974 Schmidt et a1 EMISSION FROM INTERNAL 3,827,237 8/1974 Linder 60/276 x COMBUSTION ENGINES WITH CARBURETOR SUPPLY Primary Examiner-Wendell E. Burns Attorney, Agent, or Firm-Flynn & Frishauf [57] ABSTRACT The carburetor for the engine is set to provide a slightly lean mixture, for example at an air number of )\=1.1. A fuel injection valve is located in the induction tube, or inlet manifold to the engine. The opening time of the valve is controlled in dependence on inlet manifold pressure, or other operating parameters (and may be non-linear to match nonlinearities of the carburetor) and, additionally, by sensed composition of the exhaust gases from the engine to provide additional fuel so that the overall fuel-air mixture, supplied by the carburetor as well as the injected fuel will be just under the stoichiometric level, for example at an air number of approximately 0.98, so that the exhaust gases will be reducing and permit reduction of NO compounds in a catalytic converter with presence of minimum amounts of CO and CH compounds in the exhaust.

22 Claims, 5 Drawing Figures METHOD AND SYSTEM TO REDUCE NOXIOUS COMPONENTS IN THE EXHAUST EMISSION FROM INTERNAL COMBUSTION ENGINES WITH CARBURETOR SUPPLY Cross reference to related patents: US. Pat. Nos. 3,483,851, 3,782,347, and 3,759,232.

I The present invention relates to a system to reduce the noxious components of exhaust gases from internal combustion engines in which the fuel'air mixture applied to the engine is provided by a carburetor, normally set to supply a somewhat lean mixture.

v It has previously been proposed to supply a somewhat lean mixture to internal combustion engines by so setting the carburetor controls that an excess of air is present. To obtain a stoichiometric fuel-air mixture ()t=l a second carburetor has been proposed which provides a rich fuel-air mixture. The two mixtures are then mixed together, until a stoichiometric overall mixture is provided. If the mixture applied to the internal combustion engine is at the stoichiometric level (,\=l or just below (for example A is approximately 0.98), the overall emission from the internal combustion en- -gine, including CO and CH-compounds, becomes a minimum. The remaining NO compounds in the exhaust can be reduced in a catalytic reactor connected to the exhaust from the internal combustion engine itself. Such a device becomes expensive due to the use of two carburetors, and the control of both carburetors,

one of which is controlled by a sensor in the exhaust system, is difficult to carry out and additionally includes a substantial dead time which arises in changing of mechanical control elements in the carburetor itself. I The accuracy of the setting and the composition of the SUBJECT MATTER OF THE PRESENT INVENTION Briefly, the inlet system of the engine, that is, for example the induction type or the inlet manifold thereof, has a fuel injection valve located therein which provides fuel to the engine, in addition to the fuel-air mixture supplied by the carburetor. The carburetor is set to supply a fuel-air mixture which is lean, that is, I;

fuel is added, in controlled measured quantities, by the injection valve, just sufficient to bring the fuel-air composition of the mixture actually applied to the cylinders of the engine to just below stoichiometric value, under control of a sensor located in the exhaust system of the internal combustion engine. The invention will be described by way of example with reference to the accompanying drawings, wherein: FIG. I is a graph illustrating the relative presence of quantities of the components of the exhaust gases in dependence on the air number A (abscissa);

FIG. 2 is a general block diagram of an engine system incorporating the present invention;

FIG. 3 is a schematic circuit diagram of an inductively coupled monostable flip-flop;

FIG. 4 is a schematic diagram illustrating operating characteristics of a vacuum-induction transducer ussed in the system of FIG. 3, in which the abscissa indicates pressure (or, rather, vacuum) and the ordinate injection time of the fuel injection valve, reflecting the operating characteristics of the transducer; and

FIG. 5 is a schematic circuit diagram of an integral controller.

The air number A is defined as having a value of unity (l) if a stoichiometric composition of fuel and air is present. If excess air is present. the value of A exceeds l.0, the value being determined by the mass ratio of air to fuel. Curve 10 (FIG. 1) illustrates the relationship of the CO component in the exhaust gases with respect to A. As is apparent, the value of the CO component decreases at a value below )\=l and reaches a minimum value just beyond )t=l. Thereafter, the CO value is essentially constant and very low. Curve [1 illustrates the quantity of hydrocarbons (CH compounds) in the exhaust. Up to a value of )t=l.3, approximately, curve I I has somewhat the same shape as curve 10 for CO. Above )t=l.3, the unburned hydrocarbons in the exhaust rise rapidly. This is due to ignition failures which occur from time to timeand more frequently as the mixture becomes lean-so that unburned fuel will be present in the exhaust.

Curve I2 illustrates the relationship of nitrogen oxygen compounds (NO in the exhaust. As can be seen, curve 12 has a shape which is opposite to that of the curves I0 and 11. Curve I2 has a maximum which is approximately at )t=l .05. At higher and lower values of air number A, curve 12 drops rapidly. NO, components arise at high combustion temperatures, since the NO components are generated by oxidation of the nitrogen in the air. The combustion temperatures reach their maximum value at approximately stoichiometric composition of the supplied fuel-air mixture. Curve 12 shows the relationship of the NO, components in the exhaust gases as derived from the cylinders of the engine I5 (FIG. 2), that is, as they appear in the

exhaust manifold stubs

21, 22, 23, 24 which are connected to an exhaust manifold 18, and hence to an exhaust coI- lection line 25. By connecting a catalytic reactor 26 to the exhaust line 25, the relationship of NO components in the exhaust downstream from the catalytic reactor changes substantially, as illustrated by broken curve 14. When the exhaust gas composition is reducing, that is, at air numbers less than unity, the nitrogenoxygen compounds react in the catalytic reactor 13 with the carbon monoxide and with the hydrogen in the unburned hydrocarbon remnants in the exhaust. Thus, at air numbers less than unity, the output from the catalytic reactor will have practically no NO, compounds.

If the air number exceeds unity, the exhaust gases are nno longer reducing, but rather change to be oxidizing, that is, oxygen is now present in the exhaust gas. The NO, compounds then no longer can be reduced in the catalytic reactor 26, so that for air numbers higher than unity, the two curves l2 and 14 will coincide.

The exhaust from the internal combustion engine, of course, includes all the components referred to: CO, CH-compounds and l JO compounds. These are the noxious components from the exhaust which are to be minimized. Overall minimum noxious components can be obtained at air numbers just below unity, for example at an air number A of approximately 0.97 to 0.99, for example of about 0.98. At this air number, the CO proportion is low, although not yet at its absolute minimum; the CH component is about to reach its minimum: and the NO, components can be practically completely eliminated in a catalytic reactor. Thus, operation of the internal combustion engine at an air number of approximately 098 provides for effective overall minimum noxious exhaust emission.

An internal combustion engine (FIG. 2) has inlet manifold stubs 16, 17, 18, 19 connected to an induction tube 20. For purposes of illustration. engine 15 is illustrated as a four-cylinder engine, although the invention, of course, is applicable to an engine of any number of cylinders. Exhaust manifold stubs 2124 are connected to a manifold line 25, the pipe of which connects to the catalytic reactor 26. The exhaust from reactor 26 is conducted, for example, through a muffler, to the tailpipe and then exhausted to ambient air.

The fuel-air mixture for engine 15 is supplied by a carburetor 27. The illustration is highly schematic, and the invention is applicable to any type of commercially used carburetor. Hence, the schematic illustration merely shows a nozzle extending into the induction tube. Air is supplied to the induction tube through a filter 28 and sucked in upon operation of the engine. A throttle 29 is located in convention manner, the throttle position being controlled by a suitable linkage, shown in dotted line, by a control element, for example an accelerator pedal 32.

In accordance with the present invention, a fuel injection valve 30 is provided which injects fuel into the induction pipe ofthc internal combustion engine 15. The injection valve is located behind throttle 29. Operation of the fuel injection valve 30 is controlled by an electronic controller 31 which includes a monostable flip-flop (FF) to be described below. During the un stable state of the FF, the injection valve 30 is opened; during the table state of the FF, the injection valve 30 is closed. Fuel is supplied to injection valve 30 by a fuel line, schematically indicated.

Operation of the fuel injection valve 30, controlled by the control unit 31, is governed by various operating parameters of the internal combustion engine. One of these operating parameters is pressure (or, rather, vacuum) in the induction pipe 20; another parameter is engine speed; another parameter, in accordance with the present invention, is composition of the exhaust gases. Other, further parameters may be used to influence the open-time of the fuel injection valve 30.

A pressure sensing device 46, such as a diaphragm chamber, is connected in pressure sensing relationship to the induction pipe 20, to control the duration of the unstable state of the FF 31. Circuit 31 further has a correction input 33. The circuit 31 is triggered by a pulse source 34 which may, for example, be controlled from the crankshaft of the engine 15. The pulse source may, also, be an external pulse source, such as an astable multivibrator operating at a frequency which is dependent on loading of the engine, for example by sensing air flow, The monostable FF 31 has a further input, connected to an integral controller 35 which, in turn, is supplied by signals from a threshold switch 36, which is connected to an exhaust gas composition sensor 37. Sensor 37 is sensitive to the presence of oxygen in the exhaust gases and located in the exhaust manifold, or

exhaust pipe 25 between the engine 15 and the catalytic reactor 26.

Basic operation: The carburetor 27 is so adjusted that in any usually encountered operating condition, the fuel-air composition supplied thereby will be lean, that is. the air number will be greater than unity. Preferably, the setting may be such that the air number is, on the average, approximately 1.1. This ensures that under all ordinary operating conditions of an internal combustion engine used, for example, in automotive applications. the mixture will be lean. Additional fuel is added to this lean mixture supplied by carburetor 27 by selectivcly opening the fuel injection valve 30. The quantity of fuel supplied by injection valve 30 is so controlled. or so measured that the overall total fuel-air mixture which is supplied to the cylinders of engine 15 will have an air number ofjust under unity, and preferably about 0.98.

The necessary additional fuel supplied by the injection valve 30 should bring the mixture as close to the desired air number as possible. Its operation, therefore, should be matched to the operating characteristics of the carburetor, and any variations or non-linearities in the fuel-air composition supplied by the carburetor under varying operating conditions of the engine should be balanced by the fuel injection valve. This balancing can be essentially obtained, in accordance with the present invention, since the quantity of fuel to be added by the injection valve will be small; thus, any variations in quantity of fuel to be supplied will be small, so that the control swing, or control range to which the injection valve 30 will be subjected will be small. The coarse adjustment, and the coarse supply and matching of fuel to air, according to requirements of engine 15 under varying operating conditions, is carried out by carburetor 27.

In accordance with a feature of the invention, the integral controller provides output signals which influence the fuel supply added by the injection valve in multiplicative relationship.

The electronic control unit 31 (FIG. 3) includes a monostable FF which, specifically, has an input transistor 37 and an output transistor 39, the base of which is connected over resistor 38 to the collector of transistor 37. The collector of output transistor 39 is connected to a common positive bus 40 over the primary winding 41 of a transformer 42. Transformer 42 has a movable core 43, which is suitably connected by a linkage, sche matically shown at 44, with the membrane 45 of 3 diaphragm chamber unit 46. Diaphragm chamber unit 46 is connected pneumatically to the induction pipe 20 (FIG. 2) of the engine 15 by a branch stub located behind the throttle 29.

In quiescent condition, input transistor 37 of the monostable FF is held in conductive condition by resistor 47 connected between the base of the transistor 37 and positive bus 40. The secondary winding 49 of transformer 42 is connected to the base of the transistor 37 over diode 48; the other terminal of the secondary winding 49 is connected to the tap, or division point of a voltage divider formed of

resistors

50, 51 which are connected across positive bus 40 and negative bus 53 of the system. The correction input terminal 33 is also connected to the tap point between

resistors

50, 51.

A control cam, connected to rotate in synchronism with the crankshaft of the internal combustion engine 37, as illustrated by arrow 52, opens and closes a switch 54 which has one terminal connected to the common, or negative. or ground. or chassis bus 53 of the supply source 1 not shown. and typically the battery ofan automotive vehicle). A load resistor 55 and one electrode of a coupling capacitor 56 are further connected to the switch 54. The other electrode of capacitor 56 is connected to a second load resistor 57 and then to the negative bus 53. A diode 58 connects to the base of the transistor 37.

Operation: The monostable FF is controlled, once for each rotation of the cam. as illustrated by arrow 52, for example once for each rotation of the crankshaft. to switch into astable state. When the switch 54 is openas illustrated in FIG. 3capacitor 56 can charge to the operating voltage between

lines

40 and 53 through the two

resistors

55, 57. When the control arm 54 is switched by the cam, upon rotation thereof(arrow 52) against the fixed contact connected to negative bus 53, the positively charged electrode of capacitor 56 is connected to the negative terminal, and the base of transistor 37 will receive a strong negative voltage. This blocks transistor 37 which, in turn, controls output transistor 39 to change to conductive state. The collector current. flowing over primary winding 41 of trans former 42 induces a voltage in the secondary 49, which continues to hold the input transistor 37 in blocked state. The duration of this voltage which holds the input transistor blocked is determined by the induced voltage which depends on the position of the core 43 in the transformer 42, and hence on the pressure (or. rather. vacuum) in the induction pipe of the engine to which the vacuum diaphragm chamber 46 is connected. If pressure drops sharply below ambient atmospheric pressure, for example when the throttle 29 is closed, or almost closed, membrane 46 moves the iron core 43 downwardly (H6. 3) to increase the air gap in transformer 42, thus substantially decreasing the inductivity of the primary 41. Due to the low induced volt age, input transistor 37 will rapidly change to its quiescent conductive state and. thus. rapidly block the output transistor 39. The pulse which is derived from the collector of output transistor 39. and applied over resis tor 59, will thus be only of short duration. If the accelerator 32 is substantially completely depressed, so that throttle 29 is open, or almost open, the air pressure in the induction pipe will be only slightly less than that of ambient atmospheric air. Core 43 is then moved downwardly only slightly. The primary winding will have a high inductivity. causing slow rise of the collector current in the primary winding 41, and resulting in along pulse from the resistor 59 connected to the collector of transistor 39.

The correction input 33 may have signals of various input parameters connected thereto; for example, the setting signal of integral controller 35 may be connected at this terminal, as well as other correction parameters. for example correction parameters representing speed of the engine. Changing the voltage at the tap point of the voltage divider, changes the duration of the pulse, since the voltage applied to the base of the transistor will be changed. This change is multiplicative. It is also possible to apply correction parameters to the input 60, which will also effect multiplicative change of the pulse duration of the monostable FF. If this is done. it is desirable to include a resistor in the line between the connection from the lower terminal of primary 4] and the supply voltage line 40.

The invention has been described in connection with an inductively coupled FF. lt is. ofcourse. also possible to use R/C coupled flipflops. or other circuits. Rather than using vacuum in the induction manifold, other control parameters may be used. for example a param ctcr representative of air flow through the induction tube obtained. for example. as an output voltage from a potentiometer. the slider position of which is controlled by a deflection member in the induction tube leading to the internal combustion engine. Such a defiection element will provide an output signal which is also representative of load on the engine since. in effect. flow of air-fuel mixture to the cylinders of the engine is measured. The vacuum diaphragm chamber will also respond. to some extent. to loading on the engine since, when the throttle is open and at high speed oftlie engine, the pressure in the induction tube is practically the same as that of ambient air pressure. It is an impor tant feature ofthe present invention that the correction parameter so affects the unstable time of a monostable FF that the effect is multiplicative.

FIG. 4 is a diagram of voltage or. rather. injection time ii. in milliseconds. of the injection valve 30 with respect to induction tube vacuum. The normal injection timevacuum characteristic of the transducer 46 is illustrated by solid line 61. By suitably shaping core 43 of the transformer 42, the essentially linear relationship can be distorted to obtain the relationship illus trated in broken line 62. A similar distortion of the linear relationship can be obtained by utilizing non-linear resistors in the circuit of Fl(]. 3, or by placing a nonlinear mechanical linkage between the membrane 45 of transducer 46 and core 43. Shaping the core 43. ho\ ever, to be non-linear is a simple solution. The nonlinearity of the core 62 compensates norrlinearities of the carburetor.

FIG. 5 illustrates a circuit diagram of an integral controller, for example the integral controller 35 of FIG. 2, as well as the arrangement of the threshold switch 36 (FIG. 2 The output sensor 37 is connected over a separable connector, shown schematically at 37'. over a coupling resistor 65 to the inverting input of an operational amplifier 63 which. by virtue of its feedback resistor 69, is connected to provide for proportional amplification of the output signal from the sensor 37. The direct input of operational amplifier 63 is connected over coupling resistor 66 to the tap point of a voltage divider. formed by adjustable resistor 67 and resistor 68 and connected across

buses

40, 53. The value of the feedback resistor 69 determines the amplification fac tor of the operational amplifier. The output of the operational amplifier is connected to common positive bus 40; the power supply connections to the amplifier 63 have been omitted for clarity.

The output of the operational amplifier 63 is connected over a coupling resistor 7i to the inverting input of a second operational amplifier 64, forming the integral controller. The direct input of the operational amplifier 64 is connected over coupling resistor to the tap point of a voltage divider formed of serially connected

resistors

73, 74, respectively connected across

buses

40, 53. The feedback circuit of operational amplifier 64 includes capacitor 75 which, then, causes the operational amplifier 64 to operate as an integrating amplifier. The output of operational amplifier 64 is connected to a load resistor 64', and then to positive bus 40. An output resistor 76 connects the output of operational amplifier 64 to output terminal 77, which is connected to the correction input terminal 33 of the monostable FF of circuit 3| (FIGS. 2, 3).

Operation of circuit of FIG. The sensor 37 which, preferably, is a known oxygen sensor, described in more detail in co-pending applications Ser. Nos. 259,l34; 3 l6,008; 447,475, assigned to the assignce of the present application, is amplified in operational amplifier 63. The operational amplifier 63, as is apparent from the diagram of FIG. 5, is connected as an inverting amplifier. The output voltage will, then, have a negative value when the sensor 37 provides an output. The output voltage derived from amplifier 63 jumps between discrete threshold levels, in dependence on whether the voltage at its inverting input is above, or below the voltage at the direct input. The voltage at the direct input is determined by the setting of the voltage divider formed by

resistors

67, 68, and such other signals connected to the tap point between

resistors

67, 68 to form correction signals representative of selected parameters (not shown). Operational amplifier 63, then, operates as an inverter, and the output signal therefrom is connected to the inverting input of operational amplifier 64. Operational amplifier 64, due to the presence of integrating capacitor 75, integrates when the input voltage is negative, in positive direction. The voltage at the output terminal 77 then shifts slowly in positive direction, the rate of shift being determined by the integrating constants of the circuit. The output voltage of operational amplifier 64 shifts in negative direction, that is, the operational amplifier integrates in negative direction if the output from operational amplifler 63 is positive, representative of a rich air-fuel mixture.

The voltage at terminal 77, connected to terminal 33 (FIG. 3) and hence to the tap point of the voltage divider formed by

resistors

50, 51 then modifies the unstable time of the monostable FF, and hence the open time duration ti of the fuel injection valve 30 (FIG. 2) by introducing an additional signal at the tap point between

resistors

50, 51, so that the actual open time ti of the fuel injection valve will be additionally modified over that shown in the relationship of FIG. 4, depending on sensed composition of the exhaust gases, as de' termined by sensor 37. The composition of the exhaust gases can, therefore, be maintained in the reducing range, that is, at an air number just below unity so that the engine will operate with a just slightly rich mixture and just to the left of the steep portion of curve 14, FIG. 1, resulting in minimum overall noxious components in the exhaust of the engine.

Various changes and modifications may be made within the scope of the inventive concept and the invention may be used with features described in the copending applications, the disclosure of which is hereby incorporated. The timing of the astable time of multivibrator 31 can be modified also, for example, by connecting a variable resistor 33a in parallel to terminal 33', so that the voltage division ratio of

voltage divider

50, 51 is modified. The extent of variation of resistor 33a can be controlled by a selected engine operating parameter. For example, resistor 330 can be a potentiometer, the slider of which is coupled to the gas pedal 32 (FIG. 2). The transformer 42 then may have a fixed value of inductions, so that the pressure-displacement transducer 46 need not be coupled to the movable core I claim: 1. Method to reduce noxious components in the exhaust from internal combustion engines having a carburetor to supply a fuel-air mixture to the engine, comprising the steps of supplying fuel from the carburetor to the engine in a quantity less than that forming a stoichiometric level to supply a lean fuel-air mixture to the engine;

sensing the composition of the exhaust gases by testing oxygen content therein, and providing a sensing signal representative of oxygen in the exhaust gases;

intermittently injecting fuel to the fuel-air mixture being supplied to the engine;

and controlling the injection time, for each injection of fuel, in dependence on sensed composition of the exhaust gases by injecting fuel during respectively longer, or shorter time intervals, as the sensed composition of the exhaust gases changes between lean I) and rich ()t l) mixture to provide an overall fuel-air mixture being applied to the engine which is just below stoichiometric value.

2. Method according to claim I, wherein the air number of the fuel-air mixture being supplied to the engine by the carburetor has an air number of about 1.1, and the step of intermittently injecting fuel comprises adding fuel in such a a quantity that the overall air number of the fuel-air mixture supplied to the engine by both the carburetor and the fuel injection valve is about 0.98.

3. Method according to claim 1, wherein the step of controlling the injection time comprises the step of sensing induction tube pressure and said injection time is further controlled as a function of the induction tube pressure.

4. Method according to claim l, wherein the step of controlling the injection time comprises the step of determining the relationship of operating characteristics of the engine carburetor with respect to the engine operating conditions and deriving an error function representative of non-linearities in said relationship;

and modifying the injection time in accordance with a function which compensates for said error function to provide an overall fuel-air mixture being applied to the engine in which the relative composition of fuel and air is essentially unvarying regardless of engine operating conditions.

5. Method according to claim 1, wherein the step of injecting fuel into the fuel-air mixture comprises supplying fuel at a substantially constant pressure to an injection valve to vary the quantity of fuel added by said injection step to the fuel-air mixture derived from the carburetor substantially only as a function of injection time of said valve.

6. System to reduce the noxious components in the exhaust from an internal combustion engine (15) having a carburetor (27) to supply a fuel-air mixture to the engine, in which the carburetor supplies a lean fuel-air mixture having an air number greater than unity (lt l .0)

carrying out the method of claim 1, comprising sensing means (37) responsive to the exhaust gases from the engine and providing a sensing signal rep resentative of oxygen in the exhaust from the engine',

a fuel injection valve (30) in the induction system (20) to the engine;

control means (31 connected to energize said injection valve for intermittent opening, said sensing means (37) being connected to said control means (3]) to control the time of energization of said injcction valve, and hence the duration that said valve is open and hence the quantity of fuel injected, as a function of sensed composition of the exhaust gases.

7. System according to claim 6, wherein the control means (3] comprises a monostable flip-flop (FF) (3] connected to and controlled by said sensing signal to change the duration of the unstable state of said monostable FF in dependence on sensed composition of the exhaust gases.

8. System according to claim 7, wherein the control means further comprises an integrating circuit (35) and a threshold switch (36), the sensing signal being applied to said threshold switch which changes state when the sensing signal exceeds a predetermined reference limit, the output of the threshold switch being applied to the integrator (35) to provide an integrated signal, varying in positive or negative going direction, and depending on the time that said sensing signal passes the threshold level of said threshold switch.

9. System according to claim 6, further comprising means (46) sensing pneumatic conditions in the induction tube to the internal combustion engine and further controlling the control means (31) to vary time of energization of said injection valve, and hence the duration that said valve is open, in dependence on pneumatic conditions in said induction tube.

10. System according to claim 9, wherein the means sensing pneumatic conditions in the induction tube comprises a pressure transducer sensing vacuum in the induction tube.

11. System according to claim 9, wherein the pneumatic sensing means comprises an air mass flow transducer responsive to air mass flow in the induction tube of the engine.

12. System according to claim 7, further comprising transducer means responsive to at least one of the pneumatic conditions in the induction tube of the engine: vacuum; mass air flow;

said transducer means being connected to said monostable FF and additionally controlling the unstable state of said monostable FF (31) in dependence on pressure, or air flow conditions, respectively, in said induction tube.

13. System according to claim 7, wherein said monostable FF stage comprises at least one correction terminal input (33', 60) to permit modification of the unstable time of the monostable FF (31) as a function of a correction, or operating parameter of the engine.

14. System according to claim 7, wherein the unstable state of the monostable FF is controlled in dependence on position of the throttle of the engine.

15. System according to claim 7, wherein the monostable FF includes a variable circuit element (42, 43, 44) affecting the unstable time, and hence the time of energization of said fuel injection valve, and said sensing means (37) is connected to said FF to provide for multiplicative modification of change in the unstable time of said monostable FF, with respect to the variation of value of said variable circuit element.

16. System according to claim 7, wherein the output of the monostable FF is connected to the injection valve (30) to energize the fuel injection valve when the monostable FF is in unstable state, said fuel injection valve having fuel applied thereto at a substantially constant pressure.

17. System according to claim 7, further comprising a trigger means (34) connected to and controlled by the rotation of the engine and triggering the monostable FF to change from stable to unstable state in predetermined relationship to rotation of the engine.

18. System according to claim 7, wherein the monostable flip-flop (31) is triggered from stable into unstable state by a pulse source, the pulse repetition rate of which is dependent on loading of the engine.

19. System according to claim 7, wherein the mono stable FF includes an inductive circuit (42), the inductivity of said circuit being variable, and forming one control parameter determining the unstable time of said monostable FF, the inductivity varying as a function of an operating, or operation parameter of said engine.

20. System according to claim 7, wherein the FF includes a non-linear element (43; FIG. 4) and having variable characteristics which change upon change in operating, or operation parameters of the engine;

the carburetor (27) has a non-linear operating characteristic with respect to change in an operation, or operating parameter of the engine;

and the non-linearity of said element in said monostable FF being selected to compensate for, and counteract non-linearities in the characteristics of the carburetor upon change of a respective operation, or operating parameter of the engine, said element responding to operation, or operating parameters of the engine which are affected by the nonlinearities of the characteristics of the carburetor.

2]. In combination with an internal combustion engine,

operating by carrying out the method of claim 1,

a fuel supply system comprising a carburetor set to provide an air-fuel mixture at an air number greater than unity, in quantities supplying the major portion of the air-fuel mixture requirements of the engine;

a fuel injection valve in the induction system to the engine;

sensing means (37) responsive to the exhaust gases from the engine and providing a sensing signal representative of oxygen in the exhaust from the systern;

and control means connected to energize said injection valve for intermittent opening, said sensing means being connected to said control means to control the time of energization of said injection valve, and hence the duration that said valve is open as a function of sensed composition of exhaust gases,

the quantity of fuel injected being a minor proportion of the fuel necessary to provide a fuel-air mixture having an air number just under unity.

22. Fuel supply system according to claim 21, wherein the fuel supplied by the carburetor provides about -90% of the fuel requirement of the fuel-air mixture required for operation of the engine; and the fuel injected by the fuel injection valve comprises the remaining 70-30% of total fuel requirements of the engine.

l I l

Claims

1. Method to reduce noxious components in the exhaust from internal combustion engines having a carburetor to supply a fuelair mixture to the engine, comprising the steps of supplying fuel from the carburetor to the engine in a quantity less than that forming a stoichiometric level to supply a lean fuel-air mixture to the engine; sensing the composition of the exhaust gases by testing oxygen content therein, and providing a sensing signal representative of oxygen in the exhaust gases; intermittently injecting fuel to the fuel-air mixture being supplied to the engine; and controlling the injection time, for each injection of fuel, in dependence on sensed composition of the exhaust gases by injecting fuel during respectively longer, or shorter time intervals, as the sensed composition of the exhaust gases changes between lean ( lambda >1) and rich ( lambda <1) mixture to provide an overall fuel-air mixture being applied to the engine which is just below stoichiometric value.

2. Method according to claim 1, wherein the air number of the fuel-air mixture being supplied to the engine by the carburetor has an air number of about 1.1, and the step of intermittently injecting fuel comprises adding fuel in such a a quantity that the overall air number of the fuel-air mixture supplied to the engine by both the carburetor and the fuel injection valve is about 0.98.

4. Method according to claim 1, wherein the step of controlling the injection time comprises the step of determining the relationship of operating characteristics of the engine carburetor with respect to the engine operating conditions and deriving an error function representative of non-linearities in said relationship; and modifying the injection time in accordance with a function which compensates for said error function to provide an overall fuel-air mixture being applied to the engine in which the relative composition of fuel and air is essentially unvarying regardless of engine operating conditions.

6. System to reduce the noxious components in the exhaust from an internal combustion engine (15) having a carburetor (27) to supply a fuel-air mixture to the engine, in which the carburetor supplies a lean fuel-air mixture having an air number greater than unity ( lambda >1.0) carrying out the method of claim 1, comprising sensing means (37) responsive to the exhaust gases from the engine and providing a sensing signal representative of oxygen in the exhaust from the engine; a fuel injection valve (30) in the induction system (20) to the engine; control means (31) connected to energize said injection valve for intermittent opening, said sensing means (37) being connected to said control means (31) to control the time of energization of said injection valve, and hence the duration that said valve is open and hence the quantity of fuel injected, as a function of sensed composition of the exhaust gases.

7. System according to claim 6, wherein the control means (31) comprises a monostable flip-flop (FF) (31) connected to and controlled by said sensing signal to change the duration of the unstable state of said monostable FF in dependence on sensed composition of the exhaust gases.

9. System according to claim 6, further comprising means (46) sensing pneumatic conditions in the induction tube (20) to the internal combustion engine and further controlling the control means (31) to vary time of energization of said injection valve, and hence the duration that said valve is open, in dependence on pneumatic conditions in said induction tube.

12. System according to claim 7, further comprising transducer means responsive to at least one of the pneumatic conditions in the induction tube of the engine: vacuum; mass air flow; said transducer means being connected to said monostable FF and additionally controlling the unstable state of said monostable FF (31) in dependence on pressure, or air flow conditions, respectively, in said induction tube.

13. System according to claim 7, wherein said moNostable FF stage comprises at least one correction terminal input (33'', 60) to permit modification of the unstable time of the monostable FF (31) as a function of a correction, or operating parameter of the engine.

19. System according to claim 7, wherein the monostable FF includes an inductive circuit (42), the inductivity of said circuit being variable, and forming one control parameter determining the unstable time of said monostable FF, the inductivity varying as a function of an operating, or operation parameter of said engine.

20. System according to claim 7, wherein the FF includes a non-linear element (43; FIG. 4) and having variable characteristics which change upon change in operating, or operation parameters of the engine; the carburetor (27) has a non-linear operating characteristic with respect to change in an operation, or operating parameter of the engine; and the non-linearity of said element in said monostable FF being selected to compensate for, and counteract non-linearities in the characteristics of the carburetor upon change of a respective operation, or operating parameter of the engine, said element responding to operation, or operating parameters of the engine which are affected by the non-linearities of the characteristics of the carburetor.

21. In combination with an internal combustion engine, operating by carrying out the method of claim 1, a fuel supply system comprising a carburetor set to provide an air-fuel mixture at an air number greater than unity, in quantities supplying the major portion of the air-fuel mixture requirements of the engine; a fuel injection valve in the induction system to the engine; sensing means (37) responsive to the exhaust gases from the engine and providing a sensing signal representative of oxygen in the exhaust from the system; and control means connected to energize said injection valve for intermittent opening, said sensing means being connected to said control means to control the time of energization of said injection valve, and hence the duration that said valve is open as a function of sensed composition of exhaust gases, the quantity of fuel injected being a minor proportion of the fuel necessary to provide a fuel-air mixture having an air number just under unity.

22. Fuel supply system according to claim 21, wherein the fuel supplied by the carburetor provides about 70-90% of the fuel requirement of the fuel-air mixture required for operation of the engine; and the fuel injected by the fuel injection valve comprises the remaining 70-30% of total fuel requirements of the engine.