GB2059116A - Control of fuel/air ratio in i.c. engines - Google Patents

Abstract

A control system (2) for maintaining the speed of a spark ignition internal combustion engine within predetermined limits at least at idle conditions of the engine, which control system (2) comprises air control means (4, 6), fuel control means (16), first measuring means for measuring the air:fuel ratio being used by the engine, second measuring means for measuring the engine speed, and electronic control means (12) which is programmed to contain optimum engine air and fuel requirements at idling conditions and which is adapted to generate control signals for the air control means and the fuel control means in order to vary the amount of air and/or fuel being used by the engine in order to cause the engine to receive air and fuel in agreement with the programmed optimum air and fuel requirements to keep the speed substantially constant. <IMAGE>

Description

SPECIFICATION Engine idle speed control This invention relates to a control system for maintaining the revolutions per minute of a spark ignition internal combustion engine within predetermined limits at least at idle conditions of the engine.

It is well recognised that under normal running conditions, an accurate control of the air:fuel mixture being utilised by a spark ignition internal combustion engine will improve the performance, fuel economy and exhaust emissions of the engine.

It is also recognised that the idling speed of a spark ignition internal combustion engine can play a significant part in the fuel economy and the exhaust emissions of the engine because the engine spends a substantial part of its running life operating at idling conditions.

The majority of present day spark ignition internal combustion engines receive an air:fuel mixture through one or more carburettors. Each carburettor operates to draw fuel into an air stream by the pressure drop across a fuel metering orifice. The carburettor thus operates to give a fuel flow proportional to the airflow through the carburettor.

however, spark ignition internal combustion engines typically require an air flow range of 40 to 1 from idle to maximum power. This is a very large range to provide accurate air:fuel metering, especially at the lower air and hence fuel flow rates. Indeed, when the engine is operating at idle conditions, the normally used fuel jets in the engine carburettors are insensitive and many carburettors are provided with additional idle jets and other mechanisms for improving the fuel metering capability of the carburettor at the stated low fuel rates.

Some present day spark ignition internal combustion engines are provided with electrically operated fuel injection systems as an alternative to the known carburettors. However, both the known carburettors and the fuel injection systems rely on a mechanical stop on a throttle to control the idling speed of the engine. The mechanical stop on the throttle is very insensitive and it may give unrepeatable results leading to an unreliable control of the engine idling speed.

It is an aim of the present invention to provide a control system for maintaining the revolutions per minute of a spark ignition internal combustion engine within predetermined limits at least at idle conditions of the engine, thereby to improve the fuel economy and exhaust emissions of the engine.

Accordingly, this invention provides a control system for maintaining the revolutions per minute of a spark ignition internal combustion engine within predetermined limits at least at idle conditions of the engine, which control system comprises air control means, fuel control means, first measuring means for measuring the air:fuel ratio being used by the engine, second measuring means for measuring the engine revolutions per minute, and electronic control means which is programmed to contain optimum engine air and fuel requirements at idling conditions and which is adapted to generate control signals for the air control means and the fuel control means in order to vary the amount of air and/or fuel being used by the engine in order to cause the engine to receive air and fuel in agreement with the programmed optimum air and fuel requirements of the engine.

If the engine is receiving the air and fuel in a correct air:fuel ratio but in an incorrect amount, then the air:fuel ratio can be kept constant and the combined amount of the air and fuel can be increased or decreased as required by the control system. If the engine is receiving the air and fuel in an incorrect air:fuel ratio, then the air and/or the fuel can be increased or decreased as required by the control system.

If desired, the electronic control means may ensure that the engine receives optimum air and fuel amounts at engine operating conditions above engine idling conditions. Indeed, the electronic control means may ensure that the engine receives optimum air and fuel amounts at all engine operating conditions. By way of example, it is mentioned that the control system may be such as to cause the engine to receive optimum amounts of air and fuel when it is being started up from cold when it is well established that the engine will require a higher air and fuel flow and an air:fuel ratio that is richer in fuel than would normally be employed if the engine were operating at its normal high running temperature.

The control system of the invention may thus be such that the electronic control means additionally receives information on one or more ofthefol- lowing: (a) throttle angle and throttle change rate of a throttle for the engine; (b) when a starter motorforthe engine is engaged; (c) the temperature of the coolent for the engine; and (d) the amount of air flow through the carburettor for the engine, for example by means of pistol lift in a constant depression carburettor.

The air control means may be an electrically controlled air metering device which is separate from a main air throttle for the engine.

When the engine receives its normal amount of fuel from a carburettor, then the fuel control may be advantageously effected by employing at least one electrically controlled metering device which is separate from the carburettor. The or each electrically controlled fuel metering device can then be arranged to operate at idling conditions ofthe engine when the carburettor is not sufficiently sensitive to supply exactly the required amount of fuel.

If the spark ignition internal combustion engine is of a type which has its normal fuel requirements provided by at least one electrically controlled fuel metering device, then there may be no need to employ an additional electrically controlled fuel metering device for finely metering fuel at idle conditions of the engine, providing that the main fuel metering device has sufficient metering accuracy over the range required. The same electrically controlled fuel metering device or devices can be employed.

The electronic control means may be an integrated circuit device. Preferably, electronic control means is a microprocessor.

The control system of the present invention may be arranged to operate when a throttle for the engine engages a mechanical stop, thereby indicating coarsely that the engine should be idling. As the throttle is moved away from its mechanical stop, the control system of the present invention can gradually relinquish its fine control and the normal fuel and air control apparatus for the engine, for example a carburettor or a fuel injection system, can be employed.

Generally, the electronic control means will be arranged to operate in the knowledge that the engine speed can be too high because of an excess mixture mass flow to the engine or because of too rich an air:fuel mixture. Equally, the engine speed can be too low because of an insufficient mixture mass flow or too weak an air:fuel mixture. The electronic control means will determine which of the parameters is causing the engine to idle outside the predetermined iimit before making an adjustment.

The engine can be made to idle within the predetermined limits by varying the air and fuel together or by varying the air and fuel independently of each other. Before the mixture mass flow is increased or decreased to achieve a predetermined speed, the electronic control means will carry out a check to see if the mixture ratio is correct for the particular engine operating condition at that moment in time. A comparison will then be effected with the corresponding correct programme data in the electronic control means.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic view of a control system in accordance with the invention; and Figure 2 is a diagram showing the logistic steps that will be employed in the electronic control means.

Referring to Figure 1,where is shown a control system 2 for a carburettor-operated fuel supply system. The control system 2 is for maintaining the revolutions per minute of a spark ignition internal combustion engine (not shown) within predetermined limits at idle conditions of the engine. The control system 2 comprises air control means in the form of a stepper motor 4 controlling an obturator 6, the obturator 6 controlling the amount of bleed air admitted to an air duct 8 through a conduit 10. The stepper motor4 is provided with control signals which pass from an electronic control unit 12 along an air valve control line 14. The control system 2 further comprises fuel control means in the form of an electronic metering device 16. The device 16 comprises a piezoelectric crystal element 18 positioned on a body portion 20 ofthe device 16.Fuel at low pressure is supplied along a pipe 22 into the body portion 20 of the device 16 where it passes along a passage 24 and into the duct 8 through an orifice 26. A ball valve obturator 28 is arranged to block the orifice 26 and so prevent the injection of fuel until such time as the piezoelectric crystal element 18 is electrically activated by electrical signals passing along a line 30 from an oscillator 32.

The device 16 then vibrates and the obturator 28 is vibrated away from the orifice 26 to allow fuel to be injected into the duct 8. When the electrical signals passing along the line 30 cease, the device 16 ceases to vibrate and the obturator 28 returns to the position in which it blocks the orifice 26 and thus stops fuel being injected as a spray into the duct 8.

The oscillator 32 is controlled by signals passing along a line 34 from the control unit 12. The signals passing along the line 34 may relate to pulse repetition frequency and pulse width of the fuel to be injected through the orifice 26.

The device 16 will only operate when the engine is operating at idle conditions to finely control the amount of fuel injected into the duct 8. The main fuel control for the engine will be achieved by a constant depression carburettor 36 which will be substantially insensitive at idling conditions of the engine and this is why the device 16 will be brought into operation at idling conditions of the engine. The carburettor 36 comprises a piston 38 having a needle 40 control ably blocking a pipe 42 which is effective to admit fuel at low pressure into the duct 8. Air passes in the direction of the arrow 44 along the duct 8 and, if there is a sufficient air flow, it draws the fuel out of the pipe 42.At idling conditions of the engine, the air flow through the duct 8 will usually be insufficient to withdraw the fuel in the required manner out of the pipe 42 and the control unit 12 will automatically sense that the engine is operating at an idle condition and will bring the device 16 into operation.

A normally employed butterfly valve 46 is provided in the duct 8 to control the air flow through the duct 8. This valve 46 is provided with a switch and angle transducer 48 which is connected buy a line 50 to the control unit 12. This line 50 thus provides throttle angle and change rate information to the control unit 12. The control unit 12 is also provided with information along line 52 stating when a starter motor for the engine is engaged, with information along line 54 giving engine revolutions per minute, and with information along line 56 giving the temperature of a coolent for the engine.

The piston 38 of the carburettor 36 is associated with a piston lift transducer 58 which provides information for the control unit 12 along line 60 relating to the lift of the piston 38 which is thus an indication of the main air and fuel flows to the engine.

Referring now to Figure 2, there is shown an idle speed control system 2 as applied to an electrically controlled fuel metering system, this being opposed to a conventional carburettor fuel metering system.

In Figure 2, the control system 2 comprises electrical air control means shown as a stepper motor 4 and an obturator 6. Alternative devices could be, for example, a torque motor operating a movable control element against a spring or a pulsed solenoid valve.

The obturator 6 controls the amount of bleed air admitted to an air duct 8 through a conduit 10. The stepper motor 4 is provided with control signals which pass from an electronic idle control unit 12 along an air valve control line 14. The control system 2 further comprises fuel control means which can be any electrically controlled metering valve. The precise fuel control means shown in Figure 2 is a piezoelectric ultrasonic fuel metering device 16 controlled by an oscillator 32, controlled in turn via a control line 34 from a main control unit 12a. The device 16 is constructed to be substantially the same as the device 16 illustrated in Figure 2.

Unlike the control system 2 described in Figure 1, the device 16 not only provides the fuel required for engine idle conditions as directed by the idle control unit 12, but it also provides the fuel for all other engine operating conditions as directed by the main control unit 12a. Consequently a control signal passing along the fuel control line 34a does not pass directly to the oscillator 32, but passes through the main control unit 12a, so that it may be added to any fuel control signal the main control unit may have computed, and then it passes along the control line 34 to the oscillator 32. In practice, it may be that the control signal passing along 34 will comprise a minimum base fuel requirement as directed by the main control unit 12a, plus the variable fuel amount passing along line 34a as computed by the idle control unit 12.

In order for the main fuel control unit 1 2a to compute the correct amount of fuel to pass to an engine 69, the main fuel control unit 1 2a needs to know what the air mass flow to the engine is. This can be done by measuring, for example, the air mass flow directly by a mass flow sensor 61 along a line 62 to the main control unit 12a. Alternatively, manifold pressure can be sensed by a transducer 63 situated down-stream of a throttle blade 46 in an inlet manifold 65. This pressure reading is transferred back to the control unit 12a via a signal line 64.If the control unit 12a is also supplied with either engine revolutions per minute (rpm) through an rpm sensor 54a and a signal line 54 or throttle angle through a throttle angle sensor 48 and a signal line 50, the control unit 1 2a can then compute the engine air mass flow and provide fuel through the fuel metering device.

The idle control unit 12 will operate with the main control unit 12a when the engine is in an idling condition. This idling condition may be sensed via a switch incorporated in the throttle angle sensor 48.

In the idling condition, the idle control unit 12 will meter the auxiliary air and fuel and it will add them to the air passing across the throttle blade and the fuel metered by control means 16 as directed by the main control unit 12a. The amount of auxiliary air and fuel required will be computed by the idle control unit 12 after taking measurements of engine rpm via the rpm sensor 54a, engine temperature via a sensor 56a, main air flow and main fuel flow, and referring these inputs to an idle control memory 1 2b.

In the idle control memory 1 2b, information is stored concerning the correct air:fuel ratio required for correct operation of the engine under idling conditions.

The idle control unit 12 can determine if the air:fuel ratio is too rich or too weak in the system, either by inference from the airflow meter input along line 62 and the computed fuel output along line 34, or alternatively by a sensor 66 which can be incorporated in the exhaust duct 67 and which would measure the actual air fuel ratio by measuring the properties of the exhaust gas and passing this information back to the main control unit 12a and the idle control unit 12 via line 68. For example, the sensor 66 could measure the free oxygen in the exhaust gases by a zirconia sensor, which generates an electrical signal proportional to the oxygen in the exhaust gas. Alternatively, the sensor 66 could measure carbon monoxide in the exhaust to give an indication of the air fuel ratio.

In electronic fuel control systems which employ fuel metering devices 16 with an insufficient fuel metering accuracy over the range from maximum power to idling, an additional electrical fuel metering device may be added to the system which is under direct control of the idle control unit 12. In this case, the control line 34a would pass directly to an auxiliary control valve oscillator, and in turn to the additional auxiliary fuel metering device.

By way of example, the following sensors may be used. The air mass flow meter 61 could be a vortex shedding device, an ionisation mass flow measuring device, or an ultrasonic mass flow measuring device.

The throttle sensor 48 could be a resistive potentiometer measuring the angular movement of the throttle blade. The rate of change of this resistance would give a measure of the engine acceleration required by the driver. The throttle sensor 48 may be associated with a throttle stop switch. In addition, a wide open throttle switch could be included for improved main system control. The manifold pressure sensor 63 could use various known pressure transducer means, i.e. piezoelectric devices, aeneroid bellows or strain gauges, attached to a diaphragm. The engine coolant temperature sensor 65a may be for example a thermistor. The engine rpm sensor 54a may be included in the ignition distributor and may sense pulses derived from the ignition system. Alternatively, the sensor 54a could be an electro-magnetic sensor situated on the engine main drive shaft.

Referring now to Figure 3, there is shown the idle speed control unit in simple schematic form. For simplification, the example shown uses a device for measuring air:fuel ratio directly from the engine exhaust. The engine exhaust 70 is measured by an air:fuel ratio sensor 71. The output 72 of the sensor 71 is compared with a preset air:fuel ratio 73, as contained in the idle control memory 12b, in an error measuring device 75. Any discrepancy between these two values is passed to a control logic unit 76.

Engine rpm 77 are measured by a rpm sensor 78 and compared in another error measuring device 79 with a preset rpm value 80. Any discrepancy between these two values is transferred to the control logic unit 76 via line 81. The control logic unit 76 processes this information as described below with reference to Figure 4. The control logic unit 76 controls the air and fuel independently or simultaneously for the engine via control lines 83 and 85, the fuel metering device 82 and the bleed air device 84, the loop being closed by the engine 69.

Referring now to Figure 4, there is shown logic circuitry suitable for the control unit 12 and the control unit logic 76. The logic circuitry is illustrated in diagrammatic form in Figure 4. In the example illustrated in Figure 4, the air:fuel ratio band for the engine is 14.5 to 15.0 and the engine speed band is 600 to 620 revolutions per minute. It will be seen from Figure 4 that a first check is made to see if the throttle is on its stop. If this check is positive, then the air:fuel ratio is computed, for example as indicated in Figure 1 from the main air and fuel flow coming from the carburettor 36, the bleed air coming from the conduit 10, and the bleed fuel coming from the device 16. If either or both of the air:fuel ratio and the engine revolutions per minute are outside their limits, then the bleed air passing along the conduit 10 and the bleed or the fuel passing through the device 16 will be altered to bring the engine speed and air:fuel ratio back to within the prescribed limits.

The order in which the steps are taken is based on the response of the engine to the alterations in the air and fuel flow.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected.

Claims (9)

1. A control system for maintaining the revolutions per minute of a spark ignition internal combustion engine within predetermined limits at least at idle conditions of the engine, which control system comprises air control means, fuel control means, first measuring means for measuring the air:fuel ratio being used by the engine, second measuring means for measuring the engine revolutions per minute, and electronic control means which is programmed to contain optimum engine air and fuel requirements at idling conditions and which is adapted to generate control signalsforthe air control means and the fuel control means in order to vary the amount of air and/or fuel being used by the engine in order to cause the engine to receive air and fuel in agreement with the programmed optimum air and fuel requirements.

2. A control system according to claim 1 in which the electronic control means ensures that the engine receives optimum air and fuel amounts at engine operating conditions above engine idling conditions.

3. A control system according to claim 2 in which the electronic control means ensures that the engine receives optimum air and fuel amounts at all engine operating conditions.

4. A control system according to any one of the preceding claims in which the air control means is an electrically controlled air metering device which is separate from a main airthrottleforthe engine.

5. A control system according to any one of the preceding claims in which the fuel control means is at least one electrically controlled fuel metering device which is separate from a carburettor supplying fuel to the engine under normal running conditions.

6. A control system according to any one of claims 1 to 4 in which the fuel control means is at least one fuel supply device supplying fuel to the engine under normal running conditions, the said at least one fuel supply device being an electrically controlled fuel metering device.

7. A control system according to any one of the preceding claims in which the electronic control means is an integrated circuit device.

8. A control system according to claim 7 in which the electronic control means is a microprocessor.

9. A control system substantially as herein described with reference to the accompanying drawings.