GB2064171A

GB2064171A - Control of Airfuel Ratio in an Automotive Emission Control System

Info

Publication number: GB2064171A
Application number: GB7940527A
Authority: GB
Original assignee: Rover Co Ltd
Current assignee: Rover Co Ltd
Priority date: 1979-11-23
Filing date: 1979-11-23
Publication date: 1981-06-10

Abstract

Closed loop feedback control is applied to a V-8 engine. The exhaust duct from each bank of four cylinders has an oxygen sensor 11, 12 and a three-way catalyst 13, 14. Two carburetters 9, 10 each feed some of the cylinders of one bank and some of the cylinders of the other. Experimental measurements are made of the change in voltage of each of the oxygen sensors produced by given changes in air fuel ratio of each carburetter around stoichiometric air- fuel ratio. A microprocessor stores those values and takes them into account when adjusting each of the carburetters. Engines other than V-8 engines can have a similar system. <IMAGE>

Description

SPECIFICATION Automotive Emission Control System This invention relates to automotive emission control systems.

A known method of air-fuel ratio control of in-line engines having a three-way catalyst includes anl oxygen sensor in the single exhaust duct which monitors the mixture supplied by the carburetter(s) and controls the carburetter(s) in a closed feedback loop.

However, this closed loop control has hitherto been difficult to achieve with engines having two exhaust ducts, one for each of two groups of cylinders, wherein there are two carburetters which each supply some of the cylinders of one group and some of the cylinders of the other group. For example, the normal V-8 engine has a cruciform crankshaft configuration with four 900 throws in order to achieve an improved balance compared with a flat crankshaft V-8 configuration. Because the angular movement of the crankshaft between the firing strokes of the four cylinders of a given bank is not uniform, for example, the period between firing number 1 and number 3 cylinders may be three times that between number 5 and number 7, an independent carburetter cannot satisfactorily supply one bank of cylinders.Each carburetter is therefore arranged to supply the two outer cylinders of one bank and the two inner cylinders of the other bank.

Closed loop control can be achieved by bringing the exhaust ducts together immediately downstream of the engine and controlling the two carburetters in accordance with a single oxygen sensor in the common duct but this may not be possible to achieve for reasons of space.

If, on the other hand, two oxygen sensors are provided, one in each outlet duct, and both carburetters are adjusted in accordance with the average reading of the sensors, either or both sensors and therefor catalysts may then be out of limits even though the average is correct. Further, even if each sensor is sufficiently close to the required mean to satisfy the catalyst, each carburetter may be either rich or weak and individual cylinders fed by a particular carburetter may be excessively rich or weak enough to cause misfire.

The invention provides an engine having two exhaust ducts, one for each of two groups of cylinders, an oxygen sensor in each exhaust duct, and two carburettors, each supplying at least some of the cylinders of one group and at least some of the cylinders of the other group, wherein the air-fuel ratio of each carburetter is individually controlled in accordance with the electrical output signals of both oxygen sensors and stored measurements of the change in output signals of both oxygen sensors produced by given changes in the air-fuel ratio of each individual carburetter.

The measurements are made for the particular carburetter/inlet manifold configuration over a range of engine speed and load conditions. The measurements may be obtained by running the engine on a rolling road or similar means of varying the load and speed independently.

The measurements of output electrical signals, which may be output voltage, may be made in the region of stoichiometric air-fuel ratio, say in the range of from 14 to 1 5 parts of air to one part of fuel, by weight, over which region the output signal of the sensor is tolerably linear. Preferably, the measurements stored for each sensor are two coefficients, each representing the average variation of electrical signal per unit change in air-fuel ratio of one of the carburetters over the range of values of air-fuel ratio over which the measurements are made.

Advantageously, a microprocessor is used to store the measurements and to control the air-fuel ratio of the carburetters.

The invention is especially applicable to a V-8 engine, but is also applicable to an arrangement where two banks of cylinders each having a carburetter is joined by a balance pipe.

An automotive emission control system will now be described, by way of example, with reference to the accompanying drawing, which shows the system schematically as applied to a V-8 engine.

Referring to the drawing, the engine has a cruciform crankshaft configuration and eight cylinders numbered in the firing order. The two carburetters 9, 10 each feed the two outer cylinders of one bank and the two inner cylinders of the opposite bank. All the exhaust from each bank is collected in a respective common duct which also contains an oxygen sensor 11, 12 and three-way catalyst 13, 14.

In accordance with the invention, each carburetter is individually controlled in use by a microprocessor 1 5 in accordance with the output voltages of both oxygen sensors and stored measurements of the change in output voltages of both oxygen sensors produced by given changes in the air-fuel ratio of each individual carburetter.

The measurements are made as follows. It is first necessary to determine the influence of each carburetter on each sensor. These "influence coefficients" are made for the particular carburetter inlet manifold configuration over a range of engine speed/load conditions.

To obtain the influence coefficients, the engine is run at a given condition on a rolling road or similar means of varying the load and speed independently. The microprocessor is instructed to temporarily suspend feedback control and adjust the air-fuel ratio control stepping motor of one carburetter a given number of steps whilst the setting of the other carburetter is held constant. This is done over a region of air-fuel ratio of from 14 parts of air to one part of fuel to 1 5 parts of air to one part of fuel, over which the variation of output voltage of the sensor with air-fuel ratio is tolerably constant.

The changes in voltage output of each sensor per unit change in air-fuel ratio are the influence coefficients of the first carburetter. The first carburetter is then held constant whilst the influence coefficients of the second carburetter are determined. The process is then repeated at a selected number of engine speed/load conditions and when the calibration is complete, feedback control is reinstated.

These coefficients are stored in a microprocessor together with the reference or target voltage for each sensor corresponding to stoichiometric air-fuel ratio. In operation, the microprocessor receives the actual voltage output from each sensor together with engine parameters such as speed and manifold depression and computes the appropriate adjustment N for each carburetter, using the formulae devised as follows:: Let VT,=Target voltage output for sensor 11 VT2=Target voltage output for sensor 1 2 VS,=Actual voltage output for sensor 11 VS2=Actual voltage output for sensor 12 N,=Required adjustment on carburetter 11 N2=Required adjustment on carburetter 1 2 a"=Sensor 11 influence coefficient of carburetter 9 a,2=Sensor 11 influence coefficient of carburetter 10 a2,=Sensor 12 influence coefficient of carburetter 9 a22=Sensor 12 influence coefficient of carburetter 10 Then AVS,=VT,VS,=error on sensor 11 AVS2=VT2-VS2=error on sensor 12 AVS,=a"N,+a,2N2 åVS2=a2, N, +a22N2 Solving the simultaneous equations: :- a22AVS1-a12AVS2 N, a1 1a22-a12a21 a21AVS1-a11AVS2 N2 a21a12 2"a22 In the event that the carburetter/manifold configuration gives perfect distribution to all cylinders such that all influence coefficients become equal at a particular engine condition, then the above equations are insoluble. In this event, the microprocessor is arranged to apply the same adjustment to both carburetters based on the arithmetic average of the errors on the two sensor.

Claims

1. An engine having two exhaust ducts, one for each of two groups of cylinders, an oxygen sensor in each exhaust duct, and two carburetters, each supplying at least some of the cylinders of one group and at least some of the cylinders of the other group, wherein the air-fuel ratio of each carburetter is individually controlled in use in accordance with the electrical output signals of both oxygen sensors and stored measurements of the change in output signals of both oxygen sensors produced by given changes in the air-fuel ratio of each individual carburetter.

2. An engine as claimed in claim 1, wherein the measurements of output electrical signal are made in the region of stoichiometric air-fuel ratio.

3. An engine as claimed in claim 2, wherein the measurements are made between air-fuel ratios of from 14 parts of air to one part of fuel to 1 5 parts of air to one part of fuel, the parts being by weight.

4. An engine as claimed in any one of claims 1 to 3, wherein two coefficients are measured for each sensor, each representing the average variation of electrical signal per unit change in air-fuel ratio of one of the carburetters over the region of air-fuel ratio variation.

5. An engine as claimed in any one of claims 1 to 4, wherein a microprocessor is used to store the measurements and to control the air-fuel ratio of the carburetters.

6. An engine as claimed in any one of claims 1 to 5, wherein the engine is a V-8 engine.

7. An engine substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.