Method of monitoring efficiency of a catalytic converter and a control system suitable for use in the method

In a first aspect of the present invention there is provided a method of monitoring efficiency of a catalytic converter (14) present in an exhaust system (12) of an internal combustion engine (10) which has a closed loop control system (15, 23, 24) which in normal use of the engine controls richness of fuel/air charge supplied to a combustion chamber of the engine (10). The closed loop control system uses a feedback signal an output signal of an exhaust gas oxygen sensor (15) located in the exhaust system (12) downstream of at least a part of the volume of the catalytic converter (14). The method comprises the steps of: interrupting the normal operation of he engine (10) by stopping the closed loop control of the richness of the fuel/air charge; commencing open loop control of the richness of the fuel/air charge supplied to the combustion chamber; varying the richness of the fuel/air charge in an oscillatory manner during the open loop control by generating an oscillating open loop control signal; and using the output signal of the oxygen sensor (15) during open loop control to evaluate the efficiency of the catalytic converter (14). In a second aspect the present invention provides a control system which can operate according to the method.

The present invention relates to a method of monitoring efficiency of a
 catalytic converter present in an exhaust system of an internal combustion
 engine which has a closed loop control system which in normal use of the
 engine controls richness of fuel/air charge supplied to a combustion
 chamber of the engine, the closed loop control system using as a feedback
 signal an output signal of an exhaust gas oxygen sensor located in the
 exhaust system downstream of at least a part of the volume of the
 catalytic converter. The present invention also relates to a control
 system which controls richness of fuel/air charge supplied to a combustion
 chamber of an internal combustion engine and which monitors efficiency of
 a catalytic converter in the exhaust system receiving exhaust gas from the
 combustion chamber.
 The motor industry commonly refers to Air Fuel Ratios that contain excess
 air, and thus oxygen, as weak. Conversely Air Fuel ratios with excess fuel
 are referred to as rich. Strictly speaking it is a relative term with no
 absolute values, but in the case of catalyst operation the dividing line
 is generally taken to be Stoichiometric, the theoretical ratio where all
 the fuel is burnt together with all the available oxygen. For normal
 petrol this occurs at a ratio of 14.7:1. Thus for catalyst operation a
 weak mixture will be an Air Fuel Ratio in excess of 14.7:1, a rich mixture
 will be less than 14.7:1. Throughout the specification and claims when a
 mixture is referred to as rich then it will be a mixture with an excess of
 fuel and when a mixture is referred to as weak it will be a mixture with
 an excess of oxygen.
 In a petrol internal combustion engine three main pollutants are produced:
 unburnt hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen
 (NOx). In order to remove these poisonous gases from the exhaust gases of
 the internal combustion engine the HC and CO must be oxidised to
 respectively form H.sub.2 O and CO.sub.2. Also, the Nox must be reduced to
 N.sub.2 and O.sub.2. Since oxidation and reduction are opposite chemical
 processes the removal of all pollutants from exhaust gases represents a
 significant challenge.
 In order to meet the challenge internal combustion engine control systems
 the richness of the fuel/air charge supplied to combustion chambers of the
 internal combustion engine is cyclically varied from a weak mixture to a
 rich mixture, which results in the exhaust gases produced by the
 combustion having a varying oxygen content, the percentage amount of
 oxygen in the exhaust gases being relatively higher when the fuel/air
 mixture is weak than when the fuel/air mixture is rich. The cycling of the
 Air Fuel Ratio for correct catalyst operation would be typically from a
 weak extreme of 15.2:1 to a rich extreme of 14.2:1. It is important for
 the efficiency of the engine that the fuel/air ratio of the fuel/air
 charge supplied to the combustion chambers is held very close to
 stoichiometric. To achieve this level of control it has been necessary to
 use a closed loop control system which uses as a feedback signal an output
 signal of an oxygen sensor located in the exhaust system.
 When the closed loop control system is operating, as soon as the oxygen
 sensor in the exhaust system recognises that the oxygen content of the
 exhaust gas mixture indicates that the fuel/air mixture supplied to the
 combustion chamber is weak, then the control system for controlling the
 richness of the fuel/air charge mixture to the combustion chamber acts to
 increase the richness of the fuel/air charge (i.e. to increase the ratio
 of fuel to air). The control system continues to increase the richness of
 mixture of the fuel/air charge until the oxygen sensor senses an oxygen
 content in the exhaust gases which indicates that a rich fuel/air mixture
 is being supplied to the combustion chambers.
 When the sensor recognises that the oxygen content of the exhaust gases
 indicates that there is a rich fuel/air mixture supplied to the combustion
 chambers, then the control system for controlling the richness of the
 fuel/air charge weakens the richness of the fuel/air charge (i.e. reduces
 the ratio of fuel to air). The richness of the fuel/air charge is then
 further weakened by ramping down the amount of fuel mixed with the
 incoming air until the oxygen sensor in the exhaust system again notes
 that the oxygen content of the exhaust gas mixture indicates that a weak
 fuel/air mixture is being supplied to the combustion chambers. The process
 is continuous, with the richness of the fuel/air charge continually
 oscillating about stoichiometric. This results in the desired degree of
 control. Control can be improved by careful setting of the ramping rates
 for increase and decrease of the richness of the fuel/air mixture and
 particularly important are the initial shifts in the degree of richness
 once the oxygen sensor has noted an oxygen content which indicates a rich
 or lean fuel/air mixture. These factors also have a significant effect on
 the cycling frequency and amplitude of the control system.
 In most prior art control systems, the oxygen sensor in the exhaust system
 is located in front of all of the catalyst volume in the exhaust system.
 However, in some prior art systems the oxygen sensor is located in the
 exhaust system downstream of a small starter catalytic converter but
 upstream of a larger volume normal running catalytic converter. The
 location of the oxygen sensor downstream of the small starter catalytic
 converter has the effect of slowing down the cycling frequency but the
 small starter catalytic converter will protect the oxygen sensor from
 substances which can poison the sensor, thus improving durability of the
 sensor.
 A conventional method of monitoring the efficiency of a catalytic converter
 in an exhaust system does not disturb the normal closed loop control of
 the richness of the fuel/air charge, described above. Instead, the
 conventional method passively monitors the output signal of a second
 oxygen sensor present in the exhaust system to determine the efficiency of
 the catalytic converter by using a correlation between oxygen storage and
 converter efficiency.
 In prior art systems which have a closed loop control system for
 controlling the richness of the fuel/air charge based upon the output
 signal of an oxygen sensor located upstream of all catalytic converters in
 the exhaust system, then the second oxygen sensor used for monitoring
 catalytic converter efficiency is normally located just downstream of the
 catalytic converter which is monitored. An efficient catalytic converter
 will absorb oxygen and therefore the output of an oxygen sensor mounted
 downstream of the catalytic converter will be significantly damped when
 compared to the output of the controlling oxygen sensor upstream of the
 catalytic converter. An algorithm is used to measure the degree to which
 the output of the second oxygen sensor is damped in comparison with the
 output of the controlling oxygen sensor used by the closed loop control
 system. The condition of the monitored catalytic converter can then be
 determined by making a comparison of the is degree of damping with tables
 stored in the memory of the control system. The tables will be determined
 during calibration of the system.
 In prior art systems in which the controlling oxygen sensor providing the
 feedback signal for closed loop control is located downstream of a
 catalytic converter, then the second oxygen sensor for measuring the
 performance of the catalytic converter is located upstream of the
 catalytic converter, i.e. with no catalytic converter present in the
 exhaust system upstream of the second oxygen sensor. Because the
 controlling sensor is behind the catalytic converter being monitored, the
 output of the second oxygen sensor will not be damped in comparison to the
 output of the controlling oxygen Sensor. However, the switching frequency
 of the controlling oxygen sensor will be affected by gas transit time
 through the catalytic converter and also by a phase lag resulting from
 oxygen storage in the catalytic converter. From a comparison between the
 output signal of the second oxygen sensor and the output signal of the
 controlling oxygen sensor the phase lag resulting from oxygen storage in
 the catalytic converter can be determined. The phase lag is then compared
 to stored calibration tables and thus the condition of the catalytic
 converter is ascertained.
 In U.S. Pat. No. 5,157,919, a method of monitoring the performance of a
 catalytic converter is described which is different to the two standard
 methods which have already been mentioned. The method is different because
 it is intrusive, i.e. it disturbs the normal closed loop operation of the
 closed loop control system controlling the richness of the fuel/air
 mixture supplied to the combustion chamber. In the described method the
 controlling sensor of the closed loop control system is located downstream
 of the catalytic converter. As described above, during normal control of
 the engine, the richness of the fuel/air mixture supplied to the
 combustion chambers is cyclically varied about stoichiometric with ramping
 rates chosen to maximise efficiency. As also mentioned above, variation of
 the ramping rates during closed loop control of the richness of the
 fuel/air mixture supplied to the combustion chambers will have a
 significant effect upon the cycling frequency of the closed loop control
 and also upon the variation in amplitude of the control signal produced by
 the closed loop control system.
 In U.S. Pat. No. 5,157,919 it is recognised that the catalytic converter in
 the exhaust system will impose a phase lag on the output signal of the
 controlling oxygen sensor and also it is recognised that the oxygen
 storage capacity of the catalytic converter will have an impact on the
 cycling frequency and the amplitude of the closed loop control signal. The
 monitoring method of U.S. Pat. No. 5,157,919 changes the ramping rates
 used by the closed loop controller and/or changes the amount by which the
 richness of the fuel/air charges are initially varied when the controlling
 oxygen sensor notes an oxygen content in the exhaust gases which indicates
 a rich or a weak fuel/air mixture. By observing the effects of this
 variation upon cycling frequency and on amplitude of the output signal of
 the oxygen sensor, the monitoring system in U.S. Pat. No. 5,157,919
 determines the phase lag between the control signal for controlling the
 richness of the fuel/air mixture and the output signal of the controlling
 oxygen sensor and thereby determines the oxygen storage capacity of the
 catalytic converter. The determined oxygen storage capacity of the
 catalytic converter is then used as a measure of the efficiency of the
 catalytic converter. Throughout the monitoring of the efficiency of the
 catalytic converter the closed loop control of the richness of the
 fuel/air charge is maintained.
 The present invention provides a method of controlling richness of fuel/air
 charge supplied to a combustion chamber of an internal combustion engine
 and of monitoring efficiency of a catalytic converter present in an
 exhaust system of the internal combustion engine receiving exhaust gas
 from the combustion chamber, the method comprising:
 closed loop control of the fuel/air charge supplied to the combustion
 chamber of the engine during normal use of the engine, the closed loop
 control system using as a feedback signal an output signal of an exhaust
 gas oxygen sensor located in the exhaust system;
 interrupting the normal closed loop control of the richness of the fuel/air
 charge by commencing open loop control of the richness of the fuel/air
 charge supplied to the combustion chamber; and
 varying the richness of the fuel/air charge in an oscillatory manner during
 the open loop control by generating an oscillating open loop control
 signal; and
 using the output signal of an oxygen sensor in the exhaust system during
 open loop control to evaluate the efficiency of the catalytic converter;
 characterised in that a single oxygen sensor located downstream of at
 least a part of the volume of the catalytic converter is used to provide
 the feedback signal during the closed loop control and to provide, during
 open loop control, the output signal used to evaluate the efficiency of
 the catalytic converter.
 Preferably the oscillating open loop control signal is not used to control
 the richness during normal closed loop control. Preferably during open
 loop control a control signal is used for controlling the richness which
 is a combination of the oscillating open loop control signal and a closed
 loop control signal produced by the closed loop control system.
 Alternatively, during open loop control the oscillating open loop control
 signal is used on its own to control richness.
 It can be seen that in the method of the present invention the normal
 closed loop control of the richness of the fuel/air charge supplied to the
 combustion chamber is interrupted. The method then uses a generated open
 loop control signal to control the richness of the fuel/air charge and
 this open loop control signal will oscillate the richness of the fuel/air
 charge about stoichiometric in a manner similar to the way in which the
 richness of the fuel/air charge is oscillated in prior art systems by
 closed loop control using a feedback signal from an oxygen sensor upstream
 of a catalytic converter. To operate the method only one oxygen sensor is
 needed and this is located downstream of the monitored catalytic
 converter. The output signal of the oxygen sensor is used to evaluate the
 efficiency of the catalyst in a manner similar to the way in which in some
 prior art systems two output signals are compared from a controlling
 oxygen sensor located before a catalytic converter and a monitoring sensor
 located after the catalytic converter. In the present method the output
 signal which would have been produced by the controlling sensor upstream
 of the catalyst is instead replaced by the open loop control system signal
 and the output signal which would have been provided in the prior art by
 the second monitoring oxygen sensor downstream of the catalytic converter
 which is used in the normal closed loop control of the richness of the
 fuel/air mixture.
 Preferably the method includes the steps of:
 running the engine for a period with the closed loop control system in
 normal operation in order to establish steady state operating conditions;
 and
 testing for steady state operating conditions; and
 interrupting the normal operation of the engine and commencing open loop
 control of the richness of the fuel/air charge only after the testing has
 established that steady state operating conditions exist.
 In order for the diagnostic test of the method to operate correctly without
 errors, the test has to be carried out during steady state operating
 conditions within preset parameters of engine operation.
 Preferably the engine is used to power a land vehicle and the testing for
 steady state operating conditions comprises:
 measuring temperature of the monitored catalytic converter and determining
 whether the measured catalyst temperature is within predefined catalyst
 temperature limits;
 measuring temperature of liquid coolant in the engine and determining
 whether the measured coolant temperature is within predefined coolant
 temperature limits;
 measuring rate of airflow to the combustion chamber of the engine and
 determining whether the measured airflow is within predefined rate of
 airflow limits;
 measuring throttle position of the throttle of the engine and determining
 whether the measured throttle position is above a predefined throttle
 position limit;
 measuring manifold pressure of the engine and determining whether the
 measured manifold pressure is above a predefined manifold pressure limit;
 measuring rate of change of the manifold pressure and determining whether
 the measured rate of change is below a predefined rate of change of
 manifold pressure limit;
 measuring speed of revolution of the engine and determining whether the
 measured speed of revolution is above a predefined speed of revolution
 limit; and
 measuring speed of the land vehicle and determining whether the measured
 speed is above a predefined vehicle speed limit.
 The parameters of catalyst temperature, liquid coolant temperature, rate of
 airflow, throttle position, manifold pressure/rate of change of manifold
 pressure, speed of revolution of the engine and speed of the land vehicle
 could either be measured directly or alternatively could be measured
 indirectly by deriving them from signals produced by other sensors. For
 instance, an estimate of catalyst temperature can be made by extrapolating
 from direct measurements of engine speed, inlet manifold pressure, coolant
 and time from engine starting. Also rate of air flow can be indirectly
 measured by calculating the rate of airflow from the measured engine
 speed, inlet manifold pressure and air inlet temperature. Furthermore,
 manifold pressure can be estimated from measured airflow. Throughout the
 specification and drawings any reference to measuring a parameter includes
 both direct and indirect measurement and any reference to a measured
 parameter includes both a directly and an indirectly measured parameter.
 Preferably the output signal of the downstream oxygen sensor is used to
 determine the efficiency of the catalytic converter by filtering the
 output signal of the downstream oxygen sensor and then comparing the
 filtered output signal with the unfiltered output signal.
 Preferably both the filtered and unfiltered output signals are sampled
 digitally and the comparing of the filtered output signal with the
 unfiltered output signal comprises subtracting each sample of the filtered
 output signal from each unfiltered output signal to produce remainder
 signals.
 Preferably a damping of the output signal is estimated by eliminating all
 negative remainder signals and integrating all positive remainder signals
 to produce an integrated remainder signal.
 Preferably the estimated damping is compared with a predetermined value of
 damping by comparing the value of the integrated remainder signal with a
 predefined threshold value.
 Preferably the catalytic converter is noted to have failed an efficiency
 test if the value of the integrated remainder signal is greater than the
 predefined threshold value. The catalytic converter is noted to have
 passed the efficiency test if the value of the integrated remainder signal
 is less than the threshold value.
 Preferably the normal closed loop operation of the engine is re-established
 after the efficiency of the monitored catalytic converter is established.
 It will be appreciated that the monitoring operation can occur at regular
 intervals during use of the internal combustion engine, to monitor the
 performance of the catalytic converter throughout its life. When the
 catalytic converter has failed the efficiency test then the user of the
 engine can be notified, e.g. by a warning light, that the catalytic
 converter is no longer functioning as required.
 The testing for steady state operating conditions can continue throughout
 the open loop control of the richness of the fuel/air charge and normal
 closed loop operation of the engine is re-established when the testing
 establishes that steady state operating conditions no longer exist.
 The present invention in a second aspect provides a control system which
 controls richness of fuel/air charge supplied to a combustion chamber of
 an internal combustion engine and which monitors efficiency of a catalytic
 converter in an exhaust system receiving exhaust gas from the combustion
 chamber, the control system comprising:
 a closed loop control subsystem which controls the richness of the fuel/air
 charge using as a feedback signal a signal output by an oxygen sensor
 located in the exhaust system;
 an open loop control subsystem which controls the richness of the fuel/air
 charge by generating an oscillating open loop control signal; and
 a monitoring subsystem which uses an output signal of an oxygen sensor
 located in the exhaust system to evaluate efficiency of the catalytic
 converter; wherein
 the control system in normal working of the engine uses only the closed
 loop control subsystem to control the richness of the fuel/air charge; and
 the control system when monitoring the efficiency of the catalytic
 converter uses the open loop control subsystem to at least partially
 control the richness of the fuel/air charge and uses the monitoring
 subsystem to evaluate the efficiency of the catalytic converter;
 characterised in that
 a single oxygen sensor located downstream of at least a part of the volume
 of the catalytic converter is used to provide the feedback signal used by
 the closed loop control subsystem and also to provide the output signal
 used by the monitoring subsystem to evaluate the efficiency of the
 catalytic converter during open loop control.

Referring to FIG. 1 there can be seen schematically an internal combustion
 engine 10 in a road vehicle (not shown) which receives a fuel/air charge
 via a manifold 11 and which exhausts combusted gases to an exhaust system
 12. A throttle valve 13 is used to throttle airflow through the inlet
 manifold 11. A catalytic converter 14 is present in the exhaust system 12
 and acts to remove poisonous gases from the exhaust system 12. The
 catalytic converter 14 is a three-way catalytic converter which removes
 from the exhaust gases unburnt hydrocarbons (HC), carbon monoxide (CO) and
 oxides of nitrogen (NOx).
 An oxygen sensor 15 is provided in the exhaust system 12 downstream of the
 catalytic converter 14. The oxygen sensor 15 measures the amount of oxygen
 in the exhaust gases flowing out of the catalytic converter 14.
 A sensor 16 is mounted to the catalytic converter 14 and measures the
 temperature of the catalytic converter 14. A sensor 17 is provided to
 measure the rotational speed of the engine 10. A sensor 18 is provided to
 measure the temperature of the coolant water flowing in the engine 10. A
 sensor 19 is provided to measure the pressure of the fuel/air charge in
 the inlet manifold 11 of the engine 10.
 A sensor 20 is provided to measure the position of the throttle 13.
 A sensor 21 is provided to measure the road speed of the land vehicle.
 A sensor 25 is provided to measure airflow to the combustion chambers of
 the engine 10.
 A filter 22 is provided to filter the output signal of the oxygen sensor 15
 and to supply a filtered output signal to a controller 23. In practice the
 filter 22, shown for convenience separate from the controller 23, will be
 part of the controller 23.
 The digital electronic controller 23 is provided to control the richness of
 the fuel/air mixture delivered to the engine 10 and also to monitor the
 efficiency of the catalytic converter 14. The electronic digital
 controller 23 receives digitally sampled signals from each of the
 previously described sensors 15, 16, 17, 18, 19, 20, 21 and 25. The
 digital controller 23 also receives a heavily filtered version of the
 output signal of the sensor 15 which is filtered by the filter 22. The
 filter 22 might not be a separate component, but instead could be a
 digital routine operated by the digital controller 23.
 The electronic digital controller 23 controls a fuel injection system 24
 which controls the richness of the fuel/air charge delivered through the
 inlet manifold 11 to the engine 10 for combustion in the combustion
 chambers of the engine 10. The digital electronic controller 23 also
 monitors the performance of the catalytic converter 14 and when the
 catalytic converter does not meet the required standard of performance
 then the controller 23 will activate a warning light 26 which will warn
 the driver of the land vehicle that the catalytic converter does not meet
 the required standard of performance.
 The method of operation of the control system will now be described.
 In normal operation of the engine 10 the richness of the fuel/air charge
 delivered via the inlet manifold 11 to the engine 10 is controlled by a
 closed loop control subsystem comprising the electronic digital controller
 23, the oxygen sensor 15 and the fuel injection system 24. The controller
 23 controls the fuel injection system 24 to continuously vary the richness
 of the fuel/air charge delivered via the inlet manifold 11 to the engine
 10. The richness of the mixture is controlled in dependence upon the
 output signal of the oxygen sensor 15. The closed loop richness control is
 shown in Section A of FIG. 2 for a good performance catalyst and in
 Section AA of FIG. 3 for a poor performance catalyst.
 As soon as the output signal of the oxygen sensor 15 indicates that the
 exhaust gases downstream of the catalytic converter 14 have a relatively
 high oxygen content which indicates that the fuel/air charge is a weak
 mixture then the controller 23 controls the fuel injection system 24 to
 increase the richness of the mixture of the fuel/air charge supplied to
 the engine 10 (i.e. increases the ratio of fuel to air) by first causing a
 shift in the richness of the mixture and then further ramping the mixture
 richer.
 Eventually, the richness of the fuel/air mixture in the inlet manifold 11
 will be ramped to such a rich mixture that the oxygen sensor 15 detects a
 relatively low oxygen content in the exhaust gases after the catalytic
 converter 14 which indicates that the fuel/air charge is a rich mixture.
 The output signal of the oxygen sensor 15 will indicate this. When a rich
 fuel/air charge is noted by the oxygen content sensed by the oxygen sensor
 15 then the controller 23 will cause the fuel injection system 23 to
 weaken the richness of the fuel/air charge (i.e. decrease the ratio of
 fuel to air) in the inlet manifold 11, first by an initial shift and then
 by gradually ramping the mixture weaker and weaker. Eventually, the oxygen
 sensor 15 will note that the exhaust gases after the catalytic converter
 14 again have a relatively high oxygen content indicating a weak fuel/air
 charge and then the process will start again. The process will operate
 continually during normal closed loop control of the engine so that the
 richness of the fuel/air mixture supplied to the engine 10 is varied
 cyclically with time continuously. The initial shift of the ratio of the
 fuel/air mixture both richer and weaker and also the ramping rates richer
 and weaker will have a significant effect on the engine and are critical
 for the correct operation of the catalyst and therefore will have to be
 controlled carefully by the controller 23, in a known manner.
 The digital electronic controller 23 will also operate to monitor the
 efficiency of operation of the catalytic converter 14 by carrying out
 tests from the catalytic converter at regular intervals during the
 operation of the engine 10.
 When the controller 23 commences the testing operation then first of all it
 will ensure that the engine 10 is working at steady state operating
 conditions. The controller 23 ensures this by carrying out the following
 series of tests:
 1. The controller 23 checks the output signal of the sensor 16 and ensures
 that the measured catalyst temperature is within predefined limits stored
 in memory by the controller 23 (i.e. above a first threshold value and
 below a second threshold value);
 2. the controller 23 checks whether the output signal of the sensor 16
 indicates that the temperature of the water coolant in the engine 10 is
 above a predefined threshold value stored by the controller 23 in memory;
 3. The controller 23 checks whether the output signal of the sensor 25
 indicates that the airflow is within predetermined limits stored in the
 memory of the controller 23 (i.e. above a first threshold value and below
 a second threshold value);
 4. The controller 23 checks whether the output of the sensor 20 indicates
 that the throttle position is above a threshold value stored in the memory
 of the controller 23;
 5. the controller 23 ascertains whether the output signal of the sensor 19
 indicates that the manifold pressure in the air inlet manifold 11 is
 within predetermined limits, stored in the memory of the controller 23
 (i.e. above a first threshold value and below a second threshold value);
 6. the controller 23 will differentiate the output signal of the sensor 19
 with respect to time to derive a rate of change of manifold pressure
 signal and the controller 23 will check that this rate of change of
 manifold pressure signal is below a threshold value stored in the memory
 of the controller 23;
 7. the controller 23 will check that the output signal of the sensor 18
 indicates that the rate of revolution of the engine 10 is above a
 threshold value stored in the memory of the controller 23;
 8. the controller 23 will check that the output signal of the sensor 21
 indicates that the road speed of the land vehicle is above a threshold
 value stored in the memory of the controller 23.
 When all of the above tests show that the sensor parameters are within
 predefined limits then the controller 23 will start to test the
 functioning of the catalytic converter 14. The controller 23 will do this
 by first of all interrupting the normal closed loop control of the
 richness of the fuel/air charge. The controller 23 will initiate control
 of the richness of the air/fuel mixture by an open loop control subsystem.
 The open loop control subsystem is "open loop" in that its control signal
 is produced without reference to a feedback signal, the oxygen sensor
 signal. The open loop subsystem generates an oscillating open loop control
 signal which controls the fuel injection system 24 and varies the richness
 of the fuel/air charge about stoichiometric in cyclical fashion.
 On initiation of open loop control the fuelling would be changed by an
 amount (either rich or lean) but for the purpose of this example, assumed
 rich). The fuelling would then be further changed in the same direction at
 a predetermined rate each second until either an integrated airflow limit
 was achieved or a set period of time had elapsed. The output signal of the
 airflow sensor 25 could be integrated to provide a signal to be checked
 against a preprogrammed airflow limit. Once the airflow or time limit is
 reached, then the fuelling would be changed by an amount in the lean
 direction. The fuelling would be changed now in the lean direction at a
 rate each second until the same integrated airflow limit or time limit is
 reached. The fuelling would then be enriched again, repeating the cycle
 until a specified number of cycles had been completed.
 The open loop control signal is graphically represented in Section B of
 FIG. 2 and Section BB of FIG. 3. The closed loop control subsystem could
 be stopped completely and the open loop control signal generated by the
 open loop control signal could be used in isolation as the only control
 signal controlling richness of fuel/air mixture. However, in the preferred
 embodiment the closed loop control system remains operational during the
 generating of the open loop signal and adjusts the switch envelope or the
 mean value of the control signal in response to oxygen sensor output. In
 Section B of FIG. 2 it can be seen that the average value of the control
 signal is ramped gradually during open loop control.
 The open loop control signal could be made dependent on the measured engine
 speed and inlet manifold pressure, but generally because the steady state
 operating conditions for the test are strictly monitored, this will not be
 required. The controller 23 will carry out the checks 1 to 8 above
 throughout the testing of the functioning of the catalytic converter 14
 and will stop the test of the catalytic converter 14 if any of the checks
 1 to 8 are not satisfied.
 During control of the richness of the fuel/air charge by the open loop
 control subsystem, the output signal of the oxygen sensor 15 is used to
 evaluate the performance of the catalytic converter 14. The open loop
 control signal generated by the controller 23 is of a frequency higher
 than the usual frequencies of the control signal in closed loop control.
 This can be seen by comparing Sections A and B in FIG. 2 and Sections AA
 and BB in FIG. 3. The high frequency open loop signal is combined with the
 closed loop control signal in the preferred embodiment to produce a
 control signal for controlling richness which has a high frequency
 component attributable to the open loop control system and a ramping of
 mean value of the control signal which is controlled by the closed loop
 control signal. It can be seen from FIG. 2, Section C, that a good
 performance catalyst is very good at damping high frequency changes in
 fuel/air richness, but in FIG. 3 it can be seen at Section CC that a poor
 performance catalyst is bad at damping the same high frequency changes.
 The controller 23 will receive via the filter 21 a heavily filtered output
 signal from the oxygen sensor 15 and then the heavily filtered signal,
 which is a digital signal, is subtracted from the unfiltered output signal
 of the sensor 15 to leave remainder signals. This will result usually in
 both negative and positive value remainder signals. The negative value
 signals are eliminated and the positive value signals are integrated. The
 integration continues until the richness of the fuel/air charge in the
 manifold 11 has been switched from rich to lean a specified number of
 times. After this, the integrated remainder signal is compared by the
 controller 23 with a threshold value stored in memory. If the value of the
 integrated remainder signal is greater than the threshold value then the
 catalytic converter 14 is noted to have failed the monitoring test. If the
 value of the integrated remainder signal is less than the stored threshold
 value then the condition of the catalytic converter 14 is considered to be
 satisfactory.
 Using the above described methodology, the oxygen sensor 15 is used to
 estimate the degree by which the output signal has been damped. The signal
 should be significantly damped if the catalytic converter is working
 properly because the catalytic converter 14 in its operation will absorb
 oxygen. The integrated remainder value given by the test gives an
 indication of the oxygen storage capacity of the catalytic converter 14.
 If the oxygen storage capacity is below a specified level then failure of
 the catalytic converter is noted.
 The open look control signal has a frequency of approximately 2 Hz and an
 amplitude which gives a .+-.5% fuelling perturbation. The open loop
 control would last typically for 5 seconds.
 It is important to make sure that the test is carried out at the correct
 steady state conditions, because the specified levels stored in the memory
 of the controller 23 which is used for the test will have been determined
 by running an engine with a functioning catalytic converter at specified
 steady state conditions and then determining the oxygen storage capacity
 of the catalytic converter 14.
 Once the test procedure for testing the efficiency of the catalytic
 converter 14 has been completed, then the controller 23 will control the
 richness of the fuel/air charge using only the closed loop control system,
 the oxygen sensor 15 being used to provide a feedback signal in the closed
 loop control subsystem.
 The test procedure described above would run whenever the checks 1 to 8 are
 satisfied until such time as the testing procedure has produced a result.
 The procedure would not be operated again until the engine was run once
 more after a period of the ignition having been switched off. If the test
 was interrupted due to operating conditions changing, as soon as the
 checks 1 to 8 are satisfied again the test would start once more. If the
 test runs to completion, irrespective of the result, it would not operate
 again until the next time the engine was run (after an engine off period)
 and checks 1 to 8 were satisfied.
 In the system proposed by the current invention it may be necessary to
 set,steady state condition requirements which are tighter than those
 required in the prior art in order to maintain adequate fuelling control
 during open loop operation. It is also possible that it may be necessary
 to learn a switching calibration for the open loop control system during
 normal operation of the engine, which could then be used during the test
 of the efficiency of the catalytic converter 14.
 Whilst in the embodiment described above the oxygen sensor 15 is shown in
 the exhaust system 12, separate from the catalytic converter 14, it is
 possible that the sensor 15 could be mounted in the catalytic converter 14
 to measure the oxygen in the exhaust gas flow after it has flowed through
 only a part of the volume of the catalytic converter 14. This can be
 preferable for two reasons. With the system set up to monitor the minimum
 practical volume of the catalytic converter, this will provide maximum
 integrated remainder signals at low levels of total system failure.
 However, the monitored volume of the catalyst must be chosen carefully so
 that it is not so small that catalytic converter failure is noted before
 the whole catalytic converter fails to such an extent that emission limits
 are surpassed.
 It is also advantageous to monitor the performance of only a part of the
 volume of the catalytic converter 14 because if the monitored volume is
 too large the normal closed loop control subsystem will be impaired in
 operation because of a resulting very low switching frequency.
 As mentioned previously, whilst the use of an oxygen sensor measuring
 oxygen in exhaust gas flow after the catalytic converter can cause
 problems of low switching frequencies, it does have the advantage that the
 sensor is protected by the catalytic converter from being poisoned by
 components of the exhaust gas.
 Whilst in the embodiment described above a sensor 16 is provided to
 directly measure the temperature of the catalytic converter, instead the
 temperature of the catalytic converter could be indirectly measured by the
 controller 23 determining the temperature from the directly measured
 engine speed (measured by sensor 17), the inlet manifold pressure
 (measured by sensor 19), the engine coolant temperature (measured by
 sensor 18), and the time from starting of the engine (measured by an
 unillustrated timer).
 Whilst in the embodiment described above a sensor 25 is used to directly
 measure airflow to the combustion chambers, the airflow can be indirectly
 measured by the controller 23 determining the airflow from the directly
 measured engine speed (measured by the sensor 17), the directly measured
 manifold pressure (measured by the sensor 19) and from directly measured
 air inlet temperature (measured by an unillustrated sensor).
 The invention allows the use of one oxygen sensor in an exhaust pipe to
 both provide a feedback signal for closed loop control and also a measure
 of catalyst performance. Most prior art systems required two sensors, one
 for each purpose. The present invention provides a cost saving by reducing
 the number of sensors necessary and this is particularly noticeable for
 V-engines because separate catalysts are typically used for each bank of
 cylinders.