Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-08-14
2001-06-12
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S276000, C060S277000, C060S285000
Reexamination Certificate
active
06244045
ABSTRACT:
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
2
O and CO
2
. Also, the Nox must be reduced to N
2
and O
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
Denion Thomas
Lotus Cars Limited
Tran Binh
Westman Champlin & Kelly P.A.
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