Motor vehicle exhaust catalyst regeneration

Details Motor vehicle exhaust catalyst regeneration Motor vehicle exhaust catalyst regeneration

1999-12-16

2001-05-15

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S295000

Reexamination Certificate

active

06230485

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method for regenerating an aged motor vehicle exhaust catalytic converter.
Three-way catalysts in an exhaust gas purification system degrade with use, owing to oxidation of the noble metals used in such catalysts, normally Pt, Pd and Rh. This oxidation is a result of the normally lean burn engine operating conditions, typically an air/fuel super-stoichiometric level of lambda &lgr;=1.01, which is set in order to achieve improved fuel economy, and a suitable balance between the purification efficiencies of the three catalysts for unburned hydrocarbons, CO and NOx.
Patent document EP 0 490 612 A1 discloses an engine control system in which timers measure lean and rich running periods which alternate on a time scale of seconds or less.
It has been proposed in patent document U.S. Pat. No. 4,729,220 that the rate of oxidation of Rh in a three-way catalyst system may be reduced by reducing the air/fuel ratio to stoichiometric &lgr;=1 or below, when the catalyst temperature exceeds 750° C., depending also on engine load operating conditions. However, with this system it is necessary to use continuously temperature sensors and engine load sensors in order to set the air/fuel mixture, which is relatively complicated and expensive. Furthermore, in some conditions, for example in a hot ambient environment, or heavy engine load, the catalyst may be continuously above the 750° C. limit meaning that the air/fuel mixture is set at stoichiometric or below for an extended period of time, which may result in non-optimal fuel consumption or catalyst efficiency.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a more convenient and economical apparatus and method of improving the performance of a three-way catalytic converter as the converter ages.
Accordingly, the invention provides an exhaust gas purification system for an internal combustion engine, comprising:
an internal combustion engine having an exhaust conduit;
an engine management system with control means capable of controlling the engine air/fuel ratio alternately at a super-stoichiometric level and at a sub-stoichiometric level;
a three-way catalyst in-line with the exhaust conduit and comprising noble metals selected from the platinum group and rhodium, the catalyst exhibiting a three-way oxidizing activity on the exhaust gases, and the efficiency of the three-way activity degrading over time when the engine is operated at a super-stoichiometric level;
characterized in that the engine management system comprises timer and memory means to measure and store more than one cumulative time, including a first cumulative time over which the control means controls the engine at the super-stoichiometric level and during which the efficiency of the three-way activity degrades, and also a second cumulative time over which the control means controls the engine at a sub-stoichiometric level, the alternation between levels being such that the first cumulative time is greater than the second cumulative time, and the second cumulative time is between about one and ten hours to regenerate at least partially the efficiency of the three-way activity.
The engine air/fuel mixture is therefore cycled between alternate super- and sub-stoichiometric levels at predetermined times according to the cumulative engine operating times.
The engine may therefore be operated for a time, particularly an extended time at a super-stoichiometric level for improved fuel economy and for balanced catalyst performance, during which time the catalyst performance will naturally degrade through oxidation of the noble metals. The extended period may be at least one hour, but is more likely to be considerably more, for example, typically 100 to 300 hours of normal ageing conditions which would equate roughly to 5,000 km to 30,000 km of motoring. The extended period could be as much as about 1000 hours of normal ageing conditions, although this would result in a typical three-way catalyst degrading to about 50% efficiency, and it is preferred to take action to recover partially the catalyst performance before this level of degraded efficiency is reached.
Surprisingly, it has then been found that if the air/fuel mixture is changed to a sub-stoichiometric level, the performance of a typical three-way catalyst can gradually be partially recovered. This recovery takes place over a period of between about one and ten hours of engine operating time. The recovery process consists of the oxygen-getting species in the exhaust gas, such as unburned hydrocarbons, carbon monoxide, scavenging oxygen from the partially oxidized noble metals.
The amount of recovery depends on a number of factors, but in the case where the performance has degraded to about 50% to 75% of initial performance, the recovery will be to about 80% to 90% of the initial performance. It is not possible to recover fully the catalyst performance owing to loss of active surface area from thermal fusing of the porous catalyst structure.
During the recovery stage, the fuel economy and catalyst performance will be slightly degraded from what could be achieved at the normal super-stoichiometric air/fuel levels, but this loss is outweighed by the catalyst performance improvement upon a return to these normal air/fuel levels, as long as the time spent on average at a sub-stoichiometric level is no more than {fraction (1/10)} that spent at a super-stoichiometric level. Although many different regeneration alternation strategies may be envisaged, it is therefore preferable if the alternation between levels is such that the first cumulative time is at least ten times the second cumulative time.
In one embodiment of the invention, the timer means is reset following a return to an air/fuel ratio at a super-stoichiometric level. The alternation strategy for subsequent cycles is then the same as for the first cycle. However, it would be possible for the first and second predetermined times, and the predetermined cumulative times, to be different for each super/sub-stoichiometric cycle.
The regeneration process works best if the catalyst has reached or is approaching a normal operating temperature. Therefore, there is little benefit if the air/fuel mixture is set to sub-stoichiometric while the engine is still warming up. Therefore, the timer may measure a third cumulative time from the time an engine starts to operate. The operation at a sub-stoichiometric level may then be overridden until the three-way catalyst has reached a temperature of at least about 500° C. Alternatively, it would be possible to use a temperature sensor in the catalyst, or at a suitable point in the exhaust stream. However, this would entail either the use of a nonstandard three-way catalyst, or the provision of a port or mounting point on an exhaust pipe, plus the wiring of a temperature sensor to an engine management system. This would add to the cost and complexity of the system, and it therefore preferred if the system waits and operates the engine at a super-stoichiometric level, if necessary, until a predetermined time sufficient for the catalyst to be expected to have reached a temperature of at least about 500° C.
Also according to the invention, there is provided a method of operating a motor vehicle exhaust catalytic converter, the engine comprising an engine management system with control means capable of controlling the engine air/fuel ratio alternately at a super-stoichiometric level and at a sub-stoichiometric level, a timer and memory means to store measured times, the converter comprising a three-way catalyst in-line with a motor vehicle exhaust conduit and having noble metals selected from the platinum group and rhodium, the catalyst exhibiting three-way oxidising activity on the exhaust gasses, characterised in that the method comprising the steps of:
i) using the control means to control the engine firstly at the super-stoichiometric level;
ii) using the timer and memory means to measure and store a first cumulative time du

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