Exhaust purification device for internal combustion engine

Details Exhaust purification device for internal combustion engine Exhaust purification device for internal combustion engine

1999-04-13

2001-05-29

Jeffery, John A. (Department: 3742)

Power plants

Internal combustion engine with treatment or handling of...

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

Reexamination Certificate

active

06237330

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an exhaust purification device for an internal combustion engine.
BACKGROUND OF THE INVENTION
Some catalysts for purifying the exhaust gas of an internal combustion engine absorb NOx in exhaust gas when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. When the oxygen concentration of the exhaust gas falls, i.e. when the air-fuel ratio changes over to stoichiometric or rich, the NOx absorbed on the catalyst is desorbed, and the desorbed NOx is reduced by HC and CO which are present in the exhaust gas (JPA 6-336916 published in 1994).
However, engine fuel or lubricating oil generally contains sulfur, and SOx (sulfur oxides) in the exhaust gas tends to be absorbed by or deposit on the catalyst more easily when the vehicle is being driven for long periods of time at a lean air-fuel ratio. If a large amount of SOx is deposited on the catalyst, absorption of NOx declines and exhaust gas purification performance is considerably impaired.
The SOx deposited on the catalyst is discharged from the catalyst when the catalyst temperature rises above its usual level. Hence, when the amount of SOx deposited on the catalyst increases, the catalyst temperature is increased to remove SOx.
In JPA 10-54274 (published in 1998), when the amount of SOx deposited on the catalyst increases and absorption of NOx by the catalyst declines, lean misfire of the engine is performed for a predetermined time. Due to this, the amount of unburnt components increases in the exhaust gas, the temperature of the catalyst increases when these unburnt components are burnt in the catalyst, and SOx is discharged.
Alternatively, the ignition timing of the engine is retarded to increase the temperature of the exhaust gas and discharge SOx from the catalyst.
However, although the temperature increase effect is large when a lean misfire is performed and unburnt components are burnt in the catalyst, it is difficult to correctly control the amount of unburnt components sent to the catalyst, and there is a possibility of the temperature rising too much and impairing the durability of the catalyst.
On the other hand, since the temperature increase due to the retardation angle of the ignition timing raises the catalyst temperature indirectly by raising the exhaust gas temperature, a sufficient temperature increase for discharging SOx is not necessarily achieved even if durability is maintained.
A method is known of improving exhaust gas composition immediately after engine startup by installing a three-way catalyst on the upstream side, i.e. in the exhaust manifold, where the temperature after startup increases quickly. However, if an NOx absorption catalyst is also installed downstream of the upstream three-way catalyst and it is attempted to raise the temperature in order to discharge SOx from the downstream catalyst, the temperature of the upstream catalyst increases too much, and these severe temperature conditions lead to early deterioration of the upstream catalyst.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an exhaust purification device for an internal combustion engine which increases the temperature of a downstream catalyst effectively while avoiding the temperature increase of an upstream catalyst.
It is a further object of the invention to provide an exhaust purification device for an internal combustion engine which can efficiently remove SOx from a downstream NOx absorption catalyst.
In order to achieve above the objects the present invention provides an exhaust purification device for an internal combustion engine which comprises a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group, a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group, a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of the first and second exhaust passages, a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to the first front three-way catalyst to a predetermined air-fuel ratio, a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to the above-mentioned second front three-way catalyst to a predetermined air-fuel ratio, and a microprocessor programmed to determine the conditions under which the temperature of the rear three-way catalyst should be increased, and to set the air-fuel ratio of the exhaust controlled by one of the air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and to set the air-fuel ratio of the exhaust controlled by the other of the air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of the rear three-way catalyst.
The details as well as other features and advantages of the invention are set forth in the remainder of the specification and are shown in the accompanying drawings.


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