Conventionally, arrangements are known in which a three way catalyst (which will simply be referred to as “catalyst” in this specification, as well) is provided to an exhaust path of the internal-combustion engine for detoxifying exhaust from the internal-combustion engine. The three way catalyst has a function (oxidation function) of oxidizing unburned components (HC, CO) in the exhaust flowing therethrough, as well as a function (reduction function) of reducing nitrogen oxides (NOx) in the exhaust, thereby detoxifying harmful emissions such as HC and CO, which are unburned components, NOx, and so forth.
Furthermore, in general, the three way catalyst has an oxygen adsorption function (O2 storage function) for adsorbing (storing) oxygen. Specifically, in a case that exhaust flows through the three way catalyst at a richer air/fuel ratio than the theoretical air/fuel ratio, the three way catalyst oxidizes the unburned components such as HC, CO, and so forth, contained in the exhaust using oxygen stored therein (i.e., discharge oxygen). On the other hand, in a case that exhaust flows through the three way catalyst at a leaner air/fuel ratio than the theoretical air/fuel ratio, the three way catalyst reduces oxygen and NOx contained in the exhaust and stores the oxygen, captured in the reduction, therein. This oxygen adsorption function enables effective detoxification of HC and CO, which are unburned components, and NOx, over a range of air/fuel ratios around the theoretical air/fuel combustion ratio of the exhaust of the exhaust flowing through the three way catalyst around the theoretical air/fuel combustion ratio.
With such an arrangement, in order to maintain satisfactory oxygen adsorption function over the range of the air/fuel ratio around the theoretical air/fuel ratio from a certain lean air/fuel ratio up to a certain rich air/fuel ratio, the amount of oxygen adsorbed on the catalysis (oxygen adsorption amount) is preferably kept around a predetermined suitable amount, e.g., around half of the maximum oxygen amount which the catalysis can absorb (maximum oxygen adsorption capacity).
Recently, internal-combustion engines are known having a function of performing operation without fuel injection (fuel cut) depending on the operation thereof, e.g., in a case that the driver releases the accelerator pedal during driving of the vehicle, and so forth, thereby improving the fuel efficiency. With such internal-combustion engines, exhaust flowing through the catalysis includes only air during the fuel cut operation. That is to say, in this case, exhaust continuously flows through the catalysis at an extremely lean air/fuel ratio. Note that the fuel cut operation will also be referred to as “F/C” hereafter.
This often increases the oxygen adsorption amount of the catalysis up to the maximum adsorption amount thereof at the point in time after F/C (i.e., the start point of fuel injection (combustion)). In this state, the catalysis cannot exhibit satisfactory oxygen adsorption performance, leading to difficulty in efficient detoxification of NOx in the exhaust.
In order to solve the aforementioned problems, a technique is known in which the flow of the exhaust passing through the catalysis (i.e., the flow of mixture supplied to the engine) is temporarily adjusted to a rich air/fuel ratio after F/C, thereby controlling the oxygen adsorption amount of the catalysis around the aforementioned suitable amount. Such operation will also be referred to as “rich air/fuel ratio control after F/C” hereafter. On the other hand, the air/fuel ratio supplied to the engine will also be referred to as “air/fuel ratio of the engine” hereafter.
For example, with a fuel-injection control device (fuel supply control device) which has a function of the rich air/fuel ratio control after F/C, and which has been disclosed in Japanese Unexamined Patent Application Publication No. 2003-172176, the operation of the engine is performed while gradually reducing the air/fuel ratio from a richer air/fuel ratio than the theoretical air/fuel ratio by a predetermined amount after F/C up to the theoretical air/fuel ratio over time after the end of F/C.
FIG. 17 shows an example of change in the air/fuel ratio of the engine over the period of time including the rich air/fuel ratio control after F/C, performed by the fuel-injection control device disclosed in the above-described Document. Specifically, the drawing shows an example in which the air/fuel ratio of the engine is controlled to the theoretical air/fuel ratio (stoichiometric ratio) before the point in time t1 at which F/C begins. Subsequently, in this example, F/C is executed from the point in time t1 up to the point in time t2.
In this case, the rich air/fuel ratio control after F/C is executed over the time from the point in time t2 which is the end point of F/C up to the point in time t3. This adjusts the air/fuel ratio of the engine to a level richer than the stoichiometric ratio by a predetermined amount after the point in time t2. Subsequently, the air/fuel ratio is gradually reduced so as to approach the stoichiometric ratio over the following time. Eventually, the air/fuel ratio of the engine is adjusted to the stoichiometric ratio after the point in time t3.
On the other hand, the air/fuel ratio of the engine somewhat greater than the theoretical air/fuel ratio provides the maximum output of the engine. Furthermore, a relation is known according to which, as the air/fuel ratio of the engine is richer than the theoretical air/fuel ratio of the engine, the output of the engine becomes gradually smaller. Accordingly, control of the air/fuel ratio to a considerable rich air/fuel ratio as compared with the theoretical air/fuel ratio for a considerably long duration leads to the user experiencing sensation that a desired output cannot be obtained (which will also be referred to as “lag sensation” hereafter). Here, an excessively rich air/fuel ratio that causes apparently reduced output of the engine which leads to the driver experiencing lag sensation will also be referred to as “over-rich air/fuel ratio” hereafter.
Accordingly, in order to perform operation without driver experiencing such a lag sensation due to the rich air/fuel ratio control after F/C, control of the air/fuel ratio of the engine to such an over-rich air/fuel ratio for a relatively long period of time should be prevented.
On the other hand, it has come to be known that in a case of maintaining the operation of the engine at a shallow rich air/fuel ratio leaner than the over-rich air/fuel ratio (i.e., at a rich air/fuel ratio at which the operation of the engine for a relatively long duration does not leads to the driver experiencing lag sensation) during the rich air/fuel ratio control after F/C, the catalysis cannot provide sufficient detoxification of NOx in the exhaust flowing therethrough over an extremely short period of time immediately after the end of F/C, leading to a problem that a relatively large amount of NOx passing through the catalysis without detoxification. It is thought that such a phenomenon occurs due to insufficient region of the catalysis which can exhibit the aforementioned reduction function (which will be referred to as “reducing region” hereafter) during an extremely short period of time immediately after F/C.
Furthermore, it has come to be known that such a phenomenon can be avoided by operating the engine at an over-rich air/fuel ratio immediately after the end of F/C. It is thought that the reason is that the operation of the engine at an over-rich air/fuel ratio provides a sufficient area of the reducing region in an extremely short period of time immediately after F/C (detailed description will be made later).
Now, let us consider an arrangement in which the fuel-injection control device disclosed in the aforementioned document controls rich air/fuel ratio control after F/C at an over-rich air/fuel ratio immediately after the end of F/C. Here, let us say that an air/fuel ratio which is richer than the value AFrich is an over-rich air/fuel ratio as shown in FIG. 17. In this case, the operation of the engine is controlled at an over-rich air/fuel ratio over a relatively long period of time from the point in time t2 up to the point in time tA (see the hatched region in FIG. 17).
Accordingly, the fuel-injection control device disclosed in the aforementioned document has a problem that the rich air/fuel ratio control after F/C sufficient for preventing NOx from being discharged even over an extremely short period of time immediately after the end of F/C leads to the aforementioned phenomenon involving the user's lag sensation.