Patent Description:
An exhaust gas control apparatus that purifies NOx is known as an apparatus for controlling NOx emissions in an internal combustion engine operated on a leaner air-fuel ratio than a stoichiometric air-fuel ratio. A NOx storage-reduction catalyst and a selective catalytic reduction catalyst are known as the exhaust gas control apparatus that purifies NOx. The NOx storage-reduction catalyst can store NOx in exhaust gas under a lean atmosphere with excessive oxygen and release the stored NOx when a reduction agent is supplied from the outside. Further, the NOx storage-reduction catalyst can produce reaction between the NOx and the reduction agent to reduce NOx to nitrogen (N<NUM>). The selective catalytic reduction catalyst has a function for adsorbing ammonia (NH<NUM>), and with this NH<NUM>, the selective catalytic reduction catalyst can selectively reduce NOx in the exhaust gas.

<CIT> (<CIT>) discloses a configuration in which the selective catalytic reduction catalyst is disposed downstream of the NOx storage-reduction catalyst. According to this configuration, NH<NUM> that had been formed during reduction of NOx by the NOx storage-reduction catalyst can be adsorbed by the selective catalytic reduction catalyst. Further, by using this NH<NUM>, NOx that was not adsorbed by the NOx storage-reduction catalyst or NOx that leaked out from the NOx storage-reduction catalyst can be selectively reduced by the selective catalytic reduction catalyst.

In the case of an exhaust gas control apparatus that purifies NOx by using the selective catalytic reduction catalyst, addition of urea or the like will be necessary to make the selective catalytic reduction catalyst adsorb NH<NUM>. On the other hand, in the case of an exhaust gas control apparatus that purifies NOx by using the NOx storage-reduction catalyst or by a combination of the NOx storage-reduction catalyst and the selective catalytic reduction catalyst, NOx that is generated during the lean combustion operation of the engine can be stored inside the exhaust gas control apparatus. In this case, by supplying a reduction agent from an upstream side of the exhaust flow, NOx stored inside the apparatus can be purified by reducing NOx to N<NUM>. In other words, special means such as the addition of urea or the like is unnecessary. Supplying of the reduction agent to the exhaust gas control apparatus is achieved by executing a so-called rich spike control, which is processing executed to temporarily change an in-cylinder air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio by increasing a fuel injection amount per cycle.

<CIT> discloses an apparatus for controlling a rich spike fueling event in an internal combustion engine whereby the valve overlap is extended in order to regenerate a NOx trap.

However, depending on an operation range of the engine, the rich spike control may not function effectively in some cases. A specific example of such an operation range is a high-torque range. In the high-torque range, a valve timing of an intake valve is advanced so as to enhance intake efficiency. In addition, a valve overlap amount between the intake valve and an exhaust valve is increased as the valve timing of the intake valve is advanced. The valve overlap in the high-torque range may cause air to flow from an intake port to an exhaust port (so-called scavenging). In particular, in a turbocharged engine, occurrence of scavenging becomes conspicuous when intake pressure is boosted by turbocharging.

When scavenging occurs, air with high oxygen concentration flows to the exhaust gas control apparatus. As a result, a reaction occurs between oxygen and the reduction agent generated by the rich spike control, and hinders the reduction of NOx by the reduction agent. Specifically, in the case of an exhaust gas control apparatus that purifies NOx by using the NOx storage-reduction catalyst, hydrocarbon (HC) and carbon monoxide (CO) necessary for the reduction of NOx are oxidized by oxygen. In the case of an exhaust gas control apparatus that purifies NOx by a combination of the NOx storage-reduction catalyst and the selective catalytic reduction catalyst, NO and CO necessary for forming NH<NUM> are oxidized by oxygen. As a result, NOx purification efficiency of the exhaust gas control apparatus is deteriorated, which may sometimes lead to discharge of NOx into the atmosphere.

The invention provides a control device for an internal combustion engine that can prevent deterioration of NOx purification efficiency by maintaining the effectiveness of the rich spike control.

First aspect of the invention relates to a control device for an internal combustion engine according to claim <NUM>. A lean atmosphere with excessive oxygen refers to an atmosphere with a higher oxygen concentration than that in the exhaust gas obtained by combusting an air-fuel mixture of stoichiometric air-fuel ratio. To react a reduction agent directly with NOx means to generate a chemical reaction having the reduction agent and NOx as reactants. To react a reduction agent indirectly with NOx means to generate a chemical reaction having NOx and another product formed by a reaction between the reduction agent and another substance as the reactants.

Rich spike control is processing executed to temporarily change the in-cylinder air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio by increasing a fuel injection amount per cycle when the internal combustion engine is operated with the in-cylinder air-fuel ratio controlled to a leaner air-fuel ratio than the stoichiometric air-fuel ratio. Rich spike control execution means is configured to execute this processing in accordance with a predetermined execution rules. Valve timing control means is configured to decrease an overlap amount of the intake valve and the exhaust valve when the rich spike control is executed in an operation range where a pressure of the intake port becomes higher than a pressure of the exhaust port, that is, in an operation range in which scavenging may occur, compared to that during non-execution of the rich spike control.

By decreasing the overlap amount between the intake valve and the exhaust valve, an air amount that flows from the intake port to the exhaust port by scavenging (hereinafter referred to as a scavenging amount) can be reduced. By reducing the scavenging amount during execution of the rich spike control, a reaction between oxygen and the reduction agents produced by the rich spike control is reduced. At the same time, direct or indirect reaction between the reduction agents and NOx can be promoted.

In the control device, the electronic control unit may be configured to maintain the overlap amount such that the overlap amount during execution of the rich spike control is the same as the overlap amount during non-execution of the rich spike control, in an operation range where the pressure of the exhaust port becomes higher than the pressure of the intake port. According to this configuration, an operation for varying the overlap amount between the intake valve and the exhaust valve is limited to the operation range where scavenging may occur. Accordingly, deterioration of combustion stability due to the variation in the overlap amount can be prevented.

In the control device, the electronic control unit may be configured to vary the overlap amount during execution of the rich spike control such that the overlap amount in the operation range where the pressure of the intake port becomes higher than the pressure of the exhaust port is larger than the overlap amount in the operation range where the pressure of the exhaust port becomes higher than the pressure of the intake port. According to this configuration, in the operation range where scavenging may occur, increasing the overlap amount can scavenge residual exhaust gas and improve charging efficiency of fresh air. Further, in the operation range where scavenging does not occur, an internal EGR amount can be reduced by decreasing the overlap amount, and a combustion limit of the lean combustion operation can be pushed up by increasing the rate of fresh-charged air in the cylinder gas.

In the control device, the electronic control unit may be configured to increase the overlap amount in accordance with advancing of a valve timing of the intake valve, and the electronic control unit may be configured to decrease the overlap amount in accordance with retarding of the valve timing of the intake valve. According to this configuration, when the rich spike control is executed in an operation range where scavenging may occur, the valve timing of the intake valve is retarded in addition to decreasing the overlap amount. Retarding the valve timing of the intake valve can decrease the charging efficiency of fresh air without lowering the pressure of the intake port by throttling the throttle valve. Thus, according to this configuration, when the rich spike control is terminated to return to the lean combustion operation again, the charging efficiency of fresh air can be readily increased.

In the control device, the exhaust gas control apparatus may include a NOx storage-reduction catalyst. The NOx storage-reduction catalyst may be configured to store NOx in the exhaust gas under the lean atmosphere with excessive oxygen. The exhaust gas control apparatus may be configured to react the reduction agents supplied from the upstream side of the exhaust flow of the NOx storage-reduction catalyst with NOx stored on the NOx storage-reduction catalyst to reduce NOx to N<NUM>. According to this configuration, the reduction agents supplied from the upstream side of the exhaust flow can be directly reacted with NOx to reduce NOx.

In the control device, the exhaust gas control apparatus may include a NOx storage-reduction catalyst and a selective catalytic reduction catalyst, the selective catalytic reduction catalyst may be disposed downstream of the NOx storage-reduction catalyst. According to this configuration, the exhaust gas control apparatus stores NOx in the exhaust gas in the NOx storage-reduction catalyst under the lean atmosphere with excessive oxygen, and reacts the reduction agent supplied from the upstream side of the exhaust flow with NOx stored in the NOx storage-reduction catalyst to reduce NOx to NH<NUM>. Then, NH<NUM> in the exhaust gas is adsorbed by the selective catalytic reduction catalyst, and NOx released from the NOx storage-reduction catalyst is reacted with NH<NUM> adsorbed by the selective catalytic reduction catalyst to reduce NOx to N<NUM>. Accordingly, the reduction agent supplied from the upstream side of the exhaust flow can be reacted indirectly with NOx to reduce NOx.

Second aspect of the invention relates to a control method of an internal combustion engine. The internal combustion engine includes an exhaust gas control apparatus, an exhaust port, an intake port, an exhaust valve and an intake valve, the exhaust gas control apparatus configured to store NOx in exhaust gas under a leaner atmosphere with excessive oxygen compared to an atmosphere under a stoichiometric air-fuel ratio and the exhaust gas control apparatus configured to react, directly or indirectly, NOx with a reduction agent supplied from an upstream side of an exhaust flow to reduce NOx. The control method includes: executing a rich spike control to temporarily change an in-cylinder air-fuel ratio from a leaner air-fuel ratio than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio; and varying an overlap amount of the intake valve and the exhaust valve such that the overlap amount is less during execution of the rich spike control than during non-execution of the rich spike control, in an operation range where a pressure of the intake port becomes higher than a pressure of the exhaust port.

According to the above-mentioned control unit, the overlap amount between the intake valve and the exhaust valve is decreased during execution of the rich spike control in the operation range where the pressure of the intake port becomes higher than the pressure of the exhaust port. Accordingly, the scavenging amount during execution of the rich spike control can be reduced. As a result, a reaction between oxygen and the reduction agents produced by the rich spike control is reduced. At the same time, a direct or indirect reaction between the reduction agents and NOx can be promoted to prevent deterioration of the NOx purification efficiency.

A first embodiment will be described hereinafter with reference to the attached drawings.

<FIG> is a view that illustrates a system configuration according to the first embodiment of the invention. The system according to this embodiment includes an internal combustion engine (hereinafter simply referred to as the engine) <NUM> equipped in a vehicle as a power unit. The number of cylinders the engine <NUM> has and how the cylinders are arranged will not be limited in particular.

The engine <NUM> includes a cylinder block <NUM> having a piston <NUM> arranged therein and a cylinder head <NUM>. A space defined by the cylinder head <NUM> and the piston <NUM> forms a combustion chamber <NUM>. The engine <NUM> is a spark-ignition type engine, and includes a spark plug <NUM> of an ignition device, which is attached to the cylinder head <NUM> such that the spark plug <NUM> protrudes from a top of the combustion chamber <NUM>. An intake port <NUM> and an exhaust port <NUM> formed in the cylinder head <NUM> are open to the combustion chamber <NUM>, respectively. A communication state of the combustion chamber <NUM> and the intake port <NUM> is controlled by an intake valve <NUM> provided in the cylinder head <NUM>. A communication state of the combustion chamber <NUM> and the exhaust port <NUM> is controlled by an exhaust valve <NUM> provided in the cylinder head <NUM>. An in-cylinder injection valve <NUM> that directly injects fuel into the combustion chamber <NUM> and a port injection valve <NUM> that injects the fuel to the intake port <NUM> are attached to the cylinder head <NUM>.

In the illustrated configuration, the engine <NUM> is provided with an intake variable valve mechanism <NUM> capable of varying valve opening characteristics of the intake valve <NUM> and an exhaust variable valve mechanism <NUM> capable of varying valve opening characteristics of the exhaust valve <NUM>. A valve mechanism, which can make at least a valve timing and an operating angle variable, is applicable to these variable valve mechanisms.

An intake manifold <NUM> is connected to the intake port <NUM>. The intake manifold <NUM> has a surge tank <NUM>. An intake passage <NUM> that takes in air from the outside is connected to the surge tank <NUM>. An electronically controlled throttle valve <NUM> is provided in the intake passage <NUM> near the surge tank <NUM>. An air cleaner <NUM> is provided at a tip end of the intake passage <NUM>. An exhaust manifold <NUM> is connected to the exhaust port <NUM>. An exhaust passage <NUM> that discharges the exhaust gas to the outside is connected to the exhaust manifold <NUM>. An exhaust gas control apparatus <NUM>, which will be explained later, is provided in the exhaust passage <NUM>.

The engine <NUM> has a turbocharger <NUM>. A compressor 28a of the turbocharger <NUM> is provided upstream of the throttle valve <NUM> in the intake passage <NUM>. An intercooler <NUM> that cools intake air compressed by the compressor 28a is provided in the intake passage <NUM> between the compressor 28a and the throttle valve <NUM>.

A turbine 28b of the turbocharger <NUM> is provided upstream of the exhaust gas control apparatus <NUM> in the exhaust passage <NUM>. A bypass passage <NUM> that bypasses the turbine 28b is provided in the exhaust passage <NUM>. A wastegate valve <NUM> is disposed in the bypass passage <NUM>. When the wastegate valve <NUM> opens, a part of the exhaust gas bypasses the turbine 28b and flows through the bypass passage <NUM>. The wastegate valve <NUM> is driven by an electronically controlled actuator <NUM>.

The exhaust gas control apparatus <NUM> is constituted by a start catalyst (hereinafter referred to as an SC) <NUM>, which is a three-way catalyst, a NOx storage-reduction catalyst (hereinafter referred to as an NSR) <NUM>, and a selective catalytic reduction catalyst (hereinafter referred to as an SCR) <NUM>. The SC <NUM>, the NSR <NUM>, and the SCR <NUM> are disposed in this order from an upstream side in the exhaust passage.

The SC <NUM>, under a lean atmosphere, reduces NOx in the exhaust gas to N<NUM> while adsorbing oxygen, and under a rich atmosphere, oxidizes HC and CO in the exhaust gas into H<NUM>O and CO<NUM> while releasing oxygen. In this specification, a lean atmosphere refers to an atmosphere with a higher oxygen concentration than that in the exhaust gas obtained by combusting an air-fuel mixture of stoichiometric air-fuel ratio, while a rich atmosphere refers to an atmosphere with a lower oxygen concentration than that in the exhaust gas obtained by combusting the air-fuel mixture of stoichiometric air-fuel ratio. Further, when a reduction agent is supplied from an upstream side, the SC <NUM> produces reaction between NOx contained in the exhaust gas and the reduction agent to reduce NOx to NH<NUM> and N<NUM>.

The NSR <NUM> stores NOx in the exhaust gas in the form of a nitrate salt such as a Ba(NO<NUM>)<NUM> under the lean atmosphere. Further, when a reduction agent is supplied from the upstream side, the NSR <NUM> releases the stored NOx and produces reaction between the reduction agent and NOx to reduce NOx to NH<NUM> and N<NUM>. The reduction reaction of NOx by the SC <NUM> and NSR <NUM> is expressed by the following chemical equations (<NUM>) and (<NUM>).

<NUM><NUM> + NO → NH<NUM> + H<NUM>O.

5CO + NO + <NUM><NUM>O → NH<NUM> + <NUM>. 5CO<NUM> + <NUM>.

However, when a supply amount of the reduction agent is not sufficient, NOx released by the NSR <NUM> is discharged to a downstream as is without being reduced.

The SCR <NUM> is constituted as a Fe-based zeolite catalyst. The SCR <NUM> adsorbs NH<NUM> formed by the SC <NUM> and the NSR <NUM>, then, for example, as expressed by the following chemical equations (<NUM>) and (<NUM>), reacts NOx in the exhaust gas (mostly NOx released by the NSR <NUM>) with the adsorbed NH<NUM> to reduce NOx to N<NUM>. Since NH<NUM> is a product formed by a reaction of a reduction agent (H<NUM> and CO) with other substances (NO and H<NUM>O), the reaction of NH<NUM> with NOx expressed by the following chemical equations (<NUM>) and (<NUM>) may be regarded as an indirect reaction of the reduction agent with NOx.

4NO + 4NH<NUM> + O<NUM> → 2N<NUM> + <NUM><NUM>O.

2NO + 2NO<NUM> + 4NH<NUM> → 4N<NUM> + <NUM><NUM>O.

The system of this embodiment is provided with sensors that obtain information relating to an operation state of the engine <NUM> at various locations. An airflow meter <NUM> that measures an intake air amount is disposed immediately downstream of the air cleaner <NUM> in the intake passage <NUM>. A boost pressure sensor <NUM> that measures boost pressure is disposed immediately downstream of the intercooler <NUM> in the intake passage <NUM>. A throttle position sensor <NUM> that measures opening of the throttle valve <NUM> is disposed near the throttle valve <NUM>. An intake pressure sensor <NUM> that measures intake pressure is disposed in the surge tank <NUM>.

An air-fuel ratio sensor <NUM> that outputs signals that linearly change relative to an air-fuel ratio of the exhaust gas before combustion is disposed immediately upstream of the SC <NUM> in the exhaust passage <NUM>. In addition, an oxygen sensor <NUM> that outputs signals that change in a stepped manner on an oxygen-excess side and an oxygen-deficient side, based on an oxygen concentration as a threshold in the exhaust gas obtained by the combustion of the air-fuel mixture of stoichiometric air-fuel ratio, is disposed immediately downstream of the SC <NUM> in the exhaust passage <NUM>. A first NOx sensor <NUM> that outputs signals that change in accordance with a concentration of NOx in the exhaust gas that has passed the NSR <NUM> is disposed immediately downstream of the NSR <NUM> in the exhaust passage <NUM>. Further, a second NOx sensor <NUM> that outputs signals that change in accordance with the concentration of NOx in the exhaust gas that has passed the SCR <NUM> is disposed immediately downstream of the SCR <NUM> in the exhaust passage <NUM>.

Further, the system of this embodiment includes an accelerator position sensor <NUM> that measures an accelerator pedal operation amount (accelerator operation amount) and a crank angle sensor <NUM> that measures a crank angle of the engine <NUM>.

The various sensors and actuators described above are electrically connected to a control device <NUM>. The control device <NUM> is constituted by an electronic control unit (ECU). The control device <NUM> is provided for controlling an overall system of the engine <NUM>, and is constituted mainly by a computer including a CPU, a ROM, and a RAM. Various control routines including a rich spike control routine and a valve timing control routine, which will be described later, are stored in the ROM. The control device <NUM> controls the engine <NUM> by operating each of the actuators based on the signals from the respective sensors. To be specific, the control device <NUM> first calculates a required torque in accordance with the accelerator pedal operation amount measured by the accelerator position sensor <NUM>. Then, an operation mode of the engine <NUM> is determined based on the required torque and a current engine speed calculated from signals output from the crank angle sensor <NUM>, and the actuators are operated in accordance with the determined operation mode.

The operation modes of the engine <NUM> selected by the control device <NUM> include a stoichiometric combustion operation mode in which the engine <NUM> is operated while an in-cylinder air-fuel ratio is controlled to the stoichiometric air-fuel ratio and a lean combustion operation mode in which the engine <NUM> is operated while the in-cylinder air-fuel ratio is controlled to a leaner air-fuel ratio than the stoichiometric air-fuel ratio. The stoichiometric combustion operation mode is selected in a range from a mid- to high-speed range to a high-speed range, while the lean combustion operation mode is selected in a range from a low-speed range to a low- to mid-speed range. In an operation range in which the lean combustion operation mode is selected, and particularly in the operation range where the intake pressure becomes lower than or equal to a back pressure, the lean combustion operation is conducted by port injection from the port injection valve <NUM> or by a combination of port injection and in-cylinder injection, mainly relying on the port injection. In the operation range where the intake pressure becomes higher than the back pressure, the lean combustion operation is conducted by the in-cylinder injection (also called a direct injection) from the in-cylinder injection valve <NUM>.

In the lean combustion operation mode, rich spike control is executed in accordance with a predetermined execution rules. The rich spike control is a control that temporarily changes the in-cylinder air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio by increasing a fuel injection amount per cycle. An in-cylinder air amount used for the calculation of the fuel injection amount is calculated using an air model. In the operation range where the port injection is conducted as a main fuel injection, increasing of the fuel injection amount for the rich spike control is conducted with respect to the port injection. On the other hand, in the operation range where the in-cylinder injection is conducted as the main fuel injection, increasing of the fuel injection amount for the rich spike control is conducted with respect to the in-cylinder injection.

By setting the in-cylinder air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is decreased and a large amount of reduction agents such as HC, CO, and H<NUM> are generated. When exhaust gas containing a large amount of reduction agents is supplied to the NSR <NUM>, the atmosphere around the NSR <NUM> becomes a reducing atmosphere. Then, NOx that has been stored as a nitrate salt is reduced to NO and is separated from a base. Accordingly, by executing the rich spike control during the lean combustion operation allows to separate NOx stored in the NSR <NUM> from the NSR <NUM> and to restore a NOx storing performance of the NSR <NUM>. The rich spike control is executed when the concentration of NOx measured by the NOx sensor <NUM> disposed immediately downstream of the NSR <NUM> exceeds a predetermined threshold value. Or instead, the rich spike control is executed when a storage amount of NOx estimated based on the engine speed, load, and the air-fuel ratio exceeds the predetermined threshold value.

The NOx separated from the NSR <NUM> by the rich spike control is reduced to N<NUM> and NH<NUM> on the NSR <NUM>. NH<NUM> formed by the NSR <NUM> is captured, together with NH<NUM> formed by the SC <NUM>, by the SCR <NUM> located at the most downstream position and is adsorbed on the SCR <NUM>. A part of the NOx separated from the NSR <NUM> by the rich spike control is released from the NSR <NUM> as is without being reduced. The SCR <NUM> produce reaction between the adsorbed NH<NUM> and NOx released from the NSR <NUM> without being purified so as to reduce NOx to N<NUM>. Accordingly, a case that emission is deteriorated by NOx discharged into the atmosphere can be effectively prevented.

<FIG> is a chart that summarizes an outline of a valve timing control performed in the lean combustion operation mode. In the figure, IVO indicates a timing at which the intake valve <NUM> opens, while IVC indicates a timing at which the intake valve <NUM> closes. A range between IVO and IVC indicates a range of crank angle during which the intake valve <NUM> is open, in other words, this range indicates an operation angle of the intake valve <NUM>. EVO indicates a timing at which the exhaust valve <NUM> opens, while EVC indicates a timing at which the exhaust valve <NUM> closes. A range between EVO and EVC indicates a range of crank angle during which the exhaust valve <NUM> is open, in other words, this range indicates an operation angle of the exhaust valve <NUM>.

Different controls are performed as the valve timing control between a non-scavenging range and a scavenging range. The non-scavenging range is the operation range in which scavenging does not occur, that is, a relatively low-load operation range with the intake pressure less than or equal to the back pressure. The intake pressure refers to the pressure of the intake port <NUM>, and back pressure refers to the pressure of the exhaust port <NUM>. In the non-scavenging range, the wastegate valve <NUM> is made full open, and mainly, the control of the air amount is performed in accordance with the opening of the throttle valve <NUM>. In the case of a turbocharged engine like the engine <NUM> in this embodiment, the non-scavenging range corresponds to a non-turbocharging range where effective turbocharging by the compressor 28a is not performed. The scavenging range is the operation range in which scavenging may occur. In other words, the scavenging range is a relatively high-load operation range with an intake pressure higher than back pressure. In the scavenging range, the throttle valve <NUM> is made full open, and mainly, the control of the air amount is performed in accordance with the opening of the wastegate valve <NUM>. In the case of a turbocharged engine like the engine <NUM> in accordance with this embodiment, the scavenging range corresponds to a turbocharging range where effective turbocharging by the compressor 28a is performed.

In the low-load range, that is, in the non-scavenging range in which scavenging does not occur, the control device <NUM> controls the intake variable valve mechanism <NUM> so as to retard IVC. Since IVO is also retarded when IVC is retarded, the crank angle from IVO to EVC, that is, an overlap amount (O/L), becomes small. By decreasing the overlap amount, an internal EGR amount can be reduced and the rate of fresh-charged air in the cylinder gas can be increased. Thus, a combustion limit of the lean combustion operation can be increased.

When executing the rich spike control in the non-scavenging range, the control device <NUM> does not change the valve timings of the intake valve <NUM> and the exhaust valve <NUM> before and after the rich spike control, but instead, maintains the overlap amount between the intake valve <NUM> and the exhaust valve <NUM> during non-execution of the rich spike control and during execution of the rich spike control. In other words, the control is performed such that the overlap amount during non-execution of the rich spike control and the overlap amount during execution of the rich spike control are the same. This is related to the fact that fuel injection by port injection (or a combination of port injection and in-cylinder injection, mainly relying on the port injection) is performed in the non-scavenging range including execution period of the rich spike control. By the port injection, a longer time can be taken to mix fuel and air. Accordingly, homogeneity of the air-fuel mixture can be improved. On the other hand, robustness to the fluctuation in combustion conditions is reduced compared to the in-cylinder injection. Therefore, as described above, parameters regarding the in-cylinder air amount including the valve timing and operation angle of the intake valve <NUM> and the overlap amount are prevented from varying both during non-execution of the rich spike control and during execution of the rich spike control. Accordingly, it becomes possible to maintain estimation accuracy of the in-cylinder air amount necessary for controlling the in-cylinder air-fuel ratio, and as a result, combustion stability in the non-scavenging range can be ensured.

In a high-load range, in other words, in the scavenging range in which scavenging may occur, the control device <NUM> controls the intake variable valve mechanism <NUM> so as to maintain the operation angle of the intake valve <NUM>, to advance both IVC and IVO, and to bring IVC close to a BDC (a piston bottom dead center). The valve timing of the exhaust valve <NUM> is maintained to be the same as that in the non-scavenging range. Accordingly, the overlap amount, which is the crank angle from IVO to EVC, is increased. Increasing the overlap amount can improve scavenging efficiency of the combustion gas, while bringing IVC close to the BDC can improve charging efficiency of fresh air.

When executing the rich spike control in the scavenging range, the control device <NUM> decreases the operation angle of the intake valve <NUM> and retards both IVC and IVO. The valve timing of the exhaust valve <NUM> is maintained to be the same as that during non-execution of the rich spike control. Reducing the operation angle of the intake valve <NUM> and retarding IVC deteriorates the charging efficiency of fresh air. As a result, it becomes possible to bring the in-cylinder air-fuel ratio into the stoichiometric ratio or enrich the in-cylinder air-fuel ratio without causing torque increase. It should be noted that at this time, the control device <NUM> maintains the throttle valve <NUM> at a fully open state, and maintains the opening of the wastegate valve <NUM> to be the same as that during non-execution of the rich spike control. By doing so, decrease in the pressure of intake port <NUM> can be prevented. Accordingly, when the rich spike control is terminated to return the lean combustion operation again, the charging efficiency of fresh air can be readily increased.

Also, by controlling the valve timing of the intake valve <NUM> as described above, the overlap amount, which is the crank angle from IVO to EVC is decreased compared to that during non-execution of the rich spike control. Preferably, the overlap amount is decreased such that the overlap amount becomes smaller than or equal to the overlap amount in the non-scavenging range. Thus, by decreasing the overlap amount between the intake valve <NUM> and the exhaust valve <NUM>, the scavenging amount during execution of the rich spike control can be reduced.

When the rich spike control is executed, the oxygen concentration in the exhaust gas is decreased and a large amount of reduction agents such as HC, CO, and H<NUM> are generated. However, if air with high oxygen concentration flows to the exhaust port <NUM> by scavenging, the oxygen concentration in the entire gas that flows to the exhaust gas control apparatus <NUM> increases, and it becomes impossible to bring the atmosphere of the SC <NUM> and the NSR <NUM> into a reducing atmosphere. In addition, NH<NUM> formed by the SC <NUM> and the NSR <NUM> by execution of the rich spike control plays an important role in purifying NOx. However, if air with high oxygen concentration flows into the exhaust gas control apparatus <NUM> by the scavenging, NO and CO necessary for forming NH<NUM> will be oxidized by the oxygen. As a result, formation of NH<NUM> will be prevented, greatly deteriorating the purification efficiency of NOx by the SCR <NUM>.

According to the valve timing control of the first embodiment, an increase in the oxygen concentration of exhaust gas that flows into the exhaust gas control apparatus <NUM> can be prevented by reducing the scavenging amount during execution of the rich spike control. This reduces the reaction between oxygen and the reduction agents produced by the rich spike control. By promoting the formation of NH<NUM> on the SC <NUM> and NSR <NUM> expressed by the above-mentioned chemical equation (<NUM>) and chemical equation (<NUM>), the selective reduction reaction of NOx on the SCR <NUM> expressed by the above-mentioned chemical equations (<NUM>) and (<NUM>) can be promoted. Accordingly, effectiveness of the rich spike control can be maintained, and deterioration of NOx purification efficiency caused by the scavenging can be prevented.

<FIG> is a flowchart that illustrates a flow of the valve timing control performed by the control device <NUM> in the lean combustion operation mode. When the operation mode of the engine <NUM> is in the lean combustion operation mode, the control device <NUM> performs the valve timing control in accordance with the flow shown in <FIG>. The flow shown in <FIG> is constituted by four steps from step S2 to step S8.

In step S2, the control device <NUM> determines whether the rich spike control is necessary, in other words, whether rich spike control execution conditions have been satisfied. For example, as described above, the rich spike control execution conditions include that the NOx concentration downstream of the NSR <NUM> which is measured by the NOx sensor <NUM> has exceeded a threshold value, that the NOx storage amount of the NSR <NUM> estimated and computed based on the engine speed and load and air-fuel ratio has exceeded a threshold value, or the like.

If the result of determination in step S2 is No, that is, when the rich spike control is not executed, the control device <NUM> performs the processing in step S8. In step S10, the control device <NUM> sets an advance angle of the valve timing of the intake valve <NUM> to a value A corresponding to the load of the engine <NUM> and sets the operation angle of the intake valve <NUM> to a value B corresponding to the load.

If the result of determination in step S2 is Yes, that is, when the rich spike control is executed, the control device <NUM> next performs the determination in step S4. In step <NUM>, the control device <NUM> determines whether the operation range of the engine <NUM> is in the turbocharging range, in other words, whether the operation range is in a scavenging range where scavenging may occur. Whether the operation range is in the turbocharging range (the scavenging range) or in the non-turbocharging range (the non-scavenging range) can be determined based on a relationship between the current speed and current load of the engine <NUM>. Instead, determination can also be made based on whether the boost pressure measured by the output of the boost pressure sensor <NUM> is higher than the atmospheric pressure.

If the result of determination in step S4 is No, that is, if the operation range of the engine <NUM> is in the non-turbocharging range, the control device <NUM> performs the processing in step S8. In step S8, the control device <NUM> sets the advance angle of the valve timing of the intake valve <NUM> to the value A corresponding to the load of the engine <NUM> and sets the operation angle of the intake valve <NUM> to the value B corresponding to the load.

If the result of determination in step S4 is Yes, that is, if the operation range of the engine <NUM> is in the turbocharging range, the control device <NUM> performs the processing in step S6. In step S6, the control device <NUM> sets the advance angle of the valve timing of the intake valve <NUM> to a value A' corresponding to the load of the engine <NUM> and sets the operation angle of the intake valve <NUM> to a value B' corresponding to the load. However, the value A' of the advance angle of the valve timing of the intake valve <NUM> is, when compared under the same load, a value smaller than the value A of the advance angle set in step S8. The value B' of the operation angle of the intake valve <NUM> is, when compared under the same load, a value smaller than the value B of the operation angle set in step S8. Accordingly, the overlap amount of the intake valve <NUM> and the exhaust valve <NUM> is decreased and the scavenging amount is controlled. Further, the charging efficiency of fresh air can be decreased without lowering the intake pressure by throttling the throttle valve <NUM>.

<FIG> is a timing chart that explains the valve timing control and effects thereof in the scavenging range which is performed by the control device <NUM>. The timing chart in <FIG> shows, from the top, the torque, a target air-fuel ratio, an actual air-fuel ratio, the in-cylinder air amount, ignition timing, the intake pressure, opening of the throttle valve <NUM>, opening of the wastegate valve <NUM> (WGV opening), close timing of the intake valve <NUM>, operation angle of the intake valve <NUM>, an oxygen amount passed through during scavenging, and a formation amount of ammonia at the SC <NUM> and the NSR <NUM>, all expressed in terms of change over time. In the chart, solid lines show results of the valve timing control according to this embodiment, and dotted lines show results of the valve timing control according to a comparative example.

In the comparative example, when the rich spike control is performed in the scavenging range (high-load range), the close timing and the operation angle of the intake valve <NUM> are maintained to be the same as those during non-execution of the rich spike control. Although the close timing and the operation angle of the intake valve <NUM> are also maintained to be the same as those during non-execution of the rich spike control and during execution of the rich spike control in the non-scavenging range of this embodiment, the comparative example also applies this control to the valve timing control in the scavenging range.

According to the valve timing control of the comparative example, as a result of the close timing of the intake valve <NUM> being maintained to be the same as that during non-execution of the rich spike control, the overlap amount between the intake valve <NUM> and the exhaust valve <NUM> is also maintained as is. Accordingly, the same amount of oxygen as that during non-execution of the rich spike control blows from the intake port <NUM> to the exhaust port <NUM> also during execution of the rich spike control. As a result, the reduction agents produced by the rich spike control end up reacting with the oxygen, and therefore hinder the formation of NH<NUM> by the SC <NUM> and the NSR <NUM>.

On the other hand, according to the valve timing control of this embodiment, when the rich spike control is executed, the close timing of the intake valve <NUM> is retarded and the operation angle of the intake valve <NUM> is made smaller. Accordingly, the overlap amount between the intake valve <NUM> and the exhaust valve <NUM> is decreased, and the oxygen amount that blows from the intake port <NUM> to the exhaust port <NUM> by the scavenging is decreased. As a result, the reaction between oxygen and the reduction agent produced by the rich spike control is reduced, while the formation of NH<NUM> by the SC <NUM> and the NSR <NUM> is promoted. Accordingly, deterioration of NOx purification efficiency caused by the scavenging can be prevented.

Further, according to the valve timing control of the comparative example, the close timing and operation angle of the intake valve <NUM> are maintained to be the same as those during non-execution of the rich spike control. Therefore, in order to decrease the in-cylinder air amount (an amount of fresh air sucked into the cylinder), the throttle valve <NUM> needs to be closed and the wastegate valve <NUM> needs to be opened. However, due to a delay in response of the intake pressure to the operation of the throttle valve <NUM> and the wastegate valve <NUM>, the in-cylinder air amount decreases with a time lag from enriching of the actual air-fuel ratio. Accordingly, to prevent the torque from becoming excessive, the ignition timing needs to be largely retarded until the in-cylinder air amount decreases to the target. In this case, retardation of the ignition timing may lead to deterioration of fuel efficiency.

In addition, according to the valve timing control of the comparative example, when returning to the lean combustion operation from the rich spike control, a time lag is generated in increasing of the in-cylinder air amount. This is because, even when the throttle valve <NUM> is promptly opened and the wastegate valve <NUM> is promptly closed, the intake pressure does not immediately rise once the intake pressure lowers. If the air-fuel ratio is caused to be on the lean side despite the fact that the in-cylinder air amount has not increased, the torque will rapidly drop. Consequently, in the case of the comparative example, the actual air-fuel ratio cannot be made lean to the target air-fuel ratio, and enriching of the actual air-fuel ratio needs to be continued until the in-cylinder air amount increases to the target. In addition, to prevent the torque from becoming excessive due to continuation of enriching the actual air-fuel ratio, the ignition timing also needs to be retarded until the in-cylinder air amount increases to the target. The longer enriching duration of the actual air-fuel ratio and the longer duration of ignition timing retardation lead to deterioration of fuel efficiency.

On the other hand, according to the valve timing control of this embodiment, the in-cylinder air amount is decreased by retarding the close timing of the intake valve <NUM>. Since the in-cylinder air amount is highly responsive to the change of close timing of the intake valve <NUM>, the in-cylinder air amount can be decreased without much delay from enriching of the actual air-fuel ratio. Accordingly, the ignition timing can be retarded by a small amount to prevent excessive torque, preventing deterioration of fuel efficiency thereby.

Further, according to the valve timing control of this embodiment, since the opening of the throttle valve <NUM> and the opening of the wastegate valve <NUM> are maintained, a decrease in the intake pressure due to execution of the rich spike control can be prevented. As long as the intake pressure is maintained, the in-cylinder air amount can be promptly increased by advancing the close timing of the intake valve <NUM>. As a result, it becomes possible to make the actual air-fuel ratio lean in accordance with the target air-fuel ratio, and therefore continuation of enriching of the actual air-fuel ratio will no longer be necessary. Thus, deterioration of fuel efficiency can be prevented.

Next, a second embodiment of the invention will be described with reference to the drawing.

<FIG> is a view that illustrates a system configuration according to the second embodiment of the invention. The system according to this embodiment includes the engine <NUM> and the control device <NUM> that controls the engine <NUM>. Since the configuration of the engine <NUM> and functions of the control device <NUM> are common to those of the first embodiment, detailed description thereof will be omitted herein. The difference between the system of this embodiment and the system of the first embodiment is the configuration of an exhaust gas control apparatus <NUM> provided in the exhaust passage <NUM>.

The exhaust gas control apparatus <NUM> is constituted by an SC <NUM> and an NSR <NUM>. The SC <NUM> is disposed in the exhaust passage on an upstream side thereof, and the NSR <NUM> is disposed in the exhaust passage on a downstream side thereof. An air-fuel ratio sensor <NUM> is disposed immediately upstream of the SC <NUM> in the exhaust passage <NUM>, and an oxygen sensor <NUM> is disposed immediately downstream of the SC <NUM> in the exhaust passage <NUM>. A NOx sensor <NUM> is disposed immediately downstream of the NSR <NUM> in the exhaust passage <NUM>.

The characteristics of the SC <NUM> are common to those of the SC <NUM> described for the first embodiment. The SC <NUM>, under a lean atmosphere, reduces NOx in the exhaust gas to N<NUM> while adsorbing oxygen, and under a rich atmosphere, oxidizes HC and CO in the exhaust gas to H<NUM>O and CO<NUM> while releasing oxygen.

The NSR <NUM> has a larger NOx storage capacity than the NSR <NUM> of the first embodiment (for example, twice as much storage capacity as the NSR used in combination with SCR). The NSR <NUM> stores NOx in the exhaust gas in the form of a nitrate salt such as a Ba(NO<NUM>)<NUM> under the lean atmosphere. Further, when a reduction agent is supplied from the upstream side of the exhaust flow, the NSR <NUM> releases the stored NOx and reacts, for example, the reduction agent with NOx to reduce NOx, as expressed by chemical equations (<NUM>) and (<NUM>). In the NSR <NUM>, NOx is reduced to N<NUM> by a direct reaction with NOx.

CO + NO → CO<NUM> + <NUM>.

CHy + (<NUM>+y/<NUM>) NO → CO<NUM> + (y/<NUM>) H<NUM>O + <NUM>/<NUM> (<NUM>+y/<NUM>) N<NUM>.

The control device <NUM> executes the rich spike control in the lean combustion operation mode, in a similar manner to that in the first embodiment. By executing the rich spike control during the lean combustion operation, NOx stored in the NSR <NUM> is separated therefrom, such that the NOx storing performance of the NSR <NUM> is recovered. The rich spike control is executed in accordance with a predetermined execution rules. For example, the rich spike control is executed when the concentration of NOx measured by the NOx sensor <NUM> disposed immediately downstream of the NSR <NUM> exceeds a predetermined threshold value. NOx that has separated from the NSR <NUM> by the rich spike control is reduced to N<NUM> on the NSR <NUM>.

However, when the engine <NUM> is operated in the scavenging range, HC and CO necessary for the reduction of NOx will be oxidized by oxygen if air containing a high concentration of oxygen flows into the exhaust gas control apparatus <NUM> by the scavenging while executing the rich spike control. As a result, the reduction of NOx is hindered, and the purification efficiency of NOx on the NSR <NUM> is significantly deteriorated.

In the valve timing control of this embodiment, similarly to the first embodiment, when the rich spike control is executed, the close timing of the intake valve is retarded and the operation angle of the intake valve is made smaller. Accordingly, the overlap amount between the intake valve and the exhaust valve is decreased, and the oxygen amount that blows from the intake port to the exhaust port by the scavenging is decreased. As a result, the reaction between oxygen and the reduction agent produced by the rich spike control can be reduced, while reduction reaction of NOx by the NSR <NUM> as expressed by the above-mentioned chemical equations (<NUM>) and (<NUM>) can be promoted. In other words, even in the case where the exhaust gas control apparatus <NUM> is configured as described above, effectiveness of the rich spike control can be maintained by performing the valve timing control similar to that in the first embodiment. Accordingly, deterioration of NOx purification efficiency caused by scavenging can be prevented.

Claim 1:
A control device for an internal combustion engine (<NUM>), the internal combustion engine (<NUM>) including an exhaust gas control apparatus (<NUM>, <NUM>), an exhaust port (<NUM>), an intake port (<NUM>), an exhaust valve (<NUM>) and an intake valve (<NUM>), the exhaust gas control apparatus (<NUM>, <NUM>) configured to store NOx in exhaust gas under a leaner atmosphere with excessive oxygen compared to an atmosphere under a stoichiometric air-fuel ratio and the exhaust gas control apparatus (<NUM>, <NUM>) configured to react, directly or indirectly, NOx with a reduction agent supplied from an upstream side of an exhaust flow to reduce NOx, the control device comprising:
an electronic control unit (<NUM>) configured to:
(i) execute a rich spike control, the rich spike control being a control executed to temporarily change an in-cylinder air-fuel ratio from a leaner air-fuel ratio than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio; and
(ii) vary an overlap amount of the intake valve (<NUM>) and the exhaust valve (<NUM>) such that the overlap amount is less during execution of the rich spike control than during non-execution of the rich spike control, in an operation range where a pressure of the intake port (<NUM>) becomes higher than a pressure of the exhaust port (<NUM>).