JP2015200294A - engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an engine which can cool exhaust without deteriorating fuel economy.SOLUTION: An engine comprises a supercharger (31), an intake valve (15), an intake variable valve device (27), a fuel injection valve (21), and a catalyst (9). In an operation region in which an exhaust system part including the catalyst (9) is brought into an overheat state, when the pressure of fresh air supercharged by the supercharger (31) is higher than exhaust pressure, an exhaust valve (16) and the intake valve (15) are brought into valve-overlapped states that both the valves are opened by making an opening/closing time of the intake valve (15) advance by using the intake variable valve device (27), fuel injection is performed after the exhaust valve (16) is closed, and a fuel injection amount from the fuel injection valve (21) is controlled so that an air-fuel mixture of a theoretical air-fuel ratio, or an air-fuel mixture which is leaner than the theoretical air-fuel ratio can be obtained.

Description

  The present invention relates to an internal combustion engine (hereinafter referred to as an “engine”), and more particularly to an engine that cools exhaust gas in an overheated operating region.

  In order to prevent the exhaust system parts including the catalyst from becoming overheated in the overheated operating region, the fuel injection amount from the fuel injection valve provided facing the intake port is increased and the increased fuel vaporization is performed. Some exhaust air is cooled by heat (see Patent Document 1).

JP 2013-7289 A

  By the way, if the exhaust gas is cooled by the vaporization heat of the fuel injected from the fuel injection valve as in the technique of Patent Document 1, the fuel efficiency is deteriorated.

  Therefore, an object of the present invention is to provide an engine that can cool exhaust gas without deteriorating fuel consumption.

  The engine of the present invention includes a supercharger that supercharges fresh air flowing into an intake passage, an intake valve that opens and closes an intake port that opens to a combustion chamber, and an intake valve that opens and closes according to the operating state of the engine. An intake variable valve operating apparatus capable of changing the timing, a fuel injection valve, and a catalyst are provided. The fuel injection valve directly injects fuel into the combustion chamber. The catalyst is interposed in the exhaust passage. And, in the operation region where the exhaust system parts including the catalyst, the intake / exhaust valve in the combustion chamber or the portion from the combustion chamber to the exhaust passage is overheated, the pressure of the fresh air supercharged by the supercharger is The following control is performed for a case where the pressure is higher than the exhaust pressure. That is, at the end of the exhaust stroke, the valve opening and closing timing of the intake valve is advanced by using the intake variable valve operating device, whereby a valve overlap state in which both the exhaust valve and the intake valve are open is achieved. Further, fuel injection is performed after the exhaust valve is closed, and the fuel injection amount from the fuel injection valve is controlled so that a lean air-fuel mixture is obtained.

  According to the present invention, when the valve overlap is generated, the pressure of the fresh air supercharged by the supercharger is higher than the exhaust pressure, so the fresh air flowing into the combustion chamber 5 from the intake passage is exhausted. Blow through. The average exhaust temperature is lowered by the fresh air blown through the exhaust passage. As a result, the exhaust system parts including the catalyst, the intake / exhaust valve in the combustion chamber, or the portion from the combustion chamber to the exhaust passage can be cooled more than when fresh air is not blown through the exhaust passage.

  In this case, if the fuel injection is performed while the exhaust valve is open, the fuel is blown out from the combustion chamber 5 to the exhaust passage, resulting in deterioration of fuel consumption. In addition, if the amount of fuel from the fuel injection valve is controlled so that an air-fuel mixture richer than the stoichiometric air-fuel ratio can be obtained in the valve overlap state, the fuel that remains in the combustion chamber is burned after it has flowed into the exhaust passage. As a result, the exhaust temperature becomes higher. On the other hand, according to the present invention, fuel injection is performed after the exhaust valve is closed, and the fuel injection amount from the fuel injection valve is controlled so that an air-fuel mixture leaner than the stoichiometric air-fuel ratio or the stoichiometric air-fuel ratio is obtained. This prevents fuel in the combustion chamber from blowing from the combustion chamber to the exhaust passage, thereby preventing deterioration of fuel consumption and preventing afterburning in the exhaust passage.

It is a schematic block diagram of the engine of 1st Embodiment of this invention. It is a flowchart for demonstrating an exhaust temperature fall process. FIG. It is a valve lift characteristic view for explaining valve overlap. 6 is a flowchart for explaining calculation of the degree of air-fuel ratio enrichment that is required immediately after the end of the exhaust gas temperature reduction process. It is a flowchart for demonstrating calculation of a fuel injection pulse width. It is a flowchart for demonstrating correction | amendment of fuel injection timing. It is a schematic block diagram of the engine of 2nd Embodiment. It is a flowchart for demonstrating the exhaust temperature fall process of 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic configuration diagram of an engine 1 according to a first embodiment of the present invention. Although FIG. 1 shows only one cylinder, it is actually an in-line four-cylinder four-cycle engine.

  In FIG. 1, a throttle valve 11 is provided in the intake passage 2. The throttle valve 11 is driven by a motor 12 that receives a signal from the engine controller 41. The air is metered by the throttle valve 11, stored in the intake collector 3 of the intake passage 2, and then introduced into the combustion chamber 5 of each cylinder via the intake manifold 4. In the present invention, since the EGR device is not provided, air that has passed through the air flow meter 42, that is, air that does not contain EGR gas, flows into the combustion chamber as it is. The air that does not contain the EGR gas is hereinafter referred to as “fresh air”.

  Fuel is injected and supplied from a fuel injection valve 21 disposed directly facing the combustion chamber 5 of each cylinder. The fuel injected into the combustion chamber 5 is vaporized and mixed with air to produce a working gas (air mixture). This air-fuel mixture is confined in the combustion chamber 5 when the intake valve 15 is closed, and is compressed by the rise of the piston 6.

  In order to ignite this compressed air-fuel mixture with a high-pressure spark, an ignition device 22 for an electronic power distribution system is provided in which an ignition coil with a built-in power transistor is arranged in each cylinder. That is, the ignition device 22 includes an ignition coil, a power transistor (not shown), and an ignition plug 24. The ignition coil 23 stores electrical energy from the battery, and the power transistor supplies and shuts off the primary side of the ignition coil 23. The spark plug 24 provided on the ceiling of the combustion chamber 5 receives a high voltage generated on the secondary side of the ignition coil 23 when the primary current of the ignition coil 23 is interrupted, and performs spark discharge.

  When the spark is blown by the spark plug 24 and the compressed air-fuel mixture is ignited slightly before the compression top dead center after the warm-up of the engine 1 is completed, the flame spreads into the combustion chamber 5 and eventually explosively burns. The gas pressure performs the work of pushing down the piston 6. This work is taken out as the rotational force of the crankshaft 7. The combusted gas (exhaust gas) is discharged from the exhaust manifold to the exhaust passage 8 when the exhaust valve 16 is opened.

  The exhaust passage 8 includes three-way catalysts 9 and 10. When the air-fuel ratio of the exhaust is within a narrow range centered on the stoichiometric air-fuel ratio, the three-way catalysts 9, 10 can efficiently remove harmful three components such as HC, CO, and NOx contained in the exhaust simultaneously.

  The air-fuel ratio is the ratio between the intake air amount and the fuel amount. In the engine controller 41, the fuel injection pulse width Ti [] is set so that the ratio of the fresh air amount introduced into the combustion chamber 5 per cycle of the engine 1 and the fuel injection amount from the fuel injection valve 21 becomes the stoichiometric air-fuel ratio. ms]. At a predetermined fuel injection timing, the fuel injection valve 21 is opened and fuel is directly injected into the combustion chamber 5 during the fuel injection pulse width Ti. The basic injection pulse width Tp [ms] is calculated based on the intake air amount signal from the air flow meter 42 and the signals from the crank angle sensors (43, 44).

  Further, after the three-way catalyst 9 is activated, it is determined that the air-fuel ratio feedback control region has been reached. Then, based on the output of the air-fuel ratio sensor 47 provided upstream of the three-way catalyst 9 and the output of the oxygen sensor 48 provided downstream of the three-way catalyst 9, the air-fuel ratio of the exhaust gas matches the stoichiometric air-fuel ratio. The feedback correction coefficient α is calculated. Then, the fuel injection pulse width Ti is calculated by correcting the basic injection pulse width Tp with the air-fuel ratio feedback correction coefficient α.

  The intake valve 15 and the exhaust valve 16 are opened / closed by operation of cams provided on the intake side camshaft 25 and the exhaust side camshaft 26, respectively, with the crankshaft 7 as a power source.

  On the intake valve 15 side, the rotation phase difference between the crankshaft 7 and the intake camshaft 25 is continuously variably controlled, and the opening / closing timing (opening timing and closing timing) of the intake valve 15 is advanced or retarded. A valve timing mechanism (hereinafter referred to as “intake side VTC mechanism”) 27 is provided. A cam angle sensor 44 for detecting the rotational position of the intake side camshaft 25 is also provided at the other end of the intake side camshaft 25.

  Also on the exhaust valve 16 side, a variable valve that continuously and variably controls the rotational phase difference between the crankshaft 7 and the exhaust camshaft 26 to advance or retard the opening / closing timing (opening timing and closing timing) of the exhaust valve 16. A timing mechanism (hereinafter referred to as “exhaust side VTC mechanism”) 29 is provided. A cam angle sensor 45 for detecting the rotational position of the exhaust side camshaft 26 is provided at the other end of the exhaust side camshaft 26.

  In the engine controller 41, instructions given to the intake-side VTC mechanism 27 (intake variable valve operating device) and the exhaust-side VTC mechanism 29 so that the opening / closing timings of the intake / exhaust valves 15 and 16 become appropriate opening / closing timings according to operating conditions. Adjust the amount.

  An engine-driven supercharger (mechanical supercharger) 31 that supercharges fresh air drawn into the engine 1 is provided upstream of the throttle valve 11. That is, the rotation of the crankshaft 7 is always transmitted to the supercharger 31 via a belt and a pulley (not shown). The supercharger 31 has a roots type, for example. The supercharger 31 may be driven by an electric motor or the like.

  An intercooler 35 is provided in the intake passage 2 downstream of the supercharger 31. The intercooler 35 is for cooling the fresh air that has been compressed by the supercharger 31 and has reached a high temperature.

  A bypass passage 32 that bypasses the supercharger 31 and merges upstream of the throttle valve 11 is provided, and a bypass valve 33 that is driven by a step motor 34 is provided in the bypass passage 32.

  The engine controller 41 controls the opening degree of the bypass valve 33. That is, the engine controller 41 determines whether or not the engine is in the supercharging region based on the throttle valve opening detected by a throttle sensor (not shown) and the engine rotational speed Ne. From this determination result, the upstream pressure (supercharging pressure) of the throttle valve 11 is equivalent to atmospheric pressure in the non-supercharging region, and the upstream pressure (supercharging pressure) of the throttle valve 11 is higher than atmospheric pressure in the supercharging region. The opening degree of the bypass valve 33 is controlled so as to reach the target boost pressure. Further, based on the signal of the actual boost pressure from the boost pressure sensor 46 provided immediately upstream of the throttle valve 11, when the actual boost pressure becomes lower than the target boost pressure, the steady-state bypass valve opening is set. On the other hand, correction to the side where the boost pressure increases is performed. On the contrary, the element in which the actual supercharging pressure becomes higher than the target supercharging pressure is immediately corrected so that the supercharging pressure decreases with respect to the steady-state bypass valve opening.

  There is a conventional device that prevents exhaust system parts including the three-way catalysts 9 and 10 and the air-fuel ratio sensor 47 from being overheated in an operation region that is overheated. In this conventional apparatus, the fuel injection amount from the fuel injection valve provided facing the intake port is increased, and the exhaust gas is cooled by the increased heat of vaporization of the fuel. However, if the exhaust gas is cooled by using the heat of vaporization of the fuel as in the conventional apparatus, the fuel efficiency is deteriorated.

  Therefore, in the first embodiment of the present invention, an operation region in which the exhaust system parts including the three-way catalysts 9 and 10 and the air-fuel ratio sensor 47 are overheated is set in advance as the exhaust temperature lowering region. When the pressure of the intake air supercharged by the supercharger 31 (that is, the supercharging pressure) is higher than the exhaust pressure, the exhaust valve 15 is advanced by advancing the opening / closing timing of the intake valve 15 using the intake side VTC mechanism 27. 16 and a valve overlap state in which the intake valve 15 is open. As a result, fresh air is blown from the intake passage 2 through the combustion chamber 5 to the exhaust passage 8, and the exhaust system parts are cooled by the fresh air blown into the exhaust passage 8. Even if the fresh air blown into the exhaust passage 8 does not directly hit the exhaust system parts, it has the effect of reducing the average exhaust temperature.

  Here, there is a comparative example in which a valve overlap occurs in an engine including a turbocharger configured by coaxially configuring a turbine and a compressor (see JP 2012-163447 A). In this comparative example, by generating valve overlap, scavenging of the gas from the combustion chamber 5 is improved, and the rotational speed of the turbine is increased by the scavenged gas, so that the efficiency of charging fresh air into the combustion chamber is increased. To improve. As in this comparative example, the valve overlap is generally used for engine output, catalyst warm-up promotion, etc., and the valve overlap is used for cooling the exhaust as in the present invention. There is no conventional device.

  Further, in the first embodiment, after the exhaust valve 16 is closed, fuel is injected from the fuel injection valve 21 provided facing the combustion chamber 5 and the stoichiometric air-fuel mixture is obtained from the fuel injection valve 21 so that a stoichiometric air-fuel mixture can be obtained. Control the fuel injection amount. This prevents the fuel supplied to the combustion chamber 5 from being blown from the combustion chamber 5 to the exhaust passage 8 and prevents afterburning of the fuel blown to the exhaust passage 8. In this embodiment, the fuel injection amount from the fuel injection valve 21 is controlled so that a stoichiometric air-fuel mixture is obtained. However, the fuel from the fuel injection valve 21 is obtained so that a leaner air-fuel mixture is obtained than the stoichiometric air-fuel ratio. It may be to control the injection amount.

  In the first embodiment, the fuel supplied from the fuel injection valve 21 is temporarily increased immediately after the valve overlap state is finished. This is because the oxygen in the fresh air blown out by the valve overlap is excessively stored in the three-way catalyst 9 on the upstream side of the exhaust passage 8, and this needs to be dealt with.

  In the first embodiment, the amount of fresh air remaining in the combustion chamber 5 in the valve overlap state is estimated, and the basic injection amount Tp for obtaining the stoichiometric air-fuel mixture is calculated based on the estimated fresh air amount. . This is because if the basic injection pulse width Tp is calculated without considering the fresh air disappearing from the combustion chamber 5 due to the valve overlap, it is necessary to cope with the fact that the air-fuel mixture becomes richer than the stoichiometric air-fuel ratio. Because there is.

  Control such as the exhaust temperature lowering process executed by the engine controller 41, the calculation of the air-fuel ratio enrichment degree required immediately after the exhaust temperature lowering process ends, the calculation of the fuel injection amount during and immediately after the exhaust temperature lowering process Is described based on the following flowchart.

  First, the flow of FIG. 2 is for performing the exhaust gas temperature lowering process, and is executed at regular intervals (for example, every 10 ms).

  It is determined whether or not the operating point determined from the engine speed Ne and the engine torque is in the exhaust temperature lowering region shown in FIG. In particular, the three-way catalyst 9 on the upstream side of the exhaust passage 8, the air-fuel ratio sensor 47 provided in the exhaust manifold assembly, and the like are exhaust system parts that are overheated because they are exposed to the hottest exhaust gas. Since the temperature of the exhaust gas rises almost in proportion to the amount of fuel supplied to the combustion chamber 5, the exhaust gas temperature becomes the highest on the high load side. For this reason, as shown in FIG. 3, an exhaust temperature lowering region is set in advance on the high load side, and when the operating point belongs to the exhaust temperature lowering region, the exhaust system component that is overheated because it is exposed to high-temperature exhaust gas. It is necessary to perform cooling so as not to decrease the proof stress and to prevent knocking. In the conventional device, the amount of fuel is increased in the fuel cooling region, and the exhaust temperature is lowered by vaporizing the fuel. In the first embodiment, instead of increasing the amount of fuel, fresh air is exhausted during the valve overlap period. The exhaust is cooled by blowing through to 8. The exhaust temperature lowering region is basically the same region as the fuel cooling region of the conventional device, but a new region may be determined in accordance with the recent downsizing tendency of the engine.

  A map of engine torque using the engine speed Ne and the engine load as parameters is created in advance, and the engine torque is calculated by searching the map from the engine speed Ne and the engine load. When the operating point determined from the engine torque and the engine speed Ne calculated in this way is not in the exhaust temperature lowering region shown in FIG. 3, it is determined that it is not necessary to cool the exhaust, and the routine proceeds to step 7 to proceed to valve overlap. Flag = 0.

  When the operating point determined from the engine torque and the engine rotational speed Ne is in the exhaust temperature lowering region shown in FIG. 3, in order to prevent the proof stress of exhaust system parts that are overheated due to exposure to high temperature exhaust, Go to 2 and after.

  In step 2, the target equivalent ratio DML is compared with 1.0. Here, there is a relationship of DML = theoretical air / fuel ratio / target air / fuel ratio between the target equivalent ratio DML and the target air / fuel ratio, and the target equivalent ratio DML is 1.0 when operating at the theoretical air / fuel ratio. At the time of stratified combustion when the air-fuel ratio exceeds 40, or when the combustion is lean and the air-fuel ratio is 20 to 23, the target equivalent ratio DML becomes a value smaller than 1.0. That is, during stratified combustion or homogeneous combustion, the intake air amount is increased as compared with the operation at the stoichiometric air-fuel ratio.

  In the present embodiment, as shown in FIG. 3 as well, an exhaust gas temperature lowering region is newly introduced. Therefore, the target equivalent ratio DML = 1.0 is set in the exhaust gas temperature lowering region. That is, the engine 1 is operated with a stoichiometric air-fuel ratio mixture in the exhaust gas temperature reduction region.

  Since the target equivalent ratio DML = 1.0 is set in the exhaust gas temperature lowering region, it is not necessary to determine whether or not the target equivalent ratio DML is 1.0 or less in step 2, but step 2 is provided just in case. ing. When the target equivalent ratio DML exceeds 1.0, the fuel remaining in the combustion chamber 5 is burnt after by the fresh air blown into the exhaust passage 8 and raises the exhaust temperature (that is, lowers the exhaust temperature). Is not possible). At this time, the routine proceeds to step 7 where the valve overlap flag = 0.

  When the target equivalent ratio DML is 1.0 or less, the stoichiometric air-fuel mixture burns in the combustion chamber 5. For this reason, it is determined that the fuel remaining in the combustion chamber 5 will not be burned by the fresh air blown into the exhaust passage 8, and the process proceeds to step 3.

  In step 3, the boost pressure Pb detected by the boost pressure sensor 46 is compared with the exhaust pressure Pex detected by the exhaust pressure sensor 49. The exhaust pressure Pexh need not be a sensor detection value. The exhaust pressure Pexh may be estimated based on the engine load and the rotational speed by a known method. Gas flows from higher pressure to lower pressure. When the supercharging pressure Pb is equal to or lower than the exhaust pressure Pex, it is determined that the fresh air flowing into the combustion chamber 5 cannot be blown through the exhaust passage 8 even if valve overlap occurs. At this time, the routine proceeds to step 7 where the valve overlap flag = 0.

  When the supercharging pressure Pb exceeds the exhaust pressure Pex, it is determined that fresh air flowing into the combustion chamber 5 can be blown through the combustion chamber 5 into the exhaust passage 8 when the valve overlap occurs. move on. In step 4, the opening / closing timing of the intake valve 15 is advanced by a predetermined amount in order to cause valve overlap. For example, when the exhaust temperature lowering region is reached and the process proceeds to step 4, as shown in FIG. 4, the lift characteristic of the exhaust valve 16 is in a solid line, and the lift characteristic of the intake valve 15 is in a broken line state. Assume that there is no valve overlap. At this time, the opening / closing timing of the intake valve 15 is advanced through the intake-side VTC mechanism 27 to obtain a solid line characteristic, thereby causing a valve overlap that has not existed before the advancement.

  In step 5, in order to indicate that a valve overlap has occurred, a valve overlap flag (initially set to zero when the engine is started) is set to 1.

  In step 6, a value obtained by correcting the basic value of the target boost pressure by a certain value is set as the target boost pressure. The reason for correcting the target boost pressure to the higher side is as follows. That is, when fresh air flowing into the combustion chamber 5 is blown into the exhaust passage 8 due to the valve overlap, the charging efficiency by the fresh air remaining in the combustion chamber is reduced accordingly. This is to obtain a charging efficiency equivalent to a state in which no valve overlap occurs by correcting the reduction of the charging efficiency due to the blow-through of fresh air to the side where the target supercharging pressure is increased.

  The basic value of the target boost pressure may be simply constant, but may be a variable value. For example, a map of the basic value of the target boost pressure with the engine speed Ne and the engine load as parameters is created in advance, and the map is searched from the engine speed Ne and the engine load, thereby obtaining the target boost pressure. The basic value of is calculated.

  On the other hand, when the operating point is in the exhaust temperature lowering region but the operating point is no longer in the exhaust temperature lowering region, the process proceeds from step 1 to step 7 to end the exhaust temperature lowering process, and the valve overlap flag = 0. And Further, when the boost pressure Pb exceeds the exhaust pressure Pexh, when the boost pressure Pb becomes equal to or lower than the exhaust pressure Pexh after that, the process proceeds from step 3 to step 7 to end the exhaust temperature lowering process, and the valve over It is assumed that the lap flag = 0.

  The flow in FIG. 5 is for calculating the air-fuel ratio enrichment degree R required immediately after the exhaust temperature lowering process is completed, and is executed at regular intervals (for example, every 10 ms) following the flow in FIG.

  In Steps 11 and 12, it is checked whether or not the valve overlap flag = 1 at this time, and whether or not the valve overlap flag = 1 at the previous time. When the valve overlap flag = 1 at this time and the valve overlap flag = 0 at the previous time, that is, when the valve overlap flag is switched from zero to 1 for the first time this time, the routine proceeds to steps 13 and 14. In step 13, a timer is started (timer value t = 0). This timer is for measuring the elapsed time since the start of the valve overlap.

  In step 14, the valve overlap experience flag (initially set to zero when the engine is started) = 1 in order to indicate that the valve overlap flag has been switched from 1 to zero and that there is experience of causing the valve overlap. To do.

  In steps 11 and 12, if the valve overlap flag = 1 at this time and the valve overlap flag = 1 at the previous time, that is, if the valve overlap is being generated, the process proceeds to step 15. In step 15, the timer value t is incremented.

  On the other hand, when the valve overlap flag = 0 at step 11, the routine proceeds to step 16 where the air / fuel ratio enrichment degree calculated flag is checked. If the air-fuel ratio enrichment degree calculated flag = 0, it is determined that the air-fuel ratio enrichment degree has not been calculated yet, and the process proceeds to step 17.

  In step 17, the valve overlap experience flag is viewed. When the valve overlap experience flag = 0, it is determined that it is not immediately after the end of the exhaust gas temperature lowering process, and the current process ends.

  When the valve overlap experience flag = 1 in step 17, it is determined that the exhaust temperature lowering process has just ended, and the process proceeds to steps 18 and 19. Steps 18 and 19 are parts for increasing the fuel injection amount immediately after the end of the exhaust gas temperature lowering process so that an air-fuel mixture richer than the stoichiometric air-fuel ratio can be obtained. This is due to the following reason. That is, the exhaust system parts can be cooled by causing valve overlap during the exhaust gas temperature lowering process and blowing out fresh air flowing into the combustion chamber 5 into the exhaust passage 8. On the other hand, the fresh air blown into the exhaust passage 8 contains oxygen to the same extent as the atmosphere. This amount of oxygen in the atmosphere is stored in particular in the three-way catalyst 9 on the upstream side of the exhaust passage 8, and the oxygen storage amount of the three-way catalyst 9 on the upstream side of the exhaust passage 8 exceeds the target value. Become excessive. If the oxygen storage amount of the three-way catalyst 9 becomes full due to this excess, NOx cannot be reduced (purified) immediately after the valve overlap state in the exhaust temperature lowering process is finished. Therefore, immediately after the exhaust temperature lowering process is finished, the fuel injection amount is increased so that an air-fuel mixture richer than the stoichiometric air-fuel ratio can be obtained. As a result, the oxygen storage amount of the three-way catalyst 9 is decreased, the oxygen storage amount is returned to the target value, and the purification rate (particularly the NOx purification rate) of the three-way catalyst 9 is kept high.

  Specifically, in step 18, the excess oxygen ratio OVR [unknown number] of the three-way catalyst 9 on the upstream side of the exhaust passage 8 is calculated based on the timer value t at that time, that is, the valve overlap time. Here, the oxygen excess rate OVR of the three-way catalyst 9 is a value centered at 1.0, and is a value that becomes larger than 1.0 as the valve overlap time is longer. The characteristic of the oxygen excess rate OVR with respect to the valve overlap time is obtained in advance by adaptation.

  In step 19, based on the calculated oxygen excess ratio OVR of the three-way catalyst 9, an air-fuel ratio enrichment degree R [anonymous number] is calculated. Here, the enrichment degree R of the air-fuel ratio is a value centering on 1.0, and is a value that becomes larger than 1.0 as the oxygen excess ratio OVR of the three-way catalyst 9 increases. The characteristic of the air-fuel ratio enrichment degree R with respect to the oxygen excess rate OVR is obtained in advance by adaptation. Since the calculation of the air / fuel ratio enrichment degree R is completed, the air / fuel ratio enrichment degree calculated flag is set to 1 in step 20. The calculated air-fuel ratio enrichment degree R is stored in the memory.

  Since the air-fuel ratio enrichment degree calculation completed flag is set to 1 in step 20, the process cannot proceed from step 16 to step 17 and subsequent steps, and the current process is terminated as it is.

  The flow in FIG. 6 is for calculating the fuel injection pulse width Ti, and is executed at regular intervals (for example, every 10 ms) following the flow in FIGS.

  In step 21, it is determined whether or not the operating point determined from the engine speed Ne and the engine torque is in the exhaust temperature lowering region shown in FIG. The operation in step 21 is the same as the operation in step 1 in FIG. When the operating point is not in the exhaust gas temperature lowering region shown in FIG. 3, the routine proceeds to step 22, where the fresh air amount detected by the air flow meter 42 is directly put into the intake air amount Qa for calculating the basic injection pulse width Tp.

  When the operating point is in the exhaust temperature lowering region shown in FIG. Step 23 is a part for estimating the amount of fresh air trapped in the combustion chamber when a valve overlap occurs. That is, in step 23, the inside of the combustion chamber 5, which is the amount of fresh air confined in the combustion chamber 5 at the closing timing of the intake valve based on the fresh air amount detected by the air flow meter 42 and the scavenging rate. Estimate the amount of fresh trap air. The scavenging rate is a value obtained by dividing the amount of fresh air by the amount of gas in the combustion chamber 5. As a method for calculating the scavenging rate, a known means may be used (see JP 2012-163447 A).

  In step 24, the amount of fresh air trapped in the combustion chamber is entered into the intake air amount Qa for calculating the basic injection pulse width Tp.

  In step 25, the basic injection pulse width Tp [ms] is calculated by the following equation based on the intake air amount Qa and the engine rotational speed Ne.

Tp = K × Qa / Ne (1)
The constant K in the equation (1) is a value for obtaining a stoichiometric air-fuel mixture.

  Here, when the fresh air flowing into the combustion chamber 5 is blown through the exhaust passage 8 due to the valve overlap, the basic injection pulse width Tp is calculated based on the estimated amount of trap fresh air in the combustion chamber 5. Even when fresh air flowing into the combustion chamber 8 is blown through the exhaust passage 8 due to valve overlap, the basic injection pulse width Tp is calculated using the fresh air amount detected by the air flow meter 42 as it is. The air-fuel ratio is inclined to the richer side than the stoichiometric air-fuel ratio by the amount of fresh air that disappears from the combustion chamber 5 due to the blow-through of fresh air by the lap. On the other hand, in this embodiment, since steps 21, 23, and 24 are added and the amount of fresh air that disappears from the combustion chamber 5 due to the blow-through of fresh air due to valve overlap is taken into account, even during valve overlap, A stoichiometric air-fuel ratio mixture is obtained.

  Steps 26 to 31 are parts for increasing the fuel injection amount immediately after the exhaust temperature lowering process is finished so that the air-fuel ratio is richer than the stoichiometric air-fuel ratio. As described above, the oxygen contained in the fresh air blown into the three-way catalyst 9 on the upstream side of the exhaust passage 8 due to the blow-off of the fresh air due to the valve overlap is excessively stored beyond the target value. . Therefore, the excessively stored oxygen is lost from the three-way catalyst 9.

  First, in step 26, the valve overlap experience flag is viewed. When the valve overlap experience flag = 1, the routine proceeds to step 27, where it is determined whether or not a fixed time has elapsed from the timing when the exhaust temperature lowering process is completed. Here, the predetermined time is defined in advance as the enrichment time (the time during which the fuel injection amount is increased to make the air-fuel ratio richer than the stoichiometric air-fuel ratio).

  When the fixed time has not elapsed since the timing at which the exhaust temperature lowering process is completed, that is, immediately after the exhaust temperature lowering process is completed, the routine proceeds to step 28. In step 28, a value obtained by multiplying the basic injection pulse width Tp by the air-fuel ratio enrichment degree R (calculated in FIG. 5) is again set as the basic injection pulse width Tp. If the air-fuel ratio enrichment degree R exceeds 1.0, the fuel injection amount is increased by this, and the exhaust air-fuel ratio becomes richer than the stoichiometric air-fuel ratio. The operation of step 28 is executed as long as a predetermined time has not elapsed from the timing when the exhaust temperature lowering process is completed. As a result, since the air-fuel ratio richer than the stoichiometric air-fuel ratio flows through the three-way catalyst 9 from the timing when the exhaust temperature lowering process is completed, the oxygen storage amount of the three-way catalyst 9 decreases to the target value. .

  When a certain time has passed in step 27, it is determined that the oxygen stored excessively during the valve overlap has disappeared from the three-way catalyst 9, and the oxygen storage amount of the three-way catalyst 9 has returned to the target value again. At this time, in order to prepare for the next exhaust gas temperature lowering process, the routine proceeds to steps 29, 30, and 31 and the valve overlap experience flag = 0, the air / fuel ratio enrichment degree R = 0, and the air / fuel ratio enrichment degree calculated flag = 0. To do.

  In step 32, the fuel injection pulse width Ti [ms] at the time of sequential injection is calculated by the following equation.

Ti = Tp × DML × (α + αm−1) × 2 + Ts (2)
Where DML: target equivalent ratio,
α: Air-fuel ratio feedback correction coefficient [anonymous number]
αm: Air-fuel ratio learning value [anonymous number]
Ts: invalid pulse width [ms],
Here, the target equivalent ratio DML in the equation (2) is determined based on the engine speed Ne and the accelerator opening. The accelerator opening is detected by an accelerator sensor 50 (see FIG. 1). In the exhaust gas temperature lowering region, the target equivalent ratio DML in the equation (2) is 1.0. In the exhaust gas temperature lowering region, the air-fuel ratio feedback control is stopped (α = 1.0). For this reason, the above equation (2) becomes as follows in the exhaust gas temperature lowering region.

Ti = Tp × 1.0 × (1.0 + αm−1) × 2 + Ts
= Tp × αm × 2 + Ts (3)
Here, assuming that there is no manufacturing error in the fuel injection valve 21 and the air flow meter 48, since αm = 1.0, Ti = Tp × 2 + Ts. From this, it can be seen that in the exhaust gas temperature lowering region, the fuel amount is determined only by Tp, so that the engine is operated with the stoichiometric air-fuel mixture.

  In step 33, the fuel injection pulse width Ti calculated in this way is transferred to the output register. When the predetermined fuel injection timing IT is reached, the fuel injection valve 21 is opened only during the fuel injection pulse width Ti, and a fuel injection amount corresponding to Ti is injected and supplied into the combustion chamber 5.

  The flow in FIG. 7 is for correcting the fuel injection timing IT, and is executed at regular time intervals (for example, every 10 ms) following the flow in FIGS.

  In step 41, the valve overlap flag is checked. If the valve overlap flag = 0, the current process is terminated.

  When the valve overlap flag = 1, the routine proceeds to step 42, where the fuel injection timing IT and the exhaust valve 16 closing timing EVC are compared. Here, the fuel injection timing IT is set in advance based on the engine speed Ne so as to be, for example, during the compression stroke or the exhaust stroke. The closing timing EVC of the exhaust valve 16 can be known from the instruction amount given to the exhaust side VTC mechanism 29 by the engine controller 41.

  When the preset fuel injection timing IT is equal to the closing timing EVC of the exhaust valve 16 or on the retard side of the closing timing EVC of the exhaust valve 16, the fuel is put into the combustion chamber 5 after the exhaust valve 16 is closed. The injection will be supplied. That is, it can be determined that the fuel supplied to the combustion chamber 5 does not blow through the exhaust passage 8. At this time, the current process is terminated.

  On the other hand, when the fuel injection timing IT is on the advance side of the closing timing EVC of the exhaust valve 16 in step 42, the process proceeds to step 43 so that the fuel injection timing IT is on the retarding side of the closing timing EVC of the exhaust valve 16. The fuel injection timing IT is corrected. The reason why IT is corrected so that IT is on the more retarded side than EVC when the fuel injection timing IT is on the more advanced side than the closing timing EVC of the exhaust valve 16 is as follows. That is, when the fuel injection is performed toward the combustion chamber 5 when the exhaust valve 16 is still open during the valve overlap in the exhaust temperature lowering region, the fuel supplied to the combustion chamber 5 is up to the combustion chamber 5. To the exhaust passage 8. As a result, the air-fuel mixture becomes leaner than the stoichiometric air-fuel ratio, and a stoichiometric air-fuel mixture cannot be obtained. Therefore, during the valve overlap in the exhaust temperature lowering region, the fuel injection is performed after the exhaust valve 16 is closed to prevent the fuel from being blown out.

  Here, the effect of this embodiment is demonstrated.

  In the present embodiment, a supercharger 31 (supercharger), an intake valve 15, an intake side VTC mechanism 27 (intake variable valve operating device), a fuel injection valve 21, and a three-way catalyst 9 are provided. In the present embodiment, the supercharging pressure Pb (the pressure of fresh air supercharged by the supercharger) is higher than the exhaust pressure Pexh in the exhaust temperature lowering region (the operating region where the exhaust system parts including the catalyst are superheated). The following control is performed for high cases. In other words, in this case, the opening and closing timing of the intake valve 15 is advanced by using the intake side VTC mechanism 27 to achieve a valve overlap state in which both the exhaust valve 16 and the intake valve 15 are open (step of FIG. 2). 1, 3, 4). In the present embodiment, when the valve overlap is generated, the supercharging pressure Pb is higher than the exhaust pressure Pexh, so that fresh air flowing into the combustion chamber 5 from the intake passage 2 blows through the exhaust passage 8. Due to the fresh air blown through the exhaust passage 8, the average exhaust temperature is lowered. As a result, the exhaust system parts including the three-way catalyst 9 can be cooled more than when the fresh air flowing into the combustion chamber 5 is not blown through the exhaust passage 8.

  In this case, if fuel injection is performed while the exhaust valve 16 is open, fuel up to the exhaust passage 8 is blown out from the combustion chamber 5, resulting in deterioration of fuel consumption. In addition, if the amount of fuel from the fuel injection valve 21 is controlled so that an air-fuel mixture richer than the stoichiometric air-fuel ratio can be obtained in the valve overlap state, the fuel remaining in the combustion chamber is discharged to the exhaust passage 8. Since it burns after, exhaust temperature will become still higher. On the other hand, in this embodiment, fuel injection is performed after the exhaust valve 16 is closed (see steps 41, 42, and 43 in FIG. 7). In addition, the fuel injection amount from the fuel injection valve 21 is controlled so as to obtain a stoichiometric air-fuel mixture (see steps 21, 23, 24 and 25 in FIG. 6). As a result, fuel in the combustion chamber 5 is not blown from the combustion chamber 5 to the exhaust passage 8 to prevent deterioration of fuel consumption, and afterburning in the exhaust passage 8 can be prevented. In this way, if the exhaust temperature can be lowered in the exhaust temperature lowering region, the possibility of adopting an exhaust system component having low heat resistance increases.

  Even when fresh air flowing into the combustion chamber 5 is blown through the exhaust passage 8 due to valve overlap, the basic injection pulse width Tp is calculated using the fresh air amount detected by the air flow meter 42 as it is. The air-fuel ratio is inclined to the richer side than the stoichiometric air-fuel ratio by the amount of fresh air that disappears from the combustion chamber 5 due to the blow-through of fresh air by the lap. On the other hand, in this embodiment, the amount of fresh air trapped in the combustion chamber (the amount of fresh air remaining in the combustion chamber 5 in the overlap state) is estimated, and the basic injection amount Tp is calculated based on the estimated amount of fresh air trapped in the combustion chamber. (Refer to steps 21, 23, 24, and 25 in FIG. 6). Thereby, even if a part of the amount of fresh air disappears from the combustion chamber 5 due to the blow-off of fresh air due to valve overlap, a mixture of stoichiometric air-fuel ratio can be obtained.

  Although exhaust system parts can be cooled by blowing out fresh air flowing into the combustion chamber 5 into the exhaust passage 8 in a valve overlap state, oxygen in the fresh air blown into the exhaust passage 8 is about the same as the atmosphere. include. This amount of oxygen in the atmosphere is stored in particular in the three-way catalyst 9 on the upstream side of the exhaust passage 8, and the oxygen storage amount of the three-way catalyst 9 exceeds the target value and becomes excessive. If the oxygen storage amount of the three-way catalyst 9 becomes full, the NOx cannot be reduced (purified) immediately after the valve overlap state is finished. On the other hand, in the present embodiment, the amount of fuel supplied from the fuel injection valve 21 is temporarily increased immediately after the operating point deviates from the exhaust temperature lowering region (immediately after the valve overlap state is finished) (step 26, FIG. 6). 27, 28). As a result, immediately after the operating point deviates from the exhaust temperature lowering region, the oxygen storage amount of the three-way catalyst 9 quickly decreases and the oxygen storage amount returns to the target value. In particular, the NOx purification rate can be kept high.

  In the valve overlap state, the amount of fresh air remaining in the combustion chamber 5 is reduced by the amount that fresh air flowing into the combustion chamber 5 blows into the exhaust passage 8, and the fresh air charging efficiency is lowered. On the other hand, in this embodiment, when the valve overlap state is set, correction is made to increase the target supercharging pressure (see steps 1, 3, 4, and 6 in FIG. 2). As a result, it is possible to prevent a decrease in the charging efficiency due to the blow-through of fresh air flowing into the combustion chamber 5 into the exhaust passage 8 and to obtain a charging efficiency equivalent to that in the case where the valve overlap state is not established.

(Second Embodiment)
FIG. 8 is a schematic configuration diagram of the engine 1 of the second embodiment. The same parts as those in FIG. 1 of the first embodiment are denoted by the same reference numerals.

  In the second embodiment, a turbocharger (capacitive supercharger) 61 having a function of assisting the turbo rotation speed with the electric motor 65 (this function is hereinafter referred to as “electric assist function”) instead of the supercharger 31 is provided. It is intended for the case of provision. Here, the turbocharger 61 includes a turbine 62, a compressor 63, a shaft 64, and an electric motor 65. The turbine 62 is disposed on the upstream side of the exhaust passage 8 and is driven by the inflowing exhaust. The compressor 63 is disposed in the intake passage 2 upstream of the throttle valve 11 and compresses fresh air downstream of the air flow meter 42. The shaft 64 that connects the turbine 62 and the compressor 63 transmits the rotational torque of the turbine 62 to the compressor 63.

  The electric motor 65 is provided between the turbine 62 and the compressor 63, more specifically, integrally with the shaft 64. The electric motor 65 is composed of a DC servo motor, for example. Since the electric motor 65 is provided integrally with the shaft 64, the rotational speed of the electric motor 65 is equal to the rotational speed of the turbocharger 61 (turbo rotational speed).

  The rotation speed of the electric motor 65 is controlled by the engine controller 41. When the electric motor 65 is switched from the non-driving state to the driving state by the engine controller 41, the rotation of the turbine 62 is assisted by the driving torque of the electric motor 65. Therefore, the rotational speed of the electric motor 65 (that is, the turbo rotational speed) is It rises from the driving state.

  The flow of FIG. 9 is for performing the exhaust temperature lowering process of the second embodiment, and is executed at regular time intervals (for example, every 10 ms). The flow in FIG. 9 replaces the flow in FIG. 2 of the first embodiment. The same parts as those in the flow of FIG. 2 of the first embodiment are denoted by the same reference numerals.

  The difference from the flow of FIG. 2 of the first embodiment will be mainly described. The supercharging pressure Pb detected by the supercharging pressure sensor 46 in step 3 and the exhaust pressure Pex detected by the exhaust pressure sensor 49 are calculated. Compare. When the supercharging pressure Pb is equal to or lower than the exhaust pressure Pexh, the routine proceeds to step 11 where the electric motor 65 is switched from the non-driving state to the driving state.

  Even in the case of a turbocharger that does not have an electric motor, there may be an opportunity for the supercharging pressure Pb to be higher than the exhaust pressure Pexh. However, in the turbocharger 61 having the electric assist function as in the second embodiment, the opportunity for the supercharging pressure Pb to be higher than the exhaust pressure Pexh can be increased as compared with a turbocharger that does not have an electric motor. That is, when the supercharging pressure Pb is equal to or lower than the exhaust pressure Pexh, in the second embodiment, the turbo rotation speed is increased from the non-driving state of the motor 65 by switching the electric motor 65 from the non-driving state to the driving state. The boost pressure is increased by actively reducing the pressure. As a result, when the electric motor is not driven, the boost pressure Pb may be lower than the exhaust pressure Pexh, but the boost pressure Pb may exceed the exhaust pressure Pexh. In this way, when the supercharging pressure Pb exceeds the exhaust pressure Pexh, the routine proceeds from step 3 to step 4, where valve overlap occurs, and fresh air flowing into the combustion chamber 5 is blown out into the exhaust passage 8.

  As described above, in the second embodiment, when the supercharger is the turbocharger 61 having the electric assist function, and the supercharging pressure Pb is equal to or lower than the exhaust pressure Pexh in the exhaust temperature lowering region, the electric motor 65 is moved from the non-driven state. Switching to the driving state (see steps 1, 3, 4 and 6 in FIG. 2). Thereby, the opportunity for the supercharging pressure Pb to exceed the exhaust pressure Pexh can be increased from a turbocharger without an electric motor. Thus, the opportunity to cool the exhaust system parts including the three-way catalyst 9 can be increased.

  In the embodiment, the object to be protected from overheating is the exhaust system part including the three-way catalyst 9, 10, but the present invention is not limited to this. For example, the object to be protected from overheating may be the combustion chamber 5, the intake / exhaust valves 15, 16, and the exhaust port that is a portion from the combustion chamber 5 to the exhaust passage 8.

1 Engine 2 Intake passage 4 Fuel injection valve 8 Exhaust passage 9 Three-way catalyst (exhaust system parts)
15 Intake valve 16 Exhaust valve 21 Fuel injection valve 27 Intake side VTC mechanism (intake variable valve operating device)
31 Supercharger (supercharger)
41 Engine controller 47 Air-fuel ratio sensor (exhaust system parts)
61 Turbocharger (supercharger)
65 electric motor

Claims (8)

A turbocharger that supercharges fresh air flowing into the intake passage;
An intake valve that opens and closes an intake port that opens to the combustion chamber;
An intake variable valve operating device capable of changing the opening and closing timing of the intake valve according to the operating state of the engine;
A fuel injection valve for directly injecting fuel into the combustion chamber;
A catalyst interposed in the exhaust passage,
In the operation region where the exhaust system component including the catalyst, the intake / exhaust valve in the combustion chamber or the portion from the combustion chamber to the exhaust passage is overheated, the pressure of the fresh air supercharged by the supercharger is the exhaust pressure. When the intake valve is higher, the intake valve is opened and closed by advancing the opening / closing timing of the intake valve, so that the exhaust valve and the intake valve are both open, and the exhaust valve An engine which performs fuel injection after being closed and controls the fuel injection amount from the fuel injection valve so as to obtain an air-fuel mixture having a stoichiometric air-fuel ratio or an air-fuel mixture leaner than the stoichiometric air-fuel ratio.   The engine according to claim 1, wherein the supercharger is a supercharger.   The engine according to claim 1, wherein the supercharger is a turbocharger having a function capable of assisting a turbo rotation speed with an electric motor.   4. The engine according to claim 3, wherein the electric motor is switched from a non-driving state to a driving state when a pressure of fresh air supercharged by the supercharger in the operation region is equal to or lower than an exhaust pressure.   A basic injection amount that estimates the amount of fresh air remaining in the combustion chamber in the valve overlap state and obtains the stoichiometric air-fuel mixture or a lean air-fuel mixture based on the estimated fresh air amount The engine according to claim 1, wherein the engine is calculated.   The engine according to claim 5, wherein the amount of fresh air remaining in the combustion chamber in the valve overlap state is estimated based on a fresh air amount detected by an air flow meter and a scavenging rate.   The engine according to any one of claims 1 to 6, wherein the fuel supplied from the fuel injection valve is temporarily increased immediately after the valve overlap state is finished.   The engine according to any one of claims 1 to 7, wherein when the valve overlap state is set, correction is made to increase the target boost pressure.

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