Engine Stop Control Method and Device

Abstract

When there is an engine stop request (S1), a necessary rich amount is calculated in accordance with an accumulated oxygen amount of a catalyst device (19) at that time (S2), and an in-cylinder air-fuel ratio is richened (S3). At the same time, the opening degree of a throttle valve (14) is reduced. Next, a determination is made as to whether the accumulated oxygen amount has fallen below a target predetermined threshold value (X) (S4), or whether an exhaust air-fuel ratio downstream of the catalyst as indicated by a downstream air-fuel ratio sensor (21) is less than a predetermined threshold value (Y), and if the determination of either is YES, an engine stop permission flag is set to ON from the viewpoint of the accumulated oxygen amount (S6). When the engine stop is finally permitted (S7) including other requirements, an engine 1 is stopped. After the accumulated oxygen amount reaches the target, the engine is operated at a stoichiometric air-fuel ratio until the engine 1 is stopped (S8).

Description

Technical Field

[0001] This invention relates to control when temporarily stopping an engine in response to a stop request.

Background Art

[0002] In vehicles equipped with an idle stop mechanism or so-called hybrid vehicles, etc., the engine is automatically stopped and automatically restarted. When such automatic stop and automatic restart are repeated, if the oxygen storage amount of the catalyst during the stop period, that is, immediately before restart, is excessive, NOx emissions during restart are likely to become a problem. Patent Document 1 discloses reducing the oxygen storage amount of the catalyst by enriching the air-fuel ratio immediately before stopping during a temporary stop.

[0003] However, in this conventional technique, enrichment is performed without considering the oxygen storage amount before enrichment, so the oxygen storage amount may decrease excessively.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] This invention obtains the oxygen storage amount of the catalyst during engine operation, and sets the rich amount during enrichment so that the smaller the oxygen storage amount at the time of a stop request, the smaller the rich amount, such that the smaller the oxygen storage amount, the smaller the rich amount.

[0006] By such control, the oxygen storage amount can be obtained within an appropriate range, and excessive decrease in the oxygen storage amount can be avoided.

Brief Description of the Drawings

[0007]

Figure 1

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Mode for Carrying Out the Invention

[0008] Hereinafter, an embodiment of the present invention will be described. This embodiment is, for example, an application of the present invention to the engine 1 of a series hybrid vehicle. As is well known, a series hybrid vehicle mainly includes a power generation motor generator that operates as a generator, an engine 1 that drives this power generation motor generator according to a power demand, a traveling motor generator that mainly operates as a motor and drives drive wheels, and a battery that temporarily stores the generated power. Therefore, the engine 1 will automatically stop and automatically restart relatively frequently according to the power demand based on the state of charge (SOC) of the battery, etc.

[0009] FIG. 1 shows the system configuration of the engine 1. This engine 1 is, for example, a spark ignition internal combustion engine of a four-stroke cycle, and includes a piston 4 that moves in a cylinder 3 so as to define a combustion chamber 2, an intake valve 6 that opens and closes between an intake port 5 and the combustion chamber 2, an exhaust valve 8 that opens and closes between an exhaust port 7 and the combustion chamber 2, a spark plug 9 that performs ignition, a fuel injection valve 10 that injects fuel into the cylinder, etc. The fuel injection valve 10 is an electromagnetic or piezoelectric injection valve that opens when a drive pulse signal is applied, and injects an amount of fuel substantially proportional to the pulse width of the drive pulse signal. Note that the present invention may also have a port injection type configuration with a fuel injection valve provided in the intake port 5.

[0010] An electronically controlled throttle valve 14 whose opening degree is controlled by a control signal from an engine controller 13 is interposed upstream of a collector portion 12 of an intake passage 11 connected to the intake port 5. An air flow meter 15 for detecting an intake air amount is disposed upstream of the throttle valve 14, and an air cleaner 16 is disposed further upstream. An intake pressure sensor 17 for detecting an intake pressure downstream of the throttle valve 14 is provided in the collector portion 12.

[0011] Also, a catalyst device 19 composed of a three-way catalyst is interposed in an exhaust passage 18 connected to the exhaust port 7, and an upstream air-fuel ratio sensor 20 and a downstream air-fuel ratio sensor 21 for detecting an oxygen concentration in the exhaust, that is, a so-called exhaust air-fuel ratio, are respectively arranged upstream and downstream thereof. Note that a configuration using an O2 sensor instead of an air-fuel ratio sensor capable of detecting the air-fuel ratio over a wide range may also be employed.

[0012] The engine 1 is controlled by the engine controller 13. The engine controller 13 and the vehicle-side controller 23 are connected via a vehicle internal network and exchange signals with each other. When the engine controller 13 receives a start request from the vehicle-side controller 23 according to the accelerator pedal opening degree, vehicle speed, battery SOC, etc. of the vehicle, the engine 1 is started. Thereafter, when the engine controller 13 receives a stop request from the vehicle-side controller 23, the engine 1 is stopped. In addition, the engine 1 is operated according to other requirements such as a heating request. Note that a configuration in which the vehicle-side controller 23 and the engine controller 13 are integrated as one controller may also be employed.

[0013] In addition to the above-mentioned air flow meter 15, intake pressure sensor 17, and air-fuel ratio sensors 20 and 21, the engine controller 13 receives detection signals from sensors such as a crank angle sensor 24 for detecting the engine speed and a water temperature sensor 25 for detecting the coolant temperature. Further, a vehicle speed signal, an accelerator pedal opening signal, etc. are input from the above-described vehicle controller 23. Based on these input signals, the engine controller 13 optimally controls the fuel injection amount and injection timing by the fuel injection valve 10, the ignition timing by the ignition plug 9, the opening degree of the throttle valve 14, etc.

[0014] Next, the control executed when temporarily stopping the engine 1 will be described based on the flowchart of FIG. 2. When the engine 1 is stopped, a so-called freewheeling period occurs during which air passes through the catalytic converter 19 after the fuel supply and ignition are stopped until the rotation of the crankshaft actually stops. As a result, the oxygen storage amount of the catalyst in the catalytic converter 19 increases, and the oxygen storage amount just before restart becomes excessively high. To address such a problem, in this embodiment, the in-cylinder air-fuel ratio is enriched before the engine 1 is stopped to reduce the oxygen storage amount.

[0015] The process shown in the flowchart of FIG. 2 is repeatedly executed, for example, during the operation of the engine 1. In the first step 1, it is determined whether there is an engine stop request. If there is an engine stop request, the process proceeds to step 2 and subsequent steps. If there is no engine stop request, the process proceeds to step 10 to perform normal air-fuel ratio control. Here, the fuel injection amount is controlled using a known air-fuel ratio control method so as to maintain the stoichiometric air-fuel ratio and further so that the oxygen storage amount is within a predetermined range. By controlling the oxygen storage amount within a predetermined range during normal operation, it becomes easier to control the oxygen storage amount associated with stopping and restarting.

[0016] In step 2, the required rich amount is calculated according to the oxygen storage amount at that time. For example, as shown in the characteristics of FIG. 3, the higher the oxygen storage amount (close to 100%), the larger the rich amount, and the lower the oxygen storage amount (close to 0%), the smaller the rich amount is given. Note that the oxygen storage amount is repeatedly calculated by another routine (not shown) while the main switch of the vehicle is ON. For example, by integrating the increase and decrease of oxygen storage per unit time or per unit cycle using the exhaust air-fuel ratio of the exhaust gas flowing into the catalyst device 19 detected by the upstream air-fuel ratio sensor 20 and the volume of the gas passing through the catalyst device 19, the oxygen storage amount is always estimated. Therefore, in step 2, the rich amount is determined according to the oxygen storage amount at the time when the stop request is made.

[0017] After determining the rich amount, proceed to step 3 and inject rich fuel. This may increase the injection amount at the normal injection timing, or additional injection may be performed at a timing different from the normal injection timing. At the same time, the engine load is reduced, that is, the opening degree of the throttle valve 14 is reduced. This is a process for reducing the air flow rate (gas flow rate passing through the catalyst device 19) during the free rotation period described later. Since there is a delay in the change of the air amount, the opening degree of the throttle valve 14 is reduced in advance.

[0018] Next, proceed to step 4 and determine whether the continuously required oxygen accumulation amount (not shown) has decreased below a predetermined threshold value (X) as a target. If YES here, proceed to step 6. If NO, proceed to step 5 and determine whether the exhaust air-fuel ratio indicated by the downstream air-fuel ratio sensor 21 is less than a predetermined threshold value (Y). The threshold value (Y) of this downstream air-fuel ratio sensor 21 is set to correspond to the target oxygen accumulation amount. If YES in step 5, also proceed to step 6. That is, in step 4, it is determined whether the oxygen accumulation amount has reached the target value based on the oxygen accumulation amount which is the integrated calculation value, and in step 5, it is determined whether the oxygen accumulation amount has reached the target value based on the composition of the gas passing through the catalytic device 19. By estimating the oxygen accumulation amount using the detection value of the downstream air-fuel ratio sensor 21 as in step 5, high estimation accuracy of the oxygen accumulation amount can be obtained.

[0019] If NO in both step 4 and step 5, return to step 3 and repeat the injection of rich fuel. Then, similarly, the determinations in step 4 and step 5 are made. That is, the richening is continued until the determination in step 4 or step 5 becomes YES.

[0020] In step 6, the engine stop permission flag from the viewpoint of the oxygen accumulation amount is set to ON. Even if this engine stop permission flag from the viewpoint of the oxygen accumulation amount is ON, the engine stop is not permitted unless several engine stop permission flags based on other requirements are simultaneously ON. In the next step 7, it is determined whether the engine stop is permitted including such other requirements. If the engine stop is permitted, proceed to step 9 and stop the engine 1, that is, stop the fuel injection and ignition.

[0021] If the answer in step 7 is NO, that is, when the engine stop permission flag from the perspective of the oxygen storage amount is ON but the engine stop is not permitted based on other requirements, proceed to step 8. As pre-stop stoichiometric air-fuel ratio control, maintain the in-cylinder air-fuel ratio at the stoichiometric air-fuel ratio. As a result, the oxygen storage amount will basically be maintained as it is. This pre-stop stoichiometric air-fuel ratio control continues until it is determined in step 7 that the engine stop is permitted.

[0022] In addition, as a condition for turning ON the engine stop permission flag from the perspective of the oxygen storage amount in step 6, in addition to the determination in steps 4 and 5, it may be conditioned that the intake pressure detected by the intake pressure sensor 17 has reached a predetermined negative pressure. In this case, when the oxygen storage amount reaches the target value and the intake pressure becomes the predetermined negative pressure, the engine stop permission flag from the perspective of the oxygen storage amount becomes ON.

[0023] Figure 4 is a time chart showing changes in the oxygen storage amount and the like during engine stop by the above control. The topmost (a) injection / ignition column represents a flag indicating the execution / stop of injection / ignition (in other words, the operation of engine 1). The (b) air-fuel ratio column shows three values: the in-cylinder air-fuel ratio (actually the target air-fuel ratio in injection amount control), indicated as (A / F), the exhaust air-fuel ratio at the catalyst inlet side detected by the upstream air-fuel ratio sensor 20, indicated as (Fr A / F), and the exhaust air-fuel ratio at the catalyst outlet side detected by the downstream air-fuel ratio sensor 21, indicated as (Rr A / F). The (c) oxygen storage amount column shows the change in the oxygen storage amount of the catalyst in the catalytic device 19. The circled part marked with the symbol OS is the target oxygen storage amount. The next (d) gas flow rate column represents the gas flow rate passing through the catalytic device 19.

[0024] Also, the (e) column at the bottom of FIG. 4 shows the state of the engine 1. Here, it is divided into three states: "operation", "idling", and "stop". "Operation" refers to the state where the engine 1 is rotating with combustion due to fuel injection and ignition. "Idling" refers to the state where the crankshaft is rotating due to inertia or the like after fuel injection and ignition have stopped. "Stop" refers to the state where the rotation of the crankshaft has stopped.

[0025] In the example of FIG. 4, time t1 is the timing when an engine stop request is output from the vehicle controller 23 to the engine controller 13, time t2 is the timing when the engine stop permission flag becomes ON from the perspective of the oxygen storage amount, time t3 is the timing when engine stop is finally permitted, and time t4 is the timing when the rotation of the engine 1 has stopped.

[0026] Until time t1, normal air-fuel ratio control is performed. The exhaust air-fuel ratio Fr A / F on the catalyst inlet side basically changes following the in-cylinder air-fuel ratio. In this example, during the period until time t1, the air-fuel ratio A / F is controlled slightly leaner than the stoichiometric air-fuel ratio in order to maintain the exhaust air-fuel ratio within a predetermined range. The exhaust air-fuel ratio Rr A / F on the catalyst outlet side changes slowly due to the oxygen storage capacity of the catalyst.

[0027] When an engine stop request is output at time t1, the air-fuel ratio A / F is enriched, and at the same time, the opening degree of the throttle valve 14 is reduced. As the opening degree of the throttle valve 14 is reduced, the gas flow rate passing through the catalyst device 19 decreases.

[0028] As the air-fuel ratio A / F is enriched, the oxygen storage amount of the catalyst gradually decreases and reaches the target oxygen storage amount at time t2. As a result, the engine stop permission flag from the perspective of the oxygen storage amount becomes ON, and also, the enrichment ends and the operation shifts to operation at the stoichiometric air-fuel ratio. Here, in the above embodiment, since the rich amount is set according to the oxygen storage amount at time t1, for example, when the oxygen storage amount at time t1 is low, there is no excessively rapid decrease in the oxygen storage amount, and the oxygen storage amount does not decrease too much as a so-called overshoot. Conversely, when the oxygen storage amount at time t1 is relatively high, a rapid decrease in the oxygen storage amount is achieved with an appropriate rich amount.

[0029] Thereafter, at time t3, engine stop is permitted and fuel injection and ignition stop. After this, it becomes an idling without combustion, and only air passes through the catalytic device 19. Therefore, the oxygen storage amount increases. At this time, in the above embodiment, since the opening degree of the throttle valve 14 is reduced in advance, the amount of air flowing into the catalytic device 19 along with idling is limited to a small amount. Therefore, an undesirable increase in the oxygen storage amount due to idling is minimized.

[0030] Next, FIG. 5 shows a time chart of a different embodiment. In this embodiment, in the flowchart of FIG. 3, as indicated by the broken line, when it is NO in steps 4 and 5, it returns to step 2, and the calculation of the rich amount according to the oxygen storage amount at that time is repeatedly performed. In step 2, initially, the rich amount is determined according to the oxygen storage amount at the time when a stop request is made, but thereafter, the rich amount is set according to the oxygen storage amount at that time.

[0031] Therefore, as shown in columns (b) and (c) of FIG. 5, as the oxygen storage amount decreases due to enrichment, the rich amount gradually decreases. As a result, it is suppressed that the oxygen storage amount becomes less than the target value due to overshoot. Furthermore, the air-fuel ratio step at the time of shifting to the stoichiometric air-fuel ratio at the end of enrichment (time t2) becomes smaller.

[0032] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible. For example, the present invention is applicable not only to the engine 1 of a series hybrid vehicle but also to any engine that performs temporary automatic stop. Further, in the above embodiment, whether or not the oxygen storage amount has reached the target value is determined by two different methods in steps 4 and 5, but either one may be used.

Claims

1. An engine stop control method for temporarily stopping an engine in response to a stop request for the temporary stop of the engine, the engine stop control method for reducing the oxygen storage amount of a catalyst by enriching an in-cylinder air-fuel ratio to be richer than a stoichiometric air-fuel ratio when there is a stop request, wherein the oxygen storage amount of the catalyst is obtained during the operation of the engine, in accordance with the oxygen storage amount at the time when there is a stop request, the rich amount at the time of enrichment is set so that the smaller the oxygen storage amount, the smaller the rich amount, furthermore, when there is a stop request, the opening degree of the throttle valve in the intake passage is reduced, and when the pressure in the intake passage downstream of the throttle valve reaches a predetermined negative pressure due to the reduction of the opening degree of the throttle valve and the oxygen storage amount of the catalyst reaches a target value due to enrichment, the stop of the engine is permitted, an engine stop control method.

2. The oxygen storage amount of the catalyst during engine operation is controlled within a predetermined range, the engine stop control method according to claim 1.

3. When the oxygen storage amount of the catalyst reaches a target value before engine stop due to enrichment, the in-cylinder air-fuel ratio is controlled to the stoichiometric air-fuel ratio, the engine stop control method according to claim 1 or 2.

4. Based on the detection value of an oxygen concentration sensor located downstream of the catalyst, it is determined that the oxygen storage amount of the catalyst has reached the target value, the engine stop control method according to any one of claims 1 to 3.

5. An engine stop control device for temporarily stopping an engine in response to a stop request for the temporary stop of the engine, the engine stop control device for reducing the oxygen storage amount of a catalyst by enriching an in-cylinder air-fuel ratio to be richer than a stoichiometric air-fuel ratio when there is a stop request, wherein an oxygen storage amount calculation unit that obtains the oxygen storage amount of the catalyst during the operation of the engine; a rich amount calculation unit that sets the rich amount at the time of enrichment so that the smaller the oxygen storage amount at the time when there is a stop request, the smaller the rich amount, is provided with, furthermore, when there is a stop request, the opening degree of the throttle valve in the intake passage is reduced, and when the pressure in the intake passage downstream of the throttle valve reaches a predetermined negative pressure due to the reduction of the opening degree of the throttle valve and the oxygen storage amount of the catalyst reaches a target value due to enrichment, the stop of the engine is permitted, an engine stop control device.

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