JP7498642B2 - Method for controlling an internal combustion engine - Google Patents

Description

This invention relates to an internal combustion engine that uses turbocharging to achieve lean combustion with a large air-fuel ratio (A/F or G/F), and in particular to a control method for avoiding a drop in catalyst temperature.

To reduce fuel consumption, there is known an internal combustion engine that can be switched between a stoichiometric combustion mode in which the theoretical air-fuel ratio is the target air-fuel ratio and a lean combustion mode in which the lean air-fuel ratio is the target air-fuel ratio. Patent Document 1 discloses an internal combustion engine equipped with a supercharged lean combustion mode in which the target air-fuel ratio is a lean air-fuel ratio while performing supercharging using a turbocharger, and a non-supercharged stoichiometric combustion mode in which the target air-fuel ratio is the theoretical air-fuel ratio without supercharging. In such an internal combustion engine, a large amount of intake air (fresh air and EGR gas) can be supplied into the cylinder by supercharging, making it possible to perform lean combustion with a large air-fuel ratio (A/F or G/F), for example, lean combustion with an air excess ratio λ of around 2.

JP 2008-121511 A

In internal combustion engines equipped with a turbocharger, the catalyst for purifying exhaust gas is placed downstream of the turbine in the exhaust system. In lean combustion, the exhaust gas temperature is relatively low to begin with, and as a result of exhaust energy being consumed by the work of the turbine, the exhaust gas temperature at the catalyst inlet becomes low. Therefore, depending on the operating conditions, the catalyst temperature may drop excessively during lean combustion.

In the first aspect of the present invention,
determining whether a catalyst temperature is below a predetermined temperature corresponding to a catalyst activation temperature during lean burn operation with supercharging;
When the catalyst temperature is lower than a predetermined temperature, the rotation speed is increased by changing the speed ratio, and the target load is lowered so as to keep the shaft output of the internal combustion engine substantially constant, thereby changing the operating point of the internal combustion engine from the current operating point to a high-speed low-load side;
The opening of the wastegate valve of the turbocharger is increased so as to obtain a target boost pressure which decreases as the target load decreases.

In the second aspect of the present invention,
The internal combustion engine is a power generating internal combustion engine connected to a generator in a series hybrid vehicle,
determining whether the catalyst temperature is below a predetermined temperature corresponding to a catalyst activation temperature during lean burn operation with supercharging;
when the catalyst temperature is lower than a predetermined temperature, the rotational speed of the generator is increased by controlling the number of revolutions of the generator, and the target load is lowered so as to keep the shaft output of the internal combustion engine substantially constant, thereby changing the operating point of the internal combustion engine from the current operating point to a high-speed low-load side;
The opening of the wastegate valve of the turbocharger is increased so as to obtain a target boost pressure which decreases as the target load decreases.

In one embodiment, when the boost pressure is lower than a predetermined pressure corresponding to the lower limit of the boost pressure required to maintain lean combustion, the above-mentioned change in the operating point and the increase in the opening of the wastegate valve are not performed.

According to this invention, by increasing the opening of the turbocharger wastegate valve when the catalyst temperature is low, the amount of exhaust heat recovered by the turbine is reduced, and the temperature of the exhaust gas flowing into the downstream catalyst increases. This makes it possible to suppress excessive temperature drops in the catalyst.

1 is an explanatory diagram showing a schematic system configuration of an internal combustion engine. 4 is a flowchart showing a process flow of catalyst temperature control in the first embodiment.

Below, one embodiment of the invention will be described in detail with reference to the drawings.

Figure 1 shows a schematic system configuration of an internal combustion engine 1 according to an embodiment of the present invention. The internal combustion engine 1 of this embodiment is equipped with a turbocharger 12 as a supercharging means. The internal combustion engine 1 is, for example, a four-stroke spark-ignition gasoline engine, and is particularly configured to be switchable between a stoichiometric combustion mode in which the theoretical air-fuel ratio (i.e., air excess ratio λ = 1) is set as the target air-fuel ratio, and a lean combustion mode in which the lean air-fuel ratio (e.g., near λ = 2) is set as the target air-fuel ratio. In the lean combustion mode, a larger amount of air is required than in the stoichiometric combustion mode, and the target air-fuel ratio is set on the assumption that supercharging is performed by the turbocharger 12.

In the first embodiment, the internal combustion engine 1 is mounted on a vehicle in combination with a stepped automatic transmission or a continuously variable automatic transmission (e.g., a belt-type CVT) (not shown). In other words, the vehicle basically runs using the internal combustion engine 1 as a drive source.

The internal combustion engine 1 is preferably configured for lean combustion. For example, a pair of intake valves 4 and a pair of exhaust valves 5 are arranged on the ceiling wall of the combustion chamber 3 formed by the piston 2, and an ignition plug 6 and a fuel injection valve 7 are arranged in the center surrounded by these intake valves 4 and exhaust valves 5.

The fuel injection valve 7 is an electromagnetic or piezoelectric injection valve that opens when a drive pulse signal is applied, and injects an amount of fuel into the cylinder that is substantially proportional to the pulse width of the drive pulse signal.

An electronically controlled throttle valve 11, whose opening is controlled by a control signal from an engine controller 10, is installed upstream of the collector section 9a of the intake passage 9 connected to the intake port 8, and a compressor 12A of a turbocharger 12 is installed further upstream of that. An air flow meter 13 that detects the amount of intake air and an air cleaner 14 are installed upstream of the compressor 12A. An intercooler 16 is provided between the compressor 12A and the throttle valve 11. A boost pressure sensor 17 that detects boost pressure (Pb) is installed in the collector section 9a.

Meanwhile, the turbine 12B of the turbocharger 12 is disposed in an exhaust passage 22 connected to an exhaust port 21 through which the exhaust valve 5 opens and closes, and a catalyst 23, for example a three-way catalyst, is disposed downstream of the turbine 12B. A wastegate valve 24 is provided at the inlet of the turbine 12B to guide a portion of the exhaust gas bypassing the turbine 12B for boost pressure control. The wastegate valve 24 is an electronically controlled type whose opening is controlled by an electric actuator 25.

An air-fuel ratio sensor 26 for detecting the so-called exhaust air-fuel ratio is provided upstream of the catalyst 23. A catalyst inlet temperature sensor 18 is provided at the inlet of the catalyst 23 to detect the temperature of the exhaust gas flowing into the catalyst 23 as a parameter equivalent to the catalyst temperature (T_cat). The temperature of the catalyst carrier (so-called bed temperature) may be directly detected as the catalyst temperature (T_cat). Although the catalyst 23 is illustrated as a single catalyst, it is generally composed of a pre-catalyst located in the engine room of the vehicle and a main catalyst located under the vehicle floor. When multiple catalysts are provided in this way, it is desirable to detect the temperature of each catalyst by an individual temperature sensor and execute the temperature control described below when the temperature of any of the catalysts drops.

In addition to controlling the opening of the throttle valve 11, the engine controller 10 also controls the amount and timing of fuel injection by the fuel injection valve 7, the ignition timing by the spark plug 6, etc. Air-fuel ratio control using the air-fuel ratio sensor 26 is also performed by the engine controller 10.

The engine controller 10 also controls the opening of the wastegate valve 24, thereby controlling the boost pressure and therefore the torque of the internal combustion engine 1.

The engine controller 10 receives as input signals the detection signals of the air flow meter 13, boost pressure sensor 17, catalyst inlet temperature sensor 18, and air-fuel ratio sensor 26 described above, as well as detection signals from a crank angle sensor 31 that detects the engine speed (Ne), a water temperature sensor 32 that detects the cooling water temperature (TW), and an accelerator position sensor 33 that detects the accelerator position (APO) in response to the driver's depression of the accelerator pedal.

The engine controller 10 is also connected to a transmission controller (not shown) via CAN communication or the like, and exchanges the necessary signals with each other.

The storage device of the engine controller 10 stores a control map that sets a stoichiometric burn operating region where the stoichiometric burn mode should be set and a lean burn operating region where the lean burn mode should be set, with the torque (in other words, load) and rotation speed of the internal combustion engine 1 as parameters. In addition, a target air-fuel ratio map that assigns a target air-fuel ratio to each operating point determined by the torque and rotation speed in a form that basically corresponds to the stoichiometric burn operating region and the lean burn operating region is also stored. The lean burn operating region is set to a low- and medium-speed range where the torque is relatively small, and the target air-fuel ratio in this lean burn operating region is an air-fuel ratio on the lean side where NOx emissions are somewhat low, for example, an air-fuel ratio of about 28 to 32 near "λ=2".

If the target air-fuel ratio is a lean air-fuel ratio equivalent to "λ=2", twice as much air is required compared to the theoretical air-fuel ratio, so in the above internal combustion engine 1, the required amount of intake air is ensured by supercharging with the turbocharger 2 in lean combustion mode. Note that although an exhaust gas recirculation device is not shown in FIG. 1, a relatively large amount of exhaust gas recirculation may be used to achieve a leaner combustion.

Next, we will explain the control of the temperature drop of the catalyst 23 during lean combustion, which is a key part of the present invention. As mentioned above, during lean combustion, the exhaust temperature is relatively low to begin with, and because the exhaust passes through the turbine 12B due to supercharging, exhaust energy is consumed by the work of the turbine 12B (in other words, exhaust heat is recovered), so the temperature of the catalyst 23 is likely to drop. If the temperature of the catalyst 23 falls below its activation temperature, the desired exhaust purification performance of the catalyst 23 cannot be obtained. Therefore, in the present invention, the temperature of the catalyst 23 is restored mainly by increasing the opening of the wastegate valve 24.

Figure 2 is a flowchart showing the process flow of catalyst temperature control in the first embodiment. The routine shown in this flowchart is repeatedly executed by the engine controller 10 during lean combustion assuming supercharging. In step 1, it is determined whether the supercharging pressure Pb is higher than a predetermined criterion #2. If NO, the routine is terminated. If YES, the process proceeds to step 2. The criterion #2 is set corresponding to the lower limit of the supercharging pressure required to maintain lean combustion. In step 2, it is determined whether the catalyst temperature T_cat is lower than a predetermined criterion #1. If NO, the routine is terminated. If YES, the process proceeds to step 3. The criterion #1 is set to a temperature slightly higher than the catalyst activation temperature.

In step 3, the operating point of the internal combustion engine 1 is changed from the current operating point to the high-speed, low-load side while keeping the shaft output of the internal combustion engine 1 substantially constant. For example, in the case of a continuously variable transmission, the gear is changed via the transmission controller so that the engine speed Ne increases by a predetermined constant amount β. At the same time, the target load Te of the internal combustion engine 1 is calculated as follows so that the target shaft output Peng is obtained under the increased engine speed Ne. Note that Vcc is the displacement of the internal combustion engine 1.

Te=2×Ne×Vcc/(1200×Peng)
In the case of a stepped transmission, gear shifting is performed so that the engine rotation speed Ne becomes higher, such as from "fifth gear to fourth gear,""fourth gear to third gear," etc. In this case, the amount of change β of the engine rotation speed Ne is determined according to the gear stage.

By shifting the operating point to the high speed/low load side in this manner, the target boost pressure decreases as the target load decreases, and the opening degree of the wastegate valve 24 increases.

After changing the operating point in step 3, proceed to step 4 and wait for a certain number of cycles (n cycles). A certain period of time may be used instead of the number of cycles. This is set taking into consideration the delay time from when the operating point is changed until the catalyst temperature T_cat stabilizes.

After n cycles have elapsed, in step 5, it is determined whether the boost pressure Pb is higher than a predetermined criterion #2. If NO, the routine is terminated. If YES in step 5, proceed to step 6. In step 6, it is determined whether the catalyst temperature T_cat is higher than a predetermined criterion #1'. It is desirable to set criterion #1' to a temperature slightly higher than criterion #1 in step 2. If YES, the routine is terminated.

On the other hand, if the answer is NO in step 6, the process returns to step 3, and the operating point is changed to the high-speed, low-load side so that the engine speed Ne changes by β. In other words, on the condition that the boost pressure Pb is higher than criterion #2, the operating point is repeatedly changed to the high-speed, low-load side until the catalyst temperature T_cat exceeds criterion #1'. When criterion #1' is exceeded, the routine is terminated. Note that when the routine is terminated, the operating point may be maintained as it has been when it has been changed up to that point, or the temporary change in the operating point may be terminated and the normal operating point setting may be restored when the routine is terminated.

In the first embodiment, this processing allows the opening of the wastegate valve 24 to be increased while keeping the shaft output Peng substantially constant, and the catalyst temperature T_cat is maintained higher than the activation temperature.

The target load Te, which is gradually set lower, is limited by the load value when the wastegate valve 24 is fully open.

One embodiment of the present invention has been described above .

In addition, if multiple catalysts 23 are provided, it is advisable to perform the above-mentioned temperature increase process when the temperature T_cat of any one of the catalysts 23 falls below criterion #1.

In the first embodiment, the operating point of the internal combustion engine 1 is changed to the high-speed, low-load side by changing the gears of the transmission. However, in the case of a series hybrid type vehicle in which the internal combustion engine 1 is used for generating electricity, the rotation speed of the internal combustion engine 1 can be changed by controlling the rotation speed of the generator connected to the internal combustion engine 1.

In addition, the boost pressure Pb is set to a lower limit, criterion #2, in order to maintain lean combustion, but instead of the boost pressure Pb itself, parameters such as the opening degree of the wastegate valve 24 (position of the actuator 25) and the rotation speed of the turbocharger 12 that are related to the boost pressure Pb may be used.

In addition, when the catalyst temperature T_cat reaches criterion #1' due to a change in the operating point, etc., the previous change in the operating point, etc. may be maintained, and when it reaches criterion #1'', which is set to a temperature higher than criterion #1', the operating point, etc. may be restored to the normal setting.

Reference Signs List 1 internal combustion engine 10 engine controller 12 turbocharger 17 boost pressure sensor 18 catalyst inlet temperature sensor 23 catalyst

Claims (3)

A method for controlling an internal combustion engine having a turbocharger, a catalyst located downstream of a turbine of the turbocharger, and capable of lean combustion, comprising the steps of:
determining whether a catalyst temperature is below a predetermined temperature corresponding to a catalyst activation temperature during lean burn operation with supercharging;
When the catalyst temperature is lower than a predetermined temperature, the rotation speed is increased by changing the speed ratio, and the target load is lowered so as to keep the shaft output of the internal combustion engine substantially constant, thereby changing the operating point of the internal combustion engine from the current operating point to a high-speed low-load side;
increasing an opening of a wastegate valve of the turbocharger so as to obtain a target boost pressure that decreases as the target load decreases;
A method for controlling an internal combustion engine. 2. The control method for an internal combustion engine according to claim 1 , wherein when the boost pressure is lower than a predetermined pressure corresponding to a lower limit boost pressure necessary to maintain lean combustion , the above-mentioned change in operating point and the increase in the opening of the wastegate valve are not performed . A method for controlling an internal combustion engine having a turbocharger, a catalyst located downstream of a turbine of the turbocharger, and capable of lean combustion, comprising the steps of:
The internal combustion engine is a power generating internal combustion engine connected to a generator in a series hybrid vehicle,
determining whether a catalyst temperature is below a predetermined temperature corresponding to a catalyst activation temperature during lean burn operation with supercharging;
when the catalyst temperature is lower than a predetermined temperature, the rotational speed of the generator is increased by controlling the number of revolutions of the generator, and the target load is lowered so as to keep the shaft output of the internal combustion engine substantially constant, thereby changing the operating point of the internal combustion engine from the current operating point to a high-speed low-load side;
increasing an opening of a wastegate valve of the turbocharger so as to obtain a target boost pressure that decreases as the target load decreases;
A method for controlling an internal combustion engine.

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