JP2008075633A - Combustion control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve accuracy of learning of a model function by taking engine speed into consideration in a device controlling ignition timing so as not to cause knocking by using a model function, namely, a self-ignition prediction model, and predicting self-ignition of in-cylinder unburnt gas from a detection value and an estimated value of each parameter. <P>SOLUTION: An estimated value of an in-cylinder unburnt gas temperature is calculated on the basis of an operating condition of the engine, and the model function is corrected per each engine speed area on the basis of the calculated in-cylinder unburnt gas temperature. Since correction of the model function is carried out per each engine speed area, accuracy of learning of the model function can be improved. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a combustion control device for an internal combustion engine, and more particularly, to one using a model function for predicting knocking.

  Various attempts have been made to obtain, as a general expression, the relationship between parameters such as the in-cylinder pressure, the in-cylinder unburned gas temperature, and the equivalence ratio of the internal combustion engine and the occurrence of self-ignition (knocking) in the in-cylinder unburned portion ( For example, Non-Patent Document 1). Using such a general formula as a model function, that is, a self-ignition prediction model, a method of predicting self-ignition of unburned gas in the cylinder from the detected values and estimated values of each parameter and controlling the ignition timing so that knocking does not occur is also attempted. (For example, Patent Document 1).

  On the other hand, Patent Document 2 discloses a method in which correction by learning of ignition timing is performed for each operating region specified by the rotational speed and load in an apparatus that controls ignition timing so that knocking does not occur. In this method, after correcting the entire operation region, correction is performed for each operation region. However, this method is feedback control that corrects the target value of the ignition timing set on the map, and does not correct the model function for predicting self-ignition. Avoiding knocking is not effective in terms of avoidance control, and it is necessary to conduct an enormous experiment for setting the ignition timing, resulting in an increase in development cost.

JP 2004-332584 A Japanese Patent Publication No. 6-50102 “Correlation of autoignition phenomena in internal combustion engines and rapid compression machines” J. C. Livengood and P. C. Wo, Fifth Symposium (international) on Combustion, Reinhold Publishing Corporation (1955)

  The present inventors conducted experiments for self-igniting the in-cylinder unburned portion under various operating conditions, and found that the timing of self-ignition of the in-cylinder unburned portion is strongly influenced by the engine speed. .

  The present invention has been made on the basis of such new knowledge, and an object thereof is to improve the accuracy of learning of a model function by considering the engine speed.

  According to a first aspect of the present invention, there is provided a predicting means for predicting the self-ignition occurrence timing of unburned gas based on a model function including, as a parameter, a cylinder unburned gas temperature or a physical quantity correlated with the cylinder unburned gas temperature as a parameter A combustion control device for an internal combustion engine comprising combustion control means for controlling combustion so that self-ignition does not occur based on the self-ignition occurrence time predicted by the prediction means, wherein the operating state of the internal combustion engine Temperature estimation means for calculating an in-cylinder unburned gas temperature or an estimated value of a physical quantity correlated with the in-cylinder unburned gas temperature, and for each rotation speed region of the internal combustion engine based on the calculated estimated value A combustion control device for an internal combustion engine, further comprising learning processing means for correcting the model function.

  In the present invention, the temperature estimation unit calculates the in-cylinder unburned gas temperature or the estimated value of the physical quantity correlated with the in-cylinder unburned gas temperature based on the operating state, and based on the calculated estimated value, the learning processing unit Corrects the model function for each rotation speed region. As described above, in the present invention, since the model function is corrected for each rotation speed region, the accuracy of learning of the model function can be improved. The model function in the present invention defines a relationship between a parameter indicating the operating state of the engine and the timing of occurrence of self-ignition of unburned gas, and includes a map in addition to a numerical calculation model.

  The model function in the present invention preferably further includes in-cylinder pressure as a parameter.

  The predicting means in the present invention preferably predicts the self-ignition occurrence time by integrating the reciprocal of the ignition delay time of the unburned gas.

  Preferably, the learning processing means in the present invention estimates an estimated value of the in-cylinder unburned gas temperature that is equal to or higher than a predetermined high temperature side reference value in a certain rotation speed region and the estimated value that is equal to or lower than a predetermined low temperature side reference value. If so, the correction of the model function for the rotation speed region is allowed. In this case, good accuracy can be obtained with a small number of learning times, and learning can be speeded up.

  In the present invention, it is preferable to further include setting means for setting at least one of the high temperature side reference value and the low temperature side reference value based on a past operation history. In this case, an appropriate reference value can be set based on the past driving history. In this case, the setting means preferably performs the setting based on the maximum value and the minimum value of the in-cylinder unburned gas temperature or the physical quantity correlated with the in-cylinder unburned gas temperature in the past operation history. It is.

  A preferred embodiment of the present invention will be described below. In FIG. 1, an engine 1 according to the first embodiment of the present invention is an in-cylinder direct injection type four-cylinder gasoline engine, in which a cylinder 2 is formed inside a cylinder block 1a, and a piston 3 slides therein. Inserted as possible.

  The piston 3 is connected to the crankshaft 5 by a connecting rod 4. The piston 3 is provided with a recess 3b in the center of the piston head 3a in place of the valve recess in order to improve fuel consumption and combustion efficiency. The cylinder head 6 is common to all cylinders, and an intake port 7 and an exhaust port 8 are formed in each cylinder, and an intake valve 9 and an exhaust valve 10 are set via a valve spring (not shown). Has been. A fuel injection valve 12 is provided in the intake port 7, and a throttle valve (intake air) controlled by a throttle actuator 15 a is provided in a part of an intake passage 13 including an intake manifold extending from the intake port 7 to an air flow meter 14 on the upstream side. (Throttle valve) 15 is provided.

  The camshafts 9 a and 10 a that drive the intake and exhaust valves 9 and 10 are provided with a variable valve timing system (hereinafter referred to as VVT) 11. The VVT 11 is a mechanism for continuously changing the opening and closing timings of the intake and exhaust valves 9 and 10 by changing the rotation phase of the camshafts 9a and 10a with respect to the rotation of the crankshaft 5, and is driven by hydraulic pressure.

  A catalyst device 31 is provided in the exhaust path of the engine. The catalyst device 31 is, for example, a three-way catalyst that supports a catalyst component such as platinum, rhodium, or palladium and an additive such as cerium or lanthanum, and includes CO (carbon monoxide), HC (hydrocarbon), and NOx in the exhaust gas. Purify (nitrogen oxides). The spark plug 33 is connected to an ignition coil (not shown) and is installed on the top surface of the combustion chamber 34.

  Although not shown in detail, an electronic control unit (hereinafter referred to as an ECU) 30 temporarily holds a CPU that performs various arithmetic processes, a ROM that stores control programs, initial values of control variables, and the like, and control programs and data. A RAM, an input / output port, an A / D and D / A converter, a storage device, and the like are included.

  The ECU 30 receives output signals from various sensors. Such sensors include the air flow meter 14 described above, an accelerator opening sensor 16a provided in association with the accelerator pedal 16 operated by the driver, and a crank angle provided facing a part of the crankshaft 5. Sensor 17, intake air temperature sensor 18 provided in intake passage 13, A / F (air / fuel ratio) sensor 20 provided in the exhaust manifold, water temperature sensor 21 installed in cylinder block 1 a for detecting the coolant temperature, drive (not shown) A vehicle speed sensor 22 provided adjacent to the wheel and an in-cylinder pressure sensor 23 provided in a combustion chamber of the engine are included.

  The above-described VVT 11, fuel injection valve 12, throttle valve 15, spark plug 33, and the like are controlled by a control signal from the ECU 30. The fuel from the fuel tank 32 is pressurized by a fuel pump (not shown) and supplied to the fuel injection valve 12.

  As shown in FIG. 2, when it is assumed that the air-fuel mixture in the combustion chamber 34 is divided into the burned part 36 and the unburned part 37 with the flame surface 35 as a boundary, the burned gas accompanying the progress of combustion. The unburned part 37 is compressed by the heat generation / expansion of the part 36. A state in which self-ignition occurs before the progression of the flame is completed by the compression of the unburned portion 37 is considered knocking.

  In the present embodiment, the ECU 30 predicts the self-ignition occurrence time of unburned gas based on the mathematical formula (1) which is a model function (hereinafter referred to as a knock model), and based on the predicted self-ignition occurrence time. Combustion control processing for controlling combustion so that ignition does not occur is performed. Equation (1) shows that self-ignition occurs when the integral value of the reciprocal of the self-ignition delay time τ reaches 1 (Livengood-Wu integration). This self-ignition delay time τ is calculated by Equation (2).

  In Equation (2), P is the in-cylinder pressure, Φ is the equivalence ratio, Tu is the in-cylinder unburned gas temperature, f (RON) is a predetermined function for reflecting the influence of the octane number RON, and A, B, C, and Ea are Conformity value. The conforming values A, B, C, and Ea are constants, their initial values are experimentally determined, and are corrected as described below based on the operating state of the engine.

  The integrated value of the reciprocal 1 / τ of the left side of the above equation (1), that is, the reciprocal 1 / τ of the self-ignition delay time τ, changes as shown in FIG. 3, for example, and the time (crank angle) when this reaches 1 ].

  However, even when the self-ignition timing Clgw arrives, self-ignition does not occur if there is no predetermined unburned fuel amount sufficient to reach self-ignition. Since the amount of unburned fuel can be obtained from the combustion ratio, a predetermined reference combustion ratio (determination value) and a combustion ratio are compared with each other as shown in FIG. The reference combustion ratio arrival timing Cbr1 [degATDC], which is the time (crank angle) when the unburned fuel amount is reached, can be calculated. Then, the ECU 30 determines that knocking occurs only when the predetermined amount of unburned fuel exists.

  That is, in the ECU 30, as shown in FIG. 5, based on the combustion rate estimated from the in-cylinder pressure, the time when the unburned fuel amount that gradually decreases reaches the self-ignition limit (crank angle), that is, the reference combustion rate arrival timing. Cbr1 is calculated by a predetermined limit prediction function, and on the other hand, the self-ignition timing Clgw is calculated by Equation (1) which is a knock model. Then, the ECU 30 determines (predicts) that knocking will occur when Clgw <Cbr1, that is, when there is a predetermined amount of unburned fuel that can lead to self-ignition and when the self-ignition timing has arrived. . Then, an ignition timing at which knocking does not occur is determined by calculation, and ignition is performed at this ignition timing, so that the operation based on the knock model is constantly performed. The knock function (knock model) in the present embodiment is a conditional expression (numerical calculation model) indicating the relationship between the physical quantity indicating the operating state of the engine and the timing at which knocking occurs, but the model function in the present invention is a map. It may be.

  The knock model correction process in the present embodiment configured as described above will be described with reference to FIG. The processing routine of FIG. 6 is repeatedly executed every predetermined time during the operation of the engine. First, the ECU 30 acquires engine operating conditions (S10). The operating conditions here are the in-cylinder pressure detected by the in-cylinder pressure sensor 23, the engine speed detected by the crank angle sensor 17, the load detected by the air flow meter 14, and the valve timing commanded by the ECU 30 to the VVT 11. including. The ECU 30 also stores the detected operating condition as a driving history in a predetermined area of the storage device.

  Next, the ECU 30 determines whether a predetermined knock learning condition is satisfied (S20). Here, the knock learning condition is a condition for allowing knock learning, for example, that it is not immediately after starting, oil temperature and water temperature are within a predetermined range, and that it is not in a transient operation state such as during rapid acceleration. Use under conditions. If the knock learning condition is not satisfied, the process is returned.

  When the knock learning condition is satisfied, the ECU 30 refers to the past driving pattern stored in the storage device (S30). The past operation patterns here are the rotation speed, load and unburned gas peak temperature acquired within the past predetermined period. Of these, the actual rotation speed and load used within the past predetermined period are shown in the figure. 7 is indicated by a solid line.

  Next, the ECU 30 sets a learning allowable region based on the unburned gas peak temperature and the rotation speed acquired within the past written period (S40). The learning allowable region here is an operation region in which a learning process described later is permitted, and is set for each rotational speed region within a region consisting of the rotational speed and the unburned gas peak temperature. Specifically, the high load reference values Ath1, Bth1, and Cth1 as the lower limit values are shifted from the maximum value of the unburned gas peak temperature in each of the rotation speed regions A, B, and C shown in FIG. Further, low load reference values Ath2, Bth2, and Cth2 as upper limit values are set from the lower limit value of the load (lower limit value of the load causing knocking) to the constant value high temperature side. In this manner, learning allowable areas A1, A2, B1, B2, C1, and C2 that are operation areas that allow the learning process are set for each of the rotation speed areas A to D.

  According to the experiments by the present inventors, as shown in FIG. 9, at the same rotation speed and air amount, the maximum value of the unburned gas temperature under the operating conditions where knocking occurs (unburned gas peak temperature). ) Becomes smaller as the load is higher. That is, the load and the unburned gas peak temperature are related, and the unburned gas peak temperature gradually decreases as the load increases.

  Next, the ECU 30 determines whether the engine is currently operating in any one of the learning allowable areas A1, A2, B1, B2, C1, and C2 based on the operating conditions previously acquired in step S10 (S50). . If affirmative, it is then determined whether knocking has occurred (S60). This determination is made based on the detection value of the in-cylinder pressure sensor 23, for example, by comparison with a predetermined threshold value. If knocking has occurred, the ECU 30 stores knock data in the storage device (S70). The knock data here includes in-cylinder state quantities (in-cylinder pressure, in-cylinder unburned gas temperature, combustion ratio) in addition to operating conditions (rotation speed, load, valve timing). Note that the in-cylinder unburned gas temperature and the combustion ratio are calculated based on the detection value of the in-cylinder pressure sensor 23.

  Next, the ECU 30 determines whether the knock model can be learned (S80). Here, for each of the rotational speed regions A, B, C, and D, it is determined whether knock data in both the high temperature side and low temperature side learning allowable regions are acquired. That is, in the present embodiment, learning for each rotation speed region is allowed on condition that two knock data are prepared. In the example of FIG. 8, learning is allowed because two knock data are prepared in the rotation speed region B, but learning is not allowed because two knock data are not prepared in the rotation speed regions A and C.

  If negative in step S60 or S80, the ECU determines the advance amount in the learning allowable region (S110). Here, if knocking has not occurred in the learning allowable region in step S60, the advance amount for the learning allowable region is increased. In addition, when knocking occurs in the learning allowable region in step S80 but the two knock data are not prepared, no knock data is necessary, and therefore, in order to return the ignition timing to the retard side, the learning allowable region The advance amount at is set to zero.

  If negative in step S50, that is, if the vehicle is not operating in the learning allowable region, it is determined whether knocking has occurred outside the learning allowable region (S120). This determination is made based on a detection value of the in-cylinder pressure sensor 23, for example, by comparison with a predetermined threshold value. If knocking has occurred, the knock ignition timing of the entire operating condition (that is, the entire rotational speed region) is provisionally corrected to the retard side (for example, 2 °) (S130). The provisional correction here may be performed only for the rotation speed region where knocking occurs.

  In step S80, when two knock data are obtained for a certain rotation speed region, a knock model for the rotation speed region is identified (S90). As shown in FIG. 10, the knock model is identified by the self-ignition timing estimated from the combustion rate (the timing when the combustion rate exceeds a predetermined reference value) Cbr2 and the auto-ignition prediction formula. The adaptation value Ea, which is one of the coefficients in the self-ignition prediction formula, is adjusted so that the difference (Clgw−Cbr2) from the ignition timing Clgw becomes zero. Here, the adjustment amount per processing cycle may be a constant value, or may be a variable value determined in advance according to the difference (Clgw−Cbr2).

  Finally, the ECU 30 updates the knock model for the rotation speed region with the adjusted fitness value Ea (S100) and exits this routine. As a result of the above processing, the knock model in the present embodiment is corrected for each rotation speed region.

  As described above, in the present embodiment, the estimated value of the in-cylinder unburned gas temperature is calculated based on the operating state of the engine 1, and the knock model is calculated for each rotation speed region based on the calculated in-cylinder unburned gas temperature. Correct. Thus, in the present invention, since the knock model is corrected for each rotation speed region, the learning accuracy of the knock model can be improved.

  In the present embodiment, the knock model further includes the in-cylinder pressure as a parameter, and the self-ignition occurrence timing is predicted by integrating the reciprocal of the ignition delay time of the unburned gas. The desired effect can be obtained.

  Further, in the present embodiment, when an estimated value of in-cylinder unburned gas temperature that is equal to or higher than a predetermined high temperature side reference value and the estimated value that is equal to or lower than a predetermined low temperature side reference value are estimated in a certain rotation speed region, Since the correction of the knock model for the rotation speed region is allowed, good accuracy can be obtained with a small number of learning times, and learning can be speeded up.

  In the present embodiment, the high temperature side reference values Ath1 to Cth1 and the low temperature side reference values Ath2 to Cth2 are set based on the past operation history, so that an appropriate reference value reflecting the past operation history can be set. In this case, since the setting is performed based on the past history of the in-cylinder unburned gas temperature, the expected effect of the present invention can be obtained with a simple configuration. Note that one of the high temperature side reference value and the low temperature side reference value may be set based on the past operation history.

  Next, a second embodiment of the present invention will be described. According to the experiments by the present inventors, as shown in FIG. 11, the unburned gas peak temperature under the operating condition in which knocking occurs is smaller as the overlap amount is larger at the same rotation speed and air amount. Become. That is, as the overlap amount is larger, the maximum value of the unburned gas temperature (unburned gas peak temperature) gradually decreases, and knocking occurs at a lower unburned gas temperature. The second embodiment utilizes this property. Specifically, in the second embodiment, when the engine operating condition is in the vicinity of the knock learning operating condition, the valve timing is changed to increase the overlap at a high load and reduce the overlap at a low load. Thus, the difference (difference) in unburned gas peak temperature between knock data is increased, and learning of the knock model is promoted. Since the second embodiment is different from the first embodiment only in the control process, description of the mechanical configuration is omitted.

  Processing in the second embodiment will be described. As shown in FIG. 12, the process in the second embodiment corresponds to a process in which steps S140 to S170 are newly inserted between steps S50 and S120 of the process in the first embodiment described above. For this reason, processing steps similar to those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  If the result in step S50 is negative, that is, if the vehicle is not operating in the learning allowable region, the ECU 30 determines whether the driving state is in the vicinity of the learning allowable region (S140). This determination is made depending on whether the load is within a predetermined range from the learning allowable region in the rotation speed region to which the current operating state belongs. If not, the process proceeds to step S120. If the determination is affirmative, that is, if the driving state is in the vicinity of the learning allowable region, the ECU 30 next determines whether the current load is higher than the learning allowable region (S150).

  The ECU 30 controls the VVT 11 to change the valve timing, thereby increasing the valve overlap amount when the current load is higher than the learning allowable range (S160), and when the current load is lower, the valve overflow is increased. The wrap amount is decreased (S170), and the process is temporarily returned. As a result of these processes, the unburned gas temperature is decreased by increasing the overlap amount, and the unburned gas temperature is increased by decreasing the overlap amount.

  The processes in steps S140 to S170 are repeatedly executed until the operating condition (S10) is within the learning allowable area (S50). When the operating conditions are within the learning allowable area, the processes in steps S60 to S110 are the same as in the first embodiment. Will be done.

  As described above, in the present embodiment, when an in-cylinder unburned gas temperature within a predetermined range that is outside the learning allowable region in a certain rotation speed region is estimated, the estimated value is within the learning allowable region. Change the valve overlap amount. Therefore, learning of the knock model can be promoted.

  In the second embodiment, the valve overlap amount is changed by changing the valve timing. However, in the case of an engine having a variable valve lift mechanism, the valve overlap amount is changed by changing the valve lift amount. May be.

  In the second embodiment, the operation state is controlled so that the operation condition falls within the learning allowable range by changing the valve overlap amount. Instead of or in addition to this, the engine speed and / or Alternatively, the operation state may be controlled by changing the charging efficiency so that the operation condition falls within the learning allowable region. That is, when the result in step S150 in the routine of FIG. 12 is affirmative, the engine speed is decreased and / or the charging efficiency is increased, and when the result is negative in S150, the engine speed is increased and / or the charging efficiency. May be reduced. Changing the engine speed is particularly suitable for an engine having a continuously variable transmission that can change the gear ratio to compensate for this. In addition to changing the valve timing, the charging efficiency can be changed, for example, by controlling a variable trumpet type or variable valve type intake pipe length variable mechanism (variable intake system). As a result, the influence of the pressure wave in the intake pipe is controlled to change the intake pressure, thereby changing the charging efficiency.

  In each of the above embodiments, the numerical calculation model is used as a model function that defines the relationship between the parameter indicating the operating state of the engine and the self-ignition occurrence time of the unburned gas. However, the model function in the present invention is a map. May be. In each of the above embodiments, the knock model includes the in-cylinder unburned gas temperature as a parameter. However, the model function in the present invention correlates with the in-cylinder unburned gas temperature instead of the in-cylinder unburned gas temperature. Some physical quantity, for example, in-cylinder pressure, may be included as a parameter. When adiabatic compression is assumed, the in-cylinder pressure can be considered to correlate with the in-cylinder unburned gas temperature. Therefore, the intended effect of the present invention can also be obtained by using the in-cylinder pressure as a parameter.

  Further, in each of the above embodiments, the example in which the present invention is applied to a four-cycle gasoline engine has been described. However, the present invention is not limited to an engine using liquid fuel or gaseous fuel other than gasoline as an energy source, or a so-called port injection engine. The present invention can also be applied to an internal combustion engine of the type, and both belong to the category of the present invention.

It is a schematic structure figure showing a 1st embodiment of the present invention. It is a top view which shows notionally the burned part and unburned part in a combustion chamber. It is a timing diagram which shows transition of the integral value 1 / τ of the reciprocal of the self-ignition delay time. It is a timing diagram which shows transition of a combustion ratio. It is a conceptual diagram of steady operation using a knock model. It is a flowchart which shows the knock model correction process in 1st Embodiment. It is a graph which shows an example of the past driving | operation pattern. It is a graph which shows the example of a setting of a learning permissible field. It is a graph which shows the relationship between the load on the driving | running condition which knocking generate | occur | produces, and unburnt gas peak temperature. It is a conceptual diagram which shows the procedure of learning of the knock model in 1st Embodiment. It is a graph which shows the relationship between the overlap amount on the driving | running condition which knocks generate | occur | produces, and unburnt gas peak temperature. It is a flowchart which shows the knock model correction process in 2nd Embodiment.

Explanation of symbols

1 Engine 23 In-cylinder pressure sensor 30 ECU
34 Combustion chamber

Claims (7)

A predicting means for predicting the self-ignition occurrence time of the unburned gas based on a model function including, as a parameter, an in-cylinder unburned gas temperature or a physical quantity correlated with the in-cylinder unburned gas temperature; A combustion control device for an internal combustion engine comprising combustion control means for controlling combustion so that self-ignition does not occur based on the ignition occurrence time,
A temperature estimation means for calculating an estimated value of the in-cylinder unburned gas temperature or a physical quantity correlated with the in-cylinder unburned gas temperature based on the operating state of the internal combustion engine;
Learning processing means for correcting the model function for each rotational speed region of the internal combustion engine based on the calculated estimated value;
A combustion control apparatus for an internal combustion engine, further comprising: A combustion control device for an internal combustion engine according to claim 1,
The combustion function control apparatus for an internal combustion engine, wherein the model function further includes in-cylinder pressure as a parameter. A combustion control apparatus for an internal combustion engine according to claim 1 or 2,
The combustion control device for an internal combustion engine, wherein the predicting means predicts a self-ignition occurrence time by integrating a reciprocal of an ignition delay time of unburned gas. A combustion control device for an internal combustion engine according to any one of claims 1 to 3,
The learning processing means, when the estimated value that is equal to or higher than a predetermined high temperature side reference value and the estimated value that is equal to or lower than a predetermined low temperature side reference value are estimated in a certain rotation speed region, A control device that allows correction of a function. A combustion control device for an internal combustion engine according to claim 4,
The control apparatus further comprising setting means for setting at least one of the high temperature side reference value and the low temperature side reference value based on a past operation history. A combustion control device for an internal combustion engine according to claim 5,
The setting means performs the setting based on a past in-cylinder unburned gas temperature or a maximum value or a minimum value of a physical quantity correlated with the in-cylinder unburned gas temperature in the rotation speed region. . A control device for an internal combustion engine according to any one of claims 4 to 6,
When the estimated value outside the region allowing correction by the learning processing unit is estimated in a certain rotation speed region, the combustion control unit controls the valve so that the estimated value falls within the region allowing correction. A control device that changes at least one of an overlap amount, an engine speed, and a charging efficiency.

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