BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a control system for a spark-ignition internal combustion engine.

2. Description of Related Art

[0002] A control system for a spark-ignition internal combustion engine including a tumble flow control valve that produces tumble flow in each cylinder is disclosed in, for example, Japanese Patent Application Publication No. 2012-021501 (JP 2012-021501 A). In this control system, the tumble ratio is estimated, based on a detection value of a first air flow meter provided upstream of a throttle valve, and a detection value of a second air flow meter provided right downstream of the tumble flow control valve. Then, the feedback control of the opening of the tumble flow control valve is performed so that the estimated tumble ratio follows a target tumble ratio. The target tumble ratio is set to a value within a permissible control range established for avoiding misfiring and unstable combustion.

[0003] In a spark-ignition internal combustion engine as described in Japanese Patent Application Publication No. 2009-041397 (JP 2009-041397 A), airflow drawn from two intake ports forms tumble flow that is directed toward two exhaust ports while swirling in the axial direction of the cylinder. In the internal combustion engine, twin airflows (twin vortexes) that swirl in mutually opposite directions are produced from the tumble flow.

SUMMARY OF THE INVENTION

[0004] In the meantime, it is desirable to control the flow rate of gas in the vicinity the ignition plug at the time of ignition to within a range in which the flow rate is not excessively high nor excessively low, so as to stabilize the ignition performance of the gas in the cylinder. In this connection, in the system of JP 2012-021501 A, the tumble ratio (the flow rate of the tumble flow / the engine speed) can be controlled to within a certain definite range. However, the flow rate of the tumble flow changes if the engine speed changes; therefore, the ignition performance of the gas in the cylinder may deteriorate even if the tumble ratio is controlled to within the definite range.

[0005] The invention provides a spark-ignition internal combustion engine that exhibits improved ignition performance of an air/fuel mixture without depending on the engine speed.

[0006] A control system for a spark-ignition internal combustion engine including two intake ports that communicate with a combustion chamber of the spark-ignition internal combustion engine, and an ignition plug provided in a central portion of an upper wall of the combustion chamber, is provided according to one aspect of the invention. The spark-ignition internal combustion engine is configured such that intake air flowing from the two intake ports into the combustion chamber forms tumble flow in the combustion chamber. The control system includes an intake air flow controller configured to control at least one of quantities of intake air flowing from the two intake ports, and an ECU configured to control the intake air flow controller such that, when the spark-ignition internal combustion engine operates in an twin airflows region in which twin airflows are produced in the tumble flow, a difference is produced between the quantities of intake air flowing from the two intake ports into the combustion chamber, so as to increase a flow rate of gas flowing in the vicinity of the ignition plug, as compared with the case where the quantities of intake air flowing from the two intake ports are substantially equal to each other. The twin airflows swirl in opposite directions about a cylinder axis of the internal combustion engine.

[0007] With the above arrangement, the intake air flow controller can be controlled so that a difference is produced between the quantities of intake air flowing from the two intake ports, in the operating region in which the twin airflows that swirl in opposite directions are produced from the tumble flow. By differentiating the quantities of intake air flowing from the two intake ports from each other, it is possible to produce a difference in size between the vortexes of the two airflows. When a difference is produced between the sizes of the vortexes of the two airflows, the flow rate of gas flowing in the vicinity of the ignition plug at the time of ignition by the ignition plug (which will also be called "ignition-timing flow rate") can be increased, as compared with the case where the sizes of the vortexes of the two airflows are substantially equal to each other. Accordingly, the ignition performance of the air/fuel mixture is improved in the operating region in which the twin airflows are produced.

[0008] In the control system according to the above aspect of the invention, the twin airflows region in which the twin airflows are produced may be an operating region in which a rotational speed of the spark-ignition internal combustion engine is equal to or lower than a predetermined value. Also, the ECU may be configured to control the intake air flow controller such that the quantities of intake air flowing from the two intake ports become substantially equal to each other, when the spark-ignition internal combustion engine operates in an operating region other than the twin airflows region in which' the twin airflows are produced.

[0009] If the twin airflows are produced, the ignition-timing flow rate is reduced. However, when the engine speed is lower than a given value, the ignition-timing flow rate is excessively reduced, and the ignition performance may be impaired. With the above arrangement, when the engine speed is lower than the predetermined value, the intake air flow controller is operated so that a difference is produced between the quantities of intake air flowing from the two intake ports, so that the ignition-timing flow rate is prevented from being excessively reduced. When the engine speed is equal to or higher than the predetermined value, the intake air flow controller is operated so that the quantities of intake air flowing from the two intake ports become substantially equal to each other, so that the ignition-timing flow rate can be moderately reduced. Accordingly, the ignition performance of the air/fuel mixture can be improved without depending on the engine speed.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in ^" which like numerals denote like elements, and wherein:

FIG. 1 is a view useful for explaining the system configuration of one embodiment of the invention;

FIG. 2 is a view showing the relationship among the plug vicinity flow rate at the time of ignition, the fuel concentration of an air/fuel mixture, and the ignition lag;

FIG. 3 A is a plan view of a combustion chamber as viewed from a cylinder head side (when the engine speed is low);

FIG. 3B is a side view of the combustion chamber as viewed from intake ports side

(when the engine speed is low);

FIG. 3C is a view showing the flow direction of tumble flow when the combustion chamber is viewed from the cylinder head side (and the engine speed is low);

FIG. 4A is a plan view of the combustion chamber as viewed from the cylinder head side (when the engine speed is high);

FIG. 4B is a side view of the combustion chamber as viewed from the intake ports side (when the engine speed is high);

FIG. 4C is a view showing the flow direction of tumble flow when the combustion chamber is viewed from the cylinder head side (and the engine speed is high);

FIG. 5 A and FIG. 5B are views showing changes in the flow rate of gas in the combustion chamber in the latter half of the compression stroke when tumble flow of ω shape is produced;

FIG. 6 is a view showing changes in the gas flow rate around the compression top dead center;

FIG. 7 A is a view showing lift amount control of intake valves when the engine is in a high speed region;

FIG. 7B is a view showing lift amount control of intake valves when the engine is in a middle speed region;

FIG. 8 is a flowchart illustrating a routine executed by an ECU in the embodiment; FIG. 9A is a view showing throttle opening control when the engine is in a high speed region;

FIG. 9B is a view showing throttle opening control when the engine is in a middle speed region;

FIG. 1 OA is a view showing SCV control when the engine is in a high speed region; ' and ^

FIG. 1 OB is a view showing SCV control when the engine is in a middle speed region. DETAILED DESCRIPTION OF EMBODIMENTS

[0011] A control system according to one embodiment of the invention will be described with reference to the drawings.

[0012] Initially, the system configuration will be described. FIG. 1 is a view useful for explaining the system configuration of this embodiment of the invention. The system shown in FIG. 1 includes an internal combustion engine 10, A piston 14 is received in a cylinder 12 of the engine 10 such that the piston 14 can reciprocate in the cylinder 12 in sliding contact therewith. A cylinder head 16 is mounted above the cylinder 12. A combustion chamber 18 is defined by a wall of a bore of the cylinder 12, a top face of the piston 14, and a bottom of the cylinder head 16.

[0013] The cylinder head 16 is provided with a fuel injection valve 20 for directly injecting fuel into the combustion chamber 18. The cylinder head 16 is also provided with an ignition plug 22 for igniting an air/fuel mixture in the combustion chamber 18. Namely, the internal combustion engine 10 is an in-cylinder injection type spark-ignition engine. The internal combustion engine 10 may be a port injection type spark-ignition engine.

[0014] Intake ports 24 and exhaust ports 26 are formed in a lower surface of the cylinder head 16. Two intake ports 24 and two exhaust ports 26 are provided for each cylinder 12. The combustion chamber 18 communicates with an intake passage 28 via the intake ports 24, and communicates with an exhaust passage 30 via the exhaust ports 26. The intake ports 24 are formed in such a shape as to promote production of tumble flow of intake air as vertically swirling flow in a direction denoted as "TUMBLE DIRECTION" in FIG. 1. An airflow control valve for effectively producing tumble flow may be provided in the intake passage 28.

[0015] An intake valve 32 is provided in each of the intake ports 24. The intake valve 32 is connected to an intake variable valve mechanism 34 for changing valve-opening characteristics (such as a lift amount, operation angle, and opening and closing timing) of the intake valve 32. The intake variable valve mechanism 34 is able to adjust the opening and closing timing and lift amounts of the two intake valves 32 provided in the same cylinder, independently of each other. Like the intake port 24, an exhaust valve 36 is provided in each of the exhaust ports 26. The exhaust valve 36 is connected to an exhaust variable valve mechanism 38.

[0016] The system shown in FIG. 1 includes an ECU (Electronic Control Unit) 50. Various sensors needed for control of the engine 10, including a crank angle sensor 40 for detecting the engine speed, are electrically connected to the input side of the ECU 50. Various actuators, such as the fuel injection valve 20, ignition plug 22, intake variable valve mechanism 34, and the exhaust variable valve mechanism 38, are electrically connected to the output side of the- ECU 50. The ECU 50 executes certain programs based on input information from the above-described various sensors, and operates the above-described various actuators, etc., so as to perform certain engine controls, such as fuel injection control, and also perform ignition-timing flow rate control as will be described later.

[0017] The need to control the flow rate of gas in the vicinity of the ignition plug at the ignition timing will be described. FIG. 2 shows the relationship among the plug vicinity flow rate (which refer to the flow rate of gas in the vicinity of the ignition plug) at the time of ignition, the fuel concentration of the air/fuel mixture, and the ignition lag (which refers to an interval between ignition caused by the ignition plug, and the time at which the combustion pressure in the combustion chamber starts rising due to ignition of the mixture). As shown in FIG. 2, the ignition lag increases as the fuel concentration is lowered. Also, the ignition lag increases as the plug vicinity flow rate at the ignition timing deviates from a certain flow rate value (optimum value), toward the high-flow-rate side or the low-flow-rate side.

[0018] The increase of the. ignition lag with increase in the plug vicinity flow rate at the ignition timing is caused by discharge spark cut-off. If a discharge spark is extended by high-speed flow of the air/fuel mixture before the mixture is warmed by the discharge spark, the discharge spark may cut off. If the discharge spark cut-off occurs, re-discharge takes place immediately. However, if the plug vicinity flow rate at the ignition timing is increased to a higher rate, the discharge spark cut-off takes place in a shorter time, and there is not enough time to heat the mixture and cause it to be ignited.

[0019] The increase of the ignition lag with reduction of the plug vicinity flow rate at the ignition timing is caused by a discharge path. The energy pe^ unit length of an electric spark produced by discharge is determined by characteristics of an ignition coil, and is constant irrespective of the path length of the discharge spark. Therefore, if the length of the discharge path is increased due to airflow, or the like, the energy supplied to the air/fuel mixture as a whole increases, and the volume of the mixture thus heated also increases. However, if the plug vicinity flow rate at the ignition timing is lowered, the discharge path is less likely or unlikely to be extended, and therefore, the energy supplied and the volume of the heated mixture cannot be increased.

[0020] As the ignition lag increases, torque fluctuation of the engine 10 increases.

Accordingly, it is necessary to control the plug vicinity flow rate at the ignition timing to be within a certain definite range in which the flow rate is not excessively high nor excessively low, so that the ignition lag is held in a region in which the torque fluctuation is within a permissible range. Thus, in this embodiment, control (ignition-timing flow rate control) for controlling the plug vicinity flow rate at the ignition timing within the optimum range, by Using the shape of tumble flow produced in the combustion chamber 18, is performed.

[0021] Referring to FIG. 3A through FIG. 4C, the shape of the tumble flow used in the ignition-timing flow rate control will be described. Initially, the tumble flow of normal shape (which will be also called "normal tumble") will be described with reference to FIG. 3A - FIG. 3C. FIG. 3A is a plan view of the combustion chamber 18 as viewed from the cylinder head 16 side. FIG. 3B is a side view of the combustion chamber 18 as viewed from the intake ports 24 side. FIG. 3C is a view showing the flow direction of the tumble flow when the combustion chamber 18 is viewed from the cylinder head 16 side. All of these figures are taken at a point in time in the neighborhood of the compression top dead center.

[0022] A white circle shown in each of FIG. 3A and FIG. 3B indicates a vortex center point of tumble flow in a cross-section (that extends in the intake and exhaust direction) taken at the center of the cylinder bore at which the ignition plug 22 is installed. Two black circles shown in each of FIG. 3 A and FIG. 3B indicate vortex center points of tumble flow in cross-sections each taken along the axes of the intake valve 32 and exhaust valve 36, and an axis line that passes these black circles is denoted as a tumble center axis. As shown in FIG. 3A and FIG. 3B, the tumble center axis does not bend but is in the form of a substantially straight line. Therefore, the tumble flow flows uniformly in a direction from the intake side toward the exhaust side (see FIG. 3C).

[0023] Referring next to FIG. 4A - FIG. 4C, tumble flow of ω shape (which will be also called "ω tumble") will be described. FIG. 4A - FIG. 4C correspond to FIG. 3A - FIG. 3C, respectively, and notations in FIG. 4A - FIG. 4C are the same as those in FIG. 3A - FIG. 3C. If the engine speed is low, the tumble center axis does not bend but is in the form of a substantially straight line, as shown in FIG. 3 A - FIG. 3C. However, as the engine speed increases (more specifically, becomes equal to or higher than 2000 rpm), the velocity of the tumble flow becomes higher. Then, the tumble center axis bends at the Vortex center of tumble flow at the center of the cylinder bore (the position of the ignition plug), as a bending point. As a result, the tumble flow that originally swirls as a single airflow turns into two streams of air that swirl in mutually opposite directions with respect to the cylinder axis as a center. The two swirling flows assume an ω (omega) shape (see FIG. 4C), and therefore, are called "ω tumble" in this specification.

[0024] FIG. 5A and FIG. 5B are views useful for explaining changes in the gas flow rate in the combustion chamber 18 at around the compression top dead center (the latter half of the compression stroke) when the ω tumble is produced. FIG. 5A indicate the positions of measurement point A, measurement point B, and measurement point C. FIG. 5B shows changes in the gas flow rate at the measurement point A, measurement point B, and the measurement point C, in this order as viewed from the top of FIG. 5B. The measurement point A is the position of a plug gap, and the measurement point B is a , given position located radially outwardly of the measurement point A in the cylinder bore, while the measurement point C is a given position located radially outwardly of the measurement point B in the cylinder bore. In FIG. 5B showing changes in the gas flow rate, the vertical axis indicates measurement value of the gas flow rate. More specifically; the measurement value takes a positive (+) value when the gas flows from the intake side to the exhaust side, and takes a negative (-) value when the gas flows from the exhaust side to the intake side.

[0025] As shown in FIG. 5B, when the ω tumble is produced, the direction of gas flow at the plug gap position, or the measurement point A, is reversed, at a point in the neighborhood of the compression top dead center. Namely, the flow of gas from the intake valves 32 side toward the exhaust valves 36 side turns into flow from the exhaust valves 36 side toward the intake valves 32 side. As is understood from comparison of changes in the gas flow rate at the measurement points A - C, the flow direction is less likely to change as the distance between the measurement point and the plug gap position increases, and no reverse of the flow direction takes place at the measurement point C.

[0026] Referring to FIG. 6 and FIG. 7, the ignition-timing flow rate control will be described in detail. FIG. 6 is a view showing changes in the gas flow rate at around the compression top dead center. As described above, the normal tumble is produced when the engine is in a low speed region (e.g., less than 2000 rpm). When the normal tumble is produced, the plug vicinity flow rate gradually lowers, and falls within the optimum range in the neighborhood of the compression top dead center. Therefore, in the low engine speed region, the ignition timing is set to the point in time in the neighborhood of the compression top dead center. [0027] The ω tumble is produced when the engine is in a middle to high speed region (e.g., equal to or higher than 2000 rpm). When the ω tumble is produced, the plug vicinity flow rate is largely reduced in the latter half of the compression stroke, to be lower than the optimum range, and assumes a negative value before the compression top dead center is reached. Generally, the ignition timing is advanced as the engine speed is higher. In this embodiment, too, the ignition timing is changed to be advanced according to the engine speed. In the high engine speed range (e.g., 4000 rpm or higher), the ignition timing that has been changed according to the engine speed generally coincides with a period in which the plug vicinity flow rate falls within the optimum range.

[0028] However, in the middle engine speed region (e.g., 2000 - 4000 rpm), the ignition performance of the air/fuel mixture is greatly influenced by reduction of the plug vicinity flow rate due to production of the ω tumble. Therefore, it is necessary to correct the ignition timing to the more advanced side than the ignition timing changed according to the engine speed, so that the plug vicinity flow rate at the ignition timing is held within the optimum range. Thus, in the middle engine speed region, a difference is produced between the lift amounts of the two intake valves 32, so that two swirling flows that constitute the ω tumble become different in size from each other. In this manner, the ignition timing that has been changed according to the engine speed is caused to coincide with the period in which the plug vicinity flow rate falls within the optimum range.

[0029] FIG. 7A and FIG. 7B are views useful for explaining a method of adjusting the shape of the tumble flow when the engine is in the middle to high speed region. As shown in FIG. 7A, in the high engine speed region, the lift amounts of the two intake valves 32 are made substantially equal to each other, and the ω tumble is produced. As a result, the plug vicinity flow rate is lowered, to be equal to a value within the optimum range at the time that coincides with the ignition timing that has been changed according to the engine speed. On the other hand, in the middle engine speed region, a difference is produced between the lift amounts of the two intake valves 32, so that the two swirling flows that constitute the co tumble become different in size from each other, as shown in FIG. 7B. Thus, the plug vicinity flow rate is prevented from being excessively reduced, and becomes equal to ,a value within the optimum range at the time that coincides with the ignition timing that has been changed according to the engine speed.

[0030] As explained above, according to the ignition-timing flow rate control, the tumble flow whose shape changes according to the engine speed is utilized so that the time at which the plug vicinity flow rate becomes equal to a value within the optimum range coincides with the ignition timing that has been changed according to the engine speed. Accordingly, various problems caused by prolonged ignition lag are less likely or unlikely to occur.

[0031] FIG. 8 is a flowchart illustrating a routine executed by the ECU 50 so as to implement the ignition-timing flow rate control of this embodiment. This routine is repeatedly executed every 10 cycles of the engine 10 for each cylinder.

[0032] As shown in FIG. 8, the ECU 50 initially determines whether the engine speed is in a low speed region (step 100). More specifically, the ECU 50 determines whether the engine speed obtained from a detection value of the crank angle sensor 40 is equal to or lower than 2000 rpm. If it is determined that the engine speed is equal to or lower than 2000 rpm, it can be determined that the engine is in the low speed region. Then, the ECU 50 makes the lift amounts of the two intake valves 32 equal to each other, so as to produce normal tumble in the combustion chamber 18 (step 102). Then, the ECU 50 changes the ignition timing to the advanced side according to the engine speed (step 104).

[0033] If it is determined in step 100 that the engine speed is higher than 2000 rpm, it can be determined that the engine is in a middle to high speed region. Then, the ECU 50 determines whether the engine is in a middle speed region (step 106). More specifically, the ECU 50 determines whether the engine speed obtained in step 100 is lower than 4000 rpm. If it is determined that the engine speed is lower than 4000 rpm, it can be determined that the engine is in the middle speed region. Then, the ECU 50 controls the intake variable valve mechanism 34 to provide a difference between the lift amounts of the two intake valves 32, so as to change the shape of the ω tumble produced in the combustion chamber 18 as described above (step 108). Then, the ECU 50 changes the ignition timing to the advanced side according to the engine speed (step 104).

[0034] If it is determined in step 106 that the engine speed is equal to or higher than 4000 rpm, it can be determined that the engine speed is in a high speed region. Then, the ECU 50 controls the intake variable valve mechanism 34 to make the lift amounts of the two intake valves 32 equal to each other, and produce the ω tumble in the combustion chamber 18 (step 110). Then, the ECU 50 changes the ignition timing to the advanced side according to the engine speed (step 104).

[0035] According to the routine illustrated in FIG. 8, the normal tumble is used when the engine is in the low speed region, or the ω tumble is used when the engine is in the high speed, so that the time at which the plug vicinity flow rate becomes equal to a value within the optimum range coincides with the ignition timing that has been changed according to the engine speed. Also, when the engine is in the middle speed region, a difference is produced between the lift amounts of the two intake valves 32, so that the shape of the ω tumble is changed, and the time at which the plug vicinity flow rate becomes equal to a value within the optimum range coincides with the ignition timing that has been changed according to the engine speed.

[0036] While the sizes of the two swirling flows that constitute the ω tumble are changed by providing a difference between the lift amounts of the two intake valves 32 in the above-described embodiment, the method of changing the sizes of the swirling flows may be modified in various manners. In this respect, modified examples of the above embodiment will be described with reference to FIG. 9A through FIG. lOB.

[0037] FIG. 9A and FIG. 9B are views useful for explaining a first modified example of the above-described embodiment. As shown in FIG. 9A and FIG. 9B, when throttle valves 42 are provided in the two intake ports 24 independently of each other, the openings of the throttle valves 42 are made different from each other, so that the shape of the ω tumble can be changed. As shown in FIG. 9 A, when the engine speed is in a high speed region, the openings of the two throttle valves 42 are made equal to each other, so that the ω tumble as shown in FIG. 9A can be produced. As shown in FIG. 9B, when the engine speed is, in a middle speed region,, a difference is provided between the openings of the two throttle valves 42, so that the sizes of the two swirling flows that constitute the co tumble can be changed.

[0038] FIG. 10A and FIG. 10B are views useful for explaining a second modified example of the above-described embodiment. As shown in FIG. 10A and FIG. 10B, when a swirl control valve (SCV) 44 is provided in one of the intake ports 24, the shape of the co tumble can be changed by changing the opening of the SCV 44. As shown in FIG. 10A, in a high engine speed region, the SCV 44 is operated so as to open the corresponding intake port 24, so that the co tumble as shown in FIG. 10A can be produced. As shown in FIG. 10B, in a middle engine speed region, the SCV 44 is operated so as to at least partially close the corresponding intake port 24, so that a lateral swirling component (swirl component) is introduced into the intake air flowing into the combustion chamber 18, whereby the sizes of the two swirling flows that constitute the ω tumble can be changed.

[0039] Thus, the sizes of the two swirling flows that constitute the co tumble can be made different from each other, by use of the two throttle valves 42 or the SCV 44. Namely, any mechanism can provide substantially the same effect as that of the above-described embodiment, if it can produce a difference between the quantities of intake air flowing from the two intake ports 24 into the combustion chamber 18.

[0040] In the above-described embodiment, the intake valves 32 and the intake variable valve mechanism 34 correspond to the "intake air flow controller" according to the invention.