Control apparatus for an internal combustion engine and method for controlling the same

Abstract

In a control apparatus for an internal combustion engine having a valve timing control portion controlling the timing opening and closing of an intake valve disposed in an intake port communicating with a cylinder of the internal combustion engine, when the internal combustion engine is being started from a cold condition, multi-stroke operation is set, in which one combustion cycle of the internal combustion engine includes two or more intake and compression strokes, formed by a first intake stroke and the first compression stroke and a second intake stroke and compression stroke, followed by a combustion stroke and an exhaust stroke. The valve timing control portion controls a lift of the intake valve during the first intake stroke and the first compression stroke to a low lift amount, which is smaller than the normal lift amount required for intake of a requested intake air amount, and controls the lift of the intake valve in a second intake and a second compression stroke to the normal lift amount.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus for an internal combustion engine and a method for controlling an internal combustion engine. More specifically, it relates to a control apparatus for an internal combustion engine and a method for controlling an internal combustion engine capable of controlling the timing of the opening and closing and the lift amounts of individual intake valves of an internal combustion engine.

2. Description of the Related Art

The Japanese Patent Application Publication No. JP-A-10-252511 discloses a system that controls the opening and closing of the intake valve and exhaust valve by a valve driving mechanism capable of variably adjusting the timing of the opening and closing of intake and exhaust valves disposed in each cylinder of an internal combustion engine. In this system, during normal operation, in which combustion in the internal combustion engine is stable, the internal combustion engine is operated by a four-stroke combustion cycle comprising an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. In doing this, control is performed to open and close the intake valve at a prescribed timing in the intake stroke, and control is performed to open and close the exhaust valve at a prescribed timing in the exhaust stroke.

However, in the case, for example, of a cold start of the internal combustion engine, in which there is a tendency for incomplete combustion to occur, there are cases in which the fuel is not completely combusted and non-combusted fuel remains that is exhausted from the cylinder in the exhaust stroke. Therefore, in the above-noted system, in the case in which it is determined that incomplete combustion is occurring in the internal combustion engine, the following control is performed. Specifically, the opening of the intake valve and the exhaust valve is stopped for a prescribed period of time during operation of the internal combustion engine. As a result, during the time that the valves are stopped, both the intake and the exhaust valves are closed, and the up and down motion of the piston repeatedly performs the compression stroke and the expansion stroke only. In this condition, ignition is done each time between the compression stroke and the expansion stroke.

During this repetition of the compression and explosion/expansion strokes in this manner, non-combusted fuel in the cylinder is completely combusted. Subsequently, when complete combustion of the non-combusted fuel is verified, normal valve opening and closing operation is again permitted and normal operation resumes. By doing this, the system enables complete fuel combustion and suppression of exhausting of non-combusted fuel in an operating condition in which there is a tendency for incomplete combustion to occur.

During the time when an internal combustion engine is being started from the cold condition of the internal combustion engine, in order to stabilize combustion and to improve starting characteristics, the fuel injection amount is increased by performing control to achieve a fuel-rich condition. From the standpoint of improving fuel economy and exhaust emissions, however, it is desirable to further expand the lean limit of the internal combustion engine even during the cold start. With regard to this point, when incomplete combustion occurs, the above-noted system performs repeated compression and expansion strokes with both the intake valve and the exhaust valve closed, performing ignition each time to complete combust the non-combusted fuel. That is, according to the system, by repeating the compression and explosion/expansion strokes, fuel that fills the closed cylinder is completely combusted, and is not related to control of the air-fuel ratio toward the lean side during operation when the internal combustion engine is cold started, nor is it related to extending the lean limit.

SUMMARY OF THE INVENTION

The present invention has an object to provide a control apparatus for an internal combustion engine and a method for controlling an internal combustion engine improved to achieve an extension of the lean limit, even when the internal combustion engine is being started from the cold start.

A first aspect of the present invention is a control apparatus for an internal combustion engine having a variable valve driving means for changing the timing of the opening and closing and lift amount of an intake valve disposed in an intake port communicating with a cylinder of the internal combustion engine; a valve timing control means for controlling the timing of the opening and closing and lift amount of the intake valve by the variable valve driving means; a cold start determining means for determining whether the internal combustion engine is being started from a cold start; and a multi-stroke operation setting means for setting, in which one combustion cycle of the internal combustion engine includes two or more intake and compression strokes, when the cold start determining means determines that the internal combustion is being started from the cold start, wherein the multi-stroke operation is formed by a first intake stroke and a first compression stroke and a second intake stroke and a second compression stroke, followed by a combustion stroke and an exhaust stroke. In this aspect, the valve timing control means controls a lift of the intake valve during the first intake stroke and the first compression stroke to a low lift amount, which is smaller than the normal lift amount required for intake of a requested intake air amount, and controls the lift of the intake valve in a second intake stroke and a second compression stroke to the normal lift amount.

According to the first aspect, by performing two or more intake and compression strokes, and making the lift amount in the first intake stroke small, it is possible to raise the intake temperature when an intake air flows into a combustion chamber. Even when the temperature of the internal combustion engine is low, therefore, as during cold starting, it is possible to more quickly raise the temperature in the combustion chambers and stabilize combustion.

In a second aspect, the low lift amount may be the lift amount at which the pumping loss during the first intake stroke and the first compression stroke is maximum.

According to the second aspect, when intake is done in the first intake stroke and the first compression stroke, it is possible to more effectively raise the temperature of the intake gas, enabling not only an improvement in combustion characteristics, but also earlier warm-up of the internal combustion engine.

A third embodiment is the control apparatus of either the first or second aspect, which may further have an ignition timing control means for controlling ignition timing by a spark plug disposed in the cylinder, wherein the ignition timing control means prohibits ignition during the first intake stroke and the first compression stroke.

According to the third aspect, in addition to effectively raising the temperature of the intake gas during the first intake stroke and the first compression strokes, it is possible to perform intake in accordance with a requested intake air amount in the second intake and the second compression stroke, enabling generation of a torque as required for the requested load.

A fourth aspect is a-control apparatus of any one of the first to third aspects, wherein the multi-stroke operation setting means may perform, during one combustion cycle, a plurality of repetitions of the first intake stroke and the first compression stroke, followed by performing the second intake stroke and the second compression stroke.

According to the fourth aspect, the intake temperature is more reliably raised and it is possible to warm up the internal combustion engine at an earlier stage.

A fifth aspect is the control apparatus according to any of the first to fourth aspects, which may further have a multi-stroke operation termination determining means for determining whether multi-stroke operation is to be terminated; and a four-stroke operation setting means for setting one combustion cycle of the combustion of the internal combustion engine to four-stroke operation comprising an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, if the multi-stroke operation judges that multi-stroke operation is to be terminated.

The above-noted multi-stroke operation is advantageous in improving the combustion characteristics when the internal combustion engine is cold. However, because the lift amount of the intake valve becomes small in the first intake stroke and the first compression stroke, so that the intake resistance becomes large, the torque loss is large. Therefore, after combustion stabilizes, a switch may be made to normal four-stroke operation. With regard to this point, according to the sixth through ninth aspects as noted below, it is possible to reliably determine the timing of this switching and, when it is determined that the multi-stroke operation is to be terminated, it is possible to switch to four-stroke operation, in which the normal intake stroke, compression stroke, expansion stroke, and exhaust stroke are performed.

A sixth aspect is the control apparatus of the fifth aspect, which may further have a temperature detecting means for detecting a temperature in the cylinder, wherein the multi-stroke operation termination determining means determines that multi-stroke operation is to be terminated if a temperature in the cylinder reaches a threshold cylinder temperature.

A seventh aspect is the control apparatus of the fifth aspect, which may further comprise a water temperature detection means for detecting the temperature of the coolant of the internal combustion engine, wherein the multi-stroke operation termination determining means determines that multi-stroke operation is to be terminated if the coolant temperature reaches a threshold coolant temperature.

An eighth aspect is the control apparatus of the fifth aspect, which may further have a requested load calculation means for calculating a requested load on the internal combustion engine, wherein the multi-stroke operation termination determining means determines that multi-stroke operation is to be terminated if the calculated requested load reaches or exceeds a threshold engine load.

A ninth aspect is the control apparatus of the fifth aspect, which may further have a cylinder temperature predicting means for predicting, before starting the first intake stroke and the first compression stroke in one combustion cycle, a temperature in a cylinder after performing the second intake stroke and the second compression stroke, wherein the multi-stroke operation termination determining means determines that multi-stroke operation is to be terminated if the predicted temperature in the cylinder reaches a threshold predicted cylinder temperature.

In the case of the multi-stroke operation noted above, if intake and compression strokes are repeated, the intake gas temperature will raise excessively. When the intake gas temperature rises excessively, it is also possible to envision this as a cause of abnormal combustion. With regard to this point, according to the ninth aspect, the temperature in the cylinder is predicted, and it is determined whether to switching from multi-stroke operation to six-stroke operation based on the predicted temperature. Therefore, even in the case of performing multi-stroke operation, it is possible to more reliably prevent the intake gas from reaching an excessively high temperature.

Also, in the case in which the fuel of the internal combustion engine includes an alcohol, because the volatility of the fuel will differ depending on the concentration of the alcohol fuel in the fuel, the combustion characteristics will also change. That is, even for the same operating condition, the time from starting to reach stabilized combustion will differ.

A tenth aspect is the control apparatus of any one of the sixth to ninth aspects, wherein the internal combustion engine may use a fuel including alcohol as a fuel, and the control apparatus may set any one of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, and the threshold predicted engine temperature in accordance with the concentration of alcohol fuel in the fuel.

According to the tenth aspect, the determination value of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, or the threshold predicted engine temperature used as a reference in determining whether to switch from multi-stroke operation to four-stroke operation in accordance with the concentration of alcohol fuel in the fuel. It is therefore possible to perform the switching determination reliably, in accordance with the fuel that is used.

An eleventh aspect is the control apparatus of any one of the first to tenth aspects, wherein the internal combustion engine may a first cylinder group and a second cylinder group, and wherein the control apparatus may operate only cylinders belonging to the first cylinder group and may include a reduced-cylinder operation setting means for setting cylinders belonging to the second cylinder group to reduced cylinder operation, in which the cylinders are stopped, and an all-cylinder operation setting means for setting all cylinders belonging to the first cylinder group and cylinders belonging to the second cylinder group to all-cylinder operation, in which all cylinders are operated, wherein the cold start determination means determines whether restoration of operation of the cylinders belonging to the second group of cylinders is a cold start when an engine transition is made from reduced-cylinder operation to all-cylinder operation, and the multi-stroke operation setting means sets the operation of cylinders belonging to the second cylinder group to multi-stroke operation when the cold start determining means determines that restoration of operation of the cylinders belonging to the second group of cylinders is the cold start.

According to the eleventh aspect, when return is being made from so-called reduced-cylinder operation to all-cylinder operation, even in the case in which the cylinders that had be stopped during reduced-cylinder operation are to be restored to operation when cold, it is possible to apply the above-noted multi-stroke operation. It is therefore possible to more quickly stabilize the combustion in cylinders that had been stopped, and possible to more quickly return from reduced-cylinder operation to all-cylinder operation.

A twelfth aspect is the control apparatus of any one of the first to eleventh aspects, wherein the variable valve driving means may have an intake cam driving the opening and closing of the intake valve and an electrical motor rotationally driving the intake cam, wherein the valve timing control means may control the valve timing by controlling the rotational drive of the intake cam using the electrical motor.

According to the twelfth aspect, because it is possible to control the valve timing of the intake valve using an electrical motor, it is possible to reliably control the intake valve to a set lift amount, and possible to reliably achieve control of the lift amount in the above-noted six-stroke operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features, and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG. 1 is a schematic view describing the configuration of a system according to the first embodiment of the present invention;

FIG. 2 is a drawing describing the relationship between the lift amount of an intake valve and the pumping loss;

FIG. 3A and FIG. 3B are drawings describing the opening and closing timing and the lift amounts of the intake and exhaust valves;

FIG. 4 is a flowchart describing a control routine executed by the system in the first embodiment of the present invention;

FIG. 5 is a graph describing the relationship between the coolant temperature and the threshold engine load in a second embodiment of the present invention;

FIG. 6 is a flowchart describing a control routine executed by the system in the second embodiment of the present invention;

FIG. 7 is a flowchart describing a control routine executed by the system in the third embodiment of the present invention;

FIG. 8 is a graph describing the relationship between the alcohol concentration in the fuel and the threshold coolant temperature in a fourth embodiment of the present invention;

FIG. 9 is a flowchart describing a control routine executed by the system in the fourth embodiment of the present invention; and

FIG. 10 is a flowchart describing a control routine executed by the system in a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention are described in detail below, with references made to the accompanying drawings. In the drawings, elements that are the same or corresponding elements to elements earlier described are assigned the same reference numerals and the descriptions thereof are either simplified or omitted.

FIG. 1 is a schematic view showing the configuration of the first embodiment of the present invention. The system shown in FIG. 1 has an internal combustion engine 10. The internal combustion engine 10 has a cylinder 12. Although only the cross-section of only one cylinder 12 is shown in FIG. 1, the internal combustion engine 10 actually has a plurality of cylinders 12. A piston 14 is disposed within the cylinder 12. The piston 14 is connected to a crankshaft 18 via a connecting rod 16. A rotational speed sensor 20 that generates an output responsive to the rotational speed of the crankshaft is disposed in the vicinity of the crankshaft 18. A coolant temperature sensor 22 that detects the temperature of the coolant for the internal combustion engine is provided in the internal combustion engine. A combustion chamber 24 is provided at the top of the piston 14. A temperature sensor 26 (temperature detecting means) that generates an output responsive to the temperature within the combustion chamber 24 is disposed in the combustion chamber 24. A spark plug 28 is inserted with the head thereof exposed into the combustion chamber 24.

The internal combustion engine 10 has an intake port 30 and an exhaust port 32 that communicate with the combustion chamber 24. An injector 34 is built into the intake port 30. The intake port 30 is connected to an intake passage 36. An air flow meter 38 is disposed in the intake passage 36.

The intake port 30 of each cylinder 12 of the internal combustion engine 10 has an intake valve 40 that opens and closes the intake port 30. An intake valve shaft 42 is fixed to the intake valve 40. A valve lifter 44 is mounted to the top end of the intake valve shaft 42. The impelling force of a valve spring 46 acts on the intake valve shaft 42, and the intake valve 40 is impelled in the valve-closing direction by the impelling force. An intake cam 50 is disposed above the valve lifter 44. The intake cams 50 of each cylinder 12 are connected two each to one and the same camshaft (not shown), and are linked to a variable valve timing mechanism 52 via the camshaft and the like. A cam position sensor 54 is mounted in the vicinity of the camshaft of the intake cams. The cam position sensor 54 generates an output responsive to the rotational angle and the rotational speed of the intake cam 50.

The internal combustion engine 10 has, on the exhaust ports 32 of each cylinder 12, an exhaust valve 60 that opens and closes the exhaust port 32. The exhaust valve 60 has the same configuration as the intake valve 40. That is, the exhaust valve 60 has an exhaust valve shaft 62 fixed to the exhaust valve 60, a valve lifter 64 mounted at the top of the exhaust valve shaft 62, and a valve spring 66 mounted so as to impel the exhaust valve shaft 62 in the valve-closing direction. An exhaust cam 70 is disposed at the top of the valve lifter 64. The exhaust cams 70 of each cylinder 12 are connected two each to one and the same camshaft (not shown), and are linked to a variable valve timing mechanism 72 via the camshaft and the like. A cam position sensor 74 is mounted in the vicinity of the camshaft of the exhaust cams 70. The cam position sensor 74 generates an output responsive to the rotational angle and the rotational speed of the exhaust cam 70.

The variable valve timing mechanism 52 for the intake valve 40 side utilizes a motor to control the rotational speed and rocking of the camshaft to control the rotation an the rocking of the intake cam 50. As a result, the phase, operating angle, and lift amount of the intake valve 40 can be varied independently for each cylinder 12. The variable valve timing mechanism 72 for the exhaust valve 70 utilizes a motor or the like to control the rotation and rocking of the camshaft to control the rotation and rocking of the exhaust cam 70. As a result, the phase, operating angle, and lift amount of the exhaust valve 70 can be varied independently for each cylinder 12.

By varying the phase of the intake and exhaust valves 40, 60, it is possible to change the timing of the opening and the closing of the intake and exhaust valves 40, 60. By varying the operating angle, it is possible to change the period of time of opening of the intake and exhaust valves 40, 60. By varying the lift amount, it is possible to change the size of the passage formed between the valves and the intake and output ports 30, 32 when the intake and exhaust valves 40, 60 are open. This type of control can be performed for each intake valve 40 and exhaust valve 60 of each individual cylinder 12. Because the mechanism to control the rotating and rocking of the camshaft so as to control the phase, operating angle, and lift amount of the intake valves 40 or the exhaust valve 60 is not particularly novel, it will not be described here in detail.

The internal combustion engine 10 has an ECU (electronic control unit) 80 as a control apparatus for the internal combustion engine. The ECU 80 acquires information required for control of the internal combustion engine 10 from such sensors as the rotational speed sensor 20, the coolant temperature sensor 22, the temperature sensor 26, the air flow meter 38, and the cam position sensors 54, 74, and controls the spark plug 28, the injector 34, an the variable valve timing mechanisms 52, 72, based on the acquired information.

In cases, for example, in which the internal combustion engine 10 is cold started, in which the internal combustion engine 10 has not yet been warmed up, the temperatures of various parts of the internal combustion engine 10 are low. For this reason, the intake air temperature when the engine is cold started is low. It is therefore difficult for fuel to be atomized, and the condition occurs in which there is low mixture of air and fuel, making it difficult to achieve stable combustion. For this reason, control is generally performed to increase the amount of fuel injection amount at the cold start. If this control is performed, however, the air-fuel ratio during the cold start becomes rich, and the amount of non-combusted fuel that is exhausted increases. In order to extend the lean limit and improve the fuel economy and exhaust emissions characteristics, therefore, it is desirable that the control to increase the fuel injection amount during a cold start be done over a short period of time, or be avoided. For this reason, it is desirable in cold starting that, at an earlier stage after the starting of the internal combustion engine 10 the intake air temperature is increased, atomization is promoted, and combustion is stabilized.

If the intake resistance in the intake stroke of the internal combustion engine 10 increases, heat of friction occurs when the intake gas is taken into the cylinder 12. Therefore, by making the intake resistance in the intake stroke, that is, the pumping loss, large it is possible to raise the temperature of the gas taken into the cylinder 12 by increasing heat of friction.

FIG. 2 is a drawing describing the relationship between the lift amount of the intake valve 40 and the pumping loss. In FIG. 2, the horizontal axis represents the lift amount of the intake valve 40, and the vertical axis represents the pumping loss. The solid line (i) in FIG. 2 shows the case in which the internal combustion engine 10 is in a lower rotational speed region than shown by the solid line (ii). As shown in FIG. 2, as the lift of the intake valve 40 increased from the condition in which the lift amount is zero, the pumping loss also increases, and becomes maximum at some lift amount (low lift amount). The pumping loss gradually decreases as the lift amount becomes larger than the low lift amount. The low lift amount at which the pumping loss is maximum is different depending upon the engine rotational speed and, as shown by the solid lines (i) and (ii) of FIG. 2, as the engine rotational speed increases, the low lift amount tends to increase.

The heat of friction generated when the intake gas flows in cylinder is higher, the greater is the pumping loss in the intake stroke. Therefore, by setting the lift amount to the low lift amount and making the pumping loss maximum, it is possible to more quickly raise the temperature of the intake gas when making a cold start.

In contrast, the lift amount of the intake valve 40 is set to the lift amount (normal lift amount) required to reliable intake the requested air into the cylinder 12. With regard to this point, even when the internal combustion engine is cold started, in order to generate the torque required for starting, it is necessary to intake the requested amount of air, and necessary to perform intake by controlling the lift amount of the intake valve 40 to the normal lift amount. Therefore, if the compression stroke and the explosion/combustion stroke are performed after completing the intake stroke with the control remaining at the low lift amount in order to raise the temperature as noted above, it can be presumed that the amount of intake gas filling the cylinder 12 is insufficient, so that the combustion is actually decreased.

The system of the first embodiment, in order to simultaneously achieve a rise of the intake gas temperature and a sufficient gas intake amount in performing a cold start, after repeating the intake stroke and the compression stroke two times in one combustion cycle of the internal combustion engine 10, executes a combustion stroke and an exhaust stroke. FIG. 3A and FIG. 3B describe the opening and closing timing and lift amounts of the intake and exhaust valves in the internal combustion engine 10, FIG. 3A showing operation at the time of cold starting and FIG. 3B showing the operation after combustion stabilizes.

As shown in FIG. 3A, when the intake temperature is low during a cold start, one combustion cycle is formed by the six strokes of the first intake stroke, a first compression stroke, a second intake stroke, a second compression stroke, an expansion stroke, and an exhaust stroke, with one ignition occurring with a prescribed timing in the second compression stroke. In this embodiment, this operating condition of the internal combustion engine 10 will be referred to as “six-stroke operation.” During six-stroke operation, in the initial intake stroke (first intake stroke), in order to raise the intake temperature with the pumping loss at a maximum, the lift amount of the intake valve 40 is controlled to be the low lift amount. The relationship between the low lift amount and the pumping loss differs, depending upon the timing of the opening and closing and the operating angle of the intake valve. Therefore, the low lift amount that is set at this point is the lift amount that maximizes the pumping loss under a condition in which the opening and closing timing and operating angle are set to appropriate timing and angle in relationship to other operating conditions. The intake temperature rises because of the heat of friction generated during the first intake stroke. Although the temperature rise will differ depending upon the temperature of the gas that is taken in and the engine rotational speed at that time, it is, for example, approximately 50° C. to 60° C. After performing intake in the condition of this lift amount, the intake valve 40 is closed, and the first compression stroke is entered.

After the above, the second intake stroke is entered without performing ignition. When this occurs, the lift amount of the intake valve 40 is controlled to the normal lift amount required to intake the requested air amount. The normal lift amount also differs depending upon the timing of the opening and closing and the operating angle of the intake valve 40. Therefore, the normal lift amount is set to a lift amount to intake the requested amount of air in the case in which the lift amount, the opening and closing timing, and the operating angle are properly set. With the valve timing set in this manner, the piston 14 is lowered in the second intake stroke and intake is performed. By doing this, the required amount of intake air can be acquired. The piston 14 begins to rise and the second compression stroke starts, and in the second compression stroke ignition is performed at the optimum timing. After that, the expansion stroke and the exhaust stroke are performed.

During the six-stroke operation, the exhaust valve 60 is closed during a period from the first intake stroke to the second compression stroke and during the compression stroke in the same manner as a period from the normal intake stroke to the expansion stroke. That is, during the six-stroke operation control is performed so that the exhaust valve 60 is first opened and then closed at an appropriate time in the region of the start of the exhaust stroke.

The two intake and compression strokes causes the temperature of the intake gas to raise, and enables intake of the required amount of intake gas. Therefore, it is possible to acquire the air required for combustion while promoting the mixing of fuel and air, and possible to improve the combustion condition during the cold start. Also, by a temperature rise of the intake gas, it is possible to more quickly warm-up the internal combustion engine and stabilize combustion. Because the raising of the intake gas temperature at the time of cold start stabilizes combustion, it is possible to suppress an increase in the fuel injection amount, and extend the lean limit.

A map establishing the relationship between the low lift amount and engine rotational speed at which the pumping loss is maximum and the timing of the opening and closing of the intake valve 40, and a map establishing the relationship between the lift amount, the requested air intake amount and timing of the opening and closing of the intake valve 40 are stored in the ECU 80. The low lift amount and normal lift amount in the case of six-stroke operation are established in accordance with these maps, and the ECU 80 performs control of the intake valve 40, via the variable valve timing mechanism 52, in accordance with the set low lift amount and normal lift amount.

After the internal combustion engine 10 is warmed up and combustion has stabilized, a transition is made to operation (four-stroke operation) in which one combustion cycle is formed by the normal four strokes. That is, the intake stroke is performed so that the lift amount of the intake valve 40 is controlled to the normal lift amount in response to the requested amount of intake air. After that, the intake valve 40 is closed and, after performing the compression stroke, ignition is performed at an appropriate timing, immediately after which the expansion stroke and exhaust stroke are performed. The exhaust valve 60, similar to the case of normal control, is first opened and then closed at an appropriate time in the exhaust stroke of the four strokes to perform exhausting.

The ECU 80 stores a map establishing the relationship between the normal lift amount, the amount of requested air intake, and the timing of the opening and closing and the operating angle of the intake valve 40. In the case of performing four-stroke operation, the normal lift amount is calculated using the map and, in response to the calculated normal lift amount, the ECU 80 controls the intake valve 40 via the variable valve timing mechanism 52.

The six-stroke operation is effective in the case in which, for example, during cold starting, when raising the intake temperature has priority. However, in the condition in which control is performed to the low lift amount at which the pumping loss is maximum, because two intakes are performed the torque loss increases. If the internal combustion engine 10 has been warmed up and the combustion has been stabilized, a switch is made immediately to four-stroke operation. For this reason, it is determined in the system of the first embodiment that combustion has stabilized, a transition is made from six-stroke operation to four-stroke operation.

Specifically, when the temperature in the combustion chamber 24 has risen sufficiently, it can be determined that combustion in the internal combustion engine 10 has stabilized. Therefore, the temperature in the combustion chamber 24 is detected from the output of the temperature sensor 26 mounted in the combustion chamber 24, and if the detected temperature is sufficiently high, it is determined that combustion has stabilized. The ECU 80 has stored a threshold cylinder temperature that is the minimum temperature in the combustion chamber 24 to determine that the internal combustion engine 10 has warmed up and the combustion has stabilized. If the detected temperature has reached at least the threshold cylinder temperature, the ECU 80 determines that combustion in the internal combustion engine 10 has stabilized and switches from six-stroke operation to four-stroke operation.

FIG. 4 is a flowchart describing a control routine executed by the ECU 80 in the first embodiment of the present invention. The flowchart shown in FIG. 4 is a routine that executed each time the internal combustion engine 10 is started. In the flowchart shown in FIG. 4, the temperature of the coolant in the internal combustion engine 10 is first detected (step S100). The coolant temperature is determined based on the output of the coolant temperature sensor 22. Next, it is determined whether or not cold start of the internal combustion engine 10 has been requested (step S102). Whether or not cold start has been requested is determined, for example, based on whether or not the coolant temperature determined in step S100 is below a prescribed range when starting of the internal combustion engine 10 is requested.

If it is determined at step S102 that the cold start has been requested, required information regarding the current operating condition is detected (step S104). For example, information such as the engine rotational speed, the accelerator operating amount, and the temperature in the combustion chamber 24 is detected in accordance with the output from various sensors. Next, the requested intake air amount is calculated (step S106). The requested intake air amount is calculated in accordance with requested load determined based on the output of an accelerator operating sensor.

Next, it is determined whether or not the temperature T24 in the combustion chamber 24 is greater than or equal to the threshold cylinder temperature T0 (step S108). If at step S108 the temperature T24 in the combustion chamber 24 is greater than or equal to the threshold cylinder temperature T0, it is determined that the internal combustion engine 10 has not warmed up and that combustion has not stabilized, resulting in execution of six-stroke operation (step 110).

Specifically, using a map stored in the ECU 80, the low lift amount at which the pumping loss is maximum for the current engine rotational speed is determined, and the lift amount of the intake valve 40 for the first intake stroke is determined. Using a map stored in the ECU 80, the normal lift amount for the second intake stroke is determined in accordance with the requested intake air amount determined in step S106. In accordance with the current operating condition detected in step S104, the operating angle and phase of the intake valve 40 at the time of engine starting are determined. In accordance with the valve timing, such as the calculated lift amount and the like, control of the intake valve 40 is performed by the variable valve timing mechanism 52. In this condition, the first intake stroke, the first compression stroke, the second intake stroke, and the second compression stroke are performed, after which the compression stroke and exhaust stroke are performed. Control is performed so that ignition is done at an appropriate time during the second compression stroke. During this period of time, control is performed to close the exhaust valve 60 from the first intake stroke to the second compression stroke and during the expansion stroke, and to open the exhaust valve 60 at the normal valve timing in the exhaust stroke to perform exhausting.

Return is made again to step S104, at which information regarding the current operating condition is detected after the requested amount of intake air is calculated (steps S104 and S106), at step S108 the temperature T24 in the combustion chamber 24 is compared with the threshold cylinder temperature T0. If it is not determined that the temperature T24 is greater than or equal to the threshold cylinder temperature T0 at step S108, six-stroke operation is performed (step S110), and the processing of steps S104 to S108 is performed. That is, six-stroke operation (step S110) and the processing of steps S104 to S108 are repeated until it is determined that the temperature T24 in a combustion chamber temperature reaches the threshold cylinder temperature T0 at step S108.

If, however, it is not determined at step S102 that a cold start had been requested, and if it is determined at step S108 that the temperature T24 in the combustion chamber 24 is greater than or equal to the threshold cylinder temperature T0, it is determined that the temperature T24 in the combustion chamber 24 has reached the temperature T0 at which the internal combustion engine is assumed to have already warmed up. Therefore, normal four-stroke operation is executed (step S112). Specifically, using a map stored in the ECU 80, the normal lift amount of the intake valve 40 is set in accordance with the requested amount of intake air. The opening and closing timing and operating angles of the exhaust valve 60 and the intake valve 40 are set in accordance with the current condition of the internal combustion engine. In this condition, the normal intake stroke, compression stroke, expansion stroke, and exhaust stroke are performed, and control is done to perform ignition between the compression stroke and the expansion stroke. Next, the processing is ended.

As described above, according to the first embodiment when performing cold starting, control is performed so that after performing the first intake stroke and the first compression stroke at the low lift amount at which the pumping loss is maximum, the second intake stroke and the second compression stroke are performed, after which the expansion stroke and the exhaust stroke are performed. By doing this, it is possible to raise the temperature of the intake air from the condition in which the intake temperature is low, such as when doing cold starting, thereby enabling both an improvement in the combustion condition, early warm-up of the internal combustion engine, and stabilization of combustion.

In the first embodiment, the temperature T24 in the combustion chamber 24 is directly detected and, based on whether this temperature T24 has reached the threshold temperature T0, it is determined whether to switch between six-stroke operation and four-stroke operation. The present invention, however, is not limited to performing the determination of switching between six-stroke operation and four-stroke operation in this manner. This determination can be made if it is possible to determine with some accuracy the stabilization of combustion when cold starting is done. The determination of whether or not to switch, therefore, can be made, for example, by detecting the temperature of the coolant for the internal combustion engine 10 and basing the judgment on whether or not the coolant temperature is higher than or equal to the threshold coolant temperature at which the internal combustion engine 10 is assumed to have warmed up. The threshold coolant temperature can be set based on a temperature by experimentally determining a value which is experimentally determined and whish indicates that the internal combustion engine 10 has warmed up and, based on that value, in consideration of what extent of warm-up the six-stroke operation is to be continued.

The first embodiment of the present invention is described for the case in which it is determined that a cold start has been requested. The present invention is not, however, limited in this manner, and may perform six-stroke operation in other cases in which it is effective to give priority to raising the intake temperature. Therefore, for example, six-stroke operation may be started even in cases in which it is determined that the internal combustion engine 10 is not warmed up. Also, six-stroke operation may also be performed only in a case, of a so-called fast idling condition, in which the internal combustion engine is operating at a rotational speed higher than a normal idling speed, such as during catalyst warm-up or during a cold start.

The low lift amount in the first intake stroke of six-stroke operation was described for the case of a lift amount at which the pumping loss is maximum. In the present invention, however, the low lift amount is not limited in this manner, and may be set as a small lift amount at which the pumping loss is larger than at a lift amount that is normally set in accordance with a requested amount of intake air. This is a reason why, even if two intake strokes are performed at a lift amount that is the same as the normal lift amount, it is possible to raise the intake temperature slightly.

The first embodiment was described for the case in which the lift amount in the second intake stroke during six-stroke operation and the lift amount in four-stroke operation are set in accordance with the requested amount of intake air, and valve timing control including this lift amount is performed to control the intake air amount. The present invention is not limited in this manner, however, and an electronically controlled throttle valve may be disposed in the intake passage 36 and the air intake amount may be controlled by the degree of opening of the throttle valve. In this case, the lift of the intake valve in the first intake stroke can be controlled to the low lift amount, and the normal lift amount in the second intake stroke and in four-stroke operation can be set to the maximum lift amount for the case in which the intake cam 50 is rotated one time, instead of the lift amount set in accordance with the requested amount of intake air.

Also, the first embodiment is described for case in which, during the cold start of the internal combustion engine, the first combustion cycle is performed with six-stroke operation, which includes a first intake stroke, a first compression stroke, a second intake stroke, a second compression stroke, an expansion stroke, and an exhaust stroke. However, the present invention is not limited in this manner, and multi-stroke operation may be performed that includes a plurality of repetitions of a first intake stroke and a first compression stroke, followed by performing of a second intake stroke, a second compression stroke, an expansion stroke, and an exhaust stroke. In this case, pumping loss during the first intake stroke increases, thereby enabling of effectively raising the intake temperature during one combustion cycle.

The first embodiment was described for the case in which the means for changing the valve timing of the intake valve 40 is that of connecting two intake cams 50 each to one and the same camshaft, the rotation and rocking of the camshaft being controlled by the variable valve timing mechanism 52, and value timing including the phase, lift amount, and operating angles of the intake valves 40 being controlled independently for each cylinder 12. The present invention, however, is not limited to this method of controlling the intake valve 40. In the present invention, the means for changing the valve timing of the intake valve 40 may be a different configuration that is capable of opening and closing the valve at least two times, in the intake strokes during one combustion cycle, and also changing the lift of the intake valve. Specifically, for example, by using an electromagnetically driven valve, the lift and opening and closing timing of the intake valve 40 may be independently controlled for each intake valve 40. In the same manner, the means for changing the valve timing of the exhaust valve 60 is not limited to the means described with regard to the first embodiment, and may be a different configuration that is capable of controlling the timing of the one opening and closing at appropriate times within the exhaust stroke, in accordance with the condition in six-stroke operation (or multi-stroke operation).

Although the first embodiment was described for the case in which the internal combustion engine 10 is a gasoline engine, there is no limitation in this manner, and the internal combustion engine 10 may also be, for example, a diesel engine. Although the example given is that of fuel injection by port injection, the engine may be an internal combustion engine that uses cylinder injection.

For example, in the first embodiment by executing step S102 the “cold starting determining means” may implemented, by executing step S110 the “multi-stroke operation setting means,” the “valve timing control means,” and the “ignition timing control means” may be implemented, by executing step S108 the “multi-stroke operation termination determining means” may be implemented, by executing step S110 the “four-stroke operation setting means” is implemented, and by executing step S104 the “temperature detection means” and “coolant temperature detection means” are implemented.

The second embodiment of the present invention is described below, with reference to FIG. 5 and FIG. 6. The description of the second embodiment will focus on the only the characteristic parts of the second embodiment, and the descriptions of parts that are the same as the first embodiment will be either simplified or omitted. The system in the second embodiment has the same type of configuration as the system of the first embodiment. In the system of the second embodiment, with the exception of the method for determining timing of switching from six-stroke operation to four-stroke operation being different from the method in the first embodiment, control is performed in the same manner as in the first embodiment.

Specifically, in the system of the second embodiment, the determination of the switching from six-stroke operation to four-stroke operation is made in accordance with an engine load. FIG. 5 is a graph describing the relationship between the coolant temperature and the threshold engine load to determine whether to switch from six-stroke operation to four-stroke operation in the second embodiment. In FIG. 5, the horizontal axis represents the coolant temperature and the vertical axis represents the threshold engine load. As noted above, in the six-stroke operation, the intake stroke are performed two times and, of the two intake strokes, intake is performed in the first intake stroke with the lift amount set to the lift amount at which the pumping loss is maximum. For this reason, compared with the case of normal four-stroke operation, the generated torque is small. Therefore, in the case in which the load becomes large, it is difficult to generate a torque in accordance with that load with six-stroke operation. Therefore, regardless of whether the internal combustion engine 10 has warmed up, in the case in which the requested load is above a given load, in order to generate a torque in accordance with the requested load, switching is done from the six-stroke operation to the four-stroke operation. That is, the rise of the requested load of the internal combustion engine 10 above the solid line (i) (threshold engine load (i)) in FIG. 5 is a first condition for switching from six-stroke operation to four-stroke operation.

In six-stroke operation, an increase in the intake temperature makes it easier to burn the fuel. If the engine load becomes large under this condition, abnormal combustion tends to cause knocking. Also, if such abnormal combustion occurs, it can be assumed that the warm-up of the internal combustion engine 10 has progressed to some extent. In the second embodiment, therefore, in order to give priority to suppression of knocking, six-stroke operation is only permitted only when the engine load is within a range that does not cause knocking. The limit value of requested load set in consideration of occurrence of knocking, as indicated by the solid line (ii) in FIG. 5, is smaller than when the temperature of the coolant for the internal combustion engine 10 is high, and becomes higher when the coolant temperature is low. In consideration of suppression of knocking, the rise above the solid line (ii) in FIG. 5 (threshold engine load (ii)) is a second condition for switching from six-stroke operation to four-stroke operation.

From the above, in the second embodiment six-stroke operation is executed at the cold start, and a transition is made to four-stroke operation if either of the following first and second conditions is satisfied. The first condition is (requested load)≧(threshold engine load (i)) and the second condition is (requested load)≧(threshold engine load (ii)).

That is, when the coolant temperature and the requested load are in the region below the thick line (I), six-stroke operation is executed and continued, and the value of the solid line (I) is a threshold engine load for switching from six-stroke operation to four-stroke operation. The threshold engine load is the value that is the smaller of the threshold engine load (i) and the threshold engine load (ii) at the coolant temperature at that time. The ECU 80 has stored map establishing the relationship between the coolant temperature and the threshold engine load, based on the relationship such as shown in FIG. 5. The threshold engine load is calculated using the map, based on the detected coolant temperature.

FIG. 6 is a flowchart describing a control routine executed by the ECU 80 in the second embodiment of the present invention. The routine of FIG. 6 is the same as the routine of FIG. 4, with the exception that, after step S104 of FIG. 4, step S202 is executed, and after step S106 step S204 is executed, and in place of step S108, steps S204 and S206 are executed.

Specifically, it is determined the internal combustion engine is being started from the cold condition, i.e. a cold start, at step S102 and then information regarding the operating condition is detected (step S104). In this case, the engine rotational speed, the accelerator operating amount and, in place of the coolant temperature of the combustion chamber 24, the coolant temperature are detected in accordance with outputs from various sensors. Next, the engine load is calculated (step S202). The engine load is calculated based on information regarding the operating condition of the internal combustion engine 10 detected in step S104. Next, the requested intake air amount is calculated (step S106), and the threshold engine load is calculated (step S204). The threshold engine load is determined using a map (refer to FIG. 5) stored in the ECU 80, in accordance with the coolant temperature calculated at step S104.

Next, it is determined whether or not the current load is greater than or equal to the threshold engine load (step S206). That is, the load calculated at step S202 and the load calculated at step S204 are compared, and it is determined whether the engine load is greater than or equal to the threshold engine load. At step S206 if it is determined that the engine load is greater than or equal to the threshold engine load, six-stroke operation is performed (step S110). That is, control is performed so that the first intake stroke is performed with the intake valve 40 at the low lift amount and the first compression stroke is performed, and the second intake stroke is performed with the intake valve 40 at the normal lift amount condition, the second compression stroke, and ignition are performed, followed by the expansion stroke and the exhaust stroke. The processing of steps S104, S202, S106, S204, S206, and S110 is repeated until it is determined that the engine load is greater than or equal to the threshold engine load at step S206.

If, however, it is not determined that the internal combustion engine is being started from the cold condition at step S102, or if at step S206 it is determined the engine load is greater than or equal to the threshold engine load, four-stroke operation is set (step S112), and the processing is ended.

As described above, in the second embodiment the threshold engine load for switching from six-stroke operation to four-stroke operation is set in accordance with the coolant temperature, and the engine operation is switched from six-stroke operation to four-stroke operation in accordance with the set threshold engine load. Therefore, if the requested engine load is large and the internal combustion engine can not generate an output torque in corresponding to the requested load with six-stroke operation, or if knocking is expected to occur because of abnormal combustion, it is possible to avoid six-stroke operation and perform four-stroke operation. Also, six-stroke operation is continued until either the first or second above-noted condition is satisfied. For this reason, if the intake temperature is low at the cold start of the internal combustion engine, it is possible to reliably raise the temperature of the intake gas and improve the combustion condition.

The second embodiment is described for the case in which the switching load is set to the smaller of the first condition that considers the requested engine load and the second condition that considers the occurrence of knocking. In the present invention, however, the threshold engine load need not take into consider both of these, and may be set with consideration given to either one of the first condition and the second condition.

In the second embodiment, by executing step S202 the “requested load calculation means” may be implemented, and by executing step S206, the “multi-stork operation termination determining means” may be implemented.

The third embodiment of the present invention is described below, with reference made to FIG. 7. The description as follows will focus on the only the characteristic parts of the third embodiment, and the descriptions of parts that are the same as the first embodiment will be either simplified or omitted. The system in the third embodiment has the same type of configuration as the system of the first embodiment. In the system of the third embodiment, with the exception of predicting the temperature in the combustion chamber 24 and switching from four-stroke operation to six-stroke operation based on the predicted temperature, control is performed in the same manner as in the first embodiment.

Specifically, in the system of the third embodiment as well, six-stroke operation is performed at the cold start. It is possible to predict the temperature rise ΔT after the intake stroke in six-stroke operation, based on the air intake amount detected during the six-stroke operation. Therefore, the predicted temperature Tp in the combustion chamber 24 after the second intake stroke in six-stroke operation can be expressed by the current temperature T24 in the combustion chamber and the temperature rise ΔT in the form of Equation (1).

Predicted combustion chamber temperature Tp=Combustion chamber temperature T24+ΔT  (1)

Before starting six-stroke operation, even if the temperature in the combustion chamber 24 is lower than the threshold engine load, there are cases in which an excessive rise occurs in the intake temperature when six-stroke operation is actually performed. If ignition is done under the condition, because it may cause abnormal combustion or knocking to occur, it is preferable to avoid this condition. Therefore, in the third embodiment, as noted above, the temperature Tp in the combustion chamber 24 after the second intake stroke in six-stroke operation is predicted and, if the predicted temperature Tp is at least the threshold engine load T0, a switch is made to four-stroke operation.

FIG. 7 is a flowchart describing a control routine executed by the ECU 80 in the third embodiment. The flowchart shown in FIG. 7, with the exception of having steps S302 to S310 after the step S110 of the flowchart shown in FIG. 4, is the same as the routine shown in FIG. 4. Specifically, in the first combustion when the cold start is determined at step S102, if at step S108 it is determined that the current temperature T24 in the combustion chamber 24 is lower than the threshold cylinder temperature T0, six-stroke operation is performed (step S110), after which information regarding the operating condition, such as the temperature T24 in the combustion chamber 24 or the intake air amounts and the like in the first and second intake strokes is again detected (step S302).

Next, the requested intake air amount is calculated (step S304). After that, based on the intake air amounts in the first and second intake strokes detected at step S302, the temperature rise ΔT is calculated (step S306). The temperature rise ΔT can be determined from a map establishing the relationship between the intake air amount and the temperature rise. Next, the predicted temperature Tp after the second intake stroke in the combustion chamber 24 is calculated (step S308). The combustion chamber predicted temperature Tp is calculated in accordance with the above-noted Equation (1).

Next, it is determined whether the combustion chamber predicted temperature Tp is greater than or equal to the threshold predicted cylinder temperature T0 (step S310). If it is determined that the combustion chamber predicted temperature Tp is greater than or equal to the threshold cylinder temperature T0, six-stroke operation is again performed at step S110, and the processing of steps S302 to S310 is performed. That is, as long as it is not determined that the condition combustion chamber predicted temperature Tp is greater than or equal to the threshold predicted cylinder temperature T0 at step S310, six-stroke operation is performed at step S110. If, however, it is determined that the combustion chamber predicted temperature Tp is greater than or equal to the threshold predicted cylinder temperature T0 at step S310, four-stroke operation is set and processing is ended.

As described above, according to the third embodiment, when the internal combustion engine 10 is being started from the cold condition, i.e. cold start, when six-stroke operation is performed, the combustion chamber temperature after performing six-stroke operation is predicted, and it is determined whether to switch to four-stroke operation based on the predicted temperature. It is therefore possible to suppress an excessive rise in the temperature in the combustion chamber 24, and possible to effectively prevent knocking due to abnormal combustion.

The third embodiment is described for the case in which the temperature in the combustion chamber 24 is detected by the temperature sensor 26, and the predicted temperature is calculated from the detected temperature and the temperature rise predicted from the intake air amount. However, the method for calculating the predicted temperature Tp in the combustion chamber 24 is not limited to this method, and may be a calculation by another method. For example, the initial value of the temperature in the combustion chamber 24 may be predicted from the coolant temperature at the time of starting, after which the temperature rise ΔT is calculated from the intake air amounts for the intake strokes (first intake stroke and second intake stroke) for each combustion cycle, and the temperature rise ΔT may be successively added to the initial value of the temperature in the combustion chamber 24 to predict the temperature in the combustion chamber 24. Also, for example, a combustion pressure sensor that detects the combustion pressure is provided and the temperature in the combustion chamber may be predicted from the combustion pressure and the intake air amount. Additionally, the temperature in the combustion chamber 24 is directly detected and the temperature at the next time may be predicted from the amount of variation. Alternatively, a temperature sensor is provided in the vicinity of the intake valve to directly detect the intake temperature and the temperature in the combustion chamber 24 is predicted based on the intake temperature.

The third embodiment was described for the case in which a switch is made to four-stroke operation if the predicted temperature in the combustion chamber 24 reaches or exceeds the threshold predicted cylinder temperature. The present invention, however, is not limited in this manner, and if the predicted temperature in the combustion chamber 24 is at least the threshold predicted cylinder temperature, the lift amount may be increased by a prescribed amount from the low lift amount, the low lift amount being gradually changed until it reaches the normal lift amount during which time six-stroke operation is continued. By doing this, it is possible to suppress the torque variation to a small amount when the switch is made to four-stroke operation. When such control is performed, the threshold predicted cylinder temperature may be set to lower than the normal. Additionally, amount of gradual change of the lift amount during six-stroke operation is not limited to being a fixed amount of change.

In the third embodiment, by executing steps S306 and S308, the “cylinder internal temperature predicting means” may be implemented, and by executing step S310 the “multi-stroke operation termination determining means” may be implemented.

The fourth embodiment of the present invention is described below, with reference made to FIG. 8 and FIG. 9. The description of the fourth embodiment will focus on the only the characteristic parts of the fourth embodiment, and the descriptions of parts that are the same as the first to third embodiments will be either simplified or omitted. The system in the fourth embodiment has the same type of configuration as the system of the first embodiment, with the exception that it is used as a flexible fuel vehicle (FFV). Specifically, the system of the fourth embodiment can use alcohols such as ethanol, methanol, bio-ethanol, or bio-methanol, or a mixture of these alcohols and gasoline as a fuel. Use as a fuel is possible regardless of the proportion of alcohol fuel in the fuel that is used.

The system in the fourth embodiment performs six-stroke operation at the cold start. The control executed by the system of the fourth embodiment, with the exception of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load or the threshold predicted cylinder temperature being set in accordance with the alcohol concentration in the fuel when determining whether to switch from six-stroke operation to four-stroke operation, is the same as in the first embodiment. FIG. 8 is a graph describing the relationship between the alcohol concentration in the fuel and the threshold coolant temperature for switching to four-stroke operation in a fourth embodiment of the present invention.

The proportion of alcohol fuel in the fuel used the system of the fourth embodiment as described above is not fixed. However, the concentration of alcohol included in the fuel used is a factor affecting the atomization of fuel when taken into the cylinder 12. Specifically, atomization of the fuel occurs easily even at a relatively low temperature in the case in which the alcohol concentration is low and the gasoline concentration is high, whereas, as the alcohol concentration in the fuel increases, it becomes difficult for the fuel to atomize. For this reason, the temperature at which a given amount of fuel can be atomized is higher, the higher is the alcohol concentration.

Therefore, particularly when the temperature in various parts of the internal combustion engine 10 is low at the cold start, the higher the alcohol concentration is, in order to achieve stable combustion, the more it is necessary raise the temperature of the intake gas to make it easier to atomize the fuel. For this reason, when six-stroke operation is performed for raising the intake temperature, the intake temperature is made higher, the higher is the concentration of the alcohol fuel. That is, as shown in FIG. 8, control in which the threshold coolant temperature for switching from six-stroke operation to four-stroke operation is made higher, the higher is the alcohol fuel concentration, to raise the intake temperature in six-stroke operation, is continued until the inside of the combustion chamber 24 reaches a higher temperature.

The ECU 80 has stored map establishing the relationship, as shown in FIG. 8, between alcohol concentration in the fuel and the threshold coolant temperature. When making a cold start of the internal combustion engine 10, the alcohol concentration of the fuel is detected and the threshold coolant temperature is calculated in accordance with the map, in accordance with the detected alcohol concentration. If the temperature of the coolant in the internal combustion engine 10 reaches or exceeds the threshold coolant temperature, a switch is made from six-stroke operation to four-stroke operation.

FIG. 9 is a flowchart describing a control routine executed by the ECU 80 in the fourth embodiment. The routine of FIG. 9, with the exception of execution of steps S402 to S406 in place of step S108 after step S106 of FIG. 4, is the same as the routine of FIG. 4. Specifically, if it is determined at step S102 that the internal combustion engine is being started from the cold condition, information regarding the operating condition is detected, the requested intake air amount is calculated (steps S104 and S106), and the alcohol concentration of the currently used fuel is read out (step S402). The alcohol concentration of the fuel is stored in the ECU 80. At this point, instead of reading out the alcohol concentration from the ECU, a concentration meter that detects the concentration of the alcohol fuel may be installed to detect the alcohol concentration.

Next, the threshold coolant temperature is calculated (step S404). The threshold coolant temperature is calculated as a value corresponding to the alcohol concentration read out in step S402, in accordance with the map stored beforehand in the ECU 80. Next, it is determined whether the coolant temperature detected at step S104 is greater than or equal to the threshold coolant temperature (step S406) and, if it is not determined that the coolant temperature is greater than or equal to the threshold coolant temperature, six-stroke operation is performed (step S110). Six-stroke operation is repeated in the steps S104, S106, S402 to S406 and S110 until it is determined at step S406 that the coolant temperature is greater than or equal to the threshold coolant temperature. If it is determined that the coolant temperature is greater than or equal to the threshold coolant temperature at step S406, a switch is made to four-stroke operation (step S112).

As described above, in the fourth embodiment the threshold coolant temperature is calculated in accordance with the alcohol concentration in the fuel. For this reason, it is possible to continue six-stroke operation until the coolant temperature reaches a temperature, at which the combustion stabilizes, set in accordance with the alcohol concentration, and possible to reliably perform warm-up to the required temperature. It is also possible to accommodate combustibility in accordance with the concentration of the alcohol fuel. When it is difficulty to achieve stable combustion due to the high alcohol concentration, it is possible to raise the temperature more quickly to achieve stable combustion by continuing six-stroke operation to a higher temperature. In particular when a fuel is used that includes an alcohol fuel, although there are cases in which the cold start is difficult, it is possible to raise the temperature in the combustion chamber 24 more quickly by performing six-stroke operation as in the above-described fourth embodiment. Therefore, because the temperature in the combustion chamber becomes a temperature at which the fuel can be atomized more quickly, it is possible to improve the starting characteristics of the internal combustion engine. In an engine using a fuel with a low volatility, such as in an FFV, the fourth embodiment can effectively improve starting characteristics.

The fourth embodiment is described for the case of using an alcohol fuel or a fuel mixture of an alcohol fuel and gasoline. The present invention, however, is not limited in this manner, and may also use a fuel including a so-called bio-alcohol or a light oil in place of gasoline. In this case as well, in general if the alcohol concentration is high, the threshold coolant temperature will be set to high. In this manner, by experimentally setting the relationship between the threshold coolant temperature and the alcohol concentration for each fuel beforehand, the forth embodiment can be applied to other alcohol fuels as well.

The fourth embodiment is described for the case in which the threshold coolant temperature is set in accordance with the alcohol concentration. The fourth embodiment, however, is not limited in this manner. For example, the threshold coolant temperature T0 with respect to the combustion chamber temperature T24 in the first embodiment, the threshold engine load in the second embodiment, and the threshold predicted coolant temperature T0 with respect to the combustion chamber predicted temperature Tp in the third embodiment may each be set in accordance with the alcohol concentration. Each of these thresholds can be made based on experimental maps in accordance with the alcohol concentration.

Also, for example, in the fourth embodiment by executing step S302 and step S304, the “determination value setting means” may be implemented.

The fifth embodiment of the present invention is described below, with reference made to FIG. 10. The description of the fifth embodiment will focus on the only the characteristic parts of the fifth embodiment, and the descriptions of parts that are the same as the first through the fourth embodiments will be either simplified or omitted. The system of the fifth embodiment, with the exception that the engine is a so-called V-type engine having a plurality of cylinders, is the same as the system of FIG. 1.

Specifically, the internal combustion engine 10 of the fifth embodiment has two groups (hereinafter “banks”) of cylinders. In this system, if the requested load is large, the internal combustion engine 10 is operated with all of the cylinders 12 operating (all-cylinder operation). In contrast, if the requested load is small, only one bank of cylinders is operated, with cylinders belonging to the other bank being stopped (reduced-cylinder operation).

In the case of reduced-cylinder operation, in which only one bank is operating, the cylinders belonging to the other bank are stopped. In this condition, if the requested load becomes large, transition is made from the reduced-cylinder operation to all-cylinder operation. That is, the bank that is stopped (auxiliary bank) is started. Then, even if the cylinders 12 on the operating bank side, which are operating during reduced-cylinder operation, are warm up, if the reduced-cylinder operation continued for a long period of time and when starting cold in the reduced-cylinder operation, the cylinders 12 of the auxiliary bank that are stopped during reduced-cylinder operation might not be sufficiently warmed up. In such cases, immediately after switching from reduced-cylinder operation to all-cylinder operation, it can be envisioned that a raise in the temperature of the intake air in the auxiliary bank is not sufficient, and combustion in the cylinders of the auxiliary bank degrades.

In the above-noted situation, it cylinders 12 of the auxiliary bank are not warmed up and are still cold, the system of the fifth embodiment makes one combustion cycle of temperature six-stroke operation when the auxiliary bank is returned to operation. That is, with regard to the auxiliary bank, the low lift amount at which the pumping loss is maximum is set, and the first intake stroke and the first compression stroke are performed, after which the lift amount is set to the normal lift amount and then the second intake stroke, the second compression stroke, the expansion stroke, and the exhaust stroke are performed. After that, if the temperature T24 in the combustion chambers 24 of the auxiliary bank reaches or exceeds the threshold cylinder temperature T0, the six-stroke operation is ended and the four-stroke operation is performed.

During this period, the current operating condition on the operating bank that is operating during the reduced-cylinder operation is maintained. That is, in the case of performing four-stroke operation, four-stroke operation is performed, and in the case of performing six-stroke operation, six-stroke operation is performed, for example, in accordance with the routine shown in FIG. 4. If it is determined that the six-stroke operation is to be terminated, a switch is made to four-stroke operation. When the operation of the auxiliary bank is restored by four-stroke operation, the internal combustion engine 10 starts to operate with all of the cylinders 12 operating. When the stopped bank is returned to four-stroke operation, the ignition timing is switched to ignition timing that is set for all-cylinder operation beforehand, and then, the timing of the intake valves 40 and the exhaust valves 60 of each cylinder 12 is switched to the valve timing set beforehand.

FIG. 10 is a flowchart describing a control routine executed by the system of the fifth embodiment. The routine of FIG. 10, is repeatedly executed during operation of the internal combustion engine 10. Specifically, it is determined at step S502 whether or not reduced-cylinder operation is in progress. If it is not determined that reduced-cylinder operation is in progress, the current operation is continued and processing ends.

If, however, it is determined at step S502 that reduced-cylinder operation is in progress, next information regarding the operating condition is detected (step S504). Required information, for example the engine rotational speed and intake air amount, and the coolant temperature and the like is detected based on outputs from various sensors. Next, the current requested load is calculated (step S506). The requested load is calculated based on the accelerator operating amount. Next, it is determined whether there is a request to transition from reduced-cylinder operation to all-cylinder operation (step S508). Whether or not there is a request to transition from reduced-cylinder operation to all-cylinder operation is determined, for example, based on whether the load calculated at step S506 is higher than a prescribed load. If a request for transition to all-cylinder operation is determined at step S508, the current operation is continued and processing is ended.

If it is not determined that there is a request for transition to all-cylinder operation at step S508, however, it is determined whether the cold start of the auxiliary bank is carried out (step S510). Specifically, the determination is made based on whether the temperature of the coolant in the auxiliary bank detected at step S504 is lower than a prescribed coolant temperature.

If it is determined at step S510 that the auxiliary bank is being restored from a cold start, the temperature T24 in the combustion chamber 24 of the cylinders 12 in the auxiliary bank is detected (step S512). Then, it is determined whether the temperature T24 is greater than or equal to the threshold cylinder temperature T0 for switching from the six-stroke operation to the four-stroke operation (step S514). If it is not determined that the temperature T24 is greater than or equal to the threshold cylinder temperature T0 at step S514, at step S516 the auxiliary bank is set to six-stroke operation. That is, the first intake stroke is performed with the intake valve 40 set to the low lift amount, and the first compression stroke are performed. After the first intake stroke and compression stroke, the second intake stroke is performed with the intake valve 40 set to the normal lift amount and second compression stroke are performed, after which the expansion stroke and exhaust stroke are performed. After that, processing returns to step S512. The six-stroke operation of steps S512, S514, and S516 is repeated until it is determined step S514 that the temperature T24 in the combustion chamber 24 reaches or exceeds the threshold cylinder temperature T0.

It is not determined at step S510 that the auxiliary bank of cylinders 12 is being restored from the cold condition, and it is determined at step S514 that the temperature T24 is greater than or equal to the threshold cylinder temperature T0, at step S518 normal four-stroke operation is executed, and all-cylinder operation is performed immediately. After that, the processing is ended.

As described above, according to the fifth embodiment, even when restoring the operation of the auxiliary bank that had been stopped, by performing six-stroke operation to raise the temperature, it is possible to more quickly raise the temperature in the combustion chambers 24 on the auxiliary bank, enabling stabilization of combustion.

In the system of the fifth embodiment, it is possible to start with reduced-cylinder operation even when the cold start of the internal combustion engine 10 is carried out. In the case of performing a cold start with reduced-cylinder operation, the routine of FIG. 4 is performed in the same manner as in the first embodiment, and the auxiliary bank only is operated in six-stroke operation until the temperature T24 in the combustion chambers 24 on the auxiliary bank rises to the threshold cylinder temperature T0. In this manner, even when cold start of the internal combustion engine 10 is carried out, it is possible by performing six-stroke operation to raise the temperature in the combustion chambers 24 on the auxiliary bank to more quickly stabilize combustion. Because it is possible in this manner to perform reduced-cylinder operation even in the cold condition, it is possible to improve fuel economy.

In the fifth embodiment, for example, by executing step S510 the “cold starting determining means” may be implemented, and by executing step S516 the “multi-stroke operation means” may be implemented.

In the case in which references are made to numbers of elements, quantities, and ranges in the like in the foregoing embodiments, unless the numbers are explicitly stated or clear in principle as specific numbers, there is no restriction to the stated numbers. Additionally, the structures and methods of steps described in the embodiments, unless explicitly stated or clear in principle as specified structures and methods, are not necessarily essential to the present invention.

Claims

1. A control apparatus for an internal combustion engine comprising:

a variable valve driving device that changes the timing of the opening and closing and lift amount of an intake valve disposed in an intake port that communicates with a cylinder of the internal combustion engine;

a valve timing control portion that controls the timing of the opening and closing and lift amount of the intake valve by the variable valve driving device;

a cold start determining portion that determines whether the internal combustion engine is being started from a cold start; and

a multi-stroke operation setting portion that sets a multi-stroke operation, in which one combustion cycle of the internal combustion engine includes two or more intake and compression strokes, when the cold start determining portion determines that the internal combustion engine is being started from the cold start, wherein the multi-stroke operation is formed by a first intake stroke and a first compression stroke and a second intake stroke and a second compression stroke, followed by a combustion stroke and an exhaust stroke,

wherein the valve timing control portion controls the lift of the intake valve during the first intake and the compression strokes to a low lift amount, which is smaller than the normal lift amount required for intake of a requested intake air amount, and controls the lift of the intake valve in the second intake and the second compression stroke to the normal lift amount.

2. The control apparatus according to claim 1, wherein

the low lift amount is the lift at which the pumping loss during the first intake and the first compression stroke is maximized.

3. The control apparatus according to claim 1, further comprising:

an ignition timing control portion that controls ignition timing by a spark plug disposed in the cylinder, wherein the ignition timing control portion prohibits ignition during the first intake and the first compression stroke.

4. The control apparatus according to claim 1, wherein

the multi-stroke operation setting portion sets the execution of, during one combustion cycle, a plurality of repetitions of the first intake stroke and the first compression stroke, followed by performing the second intake stroke and the second compression stroke.

5. The control apparatus according to claim 1, further comprising:

a multi-stroke operation termination determining portion that determines whether or not multi-stroke operation is to be terminated; and

a four-stroke operation setting portion that sets one combustion cycle of the combustion of the internal combustion engine to four-stroke operation, which includes an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, if the multi-stroke operation termination determining portion determines that multi-stroke operation is to be terminated.

6. The control apparatus according to claim 5, further comprising:

a temperature detector that detects a temperature in the cylinder wherein

the multi-stroke operation termination determining portion determines that multi-stroke operation is to be terminated if a temperature in the cylinder is greater than or equal to a threshold cylinder temperature.

7. The control apparatus according to claim 5, further comprising:

a coolant temperature detector that detects a temperature of the coolant of the internal combustion engine wherein

the multi-stroke operation termination determining portion determines that multi-stroke operation is to be terminated if the temperature of the coolant is greater than or equal to a threshold coolant temperature.

8. The control apparatus according to claim 5, further comprising:

a requested load calculation portion that calculates a requested load on the internal combustion engine wherein

the multi-stroke operation termination determining portion determines that multi-stroke operation is to be terminated if the calculated requested load is greater than or equal to a threshold engine load.

9. The control apparatus according to claim 5, further comprising:

a cylinder temperature predicting portion that predicts, before starting the first intake stroke and the first compression stroke, in one combustion cycle, a temperature in a cylinder after performing the second intake stroke and the second compression stroke wherein

the multi-stroke operation termination determining portion determines that multi-stroke operation is to be terminated if the predicted temperature in the cylinder is greater than or equal to a threshold predicted cylinder temperature.

10. The control apparatus according to claim 6, wherein

the internal combustion engine uses a fuel including alcohol as a fuel, and the control apparatus further includes a determining value setting portion that sets any one of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, and the threshold predicted cylinder temperature in accordance with the concentration of alcohol in the fuel.

11. The control apparatus according to claim 1, wherein

the internal combustion engine has a first cylinder group and a second cylinder group, and wherein

the control apparatus operates only cylinders belonging to the first cylinder group and includes

a reduced-cylinder operation setting portion that sets cylinders belonging to the second cylinder group to reduced cylinder operation, in which the cylinders are stopped, and

an all-cylinder operation setting portion that sets all cylinders belonging to the first cylinder group and cylinders belonging to the second cylinder group to all-cylinder operation, in which all cylinders are operated, wherein

the cold start determining portion determines whether restoration of operation of the cylinders belonging to the second group of cylinders is a cold start when the engine transitions from reduced-cylinder operation to all-cylinder operation, and

the multi-stroke operation setting portion sets the operation of cylinders belonging to the second cylinder group to multi-stroke operation when the cold start determining portion determines that restoration of operation of the cylinders belonging to the second cylinder group of cylinders is the cold start.

12. The control apparatus according to claim 1, wherein

the variable valve driving device has an intake cam that drives the opening and closing of the intake valve and an electrical motor rotationally driving the intake cam, wherein the valve timing control portion controls the valve timing by controlling the rotational drive of the intake cam using the electrical motor.

13. A method for controlling an internal combustion engine comprising:

determining whether the internal combustion engine is being started from a cold start;

executing multi-stroke operation, which includes, in one combustion cycle in the internal combustion engine, two or more intake and compression strokes, formed by a first intake stroke and a first compression stroke and a second intake stroke and compression stroke, followed by a combustion stroke and an exhaust stroke; and

controlling a lift of an intake valve during the first intake stroke and the first compression stroke to a low lift amount, which is smaller than the normal lift amount required for intake of a requested intake air amount, and controlling the lift of the intake valve in the second intake stroke and the second compression stroke to the normal lift amount.

14. The control apparatus according to claim 7, wherein

the internal combustion engine uses a fuel including alcohol as a fuel, and the control apparatus further includes a determining value setting portion that sets any one of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, and the threshold predicted cylinder temperature in accordance with the concentration of alcohol in the fuel.

15. The control apparatus according to claim 8, wherein

the internal combustion engine uses a fuel including alcohol as a fuel, and the control apparatus further includes a determining value setting portion that sets any one of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, and the threshold predicted cylinder temperature in accordance with the concentration of alcohol in the fuel.

16. The control apparatus according to claim 9, wherein

the internal combustion engine uses a fuel including alcohol as a fuel, and the control apparatus further includes a determining value setting portion that sets any one of the threshold cylinder temperature, the threshold coolant temperature, the threshold engine load, and the threshold predicted cylinder temperature in accordance with the concentration of alcohol in the fuel.