Method for operating an internal combustion engine having two inlet valves

Abstract

The invention relates to a method for controlling an internal combustion engine, which has a first inlet valve (P) and a second inlet valve (S) per cylinder, which can be operated independently of one another between maximum operation (P1, S1) and minimum operation. The change of the inlet valves takes place preferably by engine oil pressure, in a basic state (P1+S2) the first inlet valve (P) is in maximum operation (P1) and the second inlet valve (S) is in minimum operation (S2). From the basic state, the valve control device switches the first inlet valve (P) into a minimum operation (P3) at low engine rotational speeds/engine torques and sufficient engine oil pressure and switches the second inlet valve (S) into maximum operation (S1) at high engine rotational speeds/engine torques.

Description

FIELD OF THE INVENTION

[0001] The invention is a method for operating an internal combustion engine having a first and a second inlet valve in one cylinder, which are capable of being changed independently between maximum operation and minimum operation. The invention relates, furthermore, to an internal combustion engine for carrying out a method of this type.

BACKGROUND OF THE INVENTION

[0002] Motor vehicle engines with spark ignition, which, in practice, use almost exclusively gasoline as fuel, are operated predominantly with a stoichiometric air/fuel mixture, which allows highly efficient treatment of the exhaust gas emissions. Torque control is provided by throttling. In part load operation of such an engine, however, fuel consumption increases due to throttling and to stoichiometric operation. These losses can be reduced by homogeneous or stratified lean operation, although this has the disadvantage that the NOx emissions of the engine cannot be treated at high efficiency by a conventional three-way catalytic converter. Three-way catalysts with NOx trapping capability are considerably more costly than conventional three-way catalytic converters are used to remove nitrogen oxides for lean burning systems. Furthermore, when stratified direct injection is employed, the systems for the provision of air, fuel, exhaust gas recirculation, and ignition are also all considerably more complex and costly than in a conventional engine. Even pressure charging, for improving fuel consumption in stoichiometric operation, is more costly due to the turbocharger with intermediate cooler.

SUMMARY OF THE INVENTION

[0003] A more cost-effective approach to improving engine efficiency is by variable valve control in which the opening durations, closing times, and/or valve stroke of the inlet valves and/or the exhaust valves is adjusted. One alternative to provide variable valve control is for the valve control device to have two or three valve opening profiles (referred to briefly below as “valve profiles”), which differ in terms of duration and/or valve stroke.

[0004] Furthermore, in internal combustion engines having two or more inlet valves per cylinder, the inlet valves of a cylinder may be equipped in each case with variable valve controls, which are independent of one another, thus allowing a multiplicity of control possibilities for engine operation.

[0005] Against this background, the object of the present invention was to provide means for efficient operation of an internal combustion engine having a plurality of inlet valves per cylinder, which can be changed independently of one another between minimum operation and maximum operation.

[0006] A method operate an internal combustion engine which has at least one cylinder with at least one first and one second inlet valve is disclosed. Preferably, the first inlet valve is arranged in a primary inlet duct, via which the main quantity of fresh air is supplied, and the second inlet valve is arranged in a secondary inlet duct, via which a smaller quantity of air for swirling is supplied. The inlet valves are capable of being changed independently from each other between:

[0007] (a) maximum operation, which is defined by a larger valve stroke and/or a longer opening duration, and

[0008] (b) minimum operation, which is defined by a smaller valve stroke and/or a short opening duration.

[0009] The shorter valve opening durations in minimum operation preferably are such that they lie completely within the valve opening durations in maximum operation. Moreover, the valve opening duration in minimum operation is preferably shorter than the intake stroke of the internal combustion engine. The method is defined in that, in a basic state, the first inlet valve is used in maximum operation and the second inlet valve in minimum operation.

[0010] In the method described, by the relatively simple changeover of valve operation between two types of operation (minimum operation, maximum operation), a broad spectrum of control possibilities can be covered since two inlet valves of a cylinder can be changed independently. In theory, by a combination of the various types of operation of the two inlet valves, a minimum of four operating modes can be set for the internal combustion engine. According to the method, an operating mode, in which the first of the inlet valves is used in maximum operation and the other inlet valve is used in minimum operation, serves as a basic state, which is assumed during most of the time of the “normal” driving operation of a motor vehicle, i.e., the most commonly used rotational speed and torque ranges. The advantage of a basic state of this kind, with mixed maximum and minimum operation of the inlet valves, is that this constitutes an optimum starting point both for an increase in power of the internal combustion engine and for a transition to minimum power requirements, for example during idling. Furthermore, owing to the maximum operation of one of the inlet valves, the basic state makes available a certain minimum performance of the internal combustion engine, which ensures a reliable functioning of connected assemblies such as, for example, an alternator, or of pressure pumps, and therefore the controllability of the motor vehicle functions. At the same time, due to the minimum operation of the other inlet valve, an asymmetric air inlet and consequently a high swirling in the cylinder are achieved. This leads to a good mixing of air and fuel and to good combustion. The basic state thus has advantageous properties also with regard to fuel utilization and engine efficiency. The basic state is preferably implemented automatically by the inlet valves in the event of the absence of active (that is to say, energy requiring) control of the inlet valves. That is to say, the basic state is a default state which is assumed automatically in the event of a possible anomaly of valve control owing to the mixing of maximum operation and minimum operation, reliable engine operation is ensured in the basic or default state.

[0011] Alternatively, the changeover of the inlet valve between minimum operation and maximum operation is brought about by the pressure of the engine oil. In the event of a low engine oil pressure below a predetermined threshold value, the inlet valves are operated in the basic state. The basic state is therefore assumed automatically whenever the available engine oil pressure is insufficient for switching the inlet valves, for example because the engine has been idling for a relatively long time, because there is a high oil temperature or because the oil system is disrupted. In the basic state, a spontaneous increase in power of the internal combustion engine is therefore possible from idling, the increased power subsequently ensuring an increase in the engine oil pressure and thus ensuring the complete switchability of the valve control.

[0012] In one embodiment, the capacity of the inlet valves to change independently of one another is used to operate the engine optimally according to the following. In the case of a low to medium engine rotational speed and a low to medium engine torque, both inlet valves can be used in minimum operation. When both inlet valves are in minimum operation, the air quantity supplied and consequently also the associated fuel consumption are minimum. If, as described above, the changeover of the inlet valve takes place with the aid of the engine oil pressure, the operating state of minimum operation of both inlet valves is carried out preferably only in the case of a sufficiently high engine oil pressure or in the case of an engine oil temperature which has not increased, so that the capability of the inlet valves to change over and consequently the transition into another operating state remain possible at any time. In the case of a high engine rotational speed and/or a high engine torque, both inlet valves are used in maximum operation to provide as much air as possible.

[0013] The transition from one operating mode to another operating mode of the inlet valves preferably takes place in such a way that only one of the inlet valves changes its type of operation. Thereby, changes in engine behavior associated with the inlet valve mode change are kept as small as possible, so that they are noticed to the least possible extent by the driver or can be compensated relatively simply by other devices such as a change in the throttle setting, spark timing or in fuel injection. An operating mode change of this kind due to a change in the type of operation of only one inlet valve is implemented in the chain of the above-described operating modes (both inlet valves in minimum operation; basic state; both inlet valves in maximum operation).

[0014] The valve stroke and/or the valve opening duration of the first inlet valve in minimum operation are/is preferably shorter than the corresponding size when the second inlet valve is in minimum operation. In particular, the maximum valve stroke of the first inlet valve in minimum operation may be less than 40%, preferably less than one third, of the maximum valve stroke of the second inlet valve in minimum operation. Thereby, when both inlet valves are in minimum operation, most air is sucked in through the second inlet valve, thus ensuring high swirling in the cylinder due to the asymmetry.

[0015] The valve stroke of the second inlet valve in the minimum operation is preferably less than 40%, particularly less than one third, of the valve stroke of the first inlet valve in the maximum operation. During the basic state, therefore, the predominant fraction of the inlet air flows through the first inlet valve with a relatively low flow resistance. By contrast, considerably less air flows through the second inlet valve, although the corresponding quantity advantageously brings about swirling in the cylinder.

[0016] The invention relates, furthermore, to an internal combustion engine which contains the following components:

[0017] a) At least one cylinder with at least two inlet valves.

[0018] b) A variable valve control device which can changeover the inlet valves independently of one another between (a) maximum operation with a larger valve stroke and/or a longer opening duration and (b) minimum operation with a small valve stroke and/or a shorter opening duration. The valve control device may contain, in particular, a camshaft with different inlet cams for minimum operation and maximum operation, between which the changeover can be made.

[0019] c) An engine control which is connected to the valve control device and is set up to operate the internal combustion engine according to a method of the type explained above. That is to say, in a basic state, the first inlet valve is used in maximum operation and the second inlet valve in minimum operation. The internal combustion engine or the engine control may be developed in such a way that it can also implement the variants of said method.

[0020] According to the preferred development of the internal combustion engine, the valve control device is connected to the engine oil system of the internal combustion engine and is designed to change over the inlet valves by means of selective action by an engine oil pressure. The use of the engine oil pressure for switching purposes has the advantage that known hydraulic methods can be adopted. Preferably, furthermore, the valve control device is set up to operate the first inlet valve in maximum operation and the second inlet valve in minimum operation when the engine oil pressure lies below a predetermined threshold pressure. This setting thus constitutes a default state which is assumed when, for any reason, the engine oil pressure is not sufficient for changeover operation.

[0021] The invention is explained in more detail below by way of example with the aid of the Figures of which:

[0022] FIG. 1 is a graph of valve stroke versus crank angle for inlet and exhaust valves ∘ an internal combustion engine showing three operating states;

[0023] FIG. 2 is a graph of the full torque curves in an engine state graph of the torque versus engine rpm for the three operating states of the inlet valve, as illustrated in FIG. 1;

[0024] FIG. 3 is a schematic of the first and second inlet valves in the port of an internal combustion engine and graphs of the associated types of operation according to the invention of the valves; and

[0025] FIG. 4 is a graph of the full torque curve in an engine state graph of the torque versus engine rpm showing the engine control, according to the present invention, indicating operating mode.

DETAILED DESCRIPTION

[0026] FIG. 1 shows diagrammatically the valve profiles of an exhaust valve EX and of an inlet valve IN of the valve stroke H (vertical axis) against crankshaft angle (horizontal axis) where TDC is top dead center and BDC is bottom dead center of piston travel. The inlet valve, which is of interest in the present case, is operated by means of a valve control device capable of being changed over between three states 1, 2, 3, so that three valve profiles of the inlet valve are possible:

[0027] 1: Maximum operation under full torque, the valve opening duration being longer than the intake stroke;

[0028] 2: Intermediate operation for medium torques, the valve opening duration being shorter than the intake stroke;

[0029] 3: Minimum operation for engine idling with a very short opening duration and with a small maximum valve stroke.

[0030] With reduced valve opening duration in intermediate operation 2, the pumping loss normally associated with throttling of the engine can be reduced. Since the inlet valve is open for a shorter duration and/or stroke, the manifold pressure is raised. Consequently, the pressure drop across the throttle is less and the engine performs less pumping work to induct the desired amount of air into the cylinder.

[0031] FIG. 2 shows, in an engine state graph of torque, T, of the internal combustion engine versus engine rotational speed, n, the three full torque curves 1, 2 and 3 for wide open throttle operation for valve operation modes 1, 2 and 3, according to FIG. 1. Reduced valve stroke and valve opening duration reduces the achievable torque at all engine speeds. The minimum operation 3 of the inlet valve can be utilized, for example, only in a small region of area around the idling rotational speed n0. The intermediate operation 2 of the inlet valve affords the greatest advantages in the range of medium engine rotational speeds and of low torques. By contrast, maximum operation 1 opens up the entire operating field of the internal combustion engine, but has no advantages with regard to fuel consumption.

[0032] At idle, therefore, the minimum operation 3 of the inlet valve is preferable, and, in the event of an actuation of the accelerator pedal by the driver as command for spontaneous acceleration, a rapid changeover into intermediate operation 2 or, if a particularly sharp acceleration were required, even into maximum operation 3. However, in hydraulically switched valve control devices used conventionally, such a valve control leads to problems, since, during idling, engine oil pressure is typically insufficient for changing over the actuators. This is exacerbated by increased oil temperatures such that a sufficient engine oil pressure for changing over the actuators becomes available only at medium engine rotational speeds.

[0033] This could be overcome, in principle, by an oil pump having a higher performance than otherwise be used. However, this would largely nullify the advantages achieved by variable valve control. Thus, a valve control device which can change over between (at least) two types of operation of the inlet valves (minimum operation, maximum operation) is desired.

[0034] In this regard, FIG. 3 shows the valve device on which the solution according to the invention is based. The upper part of FIG. 3 illustrates a part of a cylinder head of an internal combustion engine with a first inlet valve P and with a second inlet valve S. Fresh air is supplied to the inlet valves via an intake manifold 10, the intake manifold 10 being divided into a primary inlet duct 11 to the first inlet valve P and a secondary inlet duct 12 to the second inlet valve S. The configuration of the ducts and the inlet valves P, S is preferably such that the air supplied through the second inlet valve S generates swirl in the cylinder to create turbulence to enhance the ensuing combustion of the air and fuel.

[0035] The two inlet valves are capable of being changed independently of one another between various types of operation with the aid of the engine oil pressure. The lower part of FIG. 3 illustrates these types of operation of the two inlet valves P, S as valve profiles in the graph of the valve stroke H against the crankshaft angle. Accordingly, the two inlet valves P, S have a maximum operation P1 and S1 with a large maximum valve stroke and with a long valve opening duration. In particular, the valve opening duration is preferably longer than the intake stroke of the internal combustion engine. The valve profiles P1 and S1 of the two inlet valves are otherwise similar to one another.

[0036] Furthermore, the two inlet valves may also be operated in each case in minimum operation P3 and S2. In this case, they have very much smaller maximum valve strokes and/or shorter valve opening durations than in maximum operation P1 or S1. In particular, the valve opening duration is shorter than the intake stroke of the internal combustion engine.

[0037] Moreover, FIG. 3 indicates that

[0038] in minimum operation, P3, of the first inlet valve, P, the maximum valve stroke is only about one third of the maximum valve stroke of the second inlet valve S in minimum operation S2 of the latter; and

[0039] the maximum valve stroke of the second inlet valve S in minimum operation S2 of the latter is only about one third of the maximum valve stroke of the first inlet valve P in maximum operation, P1, of the latter.

[0040] FIG. 4 shows an engine state graph (corresponding to FIG. 2) for the internal combustion engine or valve control discussed above with reference to FIG. 3. The possibility of being able to change over two inlet valves, P, S, of a multi-valve internal combustion engine independently between various cam profiles affords additional advantageous possibilities, as compared with the system of FIGS. 1 and 2.

[0041] In the combination of maximum operation, P1, of the first inlet valve P with minimum operation S2 of the second inlet valve S, designated in brief as “P1+S2”, the full torque of the internal combustion engine is available in the lower half of the rotational speed range, whereas, at higher rotational speeds, considerable losses of torque and power occur. However, in terms of fuel consumption, the asymmetric minimum/maximum operation of the inlet valves is also advantageous, since an asymmetric flow of the inlet air with turbulence is thereby generated, this being conducive to rapid and complete combustion. This, in turn, makes the engine tolerant to dilution with fresh air (lean combustion) or residual gases (exhaust gas circulation, variable camshaft control), which improve fuel efficiency.

[0042] In the lower rpm range, the P1+S2 combination satisfies, torque, acceleration response and allows for sufficient oil pressure for the valve control device. At the same time, advantages arise in regards to combustion stability and fuel consumption over double maximum operation P1+S1. The operating mode P1+S2 is suitable as a basic state or a standard setting not employing engine oil pressure. The basic state, P1+S2, is the “default state,” since this state is assumed in the absence of engine oil pressure (for whatever reasons), and sufficient driveability of the motor vehicle, while providing good fuel efficiency.

[0043] Investigations with motor vehicles show that four-cylinder gasoline engines with manual shift transmissions are operated predominantly between 1500 and 3000 rpm. Speeds between idling (approximately 700 rpm.) and 1500 rpm are not used often. Under normal operating conditions, that is to say with no excessively high oil temperature, oil pressure is much higher at 1500 rpm than during idling and is, therefore, sufficient to change over the actuators of the valve control device. According to FIG. 4, therefore, preferably the first inlet valve P is changed from maximum operation, P1, to minimum operation, P3, when the engine speed reaches approximately 1500 and approximately 3000 rpm and engine torque is light to medium. The zone of this operating mode, designated by “P3+S2,” is reached from the basic state, P1+S2, by changing one of the inlet valves, P.

[0044] At high engine speed, the second inlet valve S, too, is adjusted to maximum operation, S1, to provide a maximum of fresh air. At a predetermined medium engine rotational speed, therefore, the valve control device changes the second inlet valve S into. maximum operation S1, that is, the operating mode changes from P1+S2 to P1+S1. In the hydraulic valve control device described, this change takes place by pressure upon the inlet camshaft to the second inlet valve S.

[0045] According to FIG. 4, a distinction is thus made between three operating modes: the basic state or default operating condition, P1+S2 (achievable without engine oil pressure), the low torque operating state P3+S2 (requires engine oil pressure for the first inlet valve P), and the high torque operating mode P1+S1 (requires engine oil pressure for the second inlet valve S). It is advantageous that, in each operating mode, the use of engine oil pressure is necessary at most for one of the inlet valves.

[0046] Furthermore it is particularly advantageous that the transitions among operating modes is executed by only one of the inlet valves changing. In general, the transitions between the various operating modes, P3+S2 Û P1+S2 Û P1+S1 require not only a valve change, but also adjustment in the throttle valve position and spark timing to ensure a constant torque of the engine through the change. In the method described, such a transition control is made easier in that only the changeover is of a single inlet valve.

[0047] In summary, a preferred embodiment of the invention thus relates to a method for operating a four-stroke internal combustion engine having two inlet valves, P, S, per cylinder and a corresponding primary inlet duct 11 and secondary inlet duct 12, each inlet valve P, S being operated by an independent changeover device which provides either a higher and longer valve profile or lower and shorter valve profile, the lower/shorter profile lying completely within the higher/longer valve profile. The changeover among valve profiles is achieved using an engine oil pressure above a threshold value, by means of two independent oil control valves for the first and second inlet valves P, S of a cylinder bank. In a basic state, which doesn't relay on oil pressure, the first inlet valve, P, of each cylinder is operated with a maximum valve stroke, P1, and the second inlet valve, S, with a minimum valve stroke, S2, and with a shorter valve opening duration than the intake stroke duration. In the case of insufficient engine oil pressure for a changeover, it becomes possible, at low engine speeds, to have full torque by the provision of a full valve opening duration of the first inlet valve, while at the same time the combustion properties are improved by the generation of swirl as a result of the reduced opening of the second inlet valve.

[0048] The maximum valve stroke in minimum operation, S2, of the second inlet valve, S, is preferably smaller than the maximum valve stroke of the first inlet valve, P, in maximum operation, P1, of the latter, so that the predominant fraction of the fresh air supply takes place via the first inlet valve, P.

[0049] Under typical operating conditions with heated-up engine oil, at moderate engine speed and torque, the valve control device for the first inlet valve, P, is acted upon by pressure, so that these are changed over to minimum operation, P3, with a short valve stroke and a short valve opening duration, which are both smaller than the corresponding sizes in the case of the second inlet valve, S, in minimum operation, S2, of the latter. The second inlet valve, S, remains without action upon it by pressure and therefore in minimum operation, S2. The short valve opening durations of the two inlet valves, P, S, reduce the pumping losses in low-torque operation, and the asymmetric valve openings ensure swirls and turbulence in the cylinder to assist combustion.

[0050] The maximum valve stroke of the first inlet valve, P, in minimum operation, P3, preferably amounts to one third of the maximum valve stroke of the second inlet valve, S, in minimum operation, S2, of the latter, so that, owing to the very much higher flow resistance of the first inlet valve, P, the predominant fraction of the inlet air flows through the second inlet valve. Furthermore, under typical operating conditions of medium engine speed and medium engine torque, but an increased engine oil temperature and therefore a reduced engine oil pressure which is not sufficient for changing over the valve control device, the control device of the two inlet valves, P, S, operate in the basic state P1+S2. This makes it possible to have full engine torque and at the same time is conducive to obtaining the oil pressure in the rest of the engine.

[0051] During idling or at a low engine speed, the engine oil pressure is typically not sufficient for changing over the valve control device, so that it likewise remains in the basic state P1+S2, ensuring the availability of a full engine torque.

[0052] At medium to high engine rotational speeds, engine oil pressure is sufficient for changing the valve control device, so that the actuators for the second inlet valves, S, can be acted upon by pressure, so that the second inlet valves, S, can be used in maximum operation (similar to the maximum operation of the first inlet valve, P). The full torque and power potential of the internal combustion engine at high rotational speed is thereby available.

Claims

1. A method of operating an internal combustion engine with at least one cylinder, the cylinder having a primary intake valve and a secondary intake valve disposed therein, the method comprising:

operating the primary valve at a first lift profile and the secondary valve at the second lift profile as a default condition wherein the primary valve is capable of being switched between said first lift profile and a third lift profile and the secondary valve is capable of being switched between said second lift profile and a fourth lift profile and the primary and secondary valves are independently controllable.

2. The method of claim 1 wherein said first lift profile has a taller valve stroke than said third lift profile and said fourth lift profile has a taller valve stroke than said second lift profile.

3. The method of claim 1 wherein said first lift profile has a longer opening duration than said third lift profile and said fourth lift profile has a longer opening duration than said second lift profile.

4. The method of claim 1 wherein a valve stroke of the secondary valve operating with said second lift profile is less than 40% of a valve stroke of the primary valve with said first lift profile.

5. The method of claim 1 wherein said third lift profile of the primary valve and said fourth profile of the secondary valve are caused to be selected by oil pressure acting on a valve change device.

6. The method of claim 1 wherein the primary valve operates with said first lift profile and the secondary valve operates with said second lift profile when oil pressure is below an actuation pressure.

7. The method of claim 1, further comprising: operating the primary valve at said first lift profile and the secondary valve at said fourth lift profile in response to at least one of engine speed and torque.

8. The method of claim 1, further comprising: operating the primary valve at said first lift profile and the secondary valve at said fourth lift profile when engine speed and torque are in a higher range.

9. The method of claim 1, further comprising: operating the primary valve at said second lift profile and the secondary valve at said third lift profile in response to at least one of engine speed and torque.

10. The method of claim 1, further comprising: operating the primary valve at said second lift profile and the secondary valve at said third lift profile when engine speed and torque are in a lower range.

11. An internal combustion engine, comprising: at least one engine cylinder;

a primary inlet valve disposed in each of said cylinders;

a secondary inlet valve disposed in each of said cylinders;

a first variable valve control device allowing switching of the primary inlet valve between a first and a third lift profile;

a second variable valve control device allowing switching of the secondary inlet valve between a second lift profile and a fourth lift profile; and

an engine control unit coupled to the engine and to said first and second variable valve control devices, said engine control unit controlling said variable valve control devices based on a speed and torque of the engine.

12. The engine of claim 11 wherein said variable valve control devices are actuated by engine oil pressure.

13. The engine of claim 11 wherein said engine control unit manages oil pressure actuation of said variable valve control devices.

14. The engine of claim 11 wherein a default operating condition is when said primary inlet valve is operated at said first lift profile and said secondary valve at said second lift profile as a default condition.

15. The engine of claim 14 wherein said first lift profile has a taller valve stroke than said third lift profile and said fourth lift profile has a taller valve stroke than said second lift profile.

16. The engine of claim 14 wherein said first lift profile has a longer opening duration than said third lift profile and said fourth lift profile has a longer opening duration than said second lift profile.

17. The engine of claim 14 wherein said default operating condition is requested by said engine control unit when engine speed and torque is in a medium range.

18. The engine of claim 14 wherein said engine control unit causes switching of said first valve control device to said third lift profile at a lower range of engine speed and torque.

19. The engine of claim 14 wherein said engine control unit causes switching of said second valve control device to said fourth lift profile at a higher range of engine speed and torque.