Method for operating an internal combustion engine and control unit

Abstract

Method for operating an internal combustion engine (1) with - a crankshaft which can be driven by a piston of a cylinder (4), - an intake manifold through which the cylinder (4) can be supplied with fresh air, - an inlet valve (5) through which, when open, fresh air can flow from the intake manifold into the cylinder (4), - a variable valve train (3) by which the opening duration or the relative opening time of the inlet valve (5) is variable with respect to the crankshaft position, characterized in that - when starting the internal combustion engine (1) in the event that the intake manifold pressure deviates from the intake manifold setpoint pressure (p s ) deviates, cylinder filling pre-control (4) is achieved by the variable valve train by adjusting the fresh air supply compared to the fresh air supply at intake manifold set pressure (p s ) is reduced, - the procedure is applied when re-igniting the internal combustion engine (1) after a coasting phase in which the internal combustion engine (1) is towed without fuel supply and with reduced or no air mass flow in the cylinder (4) and - during the re-ignition of the internal combustion engine (1) after the overrun phase, the intake valve timing is continuously advanced, whereby the rate of adjustment of the intake valve timing is determined in the event that the intake manifold pressure deviates from the intake manifold setpoint pressure (p s ) deviates, compared to the adjustment speed at intake manifold setpoint pressure (p s ) is reduced.

Description

[0001] The invention relates to a method for operating an internal combustion engine with a crankshaft which can be driven by a piston of a cylinder, an intake manifold through which the cylinder can be supplied with fresh air, an intake valve through which, in the open state, fresh air can flow from the intake manifold into the cylinder, and a variable valve train by which the opening duration or the relative opening time of the intake valve is variable with respect to the crankshaft position. Background of the invention

[0002] Modern internal combustion engines in motor vehicles, hereinafter also referred to as combustion engines, are increasingly not operated continuously, but rather are towed during certain operating phases. Internal combustion engines in motor vehicles are generally connected to the vehicle's wheels via the drivetrain and a clutch. During deceleration, the internal combustion engine is towed along by the inertia of the vehicle via the closed drivetrain, with the vehicle being slowed by a drag torque exerted by the internal combustion engine. The operating mode of the internal combustion engine, in which the engine is towed without fuel injection into the cylinders, is called overrun operation.

[0003] The drag torque of the combustion engine is primarily caused by friction and charge exchange losses. However, particularly in hybrid drive systems, it may be desirable to minimize the drag torque of the combustion engine in order to utilize the torque generated by the drivetrain (resulting from the moment of inertia) for electrical energy recuperation during deceleration with the powertrain closed. In such a case, it is desirable to minimize the drag torque of the unpowered combustion engine.

[0004] It is known from the prior art to use variable valve trains to influence the drag torque. DE 199 32 665 A1 discloses a method for controlling the gas exchange valves of an internal combustion engine by means of a variable valve timing system, in which the intake valves are variably controlled during overrun.

[0005] However, solely focusing on minimal engine braking leads to undesirable side effects. For example, when the throttle valve is opened during deceleration, losses due to the flow of fresh air through the cylinders, charge exchange losses, and compression losses from engine braking still occur. The cold, oxygen-rich fresh air passing through the combustion chamber lowers the temperature in the exhaust system and causes the exhaust aftertreatment system to exceed its optimal temperature range. In the case of a three-way catalytic converter, oxygen saturation is also critical, because as soon as the engine switches from deceleration back to combustion operation, the oxygen saturation must be compensated for by a temporarily richer fuel mixture.Intermittent temperature drops the efficiency of the catalytic converter, and the subsequent enrichment of the mixture after switching to drive mode leads to increased fuel consumption and thus further increased emissions. To operate the catalytic converter within its optimal lambda range, an excess of oxygen must therefore be avoided. Furthermore, if the combustion engine is a gasoline engine equipped with a particulate filter, the oxygen can cause uncontrolled and unwanted regeneration of the filter. This can lead to thermal overload, which damages not only the gasoline particulate filter itself but also other components.

[0006] Closing the throttle valve can also be problematic. If a critical vacuum develops in the combustion chamber, a negative pressure differential to the crankcase can force air and oil into the combustion chamber. When combustion resumes, this results in increased emissions and higher oil consumption of the internal combustion engine.

[0007] Air scavenging of the engine during overrun phases can also be prevented by variable valve train systems. This can be achieved, for example, by deactivating all valve lifts, using fully variable intake lift control, or by combining exhaust valve deactivation with extended intake phase adjustment. Such systems are known from DE 10 2016 216 116 A1, DE 10 2008 036 635 A1, DE 10 2015 107 539 A1, DE 10 2013 202 196 A1, DE 10 2017 011 301 B3, DE 199 52 037 A1, or WO 2013 / 101 282 A2. DE 10 2006 031 572 B4 discloses a generic method.

[0008] All concepts that result in an effective zero mass flow during the overrun phase have in common that, within this phase, the intake manifold pressure between the throttle valve and the intake valves rises continuously until complete pressure equalization with atmospheric pressure is achieved. Due to the effective zero mass flow across the throttle valve, it loses its throttling effect and cannot be used for cylinder filling control during the first combustion cycles immediately after the engine restarts.

[0009] If the engine is restarted nonetheless, this leads to an increased filling of the cylinders with fresh air during the first few operating cycles. A high fresh air mass in conjunction with combustion at λ = 1 results in a high torque output from the internal combustion engine immediately after restarting. This condition persists until the intake manifold volume is "emptied" and the throttle valve can again ensure proper charge control. DE 10 2016 111 505 A1 presents such a method, according to which a transition control unit is provided that first restores the vacuum in the intake manifold between overrun and combustion operation in order to then switch to regular combustion operation. Passing through the phase determined by the transition control unit is mandatory and therefore occurs even if sufficient vacuum is present. Maintaining this phase is complex to implement and can be perceived as a time-delayed restart of the internal combustion engine.

[0010] One possible solution to avoid the torque peak is to adjust the ignition timing, thereby actively induced a decrease in combustion efficiency. This measure leads to increased fuel consumption. Object of the invention

[0011] The object of the invention is to resolve the aforementioned conflicting objectives and to provide a method for operating an internal combustion engine that enables a change between combustion and overrun operation with low emissions and simultaneously allows for the fastest and most convenient possible exit from overrun operation, particularly when the desired drive torque is low and the engine restart is to take place under low load. Furthermore, it is an object of the invention to provide a control unit for an internal combustion engine that enables low-emission operation.

[0012] This problem is solved by the method for operating an internal combustion engine according to claim 1 and by a control unit according to claim 8. Further embodiments are specified in the dependent claims.

[0013] The method according to the invention relates to a method for starting, preferably restarting, the internal combustion engine after a coasting phase. The method according to the invention is described below using a variable valve train with a camshaft adjuster. For this purpose, the internal combustion engine is equipped with an intake camshaft and, for example, an electromechanically adjustable camshaft adjuster for the intake valves, and an exhaust camshaft and an electromechanically adjustable camshaft adjuster for actuating the exhaust valves. The valve timing and / or the valve lift can also be varied electrohydraulically or by other means.

[0014] During the overrun phase, the air mass flow through the variable valve train is reduced to avoid the aforementioned disadvantages. This can be achieved by adjusting the timing of a camshaft adjuster to a value that is not practical for spark-ignition operation, but which represents a phase angle optimized for engine braking. The phase angle of the intake and exhaust camshafts, resulting in reduced engine braking torque, enables effective energy recuperation without generating an air mass flow through the catalytic converter or creating a critical vacuum in the combustion chamber.

[0015] When entering the overrun phase, the internal combustion engine switches from an operating point with power output to an operating point with power input. Before entering the overrun phase, the intake valves typically open shortly after top dead center (TDC). Meanwhile, the exhaust valves typically close shortly before TDC.

[0016] During overrun, the engine is dragged by the rolling vehicle via the transmission. This involves a change in the operating point. The camshaft adjusters, preferably at adjustment speeds typical for these systems, adjust to the target angles, so that the intake valves now open significantly after top dead center (TDC) and the exhaust valves close significantly before TDC. This adjustment usually occurs as quickly as possible. Simultaneously, the throttle valve is briefly opened to establish constant conditions in the intake manifold as quickly as possible. As a result of these changes, the engine valves are open near bottom dead center (BDC) during overrun. Only a small mass of air is moved, which is drawn in and expelled equally from both the intake and exhaust manifolds. The air mass flow through the respective valves is balanced at zero.This minimizes internal friction and pumping losses caused by intake, compression, expansion, and exhaust, thus reducing vehicle braking as much as possible. At the same time, it largely avoids the air mass flow induced by the combustion engine that would cool the exhaust system.

[0017] If the internal combustion engine is part of a hybrid engine unit, it is advantageous to minimize the engine's drag torque in addition to eliminating air mass flow through the exhaust system. This can be achieved by minimizing the air mass flow while simultaneously achieving the lowest possible drag torque. Alternatively, the drag torque can be minimized while simultaneously achieving the lowest possible air mass flow. It is also possible to apply one of these methods depending on an external control variable, such as the temperature of the exhaust aftertreatment system.Depending on the characteristic curve formed by the parameters drag torque and air mass flow, both parameters can also be reduced to a range near their minimum if, for example, the gradient of the parameters is low, so that the control system is particularly insensitive to changing external parameters and does not require readjustment. This simplifies the implementation of the control strategy.

[0018] When combustion restarts to deliver power from the internal combustion engine, the camshaft adjusters move to the target angles for engine restart. According to the invention, the camshaft adjustment does not always occur as quickly as possible, but is delayed, at least when the pressure in the intake manifold deviates from the target intake manifold pressure and the load requirement is low. This prevents unwanted torque peaks, which negatively affect drivability, particularly when ambient pressure has settled in the intake manifold.

[0019] While torque peaks could be avoided by adjusting the ignition timing, this leads to a decrease in combustion efficiency and is therefore energy-inefficient, as it results in unwanted increased fuel consumption. In contrast, the proposed charge control via the valve train reduces the theoretically achievable adjustment speed when the intake manifold pressure falls below a certain threshold and / or when engine restart at low load (gentle engagement) is desired.

[0020] If the engine has restarted and the intake manifold volume has not yet been emptied, the valve train can still be used for charge control after the restart phase until conventional charge control, for example via the throttle valve, can be used again effectively. If the intake manifold volume has already been emptied during the restart, conventional control can also be used. Finally, the valve train can be used for charge control in parallel with the throttle valve.

[0021] In a further development of the invention, the cylinder's fresh air supply is reduced by the variable valve train so that the resulting torque does not exceed the target torque setting or exceeds it by less than 50%. The first variant enables a particularly smooth re-engagement of the internal combustion engine without a perceptible torque peak. In the second variant, the internal combustion engine engages more quickly, but the torque peak that would occur without the charge control is reduced.

[0022] During restarting the internal combustion engine after the overrun phase, the intake valve timing is continuously advanced. The rate of this advance is reduced when the intake manifold pressure deviates from the target pressure, compared to when the target pressure is reached. This continuous advance can be controlled or regulated based on the intake manifold pressure.

[0023] In a preferred embodiment, no ignition angle adjustment is made when re-igniting the internal combustion engine.

[0024] The invention further relates to a control unit with which an internal combustion engine can be operated using the presented method.

[0025] An embodiment of the invention is described in more detail below with reference to drawings. Brief description of the drawings

[0026] They show: Fig. 1a a schematic, temporal development of the injection activity of a first internal combustion engine without variable valve train according to the state of the art during entry into and exit from the overrun phase, Fig. 1b a schematic, temporal development of the intake manifold pressure of the first internal combustion engine without variable valve train during entry into and exit from the overrun phase, Fig. 1c a schematic, temporal development of the intake valve closing time in °KW after TDC of the first internal combustion engine without variable valve train during entry into and exit from the overrun phase, Fig. 1d a schematic, temporal development of the engine torque in Nm of the first internal combustion engine without variable valve train during entry into and exit from the overrun phase, Fig. 2a a schematic, temporal development of the injection activity of a second internal combustion engine with variable valve train without charge pilot control according to the state of the art during entry into and exit from the overrun phase, Fig. 2b a schematic, temporal development of the intake manifold pressure of the second internal combustion engine with variable valve train without charge pilot control during entry into and exit from the overrun phase, Fig. 2c a schematic, temporal development of the intake valve closing time in °KW after TDC of the second internal combustion engine with variable valve train without charge pilot control during entry into and exit from the overrun phase, Fig. 2d a schematic, temporal development of the engine torque in Nm of the second internal combustion engine with variable valve train without charge control during entry into and exit from the overrun phase, Fig. 3a a schematic, temporal development of the injection activity of a third internal combustion engine according to the invention with variable valve train and charge control during entry into and exit from the overrun phase, Fig. 3b a schematic, temporal development of the intake manifold pressure of the third internal combustion engine according to the invention with variable valve train and charge control during entry into and exit from the overrun phase, Fig. 3c the temporal development of the inlet valve closing time in °KW after TDC of the third internal combustion engine according to the invention with variable valve train and charge control during entry into and exit from the overrun phase, Fig. 3d a schematic, temporal development of the engine torque in Nm of the third internal combustion engine according to the invention with variable valve train and pre-control of filling during entry into and exit from the overrun phase and Fig. 4 a schematic internal combustion engine, Fig. 5 another schematic internal combustion engine, Detailed description of the drawings

[0027] The Fig. 4 and Fig. Figure 5 shows a partially and roughly schematically represented internal combustion engine 1 as a reciprocating piston engine with cylinders 4 and a crankshaft (not shown). It is designed as a four-cylinder in-line engine, whereby the invention can also be implemented in internal combustion engines 1 with a different number of cylinders and design. The valve train of the internal combustion engine 1, i.e., the valve train, is designated by 3. As a four-valve engine, the internal combustion engine 1 has two intake valves 5 and two exhaust valves 6 per cylinder 4. An intake camshaft is designated by 7, and an exhaust camshaft by 8. The intake camshaft 7 is adjustable by an intake-side camshaft adjuster 9, and the exhaust camshaft 10 is adjustable by an exhaust-side camshaft adjuster 10.In the exemplary embodiments considered, the camshaft adjusters 9, 10 are designed as electromechanical adjusters, each with an actuating mechanism designed as a wave gear, and each has an electric motor 11 for adjusting the phase position of the respective camshaft 7, 8 relative to the crankshaft of the internal combustion engine 1. In a manner known per se, the camshafts 7, 8 are driven by the crankshaft via a wrap-around drive or a gear drive, wherein a drive wheel 13 is fixedly connected to the housing of the actuating mechanism of the camshaft adjuster 9, 10 or is an integral part of this housing.

[0028] A control unit 12, which optionally performs further control tasks, is provided for controlling the camshaft adjusters 9, 10. Data connections between the control unit 12 and the camshaft adjusters 9, 10 are designated 15. A switching device 14 allows the exhaust valves 6 to be deactivated if necessary. The switching device 14 of the internal combustion engine according to Fig. 5 is electromechanically actuated and can be equipped with switchable rocker arms.

[0029] The Fig. Figures 1a to 1d schematically show the temporal development of some characteristic values ​​of a first internal combustion engine 1 according to the state of the art, which does not have a variable valve train. In a first firing phase 21, which lasts until time t1, the internal combustion engine 1 is fired. Fig. 1a digitally represents the injection 24 of the internal combustion engine 1, which occurs during firing phases 21 and 23 (value is 1) and is omitted during deceleration phases 22 (value is 0). At time t1, the internal combustion engine 1 is switched off, and at time t3, the internal combustion engine 1 is restarted in a re-firing phase 23. Injection is omitted between these times. At time t2, which is shortly before time t3, a torque input initiated by the driver or a control unit is applied, which initiates the restart process of the internal combustion engine 1. At time t3, the data processing for the restart of the internal combustion engine 1 is completed.

[0030] As soon as the vehicle enters the overrun phase 22, the injection 24 is suspended. The intake manifold pressure is regulated by the throttle valve. In the example shown, after Fig. 1b it is kept constantly low. The intake valve closing time 26, in Fig. As shown in Figure 1c, the throttle angle remains at the target angle for engine restart and is not varied. The throttle valve typically remains wide closed during the overrun phase 22. This allows for a torque-neutral restart to occur quickly after the driver's torque input, without any overshoot at the beginning of the re-ignition phase 23. The engine torque 27 ( Fig. 1d) therefore essentially corresponds to the target torque specification. However, during the overrun phase 22, air can enter the exhaust aftertreatment system, so that after restarting the engine the mixture must be enriched, which increases emissions.

[0031] The Fig. Figures 2a to 2d schematically show the temporal development of the characteristic values ​​of a second internal combustion engine 1 according to the state of the art, which, in contrast to the first internal combustion engine 1, has a variable valve train 3. The graph according to Fig. 2a corresponds to the graph of the Fig. 1a. Thus, in the first firing phase 21, the internal combustion engine 1 is fired, in the overrun phase 22 firing is omitted, and at time t3 the refiring phase 23 begins. The variable valve train 3 is used to prevent oxygen enrichment in the exhaust aftertreatment system. To this end, it prevents air scavenging of the engine in the overrun phase 22 by deactivating the exhaust valves 6 and adjusting the intake valves 5 to an extended range. In the overrun phase 22, the exhaust valve lifts are deactivated synchronously with the interruption of injection 24 and reactivated synchronously with the start of injection 24 upon restart. On the intake side, the variable valve train 3 can be used to minimize engine drag torque in the overrun phase 22.The reduced charge exchange work, particularly in combination with P0 and P1 hybrid vehicles, enables significant energy recuperation, thus increasing the overall efficiency of the powertrain. This is achieved by setting extremely late intake valve phase positions, which are not practical for combustion operation 21, 23, so that the maximum intake valve lift is located approximately at bottom dead center (BDC). Here too, the throttle valve position preferably remains almost completely closed during the overrun phase 22.

[0032] If the internal combustion engine 1 is to provide torque again, the intake valve phase position is quickly adjusted back to the conventional target position, as shown from Fig. 2c is evident between times t2 and t3. Injection 24 is omitted between these times. Already at time t2, which is shortly before time t3, a torque input initiated by the driver or a control unit is applied, which initiates the restart process of the internal combustion engine 1. At time t3, the data processing for the restart of the internal combustion engine 1 is completed.

[0033] During overrun phases 22, the intake manifold pressure 25 increases continuously, for example due to leaks. If the overrun phase 22 lasts relatively long, such as when driving downhill, the intake manifold pressure 25 ( Fig. 2b) within the overrun phase 22, the pressure rises to such an extent that it almost corresponds to the ambient atmospheric pressure. If re-ignition is initiated at an increased intake manifold pressure 25, this leads to a brief, strong torque build-up with a torque peak 27 due to the high air mass. However, the general objective is to engage the internal combustion engine 1 with low torque. In this case, the strong torque build-up leads to a loss of comfort.

[0034] The Fig. Figures 3a to 3d schematically show the temporal development of the characteristic values ​​of a third internal combustion engine 1, which, like the second internal combustion engine 1, has a variable valve train 3 and is operated with the method according to the invention. The graph according to Fig. 3a corresponds to the graph of the Fig. 2a. Again, in the first firing phase 21, the internal combustion engine 1 is fired, in the overrun phase 22 firing is omitted, and at time t3 the refiring phase 23 begins. The variable valve train 3 is again used to prevent air scavenging, so that the operating procedure is identical to that of the second internal combustion engine until the end of the overrun phase 22. As with the second internal combustion engine 1, the development of the intake manifold pressure is therefore also identical ( Fig. 3b).

[0035] At time t2, when the restart request for internal combustion engine 1 is triggered, the intake valve phase position is not adjusted as quickly as possible, but with a delay, to the conventional target position, unlike in the second internal combustion engine. The degree of delay depends on how much the intake manifold pressure 25 is increased and what load is demanded on internal combustion engine 1. As can be seen from the Fig.As can be seen in 3a to 3d, re-firing occurs at time t3, which begins even though the inlet valve time does not yet correspond to the valve time required for continuous operation under this load requirement, target valve time 29 at intake manifold target pressure p s , corresponds. The time difference t between t3 and t2 is the time required to reach the target angle of the pilot control. The adjustment to the set valve time 29 at the intake manifold set pressure p s This occurs as long as the intake manifold pressure has not yet reached its target pressure. Typical timing can be assumed, allowing for controlled adjustment; however, regulated adjustment is preferred. This also allows the adjustment speed to be adapted to the actual intake manifold pressure. Ideally, the adjustment of the intake valve closing time 26 is such that the engine torque build-up 27 is monotonous and as rapid as possible.

[0036] The variable valve train 3 is thus used to advance the intake valve closing time 26 during restarting the internal combustion engine 1. This makes it possible to avoid a torque peak 28 at increased pressure in the intake manifold. To achieve this, the variable valve train 3 deactivates the exhaust valves during the overrun phase 22 and reduces the engine drag torque by extremely retarding the intake valve lift. Based on the driver's torque input and the relevant input variables, the stored filling model calculates the target valve timing for a torque-neutral engine restart. In the example chosen, this means that the intake valve lift phase must be continuously advanced, depending on engine speed, but more slowly than in the case without advance control, until the intake manifold pressure is back at the target value and further load control, e.g., via the throttle valve, can take place. Reference symbol list 1 internal combustion engine 2 cylinder heads 3 Valve train 4 cylinders 5 Inlet valve 6 exhaust valve 7 Intake camshaft 8 exhaust camshaft 9 camshaft adjusters, intake side 10 camshaft adjusters, exhaust side 11 Electric motor 12 Control unit 13 Drive wheel 14 Switching device 15 Data connection 21 Firing phase 22. Shear phase 23 Re-firing phase 24 Injection 25 Intake manifold pressure 26 Intake valve closing time after TDC in °KW 27 Engine torque in Nm 28 Peak torque 29 Target valve time at suction pipe target pressure t time t1 Time of shutdown of the internal combustion engine t2 Time of restart request of the internal combustion engine t3 Time after data processing for restarting the internal combustion engine t Time difference between t3 and t2 p s Intake manifold target pressure

Claims

[1] Method for operating an internal combustion engine (1) with - a crankshaft which can be driven by a piston of a cylinder (4), - an intake manifold through which the cylinder (4) can be supplied with fresh air, - an inlet valve (5) through which, when open, fresh air can flow from the intake manifold into the cylinder (4), - a variable valve train (3) by which the opening duration or the relative opening time of the inlet valve (5) is variable with respect to the crankshaft position, characterized by , that - when starting the internal combustion engine (1) in the event that the intake manifold pressure deviates from the intake manifold setpoint pressure (p s ) deviates, cylinder filling pre-control (4) is achieved by the variable valve train by adjusting the fresh air supply compared to the fresh air supply at intake manifold set pressure (p s ) is reduced, - the procedure is applied when re-igniting the internal combustion engine (1) after a coasting phase in which the internal combustion engine (1) is towed without fuel supply and with reduced or no air mass flow in the cylinder (4) and - during the re-ignition of the internal combustion engine (1) after the overrun phase, the intake valve timing is continuously advanced, whereby the rate of adjustment of the intake valve timing is determined in the event that the intake manifold pressure deviates from the intake manifold setpoint pressure (p s ) deviates, compared to the adjustment speed at intake manifold setpoint pressure (p s ) is reduced. [2] Method according to claim 1, characterized by , that the filling of the cylinder (4) with fresh air is reduced by the variable valve train (3) so that the resulting torque does not exceed the target torque specification or exceeds it by less than 50%. [3] Method according to claim 2, characterized by, that the internal combustion engine has an exhaust valve (6) which is permanently closed during the overrun phase and the inlet valve (5) is operated with an inlet valve timing that is shifted so far to “late” that it is unsuitable for combustion operation. [4] Method according to any one of claims 2 to 3, characterized by , that no ignition angle intervention takes place when the internal combustion engine (1) is restarted. [5] Method according to any one of the preceding claims, characterized by , that the variable valve train (3) has an intake camshaft (7) with an electric camshaft adjuster (9) or electro-hydraulically actuated valves. [6] Method according to any one of the preceding claims, characterized by , that the internal combustion engine (1) has a throttle valve and the charge control is carried out by the variable valve train, as long as the load control by the throttle valve is not possible or can only be carried out to a reduced extent. [7] Method according to any one of the preceding claims, characterized by , that the internal combustion engine (1) is operated as part of a hybrid drive of a motor vehicle. [8] Control unit (12) operating an internal combustion engine (1) using a method according to any of the preceding claims.

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