Method for performing a charge exchange in an internal combustion engine

Abstract

Method for carrying out a charge exchange in a turbocharged internal combustion engine with - at least one cylinder head with at least one cylinder, each cylinder having at least one intake port for supplying fresh air via an intake system and at least one exhaust port for removing exhaust gases via an exhaust system, and having a piston that is movable along a longitudinal axis between a bottom dead center and a top dead center, and - at least two valve trains with at least two valves that are movable along their longitudinal axis between a valve closing position and a valve opening position while performing a maximum valve lift in order to open and close the at least one intake port and the at least one exhaust port of the at least one cylinder during a charge exchange, and with at least two actuating devices to open the valves against a preload force, - the intake valve of the at least one intake port is actuated in such a way that this intake valve performs an additional valve lift before the piston reaches top dead center during the charge exchange and before the intake valve performs its maximum valve lift during the charge exchange, wherein the intake valve of the at least one intake port is actuated to perform the additional valve lift only if there is a pressure differential such that the pressure in the intake system is greater than the pressure in the exhaust system, and - the exhaust valve of the at least one exhaust port is actuated in such a way that this exhaust valve performs an additional valve stroke before the piston reaches bottom dead center during the charge exchange and after the exhaust valve has performed the maximum valve stroke during the charge exchange, characterized in that the inlet valve of the at least one inlet port is not moved to the closed position after the additional valve stroke has been completed, but is opened further in order to complete the maximum valve stroke during the charge exchange if there is not enough time to move the inlet valve to the closed position after the additional valve stroke has been completed.

Description

[0001] The invention relates to a method for carrying out a charge exchange in a supercharged internal combustion engine with - at least one cylinder head with at least one cylinder, each cylinder having at least one inlet opening for supplying fresh air via an intake system and at least one outlet opening for discharging the exhaust gases via an exhaust gas removal system and having a piston which is movable along a piston longitudinal axis between a bottom dead center and a top dead center, and - at least two valve trains with at least two valves which are movable along their longitudinal axis between a valve closing position and a valve opening position while performing a maximum valve lift in order to open and close the at least one inlet opening and the at least one outlet opening of the at least one cylinder as part of a gas exchange, and with at least two actuating devices for opening the valves against a preload force.

[0002] An internal combustion engine of the type mentioned is used as a drive for motor vehicles. For the purposes of the present invention, the term "internal combustion engine" encompasses gasoline engines, diesel engines, but also hybrid internal combustion engines that utilize a hybrid combustion process, and hybrid drives that, in addition to the internal combustion engine, comprise an electric motor that can be connected to the internal combustion engine for driving purposes, which absorbs power from the internal combustion engine or delivers additional power as a switchable auxiliary drive.

[0003] Internal combustion engines have a cylinder block and at least one cylinder head, which are connected to form at least one cylinder. The cylinder block has a corresponding number of cylinder bores to accommodate the pistons or cylinder barrels. The cylinder head typically serves to accommodate the valve train. To control the gas exchange, an internal combustion engine requires control elements and actuating devices to operate the control elements. During the gas exchange of a cylinder, the combustion gases are expelled through at least one exhaust port and the combustion chamber is filled, i.e., fresh air is drawn in through at least one inlet port.To control the gas exchange, four-stroke engines almost exclusively use lift valves as control elements. These perform an oscillating stroke along their longitudinal axis during operation of the internal combustion engine, thereby opening and closing the intake and exhaust ports. The actuating device required to move a valve, including the valve itself, is called the valve train.

[0004] The valve train's task is to open or close at least one intake and / or exhaust port of a cylinder in a timely manner. The goal is to achieve rapid opening of the largest possible flow cross-sections in order to minimize throttling losses in the incoming and outgoing gas flows and ensure the best possible filling of the cylinder and complete removal of the exhaust gases. Therefore, according to the state of the art, a cylinder is often and increasingly equipped with two or more intake and exhaust ports.

[0005] According to the state of the art, the intake lines leading to the inlet ports and the exhaust lines connecting to the outlet ports are at least partially integrated into the cylinder head.

[0006] Minimizing fuel consumption is a fundamental goal in the development of internal combustion engines. In this context, turbocharging is becoming increasingly important. Turbocharging is a suitable means of increasing the power of an internal combustion engine while maintaining the same displacement, or reducing the displacement while maintaining the same power. Under the same vehicle conditions, the load spectrum can thus be shifted toward higher loads, where specific fuel consumption is lower. Turbocharging an internal combustion engine therefore supports efforts to minimize fuel consumption, i.e., improve the efficiency of the internal combustion engine.

[0007] The design of exhaust gas turbocharging often presents difficulties, even though the goal is generally to achieve a noticeable increase in power across all engine speed ranges. However, current technology observes a drop in torque below a certain engine speed. This drop in torque becomes understandable when one considers that the boost pressure ratio depends on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a lower exhaust gas mass flow and thus to a lower turbine pressure ratio. As a result, the boost pressure ratio and boost pressure also decrease at lower engine speeds, which equates to a drop in torque.

[0008] In principle, the drop in boost pressure can be counteracted by using a small exhaust turbocharger, i.e., one with a small turbine cross-section, in order to generate sufficient boost pressure even at low exhaust flows. Ultimately, this simply shifts the torque drop to lower engine speeds. Furthermore, this approach, i.e., reducing the turbine cross-section, has its limitations, since the desired boosting and power increase must be possible without restriction and to the desired extent even at high engine speeds.

[0009] According to the state of the art, attempts are made to improve the torque characteristics of a turbocharged internal combustion engine by various measures, for example by designing a small turbine cross-section in combination with exhaust gas blow-off or by using several exhaust gas turbochargers.

[0010] The torque characteristics can be advantageously influenced by connecting several exhaust turbochargers in series. By connecting two exhaust turbochargers in series, one serving as the high-pressure stage and one as the low-pressure stage, the engine characteristic map can be advantageously expanded, both to accommodate smaller and larger compressor flows.

[0011] In particular, with the exhaust gas turbocharger serving as the high-pressure stage, it is possible to shift the surge limit towards smaller compressor flows, whereby high boost pressure ratios can be achieved even at low compressor flows, which significantly improves the torque characteristics in the lower speed range. This is achieved by designing the high-pressure turbine for small exhaust gas mass flows and providing a bypass line through which more exhaust gas is directed past the high-pressure turbine as the exhaust gas mass flow increases. For this purpose, the bypass line branches off from the exhaust gas removal system upstream of the high-pressure turbine and flows back into the exhaust gas removal system upstream of the low-pressure turbine. A shut-off element is arranged in the bypass line to control the exhaust gas flow past the high-pressure turbine.

[0012] The response of an internal combustion engine charged in this way is significantly improved compared to a comparable internal combustion engine with single-stage charging, since the smaller high-pressure stage is less sluggish, i.e. the rotor of a smaller-sized exhaust gas turbocharger can be accelerated more quickly.

[0013] The torque characteristics of a supercharged internal combustion engine can be further improved by several turbochargers arranged in parallel, i.e. by several turbines arranged in parallel with a smaller turbine cross-section, with turbines being successively switched on as the exhaust gas quantity increases.

[0014] The torque characteristics of a supercharged internal combustion engine can also be improved by supplementing the at least one exhaust gas turbocharger with a mechanical charger, i.e. a compressor.

[0015] In addition to a conceptual improvement in exhaust gas turbocharging, as described above, the prior art also includes methods for improving the torque characteristics of an internal combustion engine turbocharged by exhaust gas turbocharging, which are intended to increase torque and, in particular, to overcome the torque weakness at low engine speeds. The cause of the torque weakness at low engine speeds is generally considered to be the insufficient exhaust gas volume at low engine speeds.

[0016] Variable valve timing allows the valve overlap—that is, the crank angle range in which the exhaust is not yet closed when the intake is open—to be varied. During valve overlap, scavenging losses can occur, whereby a portion of the intake fresh air flows through the cylinder without participating in the subsequent combustion. Variable valve timing allows the valve overlap to be varied depending, among other things, on engine speed.

[0017] In spark-ignition internal combustion engines turbocharged by exhaust gas, a large valve overlap at low engine speeds is suitable for significantly increasing maximum torque and improving transient operating behavior. A pressure gradient between the intake and exhaust sides at low engine speeds supports an effective scavenging process of the cylinders with fresh air, ensuring a greater cylinder charge and thus higher performance.

[0018] In gasoline engines with a comparatively low compression ratio of, for example, ε ≈ 10 for naturally aspirated engines or ε ≈ 8 ... 9 for turbocharged engines, the torque weakness at low speeds can be counteracted with a large valve overlap. In high-compression internal combustion engines, such as diesel engines, a large valve overlap is often not feasible because the piston moves very close to the intake or exhaust valves when passing top dead center, and the valves cannot open completely during valve overlap.

[0019] The document US 2009 / 0 194 080 A1 describes a method for performing a charge cycle in a turbocharged internal combustion engine. The intake valve of at least one intake port is actuated in such a way that it performs an additional valve lift before the piston reaches top dead center during the charge cycle and before the intake valve performs its maximum valve lift during the charge cycle. The intake valve of at least one intake port is actuated to complete the additional valve lift only if a pressure drop exists such that the pressure in the intake system is greater than the pressure in the exhaust gas removal system.

[0020] The document DE 10 2009 034 763 A1 discloses a method for operating an internal combustion engine, in particular with direct fuel injection, by performing at least a second opening and closing of an exhaust valve during an intake stroke during or after a first opening and closing of at least one intake valve for controlling the charge exchange of a combustion chamber defined by a piston and a cylinder, and / or by performing at least a second opening and closing of the at least one intake valve during an exhaust stroke during or after a first opening and closing of the at least one exhaust valve for controlling the charge exchange. Further relevant prior art is represented by the documents DE 10 2013 008 826 A1, DE 10 2012 204 682 A1, DE 10 2010 033 005 A1, and DE 10 2013 015 012 A1.

[0021] Against the background of the above, it is the object of the present invention to provide a method according to the preamble of claim 1, with which the torque weakness at low speeds can be counteracted and the torque characteristics can be improved, in particular in an internal combustion engine charged by means of exhaust gas turbocharging.

[0022] This task is solved by a method for carrying out a charge exchange in a supercharged internal combustion engine with - at least one cylinder head with at least one cylinder, each cylinder having at least one inlet opening for supplying fresh air via an intake system and at least one outlet opening for discharging the exhaust gases via an exhaust gas removal system and having a piston which is movable along a piston longitudinal axis between a bottom dead center and a top dead center, and - at least two valve trains with at least two valves which are movable along their longitudinal axis between a valve closing position and a valve opening position while performing a maximum valve lift in order to open and close the at least one inlet opening and the at least one outlet opening of the at least one cylinder as part of a gas exchange, and with at least two actuating devices to open the valves against a preload force, which is characterized in that - the inlet valve of the at least one inlet opening is actuated in such a way that this inlet valve performs an additional valve lift before the piston reaches top dead center during the gas exchange and before the inlet valve performs the maximum valve lift during the gas exchange, wherein the inlet valve of the at least one inlet opening is actuated for the purpose of completing the additional valve lift only if a pressure gradient exists such that the pressure in the intake system is greater than the pressure in the exhaust gas discharge system, and - the exhaust valve of the at least one exhaust port is actuated in such a way that this exhaust valve performs an additional valve lift before the piston reaches bottom dead centre during the gas exchange and after the exhaust valve has performed the maximum valve lift during the gas exchange.

[0023] According to the invention, the intake valve of the at least one intake opening performs at least two valve strokes, wherein the actual, larger valve stroke, which primarily serves the charge exchange, is preceded by an additional smaller valve stroke.

[0024] The additional, shorter valve lift serves to flush residual gas from the cylinder. Residual gas flushing is assisted by the piston moving toward top dead center, passing through top dead center, or being close to top dead center, so that the instantaneous combustion chamber volume during the full additional valve lift is comparatively small or only slightly larger than the compression volume at which the piston is at top dead center.

[0025] The greatest possible flushing out of the residual gas ensures the greatest possible cylinder fresh charge in the next working cycle and thus a higher energy yield in the subsequent combustion cycle, i.e. an increase in power and thus an increase in torque at the same, especially low, speed.

[0026] In internal combustion engines turbocharged by exhaust gas turbocharging, the scavenging process has another beneficial effect: the effect that a larger mass flow is available or provided to the turbine of an exhaust gas turbocharger located in the exhaust gas removal system. The larger mass flow through the turbine results in a higher turbine pressure ratio, a higher boost pressure ratio, and thus a higher boost pressure on the intake side, which also contributes to better cylinder filling, i.e., the greatest possible cylinder charge in the next combustion cycle. This effect also leads to an increase in power, i.e., an increase in torque at a constant engine speed.

[0027] The fresh air introduced into the cylinder or passed through the cylinder during the residual gas purge process is also heated by convection as it flows over the piston crown and the hot combustion chamber interior walls, resulting in a higher temperature and thus increased enthalpy of the mass flow introduced into the exhaust gas removal system. Furthermore, the hot purged residual gas further contributes to increasing the enthalpy of the mass flow fed to the turbine. In addition to increasing the mass flow, the increase in enthalpy—as described above—leads to higher boost pressure and thus to increased torque.

[0028] While with a larger valve overlap, where the intake valve has to be opened earlier or the exhaust valve has to be closed later, there is a risk that the piston passing through the top dead center comes into contact with an open intake valve or an open exhaust valve, the additional stroke according to the invention can be carried out before reaching the top dead center and at a distance from the top dead center without the piston touching and damaging the open intake valve.

[0029] According to the invention, the intake valve of the at least one intake port is actuated to complete the additional valve lift only if a pressure gradient exists such that the pressure in the intake system is greater than the pressure in the exhaust gas removal system. The decisive factor here is the pressure gradient between the cylinder inlet and the cylinder outlet, i.e., between the at least one intake port and the at least one exhaust port of a cylinder.

[0030] A pressure gradient between the intake system and the exhaust gas removal system supports the scavenging process with fresh air, i.e., effective cylinder purging. In this context, it should be noted that gas-dynamic wave processes occur in the intake system and the exhaust gas removal system, particularly due to the intermittent gas exchange, which is why the pressure on the intake and exhaust sides generally changes over time.

[0031] The internal combustion engine according to the invention is a supercharged internal combustion engine. A pressure gradient between the intake system and the exhaust gas removal system supports the purging of the cylinder with fresh air, i.e., the purging process, so it is advantageous to ensure high pressure in the intake system. This can be achieved by supercharging the internal combustion engine. The boost pressure generated by supercharging then prevails in the intake system, which at low engine speeds is often higher than the exhaust backpressure in the exhaust gas removal system.

[0032] The pressures in the intake system and the exhaust gas removal system change continuously due to the intermittent gas exchange. At the beginning of the gas exchange, the evacuation of the combustion gases is based on the high pressure difference between the combustion chamber and the exhaust gas removal system. This pressure-driven flow process is accompanied by a high pressure peak. As the gas exchange progresses, the pressures in the combustion chamber and the exhaust gas removal system equalize, and the combustion gases are expelled as a result of the piston's reciprocating movement. Significant pressure changes also occur on the intake side. Consequently, the pressure difference between the intake system and the exhaust gas removal system—i.e., between the cylinder's intake port and exhaust port—also changes constantly.

[0033] According to the invention, the exhaust valve of the at least one exhaust port is also actuated in such a way that this exhaust valve performs an additional valve lift before the piston reaches the bottom dead center during the gas exchange and after the exhaust valve has performed the maximum valve lift during the gas exchange.

[0034] If an exhaust valve is opened during the intake process of the gas exchange, in which the piston moves downward, before the piston reaches bottom dead center, at least one intake valve is still open. The fresh air introduced into the cylinder via the intake port can then flow through the cylinder and exit again via the exhaust port due to the additional valve lift provided by the exhaust valve. The associated effects are partly those described in connection with the additional valve lift of the intake valve.

[0035] In this case, purging of residual gas does not occur or at most only to an insignificant extent, which is why the advantages associated with purging of residual gas, namely the generation of a large cylinder fresh charge in the next working cycle and the increase in enthalpy due to the purged residual gas, are lost.

[0036] In internal combustion engines turbocharged by exhaust gas turbocharging, however, the scavenging process has the same beneficial effect, albeit not to the same extent. The turbine of an exhaust gas turbocharger, located in the exhaust gas removal system, is provided with a larger mass flow, thereby increasing torque at a constant engine speed. However, this larger mass flow has a lower temperature, since the fresh air flowing through the cylinder is neither heated by purged residual gas nor by a piston positioned near bottom dead center.

[0037] Although additional valve lift on the intake side offers more advantages than on the exhaust side, performing additional valve lift on the exhaust side has the advantage of reducing the sensitivity of the process to changes in exhaust back pressure.

[0038] In addition, the lack of heating of the fresh air can be advantageously used to specifically cool the mass flow supplied to the turbine, i.e., to reduce the turbine inlet temperature. In this context, it must be considered that the operation of an internal combustion engine is often limited, especially at full load, by the maximum permissible temperature at the turbine inlet.

[0039] In this respect, the exhaust valve of the at least one exhaust port can also perform an additional valve lift solely with the aim of cooling the mass flow supplied to the turbine.

[0040] The method according to the invention solves the problem underlying the invention, namely to demonstrate a method with which the torque weakness at low speeds can be counteracted and the torque characteristics can be improved, in particular in an internal combustion engine charged by means of exhaust gas turbocharging.

[0041] Further advantageous embodiments of the method according to the invention are discussed in connection with the subclaims.

[0042] Advantageous embodiments of the method include those in which the intake valve of the at least one intake port is actuated to complete the additional valve lift, while at least one exhaust port of the at least one cylinder is at least temporarily open. The fresh air introduced via the intake port during the additional valve lift can flow unhindered through the cylinder and exit the cylinder via the exhaust port, thereby advantageously supporting the scavenging process.

[0043] Embodiments of the method are advantageous in which the additional valve lift of the intake valve is carried out up to 50°CA before top dead center.

[0044] Embodiments of the method are advantageous in which the additional valve lift of the intake valve is carried out up to 30°CA before top dead center.

[0045] The closer the additional valve lift is performed to top dead center, the smaller the instantaneous combustion chamber volume at the full additional valve lift, and the more effective and pronounced the scavenging process, or rather, the temperature increase of the scavenging air as it passes over the inner walls of the combustion chamber, especially the piston crown. Both of these factors contribute to increasing torque, especially at low engine speeds. The crank angle mark at which the additional valve lift is performed must be coordinated with the height of the additional valve lift.

[0046] Embodiments of the method are advantageous in which the inlet valve of the at least one inlet opening is returned to the closed position after completion of the additional valve lift, before this inlet valve is opened again in order to complete the maximum valve lift within the scope of the charge exchange.

[0047] Advantageously, in the method, the inlet valve of the at least one inlet opening is not moved into the closed position after the additional valve lift has been completed, but is opened further in order to complete the maximum valve lift within the scope of the charge exchange.

[0048] There may not be enough time to move the intake valve to the closed position after completing the additional valve lift, as this intake valve must be immediately opened wide and completely for the purpose of gas exchange. This can be the case, for example, if the additional valve lift is performed very close to top dead center.

[0049] However, embodiments of the method may be advantageous in this context in which the inlet valve of the at least one inlet opening is moved towards the closed position after the additional valve lift has been completed in order to reduce the valve lift by half or less.

[0050] Embodiments of the method may also be advantageous in which the inlet valve of the at least one inlet opening is moved towards the closed position after the additional valve lift has been completed in order to reduce the valve lift by at least one third.

[0051] Embodiments of the method may also be advantageous in which the inlet valve of the at least one inlet opening is moved towards the closed position after the additional valve lift has been completed in order to reduce the valve lift by at least a quarter.

[0052] According to the above process variants, a valve lift curve with at least two maximums is formed. This means that when the additional valve lift is reached, the intake valve is not immediately opened further to achieve the maximum valve lift for the purpose of gas exchange, but is first moved toward the closed position. This means that the valve lift is initially reduced before further opening is initiated.

[0053] This procedure may be necessary, for example, if the additional valve lift is performed very close to top dead center to prevent piston contact with the intake valve. In such cases, it is advantageous to reduce the valve lift before the intake valve is opened further or fully during the gas exchange process.

[0054] Nevertheless, when the additional valve lift is reached, the intake valve can also be opened further, either directly or with a delay, in order to achieve the maximum valve lift for the purpose of a charge exchange.

[0055] Embodiments of the method in which an additional valve lift is performed which is less than a quarter of the maximum valve lift are advantageous.

[0056] Embodiments of the method in which an additional valve lift is performed which is less than one sixth of the maximum valve lift are advantageous.

[0057] Embodiments of the method in which an additional valve lift is performed which is less than one eighth of the maximum valve lift are advantageous.

[0058] The amount of additional valve lift must generally be coordinated with the crank angle mark at which the additional valve lift is performed. The closer the additional valve lift is performed to top dead center, the smaller the additional valve lift must be set to prevent the intake valve from being touched by the piston during the gas exchange. However, a variety of other parameters may need to be considered, such as the piston shape, the shape of the combustion chamber roof, and, in particular, the compression ratio ε of the cylinder or internal combustion engine.

[0059] Embodiments of the method in which an additional valve lift of less than 2 mm is performed are advantageous.

[0060] Embodiments of the method in which an additional valve lift of less than 1.5 mm is performed are advantageous.

[0061] The information given for the above process variants for the additional valve lift is only an example and is intended to illustrate the relationship to the maximum valve lift, which can easily be 8 mm to 12 mm.

[0062] Embodiments of the method in which a diesel engine is used as the internal combustion engine are advantageous. As already explained, the method according to the invention is particularly advantageous in high-compression internal combustion engines, such as diesel engines, where the valves cannot be actuated and opened arbitrarily and in the desired manner to achieve valve overlap, since the piston moves very close to the valves when passing top dead center. The risk of the piston coming into contact with an open intake valve does not exist with the method according to the invention.

[0063] Embodiments of the method in which an internal combustion engine charged by means of exhaust gas turbocharging is used as the internal combustion engine are advantageous.

[0064] In internal combustion engines turbocharged by exhaust gas turbocharging, the scavenging process has the special effect of providing a larger mass flow to the turbocharger's turbine located in the exhaust gas removal system. This increased mass flow through the turbine leads to higher boost pressure on the intake side and ultimately to an increase in torque.

[0065] The heating of the fresh air used for purging as well as the purged residual gas lead to an increase in the enthalpy of the exhaust gas mass flow provided to the turbine and thus also to an increase in torque.

[0066] In internal combustion engines in which each cylinder has at least two inlet openings for supplying fresh air via the intake system and at least two outlet openings for discharging the exhaust gases via the exhaust gas discharge system, embodiments are advantageous which are characterized in that the inlet valves of at least two inlet openings are actuated in such a way that these inlet valves perform an additional valve lift.

[0067] In internal combustion engines in which each cylinder has at least two inlet openings for supplying fresh air via the intake system and at least two outlet openings for discharging the exhaust gases via the exhaust gas removal system, embodiments are advantageous which are characterized in that the inlet valve is actuated by at least one inlet opening in such a way that this inlet valve performs an additional valve lift.

[0068] Advantageous embodiments of the method are those in which the exhaust valve of the at least one exhaust port is actuated to complete the additional valve lift, while at least one intake port of the at least one cylinder is at least temporarily open. The fresh air introduced via the intake port can flow unhindered through the cylinder and exit the cylinder via the exhaust port during the additional valve lift, thereby advantageously supporting the flow through the cylinder and thus the introduction of fresh air into the exhaust gas removal system.

[0069] Embodiments of the method are advantageous in which the exhaust valve of the at least one exhaust opening is actuated for the purpose of completing the additional valve lift only if there is a pressure gradient such that the pressure in the intake system is greater than the pressure in the exhaust gas discharge system.

[0070] A pressure gradient between the intake system and the exhaust gas removal system promotes the flow of fresh air through the cylinder. In this context, it is important to consider that gas-dynamic wave processes occur in the intake system and the exhaust gas removal system, particularly due to the intermittent gas exchange, which is why the pressure on the intake and exhaust sides generally changes over time.

[0071] Embodiments of the method are advantageous in which the additional valve lift of the exhaust valve is carried out up to 50°CA before bottom dead center.

[0072] Embodiments of the method are advantageous in which the additional valve lift of the exhaust valve is carried out up to 30°CA before bottom dead center.

[0073] The closer the additional valve lift is performed to bottom dead center, the lower the heating or temperature increase of the fresh air as it passes over the inner walls of the combustion chamber, especially the piston crown. The crank angle mark at which the additional valve lift is performed must be coordinated with the height of the additional valve lift.

[0074] Embodiments of the method are advantageous in which the exhaust valve of the at least one exhaust opening is returned to the closed position after the maximum valve lift has been completed during the charge exchange, before this exhaust valve is opened again in order to complete the additional valve lift.

[0075] Embodiments of the method are advantageous in which the exhaust valve of the at least one exhaust opening performs an additional valve lift which is less than a quarter of the maximum valve lift.

[0076] Embodiments of the method are advantageous in which the exhaust valve of the at least one exhaust opening performs an additional valve lift which is less than one sixth of the maximum valve lift.

[0077] In the following, the invention is described using a variant of the method and according to the Fig. 1 is described in more detail. Here: Fig. 1 in a diagram the valve lift curves h intake , h exhaust an inlet valve and an outlet valve according to a first embodiment of the method.

[0078] Fig. 1 shows in a diagram the valve lift curves h intake , h exhaust an intake valve and an exhaust valve according to a first embodiment of the method. Also shown is the pressure difference Δp between the intake system and the exhaust gas removal system, i.e., between the intake side and the exhaust side.

[0079] During the charge exchange, the exhaust gas is evacuated from the cylinder by opening the exhaust valve and the maximum valve lift Δh max,exhaust The intake valve is then opened to fill the cylinder with fresh air again, whereby the intake valve reaches the maximum valve lift Δh max,intake performed.

[0080] The intake valve is actuated in such a way that this intake valve has an additional valve lift Δh add,intake before the intake valve reaches the maximum valve lift Δh during the gas exchange max,intake and before the piston reaches top dead center during the gas exchange. Consequently, the intake valve performs two valve strokes Δh max,intake , Δh add,intake , where the actual, larger valve lift Δh max,intake, which primarily serves the charge exchange, an additional smaller valve lift Δh add,intakeThe additional smaller valve lift Δh add,intake serves to flush residual gas from the cylinder. Residual gas flushing is assisted by the piston moving toward top dead center, which reduces the combustion chamber volume. A positive pressure gradient Δp between the intake system and the exhaust gas removal system further supports the flushing process with fresh air. The greatest possible cylinder charge in the next working cycle ensures increased power and torque at unchanged engine speed.

[0081] In internal combustion engines turbocharged by exhaust gas turbocharging, the purging process provides a larger mass flow to the turbine located in the exhaust gas removal system, allowing higher boost pressure to be generated on the intake side. The higher boost pressure also leads to an increase in torque at constant engine speed. The hot purged residual gas increases the enthalpy of the mass flow supplied to the turbine, which is further increased by the fact that the fresh air passed through the cylinder during the residual gas purge process heats up due to convection as it flows through the cylinder.

[0082] In this case, the intake valve is opened after completing the additional valve lift Δh add,intake not returned to the closed position, but in order to complete the maximum valve stroke Δh max,intakeThe intake valve is opened further during the gas exchange. However, after completing the additional valve lift Δh add,intake first moved towards the closed position to reduce the valve lift by almost half, before this inlet valve is moved to complete the maximum valve lift Δh max,intake is opened further. The resulting valve lift curve has two maxima.

[0083] According to the first procedure variant according to Fig. 1a, the exhaust valve is actuated in such a way that this exhaust valve also has an additional valve lift Δh add,exhaust performed.

[0084] After the exhaust valve has reached the maximum valve lift Δh during the charge exchange max,exhaust and before the piston reaches bottom dead center during intake, the exhaust valve is opened again.

[0085] While the exhaust valve has the additional valve lift Δh add,exhaust, the intake valve is still open. The fresh air introduced into the cylinder via the intake port can leave the cylinder again via the exhaust. A positive pressure gradient Δp between the intake system and the exhaust gas removal system supports the flow through the cylinder. The additional valve lift Δh add,exhaust The exhaust valve's function is less to purge residual gas and more to cool the mass flow supplied to the turbine, i.e., to lower the turbine inlet temperature. The fresh air flowing through the cylinder is heated neither by purged residual gas nor by a piston positioned near bottom dead center. Due to this lack of heating of the fresh air as it flows through the cylinder, a cooling effect can be achieved.

[0086] The additional valve lift Δh add,exhaustThe exhaust valve is also suitable, although not to the same extent, for increasing the mass flow supplied to the turbine and thereby increasing the torque at constant speed, even though this increased mass flow has a lower temperature. In this respect, an additional valve lift Δh on the exhaust side supports add,exhaust the additional valve lift Δh on the intake side add,intake . Reference symbol h exhaust Valve lift of an exhaust valve h intake Valve lift of an intake valve Δh valve lift Δh max,exhaust maximum valve lift of an exhaust valve Δh max,intake maximum valve lift of an intake valve Δh add,exhaust additional valve lift of an exhaust valve Δh add,intake additional valve lift of an intake valve Δp pressure difference between intake system and exhaust gas discharge system °CA degrees crank angle

Claims

[1] Method for carrying out a charge exchange in a supercharged internal combustion engine with - at least one cylinder head with at least one cylinder, each cylinder having at least one inlet opening for supplying fresh air via an intake system and at least one outlet opening for discharging the exhaust gases via an exhaust gas removal system and having a piston which is movable along a piston longitudinal axis between a bottom dead center and a top dead center, and - at least two valve trains with at least two valves which are movable along their longitudinal axis between a valve closing position and a valve opening position while performing a maximum valve lift in order to open and close the at least one inlet opening and the at least one outlet opening of the at least one cylinder during a gas exchange, and with at least two actuating devices for opening the valves against a preload force, - the inlet valve of the at least one inlet opening is actuated in such a way that this inlet valve performs an additional valve lift before the piston reaches top dead center during the gas exchange and before the inlet valve performs the maximum valve lift during the gas exchange, wherein the inlet valve of the at least one inlet opening is actuated for the purpose of completing the additional valve lift only if a pressure gradient exists such that the pressure in the intake system is greater than the pressure in the exhaust gas discharge system, and - the exhaust valve of at least one exhaust port is actuated in such a way that this exhaust valve performs an additional valve lift before the piston reaches bottom dead center during the gas exchange and after the exhaust valve has performed the maximum valve lift during the gas exchange characterized bythat the inlet valve of the at least one inlet opening is not moved into the closed position after completion of the additional valve lift, but is opened further in order to complete the maximum valve lift within the scope of the gas exchange if there is no time available to move the inlet valve into the closed position after completion of the additional valve lift. [2] Method according to claim 1, characterized by that the inlet valve of the at least one inlet opening is actuated in order to complete the additional valve lift, while at least one exhaust opening of the at least one cylinder is at least temporarily open. [3] Method according to claim 1 or 2, characterized by that the additional valve lift of the intake valve is carried out up to 50°CA before top dead center. [4] Method according to one of the preceding claims, characterized bythat the additional valve lift of the intake valve is carried out up to 30°CA before top dead center. [5] Method according to one of the preceding claims, characterized by that the inlet valve of at least one inlet opening is moved towards the closed position after completion of the additional valve lift in order to reduce the valve lift to half or less. [6] Method according to one of the preceding claims, characterized by that the inlet valve of at least one inlet opening is moved towards the closed position after completion of the additional valve lift in order to reduce the valve lift by at least one third. [7] Method according to one of the preceding claims, characterized by , the inlet valve of at least one inlet opening is moved towards the closed position after completion of the additional valve lift in order to reduce the valve lift by at least a quarter. [8] Method according to one of the preceding claims, characterized by that an additional valve lift is performed which is less than a quarter of the maximum valve lift. [9] Method according to one of the preceding claims, characterized by that an additional valve lift is performed which is less than one sixth of the maximum valve lift. [10] Method according to one of the preceding claims, characterized by that an additional valve lift of less than 2mm is performed. [11] Method according to one of the preceding claims, characterized by that an additional valve lift of less than 1.5mm is performed. [12] Method according to one of the preceding claims, characterized by that a diesel engine is used as an internal combustion engine. [13] Method according to one of the preceding claims, characterized by that an internal combustion engine charged by exhaust gas turbocharging is used as the internal combustion engine. [14] Method according to one of the preceding claims for carrying out a charge exchange in an internal combustion engine, in which each cylinder has at least two inlet openings for supplying fresh air via an intake system and at least two outlet openings for discharging the exhaust gases via an exhaust gas discharge system, characterized by that the intake valves of at least two intake ports are actuated in such a way that these intake valves perform an additional valve lift.

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