Internal combustion engine that can be operated using either a two-stroke or four-stroke process, and a method for operating such an internal combustion engine

Abstract

Internal combustion engine, which can be operated either according to a first operating mode using a two-stroke process or according to a second operating mode using a four-stroke process, with - at least one cylinder, which has at least one exhaust port for removing exhaust gases via an exhaust system and at least one intake port for supplying charge air via an intake system, and - a valve train comprising a valve for each opening and a valve actuation device for actuating the valves, comprising at least one camshaft with several cams, wherein - at least one mechanical supercharger is arranged in the intake system, wherein a bypass line is provided to bypass the mechanical supercharger, which branches off from the intake system upstream of the supercharger and re-enters the intake system downstream of the supercharger and in which a shut-off element is arranged, - at least one exhaust gas turbocharger is provided, comprising a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system, the compressor being located upstream of the mechanical turbocharger in the intake system, - at least one camshaft for actuating at least one mode-adaptable valve has two different cams, wherein a first cam serves to actuate this mode-adaptable valve in the first operating mode and a second cam serves to actuate this mode-adaptable valve in the second operating mode, - the valve train (1) is switchable in such a way that the actuation of the at least one mode-adaptable valve is carried out in the second operating mode by means of a second cam and in the first operating mode by means of a first cam, and - the valve actuation device for actuating the valves comprises several valve-associated cam follower elements (2), wherein each valve-associated cam follower element (2) is arranged in the force flow between a cam and a valve in such a way that the valve performs an oscillating stroke movement when the camshaft rotates, wherein two cam follower elements (3, 4) are provided for the two different cams of the at least one mode-adaptable valve, which are mechanically coupled to the valve-associated cam follower element (2), wherein a first cam follower element (3) associated with the first cam is mechanically coupled to the valve-associated cam follower element (2) in order to actuate the mode-adaptable valve in the first operating mode by means of the first cam, and a second cam follower element (4) associated with the second cam is mechanically coupled to the valve-associated cam follower element (2),to actuate the valve in the second operating mode using a second cam.

Description

[0001] The invention relates to an internal combustion engine which can be operated either according to a first operating mode using a two-stroke process or according to a second operating mode using a four-stroke process, with - at least one cylinder, which has at least one exhaust port for removing exhaust gases via an exhaust system and at least one intake port for supplying charge air via an intake system, and - a valve train comprising a valve for each opening and a valve actuation device for actuating the valves, comprising at least one camshaft with multiple cams, - at least one charging device is arranged in the intake system, wherein a bypass line is provided to bypass this charging device, which branches off from the intake system upstream of the charging device and re-enters the intake system downstream of the charging device and in which a shut-off element is arranged, and - at least one exhaust gas turbocharger is provided, comprising a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system, the compressor being located upstream of the charging device in the intake system.

[0002] Furthermore, the invention relates to a method for operating such an internal combustion engine.

[0003] An internal combustion engine of the type mentioned above is described in US 2010 / 0300383 A1 and is used, for example, as a drive for a motor vehicle. Within the scope of the present invention, the term internal combustion engine includes, in particular, diesel engines, but also gasoline engines and hybrid internal combustion engines.

[0004] Due to the intermittent combustion of modern internal combustion engines, it is necessary to remove the exhaust gases from at least one cylinder after combustion as part of a charge exchange process and to supply intake air or a fresh fuel-air mixture. A fundamental distinction must be made between two-stroke and four-stroke processes. The following section will first discuss the charge exchange process of the four-stroke process in more detail.

[0005] In the four-stroke engine, the exhaust gases are expelled through the exhaust ports during the charge exchange, while the cylinder is filled with fresh air-fuel mixture or charge air through the intake ports. The exhaust ports and the intake ports are also referred to simply as ports and are collectively called by this term.

[0006] To control the charge exchange, four-stroke engines almost exclusively use lift valves, which perform an oscillating lifting motion during the operation of the internal combustion engine and thus open or close the intake and exhaust ports.

[0007] The valve actuation mechanism required for the movement of the valves, including the valves themselves, is called the valve train. The function of the valve train is to open and close the intake and exhaust ports of the cylinders in a timely manner, aiming for rapid opening of the largest possible flow cross-sections to minimize throttling losses in the incoming and outgoing gas flows and to ensure optimal cylinder filling with the fresh mixture and effective, i.e., complete, exhaust of the combustion gases. According to the prior art, cylinders are therefore often equipped with two or more intake and / or exhaust ports. The internal combustion engine that is the subject of the present invention should also be able to operate in a four-stroke cycle when required; therefore, the at least one cylinder has at least one exhaust port and at least one intake port.

[0008] The commonly used poppet valves are movable along their longitudinal axis between a closed and an open position, i.e., they can be shifted to open or close a cylinder. To actuate a valve, valve springs are provided to bias the valve towards the closed position, and a valve actuation device is used to open the valve against the bias force of the valve springs.

[0009] The valve actuation device comprises at least one camshaft on which a plurality of cams, in this case at least two cams, are arranged and which is set in rotation by the crankshaft – for example, by means of a chain drive – in the four-stroke process such that the camshaft, together with the cams, rotates at half the crankshaft speed. One working cycle extends over two crankshaft revolutions and comprises four working strokes, namely, in addition to intake and exhaust, also compression and expansion as a result of the combustion of the fuel-air mixture.

[0010] Basically, a distinction is made between a bottom camshaft and an overhead camshaft, with reference to the dividing plane between the cylinder head and the cylinder block.

[0011] Overhead camshafts are suitable for actuating so-called upright valves, but also, with the aid of pushrods and levers, such as rocker arms or rocker arms, for actuating overhead valves. Upright valves are opened by moving them upwards, whereas overhead valves are opened by a downward movement. A tappet is typically used as an intermediate element, which should be in contact with the camshaft lobe at least during the opening and closing process.

[0012] Overhead camshafts, on the other hand, are used exclusively for actuating overhead valves. A valve train with an overhead camshaft includes a rocker arm, a rocker arm, or a tappet as an additional component. The rocker arm rotates around a fixed pivot point and, when deflected by the cam, moves the valve towards the open position against the preload force of the valve springs. In a rocker arm that pivots around a central pivot point, the cam engages at one end of the rocker arm, with the valve located at the opposite end. An advantage of using overhead camshafts is that, particularly due to the elimination of the pushrod, the moving mass of the valve train is reduced, and the valve train is more rigid, i.e., less elastic.At least two bearings are required to mount and support the camshaft; in the case of overhead camshafts, these are generally located in or on the cylinder head. When a tappet is used as the cam follower, the tappet is placed on the end of the poppet valve facing away from the combustion chamber, so that the tappet participates in the oscillating stroke of the valve when the cam engages with the tappet in the area of ​​the cam nose and deflects it.

[0013] Within the scope of the present invention, intermediate elements of the valve actuation device, i.e. valve drive components, which are located in the force flow between cam and valve, i.e. are arranged, are referred to as cam follower elements, i.e. are summarized under this term.

[0014] In contrast to the four-stroke process described above, the charge exchange in the two-stroke process does not take place by pushing out and sucking in using an oscillating piston, but rather by scavenging the combustion chamber with charge air or fresh mixture using a pressure gradient generated across the combustion chamber, the so-called scavenging pressure gradient.

[0015] For the charge exchange in a two-stroke engine, according to the state of the art, slots are often provided in the cylinder, i.e., in the cylinder tube, which are controlled by an oscillating piston. This means they are opened to supply charge air and / or expel exhaust gases, and closed for compression and expansion. One working cycle extends over one crankshaft revolution and comprises two power strokes: compression and expansion resulting from the combustion of the fuel-air mixture. The intake and exhaust strokes are either omitted or replaced by a scavenging process.

[0016] The two-stroke cycle offers several advantages over the four-stroke cycle, some of which stem from the fact that the combustion cycle in the two-stroke cycle lasts only one crankshaft revolution, whereas in the four-stroke cycle it lasts two. By its very nature, the two-stroke cycle is characterized by lower friction. Furthermore, the engine speed can be reduced, for example, the idle speed can be halved. Due to the doubled number of combustion cycles at the same crankshaft speed, it is also possible to significantly increase the maximum torque or—assuming the same torque output—to lower the mean effective pressure. This results in advantages related to the acoustics of the internal combustion engine, which contributes significantly to the overall noise emissions of the vehicle.Reducing the mean effective pressure lowers combustion temperatures, thereby counteracting the formation of nitrogen oxides and / or soot. Due to the reduced mechanical and thermal stress, the internal combustion engine could be designed more delicately, saving material and thus reducing both weight and cost.

[0017] The two-stroke engine also has disadvantages compared to the four-stroke engine, most of which can be attributed to the less efficient charge exchange, namely scavenging. Due to scavenging, the two-stroke engine leads to significantly higher emissions of unburned hydrocarbons and carbon monoxide, with the scavenging of the combustion chamber with charge air or fresh fuel mixture increasing with engine speed. mot It becomes increasingly worse, i.e., less efficient, which is why two-stroke processes are generally less suitable for high speeds.

[0018] To take advantage of the benefits of the two-stroke cycle, internal combustion engines are being developed that can operate using both two-stroke and four-stroke cycles. Various concepts have been developed to convert an internal combustion engine from a two-stroke to a four-stroke cycle.

[0019] The translation of European patent DE 601 17 553 T2 describes a selectable camshaft drive for an internal combustion engine that can be operated either in a two-stroke or four-stroke cycle. The at least one camshaft is permanently engaged with a planetary gear set driven by the crankshaft. The camshaft is driven either at crankshaft speed, as in the two-stroke cycle, or at half crankshaft speed, as in the four-stroke cycle. The charge exchange occurs via the intake and exhaust ports of the cylinders, which are controlled by cams in both operating modes using the at least one camshaft.

[0020] US Patent 2012 / 0167841 A1 describes a valve train in which two different cams for actuating a valve are spaced apart from each other on a camshaft. The transition from a first operating mode, the two-stroke cycle, to a second operating mode, the four-stroke cycle, or vice versa, is achieved by moving the camshaft along its longitudinal axis, i.e., its axis of rotation. This engages either one cam or the other cam with its corresponding valve. The transition from one operating mode to the other is achieved by moving the camshaft.

[0021] The use of different cam profiles for two-stroke and four-stroke operation is also revealed in DE 10 2007 002 802 A1 and US 2006 / 0272598 A1.

[0022] The JP S60-88810 A describes an additional exhaust port for two-stroke operation.

[0023] In light of the above, it is an object of the present invention to provide an internal combustion engine which can be easily converted from the two-stroke process to the four-stroke process - and vice versa - and which is improved with regard to its operating behavior.

[0024] Another sub-objective of the invention is to demonstrate a method for operating such an internal combustion engine.

[0025] The first subtask is solved by an internal combustion engine that can be operated either according to a first operating mode using a two-stroke process or according to a second operating mode using a four-stroke process, with - at least one cylinder, which has at least one exhaust port for removing exhaust gases via an exhaust system and at least one intake port for supplying charge air via an intake system, and - a valve train comprising a valve for each opening and a valve actuation device for actuating the valves, comprising at least one camshaft with several cams, in which - at least one mechanical supercharger is arranged in the intake system, wherein a bypass line is provided to bypass the mechanical supercharger, which branches off from the intake system upstream of the supercharger and re-enters the intake system downstream of the supercharger and in which a shut-off element is arranged, - at least one exhaust gas turbocharger is provided, comprising a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system, the compressor being located upstream of the mechanical turbocharger in the intake system, - at least one camshaft for actuating at least one mode-adaptable valve has two different cams, wherein a first cam serves to actuate this mode-adaptable valve in the first operating mode and a second cam serves to actuate this mode-adaptable valve in the second operating mode, - the valve train is designed to be switchable in such a way that the actuation of the at least one mode-adaptable valve is carried out in the second operating mode by means of a second cam and in the first operating mode by means of a first cam, and - the valve actuation device for actuating the valves comprises several valve-associated cam follower elements, each valve-associated cam follower element being arranged in the force flow between a cam and a valve in such a way that the valve performs an oscillating stroke movement when the camshaft rotates, wherein two cam follower elements are provided for the two different cams of the at least one mode-adaptable valve, which are mechanically coupled to the valve-associated cam follower element, wherein a first cam follower element associated with the first cam is mechanically coupled to the valve-associated cam follower element in order to actuate the mode-adaptable valve in the first operating mode by means of the first cam, and a second cam follower element associated with the second cam is mechanically coupled to the valve-associated cam follower element.to actuate the valve in the second operating mode using a second cam.

[0026] In the internal combustion engine according to the invention, the at least one camshaft of the valve actuation device for actuating at least one valve has different cams which are used according to the respective current working method, i.e. are active.

[0027] Since the internal combustion engine in this case may only have a single cylinder with a single exhaust port, and since the charge exchange according to the two-stroke process does not necessarily have to use the at least one intake port of the cylinder, but can also take place via intake slots, the camshaft according to the invention has a pair of different cams for at least one valve, namely in this case for the at least one exhaust valve of the at least one cylinder.

[0028] Such a valve, which can be selectively actuated by means of a first cam or by means of a second cam in order to be controlled and actuated according to the two-stroke process or according to the four-stroke process, is referred to in the context of the present invention as a mode-adaptable valve, since the valve actuation or the valve actuation device is adapted to the operating mode, i.e. the cam used is selected according to the chosen operating mode.

[0029] Nevertheless, embodiments are also advantageous and within the scope of the present invention in which both the intake valves and the exhaust valves are designed as mode-adaptable valves, wherein the actuation of the intake valves and the exhaust valves can be effected via a common camshaft or via separate camshafts, i.e., on the intake side via an intake camshaft and on the exhaust side via an exhaust camshaft. Consequently, the valve actuation device of the internal combustion engine according to the invention must comprise at least one camshaft with several cams for actuating the valves.

[0030] In order to be able to exchange, i.e., change, the cam used for actuation in a mode-adaptable valve, the valve train must be switchable, specifically, according to the invention, such that the valve is actuated by means of a second cam in the second operating mode and by means of a first cam in the first operating mode. Examples of such a switchable valve train are discussed in the description of the advantageous embodiments.

[0031] According to the invention, a mechanical supercharger is arranged in the intake system of the internal combustion engine, with which a positive pressure gradient can be generated across the combustion chamber as needed, i.e. at any time and at any operating point of the internal combustion engine, in order to be able to carry out an effective scavenging process according to the two-stroke method.

[0032] The mechanical supercharger can also be used to compress the charge air before it enters at least one cylinder, if necessary.

[0033] Additionally, at least one exhaust gas turbocharger is provided, comprising a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system, with the compressor being located upstream of the mechanical turbocharger in the intake system.

[0034] Turbocharging is primarily a method for increasing performance, in which the intake air required for the engine's combustion process is compressed, allowing a larger mass of intake air to be supplied to each cylinder per combustion cycle. This increases the fuel mass and thus the mean effective pressure.

[0035] In this system, in addition to the mechanical supercharger, at least one exhaust gas turbocharger is used for charging. In this turbocharger, a compressor and a turbine are arranged on the same shaft. The hot exhaust gas stream is fed to the turbine via the exhaust system, expands within the turbine, and sets the shaft in rotation, releasing energy. The energy transferred from the exhaust gas stream to the shaft is used to drive the compressor, which is also arranged on the shaft. The compressor forces and compresses the charge air supplied to it via the intake system, thereby achieving turbocharging of the at least one cylinder. Preferably, an intercooler is provided in the intake manifold downstream of the compressor. This intercooler cools the compressed charge air before it enters the at least one cylinder. The intercooler reduces the temperature and thus increases the density of the charge air, thereby also contributing to better cylinder filling.This contributes to a larger air mass. In a sense, compression occurs through cooling. If an intercooler is used, it is preferably equipped with a bypass.

[0036] The advantage of an exhaust gas turbocharger, for example, compared to a mechanical supercharger, is that no mechanical connection is required for power transmission between the turbocharger and the internal combustion engine. While a mechanical supercharger draws the energy required for its operation directly from the internal combustion engine, the exhaust gas turbocharger utilizes the energy of the hot exhaust gases.

[0037] On the other hand, an exhaust gas turbocharger requires a sufficiently large exhaust gas mass flow to generate the required boost pressure. When the engine speed drops below a certain level, a drop in torque is often observed. This becomes clear when one considers that the boost pressure ratio depends on the turbine pressure ratio. For example, if the engine speed of a diesel engine is reduced, this leads to a smaller exhaust gas mass flow and thus a smaller turbine pressure ratio. Consequently, the boost pressure ratio also decreases at lower engine speeds, which is equivalent to a drop in torque.

[0038] The torque characteristics of a turbocharged internal combustion engine are often improved by using several turbines arranged in parallel or in series, with the turbines being designed for specific exhaust gas volumes.

[0039] In principle, it is possible to compress the charge air in two stages using a mechanical supercharger and an exhaust gas turbocharger. For the reasons mentioned above, a bypass line is provided to bypass the mechanical supercharger. This bypass line branches off from the intake system upstream of the supercharger and rejoins the intake system downstream of the supercharger, and incorporates a shut-off element. The aim is to divide the tasks such that the mechanical supercharger handles the compression of the charge air at low engine speeds and generates a scavenging gradient, while the exhaust gas turbocharger compressor is responsible for boosting at higher engine speeds, with the mechanical supercharger preferably bypassed via the bypass line.

[0040] This approach allows the turbocharger to be designed for small charge air mass flows, i.e., for low speeds, and the compressor of the turbocharger to be designed for large charge air mass flows, as occur at higher speeds.

[0041] For the reasons mentioned above, embodiments of the internal combustion engine are advantageous in which the compressor of the at least one exhaust gas turbocharger is designed for larger charge air mass flows, for example charge air mass flows that occur above a predefinable speed n. mot,limit appear.

[0042] The rotational speed is n. mot,limit preferably a predefinable speed of the internal combustion engine, at which the at least one mode-adaptable valve is switched from one operating mode to the other operating mode.

[0043] According to the invention, the valve actuation device for actuating the valves comprises several valve-associated cam follower elements, wherein each valve-associated cam follower element is arranged in the force flow between a cam and a valve in such a way that the valve performs an oscillating stroke movement when the camshaft rotates.

[0044] As already explained, so-called cam follower elements can be used in the actuation of the valves, which are arranged in the force flow between cam and valve, for example tappets, rocker arms or rocker arms.

[0045] A key advantage of using cam follower elements is their suitability for being designed and operated as switchable components. For example, a tappet can be easily configured as a hydraulically actuated cam follower element. This element is pressurized with oil so that, when activated, the tappet, acting as a cam follower element within the force flow, transmits forces from the cam to the mode-adaptable valve. Conversely, when deactivated, it is disconnected from the oil pressure to prevent force transmission from the cam to the valve.

[0046] According to the invention, two cam-associated cam follower elements are provided for the two different cams of the at least one mode-adaptable valve, which are mechanically coupled to the valve-associated cam follower element, wherein a first cam follower element associated with the first cam is mechanically coupled to the valve-associated cam follower element in order to actuate the mode-adaptable valve in the first operating mode by means of the first cam, and a second cam follower element associated with the second cam is mechanically coupled to the valve-associated cam follower element in order to actuate the valve in the second operating mode by means of the second cam.

[0047] The internal combustion engine according to the invention solves the first problem underlying the invention, namely providing an internal combustion engine that can be easily converted from the two-stroke process to the four-stroke process - and vice versa - and that is improved with regard to its operating behavior.

[0048] Advantageous embodiments of the internal combustion engine according to the invention and the dependent claims are discussed below.

[0049] Advantageous are embodiments of the internal combustion engine in which the at least one valve-associated cam follower element is a rocker arm and / or the at least two cam follower elements are rocker arms.

[0050] The use of rocker arms provides sufficient installation space to design the valve train as a switchable valve train, i.e., to provide a mechanism with which a cam-associated cam follower element can be mechanically coupled to a valve-associated cam follower element.

[0051] Advantageous are embodiments of the internal combustion engine in which both the at least one inlet opening and the at least one outlet opening of the at least one cylinder are equipped with a mode-adaptable valve.

[0052] This embodiment is advantageous for implementing concepts in which the charge exchange occurs via the openings of at least one cylinder in both the four-stroke and two-stroke processes, i.e., the intake air is drawn in via intake ports and the exhaust gases are discharged via exhaust ports. Here, too, a pressure differential is preferably generated by means of a mechanical supercharger to scavenge the combustion chamber during the charge exchange, in order to support or enable the charge exchange in the two-stroke process.

[0053] Nevertheless, during the two-stroke cycle, i.e., in the first operating mode, the charge exchange can also utilize openings other than the cylinder's intake and exhaust ports. For example, slots can be provided in the cylinder bore for introducing charge air and / or for expelling exhaust gases. The piston, oscillating within the cylinder bore, then preferably controls the at least one slot in such a way that, upon passing bottom dead center, the piston opens the slot to allow the charge exchange, i.e., the gas exchange.

[0054] Alternative concepts for charge exchange during the two-stroke cycle combine the cylinder's intake and / or exhaust ports with additional openings. For example, a slot in the cylinder bore can be used to introduce charge air, and at least one exhaust port can be used to expel the exhaust gas. In this case, only the at least one exhaust port of the cylinder, and not the intake port, would need to be equipped with a mode-adaptable valve to allow this exhaust port to be used for charge exchange in both the two-stroke and four-stroke cycles.

[0055] Therefore, embodiments of the internal combustion engine in which at least one exhaust port of at least one cylinder is equipped with a mode-adaptable valve are also advantageous.

[0056] For the reasons mentioned above, embodiments of the internal combustion engine are also advantageous in which the at least one cylinder has at least one further opening for supplying charge air.

[0057] If the at least one cylinder has at least one further opening for supplying charge air, embodiments are advantageous in which the at least one cylinder has at least one slot serving as a further opening, which can be controlled by means of a piston oscillating in the cylinder for supplying charge air.

[0058] In current technology, two-stroke engines often use slots for charge exchange. Therefore, it is a proven concept.

[0059] Turbocharging is a suitable means of increasing the power output of an internal combustion engine without changing its displacement, or of reducing the displacement while maintaining the same power output. In either case, turbocharging leads to an increase in power output within the available space and a more favorable power-to-weight ratio. Given the same vehicle parameters, this allows the load profile to be shifted towards higher loads, at which specific fuel consumption is lower.

[0060] Turbocharging therefore supports the ongoing effort in the development of internal combustion engines to minimize fuel consumption, i.e., to improve the efficiency of the combustion engine. With targeted turbocharging design, advantages can also be achieved in terms of exhaust emissions. For example, with suitable turbocharging, nitrogen oxide emissions from diesel engines can be reduced without compromising efficiency.

[0061] To comply with future emission limits, further measures are necessary. A key focus of development work is the reduction of nitrogen oxide emissions, which are particularly relevant for diesel engines. Since the formation of nitrogen oxides requires not only excess air but also high temperatures, one approach to reducing nitrogen oxide emissions involves developing combustion processes with lower combustion temperatures.

[0062] Exhaust gas recirculation (EGR), i.e., the recirculation of combustion gases from the exhaust side to the intake side, is a promising approach, as nitrogen oxide emissions can be significantly reduced with an increasing exhaust gas recirculation rate. The exhaust gas recirculation rate x AGR is determined to x AGR = m AGR / (m AGR + m Frischluft ), where m AGR the mass of recirculated exhaust gas and m Frischluftrefers to the supplied fresh air.

[0063] To achieve a significant reduction in nitrogen oxide emissions, high exhaust gas recirculation rates are required, on the order of x AGR The percentage can be approximately 60% to 70%.

[0064] When operating an internal combustion engine with exhaust gas turbocharging and simultaneously using exhaust gas recirculation, a conflict can arise if the recirculated exhaust gas is extracted from the exhaust gas discharge system upstream of the turbine via high-pressure EGR and is no longer available to drive the turbine.

[0065] When the exhaust gas recirculation rate increases, the exhaust gas mass flow rate introduced into the turbine decreases simultaneously. This reduced exhaust gas mass flow through the turbine results in a lower turbine pressure ratio, which in turn reduces the boost pressure ratio, thus equating to a lower compressor mass flow rate. In addition to the decreasing boost pressure, further problems can arise during compressor operation, particularly regarding the compressor's surge line. Disadvantages can also result with regard to pollutant emissions, for example, increased soot formation in diesel engines during acceleration.

[0066] For this reason, concepts are needed that ensure sufficiently high boost pressures, especially under partial load, while simultaneously maintaining high exhaust gas recirculation rates. One solution is the so-called low-pressure EGR.

[0067] In contrast to the previously mentioned high-pressure EGR, which extracts exhaust gas from the exhaust system upstream of the turbine and introduces it into the intake system downstream of the compressor, a low-pressure EGR system recirculates exhaust gas that has already passed through the turbine to the intake side. For this purpose, the low-pressure EGR system includes a recirculation line that branches off from the exhaust system downstream of the turbine and connects to the intake system upstream of the compressor.

[0068] The exhaust gas recirculated to the intake side via low-pressure EGR is mixed with fresh air upstream of the compressor or within the compressor itself. This mixture of fresh air and recirculated exhaust gas forms the charge air, which is compressed in the compressor. Downstream of the compressor, the compressed charge air may be further compressed in the mechanical supercharger and / or cooled in the charge air cooler.

[0069] It is harmless that exhaust gas is routed through the compressor during low-pressure EGR, since the exhaust gas used is generally one that has undergone exhaust aftertreatment downstream of the turbine, particularly in a particulate filter. Therefore, deposits in the compressor that could alter its geometry, especially the flow cross-sections, and thus reduce its efficiency, are not a concern.

[0070] For the reasons stated above, embodiments are advantageous in which an exhaust gas recirculation is provided, which includes a recirculation line that branches off downstream of the turbine from the exhaust gas discharge system and enters the intake system upstream of the compressor.

[0071] The second sub-problem underlying the invention is solved by a method for operating an internal combustion engine according to a type described above, characterized in that, starting from an internal combustion engine operated in the second operating mode, in which the at least one mode-adaptable valve is actuated by means of a second cam, the valve train is switched to transition the internal combustion engine into the first operating mode if the speed n mot a predetermined rotational speed n of the internal combustion engine mot,limit falls below this value in such a way that at least one mode-adaptable valve is actuated by means of the first cam.

[0072] What has already been said regarding the internal combustion engine according to the invention also applies to the method according to the invention. Corresponding to the different embodiments of the internal combustion engine according to the invention, different method variants are also necessary or expedient, for which reference is made to the corresponding material features of the internal combustion engine and the associated embodiments.

[0073] Advantageous are process variants in which the valve train is switched to the first operating mode to transfer the internal combustion engine if the speed n mot a predetermined rotational speed n of the internal combustion engine mot,limit falls below and remains lower than this predetermined rotational speed n for a predefined time period Δt1 mot,limit .

[0074] The introduction of an additional condition for switching the valve train or transferring the internal combustion engine is intended to prevent excessively frequent switching, in particular switching when the engine speed only briefly falls below the specified value and then rises again or fluctuates around the specified value, without the falling below the specified value justifying or requiring a switching of the valve train.

[0075] If the rotational speed n mot a predefinable rotational speed n mot,limit,2 If the limit is exceeded again, the valve train is switched again to transfer the internal combustion engine from the first operating mode back to the second operating mode.

[0076] For the reasons already mentioned, process variants are also advantageous in the present case in which switching only occurs if the rotational speed n mot a predefinable rotational speed n mot,limit,2 exceeds and the rotational speed n motfor a predefined time interval Δt2 is higher than this predefined rotational speed n mot,limit,2 .

[0077] Advantageous are variants of the method in which the internal combustion engine is only switched from the second operating mode to the first operating mode if the load T mot the internal combustion engine is greater than a predefinable load T mot,limit .

[0078] This method variant takes into account the fact that the four-stroke process is better suited for providing smaller loads than the two-stroke process.

[0079] Advantageous are process variants in which the shut-off element is adjusted towards the closed position during the transition of the internal combustion engine from the second operating mode to the first operating mode in order to increase the charge air flow guided by the mechanical supercharger.

[0080] The primary function of the mechanical supercharger is to provide a scavenging gradient for the purpose of charge exchange during the two-stroke cycle. Therefore, it is advisable to adjust the shut-off element towards the closed position when the internal combustion engine is transitioned to or operated in its first operating mode.

[0081] Advantageous are process variants in which the compressor of the at least one exhaust gas turbocharger is designed for larger charge air mass flows above the predefinable speed n. mot,limit appear.

[0082] Advantageous are process variants in which the shut-off element in the bypass line is adjusted towards the open position if the rotational speed n mot a predetermined rotational speed n of the internal combustion engine mot,limit exceeds.

[0083] The invention is described below using an exemplary embodiment according to the Fig. 1a, Fig. 1b and Fig. 2 described in more detail. This shows: Fig. 1a Schematic representation of parts of the switchable valve train of a first embodiment of the internal combustion engine in a switching state corresponding to the first operating mode, Fig. 1b schematically parts of the switchable valve train of a first embodiment of the internal combustion engine in a switching state corresponding to the second operating mode, and Fig. 2 schematically the engine map of a first embodiment of the internal combustion engine, in which the application areas of the two-stroke process and the four-stroke process are characterized.

[0084] Fig. Figure 1a schematically shows parts of the switchable valve train 1 of a first embodiment of the internal combustion engine in a first switching state. Fig. Figure 1b shows this valve train 1 in a second switching state.

[0085] Two cams can be used to actuate the mode-adaptive valve (not shown). A first cam actuates the mode-adaptive valve in the first operating mode, and a second cam actuates the mode-adaptive valve in the second operating mode.

[0086] A rocker arm 2a is shown in the center, serving as a valve-associated cam follower element 2 and arranged in the force flow between the activated cam and the valve. Adjacent to this rocker arm 2a are two further rocker arms 3a, 4a, which serve as cam follower elements 3, 4 and can be mechanically coupled to the valve-associated cam follower element 2. A pin 6, which is slidably mounted in a chamber 5 and hydraulically actuated, serves this purpose.

[0087] According to Fig. 1a is the first cam follower element 3 belonging to the first cam mechanically coupled with the valve-associated cam follower element 2 in order to actuate the mode-adaptable valve in the first operating mode of the internal combustion engine according to the two-stroke process by means of the first cam.

[0088] According to Fig. 1b the second cam follower element 4 belonging to the second cam is mechanically coupled to the valve-associated cam follower element 2 in order to actuate the mode-adaptable valve in the second operating mode according to the four-stroke procedure by means of a second cam, i.e. deflect it.

[0089] The one in the Fig. 1a and Fig. The valve train 1 shown in 1b is hydraulically switchable in such a way that the actuation of the at least one mode-adaptable valve is carried out in the second operating mode by means of a second cam and in the first operating mode by means of a first cam.

[0090] Fig. Figure 2 schematically shows the engine map of a first embodiment of the internal combustion engine, in which the application areas of the two-stroke process and the four-stroke process are indicated.

[0091] As in Fig. As shown in section 2, the two-stroke cycle is used in the lower speed range if the load T mot the internal combustion engine is greater than a predefinable load T mot,limit (unhatched area of ​​the characteristic map). If the load falls below T mot the indicated limit load T mot,limit and / or exceeds the rotational speed n mot the internal combustion engine at a specific speed n mot,limit The internal combustion engine is switched to the second operating mode by changing the valve train, i.e., it operates using the four-stroke cycle. The application range of the four-stroke cycle is indicated by hatching.

[0092] The full load curves are also shown, with dashed lines for the two-stroke process and solid lines for the four-stroke process. Reference sign 1 Valve train 2 valve-associated cam sequence element 2a rocker arm 3 first cam-associated cam sequence element 3a rocker arm 4 second cam-associated cam sequence element 4a Rocker arm 5 chamber 6 pens n mot Engine speed n mot,limit Maximum engine speed T mot load of the internal combustion engine T mot,limit Maximum load of the internal combustion engine

Claims

[1] Internal combustion engine which can be operated either according to a first operating mode using a two-stroke process or according to a second operating mode using a four-stroke process, with - at least one cylinder, which has at least one exhaust port for removing exhaust gases via an exhaust system and at least one intake port for supplying charge air via an intake system, and - a valve train comprising a valve for each opening and a valve actuation device for actuating the valves, comprising at least one camshaft with several cams, wherein - at least one mechanical supercharger is arranged in the intake system, wherein a bypass line is provided to bypass the mechanical supercharger, which branches off from the intake system upstream of the supercharger and re-enters the intake system downstream of the supercharger and in which a shut-off element is arranged, - at least one exhaust gas turbocharger is provided, comprising a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system, the compressor being located upstream of the mechanical turbocharger in the intake system, - at least one camshaft for actuating at least one mode-adaptable valve has two different cams, wherein a first cam serves to actuate this mode-adaptable valve in the first operating mode and a second cam serves to actuate this mode-adaptable valve in the second operating mode, - the valve train (1) is switchable in such a way that the actuation of the at least one mode-adaptable valve is carried out in the second operating mode by means of a second cam and in the first operating mode by means of a first cam, and - the valve actuation device for actuating the valves comprises several valve-associated cam follower elements (2), wherein each valve-associated cam follower element (2) is arranged in the force flow between a cam and a valve in such a way that the valve performs an oscillating stroke movement when the camshaft rotates, wherein two cam follower elements (3, 4) are provided for the two different cams of the at least one mode-adaptable valve, which are mechanically coupled to the valve-associated cam follower element (2), wherein a first cam follower element (3) associated with the first cam is mechanically coupled to the valve-associated cam follower element (2) in order to actuate the mode-adaptable valve in the first operating mode by means of the first cam, and a second cam follower element (4) associated with the second cam is mechanically coupled to the valve-associated cam follower element (2),to actuate the valve in the second operating mode using a second cam. [2] Internal combustion engine according to claim 1, characterized by , that at least one valve-associated cam sequence element (2) is a rocker arm (2a) and / or the at least two cam-associated cam sequence elements (3,4) are rocker arms (3a,4a). [3] Internal combustion engine according to any one of the preceding claims, characterized by , that both the at least one inlet port and the at least one outlet port of the at least one cylinder are equipped with a mode-adaptable valve. [4] Internal combustion engine according to any one of claims 1 to 3, characterized by that at least one exhaust port of at least one cylinder is equipped with a mode-adaptable valve. [5] Internal combustion engine according to any one of the preceding claims, characterized by that at least one cylinder has at least one further opening for supplying charge air. [6] Internal combustion engine according to claim 5, characterized by , that the at least one cylinder has at least one slot serving as a further opening, which can be controlled by means of a piston oscillating in the cylinder to supply charge air. [7] Internal combustion engine according to any one of the preceding claims, characterized by , that an exhaust gas recirculation is provided, which includes a recirculation line that branches off downstream of the turbine from the exhaust gas discharge system and enters the intake system upstream of the compressor. [8] Method for operating an internal combustion engine according to any of the preceding claims, characterized by , that starting from an internal combustion engine operating in the second operating mode, in which at least one mode-adaptable valve is actuated by means of a second cam, the valve train is switched to transition the internal combustion engine into the first operating mode if the speed n mota predetermined rotational speed n of the internal combustion engine mot,limit falls below this value in such a way that at least one mode-adaptable valve is actuated by means of the first cam. [9] Method according to claim 8, characterized by that the internal combustion engine transitions from the second operating mode to the first operating mode only if the load T mot the internal combustion engine is greater than a predefinable load T mot,limit . [10] Method according to claim 8 or 9, characterized by , that the shut-off element is adjusted towards the closed position during the transition of the internal combustion engine from the second operating mode to the first operating mode in order to increase the charge air flow guided by the mechanical supercharger. [11] Method according to claim 8, 9 or 10, characterized by , that the compressor of the at least one exhaust gas turbocharger is designed for larger charge air mass flows above the predefinable speed n mot,limitappear. [12] Method according to any one of claims 8 to 11, characterized by , that the shut-off element is moved towards the open position if the rotational speed n mot a predetermined rotational speed n of the internal combustion engine mot,limit exceeds.

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