Two-stroke opposed piston internal combustion engine

Abstract

A two-stroke opposed piston internal combustion engine including a plurality of cylinders, each cylinder being provided with a first piston and a second piston adapted to perform opposed motions in the cylinder, each cylinder being provided with at least one intake port, a communication between an air intake arrangement and the cylinder via the intake port being dependent on the position of the first piston, each cylinder further being provided with at least one exhaust port, a communication between an exhaust guiding arrangement and the cylinder via the exhaust port being dependent on the position of the second piston, at least one of the cylinders being provided with an additional port and an additional port valve, a communication between the cylinder and an additional conduit externally of the cylinder, via the additional port, being controllable with the additional port valve, the air intake arrangement including at least one intake valve for selectively reducing or inhibiting air admittance to at least one of the cylinders.

Claims

1. A two-stroke opposed piston diesel internal combustion engine comprising: a plurality of cylinders, wherein each cylinder of the plurality of cylinders includes a first piston and a second piston, the first piston and the second piston configured to perform opposed motions within the cylinder; an air intake arrangement, wherein a communication between the air intake arrangement and each cylinder via a respective intake port is dependent on a position of the first piston; and an exhaust guiding arrangement, wherein a communication between the exhaust guiding arrangement and each cylinder via a respective exhaust port is dependent on a position of the second piston, wherein the plurality of cylinders comprises a first subgroup of cylinders and a second subgroup of cylinders, such that each cylinder of the first subgroup of cylinders is provided with a cylinder port valve, the cylinder port valve configured to control an amount of air expelled from the cylinder to a location directly upstream of the respective cylinder of the first subgroup of cylinders via a cylinder port conduit, and such that each cylinder of the second subgroup of cylinders is provided with an air intake valve, the air intake valve configured to control an amount of air admitted to the respective cylinder of the second subgroup of cylinders via the air intake arrangement.

2. The engine according to claim 1, wherein the cylinder port conduit is located such that, in an open position of the cylinder port valve, the cylinder port valve allows communication between the respective cylinder of the first subgroup of cylinders and the location directly upstream of the respective cylinder of the first subgroup of cylinders via the cylinder port conduit when communication between the respective cylinder of the first subgroup of cylinders and the air intake arrangement via the respective intake port is blocked by the first piston, and communication between the respective cylinder of the first subgroup of cylinders and the exhaust guiding arrangement via the respective exhaust port is blocked by the second piston.

3. The engine according to claim 1, wherein the respective intake port and the respective exhaust port are spaced apart in a longitudinal direction of the cylinder, and the cylinder port valve is located between the respective intake port and the respective exhaust port.

4. The engine according to claim 1, wherein the cylinder port conduit is an extension of, or is arranged to communicate with, the air intake arrangement.

5. The engine according to claim 1, wherein the cylinder port valve is a poppet valve.

6. The engine according to claim 1, wherein the cylinder port valve is arranged such that a part of the cylinder port valve extends, in an open position of the cylinder port valve, into the cylinder, and the first and second pistons each comprise a respective recess to accommodate the part of the cylinder port valve.

7. The engine according to claim 1, wherein the cylinder port conduit is located such that top dead centre positions of the first and second pistons are symmetrically distributed with respect to the cylinder port conduit.

8. The engine according to claim 1, wherein the cylinder port valve is arranged such that a part of the cylinder port valve extends, in an open position of the cylinder port valve, into the cylinder, and top dead centre positions of the first and second pistons are symmetrically located with respect to the cylinder port conduit of the cylinder port valve.

9. The engine according to claim 1, wherein the cylinder port conduit is offset from a symmetry plane of the respective cylinder of the first subgroup of cylinders, such that a longitudinal direction of the cylinder is perpendicular to the symmetry plane, and top dead centre positions of the first and second pistons are symmetrically located with respect to the symmetry plane of the respective cylinder of the first subgroup of cylinders.

10. The engine according to claim 9, wherein the cylinder port conduit is external to the respective cylinder of the first subgroup of cylinders, such that communication between the respective cylinder of the first subgroup of cylinders and the cylinder port conduit is dependent on the position of at least one of the first piston and the second piston.

11. The engine according to claim 9, wherein a space is provided between the cylinder port conduit and the symmetry plane of the respective cylinder of the first subgroup of cylinders.

12. The engine according to claim 9, wherein the respective intake port and the respective exhaust port are located on opposite sides of the symmetry plane of the respective cylinder of the first subgroup of cylinders, and the cylinder port conduit is located on a same side of the symmetry plane of the respective cylinder of the first subgroup of cylinders as the respective intake port.

13. The engine according to claim 12, wherein the cylinder port conduit extends, in the longitudinal direction of the cylinder, closer to the symmetry plane of the respective cylinder of the first subgroup of cylinders than the respective intake port.

14. The engine according to claim 9, wherein the respective intake port and the respective exhaust port are located on opposite sides of the symmetry plane of the respective cylinder of the first subgroup of cylinders, and the cylinder port conduit is located on a same side of the symmetry plane of the respective cylinder of the first subgroup of cylinders as the respective exhaust port.

15. The engine according to claim 14, wherein the cylinder port conduit extends, in the longitudinal direction of the cylinder, closer to the symmetry plane of the respective cylinder of the first subgroup of cylinders than the respective exhaust port.

16. A method of controlling a two-stroke opposed piston diesel internal combustion engine, the method comprising: providing a plurality of cylinders, wherein each cylinder of the plurality of cylinders includes a first piston and a second piston, the first piston and the second piston configured to perform opposed motions within the cylinder, an air intake arrangement, wherein a communication between the air intake arrangement and each cylinder via a respective intake port is dependent on a position of the first piston, and an exhaust guiding arrangement, wherein a communication between the exhaust guiding arrangement and each cylinder via a respective exhaust port is dependent on a position of the second piston, wherein the plurality of cylinders comprises a first subgroup of cylinders and a second subgroup of cylinders, such that each cylinder of the first subgroup of cylinders is provided with a cylinder port valve, the cylinder port valve configured to control an amount of air expelled from the cylinder to a location directly upstream of the respective cylinder of the first subgroup of cylinders via a cylinder port conduit, and such that each cylinder of the second subgroup of cylinders is provided with an air intake valve, the air intake valve configured to control an amount of air admitted to the respective cylinder of the second subgroup of cylinders via the air intake arrangement; providing a fuel system for controlling a supply of fuel to the plurality of cylinders via at least one fuel injector provided at each cylinder of the plurality of cylinders; opening, in at least one of the cylinders of the first subgroup of cylinders, the cylinder port valve between consecutive first and second top dead centre positions of the first piston, and subsequently, during a movement of the first piston towards the second top dead centre position, before the first piston has reached the second top dead centre position, closing the cylinder port valve after a transition from a condition where the respective intake port is not blocked by the first piston, to a condition where the respective intake port is blocked by the first piston; and reducing or inhibiting a supply of air via the air intake valve of the respective cylinder of the second subgroup of cylinders and fuel via the at least one fuel injector at each cylinder of the second subgroup of cylinders so as to reduce or inhibit combustion in the second subgroup of cylinders, wherein reducing or inhibiting combustion in the second subgroup of cylinders is performed simultaneously with the steps of opening and closing the cylinder port valve in the at least one cylinder of the first subgroup of cylinders.

17. The method according to claim 16, wherein the step of opening the cylinder port valve is performed during a movement of the first piston away from the first top dead centre position, and after a transition from the condition where the respective intake port is blocked by the first piston, to the condition where the respective intake port is not blocked by the first piston.

18. A computer programmed to perform the steps of the method according to claim 16.

19. A non-transitory computer readable medium for a computer programmed to perform the steps of the method according to claim 16.

20. A controller for a two-stroke opposed piston diesel internal combustion engine, the engine comprising: a plurality of cylinders, wherein each cylinder of the plurality of cylinders includes a first piston and a second piston, the first piston and the second piston configured to perform opposed motions within the cylinder; an air intake arrangement, wherein a communication between the air intake arrangement and each cylinder via a respective intake port is dependent on a position of the first piston; and an exhaust guiding arrangement, wherein a communication between the exhaust guiding arrangement and each cylinder via a respective exhaust port is dependent on a position of the second piston, wherein the plurality of cylinders comprises a first subgroup of cylinders and a second subgroup of cylinders, such that each cylinder of the first subgroup of cylinders is provided with a cylinder port valve, the cylinder port valve configured to control an amount of air expelled from the cylinder to a location directly upstream of the respective cylinder of the first subgroup of cylinders via a cylinder port conduit, and such that each cylinder of the second subgroup of cylinders is provided with an air intake valve, the air intake valve configured to control an amount of air admitted to the respective cylinder of the second subgroup of cylinders via the air intake arrangement, and wherein the controller is configured to control a respective valve actuator such that the cylinder port valve will assume an open position or a closed position, the open position allowing communication between the respective cylinder of the first subgroup of cylinders and the cylinder port conduit, and the closed position preventing communication between the respective cylinder of the first subgroup of cylinders and the cylinder port conduit, the controller being further configured to perform the steps of the method according to claim 16.

Description

DESCRIPTION OF FIGURES

(1) Below embodiments of the invention will be described with reference to the drawings, in which

(2) FIG. 1 shows a truck with an engine as depicted in FIG. 2,

(3) FIG. 2 shows components of a four cylinder engine according to an embodiment of the invention,

(4) FIG. 3 shows a longitudinally cross-sectional view of one of the cylinders of the engine in FIG. 2,

(5) FIG. 4 shows the cylinder in FIG. 3 during a compression stroke,

(6) FIG. 5 shows the cylinder in FIG. 3 when pistons therein are at top dead centres,

(7) FIG. 6 depicts steps in a first method for controlling the engine in FIG. 2,

(8) FIG. 7 depicts steps in a second method for controlling the engine in FIG. 2,

(9) FIG. 8 depicts steps in a third method for controlling the engine in FIG. 2,

(10) FIG. 9 shows components of an engine according to an alternative embodiment of the invention, and

(11) FIG. 10 shows a longitudinally cross-sectional view of a cylinder of an engine according to a further embodiment of the invention.

DETAILED DESCRIPTION

(12) FIG. 1 shows a vehicle 500 in the form of a truck comprising a two-stroke opposed piston (2SOP) diesel engine 1 according to an embodiment of the invention. As can be seen in FIG. 2, the engine comprises four cylinders 2, air intake arrangement 5 with a single intake manifold, an exhaust guiding arrangement 6 with a single exhaust manifold. An exhaust after treatment system (EATS) 16 is provided in the exhaust guiding arrangement 6. Further a turbo charger 17 is provided upstream of the EATS 16, to pressurise air in the intake arrangement 5 using energy of the exhaust gases in the exhaust guiding arrangement 6. Alternatively, the turbo charger may be powered by an electric motor. The intake arrangement is provided with an intercooler 18. A supercharger could also be provided as is known in the art.

(13) As suggested in FIG. 3, the cylinders are presented in an engine block 110. Each cylinder is provided with a first piston 3 and a second piston 4 adapted to perform opposed motions in the cylinder. As can be seen in FIG. 3, the engine presents two crankshafts 302, 402, and the first and second pistons 3, 4 are connected to a respective of the crankshafts 302, 402.

(14) As can be seen in FIG. 3, each cylinder 2 is, at one of its ends, herein referred to as the intake end, provided with a plurality of intake ports 7. The intake ports 7 are distributed circumferentially around the cylinder interior wall. The intake ports 7 are located such that a communication between the air intake arrangement 5 and the cylinder 2 via the intake ports 7 is dependent on the position of the first piston 3. Each cylinder 2 is, at its opposite end, herein referred to as the exhaust end, provided with a plurality of exhaust ports 8. The exhaust ports 8 are distributed circumferentially around the cylinder interior wall. The exhaust ports 8 are located such that a communication between the exhaust guiding arrangement 6 and the cylinder 2 via the exhaust ports 7 is dependent on the position of the second piston 4.

(15) The first and second pistons 3, 4 are as stated adapted to perform opposed motions in the cylinder 2, but as is known in 2SOP engines, the piston at exhaust end can be slightly advanced in its oscillating movement compared to the piston at the intake end. For this presentation, the pistons 3, 4 are regarded as adapted to perform opposed motions in the cylinder 2, regardless whether the phase of movement of one of the pistons is slightly offset in relation to the phase of the movement of the other piston.

(16) As can be seen in FIG. 3, a fuel system 15 is provided for controlling the supply of fuel to the cylinders 2. The fuel system 15 comprises a fuel injector at each cylinder, adapted to inject fuel between the top dead centres of the pistons.

(17) As can be seen in FIG. 3, each cylinder 2 is provided with an additional port 9 and an additional port valve 10. A communication between the cylinder 2 and an additional conduit 11 externally of the cylinder, via the additional port 9, is controllable with the additional port valve 10. The additional conduit 11 is in communication with the air intake arrangement 5.

(18) A controller 13 for the engine is provided. The controller 13 comprises a computer program comprising program code means for performing steps of methods described below. The controller 13 is adapted to control a valve actuator 14, (FIG. 3), for the additional port valves 10. The valve actuator 14 operates pneumatically, e.g. as described in EP 1299622 incorporated herein by reference. Alternatively, the valve actuator 14 can be an electromechanical actuator or a hydraulic actuator, it can include a stepper motor, or it can include a cam shaft with cam switching, cam phasing, or an oscillating cam.

(19) As can be seen in FIG. 3, the intake ports 7 and the exhaust ports 8 are spaced apart in the longitudinal direction of the cylinder 2, and the additional port 9 is located between the intake ports 7 and the exhaust ports 8. Thus, as can be seen in FIG. 4, in an open position of the additional port valve 10, it allows communication between the cylinder 2 and the additional conduit 11 via the additional port 9, when communication between the cylinder 2 and the air intake arrangement 5 via the intake ports 7 is blocked by the first piston 3, and communication between the cylinder 2 and the exhaust guiding arrangement 6 via the exhaust ports 8 is blocked by the second piston 4.

(20) The additional port valve 10 is a poppet valve, and as can be seen in FIG. 4, it is arranged so that a part 101 of it extends, in the open position of the additional port valve 10, into the cylinder 2. The first and second pistons 3, 4 present respective recesses 301, 401 to accommodate said part 101 of the additional port valve 10.

(21) As can be seen in FIG. 5, the top dead centres of the first and second pistons 3, 4 are symmetrically distributed with respect to the additional port 9. As suggested also in FIG. 3 and FIG. 4, the part 101 of the additional port valve 10, which extends, in the open position of the additional port valve 10, into the cylinder 2, is symmetric with respect to a symmetry plane S, (FIG. 3). The normal of the symmetry plane S is parallel to the longitudinal direction of the cylinder 2, and the piston top dead centres are symmetrically located with respect to the symmetry plane S.

(22) Below three methods of controlling the engine described above are presented with reference to FIG. 6, FIG. 7 and FIG. 8.

(23) Reference is made to FIG. 6. The controller 13 is adapted to control steps of a first method as follows: The controller determines whether the engine is in a low load operational mode, SI. This mode can be defined for example by one or more thresholds of one or more operational parameters, such as an engine torque request from a throttle pedal operated by a driver of the vehicle. If it is determined that the engine is not in the first low load mode, the additional port valves in the cylinders are kept closed during the entire cycles of the cylinders, S2.

(24) If it is determined that the engine is in the first low load mode, the controller controls the additional port valves 10 in the cylinders 2, in the following steps:

(25) At each cycle in the respective cylinder, the additional port valve 10 is opened between consecutive first and second top dead centre (TDC) positions of the first piston 3. More specifically, the additional port valve 10 is opened during a movement of the first piston 3 away from the first top dead centre position, (downwards in FIG. 3), S3. Further, the opening does not start until the first piston has started to expose the intake ports 7 to the cylinder interior. In other words, the opening of the additional port valve 10 takes place after a transition from a condition where a communication between the air intake arrangement 5 and the cylinder 2 via the intake ports 7 is blocked by the first piston 3, to a condition with a communication between the air intake arrangement 5 and the cylinder 2 via the intake port 7.

(26) Subsequently, the additional port valve 10 is closed during a movement of the first piston 3 towards the second top dead centre position, (upwards in FIG. 3), before the first piston 3 has reached the second top dead centre position, after the first piston 3 has again blocked the exposure of the intake ports 7 to the cylinder interior, S4. In other words, the additional port valve 10 is closed after a transition from a condition with a communication between the air intake arrangement 5 and the cylinder 2 via the intake ports 7, to a condition where the communication between the air intake arrangement 5 and the cylinder 2 via the intake ports 7 is blocked by the first piston 3.

(27) Keeping the additional port valve 10 open after the intake ports 7 have been blocked will result in a portion of the air introduced to the cylinder, being expelled through the additional port 9. Thereby less air will be provided for the subsequent combustion with the relative low amount of fuel injected in the first low load mode, and this will contribute to a high exhaust gas temperature, keeping the processes in the EATS efficient in the first low load mode.

(28) In this example, the additional port valve opening and closing steps S3, S3 are performed in all cylinders. Between each cycle in each cylinder, the determination whether the engine is in the first low load mode, SI, is repeated. Alternatively, the determination whether the engine is in the first low load mode can be performed at predetermined time intervals.

(29) As shown in FIG. 2, the air intake arrangement 5 comprises for each cylinder an intake valve 12 for selectively reducing or inhibiting air admittance to the cylinders 2.

(30) Reference is made also to FIG. 3 and FIG. 7. The controller is adapted to control the additional port valves 10, the intake valves 12, and the fuel system 15, so as to control a second method as follows:

(31) The controller determines whether the engine is in a second low load mode, S5. If it is determined that the engine is not in the second low load mode, controller determines whether the engine is in a first low load mode, SI. The engine load at the second low load mode is lower than the load at the first low load mode. For example, the first low load mode can include a first interval of engine torque requests and the second low load mode can include a second interval of engine torque requests, where the highest value of the second torque request interval is lower than the lowest value of the first torque request interval.

(32) If it is determined that the engine is not in the first low load mode, all additional port valves 10 in the cylinders are kept closed during the entire cycles of the cylinders, and all intake valves 12 are kept open, S2.

(33) If it is determined that the engine is in the first low load mode, the additional port valve 10 in all cylinders are opened and closed as described above with reference to FIG. 6, S3, S4. This will keep the exhaust temperature sufficiently high for efficient EATS processes during the first low load mode.

(34) If it is determined in step S5 that the engine is in the second low load mode, the intake valves 12 are controlled so that air supply to two of the cylinders, herein referred to as cylinders C2 and C4, is terminated, S6. More specifically, referring to FIG. 2, the intake valves 12 in respective air intake arrangement branches for air supply to cylinders C2 and C4 are closed. In addition, the fuel system 15 is controlled so that the fuel supply to cylinders C2 and C4 is terminated. Further, the additional port valves in cylinders C2 and C4 are kept closed. Further, if it is determined that the engine is in the second low load mode, the additional port valves 10 in the remaining cylinders, herein referred to as cylinders C1 and C3, are opened and closed, S7, S8, in the same manner as has been described above with reference to FIG. 6, (steps S3, S4).

(35) Thereby cylinders C2 and C4 are deactivated, while cylinders CI and C3 are controlled with a reduced air supply by means of the additional port valves 10. The combination of cylinder deactivation and air reduction in active cylinders substantially reduces the air transport to the exhaust guiding arrangement 6 during the second low load mode. Thereby, a particularly effective manner of maintaining a high exhaust temperature is provided. This will keep the exhaust temperature sufficiently high for efficient EATS processes also during the second low load mode.

(36) It should be noted that the choice of cylinders C2 and C4 for deactivation is an example. In some embodiments, the cylinder deactivation can be rotated, such that at some time intervals, a certain subgroup of the cylinders is deactivated, while at other time intervals another subgroup of the cylinders is deactivated. In the respective time intervals, the additional port valves 10 in the remaining cylinders can be opened and closed during the cylinder cycles.

(37) At predetermined time intervals the determinations whether the engine is in the second low load mode, and where applicable whether the engine is in the first low load mode, are repeated.

(38) The method described with reference to FIG. 7 involves stepwise exhaust temperature increasing measures. In this example, at a relatively low engine load, such measures are limited to the additional port valve operation, while at an even lower engine load, the cylinder deactivation is added as a further measure. In alternative embodiments, different stepwise strategies could be adopted. For example cylinder deactivation could be a measure at the higher of the low load intervals, while the mix of cylinder deactivation and additional port valve operation could be activated at the lower of the low load intervals.

(39) Reference is made also to FIG. 8 and FIG. 9. In the embodiment in FIG. 9, the EATS 16 is located upstream of the turbo charger 17. A pressure sensor 601 is adapted to detect the pressure in the exhaust guiding arrangement 6, upstream of the EATS 16. It should be noted that the pressure sensor 106 can alternatively be located downstream of the EATS 16 but nevertheless upstream of the turbo charger 17. The pressure sensor is connected to the controller 13, and can thereby produce data corresponding to pressure feedback to the controller 13. The controller is adapted to control the intake valves 12 and the fuel system 15, so as to control a third method as follows:

(40) In the method described with reference to FIG. 8, the controller 13 determines based on feedback from the pressure sensor 601 the pressure in the exhaust guiding arrangement 6, SI 1. The controller 13 compares the determined pressure value to a predetermined pressure threshold value, SI 2. If the pressure is above the threshold value, all intake valves 12 are kept open, S2. If the pressure is below the threshold value, the intake valves 12 are controlled so that air supply to cylinders C2 and C4 is terminated, S6. More specifically, referring to FIG. 9, the intake valves 12 adapted to control the air supply to cylinders C2 and C4 are closed, and the fuel system 15 is controlled so that the fuel supply to cylinders C2 and C4 is terminated. Thereby cylinders C2 and C4 are deactivated, while cylinders CI and C3 remain active. The additional port valves 10 in cylinders CI and C3 are kept closed.

(41) Thus, upon detecting a low pressure in the exhaust guiding arrangement 6, deactivating cylinders C2 and C4 will increase the pressure in the exhaust guiding arrangement, and this will ensure that catalytic reactions in the EATS are maintained. Also, preventing a pressure drop in the exhaust guiding arrangement 6 by cylinder deactivation, will prevent the turbo charger 17 from stopping to operate. By keeping the turbo charger operating, there will be a further contribution to keeping the pressure in the exhaust guiding arrangement relatively high, which again is beneficial to the processes in the EATS.

(42) As an alternative in the method described with reference to FIG. 8, instead of measuring the exhaust guiding arrangement pressure directly, an engine operation parameter which is correlated to the exhaust guiding arrangement pressure can be used. In such an embodiment, the method comprises providing correlation data which correlates the values of the correlated engine operation parameter to the respective values of the exhaust pressure. The correlation data is before operation of the engine stored in a data storage unit accessible to the controller 13. Thus, in step SI 1 in FIG. 8, instead of determining the exhaust pressure, the controller 13 determines the correlated engine operation parameter, and in step SI 2, the determined correlated engine operation parameter is compared to a correlated engine operation parameter threshold value. The steps S2 or S6 are selected based on the comparison. The correlated engine operation parameter could be for example the engine load, values of which are correlated to respective values of the exhaust gas pressure upstream of the EATS.

(43) It should be noted that the method described with reference to FIG. 8 can also be carried out in an engine where a part of the EATS 16 is located downstream of the turbo charger 17.

(44) In the embodiment in FIG. 9, only cylinders C1 and C3 are provided with additional ports and additional port valves 10. Further, intake valves 12 are provided for only cylinders C2 and C4. In this engine, the method described above with reference to FIG. 6 can be performed in the cylinders with the additional port valves 10. Also, the method described with reference to FIG. 7 can be performed in the engine in FIG. 9. The engine in FIG. 9 entails a reduced complexity and cost, since the additional port valves with actuators, and the intake valves are provided in half the number compared to the engine in FIG. 2.

(45) It should also be noted that the method described with reference to FIG. 8 can be carried out with an engine where all cylinders are provided with additional ports, additional port valves 10, and intake valves 12, as in the engine in FIG. 2.

(46) Reference is made to FIG. 10. The diesel engine depicted in FIG. 20 differs from the engine described above with reference to FIG. 2-4 in that the additional port 9 is offset from a symmetry plane S of the cylinder. The symmetry plane S is located and oriented such that the normal of the symmetry plane S is parallel to the longitudinal direction of the cylinder 2, and the top dead centres of the first and second pistons 3, 4 are symmetrically located with respect to the symmetry plane S. Thus, the additional port 9 is offset from the symmetry plane S in the longitudinal direction of the cylinder.

(47) The communication between the cylinder 2 and an additional conduit 11 is controllable with an additional port valve 10. The additional port valve 10 comprises a spring loaded valve piston 102. A valve actuator 14 is hydraulic and comprises a hydraulic fluid conduit 141 adapted to provide, by means of a hydraulic pump and a hydraulic valve (not shown), a hydraulic pressure against a flange 141 on the valve piston. The pressure on the flange 142 can bias the piston against a valve spring 143 in a direction away from the cylinder 2, so as to open the communication between the cylinder 2 and the additional conduit 11.

(48) The additional port 9 is located on the same side of the symmetry plane S as the intake ports 7. The additional port 9 slightly overlaps the intake ports 7 in the longitudinal direction of the cylinder 2, but extends, again in the longitudinal direction of the cylinder 2, closer to the symmetry plane S than the intake ports 7. A distance is provided between the additional port 9 and the symmetry plane S.

(49) Thereby, the communication between the cylinder 2 and the additional conduit 11 via the additional port 7 is, similarly to the intake ports 7, dependent on the position of the first piston 3.

(50) Thus, for activation of the exhaust temperature increasing operation, described above, in this embodiment, the additional port valve can be opened and remain open throughout the cycles in the cylinder. The additional port 9 will be exposed to the cylinder interior during the movement of the first piston 3 away from the top dead centre, before the ports 7 have been exposed.

(51) Further, additional port 9 will be blocked from the cylinder interior during the movement of the first piston 3 towards the top dead centre, after the ports 7 have been blocked. Thereby, during a compressions stroke during an engine low load mode, air can be expelled through the additional port 9, so that the total amount of air captured in the cylinder is reduced in order to increase the exhaust temperature. After the additional port 9 has been blocked by the first piston 3, during the compression stroke, the remaining air is compressed during the remaining travel of the first piston towards the top dead centre.

(52) It should be noted that in an aspect of the invention, the engine is arranged so that the additional port valve 10 has exclusive control over the communication between the cylinder 2 and the air intake arrangement 5 via the additional port 9. Such an engine could be provided without the intake valves 12, (FIG. 2), for selectively blocking communication between the cylinders 2 and the air intake arrangement 5.