JP5965234B2 - Uniflow 2-stroke engine - Google Patents

Description

  The present invention relates to a uniflow type two-stroke engine equipped with a mechanism for suppressing generation of NOx.

  Uniflow type two-stroke diesel engines mounted on large ships and the like tend to have higher NOx emission regulations as other engines such as automobile engines. As a method for suppressing the generation of NOx, there is exhaust gas recirculation (hereinafter referred to as “EGR”) in which a part of the exhaust gas (burned gas) is returned to the combustion chamber to reduce the oxygen concentration in the cylinder. EGR performed in a uniflow type two-stroke engine is equivalent to external EGR, and the exhaust discharged from the combustion chamber is cooled and washed and returned to the inlet side (scavenging pipe side) through the outside of the combustion chamber. (For example, see Patent Document 1).

  On the other hand, some 4-stroke engines perform not only external EGR but also internal EGR (see Patent Document 2). As the internal EGR, for example, there is a method in which the intake valve is opened (so-called overlap) while the exhaust valve is opened (exhaust stroke), and the burned gas flows backward to the intake passage. Such an internal EGR can return burnt gas to the combustion chamber again without using external equipment. Therefore, the burden on the external EGR device or the like (including an external NOx reduction device other than the external EGR is also included) can be reduced, and the external EGR device or the like can be reduced. Can be eliminated.

JP 2010-133413 A JP-A-7-133726

  In the case of a 4-stroke engine, if an apparatus (so-called variable valve apparatus) that can open and close the exhaust valve and the intake valve at any timing is installed, internal EGR can be performed relatively easily, and the oxygen concentration in the cylinder can be reduced. Can be reduced. On the other hand, in the case of a uniflow type two-stroke engine, the scavenging port corresponding to the intake valve of the four-stroke engine is directly opened and closed by the piston, so that the opening / closing control cannot be substantially performed. That is, in the case of a uniflow type two-stroke engine, even if a variable valve device is used, only an exhaust valve can be opened and closed at an arbitrary timing. Therefore, control of a 4-stroke engine cannot be applied to a uniflow 2-stroke engine as it is.

  The present invention has been made in view of the above circumstances, and provides a uniflow type two-stroke engine capable of reducing the oxygen concentration in a cylinder by adjusting the opening / closing timing of an exhaust valve. Objective.

  A uniflow type two-stroke engine according to an embodiment of the present invention temporarily stores a cylinder in which a scavenging port is formed in a lower portion, a piston that reciprocates in the cylinder and opens and closes the scavenging port, and compressed fresh air. A scavenging pipe for supplying to the scavenging port; an exhaust valve disposed at an upper portion of the cylinder; and a variable valve device for opening and closing the exhaust valve at an arbitrary timing, wherein the exhaust valve is more than normal after a combustion stroke. By starting to open at a later timing, a part of the burned gas is configured not to be discharged from the exhaust valve. According to such a configuration, the burnt gas after the combustion stroke flows back to the scavenging pipe, or at least a lot of burned gas remains in the cylinder. Therefore, in the next combustion stroke, many burned gases can be included in the cylinder, and the oxygen concentration can be reduced.

  In the uniflow type two-stroke engine, when the exhaust valve opens at the timing, the pressure in the cylinder is higher than the pressure in the scavenging pipe when the scavenging port starts to open. The timing may be such that the burned gas flows backward from the cylinder to the scavenging pipe. According to such a configuration, the burned gas that has flowed back from the cylinder to the scavenging pipe flows into the cylinder again, so that the oxygen concentration in the cylinder can be reduced.

  In the uniflow two-stroke engine, the timing that is later than the normal timing may be substantially the same as the timing at which the scavenging port starts to open. According to such a configuration, the burned gas can be more reliably returned from the cylinder to the scavenging pipe.

  In the uniflow type two-stroke engine, the exhaust valve may be configured to open at a normal timing when in a low load region. According to such a configuration, since the backflow of burned gas does not occur in the low load region, it is possible to prevent a scavenging pipe fire that is likely to occur in the low load region.

  The uniflow two-stroke engine may be configured such that the exhaust valve opens at a normal timing when the oil mist concentration in the scavenging pipe exceeds a predetermined upper limit concentration. According to this configuration, when the oil mist concentration in the scavenging pipe is high and fire is likely to occur, the engine is operated more safely because control is performed so that no backflow occurs.

  The uniflow two-stroke engine may be configured such that the opening degree of the exhaust valve is reduced once or a plurality of times during the scavenging stroke. According to such a configuration, a large amount of burned gas can be left in the cylinder by reducing the amount of burnt gas discharged from the exhaust valve during the scavenging stroke. Thereby, the oxygen concentration in a cylinder can be reduced.

  In the uniflow type two-stroke engine, the cycle in which the opening degree of the exhaust valve is reduced during the scavenging stroke and the cycle in which the opening degree of the exhaust valve remains constant during the scavenging stroke are mixed. May be. According to this configuration, for example, even if the exhaust valve is very large and the response is relatively slow, the amount of exhausted NOx can be appropriately controlled.

  In the uniflow type two-stroke engine, when the temperature of the cylinder, the piston, or the exhaust valve exceeds a predetermined upper limit temperature, the opening degree of the exhaust valve is not reduced during the scavenging stroke. The exhaust valve may be configured such that the minimum opening degree of the exhaust valve is reduced, the number of times the opening degree of the exhaust valve is reduced during the scavenging stroke, or a combination thereof. According to such a configuration, when the temperature of the cylinder or the like exceeds the upper limit temperature, a large amount of fresh air flows to cool those members, and as a result, problems that may occur in the cylinder or the like can be prevented. The “maximum temperature” here may be different for each member, or may be common to all members (the same applies hereinafter).

  In the above uniflow type two-stroke engine, when the temperature of the cylinder, the piston, or the exhaust valve exceeds a predetermined upper limit temperature, the frequency of the cycle in which the opening degree of the exhaust valve is reduced during the scavenging stroke. It may be configured to decrease. Even in such a configuration, when the temperature of the cylinder or the like rises too much, a large amount of fresh air flows in, so that it is possible to prevent problems due to excessive rise in the temperature of the cylinder or the like.

  The uniflow two-stroke engine further includes an external NOx reduction device, and when the low-load region is shifted to the medium-load region, the use load of the external NOx reduction device is reduced and the medium-load region is changed to the low-load region It may be configured to increase the usage load of the external NOx reduction device when the transition is made. According to such a configuration, even when the amount of burned gas remaining inside is reduced by the above-described control when shifting from the middle load region to the low load region, the amount of NOx discharged using the external NOx reduction device is reduced. Can be suppressed.

  The uniflow two-stroke engine further includes an external NOx reduction device, and when the temperature of the cylinder, the piston, or the exhaust valve exceeds the predetermined upper limit temperature, the usage load of the external NOx reduction device It may be configured to raise. According to such a configuration, when the temperature in the cylinder exceeds the upper limit value, even if the amount of burnt gas remaining inside decreases, the external NOx reduction device can be used to suppress the NOx emission amount. it can.

  According to the above-described uniflow type two-stroke engine, the oxygen concentration in the cylinder can be reduced by adjusting the opening / closing timing of only the exhaust valve.

FIG. 1 is a block diagram of a uniflow two-stroke engine according to an embodiment of the present invention. FIG. 2 is a diagram showing a normal operation of the exhaust valve. FIG. 3 is a diagram showing the relationship between the crank angle and the opening of the exhaust valve in the case shown in FIG. FIG. 4 is a diagram showing the operation of the exhaust valve in the low load region in the embodiment of the present invention. FIG. 5 is a diagram showing the relationship between the crank angle and the opening of the exhaust valve in the case shown in FIG. FIG. 6 is a flowchart for determining whether to perform preclose control in the embodiment of the present invention. FIG. 7 is a diagram showing the operation of the exhaust valve in the middle load region and the high load region in the embodiment of the present invention. FIG. 8 is a diagram showing the relationship between the crank angle and the opening of the exhaust valve in the case shown in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Below, the same code | symbol is attached | subjected to the element which is the same or it corresponds through all the drawings, and the overlapping description is abbreviate | omitted.

<Outline of engine>
First, an outline of a uniflow two-stroke engine (hereinafter simply referred to as “engine”) 100 according to an embodiment of the present invention will be described. FIG. 1 is a block diagram of an engine 100 according to the present embodiment. The engine 100 according to the present embodiment is a diesel engine for a large ship, and includes a combustion chamber 10, a supercharger 20, an auxiliary blower 30, a scavenging pipe 40, an exhaust pipe 50, and an external EGR unit 60. And a control device 70. Hereinafter, each of these components will be described in order.

  The combustion chamber 10 is a component that is the center of the engine 100. Fresh air is supplied to the combustion chamber 10 from the scavenging pipe 40 (however, when EGR is performed, fresh air and burned gas are supplied. The same applies hereinafter). In addition, burned gas generated in the combustion chamber 10 is discharged to the exhaust pipe 50. The configuration of the combustion chamber 10 is completely different from the configuration of the combustion chamber of a four-stroke engine. Details of the configuration of the combustion chamber 10 will be described later.

  The supercharger 20 is a device that compresses the atmosphere (fresh air) and supplies it to the combustion chamber 10. The supercharger 20 has a turbine part 21 and a compressor part 22. Exhaust gas is supplied to the turbine unit 21 from the combustion chamber 10, and the turbine unit 21 is rotated by the energy of the exhaust gas. The turbine part 21 and the compressor part 22 are connected by a shaft part 23, and the compressor part 22 also rotates as the turbine part 21 rotates. When the compressor unit 22 rotates, the air taken in from the outside is compressed, and the compressed air is supplied to the combustion chamber 10 through the scavenging pipe 40.

  The auxiliary blower 30 is a device that assists the supercharger 20. A bypass pipe 32 is provided in the pipe 31 extending from the supercharger 20 to the scavenging pipe 40, and the auxiliary blower 30 is attached to the bypass pipe 31. The auxiliary blower 30 normally operates only in the low load region of the engine 100. This is because the supercharger 20 uses the energy of the exhaust as a drive source, and the flow rate of the exhaust is small in the low load region and the atmosphere cannot be sufficiently compressed. Since auxiliary blower 30 is driven by an electric motor, its capacity is constant regardless of the load area of engine 100.

  The scavenging pipe 40 is a device that once stores compressed fresh air and supplies it to the combustion chamber 10. The conventional engine (uniflow two-stroke engine) is designed so that the burned gas does not flow backward from the combustion chamber 10 to the scavenging pipe 40. This is because if burnt gas flows backward from the combustion chamber 10 to the scavenging tube 40, a fire may occur in the scavenging tube 40. Since the scavenging pipe 40 is located below the combustion chamber 10, residues such as lubricating oil and fuel tend to accumulate, and the accumulated residues may ignite. Further, a fire in the scavenging tube 40 is likely to occur particularly in a low load region of the engine 100. This is because a residue is likely to be generated in the low load region of the engine 100, and the auxiliary blower 30 operating in the low load region stirs the air in the scavenging pipe 40, so that the residue becomes mist-like and easily ignites. It is. Further, the scavenging tube 40 is provided with an oil mist concentration measuring device 41. The oil mist concentration measuring device 41 is a device that measures the concentration (oil mist concentration) of lubricating oil or fuel floating in the scavenging pipe 40. The oil mist concentration measuring device 41 transmits a signal related to the measured oil mist concentration to the control device 70.

  The exhaust pipe 50 is a device that temporarily stores exhaust gas (burned gas) discharged from the combustion chamber 10 and supplies it to the supercharger 20. The exhaust pipe 50 can not only suppress the pulsation of the exhaust discharged from the combustion chamber 10, but also can average the amount of NOx discharged. That is, even if the NOx emission amount differs for each cycle of the combustion chamber 10, stable exhaust gas with a small fluctuation in the NOx amount is discharged from the exhaust pipe 50. A temperature sensor 51 for measuring the exhaust temperature is provided on the downstream side of the exhaust pipe 50. The temperature sensor 51 transmits a signal related to the measured exhaust temperature to the control device 70.

  The external EGR unit 60 is a unit that supplies a part of burned gas (exhaust gas) discharged from the exhaust pipe 50 to the scavenging pipe 40 side. The external EGR unit 60 removes soot from the burned gas taken in by the scrubber 61, cools the burned gas with the cooling device 62, boosts the pressure with the EGR blower 63, and then sends fresh air sent from the supercharger 20. And join. The EGR blower 63 may be arranged downstream of the cooling device 62 or may be arranged upstream. The EGR blower 63 constituting the external EGR unit 60 and the adjusting valves 64 and 65 provided in the internal piping are controlled by the control device 70. Thereby, the control apparatus 70 can adjust the flow volume of the burned gas supplied through the external EGR unit 60 to the scavenging pipe 40.

  The control device 70 includes a CPU and the like, and is a device that controls the entire engine 100. The control device 70 can receive signals related to the rotational speed of the engine 100 and the fuel injection amount, respectively, and can calculate the load of the engine 100 based on the rotational speed of the engine 100 and the fuel injection amount. Further, the control device 70 receives a signal related to the exhaust temperature from the temperature sensor 51, and based on the exhaust temperature, the temperature of the cylinder 11, the piston 12 or the exhaust valve 13 constituting the combustion chamber 10 (hereinafter referred to as "member temperature"). Can be estimated. However, the member temperature may be measured directly or estimated by other methods. Further, the control device 70 can transmit a control signal to the variable valve device 14 to adjust the opening degree of the exhaust valve 13. Specific control contents by the control device 70 will be described later.

<Combustion chamber structure>
Next, the structure of the combustion chamber 10 will be described. As shown in FIG. 1, the combustion chamber 10 of the present embodiment includes a cylinder 11, a piston 12, and an exhaust valve 13. The exhaust valve 13 is opened and closed by a variable valve device 14. Hereinafter, each of these components will be described in order.

  The cylinder 11 is a cylindrical member, in which a mixture of fresh air and fuel burns. Although only one cylinder 11 is illustrated in FIG. 1, a plurality of cylinders 11 are actually arranged in the engine 100 side by side. Since the engine 100 according to this embodiment is a uniflow type, a scavenging port 15 is formed in the lower portion of the cylinder 11. Fresh air flows into the cylinder 11 from the scavenging pipe 40 through the scavenging port 15. A fuel injection nozzle 16 is provided at the top of the cylinder 11. The fuel injection nozzle 16 injects fuel into the cylinder 11 at a predetermined timing.

  The piston 12 is a member that reciprocates in the cylinder 11. The lower end portion of the piston 12 is attached to a crankshaft (none of which is shown) connected to the propeller shaft, and the crankshaft rotates as the piston 12 moves up and down, and consequently the propeller for propulsion of the hull rotates. To do. The piston 12 of the present embodiment not only compresses the air in the cylinder 11 and converts the expansion due to combustion into work, but also opens and closes the scavenging port 15 formed in the cylinder 11. That is, the opening degree of the scavenging port 15 is determined by the vertical position (crank angle) of the piston 12, and the relative relationship between the two does not change.

  The exhaust valve 13 is a valve that discharges the burned gas in the cylinder 11 to the exhaust pipe 50. The exhaust valve 13 of the present embodiment is disposed at the upper part of the cylinder 11. The exhaust valve 13 is mainly configured by a cylinder cover 17 formed on the upper portion of the cylinder 11 and an exhaust valve rod 18. When the exhaust valve rod 18 is driven, the exhaust valve 13 is opened and closed. The exhaust valve rod 18 is driven by the variable valve device 14. That is, as described above, the exhaust valve 13 is opened and closed by the variable valve device 14.

  The variable valve device 14 is a device that opens and closes the exhaust valve 13 at an arbitrary timing. As the variable valve device 14, a known one can be adopted. For example, when the exhaust valve rod 18 is driven by a camshaft, a configuration in which the phase of the camshaft is shifted with respect to the crankshaft or a configuration in which a plurality of camshafts are switched may be employed. However, the variable valve device 14 of the present embodiment is configured to drive the exhaust valve rod 18 with an electronically controlled hydraulic actuator. Further, as described above, the variable valve device 14 is controlled by the control device 70. The variable valve device 14 can perform variable timing control that can arbitrarily change the opening / closing timing of the exhaust valve 13 and variable lift amount control that can arbitrarily change the opening degree of the exhaust valve 13.

<Normal operation>
Next, the operation of the exhaust valve 13 will be described. Before describing the operation of the exhaust valve 13 in the present embodiment, first, a normal (conventional) operation will be described. In the two-stroke engine, one cycle of scavenging, compression, combustion, and exhaust is performed in two strokes (while the piston 12 makes one reciprocation). FIG. 2 is a diagram showing a normal operation of the exhaust valve 13. 2A to 2H show the position of the exhaust valve 13 until the piston 12 reaches the top dead center from the bottom dead center and further reaches the bottom dead center. As shown in FIG. 2, when the piston 12 is at the bottom dead center (A), the exhaust valve 13 is fully open (opening degree 100%). At this time, the engine 100 is in the scavenging stroke. That is, both the scavenging port 15 and the exhaust valve 13 are open, and fresh air flowing in from the scavenging port 15 pushes the burned gas toward the exhaust valve 13 side. This scavenging stroke continues for a certain period even when the piston 12 starts to rise (B, C).

  When the piston 12 further rises, the exhaust valve 13 is fully closed (opening degree 0%) and enters the compression stroke (D). Subsequently, fuel is injected into the cylinder 11 in the vicinity of the piston 12 reaching top dead center, and the combustion stroke is started (E). Thereafter, the exhaust valve 13 is fully opened while the piston 12 is lowered, and the exhaust stroke is started (F). That is, burnt gas is discharged from the exhaust valve 13. This exhaust stroke continues for a certain period while the piston 12 descends (G). When the piston 12 is further lowered, the scavenging port 15 starts to open (H). At this time, part of the burned gas in the cylinder 11 has already been discharged from the exhaust valve 13, so that the pressure in the cylinder 11 is lower than the pressure in the scavenging pipe 40. Therefore, fresh air flows from the scavenging port 15 into the cylinder 11 and the scavenging stroke starts.

  Note that the pressure in the scavenging pipe 40 and the pressure in the exhaust pipe 50 do not vary greatly as long as the load on the engine 100 is constant. For example, at a certain load, the pressure in the scavenging pipe 40 is maintained at 4.0 atm, and the pressure in the exhaust pipe 50 is maintained at 3.7 atm. And if it is normal operation | movement (if the exhaust valve 13 opens at a normal timing), when the scavenging port 15 begins to open (in the state of H shown in FIG. 2), the pressure in the cylinder 11 is the scavenging pipe. It is lower than the pressure in 40 and higher than the pressure in the exhaust pipe 50. In the above example, the pressure in the cylinder 11 when the scavenging port 15 starts to open is, for example, 3.8 atmospheres. With such a pressure relationship, the scavenging port 15 starts to open, and at the same time, fresh air flows into the cylinder 11 from the scavenging pipe 40, and can quickly enter the scavenging stroke.

  Here, in the case of normal operation, the relationship between the crank angle (the position of the piston 12) and the opening degree of the exhaust valve 13 is, for example, as shown in FIG. In FIG. 3, the horizontal axis represents the crank angle. When the crank angle is 0 degrees, the piston 12 is at top dead center (E in FIG. 2), and when it is 180 degrees and −180 degrees, the piston 12 is at bottom dead center (A in FIG. 2). In FIG. 3, the vertical axis represents the lift amount (opening degree) of the exhaust valve 13. When the lift amount is 0%, it is in a fully closed state, and when it is 100%, it is in a fully open state. As shown in FIG. 3, in a normal operation, the exhaust valve 13 remains fully open when the crank angle is in the vicinity of -180 degrees. Thereafter, the exhaust valve 13 is fully closed at an optimal timing while the piston 12 is raised so that the inside of the cylinder 11 has a required pressure at the top dead center. Further, the exhaust valve 13 is fully opened again at a certain timing while the piston 12 is lowered. And the exhaust valve 13 maintains a fully open state from there. The above is the normal operation of the exhaust valve 13.

  Note that the opening timing (crank angle) of the exhaust valve 13 is determined for each load of the engine 100 from the relationship between the pressure in the cylinder 11 and the pressure in the scavenging pipe 40. Specifically, after the combustion stroke, the crank angle at which the pressure in the cylinder 11 starts to decrease and becomes the same as the pressure in the scavenging pipe 40 is calculated at the time of design. ing. The reason for this setting is that during actual engine operation, even if the load is constant, the pressure in the cylinder 11 and the pressure in the scavenging pipe 40 fluctuate in each cycle. It is.

<Operation in low load area>
Next, the operation of the exhaust valve 13 when the engine 100 of the present embodiment is in the low load region will be described. Note that the low load region refers to a time when the maximum load of the engine 100 is, for example, 35% or less. Here, FIG. 4 is a view showing the operation of the exhaust valve 13 when the engine 100 is in the low load region. As in the case of FIG. 2, A to H in FIG. 4 indicate the positions of the exhaust valve 13 until the piston 12 reaches the top dead center from the bottom dead center and further reaches the bottom dead center. The opening and closing of the exhaust valve 13 is performed by the control device 70 controlling the variable valve device 14. As shown in FIG. 4, when the piston 12 is at bottom dead center (A), the exhaust valve 13 is fully open (opening degree 100%). The scavenging port 15 is also open. For this reason, fresh air (exactly including burned gas) flows from the scavenging port 15 into the cylinder 11 and burned gas in the cylinder 11 is pushed out from the exhaust valve 13 to the outside (exhaust pipe 50). . That is, at this time, the inside of the cylinder 11 is in the scavenging stroke.

  As described above, the exhaust valve 13 is originally fully opened during the scavenging stroke (A to C in FIG. 2). However, in this embodiment, during the scavenging stroke, the opening of the exhaust valve 13 once decreases from 100% to 50% (B), and then becomes 100% (fully opened) again (C). As described above, in the present embodiment, the opening degree of the exhaust valve 13 is not constant during the scavenging stroke, and the preclose control is performed to temporarily reduce the opening degree of the exhaust valve 13. The compression stroke (D), combustion stroke (E), and exhaust stroke (F) after the scavenging stroke are the same as the normal operation shown in FIG.

  Here, FIG. 5 is a diagram showing the relationship between the crank angle and the opening of the exhaust valve 13 when the engine 100 is in the low load region. The vertical and horizontal axes in FIG. 5 are the same as those in FIG. In FIG. 5, the broken line indicates the case of the normal operation shown in FIG. 3, and the solid line indicates the case when the engine 100 of the present embodiment is in the low load region. As shown in FIG. 5, the lift amount (opening) of the exhaust valve 13 is once reduced to 50% between −180 degrees and −90 degrees, which is fully open in normal operation in this embodiment, and then again. It is 100%.

  As described above, in the present embodiment, when the engine 100 is in the low load region, the opening degree of the exhaust valve 13 is temporarily reduced in the scavenging stroke. With this configuration, the burned gas is hardly discharged from the exhaust valve 13 during the scavenging process, and a part of the burned gas remains in the cylinder 11. As a result, the oxygen concentration in the cylinder 11 is reduced, and generation of NOx can be suppressed. In addition, as a method for causing the burnt gas to remain, the opening degree of the exhaust valve 13 can be reduced and fully closed (entering the compression stroke), but in this case, the pressure in the cylinder 11 may increase excessively. Therefore, after reducing the opening, it is desirable to increase the opening of the exhaust valve 13 to full open or close to full open again.

  In the present embodiment, the opening of the exhaust valve 13 is reduced from 100% to 50% in the scavenging stroke, but the opening may be reduced from 100% to 0%, for example. That is, the exhaust valve 13 may be completely closed once during the scavenging stroke. Furthermore, in this embodiment, the opening degree is reduced only once during the scavenging stroke, but may be reduced a plurality of times. Further, the above description has been made with a focus on a certain cycle of the engine 100. However, in addition to the cycle in which the opening degree of the exhaust valve 13 is temporarily reduced during the scavenging stroke, the opening degree of the exhaust valve 13 is separately during the scavenging stroke. Cycles that remain constant may be mixed.

  Here, the engine 100 according to the present embodiment is a large-sized engine mounted for a large vessel, and the exhaust valve rod 18 is very heavy. Also, hydraulic pressure is used to drive the exhaust valve rod 18. Therefore, a time lag is likely to occur between the control signal transmitted from the control device 70 to the variable valve device 14 and the actual operation of the exhaust valve rod 18. In such a case, a cycle in which the opening degree of the exhaust valve 13 is temporarily reduced during the scavenging stroke and a cycle in which the opening degree of the exhaust valve 13 remains constant may be mixed. For example, even if the exhaust valve 13 takes time to open and close, the movement of the piston 12 cannot be followed, and as a result, the exhaust valve 13 remains closed for a relatively long time. This is because the average value of the amount of NOx generated can be reduced compared with the normal operation while suppressing the increase in the member temperature. Note that, as described above, the NOx generation amount can be averaged by the exhaust pipe 50.

  In the present embodiment, the pre-close period, timing, opening, and frequency vary depending on the load of the engine 100. This is because the amount of burned gas to be left varies depending on the load of engine 100. For example, when the engine 100 is in the high load region, control such as setting a longer opening reduction period is performed than when the engine 100 is in the medium load region. In addition, the specific period, timing, opening degree, and frequency of the pre-close are determined based on the result of a test performed in advance with the same type engine.

<Complementary control>
As described above, the pre-close control is performed in the present embodiment. However, as a result of performing the pre-close control, it is difficult for fresh air to enter, and the member temperature (the temperature of the cylinder 11, the piston 12, or the exhaust valve 13) increases. There is a case. This is because the fresh air has a lower temperature than the burned gas, and if the fresh air does not flow into the cylinder 11, the cooling effect of the fresh air is reduced. And if member temperature rises too much, there exists a possibility that the cylinder 11, piston 12, or the exhaust valve 13 may deform | transform or break, and a malfunction may arise. For this reason, it is necessary to control the member temperature to be below a certain level. Specifically, when the member temperature exceeds the upper limit temperature, control for increasing the inflow amount of fresh air, that is, control for increasing the discharge amount of burned gas is performed.

  In the present embodiment, the pre-close control is temporarily stopped in order to increase the amount of burned gas discharged. That is, when the member temperature exceeds the upper limit temperature, the opening degree of the exhaust valve 13 during the scavenging stroke is always set to 100%. However, instead of stopping the pre-close control itself, the minimum opening of the exhaust valve 13 may be increased. In the above example, the minimum opening may be changed from 50% to 70%. Further, if the opening degree of the exhaust valve 13 is reduced a plurality of times in one scavenging stroke, the number of times may be reduced. Further, if a cycle for reducing the opening degree of the exhaust valve 13 and a cycle that remains constant are mixed, the frequency of the reduction cycle may be reduced. Furthermore, these may be combined.

  Further, whether or not to stop the pre-close control (whether or not to reduce the amount of burnt gas emission) is determined as follows, for example. FIG. 6 is a flowchart for determining whether to stop the preclose control. Note that the control device 70 determines whether to stop the pre-close control. First, the control device 70 determines whether or not pre-close control has already been performed (step S1). If it is determined that pre-close control is being performed (YES in step S1), it is determined whether the member temperature is lower than a predetermined upper limit temperature (step S2). This upper limit temperature is set to a temperature slightly lower than the temperature at which each member may be defective. Further, as described above, the member temperature can be estimated from the exhaust temperature. Therefore, actually, in step S2, it is determined whether or not the exhaust gas temperature is lower than a predetermined temperature.

  When determining that the member temperature is lower than the upper limit temperature (YES in step S2), the control device 70 continues the preclose control as it is (step S3). On the other hand, when it is determined that the member temperature is higher than the upper limit temperature (NO in step S2), the pre-close control is stopped (step S4). At this time, the control device 70 increases the flow rate of the burned gas supplied to the scavenging pipe 40 through the external EGR unit 60. The reason why the flow rate of the burned gas passing through the external EGR unit 60 is increased in this way is that the amount of burnt gas remaining in the cylinder 11 is reduced when the pre-close control is stopped. Thereby, even if the preclose control is stopped, the amount of burned gas in the cylinder 11 can be maintained, and an increase in the amount of NOx generated can be suppressed.

  Further, when it is determined that the pre-close control is not performed in Step S1 (NO in Step S1), the control device 70 determines whether or not the member temperature is higher than a predetermined restart temperature (Step S5). The restart temperature is set lower (stricter) than the upper limit temperature. When it is determined in step S5 that the member temperature is higher than the restart temperature (YES in step 5), the control device 70 stops the pre-close control as it is (step S4). On the other hand, when it is determined that the member temperature is lower than the restart temperature (NO in step 5), the pre-close control is restarted. At this time, the control device 70 reduces the flow rate of the burned gas supplied to the scavenging pipe 40 through the external EGR unit 60.

  As described above, in the present embodiment, when the member temperature is higher than the upper limit temperature, the pre-close control is stopped to avoid a problem that may occur in the cylinder 11. The amount of burned gas in the cylinder 11 is reduced by increasing the usage load of the external EGR unit 60 with the stop of the preclose control and decreasing the usage load of the external EGR unit 60 with the resumption of the preclose control. An appropriate amount is maintained, and the amount of NOx produced is appropriately suppressed.

  In the present embodiment, the external EGR unit 60 is provided as an external NOx reduction device, but another external NOx reduction device may be provided together with or instead of the external EGR unit 60. For example, the engine 100 may include a water addition device or an SCR (selective catalyst denitration) device as an external NOx reduction device. For example, the water addition device is such that by adding water to the fuel, the local combustion temperature is lowered by the latent heat of evaporation of water during combustion, and NOx generation is suppressed. When using a water addition device, increase the water addition rate with the stop of the preclose control, decrease the water addition rate with the resumption of the preclose control, or stop the water addition, so that the amount of NOx generated can be accurately determined. Keep it down. The SCR device is generally installed after the turbine unit 21 and converts NOx contained in the exhaust gas into nitrogen and water by a chemical reaction. When using an SCR device, the amount of the reducing agent is increased with the stop of the preclose control, the amount of the reducing agent is decreased with the resumption of the preclose control, or the introduction of the reducing agent is stopped, thereby reducing the NOx emission amount. Keep it accurate.

<Operations in high load area and medium load area>
Next, the operation of the exhaust valve 13 when the engine 100 of the present embodiment is in the high load region and the medium load region will be described. The high load region refers to when the maximum load of the engine 100 is, for example, 85% or more, and the medium load region refers to when the maximum load of the engine 100 is, for example, 35% to 85%. Here, FIG. 7 is a view showing the operation of the exhaust valve 13 when the engine 100 is in the low load region. 2 and FIG. 4, A to H in FIG. 7 indicate the position of the exhaust valve 13 from the bottom dead center of the piston 12 to the top dead center and further to the bottom dead center. As shown in FIG. 7, in the operation of the exhaust valve 13 in the high load region and the medium load region, preclose control is performed in which the opening degree of the exhaust valve 13 is temporarily reduced during the scavenging stroke. Thus, A to C in FIG. 7 are the same as A to C in FIG. The compression process (D) is the same as the normal operation. Therefore, here, the operation of the exhaust valve 13 shown from E to H in FIG. 7 will be described.

  As in normal operation, the exhaust valve 13 is fully closed during the combustion stroke when the engine 100 is in the high load region and the medium load region (E). Thereafter, the piston 12 starts to descend, but the exhaust valve 13 remains fully closed (F), and the exhaust valve 13 opens just before the scavenging port 15 opens (G). That is, in the high load region and the medium load region, the exhaust valve 13 starts to open at a timing later than usual (F and G in FIG. 2). Thus, since the exhaust valve 13 opens at a timing later than usual, the state in which the pressure in the cylinder 11 is high is maintained longer than usual.

  Then, immediately after the exhaust valve 13 starts to open, the scavenging port 15 opens (H). Then, when the scavenging port 15 starts to open, the pressure in the cylinder 11 becomes higher than the pressure in the scavenging pipe 40, and the burned gas flows backward from the cylinder 11 to the scavenging pipe 40. The burned gas in the cylinder 11 is also discharged from the exhaust valve 13. Further, as described above, the pressure in the scavenging pipe 40 and the pressure in the exhaust pipe 50 are constant. Similarly, for example, the pressure in the scavenging pipe 40 is maintained at 4.0 atm. Is maintained at 3.7 atm, the pressure in the cylinder 11 when the scavenging port 15 starts to open (in the H state shown in FIG. 7) is set to a value of 4.1 atm.

  Note that after a while after the scavenging port 15 starts to open, the pressure in the cylinder 11 becomes lower than the pressure in the scavenging pipe 40, and air flows into the cylinder 11 from the scavenging pipe 40 in the same manner as the normal scavenging stroke ( A). The air flowing into the cylinder 11 naturally includes the burnt gas that has flowed back, and a lot of burned gas remains in the cylinder 11. As a result, the oxygen concentration in the cylinder 11 decreases, and the generation of NOx is suppressed. If the opening of the exhaust valve 13 starts slowly, the period during which the pressure in the cylinder 11 is high is long, and the piston 12 can be continuously pushed downward by the internal pressure of the cylinder 11 for a relatively long time. As a result, the fuel consumption of engine 100 is also improved.

  Here, FIG. 8 is a diagram showing the relationship between the crank angle and the opening of the exhaust valve 13 when the engine 100 is in the middle load region and the high load region. The vertical and horizontal axes in FIG. 8 are the same as those in FIGS. 3 and 5. In FIG. 8, the broken line indicates the case of normal operation, and the solid line indicates the case of the high load region and the medium load region of the present embodiment. As shown in FIG. 8, the lift amount (opening degree) of the exhaust valve 13 is once reduced to 50% between −180 degrees and −90 degrees, which is the same as in the low load region. Further, in normal operation, the exhaust valve 13 is fully open when the crank angle is around 90 degrees. In the middle load region and the high load region of the present embodiment, the opening degree of the exhaust valve 13 is delayed at a timing slower than normal. Is fully open.

  In this embodiment, when the scavenging port 15 starts to open, the burned gas flows backward from the cylinder 11 to the scavenging pipe at a timing when the pressure in the cylinder 11 becomes higher than the pressure in the scavenging pipe 40. The exhaust valve 13 opens at the right timing. In order to realize this, in this embodiment, the exhaust valve 13 is opened immediately before the scavenging port 15 is opened. However, the exhaust valve 13 is not necessarily opened before the scavenging port 15 is opened. The timing at which the exhaust valve 13 starts to open and the timing at which the scavenging port 15 starts to open may be substantially the same, or may be later than the timing at which the scavenging port 15 starts to open. If the exhaust valve 13 is opened at such timing, a back flow from the cylinder 11 to the scavenging pipe 40 can be generated more reliably.

  In the present embodiment, the timing for opening the exhaust valve 13 after the combustion stroke is configured to be different depending on the load of the engine 100. This is because, as described above, the amount of burned gas to be left varies depending on the load of engine 100. For example, when the engine 100 is in a high load region, control is performed such that the opening timing is delayed compared to when the engine 100 is in a medium load region. Note that the timing of opening the exhaust valve 13 is determined based on the result of a test performed in advance with the same type of engine.

  As described above, control for opening the exhaust valve 13 at a timing later than usual (hereinafter referred to as “delay open control”) is performed in the medium load region and the high load region, but is not performed in the low load region. . This is because, as described above, a fire tends to occur in the scavenging tube 40 when a backflow of burned gas occurs particularly in a low load region. However, it is possible to prevent a fire in the scavenging pipe 40 by other methods such as providing the outlet of the auxiliary blower 30 apart from the scavenging pipe 40 or configuring the scavenging pipe 40 so that no residue enters the scavenging pipe 40. For example, the delay release control may be performed even in the low load region.

  In addition, if the delay release control is performed only in the medium load region and the high load region, it remains in the cylinder 11 when shifting from the low load region to the medium load region or when shifting from the medium load region to the low load region. The amount of burnt gas to be changed greatly. Therefore, in the present embodiment, when the engine 100 shifts from the low load region to the medium load region, the control device 70 performs control so that the flow rate of the burned gas supplied to the scavenging pipe 40 through the external EGR unit 60 is reduced. To do. Further, when the engine 100 shifts from the middle load region to the low load region, the control device 70 performs control so that the flow rate of the burned gas supplied to the scavenging pipe 40 through the external EGR unit 60 is increased. With this configuration, the amount of burned gas in the cylinder 11 can be maintained at an appropriate value, and the amount of NOx generated can be accurately suppressed. If engine 100 includes an external NOx reduction device other than external EGR unit 60, the usage load of the external NOx reduction device may be adjusted in the same manner.

<Complementary control 2>
As described above, the delay release control is not performed in the low load region where there is a possibility of causing a fire in the scavenging pipe 40, and the delay release control is performed only in the medium load region and the high load region. However, in the present embodiment, for safety, control is performed so that a fire does not occur in the scavenging tube 40 even in the middle load region and the high load region. Specifically, the control device 70 acquires the oil mist concentration in the scavenging pipe 40 from the oil mist concentration measuring device 41, and stops delay open control when it is determined that the oil mist concentration exceeds a predetermined value (upper limit concentration). To do. That is, when the oil mist concentration in the scavenging pipe 40 exceeds a certain concentration and a fire is likely to occur, the delay opening control is performed even if the engine 100 is in the middle load region or the high load region. And the timing at which the exhaust valve 13 is opened is returned to the normal operation timing. Thereby, safer operation of engine 100 becomes possible.

  The above is the configuration of the engine 100 according to the present embodiment. As described above, according to the present embodiment, the oxygen concentration in the cylinder 11 can be reduced only by adjusting the opening / closing timing of the exhaust valve 13 while being a uniflow type two-stroke engine. In the above, the control device 70 performs the pre-close control and the delay release control independently of each other. However, both controls may be chained. For example, when the shift from the middle load region to the low load region is performed, the delay opening control is stopped, but the control device 70 may be configured to reduce the minimum opening of the exhaust valve 13 in the preclose control accordingly. Good. According to such a configuration, it is possible to increase at least a part of the burned gas in the cylinder 11 that decreases due to the stop of the delayed release control by the preclose control.

  As described above, the embodiments of the present invention have been described with reference to the drawings. However, the specific configuration is not limited to these embodiments, and even if there is a design change or the like without departing from the gist of the present invention. It is included in the present invention. For example, in the present embodiment, the case where the burned gas flows backward from the cylinder 11 to the scavenging pipe 40 has been described, but the present invention includes the case where the burned gas does not flow backward. If the timing of closing the exhaust valve 13 is later than usual, burned gas tends to remain in the cylinder 11 without backflow, and the oxygen concentration can be reduced.

  The uniflow type two-stroke engine according to the present invention can reduce the oxygen concentration in the cylinder by adjusting the opening / closing timing of the exhaust valve. Therefore, it is useful in the technical field of a uniflow type two-stroke engine.

10 Combustion chamber 11 Cylinder 12 Piston 13 Exhaust valve 14 Variable valve device 15 Scavenging port 40 Scavenging pipe 60 External EGR unit (external NOx reduction device)
100 engine (uniflow 2-stroke engine)

Claims (9)

A cylinder with a scavenging port formed at the bottom;
A piston that reciprocates in the cylinder and opens and closes the scavenging port;
A scavenging pipe that once accommodates the compressed fresh air and supplies the scavenging port;
An exhaust valve disposed at the top of the cylinder;
A variable valve device that opens and closes the exhaust valve at an arbitrary timing,
A uniflow type two-stroke engine configured such that after the combustion stroke, the exhaust valve starts to open at the same timing as the scavenging port starts to open, so that a part of the burned gas is not discharged from the exhaust valve. 2. The uniflow two-stroke engine according to claim 1 , wherein when in a low load region, the exhaust valve is configured to open at a timing earlier than a timing at which the scavenging port starts to open. The uniflow type according to claim 1 , wherein when the oil mist concentration in the scavenging pipe exceeds a predetermined upper limit concentration, the exhaust valve is configured to open at a timing earlier than the timing at which the scavenging port starts to open. 2-stroke engine. The uniflow two-stroke engine according to any one of claims 1 to 3 , wherein the opening degree of the exhaust valve is reduced once or a plurality of times during a scavenging stroke. The uniflow according to claim 4 , wherein a cycle in which the opening degree of the exhaust valve is reduced during the scavenging stroke and a cycle in which the opening degree of the exhaust valve remains constant during the scavenging stroke are mixed. Formula 2 stroke engine. When the temperature of the cylinder, the piston, or the exhaust valve exceeds a predetermined upper limit temperature, the opening degree of the exhaust valve is not reduced during the scavenging stroke, and the minimum opening degree of the exhaust valve that is reduced during the scavenging stroke is The uniflow type two-stroke engine according to claim 4 , wherein the number of times that the opening of the exhaust valve is reduced during the scavenging stroke is reduced, or a combination thereof is performed. When the cylinder temperature of the piston, or said exhaust valve exceeds a predetermined upper limit temperature, the opening degree of the exhaust valve is configured to reduce the frequency of the cycle to reduce during the scavenging stroke, claim 5 Uniflow type 2-stroke engine as described in 1. An external NOx reduction device,
It is configured to reduce the use load of the external NOx reduction device when shifting from the low load region to the medium load region, and to increase the use load of the external NOx reduction device when shifting from the medium load region to the low load region. The uniflow two-stroke engine according to claim 2 . An external NOx reduction device,
The uniflow type according to claim 6 or 7 , wherein when the temperature of the cylinder, the piston, or the exhaust valve exceeds the predetermined upper limit temperature, the use load of the external NOx reduction device is increased. 2-stroke engine.

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