engine

Abstract

To efficiently suppress strong knocking generated in a high-load and high-rotation operation region in a specified engine equipped with a pre-chamber in a combustion chamber.SOLUTION: An engine 1 includes a piston 5 which defines a combustion chamber 6 with a cylinder block 2 and a cylinder head 3. The combustion chamber 6 includes an auxiliary chamber 15, and a main chamber 14 which is partitioned from the auxiliary chamber 15 by a pre-chamber 17. A specified ratio obtained by dividing a volume Vpc of the auxiliary chamber 15 with a stroke volume Vst of a cylinder 4 is 0.00005 or more and 0.00045 or less.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The disclosed technology relates to an engine in which the combustion chamber is divided into a large main chamber and a small pre-chamber by a partition wall with holes, and an ignition plug is installed in the pre-chamber. [Background technology]

[0002] In recent years, attention has been drawn to an ignition system (pre-chamber ignition) in which a combustion chamber is provided with both a pre-chamber and a main chamber, and the air-fuel mixture in the pre-chamber is ignited to eject a flame from a hole in the partition into the main chamber, thereby combusting the air-fuel mixture in the main chamber. The use of pre-chamber ignition improves combustibility in the main chamber and can improve the thermal efficiency of the engine.

[0003] For example, Patent Document 1 discloses an engine system that employs this pre-chamber ignition method. This engine system is equipped with a pre-chamber plug 30 (corresponding to the sub-chamber and its ignition plug) and an injector 28 that injects fuel into a main combustion chamber 26 (corresponding to the main chamber).

[0004] When the injector 28 injects fuel, an air-fuel mixture is formed in the main combustion chamber 26. Part of this mixture also flows into the auxiliary chamber 60 through the hole in the partition wall. When the mixture that has flowed into the auxiliary chamber 60 is ignited, a flame is ejected from the auxiliary chamber 60 into the main combustion chamber 26. This causes the mixture in the main combustion chamber 26 to combust.

[0005] Patent Document 2 also discloses a gas engine that employs this pre-chamber ignition system. In this gas engine, an injector 30 injects gas, which is fuel, into a sub-chamber 20.

[0006] Therefore, in the case of this gas engine, when the injector 30 injects fuel, the fuel flows into the main chamber 8 through the hole in the partition wall, thereby forming an air-fuel mixture in the main chamber 8. By igniting the rich air-fuel mixture in the pre-chamber 20, which is richer in fuel than the main chamber 8, a flame is ejected from the pre-chamber 20 into the main chamber 8, thereby combusting the lean air-fuel mixture in the main chamber 8.

[0007] In Patent Document 2, in order to stably ignite the lean mixture in the main chamber 8, the velocity of the gas injected into the main chamber 8 is set to be 4 m / s or more and 7 m / s or less based on a predetermined pre-chamber index (the value obtained by dividing the volume of the pre-chamber by the sum of the opening areas of the pre-chambers).

[0008] Generally, among pre-chamber ignition systems, the type in which fuel is injected into the auxiliary chamber, as in Patent Document 2, is called an "active pre-chamber," while the type in which fuel is injected into the main chamber, as in Patent Document 1, is called a "passive pre-chamber." [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent Publication No. 2021-113549 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-66369 Summary of the Invention [Problem to be solved by the invention]

[0010] Knock (also known as "knocking") is an abnormal combustion that produces noise and impact, and is a phenomenon that is particularly problematic in spark-ignition engines. Knock typically occurs in high-load, low-speed operating ranges. More specifically, once combustion of the air-fuel mixture begins with ignition by the spark plug, the combustion spreads due to flame propagation. During this time, unburned air-fuel mixture (end gas) may self-ignite locally. Because combustion due to self-ignition is more rapid than combustion due to flame propagation, the resulting pressure oscillations create noise and impact, resulting in knock.

[0011] Knock that occurs in the high-load, low-rpm operating range is resolved as the rotation speed increases and flame propagation becomes faster. However, knock can occur even in the high-load, high-rpm operating range, although it is less frequent. Knock that occurs in the high-load, high-rpm operating range tends to be stronger than knock that occurs in the high-load, low-rpm operating range (this knock is also called strong knock). Therefore, strong knock is likely to damage the engine and cause a decrease in engine reliability. Strong knock is particularly likely to occur in engines with a high compression ratio, and therefore it also hinders improvements in thermal efficiency.

[0012] The problem of knock, including strong knock, is also important in the pre-chamber ignition system described above. Specifically, if the flame ejected from the pre-chamber is too strong in the high-load, high-speed operating range, the flame may propagate excessively quickly in the main combustion chamber, resulting in abnormal combustion. Such abnormal combustion can cause air column resonance in the combustion chamber, resulting in strong knock due to the pre-chamber ignition system.

[0013] Therefore, it is thought that the occurrence of strong knock can be suppressed by weakening the momentum of the flame ejected from the pre-chamber. However, when the inventors investigated the relationship between strong knock and flame momentum, they found that the occurrence of strong knock cannot be suppressed by simply weakening the momentum of the flame, and that there is an optimal condition for this relationship.

[0014] The disclosed technology is based on this finding, and aims to effectively suppress strong knocking that occurs in a high-load, high-speed operating range in a specific engine equipped with a pre-chamber in the combustion chamber. [Means for solving the problem]

[0015] The disclosed technology relates to an engine including a cylinder block in which cylinders are formed, a cylinder head assembled on top of the cylinder block and covering the top of the cylinder, a piston arranged to reciprocate inside the cylinder and defining a combustion chamber together with the cylinder block and the cylinder head, and an ignition plug that ignites in the combustion chamber.

[0016] The combustion chamber has an auxiliary chamber that houses the electrode of the spark plug, and a main chamber that is separated from the auxiliary chamber by a partition wall with a through hole and has a larger volume than the auxiliary chamber, and is configured so that a specific ratio obtained by dividing the volume of the auxiliary chamber by the stroke volume of the cylinder is 0.00005 or more and 0.00045 or less.

[0017] The combustion chamber of this engine is divided into a main chamber and an auxiliary chamber that houses the spark plug electrode by a partition wall with a through hole, and this engine can perform combustion using the pre-chamber ignition method. Therefore, by igniting in the auxiliary chamber, the mixture in the main chamber can be combusted using the flame ejected from the through hole, which improves combustibility in the main chamber and improves the thermal efficiency of the engine.

[0018] However, as mentioned above, in combustion using the pre-chamber ignition method, if the flame becomes too strong in the high-load, high-speed operating range, there is a risk of strong knock occurring. In response to this, in this engine, specific ratios that determine important physical conditions of the engine are set based on the knowledge of the inventors. The engine is configured so that these ratios fall within the specific ranges mentioned above.

[0019] These specific ratios and ranges are based on the optimum condition found for the relationship between strong knock and flame intensity, and are set to obtain appropriate flame intensity in the high-load, high-speed operating range. Therefore, strong knock can be effectively suppressed in this engine.

[0020] The ratio may be configured to be equal to or greater than 0.00013 and equal to or less than 0.00035.

[0021] This range corresponds to a range in which more appropriate flame strength can be obtained than in the previous range, and therefore strong knocking can be suppressed even more effectively.

[0022] The bore-stroke ratio may be set to be equal to or greater than 1 and equal to or less than 1.5.

[0023] Engines with a large bore-stroke ratio are undesirable from the perspective of suppressing strong knocking because the combustion speed and in-cylinder temperature tend to become excessive. Therefore, a bore-stroke ratio of less than 1 is preferable, but engines with a small bore-stroke ratio are undesirable from the perspective of improving fuel economy.

[0024] In contrast, with this engine, by setting a specific ratio within a specific range, the flame intensity can be set within an appropriate range. Therefore, strong knock can be suppressed even if the bore-stroke ratio is set within a relatively large range of 1 to 1.5. This makes it possible to achieve both improved fuel economy and suppression of strong knock.

[0025] The engine may have a compression ratio of 14 to 25, inclusive.

[0026] If the compression ratio is low, the mixture in the pre-chamber will be less filled. As a result, the flame will be weaker, which may lead to misfires during medium-load operation. Therefore, a compression ratio of 14 or higher is preferable. On the other hand, if the compression ratio is high, the mixture in the pre-chamber will be more filled. As a result, the flame will be stronger, which may lead to strong knocking. Therefore, a compression ratio of 25 or lower is preferable.

[0027] The engine may also further include an injector that injects fuel into the main chamber, and the mixture in the auxiliary chamber may be formed by the fuel injected by the injector flowing into the auxiliary chamber through the through-hole.

[0028] That is, this engine uses a passive prechamber. Unlike an active prechamber, which injects fuel into the prechamber, a passive prechamber injects fuel into the main chamber, and the amount of fuel in the prechamber can only be adjusted indirectly. Therefore, the intensity of the flame is almost exclusively determined by the physical conditions of the engine, such as the size of the through-hole. Therefore, it is more difficult to achieve stable combustion with good fuel economy with a passive prechamber than with an active prechamber.

[0029] However, as mentioned above, by configuring the engine so that a specific ratio falls within a specific range, it is possible to create an appropriate flame intensity in the high-load, high-speed operating range, thereby achieving stable combustion with good fuel economy even with a passive pre-chamber.

[0030] The injector may inject liquid fuel.

[0031] Gaseous fuel can mix with air immediately after being injected from the injector. Therefore, even when injected into a small pre-chamber, a uniform mixture can be formed within the pre-chamber, and the fuel can be smoothly injected into the main chamber through the through-hole. In contrast, when liquid fuel is injected into a small pre-chamber, droplets of the fuel adhere to the partition wall, making it difficult to form a uniform mixture within the pre-chamber. It is also not easy to inject the fuel into the main chamber through the through-hole.

[0032] Therefore, when the fuel is liquid, a passive pre-chamber is preferable to an active chamber. Because fuel is injected into the main chamber, which has a larger volume, a uniform mixture can be formed. Because part of the mixture flows into the pre-chamber, the mixture in the pre-chamber can also be made uniform.

[0033] The partition wall may have four to six through holes formed therein, and these through holes may be arranged at intervals in the circumferential direction around the electrode of the spark plug.

[0034] If there are three or fewer through-holes, the range in which the flame ejected from the auxiliary chamber is distributed in the circumferential direction will be narrow, which may result in uneven combustion in the main chamber. Therefore, four or more through-holes is preferable. On the other hand, if there are seven or more through-holes, the range in which the flame is distributed in the circumferential direction will be wider, but this will cause problems such as a decrease in the strength of the partition wall, making it difficult to achieve.

[0035] Furthermore, the number of through-holes also affects the flame strength. However, if the number of through-holes is between four and six, the size of the nozzle holes can be set within an appropriate range, and the jet potential can be set within the above-mentioned optimum range while distributing the flame appropriately in the circumferential direction. [Effects of the Invention]

[0036] The disclosed technology effectively suppresses strong knocking that occurs in high-load, high-speed operating regions in specific engines equipped with a pre-chamber in the combustion chamber, thereby improving the thermal efficiency of the engine. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 1 is a schematic configuration diagram of an engine. [Figure 2] 1 shows the pre-chamber plug, (a) is a side view with a partial cross section, and (b) is a view from below. [Figure 3] FIG. 1 is a diagram showing changes in pressure (main chamber and sub-chamber) over time during combustion using the pre-chamber ignition method. [Figure 4] This is a formula for calculating the jet potential. [Figure 5] FIG. 4 is a diagram showing the change over time of the jet potential corresponding to FIG. 3. [Figure 6] 1 is a table showing the main specifications of the engine used in the test. [Figure 7] FIG. 10 is a diagram showing the relationship between the occurrence of strong knock and jet potential. [Figure 8] 8 is an example of vibration data at points P1 and P2 in FIG. 7. [Figure 9] FIG. 2 is a diagram for explaining a bore-stroke ratio. [Figure 10] FIG. 10 is a diagram showing the relationship between the volume of the sub-chamber and the jet potential that enables suppression of strong knocking. [Figure 11] FIG. 10 is a diagram showing the relationship between the specification determination ratio and the jet potential. DETAILED DESCRIPTION OF THE INVENTION

[0038] The disclosed technology will now be described, however, the following description is merely exemplary in nature.

[0039] <Engine configuration> Figure 1 shows the main components of an engine 1 to which the disclosed technology is applied. This engine 1 is a reciprocating engine that is mounted on a vehicle and used to drive the vehicle. Engine 1 uses gasoline, which is a liquid fuel, and repeatedly performs four strokes consisting of intake, compression, combustion, and exhaust (a four-stroke engine).

[0040] This engine 1 is configured to have a higher compression ratio ε (geometric compression ratio) than a normal engine to improve thermal efficiency and to burn efficiently with less fuel.The engine 1 also employs a pre-chamber ignition system (passive pre-chamber) to achieve more rapid combustion than a normal engine.

[0041] The engine 1 includes a cylinder block 2 and a cylinder head 3. The cylinder block 2 has four cylinders 4 (only one is shown in FIG. 1). The cylinder head 3 is mounted on top of the cylinder block 2 and covers the top of the cylinders 4.

[0042] Pistons 5 are installed inside the cylinders 4. Connected to the pistons 5 are connecting rods 7 that are linked to the crankshaft, causing the pistons 5 to reciprocate within the cylinders 4. The cylinder block 2, cylinder head 3, and pistons 5 define a combustion chamber 6 where combustion takes place. In the case of this engine 1, the combustion chamber 6 is further divided into a main combustion chamber 14 and an auxiliary combustion chamber 15 by a pre-chamber 17, as will be described later.

[0043] The compression ratio ε of the engine 1 is set based on the required specifications. In the case of this engine 1, since the aim is to achieve even higher thermal efficiency than conventional engines, the compression ratio ε is preferably set to 14 or more and 25 or less, and more preferably 16 or more and 18 or less.

[0044] If the compression ratio ε is small, the degree of filling of the mixture, i.e., air and fuel (and possibly exhaust gas), from the main combustion chamber 14 to the auxiliary combustion chamber 15 will be low. If the degree of filling of the mixture in the auxiliary combustion chamber 15 is low, the density of the mixture in the auxiliary combustion chamber 15 will be low, and as will be described later, the intensity of the flame ejected from the pre-chamber 17 will be weak. This may result in misfires during operation under medium loads where the amount of fuel is relatively small. Therefore, from the perspective of preventing such misfires, it is preferable that the compression ratio ε be 14 or higher.

[0045] On the other hand, if the compression ratio ε is large, the degree of filling of the mixture from the main combustion chamber 14 to the auxiliary combustion chamber 15 increases. As a result, strong knocking is more likely to occur during high-load, high-speed operation with a relatively large amount of fuel. Therefore, from the perspective of suppressing strong knocking, it is preferable that the compression ratio ε be 25 or less.

[0046] An intake port 8 and an exhaust port 9 are formed in the cylinder head 3. Although not shown, intake valves and exhaust valves are provided in the intake port 8 and the exhaust port 9 to open and close the openings on the combustion chamber 6 side.

[0047] Although not shown in the figure, an intake passage is connected to the intake port 8, and an exhaust passage is connected to the exhaust port 9. The engine 1 is also provided with an EGR system. That is, an EGR passage is connected to the exhaust passage, which recirculates a portion of the exhaust gas that has passed through the three-way catalyst back into the intake passage. The EGR passage is provided with an EGR cooler and an EGR valve that controls the flow rate of exhaust gas flowing through the EGR passage.

[0048] From the viewpoint of improving thermal efficiency, it is preferable to provide an EGR system to the engine 1, but this is not essential. Also, the engine 1 is not provided with a supercharger. In other words, the engine 1 is a naturally aspirated engine. However, depending on the specifications of the engine 1, a supercharger may be provided.

[0049] An injector 11, a pre-chamber plug 12, and a normal plug 13 (second spark plug) are attached to the cylinder head 3. The injector 11 is provided on the axis of the cylinder 4 and is located facing the center of the combustion chamber 6 when the combustion chamber 6 is viewed from above. The pre-chamber plug 12 and the normal plug 13 are located on either side of the injector 11.

[0050] The pre-chamber plug 12 extends diagonally downward from the intake port 8 side, and is positioned so that its tip faces the combustion chamber 6. The normal plug 13 extends diagonally downward from the exhaust port 9 side, and is positioned so that its tip faces the combustion chamber 6. It is also possible to provide the pre-chamber plug 12 on the exhaust port 9 side, and the normal plug 13 on the intake port 8 side.

[0051] The normal spark plug 13 has an electrode 13a at its tip. The electrode 13a of the normal spark plug 13 faces the main combustion chamber 14. The main combustion chamber 14 occupies most of the volume of the combustion chamber 6 and constitutes its main body. The actual combustion performed by the engine 1 takes place in this main combustion chamber 14.

[0052] (Pre-chamber plug 12) The pre-chamber plug 12 has an ignition plug 16 (first ignition plug) and a pre-chamber 17 provided at the tip of the ignition plug 16. The pre-chamber 17 defines a part of the combustion chamber 6, thereby forming a sub-chamber 15 inside the pre-chamber 17.

[0053] Figure 2 shows the tip portion of the pre-chamber plug 12. The pre-chamber 17 is made up of a hemispherical partition wall that covers the tip portion of the spark plug 16, and has a sub-chamber 15 formed inside it. The electrodes of the spark plug 16 (center electrode 16a and side electrode 16b) are housed in the sub-chamber 15.

[0054] More specifically, the electrode is disposed approximately in the center of the sub-chamber 15 so as to be located on the axis of the spark plug 16. As a result, the distance from the ignition point of the electrode to the pre-chamber 17 is approximately the same throughout the entire area of the pre-chamber 17.

[0055] The pre-chamber 17 is formed with a plurality of (four in the illustrated example) injection holes 18 (corresponding to through holes) that penetrate the pre-chamber 17. The auxiliary chamber 15 communicates with the main chamber 14 through these injection holes 18. These injection holes 18 are arranged at intervals in the circumferential direction around the electrode.

[0056] Specifically, as shown in Figure 2(b), when the tip portion of the pre-chamber plug 12 is viewed from its axial direction, the injection holes 18 are arranged at 90° intervals around the circumferential direction, centered on the axis of the pre-chamber plug 12 which passes through the apex A of the pre-chamber 17.

[0057] As shown in Figure 2(a), each of these injection holes 18 is formed to extend in a direction at an angle of approximately 45 degrees from the apex A of the pre-chamber 17 in a side view. This allows a flame to be ejected from each injection hole 18 at an angle of approximately 45 degrees with respect to the axis of the spark plug 16. The number of injection holes 18 is not limited to four. As will be described later, the number of injection holes 18 is preferably four to six.

[0058] These injection holes 18 have the first function of allowing the air-fuel mixture formed in the main chamber 14 to flow into the pre-chamber 15. Secondly, they have the function of igniting the air-fuel mixture that has flowed in, thereby causing the flame generated in the pre-chamber 15 to be ejected / radiated into the main chamber 14. As a result, the flame ejected from the pre-chamber 17 ignites the air-fuel mixture in the main chamber 14 and promotes flame propagation, thereby accelerating the combustion of the air-fuel mixture in the main chamber 14.

[0059] That is, in this engine 1 equipped with the pre-chamber plug 12, ignition can be performed by the pre-chamber ignition method (passive pre-chamber).

[0060] (Features of the pre-chamber ignition system) In this engine 1, which uses pre-chamber ignition, fuel is injected at least in the middle of the intake stroke (for example, at a crank angle of −300° ATDC). Depending on the operating range of the engine 1, part of the fuel may be injected in parts during the compression stroke.

[0061] Fuel is injected during the intake stroke, atomizing the fuel and forming an air-fuel mixture in the main combustion chamber 14. The air-fuel ratio of the mixture is preferably controlled between the stoichiometric air-fuel ratio (λ=1) and a lean air-fuel ratio (λ>1). For example, to be advantageous for both improving fuel economy and suppressing knocking, the mixture may be controlled to have a stoichiometric air-fuel ratio in the medium load operating range with the introduction of EGR gas, and may be controlled to have a lean air-fuel ratio in the high load, high rotation operating range.

[0062] A portion of the air-fuel mixture formed in the main combustion chamber 14 flows into the auxiliary combustion chamber 15 through the nozzle hole 18. Then, near the top dead center of the compression stroke, the spark plug 16 ignites, causing the air-fuel mixture in the auxiliary combustion chamber 15 to combust, and a flame is ejected from the nozzle hole 18. The flame ignites the air-fuel mixture in the main combustion chamber 14 and causes it to combust.

[0063] Furthermore, when the temperature of the engine 1 is low, such as during startup, it is difficult to atomize the fuel, making it difficult to achieve stable combustion using the pre-chamber ignition method. Therefore, in such cases, it is preferable to ignite the air-fuel mixture in the main combustion chamber 14 using the standard spark plug 13, as in the past.

[0064] Figure 3 shows the change in pressure over time during combustion using the pre-chamber ignition method. This data was obtained when combustion was performed using the pre-chamber ignition method under specified conditions in the medium load operating range. In Figure 3, graph G1 shows the change in pressure in the main combustion chamber 14, and graph G2 shows the change in pressure in the sub-combustion chamber 15.

[0065] As described above, from the intake stroke to the compression stroke, part of the air-fuel mixture formed in the main combustion chamber 14 flows into the auxiliary combustion chamber 15 through the injection holes 18. At that time, the injection holes 18 in the auxiliary combustion chamber 15 create a flow resistance. Therefore, although the pressure in the main combustion chamber 14 increases as the piston 5 rises, the pressure rises more slowly in the auxiliary combustion chamber 15 than in the main combustion chamber 14. In other words, the pressure in the auxiliary combustion chamber 15 is lower than in the main combustion chamber 14. The greater the pressure difference between the auxiliary combustion chamber 15 and the main combustion chamber 14, the lower the degree of filling of the air-fuel mixture flowing into the auxiliary combustion chamber 15.

[0066] The air-fuel mixture that flows into the pre-chamber 15 is ignited near the top dead center of the compression stroke (for example, -10° ATDC). As a result, the air-fuel mixture burns in the pre-chamber 15, causing the pressure in the pre-chamber 15 to rise sharply. After the top dead center of the compression stroke, the pressure in the pre-chamber 15 becomes higher than the pressure in the main chamber 14. The pressure in the pre-chamber 15 then reaches its peak, and the pressure difference between the main chamber 14 and the pre-chamber 15 becomes maximum (ΔPmax). The greater the pressure difference between the pre-chamber 15 and the main chamber 14, the stronger the force of the flame ejected from the pre-chamber 17.

[0067] The momentum of the flame ejected from the pre-chamber 17 significantly affects the combustion speed of the mixture in the main combustion chamber 14. In the disclosed technology, the jet potential (RET) is used as an index for determining the momentum of the flame ejected from the pre-chamber 17. The jet potential corresponds to the rate of energy transfer between the auxiliary combustion chamber 15 and the main combustion chamber 14. Figure 4 shows the formula for calculating the jet potential.

[0068] Figure 5 shows the change over time in the jet potential corresponding to Figure 3. The jet potential can be used to determine the momentum of the flame ejected from the pre-chamber 17. That is, if the jet potential is large, it can be determined that the momentum of the flame is strong, and if the jet potential is small, it can be determined that the momentum of the flame is weak.

[0069] Specifically, the timing of ΔPmax shown in FIG. 3 corresponds to the timing when the flame ejects from the pre-chamber 17, so the momentum of the flame ejecting from the pre-chamber 17 can be determined from the jet potential (RETmax) at that time.

[0070] RETmax increases as the compression ratio ε increases, since the density of the air-fuel mixture in the pre-combustion chamber 15 increases. Also, as the volume of the pre-combustion chamber 15 increases, more heat energy is generated in the pre-combustion chamber 15, and RETmax increases accordingly. RETmax is also affected by the size and number of injection holes 18, the volume and shape of the cylinder 4, etc.

[0071] (Issues with the pre-chamber ignition system) As mentioned above, the momentum of the flame ejected from the pre-chamber 17 is affected by the physical conditions of the engine 1 (these are also called engine components), such as the compression ratio ε, the size and number of the injection holes 18, and the volume of the sub-chamber 15. For this reason, it is difficult to achieve stable combustion with good fuel economy in both the medium load and high load / high rotation speed operating ranges, where the combustion conditions are significantly different.

[0072] That is, because the amount of fuel is basically determined according to the power demand of the engine 1, the momentum of the flame ejected from the pre-chamber 17 is strongly affected by the engine components. For this reason, it is difficult to find engine components that can achieve both fuel-efficient and stable combustion in both operating ranges: medium load, where the amount of fuel is relatively small, and high load and high rotation, where the amount of fuel is relatively large and the stroke interval is short.

[0073] In the medium-load operating range, in order to improve thermal efficiency, it is preferable to minimize the amount of fuel and, in that state, to prevent misfire of the air-fuel mixture, to speed up flame propagation in the main combustion chamber 14. Therefore, if the engine components are designed to increase the force of the flame ejected from the pre-chamber 17 in order to speed up flame propagation in the main combustion chamber 14, then in the high-load, high-speed operating range, flame propagation in the main combustion chamber 14 will be excessively fast, making it more likely that strong knock will occur.

[0074] If ignition timing is retarded to suppress strong knock, thermal efficiency will decrease. Therefore, it is difficult to achieve stable combustion with good fuel economy in both medium-load and high-load, high-speed operating ranges. With a passive prechamber, this is even more difficult than with an active prechamber.

[0075] That is, in the case of an active pre-chamber, fuel is injected into the pre-chamber 15, so the amount of fuel in the pre-chamber 15 can be directly adjusted. Therefore, although there are limitations, by changing the fuel injection conditions, such as split injection, it is possible to weaken the momentum of the flame ejected from the pre-chamber 17 and suppress strong knocking.

[0076] In contrast, in the case of a passive prechamber, fuel is injected into the main chamber 14, so the amount of fuel in the prechamber 15 can only be adjusted indirectly. The flame intensity in a passive prechamber is almost exclusively determined by the engine components. Therefore, the range and conditions under which the amount of fuel in the prechamber 15 can be adjusted are significantly more limited in the passive prechamber than in the active prechamber.

[0077] Therefore, the inventors have been studying measures to suppress strong knock in this engine 1 that employs a passive pre-chamber. In the course of their investigation, they have found that simply weakening the force of the flame ejected from the pre-chamber 17 may actually promote strong knock, and that there are optimal conditions for the relationship between these factors.

[0078] The technology to be disclosed is based on this finding, and the engine 1 is configured based on this finding to effectively suppress strong knocking while achieving stable combustion with good fuel economy in the medium load operating range.

[0079] <Suppression of strong knocking> As described above, the engine 1 employs a pre-chamber ignition system that can accelerate the combustion speed of the air-fuel mixture in the main combustion chamber 14 and promote flame propagation in the medium-load operating range. This allows stable combustion even when the concentration of the air-fuel mixture in the main combustion chamber 14 is at the minimum required, improving thermal efficiency.

[0080] On the other hand, the adoption of the pre-chamber ignition system makes strong knocking more likely to occur in high-load, high-speed operating ranges. Moreover, this engine 1 is more susceptible to strong knocking because it has a higher compression ratio ε than a normal engine 1. To address this issue, it is conceivable to suppress strong knocking without deteriorating fuel economy by weakening the momentum of the flame injected from the pre-chamber 17.

[0081] However, when the inventors conducted tests (including thermal analysis, which will be described later) to investigate the relationship between strong knock and the momentum of the flame ejected from the pre-chamber 17, they discovered that weakening the flame momentum too much actually promotes strong knock, and that in order to effectively suppress strong knock, the flame needs to have an appropriate momentum.

[0082] Figure 6 shows the main specifications (engine components) of the engine used in the test. The engine used in the test was a naturally aspirated gasoline engine, similar to engine 1 of the above-described embodiment, and was configured so that fuel was injected into the main chamber 14 (passive pre-chamber). In the test, conditions were set appropriately within the range of each engine component shown in Figure 6 depending on the content of the test.

[0083] (Relationship between strong knock and jet potential) Figure 7 shows the results of an investigation into the relationship between the occurrence of strong knock and the jet potential.

[0084] Figure 7 shows the relationship between the Ki value and the jet potential in the high-load, high-speed operating range. Specifically, it shows the relationship between the Ki value and the jet potential when an engine with a compression ratio ε of 17 is operated at 6000 rpm and full load (full throttle).

[0085] In the test, conditions were set so that different jet potentials could be obtained by changing the volume of the sub-chamber 15 and the diameter of the nozzle hole 18. The bore-stroke ratio of the engine used in this test was set to 1.1.

[0086] The Ki value is an index that represents the strength of a knock (knock intensity). The Ki value is calculated based on vibration data of pressure waves generated inside the cylinder. The vibration data is detected using a knock sensor, an in-cylinder pressure sensor, etc. The Ki value here indicates the average value of the knock intensity that occurred over 300 combustion cycles. Therefore, if a strong knock occurs during the sampling period, even if it is infrequent, the Ki value will increase depending on the strength and frequency of the knock.

[0087] Fig. 8 shows an example of vibration data at points P1 and P2 on the graph shown in Fig. 7. The solid line graph D1 is the vibration data at point P1 where the jet potential is 0 (zero). The dashed line graph D2 is the vibration data at point P2 where the jet potential is 1.2. The vertical axis represents the heat release rate of the combustion chamber 6.

[0088] Graph D1 shows the test results for an engine without a pre-chamber 17, in other words, an engine in which the spark plug 16 is exposed to the main combustion chamber 14 (corresponding to a normal ignition system). Graph D2 shows the test results for an engine equipped with a pre-chamber 17 having four nozzle holes 18 with a diameter of 1.0 mm and a sub-chamber 15 with a volume of 0.31 cc. The ignition timing was always just before top dead center (-10° ATDC).

[0089] In graph D1, a large pressure fluctuation, i.e., the occurrence of strong knocking, is observed after a certain period of time has passed after ignition. In contrast, in graph D2, the large pressure fluctuations seen in graph D1 are not observed. In graph D2, it can be seen that strong knocking is suppressed. The Ki value is calculated based on this vibration data.

[0090] As shown in Fig. 7, an inflection point was observed in the Ki value as the jet potential decreased. Specifically, the Ki value decreased as the jet potential decreased from 2, and reached a minimum around 1.2. Thereafter, the Ki value increased as the jet potential decreased. The change in Ki value tended to be greater in the range of jet potential below the minimum value than in the range of jet potential above the minimum value.

[0091] When the flame is strong (in a region where the jet potential is equal to or greater than the minimum value), flame propagation in the main combustion chamber 14 is accelerated in proportion to the flame's strength, making abnormal combustion more likely to occur. As a result, if the combustion pressure rises sharply and air column resonance occurs in the main combustion chamber 14, strong knock due to the pre-chamber ignition system occurs. Therefore, in such cases, strong knock can be suppressed by reducing the flame strength that is causing the problem, i.e., the jet potential.

[0092] On the other hand, when the flame is weak (in the region where the jet potential is below the minimum value), the flame propagation in the main chamber 14 slows down accordingly, making it difficult for abnormal combustion to occur. Therefore, strong knocking caused by the pre-chamber ignition system can be suppressed.

[0093] On the other hand, when the flame strength weakens, it becomes more difficult for the flame to reach the peripheral areas of the main chamber 14. As a result, in the high-speed operating range, the flame is unable to reach the peripheral areas of the main chamber 14, which in turn induces auto-ignition of end gas present in the peripheral areas of the main chamber 14, making it more likely that strong knock will occur.

[0094] Based on the data accumulated so far, including measurement error, the practically preferable range of Ki value is 1 or less, and more preferably 0.5 or less. Figure 7 shows the range where Ki value is 1 or less and the range where Ki value is 0.5 or less.

[0095] The range where Ki is 1 or less corresponds to the range where the jet potential is 0.95 to 1.6, and the range where Ki is 0.5 or less corresponds to the range where the jet potential is 1.05 to 1.5.

[0096] Therefore, strong knock can be effectively suppressed by setting the engine components so that a range of jet potential corresponding to these appropriate Ki value ranges is obtained. Moreover, within these ranges of jet potential, a certain degree of flame momentum can be secured, making it possible to suppress misfires even in the medium-load operating range. In other words, strong knock can be effectively suppressed while achieving stable combustion with good fuel economy in the medium-load operating range. Appropriate combustion can be achieved in both the medium-load and high-load, high-speed operating ranges.

[0097] <Engine components related to jet potential> The engine components related to the jet potential include the compression ratio ε, the size of the nozzle hole 18, the number of the nozzle holes 18, the bore, the stroke, the volume of the sub-chamber 15, and the swept volume.

[0098] (Compression ratio ε) The compression ratio ε of the engine is set based on the required engine specifications. From the viewpoint of suppressing knocking, a lower compression ratio ε is preferable, but a low compression ratio ε is disadvantageous from the viewpoint of improving thermal efficiency. Therefore, this engine 1 employs a compression ratio ε that is higher than usual, and as described above, is set appropriately within the range of 14 to 25.

[0099] (Size and number of nozzle holes 18) The injection holes 18 are preferably formed with a diameter of 0.7 mm to 1.5 mm, and the number of injection holes 18 is preferably 4 to 6. These injection holes 18 are preferably arranged at intervals in the circumferential direction around the electrode of the spark plug 16. Although the injection holes 18 are circular, they do not have to be perfect circles.

[0100] Regarding the size of the injection hole 18, the larger the injection hole 18, the smaller the airflow resistance, and the smaller the injection hole 18, the greater the airflow resistance. If the airflow resistance is small, the air-fuel mixture will flow more easily into the auxiliary combustion chamber 15 and the exhaust gas will flow more easily out of the auxiliary combustion chamber 15. Therefore, from the viewpoint of intake and exhaust of the auxiliary combustion chamber 15, it is preferable that the injection hole 18 is large.

[0101] On the other hand, the momentum of the flame ejected from the pre-chamber 17 weakens as the nozzle hole 18 becomes larger, and strengthens as the nozzle hole 18 becomes smaller. Therefore, the size of the nozzle hole 18 directly affects the magnitude of the jet potential. In contrast, by setting the size of the nozzle hole 18 within the above-mentioned range, the jet potential can be set within the above-mentioned optimal range while ensuring an appropriate airflow resistance.

[0102] Regarding the number of injection holes 18, if there are three or fewer injection holes 18, the range over which the flame ejected from the pre-chamber 17 is distributed in the circumferential direction will be narrow, which may result in non-uniform combustion in the main chamber 14. If there are seven or more injection holes 18, the range over which the flame is distributed in the circumferential direction will be wider, but this is difficult to achieve due to problems such as a decrease in the strength of the pre-chamber 17.

[0103] Moreover, the number of injection holes 18 also affects the jet potential. For example, if the opening area of the pre-chamber 17 is the same, if there are fewer injection holes 18, the injection holes 18 must be larger, and if there are more injection holes 18, the injection holes 18 must be smaller. In contrast, if the number is between four and six, the size of the injection holes 18 can be set within an appropriate range, and the jet potential can be set within the above-mentioned optimal range while appropriately distributing the flame in the circumferential direction of the combustion chamber 6.

[0104] (Bore, stroke) As shown in simplified form in Figure 9, bore B is the inside diameter of cylinder 4. Stroke S is the distance that piston 5 travels in cylinder 4 from bottom dead center (position indicated by the solid line) to top dead center (position indicated by the two-dot chain line).

[0105] The ratio of stroke S to bore B (S / B) is generally called the "bore-stroke ratio." For the same volume, the larger the bore-stroke ratio, the larger the stroke S and the smaller the bore B. And the smaller the bore-stroke ratio, the smaller the stroke S and the larger the bore B.

[0106] As the stroke S increases, the movement speed of the piston 5 increases, which increases the flow inside the combustion chamber 6 and promotes combustion. As the bore B decreases, the surface area of the combustion chamber 6 decreases, which suppresses heat loss. Therefore, an engine with a large bore-stroke ratio is preferable from the perspective of improving thermal efficiency in operating ranges such as medium load, but is undesirable from the perspective of suppressing strong knock in operating ranges of high load and high revolution, as the combustion speed and in-cylinder temperature tend to become excessive.

[0107] Conversely, as the stroke S decreases, the movement speed of the piston 5 decreases, reducing the flow inside the combustion chamber 6, and as the bore B increases, heat loss also increases. Therefore, an engine with a small bore-stroke ratio is preferable from the perspective of suppressing strong knocking in high-load, high-rpm operating ranges, as the combustion speed and in-cylinder temperature are suppressed. However, it is not preferable from the perspective of improving thermal efficiency in medium-load and other operating ranges.

[0108] In contrast, the bore-stroke ratio of this engine 1 is set to a relatively large range of 1 to 1.5. From the perspective of suppressing strong knocking, a bore-stroke ratio of less than 1 is preferable, but in the case of this engine 1, strong knocking can be suppressed by setting the jet potential within an optimal range. Therefore, the setting can be made with priority given to improving fuel economy.

[0109] Furthermore, when the bore B is small, the distance from the pre-chamber 17 to the outer edge of the combustion chamber 6 is short, making it easier for the flame to reach the peripheral parts of the combustion chamber 6. This makes it possible to effectively combust end gases even in high-speed operating ranges. Therefore, by setting the bore-stroke ratio within the above-mentioned range, it is possible to achieve stable combustion with good fuel economy in the medium-load operating range while effectively suppressing strong knocking.

[0110] (Volume of auxiliary chamber 15) The flame ejected from the pre-chamber 17 is formed by igniting the air-fuel mixture in the pre-chamber 15. Therefore, the greater the thermal energy obtained by the mixture, the stronger the flame. The larger the volume of the pre-chamber 15, the greater the amount of air-fuel mixture that can be accommodated, and therefore the greater the thermal energy obtained by the mixture. Therefore, the volume of the pre-chamber 15 has a significant effect on the jet potential.

[0111] Figure 10 shows the relationship between the volume Vpc of the auxiliary combustion chamber 15 and the jet potential, which was obtained through thermal analysis and enables the suppression of strong knock. The volume Vpc of the auxiliary combustion chamber 15 increases by a predetermined amount as the jet potential increases. As shown by the two-dot chain line, the volume Vpc of the auxiliary combustion chamber 15 changes in accordance with the compression ratio ε of the engine 1. Specifically, as the compression ratio ε increases, the volume Vpc of the auxiliary combustion chamber 15 decreases, and as the compression ratio ε decreases, the volume Vpc of the auxiliary combustion chamber 15 increases.

[0112] For example, under the optimum condition where the jet potential is 1.2, in an engine 1 with a compression ratio ε of 14, it is preferable to set the volume Vpc (point P3) of the auxiliary combustion chamber 15 to 0.31 cc, and in an engine 1 with a compression ratio ε of 25, it is preferable to set the volume Vpc (point P4) of the auxiliary combustion chamber 15 to 0.12 cc.

[0113] (stroke volume) The stroke volume is the volume displaced when the piston 5 moves from bottom dead center to top dead center, and in Figure 9, it corresponds to the volume of the cylinder 4 in the range indicated by stroke S. The larger the stroke volume, the greater the amount of mixture that can be accommodated in the combustion chamber 6. If the amount of fuel increases accordingly, strong knocking becomes more likely to occur. Therefore, if the stroke volume increases, the range of engine components that can be designed accordingly becomes limited.

[0114] <Specific ratio corresponding to suppression of strong knock> In the disclosed technology, a specific ratio (specification determination ratio) is set by thermal analysis based on the above-mentioned engine components.

[0115] That is, as mentioned above, the positional relationship between the flame and the end gas is important for strong knock that occurs when the flame is weak (strong knock when the jet is weak), and is therefore affected by the shape of the combustion chamber 14 and the state of the air-fuel mixture.

[0116] For example, if the volume of the auxiliary chamber 15 is the same, reducing the bore or stroke volume makes it easier for the flame to reach the peripheral parts of the combustion chamber 14, making it less likely that strong knock will occur when the jet is weak, whereas increasing the bore or stroke volume makes it more difficult for the flame to reach the peripheral parts of the combustion chamber 14, making it more likely that strong knock will occur when the jet is weak.

[0117] Therefore, in order to appropriately and effectively identify such conditions related to the suppression of strong knock during weak jet, it is necessary to organize them using a specific ratio, and a specification ratio is set. Then, by configuring the engine so that this specification ratio falls within a predetermined range, it becomes possible to keep the jet potential within the above-mentioned appropriate range, and strong knock can be effectively suppressed.

[0118] Specifically, the ratio (Vpc / Vst) obtained by dividing the volume Vpc of the auxiliary chamber 15 by the stroke volume Vst is set as the specification determination ratio, and the engine 1 is configured so that this specification determination ratio falls within the range of 0.00005 to 0.00045.

[0119] Figure 11 shows the relationship between the specification ratio and the jet potential obtained by thermal analysis. In the thermal analysis, the compression ratio ε is set to satisfy the above-mentioned predetermined range, that is, 14 to 25. Similarly, the bore-stroke ratio is set to satisfy 1 to 1.5, the diameter of the injection holes 18 is set to satisfy 0.7 mm to 1.5 mm, and the number of injection holes 18 is set to satisfy 4 to 6.

[0120] The volume Vpc of the auxiliary combustion chamber 15 is set to satisfy the predetermined range shown in Fig. 10 in accordance with the compression ratio ε with the stroke volume Vst set to 500 cc, thereby setting the specification ratio.

[0121] As shown in Figure 11, the specification determination ratio that can satisfy each of these conditions increases as the jet potential increases. Of the conditions described above, the graph shown with a solid line corresponds to the condition under which strong knock is most likely to occur, such as a compression ratio of 25, while the graph shown with a dashed line corresponds to the condition under which strong knock is least likely to occur, such as a compression ratio of 14. When compared at the same jet potential, the specification determination ratio is higher under conditions under which strong knock is less likely to occur than under conditions under which strong knock is more likely to occur.

[0122] In contrast, in this engine 1, the range of the specification determination ratio corresponding to the range in which the Ki value is equal to or less than 1 is set to be equal to or greater than 0.00005 and equal to or less than 0.00045. In Figure 11, the upper limit value 0.00045 corresponds to point P5, and the lower limit value 0.00005 corresponds to point P6.

[0123] That is, in this engine 1, the range of specification ratios is set under the strictest condition among the above-mentioned conditions that is most likely to cause strong knock. Specifically, the compression ratio ε is 25, and the bore-stroke ratio is 1.5, and the upper limit values of each are adopted. The number of injection holes 18 is four.

[0124] Therefore, if the above-mentioned conditions are within other ranges, the conditions are not stricter than these, so the range of the specification determination ratio can be set more easily and strong knocking can be effectively suppressed.

[0125] Furthermore, it is preferable to set the specification ratio to be 0.00013 or more and 0.00035 or less. That is, this range corresponds to the range in which the Ki value described above is 0.5 or less, and strong knock can be stably and effectively suppressed. In Figure 11, the upper limit value of 0.00035 corresponds to point P7, and the lower limit value of 0.00013 corresponds to point P8.

[0126] In this way, engine 1 incorporating the disclosed technology can effectively suppress the occurrence of strong knocking in high-load, high-speed operation while achieving stable combustion with good fuel economy in the medium-load operation range, thereby achieving an engine with excellent thermal efficiency.

[0127] The disclosed technology is not limited to the above-described embodiment, but includes various other configurations. For example, the structure of the engine 1 shown in the above-described embodiment is an example, and the normal spark plug 13 facing the main chamber 14 may be omitted. The number of cylinders 4 is also not limited to four. [Explanation of symbols]

[0128] 1 engine 2 Cylinder block 3. Cylinder head 4 cylinders 5 pistons 6 Combustion chamber 11 Injector 12 Pre-chamber plug 13 Normal plug 14 Main room 15 Antechamber 16 Spark plug 17 Pre-chamber 18 nozzle hole (through hole)

Claims

1. a cylinder block in which a cylinder is formed; a cylinder head that is assembled onto the cylinder block and covers an upper portion of the cylinder; a piston provided so as to reciprocate within the cylinder and defining a combustion chamber together with the cylinder block and the cylinder head; a spark plug that ignites in the combustion chamber; an injector that injects liquid fuel into the combustion chamber; An engine comprising: The combustion chamber is a sub-chamber that accommodates an electrode of the spark plug; a main chamber which is separated from the sub-chamber by a partition wall having a through hole and has a larger volume than the sub-chamber; and a passive pre-chamber ignition system in which a part of the air-fuel mixture formed in the main chamber flows into the auxiliary chamber through the through-hole when the injector injects the fuel into the main chamber, the air-fuel mixture in the auxiliary chamber is burned when the spark plug is ignited, and the air-fuel mixture in the main chamber is ignited and burned by a flame ejected from the through-hole; In order to suppress strong knocking that may occur in an operating range of high load and high revolutions, an engine is configured such that the bore-stroke ratio, which is the value obtained by dividing the stroke of the cylinder by the bore, is between 1 and 1.5, the compression ratio is between 14 and 25, the through holes have a diameter of between 0.7 mm and 1.5 mm and the number of through holes is between 4 and 6, and the volume of the auxiliary chamber is set to satisfy a predetermined range corresponding to the compression ratio with the stroke volume set to 500 cc, and the specific ratio obtained by dividing the volume of the auxiliary chamber by the stroke volume of the cylinder is between 0.00005 and 0.00045.

2. 2. The engine of claim 1, The engine is configured so that the ratio is equal to or greater than 0.00013 and equal to or less than 0.00035.

3. 3. The engine according to claim 1 or 2, The through holes are arranged at intervals in a circumferential direction around the electrode of the spark plug.

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