Engine

Abstract

To make a heat efficiency improvement request in a middle load operation of an engine and a knocking suppression request in a high load operation compatible.SOLUTION: An engine 1 is equipped with a main combustion chamber 6 that is formed by a cylinder block 2, a cylinder head 3, and a piston 5, a sub chamber 15 that has a plurality of injection holes 18 opening to the main combustion chamber 6, and an ignition plug 16 that ignites mixture in the sub chamber 15. A compression ratio ε of the main combustion chamber 6 is 14 to 24. A first index St×ε which is a product of the total cross-sectional area St of a plurality of injection holes 18 and the compression ratio ε is 0.1496 cm2 to 0.8449 cm2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an engine.

Background Art

[0002] As a technology for improving the thermal efficiency of a reciprocating engine, prechamber ignition is known. This is a system in which a small auxiliary chamber (prechamber) is provided separately from the main combustion chamber formed by a cylinder block, a cylinder head, and a piston. The air-fuel mixture is ignited in the auxiliary chamber, and a flame is ejected from the injection holes in the auxiliary chamber as a jet stream into the main combustion chamber. Since the combustion of the air-fuel mixture in the main combustion chamber becomes faster than normal flame propagation, it has attracted attention for improving thermal efficiency. The auxiliary chamber is formed by covering the tip of the spark plug with a cap-shaped partition wall that separates it from the main combustion chamber, and a plurality of injection holes are formed in the partition wall.

[0003] Prechamber ignition is divided into an active type and a passive type. The active type has an injector in the auxiliary chamber and forms the air-fuel mixture necessary for ignition in the auxiliary chamber. The passive type does not have an injector in the auxiliary chamber and guides the air-fuel mixture formed in the main combustion chamber during the compression stroke from the injection holes around the spark plug. Patent Document 1 shows an example of an active type prechamber ignition, and Patent Document 2 shows an example of a passive type prechamber ignition.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the case of a passive type, when the volume of the auxiliary chamber, the diameter and the number of the injection holes are determined, the jet potential of the flame ejected from the auxiliary chamber into the main combustion chamber, that is, the maximum value RETmax of the energy transfer rate from the auxiliary chamber to the main combustion chamber is uniquely determined. Therefore, it is said that it is difficult to achieve both the requirement for improving the thermal efficiency during medium load operation of the engine and the requirement for suppressing knocking during high load operation.

[0006] For example, during medium load operation, EGR (exhaust gas recirculation) is performed to reduce NOx emissions and improve fuel consumption performance. Since the EGR gas flows into the auxiliary chamber, it is likely to misfire even when ignited. In order to meet the requirements for preventing misfire and improving thermal efficiency during medium load operation, it is conceivable to determine the specifications of the auxiliary chamber from the viewpoint of making it easier for the air-fuel mixture to flow into the auxiliary chamber and increasing the jet potential. However, in that case, during high load operation, since a rich air-fuel mixture enters the auxiliary chamber, the jet potential becomes excessively large and knocking is likely to occur. On the other hand, if the specifications of the auxiliary chamber are determined from the viewpoint of suppressing the jet potential in order to suppress knocking at high load, the above requirements during medium load operation cannot be met.

[0007] An object of the present disclosure is to achieve both the requirement for improving the thermal efficiency during medium load operation of the engine and the requirement for suppressing knocking during high load operation.

Means for Solving the Problems

[0008] As a result of various studies on pre-chamber ignition by the inventor of the present application, it has been found that even in this ignition combustion method, it is possible to achieve both the requirement for improving the thermal efficiency during medium load EGR operation and the requirement for suppressing knocking during high load high speed operation. In addition, the inventor of the present application has discovered that the knocking index (knock intensity) during high load high speed operation is a function of the jet potential, and that there is a minimum value (inflection point) in this function. This means that by keeping the jet potential within the range near the minimum value, the occurrence of knocking during high load high speed operation can be suppressed.

[0009] The engine according to the present disclosure includes a main combustion chamber formed by a cylinder block, a cylinder head, and a piston, a sub-chamber having a plurality of injection holes opening into the main combustion chamber, and a spark plug for igniting an air-fuel mixture in the sub-chamber. The compression ratio ε of the main combustion chamber is 14 or more and 24 or less, and an index St·ε, which is the product of the total cross-sectional area St of the plurality of injection holes and the compression ratio ε, is 0.1496 cm 2 or more and 0.8449 cm 2 or less.

[0010] The compression ratio ε can be determined from requirements such as what thermal efficiency to aim for or how to set the ignition timing. On the other hand, when the compression ratio ε is small, the filling degree of air or the air-fuel mixture from the main combustion chamber to the sub-chamber becomes low. Therefore, from the viewpoint of obtaining an air-fuel mixture that can be surely ignited by ignition during medium-load EGR operation in the sub-chamber, the compression ratio ε is set to 14 or more. However, since knocking is likely to occur at high load and high speed when the compression ratio ε increases, the upper limit is set to 24.

[0011] Regarding the index St·ε, from the viewpoints of knocking suppression during high-load and high-speed operation and improvement of thermal efficiency during medium-load EGR operation, it is 0.1496 cm 2 or more and 0.8449 cm 2 or less.

[0012] Here, in order to increase the jet potential, it is necessary to increase the total cross-sectional area St from the viewpoint of improving the gas exchange in the sub-chamber. Conversely, in order to decrease the jet potential, it is necessary to decrease the total cross-sectional area St. For this reason, the index St·ε is larger as the jet potential is larger, and smaller as the jet potential is smaller.

[0013] The jet potential tends to increase as the compression ratio ε increases because the filling degree of the air-fuel mixture from the main combustion chamber to the auxiliary chamber increases. In order to obtain the same jet potential, when the compression ratio ε is large, it is necessary to reduce the total cross-sectional area St, while when the compression ratio ε is small, it is necessary to increase the total cross-sectional area St. Therefore, in the index St·ε obtained by multiplying the compression ratio ε by the total cross-sectional area St, the change amount (increase amount / decrease amount) of the compression ratio ε is balanced by the change amount (decrease amount / increase amount) of the total cross-sectional area St.

[0014] As a result, the index St·ε is less affected by the compression ratio ε because the total cross-sectional area St increases or decreases to offset the increase or decrease in the compression ratio ε. The index St·ε mainly depends on the jet potential.

[0015] Therefore, by obtaining the index St·ε based on the total cross-sectional area St (number of injection holes × injection hole diameter) and the compression ratio ε, the jet potential can be uniquely determined regardless of the magnitude of the compression ratio ε.

[0016] And by setting the index St·ε to be 0.1496 cm 2 or more and 0.8449 cm 2 or less, the jet potential can be kept within the range near the above-mentioned minimum value in the function of the knocking index during high-load and high-speed operation. Thereby, it is possible to suppress the occurrence of knocking due to an excessive increase in the jet potential during high-load and high-speed operation.

[0017] Furthermore, since the index St·ε is 0.1496 cm 2 or more, it is possible to avoid an excessive decrease in the jet potential during medium-load EGR operation. That is, it is advantageous for obtaining the desired jet potential and improving the thermal efficiency during medium-load EGR operation.

[0018] As described above, it is possible to satisfy both the requirement for improving the thermal efficiency during medium-load EGR operation of the engine and the requirement for suppressing knocking during high-load and high-speed operation.

[0019] In one embodiment, the above index St·ε is 0.2829 cm 2 or more and 0.7590 cm 2 or less. According to this, it becomes more advantageous in suppressing knocking during high-load and high-speed operation.

[0020] In one embodiment, the above index St·ε is 0.4611 cm 2 or more and 0.5817 cm 2 or less. According to this, it becomes even more advantageous in suppressing knocking during high-load and high-speed operation.

[0021] In one embodiment, under the high-load and high-speed operation conditions of the engine, the maximum value RETmax of the energy transfer rate from the above auxiliary chamber to the above main combustion chamber is 0.95 J / deg. or more and 1.6 J / deg. or less. According to this, the certainty of knocking suppression during high-load and high-speed operation can be further increased.

[0022] In one embodiment, the volume V of the above auxiliary chamber is 0.12 cm 3 or more and 0 .3 28 cm 3 or less.

[0023] The volume V of the auxiliary chamber itself affects the magnitude of the jet potential. Since the volume V is 0.12 cm 3 or more, a relatively large jet potential can be obtained even during medium-load EGR operation. On the other hand, since the upper limit of the volume V is set to 0 .3 28 cm 3 it is possible to avoid the jet potential from becoming excessive during high-load and high-speed operation.

[0024] In one embodiment, the number of the above injection holes is 4 or more and 6 or less. According to this, it becomes easier to keep the index St·ε within the above range.

[0025] In one embodiment, an injector that injects fuel for forming the above-described air-fuel mixture is provided to inject the fuel into the above-described main combustion chamber. According to this, as a passive pre-chamber ignition, even without an injector in the sub-chamber, the air-fuel mixture can be guided from the injection holes to around the spark plug in the sub-chamber.

Advantages of the Invention

[0026] According to the present disclosure, it is possible to satisfy both the requirement for improving the thermal efficiency during medium load operation of the engine and the requirement for suppressing knocking during high load operation.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Embodiments for Carrying Out the Invention

[0028] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications, or its uses.

[0029] <Configuration of the Engine> This embodiment relates to a reciprocating engine for vehicle drive mounted on a vehicle.

[0030] As shown in FIG. 1, the engine 1 includes a cylinder block 2 and a cylinder head 3. A cylinder 4 is formed in the cylinder block 2. A piston 5 provided in this cylinder 4 is connected to a connecting rod 7 connected to a crankshaft, whereby the piston 5 reciprocates in the cylinder 4. The main combustion chamber 6 of the engine is formed by the cylinder block 2, the cylinder head 3, and the piston 5.

[0031] 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 these intake port 8 and exhaust port 9 so as to open and close the openings on the main combustion chamber 6 side.

[0032] Although illustration is omitted, an intake passage is connected to the intake port 8, and an exhaust passage is connected to the exhaust port 9. An EGR passage for recirculating a part of the exhaust gas that has passed through the three-way catalyst into the intake passage is connected to the exhaust passage. An EGR cooler and an EGR valve for controlling the flow rate of the exhaust gas flowing through the EGR passage are provided in this EGR passage.

[0033] An injector 11 for injecting fuel for forming an air-fuel mixture into the main combustion chamber 6, a prechamber plug 12 having a prechamber described later, and a normal spark plug 13 without a prechamber are attached to the cylinder head 3. The injector 11 is provided on the cylinder axis and is provided such that its tip faces the central portion of the main combustion chamber 6. The prechamber plug 12 and the normal spark plug 13 are arranged on both sides of the injector 11 with the injector 11 interposed therebetween.

[0034] The prechamber plug 12 is provided on the intake port 8 side, extends obliquely downward from the intake port 8 side, and is arranged such that its tip faces the main combustion chamber 6. The normal spark plug 13 is provided on the exhaust port 9 side, extends obliquely downward from the exhaust port 9 side, and is arranged such that its tip faces the main combustion chamber 6. Note that the prechamber plug 12 may be provided on the exhaust port 9 side and the normal spark plug 13 may be provided on the intake port 8 side.

[0035] As shown in FIG. 2, a prechamber 15 is formed at the tip of the prechamber plug 12, and a spark plug 16 is provided in the prechamber 15. The spark plug 16 has a center electrode 16a and a side electrode (ground) 16b, similar to the normal spark plug 13.

[0036] The prechamber 15 is provided in the main combustion chamber 6, but is capable of burning the air-fuel mixture in the prechamber 15 independently of the main combustion chamber 6. More specifically, it functions as a sub-combustion chamber that ignites the air-fuel mixture in the prechamber 15 with the spark plug 16 to cause flame propagation in the prechamber 15.

[0037] As shown in FIGS. 2(a) and 2(b), the auxiliary chamber 15 is formed by a hemispherical auxiliary chamber forming portion 17 having a predetermined diameter and thickness. A plurality of injection holes 18 that open (communicate) with the main combustion chamber 6 are formed in the auxiliary chamber forming portion 17.

[0038] These injection holes 18 are provided to allow the air-fuel mixture in the main combustion chamber 6 to flow into the auxiliary chamber 15, and by igniting the inflowing air-fuel mixture, the flame generated in the auxiliary chamber 15 is ejected / radiated into the main combustion chamber 6, thereby accelerating the combustion of the air-fuel mixture in the main combustion chamber 6.

[0039] Basically, the air-fuel mixture is a mixture of fresh air from the intake port 8 and fuel injected from the injector 11 when the EGR valve is closed, and a mixture of fresh air from the intake port 8, exhaust gas from the EGR passage, and fuel injected from the injector 11 when the EGR valve is open. The fuel is gasoline as a liquid fuel.

[0040] In the present embodiment, four of these injection holes 18 are provided at 90° intervals around the axis passing through the vertex A of the auxiliary chamber forming portion 17 in a plan view seen from below as shown in FIG. 2(b). As shown in FIG. 2(a), each of the injection holes 18 is formed at a position 45° from the vertex A of the hemispherical auxiliary chamber forming portion 17 and extends in a 45° direction. Thereby, the flame jets out from the injection holes 18 at an angle of 45° with respect to the axis passing through the vertex A.

[0041] Note that the number and position of the injection holes 18 are not limited to these numerical values. For example, in a plan view seen from below, five or six injection holes 18 may be provided at equal intervals around the axis passing through the vertex A, and further, the number of injection holes may be less than 4 or 7 or more. It is preferable that the number of injection holes 18 is 4 or more and 6 or less. Also, the injection holes 18 may be straight holes extending in the above 45° direction or swirl holes extending obliquely laterally with respect to the 45° direction. The diameter of the injection holes 18 is preferably 0.7 mm or more and 1.5 mm or less.

[0042] <Specifications of Engine and Auxiliary Chamber> In this embodiment, the bore stroke ratio (S / B), which is the ratio of the cylinder bore (inner diameter B) to the piston stroke (stroke S), is preferably 1 or more and 1.5 or less. The stroke volume is preferably 500 cc or more and 700 cc or less, and more preferably 500 cc. The compression ratio ε of the main combustion chamber 6 is preferably 14 or more and 24 or less, and further preferably 16 or more and 18 or less.

[0043] The volume V of the auxiliary chamber 15 is 0.12 cm 3 or more and 0.328 cm 3 or less, and further preferably 0.2 cm 3 or more and 0.328 cm 3 or less. The throttle ratio β = St / V, which is the ratio of the total cross-sectional area St (sum of the cross-sectional areas of the plurality of injection holes 18) of the plurality of injection holes 18 to the volume V of the auxiliary chamber 15, is 0.0078 mm -1 or more and 0.0145 mm -1 or less, and further preferably 0.0078 mm -1 or more and 0.011 mm -1 or less.

[0044] <Manufacture of Engine> A method for manufacturing an engine will be described.

[0045] As shown in FIG. 3, in the first step S1, the compression ratio ε is set within a range of 14 or more and 24 or less. From the viewpoints of what thermal efficiency to aim for and how to set the ignition timing, the compression ratio ε is determined to be, for example, 15 or 17.

[0046] In the second step S2, by utilizing the fact that the pressure difference between the main combustion chamber and the auxiliary chamber depends on the compression ratio ε and the throttle ratio β, the upper limit βmax and the lower limit βmin of the throttle ratio β are determined based on the compression ratio ε determined in the first step S1. The throttle ratio β is set within the range of the upper limit βmax and the lower limit βmin.

[0047] In the third step S3, the auxiliary chamber volume V is set based on the compression ratio ε set in the first step S1 and the throttle ratio β set in the second step S2, taking advantage of the fact that the jet potential depends on the compression ratio ε, the throttle ratio β, and the auxiliary chamber volume V.

[0048] In the fourth step S4, the specifications of other engine components (such as the stroke volume of the engine, the clearance volume, the piston stroke S, the bore diameter B, the number and arrangement of the injection holes that communicate the main combustion chamber and the auxiliary chamber, etc.) are set. Since the auxiliary chamber volume V and the throttle ratio β are determined, when the injection holes are circular, if the number of injection holes is determined, the diameter of each injection hole is determined.

[0049] In the fifth step S5, based on the specifications set in the second step S2 to the fourth step S4, the design, manufacture, and assembly of each component constituting the engine are performed.

[0050] Note that the specifications of the engine components not involved in the compression ratio ε, the throttle ratio β, and the auxiliary chamber volume V can be set in parallel with or prior to the first step S1 to the third step S3.

[0051] (Regarding the second step S2 (determination of βmax and βmin)) The upper limit throttle ratio βmax and the lower limit throttle ratio βmin are determined on the premise that the pressure difference ΔP between the main combustion chamber 6 and the auxiliary chamber 15 depends on the compression ratio ε and the throttle ratio β. The following is a specific explanation.

[0052] The upper limit throttle ratio βmax is obtained based on the compression ratio ε such that the pressure difference ΔPmax between the main combustion chamber 6 and the auxiliary chamber 15 becomes a predetermined value or more when the pressure in the auxiliary chamber 15 is maximized under the conditions of medium load and medium speed EGR operation of the engine. In pre-chamber ignition, the pressures in the main combustion chamber 6 and the auxiliary chamber 15 basically change as shown in FIG. 4.

[0053] Regarding this pressure change, in the compression stroke, since the injection hole 18 of the auxiliary chamber 15 becomes a ventilation resistance, the rise of the pressure in the auxiliary chamber 15 becomes gentler than that in the main combustion chamber 6. That is, the pressure in the auxiliary chamber 15 is lower than the pressure in the main combustion chamber 6. Then, the air-fuel mixture in the auxiliary chamber 15 is ignited by the ignition before top dead center in the compression stroke, and the pressure in the auxiliary chamber 15 rapidly increases. After top dead center in the compression stroke, the pressure in the auxiliary chamber 15 becomes higher than the pressure in the main combustion chamber 6. The pressure difference between the main combustion chamber 6 and the auxiliary chamber 15 when the pressure in the auxiliary chamber 15 reaches its maximum is ΔPmax.

[0054] As described above, since the injection hole 18 becomes a ventilation resistance, before ignition, the larger the compression ratio ε is, the larger the pressure difference between the main combustion chamber 6 and the auxiliary chamber 15 becomes (the pressure in the main combustion chamber 6 becomes higher). Therefore, the ΔPmax after ignition becomes smaller as the compression ratio ε increases. On the other hand, looking at the throttle ratio β, in medium load and medium speed operation, when the throttle ratio β increases, during the pressure rise after ignition in the auxiliary chamber 15, the pressure is more likely to escape through the injection hole 18. Therefore, the larger the throttle ratio β is, the smaller the ΔPmax becomes.

[0055] That is, ΔPmax depends on the compression ratio ε and the throttle ratio β. Therefore, using the function F1(ε) of the compression ratio ε and the function F1(β) of the throttle ratio β, ΔPmax can be expressed as follows.

[0056] ΔPmax = F1(ε) × F1(β) ……(1) F1(ε) = -0.0062 × ε + 0.1949 …… (2) F1(β) = 9.51×10 -5 ×β -1.754 ……(3)

[0057] Here, F1(ε) and F1(β) are derived by a parametric study using a zero-dimensional simulation tool. That tool prepares a small room that simulates the auxiliary chamber in the main combustion chamber of the engine, and calculates the pressure, temperature, and density of each of the main combustion chamber and the auxiliary chamber considering the energy exchange between the auxiliary chamber and the main combustion chamber, heat dissipation from each of the main combustion chamber and the auxiliary chamber, etc.

[0058] In the calculation, the bore diameter B, stroke S, compression ratio ε, auxiliary chamber volume V, throttle ratio β, engine speed, fuel injection amount, air excess ratio, EGR rate, intake valve closing timing, exhaust valve opening timing, temperature and pressure at the intake valve closing timing, wall temperature and heat generation rate of each of the main combustion chamber and the auxiliary chamber are set as conditions. For the fuel injection amount, EGR rate, heat generation rate, etc., the results of a single-cylinder actual engine are simulated and inputted.

[0059] Figures 4 and 5 show an example of the results calculated under predetermined medium-load EGR operation conditions. Figure 4 shows the pressure changes in each of the main combustion chamber and the auxiliary chamber as described above, and ΔPmax and ΔPig are the evaluation indices. ΔPig is the pressure difference between the main combustion chamber and the auxiliary chamber at 10° before top dead center of the compression stroke, specifically at the time of ignition of the air-fuel mixture in the auxiliary chamber during the compression stroke of the engine. Figure 5 shows the change in the energy transfer rate from the auxiliary chamber to the main combustion chamber, and the maximum value RETmax of the energy transfer rate is the evaluation index. Hereinafter, RETmax may be referred to as the jet potential.

[0060] The above calculation is performed over several hundred conditions, and the degree of influence of the compression ratio ε and the throttle ratio β on ΔPmax is regression-analyzed from the calculation results to derive the mathematical formulas of the above F1(ε) and F1(β).

[0061] Figure 6 shows the correlation between the calculation result of ΔPmax by the simulation tool and the calculation result of F1(ε)×F1(β) by the above equations (1) to (3). It can be seen that the derived equations (2) and (3) are valid.

[0062] In medium-load EGR operation, it has been experimentally confirmed that when ΔPmax becomes 0.02 MPa or more, the thermal efficiency of the engine becomes higher than the standard. Therefore, substituting the compression ratio ε set in the first step S1 into equation (2) to obtain F1(ε), substituting the value of F1(ε) into equation (1), and setting ΔPmax to a predetermined value of 0.02 MPa, F1(β) is obtained, and the upper limit throttle ratio βmax is obtained from equation (3). Incidentally, when the compression ratio ε = 17, the upper limit throttle ratio βmax is 0.011.

[0063] Next, the lower limit throttle ratio βmin will be described. The lower limit throttle ratio βmin is obtained based on the compression ratio ε set in the first step S1 so that the pressure difference ΔPig between the main combustion chamber and the auxiliary chamber becomes equal to or less than a predetermined value under the high load and high rotation operation conditions of the engine.

[0064] As described above, when the nozzle hole 18 becomes a ventilation resistance, the pressure difference ΔPig between the main combustion chamber and the auxiliary chamber increases as the compression ratio ε increases, and the ΔPig decreases as the throttle ratio β increases. That is, ΔPig depends on the compression ratio ε and the throttle ratio β. Therefore, ΔPig can be expressed as follows using the function F2(ε) of the compression ratio ε and the function F2(β) of the throttle ratio β.

[0065] ΔPig = F2(ε) × F2(β) ……(4) F2(ε) = 0.1727 × exp(0.1309 × ε) …… (5) F2(β) = -1.0408×10 -5 ×β -2.0994 ……(6)

[0066] Similar to the previous F1(ε) and F1(β), F2(ε) and F2(β) are derived by a parametric study using the above-described zero-dimensional simulation tool. That is, the degrees of influence of the compression ratio ε and the throttle ratio β on ΔPig are regression-analyzed from the calculation results to derive the mathematical formulas of the above F2(ε) and F2(β).

[0067] Fig. 7 shows the correlation between the calculation result of ΔPig by the simulation tool and the calculation result of F2(ε) × F2(β) by the above formulas (4) to (6). It can be seen that the derived formulas (5) and (6) are appropriate. In Fig. 7, since ΔPig is the value obtained by subtracting the main combustion chamber pressure from the auxiliary chamber pressure, it has a negative value.

[0068] In high-load and high-speed operation, gas transfer from the main combustion chamber to the auxiliary chamber is important (ideally, ΔPig = 0). However, through experiments conducted under several conditions where the throttle ratio β was changed, it has been found that if ΔPig is -0.3 MPa or more (the pressure difference is small), the reliability of gas exchange in the auxiliary chamber is high and excellent thermal efficiency can be obtained. Therefore, substituting the compression ratio ε set in the first step S1 into equation (5) to obtain F2(ε), substituting the value of F2(ε) into equation (4), and setting ΔPig to the predetermined value of -0.3 MPa, F2(β) can be obtained, and the lower limit throttle ratio βmin can be obtained from equation (6). Incidentally, when the compression ratio ε = 17, the lower limit throttle ratio βmin is 0.007.

[0069] (Regarding the third step S3 (determination of the auxiliary chamber volume V)) As described above, the auxiliary chamber volume V is set based on the compression ratio ε set in the first step S1 and the throttle ratio β set in the second step S2 by utilizing the fact that the jet potential depends on the compression ratio ε, the throttle ratio β, and the auxiliary chamber volume V.

[0070] Under the high-load and high-speed operation conditions of the engine, as the compression ratio ε increases, the density of the air-fuel mixture in the auxiliary chamber increases, so the jet potential, that is, the maximum value of the energy transfer rate RETmax from the auxiliary chamber to the main combustion chamber increases. Also, as the throttle ratio β increases, the gas exchange in the auxiliary chamber improves, so RETmax increases. Further, as the auxiliary chamber volume V increases, the amount of heat generated in the auxiliary chamber increases, so RETmax increases. That is, RETmax depends on the compression ratio ε, the throttle ratio β, and the auxiliary chamber volume V. Therefore, RETmax can be expressed as follows using the function F3(ε) of the compression ratio ε, the function F3(β) of the throttle ratio β, and the function F3(V) of the auxiliary chamber volume V.

[0071] RETmax = F3(ε) × F3(β) × F3(V) ……(7) F3(ε) = 0.9698 × ln(ε) - 1.5623 ……(8) F3(β) = 0.8494 × ln(β) + 5.1483 ……(9) F3(V) = 0.3600 × exp(2.6080 × V) ……(10)

[0072] Similar to the previous F1(ε) and F1(β), F3(ε), F3(β), and F3(V) are derived through parametric studies using the above-mentioned zero-dimensional simulation tool. That is, the degrees of influence of the compression ratio ε, throttle ratio β, and auxiliary chamber volume V on RETmax are analyzed by regression from the calculation results to derive the mathematical formulas for the above F3(ε), F3(β), and F3(V).

[0073] Figure 8 shows the correlation between the calculation results of RETmax by the simulation tool and the calculation results of F3(ε)×F3(β)×F3(V) by the above equations (7) to (10). It can be seen that the derived equations (8) to (10) are reasonable.

[0074] Therefore, when the RETmax requirement as the engine performance is determined, the required auxiliary chamber volume V can be obtained from the above equations (7) to (10). Here, treating RETmax as a constant, let RETmax = a, then the auxiliary chamber volume V can be expressed as follows from the above equations (7) to (10).

[0075] V=(1 / 2.6080)×ln(A) ……(11) A=a / (0.3600×(0.9698×ln(ε)-1.5623)×(0.8494×ln(β)+5.1483))……(12)

[0076] That is, when the compression ratio ε set in the first step S1, the throttle ratio β set in the second step S2, and the a value as RETmax are substituted into equation (12), A is obtained, and the auxiliary chamber volume V is obtained from equation (11). According to equations (11) and (12), as the compression ratio ε increases, the auxiliary chamber volume V decreases; as the throttle ratio β increases, the auxiliary chamber volume V decreases; as the constant a increases, the auxiliary chamber volume V increases.

[0077] (Regarding the RETmax requirement) In high-load and high-speed operation of an engine with a high compression ratio ε, suppressing knocking (severe knock) is crucial. Through experiments conducted under several conditions where RETmax was varied, it was found that when RETmax is kept within a predetermined range, the occurrence of knocking can be suppressed. Figure 9 shows the relationship between RETmax and the knocking index (knock intensity) Ki at 6000 rpm WOT (wide open throttle) obtained from the experiment. The compression ratio ε of the engine is 17. In the experiment, RETmax was changed by varying the volume V of the auxiliary chamber and the diameter of the fuel injection hole.

[0078] The Ki value is calculated based on the vibration data of the pressure wave generated in the cylinder. The vibration data is detected using a knock sensor, an in-cylinder pressure sensor, etc. The Ki value here represents the average value of the intensities of the knocks that occurred during 300 combustion cycles. Therefore, if severe knock occurs during that sampling period, the Ki value will increase according to the intensity and frequency.

[0079] As shown in Figure 9, an inflection point was observed in the Ki value with respect to the change in the range of RETmax from 0 to 2. Specifically, the Ki value reached a minimum near RETmax = 1.2 J / deg. When RETmax increases, a steep heat generation accompanied by strong air column vibration occurs, and it is recognized that the Ki value increases. On the other hand, when RETmax decreases, the flame propagation in the main combustion chamber slows down, inducing the auto-ignition of the end gas, and it is recognized that the Ki value increases.

[0080] Figure 10 shows an example of the vibration data D1 and D2 at points P1 and P2 on the graph shown in Figure 9. The solid-line graph is the vibration data D1 at point P1 where RETmax is 0 (zero) J / deg. The dashed-line graph is the vibration data D2 at point P2 where RETmax is 1.2 J / deg. The vertical axis represents the heat generation rate, and the horizontal axis represents the crank angle, respectively.

[0081] The vibration data D1 are the test results of an engine that ignites the air-fuel mixture with a normal spark plug without a sub-chamber. The vibration data D2 are the test results of an engine that ignites the air-fuel mixture with a pre-chamber having four injection holes with a diameter of 1.0 mm and a sub-chamber volume V of 0.31 cc. The ignition timing is just before top dead center (-10° ATDC).

[0082] In the vibration data D1, large pressure fluctuations, that is, the occurrence of strong knocking, are recognized after a predetermined period has elapsed after ignition. In contrast, in the vibration data D2, no large pressure fluctuations such as those in the vibration data D1 are recognized. It can be seen that strong knocking is suppressed in the vibration data D2.

[0083] Based on the data accumulated so far, including measurement errors, the practically preferable range of the Ki value is 1 or less, and more preferably 0.5 or less. Fig. 9 shows the ranges where the Ki value is 1 or less and 0.5 or less.

[0084] Under the high-load and high-speed operation conditions of the engine, the range where the Ki value is 1 or less corresponds to the range where RETmax is 0.95 J / deg or more and 1.6 J / deg or less (a in Equation (12) is 0.95 or more and 1.65 or less). Under the high-load and high-speed operation conditions of the engine, the range where the Ki value is 0.5 or less corresponds to the range where RETmax is 1.05 or more and 1.5 or less (a in Equation (12) is 1.05 or more and 1.5 or less).

[0085] According to the above manufacturing method, the upper limit βmax of the throttle ratio β is set so that ΔPmax becomes a predetermined value (0.02 MPa at which the thermal efficiency of the engine becomes higher than the reference) or more under the medium-load EGR operation conditions, and the lower limit βmin of the throttle ratio β is set so that ΔPig becomes a predetermined value (-0.3 MPa at which gas exchange in the sub-chamber is easy) or more under the high-load and high-speed operation conditions. Since the throttle ratio β is set within the range of this upper limit βmax and the lower limit βmin, it becomes possible to ensure the desired thermal efficiency in the medium-load EGR operation and the high-load and high-speed operation.

[0086] Thus, the sub-chamber volume V is set so that RETmax under high-load and high-speed operation conditions falls within a predetermined range of 0.95 J / deg or more and 1.6 J / deg or less (a = 0.95 or more and 1.65 or less in Equation (12)), thereby suppressing knocking during high-load and high-speed operation.

[0087] That is, according to the above manufacturing method, regardless of the size of the engine's stroke volume, it is possible to achieve both an improvement in thermal efficiency during medium-load EGR operation and suppression of knocking during high-load and high-speed operation.

[0088] Here, regarding the throttle ratio β, when ΔPmax = 0.02 MPa and ΔPig = -0.3 MPa, the upper limit throttle ratio βmax when the compression ratio ε = 14 is 0.0133 mm -1 and the lower limit throttle ratio βmin is 0.0078 mm -1 is obtained.

[0089] When ΔPmax = 0.02 MPa and ΔPig = -0.3 MPa, the upper limit throttle ratio βmax when the compression ratio ε = 24 is 0.0083 mm -1 and the lower limit throttle ratio βmin is 0.0145 mm -1 is obtained.

[0090] That is, when the compression ratio ε is 14 or more and 24 or less, the throttle ratio β is 0.0078 mm -1 or more and 0.0145 mm -1 or less.

[0091] From the perspective of ensuring the desired thermal efficiency during medium-load EGR operation and high-load and high-speed operation, when the compression ratio ε is 14 or more and 24 or less, the throttle ratio β is 0.0078 mm -1 or more and 0.0145 mm -1 or less, and further preferably 0.0078 mm -1 or more and 0.011 mm -1 or less.

[0092] Regarding the sub-chamber volume V, when RETmax = 0.95 J / deg, the compression ratio ε = 24, and the throttle ratio β = 0.0145 mm-1 When it is, V = 0.0430 cm 3 results in a compression ratio ε = 14 and a throttle ratio β = 0.0145 mm -1 When it is, V = 0.2045 cm 3 results in a compression ratio ε = 24 and a throttle ratio β = 0.0078 mm -1 When it is, V = 0.2018 cm 3 results in a compression ratio ε = 14 and a throttle ratio β = 0.0078 mm -1 When it is, V = 0.3635 cm 3 results in

[0093] When RETmax = 1.6 J / deg, a compression ratio ε = 24 and a throttle ratio β = 0.0145 mm -1 When it is, V = 0.2428 cm 3 results in a compression ratio ε = 14 and a throttle ratio β = 0.0145 mm -1 When it is, V = 0.4045 cm 3 results in a compression ratio ε = 24 and a throttle ratio β = 0.0078 mm -1 When it is, V = 0.4017 cm 3 results in a compression ratio ε = 14 and a throttle ratio β = 0.0078 mm -1 When it is, V = 0.5631 cm 3 results in

[0094] According to the above, the auxiliary chamber volume V ranges from 0.0430 cm 3 to 0.5631 cm 3 or less. From the viewpoint of suppressing knocking occurrence during high-load and high-speed operation, the auxiliary chamber volume V is preferably 0.12 cm 3 or more and 0 .3 to 0.28 cm 3 or less, and more preferably 0.2 cm 3 or more and 0 .3 to 0.28 cm 3 or less.

[0095] <First Index> The product of the total cross-sectional area St [cm 2 of a plurality of injection holes in the auxiliary chamber and the compression ratio ε is defined as the first index St·ε [cm 2Let it be so. The first index St·ε is obtained based on the larger of the upper limit aperture ratio βmax and the lower limit aperture ratio βmin. When ε = 24, β = 0.0145 mm -1 and when ε = 14, β = 0.0133 mm -1 It is so.

[0096] When RETmax = 0.95 J / deg, for ε = 24, β = 0.0145 mm -1 in this case, V = 0.0430 cm 3 and St = 0.006235 cm 2 and St·ε = 0.1496 cm 2 It becomes so. When RETmax = 0.95 J / deg, for ε = 14, β = 0.0133 mm -1 in this case, V = 0.222 cm 3 and St = 0.02956 cm 2 and St·ε = 0.4139 cm 2 It becomes so.

[0097] When RETmax = 1.6 J / deg, for ε = 24, β = 0.0145 mm -1 in this case, V = 0.2428 cm 3 and St = 0.0352 cm 2 and St·ε = 0.8449 cm 2 It becomes so. When RETmax = 1.6 J / deg, for ε = 14, β = 0.0133 mm -1 in this case, V = 0.4225 cm 3 and St = 0.05626 cm 2 and St·ε = 0.7877 cm 2 It becomes so.

[0098] When RETmax = 1.05 J / deg, for ε = 24, β = 0.0145 mm -1 in this case, V = 0.0813 cm 3 and St = 0.01179 cm 2 and St·ε = 0.2829 cm 2 It becomes so. When RETmax = 1.05 J / deg, for ε = 14, β = 0.0133 mm -1 in this case, V = 0.2614 cm 3 and St = 0.03481 cm2 、St·ε = 0.4873 cm 2 It becomes as follows.

[0099] When RETmax = 1.5 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.2181 cm 3 、St = 0.03162 cm 2 、St·ε = 0.7590 cm 2 It becomes as follows. When RETmax = 1.5 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.398 cm 3 、St = 0.053 cm 2 、St·ε = 0.742 cm 2 It becomes as follows.

[0100] When RETmax = 1.2 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.1325 cm 3 、St = 0.01921 cm 2 、St·ε = 0.4611 cm 2 It becomes as follows. When RETmax = 1.2 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.312 cm 3 、St = 0.04155 cm 2 、St·ε = 0.5817 cm 2 It becomes as follows.

[0101] As described above, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value 1 or less, that is, in order to make RETmax 0.95 J / deg or more and 1.6 J / deg or less, the first index St·ε is 0.1496 cm 2 or more and 0.8449 cm 2 or less is preferable.

[0102] Also, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value 0.5 or less, that is, in order to make RETmax 1.05 J / deg or more and 1.5 J / deg or less, the first index St·ε is 0.2829 cm 2 or more and 0.7590 cm 2The following are preferred.

[0103] Furthermore, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value near 0 (near the minimum value), that is, in order to make RETmax near 1.2 J / deg, the first index St·ε is preferably 0.4611 cm 2 or more and 0.5817 cm 2 or less.

[0104] FIG. 11 is a graph showing the relationship between the jet potential RETmax and the total cross-sectional area St of the nozzle holes. FIG. 12 is a graph showing the relationship between the jet potential RETmax and the first index St·ε.

[0105] As shown in FIG. 11, the larger the jet potential RETmax, the larger the total cross-sectional area St needs to be. In the case of the compression ratio ε = 24, compared with the case of the compression ratio ε = 14, the total cross-sectional area St required to obtain the same jet potential RETmax is smaller. That is, the larger the compression ratio ε, the smaller the required total cross-sectional area St.

[0106] As shown in FIG. 12, the larger the jet potential RETmax, the larger the first index St·ε. The curve of the first index St·ε related to ε = 14 and the curve of the first index St·ε related to ε = 24 intersect with each other on the way. Specifically, when the jet potential RETmax is small, the first index St·ε related to ε = 24 is smaller than the first index St·ε related to ε = 14. When the jet potential RETmax is large, the first index St·ε related to ε = 24 is larger than the first index St·ε related to ε = 14.

[0107] <Second Index> The product of the auxiliary chamber volume V [cm 3 and the compression ratio ε is defined as the second index V·ε [cm 3 . The second index V·ε is obtained based on the larger one of the upper limit throttle ratio βmax and the lower limit throttle ratio βmin. When ε = 24, β = 0.0145 mm -1 and when ε = 14, β = 0.0133 mm -1It is.

[0108] When RETmax = 0.95 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.0430 cm 3 , V·ε = 1.03 cm 3 When RETmax = 0.95 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.222 cm 3 , V·ε = 3.11 cm 3 It becomes.

[0109] When RETmax = 1.6 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.2428 cm 3 , V·ε = 5.827 cm 3 When RETmax = 1.6 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.4225 cm 3 , V·ε = 5.92 cm 3 It becomes.

[0110] When RETmax = 1.05 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.0813 cm 3 , V·ε = 1.95 cm 3 When RETmax = 1.05 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.2614 cm 3 , V·ε = 3.66 cm 3 It becomes.

[0111] When RETmax = 1.5 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.2181 cm 3 , V·ε = 5.234 cm 3 When RETmax = 1.5 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.398 cm 3 , V·ε = 5.57 cm 3 It becomes.

[0112] When RETmax = 1.2 J / deg, ε = 24, β = 0.0145 mm -1 In this case, V = 0.1325 cm 3 , V·ε = 3.18 cm 3 When RETmax = 1.2 J / deg, ε = 14, β = 0.0133 mm -1 In this case, V = 0.312 cm 3 , V·ε = 4.37 cm 3 It becomes like this.

[0113] As described above, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value 1 or less, that is, in order to make RETmax 0.95 J / deg or more and 1.6 J / deg or less, the second index V·ε is 1.03 cm 3 or more and 5.92 cm 3 or less is preferable.

[0114] Also, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value 0.5 or less, that is, in order to make RETmax 1.05 J / deg or more and 1.5 J / deg or less, the second index V·ε is 1.95 cm 3 or more and 5.57 cm 3 or less is preferable.

[0115] Furthermore, when the compression ratio ε is 14 or more and 24 or less, in order to make the Ki value near 0 (near the minimum value), that is, in order to make RETmax near 1.2 J / deg, the second index V·ε is 3.18 cm 3 or more and 4.37 cm 3 or less is preferable.

[0116] Figure 13 is a graph showing the relationship between the jet potential RETmax and the auxiliary chamber volume V. Figure 14 is a graph showing the relationship between the jet potential RETmax and the second index V·ε.

[0117] As shown in Fig. 13, the larger the jet potential RETmax is, the larger the sub-chamber volume V needs to be. When the compression ratio ε = 24, compared with the case where the compression ratio ε = 14, the sub-chamber volume V required to obtain the same jet potential RETmax is smaller. That is, the larger the compression ratio ε is, the smaller the required sub-chamber volume V is.

[0118] As shown in Fig. 14, the larger the jet potential RETmax is, the larger the second index V·ε becomes. The curve of the second index V·ε related to ε = 14 and the curve of the second index V·ε related to ε = 24 intersect with each other on the way. Specifically, when the jet potential RETmax is small, the second index V·ε related to ε = 24 is smaller than the second index V·ε related to ε = 14. When the jet potential RETmax is large, the second index V·ε related to ε = 24 is larger than the second index V·ε related to ε = 14.

[0119] <Operational Effect> As a result of various studies on pre-chamber ignition, the inventor of the present application has found that even in this ignition combustion method, it is possible to achieve both the requirement for improving the thermal efficiency in medium-load EGR operation and the requirement for suppressing knocking in high-load high-speed rotation operation. In addition, the inventor of the present application has discovered that the knocking index (knock intensity, Ki value) during high-load high-speed rotation operation is a function of the jet potential RETmax, and there is a minimum value (inflection point) in this function (see Fig. 9). This means that by keeping the jet potential RETmax within the range near the minimum value, the occurrence of knocking during high-load high-speed rotation operation can be suppressed.

[0120] The compression ratio ε can be determined according to the requirements of what thermal efficiency to aim for or how to set the ignition timing. On the other hand, when the compression ratio ε is small, the filling degree of air or the air-fuel mixture from the main combustion chamber to the sub-chamber becomes low. Therefore, from the perspective of obtaining a mixture that can be surely ignited by ignition during medium-load EGR operation in the sub-chamber, the compression ratio ε is set to 14 or more. However, when the compression ratio ε becomes large, knocking is likely to occur during high-load high-speed rotation, so the upper limit is set to 24.

[0121] Regarding the first index St·ε, from the viewpoints of knocking suppression during high-load and high-speed operation and thermal efficiency improvement during medium-load EGR operation, it is set to be 0.1496 cm 2 or more and 0.8449 cm 2 or less.

[0122] Here, as shown in FIG. 11, in order to increase the jet potential RETmax, it is necessary to increase the total cross-sectional area St from the viewpoint of improving the gas exchange in the auxiliary chamber. Conversely, in order to decrease the jet potential RETmax, it is necessary to decrease the total cross-sectional area St. For this reason, as shown in FIG. 12, the first index St·ε is larger as the jet potential RETmax is larger, while it is smaller as the jet potential RETmax is smaller.

[0123] The jet potential RETmax tends to increase as the compression ratio ε increases because the filling degree of the air-fuel mixture from the main combustion chamber to the auxiliary chamber increases. As shown in FIG. 11, in order to obtain the same-sized jet potential RETmax, it is necessary to decrease the total cross-sectional area St when the compression ratio ε is large (ε = 24), while it is necessary to increase the total cross-sectional area St when the compression ratio ε is small (ε = 14). For this reason, in the first index St·ε obtained by multiplying the compression ratio ε by the total cross-sectional area St, the change amount (increase amount / decrease amount) of the compression ratio ε is balanced by the change amount (decrease amount / increase amount) of the total cross-sectional area St.

[0124] As a result, as shown in FIG. 12, the first index St·ε is less affected by the change in the compression ratio ε because the total cross-sectional area St increases or decreases so as to cancel out the increase or decrease in the compression ratio ε. The first index St·ε mainly depends on the jet potential RETmax.

[0125] Therefore, by obtaining the first index St·ε based on the total cross-sectional area St (number of nozzle holes × nozzle hole diameter) and the compression ratio ε, the jet potential RETmax can be uniquely determined regardless of the magnitude of the compression ratio ε.

[0126] The first index St·ε is 0.1496 cm2 Above 0.8449 cm 2 By making it the following, it becomes advantageous in terms of keeping the jet potential RETmax within the range near the above-mentioned minimum value in the function of the knocking index during high-load and high-speed operation (specifically, within the range where the jet potential RETmax is 0.95 J / deg or more and 1.6 J / deg or less) (see Fig. 9). Thereby, it becomes advantageous in terms of making the Ki value 1 or less. Therefore, it is possible to suppress the occurrence of knocking due to the jet potential RETmax becoming excessively large during high-load and high-speed operation.

[0127] Furthermore, since the first index St·ε is 0.1496 cm 2 or more, it is possible to avoid the jet potential RETmax becoming excessively small during medium-load EGR operation. That is, it becomes advantageous in terms of obtaining the desired jet potential RETmax and increasing the thermal efficiency during medium-load EGR operation.

[0128] As described above, it is possible to achieve both the requirement for improving the thermal efficiency during medium-load EGR operation of the engine and the requirement for suppressing knocking during high-load and high-speed operation.

[0129] By making the first index St·ε 0.2829 cm 2 or more and 0.7590 cm 2 or less, it becomes even more advantageous in terms of suppressing knocking during high-load and high-speed operation. Specifically, it becomes advantageous in terms of keeping the jet potential RETmax within the range of 1.05 J / deg or more and 1.5 J / deg or less (see Fig. 9). Thereby, it becomes advantageous in terms of making the Ki value 0.5 or less.

[0130] By making the first index St·ε 0.4611 cm 2 or more and 0.5817 cm 2 or less, it becomes even more advantageous in terms of suppressing knocking during high-load and high-speed operation. Specifically, it becomes advantageous in terms of keeping the jet potential RETmax within the range near 1.2 J / deg (see Fig. 9). Thereby, it becomes advantageous in terms of making the Ki value near 0 (near the minimum value).

[0131] By designing the auxiliary chamber so that the jet potential RETmax falls within a predetermined range (for example, within the range of 0.95 J / deg or more and 1.6 J / deg or less), the certainty of knocking suppression during high-load and high-speed operation can be further enhanced.

[0132] The volume V of the auxiliary chamber itself affects the magnitude of the jet potential RETmax. Since the volume V is 0.12 cm 3 or more, a relatively large jet potential RETmax can be obtained even during medium-load EGR operation. On the other hand, since the upper limit of the volume V is 0 .3 28 cm 3 it is possible to avoid the jet potential RETmax from becoming excessive during high-load and high-speed operation.

[0133] By setting the number of injection holes 18 to be 4 or more and 6 or less, it becomes easier to keep the first index St·ε within the above range.

[0134] As a passive pre-chamber ignition (pre-chamber plug 12), even if the injector 11 is not provided in the auxiliary chamber 15, the air-fuel mixture can be guided from the injection holes 18 around the ignition plug 16 in the auxiliary chamber 15.

Explanation of symbols

[0135] ε Compression ratio β Throttle ratio V Auxiliary chamber volume St Total cross-sectional area ΔPmax Pressure difference ΔPig Pressure difference RETmax Jet potential Ki Knock index St·ε First index V·ε Second index 1 Engine 2 Cylinder block 3 Cylinder head 4 Cylinder 5 Piston 6 Main combustion chamber 11 Injector 12 Prechamber plug 13 Normal ignition plug 15 Auxiliary chamber 16 Ignition plug 17 Auxiliary chamber forming part 18 Nozzle hole

Claims

1. A main combustion chamber formed by a cylinder block, a cylinder head, and a piston, A sub-chamber having a plurality of injection holes opening into the main combustion chamber, An engine comprising an ignition plug for igniting an air-fuel mixture in the sub-chamber, wherein The compression ratio ε of the main combustion chamber is 14 or more and 24 or less, An index St·ε which is the product of the total cross-sectional area St of the plurality of injection holes and the compression ratio ε is 0.1496 cm ２ or more and 0.8449 cm ２ or less. The engine.

2. In the engine according to Claim 1, The index St·ε is 0.2829 cm ２ or more and 0.7590 cm ２ or less. The engine.

3. In the engine according to Claim 2, The index St·ε is 0.4611 cm ２ or more and 0.5817 cm ２ or less. The engine.

4. In the engine according to any one of Claims 1 to 3, Under high-load and high-speed operation conditions of the engine, the maximum value RETmax of the energy transfer rate from the sub-chamber to the main combustion chamber is 0.95 J / deg. or more and 1.6 J / deg. or less. The engine.

5. In the engine according to any one of Claims 1 to 4, The volume V of the sub-chamber is 0.12 cm ３ or more and 0.328 cm ３ or less. The engine.

6. In the engine according to any one of Claims 1 to 5, The number of the injection holes is 4 or more and 6 or less. The engine.

7. In the engine according to any one of claims 1 to 6, an injector that injects fuel for forming the air-fuel mixture is provided to inject the fuel into the main combustion chamber.

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