JP2020112058A - diesel engine - Google Patents

Abstract

To appropriately perform exhaust emission control by a three-way catalyst while suppressing the amount of smoke in exhaust gas below a smoke limit threshold value in a diesel engine.SOLUTION: A diesel engine includes a combustion chamber (15), an injection nozzle (16a), and a three-way catalyst. A combustion chamber cavity (14b) is formed in the top of a piston. Representing the maximum velocity of a reverse squish flow as Vs and a fuel spray velocity near the opening peripheral edge of the combustion chamber cavity 14b as Vsp, the opening diameter of the combustion chamber cavity (14b) and the injection hole diameter of the injection nozzle (16a) are determined so that a velocity ratio Vs/Vsp becomes 0.4 or less in the operation condition in which an equivalent ratio is 0.95 or more.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to a direct injection diesel engine having a piston with a combustion chamber cavity formed at the top.

Japanese Patent No. 3751462 (Patent Document 1) discloses a direct injection diesel engine having a piston having a combustion chamber cavity recessed in a shallow dish shape formed at the top. In this diesel engine, when the velocity of the reverse squish flow from the combustion chamber cavity toward the squish area near the compression top dead center is V _s and the fuel spray velocity near the opening of the combustion chamber cavity is V _sp , the velocity is The opening diameter of the combustion chamber cavity is determined so that the ratio V _s /V _sp is 1.25 or less under all operating conditions. As a result, the fuel is uniformly distributed in the combustion chamber cavity and the squish area and is burned uniformly.

Japanese Patent No. 3751462

Generally, a diesel engine is operated in a lean state in which an air-fuel ratio indicating a ratio of an air amount to a fuel amount is larger than a stoichiometric air-fuel ratio (Stoichi). For this reason, in a diesel engine operated in a lean state, in order to purify harmful substances in exhaust gas, particularly nitrogen oxides (NOx), the NOx purification rate is high even in a lean state, for example, a urea addition type A catalyst (so-called urea SCR (Selective Catalytic Reduction) is often used. However, in order to use a urea addition type catalyst, in addition to the catalyst body, a urea tank, an addition valve, and the added urea are mixed with exhaust gas. There is a problem that the cost of post-treatment (exhaust gas purification treatment) will be high because many auxiliary devices such as a mixer for performing the treatment will be required. The inventors of the present application considered applying an inexpensive three-way catalyst, which is often used in a gasoline engine, to a diesel engine.

Here, in order to properly purify engine exhaust with a three-way catalyst, it is necessary to perform stoichiometric operation in which the air-fuel ratio becomes the stoichiometric air-fuel ratio. However, as described above, the diesel engine is generally operated in a lean state, and when a stoichiometric operation is performed with a diesel engine having a conventional structure, there is a possibility that a large amount of smoke exceeding an allowable amount is generated.

The present disclosure has been made to solve the above-described problems, and an object thereof is to appropriately control exhaust purification by a three-way catalyst in a diesel engine while suppressing the amount of smoke contained in the exhaust to an allowable amount or less. Is to be able to do.

(1) A diesel engine according to the present disclosure includes a combustion chamber that is formed by being surrounded by a piston, a cylinder head, and a cylinder, a nozzle that sprays fuel into the combustion chamber, and a exhaust gas after combustion in the combustion chamber. And a former catalyst. At the top of the piston, a combustion chamber cavity is formed which is recessed from the outer peripheral portion of the top surface of the piston and is open on the side facing the cylinder head. Let V _{s be} the maximum velocity of the reverse squish flow from the combustion chamber cavity toward the outer peripheral portion of the top surface of the piston at the time of combustion expansion near the compression top dead center, and let V _sp be the fuel spray velocity near the opening peripheral edge of the combustion chamber cavity. In this case, the opening diameter of the combustion chamber cavity and the nozzle hole diameter are determined so as to satisfy the speed ratio condition that the speed ratio V _s /V _sp is 0.4 or less under the operating condition that the equivalence ratio is 0.95 or more. Has been.

In the above diesel engine, the opening diameter of the combustion chamber cavity and the nozzle are set so as to satisfy the speed ratio condition that the speed ratio V _s /V _sp is 0.4 or less under the operating condition where the equivalence ratio is 0.95 or more. The injection hole diameter is fixed. As a result, in the above diesel engine, an operating range in which the equivalence ratio is 0.95 or more (that is, the actual air-fuel ratio is almost the theoretical air-fuel ratio) is secured while suppressing the smoke amount to less than the allowable amount, and this operation is performed. By performing the stoichiometric operation in the region, it becomes possible to appropriately perform exhaust gas purification by the three-way catalyst while suppressing the amount of smoke below the allowable amount.

(2) In one form, a tapered portion that is gradually inclined from the radially outer side to the radially inner side of the piston is formed at the peripheral edge portion of the opening of the combustion chamber cavity. The shape of the taper portion is determined such that the velocity V _s of the reverse squish flow calculated based on the value between the inner diameter and the outer diameter of the taper portion satisfies the velocity ratio condition.

It is known that the velocity (hereinafter, also simply referred to as “reverse squish velocity”) V _s of the reverse squish flow can be calculated using the opening diameter of the combustion chamber cavity (hereinafter also simply referred to as “cavity opening diameter”). There is. However, when a taper portion is formed on the peripheral edge of the opening of the combustion chamber cavity, there is a problem of what value to set the cavity opening diameter used to calculate the reverse squish velocity V _s. It was newly found by the experiment of. Specifically, it was found that if the cavity opening diameter were to be the inner diameter of the tapered portion, the reverse squish velocity V _s would be calculated higher than the actual velocity. On the contrary, if the cavity opening diameter is set to the outer diameter of the tapered portion, the reverse squish velocity V _s is calculated to be lower than the actual velocity.

In view of these results, in the above embodiment, the reverse squish speed V _s is calculated based on the value between the inner diameter and the outer diameter of the tapered portion (for example, the average value of the inner diameter and the outer diameter), and the reverse squish speed is calculated. The shape of the tapered portion is determined so that V _s satisfies the speed ratio condition. As a result, even when the tapered portion is formed at the peripheral edge portion of the opening of the combustion chamber cavity, the reverse squish velocity V _s is calculated more accurately, and the result is used to accurately determine the shape of the tapered portion that satisfies the velocity ratio condition. I can make a good decision.

(3) In one aspect, when the diesel engine is operated in an operating region where the speed ratio V _s /V _sp is 0.4 or less, the intake ratio is 1 to the combustion chamber so that the equivalence ratio becomes 1. At least one of the amount of air to be injected and the fuel injection amount of the nozzle is adjusted.

According to the above embodiment, the amount of air taken into the combustion chamber (diesel throttle opening, nozzle vane opening, etc.) so that the equivalence ratio becomes 1 in the operating region where the speed ratio V _s /V _sp becomes 0.4 or less. ) And the fuel injection amount of the nozzle (pilot injection amount, post injection amount, etc.) are adjusted. Therefore, it becomes possible to appropriately perform exhaust gas purification by the three-way catalyst while suppressing the smoke amount to be equal to or less than the allowable amount.

According to the present disclosure, it is possible to appropriately perform exhaust gas purification by a three-way catalyst in a diesel engine while suppressing the amount of smoke contained in exhaust gas to an allowable amount or less.

It is a figure which shows schematic structure of the engine and its control system by this Embodiment. It is a figure which shows the detailed shape of a piston. It is a diagram for explaining a method of setting the cavity opening diameter d _c used for calculation of the reverse squish velocity V _s. It is a diagram showing a correspondence relationship between speed ratio _V s _{/ V sp} and limitations equivalent ratio [Phi _lim. FIG. 6 is a diagram showing a correspondence relationship between a speed ratio Vs/Vsp and a smoke amount when the engine is operated in a state where the equivalence ratio is 0.95 and the EGR rate is 0%. It is a figure which shows the stoichiometric operation possible area|region A in the engine by this Embodiment typically. It is a flow chart which shows an example of the processing procedure of a control device. It is a flow chart which shows an example of the outline of the processing procedure which a control device performs during a stoichiometric operation mode.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following description, the same parts are designated by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

FIG. 1 is a diagram showing a schematic configuration of an engine 1 and a control system thereof according to the present embodiment. The engine 1 is a direct injection diesel engine. The engine 1 may be an in-line engine or may be an engine having another cylinder layout (for example, V type or horizontal type).

The engine 1 includes an engine body 10, a supply pump 17, a common rail 18, an air cleaner 20, an intercooler 26, an intake manifold 28, a supercharger 30, an exhaust manifold 50, an exhaust treatment device 55, and an EGR. A device (exhaust gas recirculation device) 60, an engine speed sensor 102, an air flow meter 104, a boost pressure sensor 106, a fuel pressure sensor 108, and a control device 200.

The engine body 10 includes a cylinder head 11, a cylinder 12, a piston 14, and an injector 16. The cylinder 12 is arranged below the cylinder head 11. The piston 14 is vertically reciprocally inserted in the cylinder 12. A combustion chamber 15 is formed by a space surrounded by the top of the piston 14, the cylinder head 11, and the cylinder 12. The shape of the piston 14 will be described in detail later.

The injector 16 is a fuel injection device that is provided in the cylinder head 11 and sprays fuel into the combustion chamber 15. The fuel stored in the fuel tank (not shown) is pressurized to a predetermined pressure by the supply pump 17 and supplied to the common rail 18. The fuel supplied to the common rail 18 is supplied to the injector 16 and injected into the combustion chamber 15 from the injection nozzle 16a of the injector 16 at a predetermined timing. The injector 16 supplies the fuel injection amount commanded according to the control signal from the control device 200 into the combustion chamber 15.

The air cleaner 20 removes foreign matter from the air sucked from the outside of the engine 1. One end of the first intake pipe 22 is connected to the air cleaner 20.

The other end of the first intake pipe 22 is connected to the intake inlet of the compressor 32 of the supercharger 30. One end of the second intake pipe 24 is connected to the intake air outlet of the compressor 32. The compressor 32 supercharges the air flowing from the first intake pipe 22 and supplies it to the second intake pipe 24. One end of the intercooler 26 is connected to the other end of the second intake pipe 24. The intercooler 26 is an air-cooled or water-cooled heat exchanger that cools the air flowing through the second intake pipe 24.

One end of the third intake pipe 27 is connected to the other end of the intercooler 26. An intake manifold 28 is connected to the other end of the third intake pipe 27. The intake manifold 28 is connected to the intake port of the engine body 10. A diesel throttle 25 is provided in the middle of the third intake pipe 27 and closer to the intercooler 26 than a branch point with the EGR 60 described later. The diesel throttle 25 adjusts the flow rate of intake air according to a control signal from the control device 200.

The exhaust manifold 50 is connected to the exhaust port of the engine body 10. One end of the first exhaust pipe 52 is connected to the exhaust manifold 50. The other end of the first exhaust pipe 52 is connected to the exhaust inlet of the turbine 36 of the supercharger 30. Therefore, the exhaust gas discharged from the exhaust port of each cylinder is supplied to the turbine 36 via the exhaust manifold 50 and the first exhaust pipe 52.

The exhaust outlet of the turbine 36 is connected to one end of the second exhaust pipe 54. An exhaust treatment device 55 including a three-way catalyst 56, a PM (Particulate Matter) removal filter 57, and an SCR catalyst 58 is connected to the other end of the second exhaust pipe 54. The exhaust gas discharged from the exhaust gas outlet of the turbine 36 is discharged to the outside of the vehicle via the second exhaust pipe 54 and the exhaust treatment device 55. It should be noted that the SCR catalyst 58 may be omitted in the present embodiment.

The three-way catalyst 56 oxidizes and reduces hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), which are harmful substances contained in the exhaust gas, by a catalyst using platinum, palladium, and rhodium. At the same time by removing. Specifically, the three-way catalyst 56 oxidizes hydrocarbons into water and carbon dioxide, oxidizes carbon monoxide into carbon dioxide, and reduces nitrogen oxides into nitrogen.

In order to properly purify (oxidize and reduce) the harmful substances contained in the exhaust by the three-way catalyst 56, the engine 1 needs to be operated in a stoichiometric state where the air-fuel ratio becomes the stoichiometric air-fuel ratio. In a diesel engine, it is general that the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio. However, lean operation requires an expensive aftertreatment device to purify the exhaust gas, which is costly. Although the engine 1 according to the present embodiment is a diesel engine, an inexpensive three-way catalyst 56 that is often used in a gasoline engine is adopted in order to reduce the post-treatment cost. In order to appropriately purify the exhaust gas by the three-way catalyst 56, the engine 1 according to the present embodiment is provided with a device for performing the stoichiometric operation while suppressing the smoke amount below the allowable value, as described later. There is.

The third intake pipe 27 and the exhaust manifold 50 are connected by the EGR device 60 without passing through the combustion chamber 15 of the engine body 10. The EGR device 60 includes an EGR valve 62 and an EGR passage 66. The EGR passage 66 connects the third intake pipe 27 and the exhaust manifold 50. The EGR valve 62 is provided in the middle of the EGR passage 66.

The EGR valve 62 is an adjustment valve that adjusts the flow rate of the EGR gas flowing through the EGR passage 66 according to the control signal from the control device 200. By adjusting the opening degree of the EGR valve 62, the EGR rate (the ratio of the EGR gas amount in the intake gas amount supplied to the engine body) is adjusted. The exhaust gas in the exhaust manifold 50 is returned to the intake side as EGR gas via the EGR device 60, whereby the combustion temperature in the combustion chamber 15 is lowered and the amount of NOx produced is reduced.

The supercharger 30 includes a compressor 32, a turbine 36, a variable nozzle mechanism 40, and an actuator 44. A compressor wheel 34 is housed in the housing of the compressor 32, and a turbine wheel 38 is housed in the housing of the turbine 36. The compressor wheel 34 and the turbine wheel 38 are connected by a connecting shaft 42 and rotate integrally. Therefore, the compressor wheel 34 is rotationally driven by the exhaust energy of the exhaust gas supplied to the turbine wheel 38.

The variable nozzle mechanism 40 is arranged in an exhaust gas inflow portion around the rotation axis of the turbine wheel 38, and guides the exhaust gas supplied from the first exhaust pipe 52 to the turbine wheel 38 and a plurality of vanes. And a link mechanism that changes the gap between adjacent vanes (the size of this gap is also referred to as “vane opening” in the following description) by rotating each. The actuator 44 changes the vane opening degree of the variable nozzle mechanism 40 by operating the link mechanism according to the operation instruction from the control device 200.

By changing the vane opening degree of the variable nozzle mechanism 40, the exhaust gas passage in the exhaust gas inflow portion to the turbine wheel 38 is narrowed or expanded. Thereby, the flow velocity of the exhaust gas blown to the turbine wheel 38 can be changed.

The engine rotation speed sensor 102 detects the rotation speed (rotation speed) of the crankshaft that is the output shaft of the engine 1 as the engine rotation speed NE. The air flow meter 104 detects a flow rate (intake air amount) Qin of fresh air introduced into the first intake pipe 22. The supercharging pressure sensor 106 detects the pressure in the intake manifold 28 as the supercharging pressure. The fuel pressure sensor 108 detects the pressure of fuel in the common rail 18 (hereinafter also referred to as “common rail pressure”). These sensors output a signal indicating the detection result to the control device 200.

The control device 200 controls the operation of the engine 1. The control device 200 includes a CPU (Central Processing Unit) 201 that performs various kinds of processing, a ROM (Read Only Memory) that stores programs and data, and a memory 202 that includes a RAM (Random Access Memory) that stores processing results of the CPU. And an input/output port (not shown) for exchanging information with the outside. The sensors described above (for example, the engine speed sensor 102, the air flow meter 104, the boost pressure sensor 106, the fuel pressure sensor 108, etc.) are connected to the input port. Devices to be controlled (for example, the injector 16, the actuator 44, the EGR valve 62, the supply pump 17, etc.) are connected to the output port.

The control device 200 executes predetermined arithmetic processing based on signals from the respective sensors and devices, and maps and programs stored in the memory. Then, the control device 200 controls the injector 16, the actuator 44, the EGR valve 62, the supply pump 17, and the like based on the result of the arithmetic processing.

For example, when operating engine 1, control device 200 performs pilot injection and main injection as one cycle of fuel injection into each cylinder. Specifically, the control device 200 outputs a main injection command corresponding to the required power to the injector 16 at a predetermined timing. As a result, "main injection" is performed in which fuel corresponding to the main injection command is injected from the injector 16. Further, the control device 200 outputs a pilot injection command for injecting a very small amount of fuel to the injector 16 prior to the main injection command in order to reduce combustion noise and purify exhaust gas. As a result, prior to the main injection, "pilot injection" in which a very small amount of fuel corresponding to the pilot injection command is injected from the injector 16 is performed. Note that injection other than pilot injection and main injection (for example, post injection after main injection) may be performed as one cycle of fuel injection into each cylinder.

<Shape of piston>
FIG. 2 is a diagram showing a detailed shape of the piston 14. An uppermost surface 14 a and a combustion chamber cavity 14 b are formed on the top (upper part) of the piston 14.

The uppermost surface 14 a of the piston 14 is located on the outer peripheral portion of the top surface of the piston 14 and is formed to be substantially parallel to the lower surface of the cylinder head 11. Hereinafter, the space sandwiched between the uppermost surface 14a of the piston 14 and the lower surface of the cylinder head 11 is also referred to as a squish area 15a.

The combustion chamber cavity 14b has a shallow dish shape that is recessed below the uppermost surface 14a and is open on the side facing the lower surface of the cylinder head 11. A tapered portion T that gradually inclines downward from the radially outer side of the piston 14 toward the radially inner side of the piston 14 is provided at the peripheral edge of the opening of the combustion chamber cavity 14b for the purpose of reducing cooling loss in the engine cylinder due to spray flame. It is formed. A lip portion L slightly protruding inward in the radial direction is formed at a tip end portion of the taper portion T on the radially inner side. Fuel is sprayed from the injection nozzle 16a of the injector 16 toward the combustion chamber cavity 14b having such a shape.

Hereinafter, a representative fuel spray velocity near the lip L near the compression top dead center is defined as “spray velocity V _sp ”, which represents a reverse squish flow from the combustion chamber cavity 14b toward the squish area 15a during combustion expansion. a specific flow rate is defined as a "reverse squish velocity V _s". Further, the size ratio of the reverse squish velocity _{V s} to the size of the spray rate _{V sp} is defined as "velocity ratio _V s _{/ V sp".}

In the engine 1 according to the present embodiment, the opening diameter of the combustion chamber cavity 14b (hereinafter, also referred to as “cavity opening diameter d _c ”) is set so that the engine operating region in which the speed ratio V _s /V _sp is less than the reference value R _ref exists. And the injection hole diameter of the injection nozzle 16a (hereinafter, also referred to as “nozzle injection hole diameter d ₀ ”). Hereinafter, this point will be described in detail.

First, a method of calculating the reverse squish velocity V _s and the spray velocity V _sp , which are parameters for determining the cavity opening diameter d _c and the nozzle injection hole diameter d ₀ , will be described. It is known that the reverse squish velocity V _s is expressed by the following equation (1) using the cavity opening diameter d _c .

In Expression (1), “x” is the distance from the lower surface of the cylinder head 11 to the uppermost surface 14a of the piston 14 at the crank angle θ. “V _k ”is the combustion chamber volume. “D” is the diameter of the piston 14. “Dx/dt” is represented by the following equation (2).

The meanings of the symbols used in formula (2) are as follows.
θ: Crank angle r: Stroke/2
ω: 2π・NE/60
ρ: r/connecting rod length “NE” is the engine speed (unit: rpm) as described above.

As can be understood from the above equations (1) and (2), the reverse squish velocity V _s can change depending on the cavity opening diameter d _c . In the present embodiment, the inverse squish velocity V _s is reduced by providing the tapered portion T and increasing the cavity opening diameter d _c .

FIG. 3 is a diagram for explaining a method of setting the cavity opening diameter d _c used to calculate the reverse squish velocity V _s . As described above, in the piston 14 according to the present embodiment, the tapered portion T is provided at the peripheral edge portion (spray collision portion) of the combustion chamber cavity 14b.

In the piston 14 having such a taper portion T, there is a problem of what value to set the cavity opening diameter d _c used for calculation of the reverse squish velocity V _s . Newly found. Specifically, it has been found that if the cavity opening diameter d _c is the innermost inner diameter d _in of the tapered portion T, the reverse squish velocity V _s is calculated to be higher than the actual velocity. On the contrary, if the cavity opening diameter d _c is the outermost diameter d _OUT of the tapered portion T, it is found that the reverse squish velocity V _s is calculated to be lower than the actual velocity. From these results, in order to calculate the typical reverse squish velocity V _s in the combustion chamber, the cavity opening diameter d _c should be set to a value between the inner diameter d _in and the outer diameter d _OUT of the tapered portion T. Is considered desirable.

Therefore, in the present embodiment, as the cavity opening diameter d _c used to calculate the reverse squish velocity V _s , the diameter of the central portion of the taper portion T, that is, the average value (=(the inner diameter d _in and the outer diameter d _OUT. d _in +d _OUT )/2) is used. Thereby, the reverse squish velocity V _s can be calculated more accurately. The parameter used to calculate the reverse squish velocity V _s may be a value between the inner diameter d _in and the outer diameter d _OUT of the tapered portion T, and is not necessarily the average value of the inner diameter d _in and the outer diameter d _OUT. It is not limited to being.

The spraying speed V _sp is calculated based on the “Kouan” formula, which is a highly reliable empirical formula, as described in Patent Document 1. First, the spray arrival distance S at time t after the start of injection is represented by the following equation (3).

In Expression (3), “ΔP” is the pressure difference between the injection pressure and the gas pressure. Here, the injection pressure corresponds to the common rail pressure in the present embodiment. The gas pressure can be represented by the cylinder gas pressure at the start of fuel injection. In addition, when injecting fuel multiple times (pilot injection, main injection, etc.) during one cycle, the cylinder gas pressure at the start timing of the main injection, which is the injection at the timing closest to top dead center, is representative. Let “Ρ _a ”is the atmosphere density. “Ρ _a ”can be represented by the in-cylinder atmosphere density at the start of fuel injection. Incidentally, in the present embodiment, the in-cylinder atmosphere density at the start timing of the main injection, which is the injection closest to the top dead center, is represented. “D ₀ ”is the injection hole diameter of the injection nozzle 16 a of the injector 16.

From the above equation (3), the spray velocity V _sp is expressed by the following equation (4) as a function of the spray reach distance S.

In the above formula (4), “C” is represented by the following formula (5).

From the above, the spray velocity V _sp is the spray velocity dS/dt when the spray arrival distance S is the diameter (dc=(d _in +d _OUT )/2) of the central portion of the tapered portion T, and the above formula (3) is used. )-Equation (5).

In this way, the reverse squish velocity V _s and the spray velocity V _sp are calculated, and the cavity opening diameter d _c and the cavity opening diameter d _c are set so that there exists an engine operating region in which their velocity ratio V _s /V _sp is less than the reference value R _ref. The nozzle hole diameter d ₀ is determined.

Next, a method of setting the “reference value R _ref ”used as a condition that the speed ratio V _s /V _sp should satisfy will be described. In the engine 1 according to the present embodiment, as described above, the inexpensive three-way catalyst 56 is used to reduce the post-treatment cost, although it is a diesel engine. In order for the three-way catalyst 56 to properly purify the exhaust gas, the engine 1 needs to be operated in a stoichiometric state where the air-fuel ratio becomes the stoichiometric air-fuel ratio. However, a diesel engine is generally operated in a lean state, and a large amount of smoke exceeding a predetermined allowable amount (hereinafter also referred to as "smoke limit threshold value") when performing stoichiometric operation with a diesel engine of a conventional structure. When it occurs, there may be a conflict.

As a result of repeated experiments conducted by the inventors of the present application in order to examine measures against such a problem, as a result, the equivalent ratio Φ (with respect to the actual air-fuel ratio) when the smoke amount contained in the exhaust gas of the engine 1 becomes the smoke limit threshold value It has been found that there is a high correlation between the limit equivalence ratio Φ _lim and the speed ratio V _s /V _sp when the ratio of the theoretical air-fuel ratio) is set to the “limit equivalence ratio Φ _lim ”.

FIG. 4 is a diagram showing a correspondence relationship between the speed ratio V _s /V _sp and the limit equivalent ratio Φ _lim . In FIG. 4, the horizontal axis represents the speed ratio V _s /V _sp , and the vertical axis represents the limit equivalent ratio Φ _lim . In FIG. 4 and FIG. 5 to be described later, the “reverse squish velocity V _s ”used for the “velocity ratio V _s /V _sp ” on the horizontal axis is a function of time as expressed by the above equation (1). And changes over time. Accordingly, the present inventors have 5 of FIG. 4 and described below, as a reverse squish velocity V _s, it was decided to use the maximum flow rate of the reverse squish velocity V _s (maximum becomes one as an absolute value).

The inventors of the present application reduced the amount of air with respect to a certain amount of fuel, and measured the equivalence ratio Φ when the amount of smoke became a smoke limit threshold as a limit equivalence ratio Φ _lim , and conducted an experiment at a plurality of speeds. This was done for each ratio V _s /V _sp . "◇" (diamond mark) shown in FIG. 4 is a point where the experimental result is plotted. The correspondence relationship between the speed ratio V _s /V _sp and the limit equivalent ratio Φ _lim can be represented by a straight line L1 that approximates the experimental result, as shown in FIG. That is, there is a negative correlation between the speed ratio V _s /V _sp and the limit equivalent ratio Φ _lim, and the limit equivalent ratio Φ _lim has a characteristic of monotonically decreasing with respect to the speed ratio V _s /V _sp . Can understand.

If this characteristic is used, the speed ratio V _s /V _sp when the limit equivalent ratio Φ _lim becomes a predetermined value Φ _u which is 1 or slightly smaller than 1 is obtained as a “reference value R _ref ”by experiments or the like in advance. , It is possible to easily calculate the cavity opening diameter d _c , the nozzle injection hole diameter d ₀ , and the common rail pressure for realizing stoichiometric combustion while suppressing the amount of smoke below the smoke limit threshold value. For example, when it is desired to realize stoichiometric combustion while suppressing the amount of smoke to the smoke limit threshold value or less in an engine in which the maximum controllable common rail pressure is determined, the speed ratio V _s /V _sp is set to be less than the reference value R _ref. In addition, the cavity opening diameter d _c and the nozzle injection hole diameter d ₀ can be uniquely determined. Further, for example, in an engine in which the cavity opening diameter d _c and the nozzle injection hole diameter d ₀ have already been determined, when it is desired to realize stoichiometric combustion while suppressing the smoke amount below the smoke limit threshold value, the speed ratio V _s /V _sp is set to the reference value R _The common rail pressure can be uniquely determined so as to be less than _ref .

FIG. 5 is a diagram showing a correspondence relationship between the speed ratio V _s /V _sp and the amount of smoke when the engine 1 is operated in a state where the equivalence ratio Φ is “0.95” and the EGR rate is “0%”. Is. In FIG. 5, the horizontal axis represents the speed ratio V _s /V _sp and the vertical axis represents the smoke amount. The unit of the smoke amount is "FSN (Filter Smoke Number)".

The inventors of the present application conducted an experiment to measure the amount of smoke discharged by the engine 1 when the engine 1 was operated in the state where the equivalence ratio Φ was 0.95 and the EGR rate was 0%, and a plurality of speed ratios V _s / _Performed every _Vsp . “O” (circles) shown in FIG. 5 are points where the experimental results are plotted. The correspondence between the speed ratio V _s /V _sp and the amount of smoke can be represented by a curve L2 that approximates the experimental results, as shown in FIG. That is, it can be understood that there is a positive correlation between the speed ratio V _s /V _sp and the smoke amount, and the smoke amount has a characteristic of monotonically increasing with respect to the speed ratio V _s /V _sp .

Further, from the experimental result of FIG. 5, the speed ratio V _s /V _sp when the smoke amount becomes the smoke limit threshold value (for example, 2 FSN) in the state where the equivalence ratio Φ is “0.95” is “0.4”. I can understand. This is because in FIG. 4 described above, the limit equivalent ratio Φ _lim is set to a predetermined value Φ _u smaller than 1, and the reference value R _ref when the predetermined value Φ _u is “0.95” is “0. 4”.

Based on this experimental result, in the present embodiment, the condition that the speed ratio V _s /V _sp is 0.4 or less under the operating condition where the equivalence ratio Φ is 0.95 or more (hereinafter also referred to as “speed ratio condition”). The cavity opening diameter d _c and the nozzle injection hole diameter d ₀ are determined so as to satisfy the above.

Here, the reason why the equivalence ratio Φ used for the above speed ratio condition is set to “0.95” instead of “1” (stoichiometric) will be described. In order to purify nitrogen oxides (NOx) with a three-way catalyst, it is necessary to leave no oxygen as the gas composition at the catalyst inlet. For this reason, a simple method is to realize stoichiometry (equivalent ratio Φ=1) by combustion and guide the exhaust gas discharged from the exhaust port to the catalyst inlet as it is.

On the other hand, the diesel engine has a problem peculiar to diesel combustion in that the fuel is mixed with air in a short time, resulting in an insufficient mixing region and causing smoke, and the equivalence ratio Φ during combustion is set to “1”. It is very difficult to suppress smoke in the (stoic) state. For this reason, the inventors of the present application set the equivalence ratio Φ to about 0.95 during combustion, and set the equivalence ratio Φ to about 0.95, and for the shortage, a fuel addition valve (for example, near the outlet of the turbine wheel 38) installed in the exhaust pipe. An equivalent fuel ratio Φ of the gas composition at the inlet of the three-way catalyst is set to "1" by injecting additional fuel from (not shown) to generate hydrocarbon (HC) and carbon monoxide (CO) by the high-temperature exhaust gas. (Stoiki) was considered effective. If such a method of controlling the equivalence ratio by adding fuel to the exhaust pipe is adopted, the equivalence ratio Φ (air-fuel ratio) at the time of combustion is changed from the aimed value due to the influence of the response of the supercharger 30 during transient operation or the like. Even if there is a deviation, there is an advantage that stoichiometry can be realized at the inlet of the three-way catalyst.

Further, in the combustion, as in the conventional case, the fuel consumption is deteriorated by the method of operating in the lean state as in the conventional case and adding the fuel to the exhaust pipe to set the equivalence ratio Φ of the gas composition at the inlet of the three-way catalyst to “1” (stoichiometric). There may be disadvantages such as the catalyst temperature becoming excessively high.

In consideration of such a situation, in the present embodiment, the equivalence ratio Φ at the time of combustion is set to about 0.95, and for the shortage, fuel is added to the exhaust pipe to set the equivalence ratio Φ near the inlet of the three-way catalyst to “ 1” (stoiki). As a result, while suppressing the smoke amount to be equal to or less than the smoke limit threshold value, the equivalence ratio Φ becomes 0.95 or more (that is, the actual air-fuel ratio becomes substantially the stoichiometric air-fuel ratio). It is possible to secure a drivable area A”).

FIG. 6 is a diagram schematically showing stoichiometric operation possible region A in engine 1 according to the present embodiment. In FIG. 6, the horizontal axis represents the engine speed NE and the vertical axis represents the engine load (engine torque). The engine 1 can be operated in a region on the lower torque side than the full load line shown in FIG. 6, but most of it is in the stoichiometric operation region A. It should be noted that the engine may be designed so that the entire stoichiometric operation area A is on the lower torque side than the full load line shown in FIG.

<Engine control>
In the memory 202 of the control device 200 according to the present embodiment, the stoichiometric operation area A described above is stored in advance. Then, when the operating region of the engine 1 is included in the stoichiometric operation possible region A, the control device 200 performs the stoichiometric operation in which the equivalence ratio Φ becomes 0.95 (that is, the actual air-fuel ratio becomes substantially the stoichiometric air-fuel ratio). .. This makes it possible to appropriately perform exhaust gas purification by the three-way catalyst 56 while suppressing the amount of smoke below the smoke limit threshold value.

FIG. 7 is a flowchart showing an example of the processing procedure of the control device 200. The control device 200 determines whether or not the current operating region of the engine 1 is included in the stoichiometric operating region A (step S10).

When the current operating region of engine 1 is included in stoichiometric operation possible region A (YES in step S10), control device 200 sets the operating mode of engine 1 to the stoichiometric operating mode (step S20).

FIG. 8 is a flowchart showing an example of an outline of a processing procedure performed by the control device 200 in the stoichiometric operation mode.

The control device 200 calculates the speed ratio V _s /V _sp corresponding to the current engine speed NE and the common rail pressure by referring to the above equations (1) to (5) (step S21).

Next, the control device 200 adjusts the common rail pressure so that the speed ratio V _s /V _sp calculated in step S21 becomes equal to or lower than the reference value R _ref (=0.4) (step S22).

Next, the control device 200 calculates the target main injection amount corresponding to the torque required by the user (step S23).

Next, the control device 200 sets the value obtained by multiplying the target main injection amount by the theoretical air-fuel ratio by 0.95 as the target air amount (step S24). As a result, the target air amount is set so that the equivalence ratio Φ becomes 0.95.

Next, the control device 200 controls the injector 16 so that the main injection amount becomes the target main injection amount (step S25).

Next, the control device 200 controls at least one of the opening degree of the diesel throttle 25 and the vane opening degree of the variable nozzle mechanism 40 so that the amount of air taken into the combustion chamber 15 becomes the target amount of air (step S26). .. Thereby, the equivalence ratio Φ is controlled to 0.95.

By the control as described above, the stoichiometric operation is performed when the operation area of the engine 1 is included in the stoichiometric operation possible area A. This makes it possible to appropriately perform exhaust purification by the three-way catalyst 56 while suppressing the amount of smoke below the smoke limit threshold value.

Returning to FIG. 7, when the current operation area of engine 1 is not included in stoichiometric operation area A (NO in step S10), control device 200 sets the operation mode of engine 1 to the normal mode (step S30). ). In the normal mode, the control device 200 controls the injector 16 so that the main injection amount becomes the target main injection amount corresponding to the torque required by the user, but does not perform the process for performing the stoichiometric operation. That is, in the normal mode, the control device 200 performs steps S21 and S22 of FIG. 8 performed during the stoichiometric operation mode so that the speed ratio V _s /V _sp becomes less than or equal to the reference value R _ref (=0.4). Common rail pressure adjustment process), step S24 (process of setting target air injection amount by multiplying theoretical air-fuel ratio to target air amount), step S26 (throttle opening so that intake air amount becomes target air amount) And processing for controlling at least one of the vane opening) is not performed. As a result, in the normal mode, the air-fuel ratio has a natural value, and generally lean operation is performed as in the conventional case.

As described above, in the engine 1 according to the present embodiment, the maximum velocity of the reverse squish flow from the combustion chamber cavity 14b to the squish area 15a (outer peripheral portion of the piston top surface) at the time of combustion expansion near the compression top dead center is V. _s, and the fuel spray velocity in the vicinity of the opening peripheral edge of the combustion chamber cavity 14b is V _sp , the velocity ratio V _s /V _sp is 0.4 or less under the operating condition where the equivalence ratio Φ is 0.95 or more. The cavity opening diameter d _c and the nozzle injection hole diameter d ₀ are determined so as to satisfy the following velocity ratio condition. Accordingly, it is possible to secure the stoichiometric operation possible region A in which the stoichiometric operation can be executed while suppressing the amount of smoke to the smoke limit threshold value or less.

Further, in the engine 1 according to the present embodiment, the tapered portion T is formed at the peripheral edge portion of the opening of the combustion chamber cavity 14b. Then, the reverse squish speed V _s is calculated based on the average value of the inner diameter d _in and the outer diameter d _OUT of the taper portion T, and the taper so that the calculated reverse squish speed V _s satisfies the above speed ratio condition. The shape of the section T is determined. As a result, even in the engine 1 in which the tapered portion T is formed at the opening peripheral edge portion of the combustion chamber cavity 14b, the reverse squish velocity V _s is calculated more accurately, and the result is used to determine the tapered portion T that satisfies the speed ratio condition. The shape can be accurately determined.

Further, in the engine 1 according to the present embodiment, when the engine 1 is operated in the stoichiometric operation range A, the equivalence ratio Φ in the combustion chamber 15 becomes 1 (that is, the air-fuel ratio is equal to the stoichiometric air-fuel ratio). Therefore, the amount of air taken into the combustion chamber 15 (throttle opening, vane opening) is adjusted. Therefore, it becomes possible to appropriately perform exhaust gas purification by the three-way catalyst 56 while suppressing the amount of smoke below the smoke limit threshold value.

<Modification 1>
In the above-described embodiment, the “reference value R _ref ”used for the speed ratio condition is set to “0.4” which is the speed ratio V _s /V _sp when the smoke amount becomes the smoke limit threshold value.

However, the reference value R _ref may be the speed ratio V _s /V _sp when the smoke amount becomes equal to or less than the smoke limit threshold value. That is, the reference value R _ref may be 0.4 or less.

<Modification 2>
In the above-described embodiment, the intake air amount (throttle opening, vane opening) so that the equivalence ratio Φ becomes 1 (that is, the air-fuel ratio becomes the stoichiometric air-fuel ratio) in the stoichiometric operation possible region A. Was adjusted.

However, the target to be adjusted so that the equivalence ratio Φ becomes 1 in the stoichiometric operation area A is not limited to the intake air amount, and may be the fuel injection amount. Since the main injection amount of the fuel injection amount in one cycle is determined by the torque required by the user, the equivalence ratio Φ becomes 1 by adjusting the pilot injection amount or the post injection amount other than the main injection amount. Good.

For example, when the equivalence ratio Φ is smaller than 1, it means that there is an excess of oxygen, so the pilot injection amount or the post injection amount may be corrected to be increased. Further, when the equivalence ratio Φ is larger than 1, it means that the fuel is excessive, and therefore the pilot injection amount or the post injection amount may be corrected so as to be reduced.

The embodiments disclosed this time are to be considered as illustrative in all points and not restrictive. The scope of the present disclosure is shown not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

1 engine, 10 engine body, 11 cylinder head, 12 cylinder, 14 piston, 14a top surface, 14b combustion chamber cavity, 15 combustion chamber, 15a squish area, 16 injector, 16a injection nozzle, 17 supply pump, 18 common rail, 20 air cleaner , 22 first intake pipe, 24 second intake pipe, 25 diesel throttle, 26 intercooler, 27 third intake pipe, 28 intake manifold, 30 supercharger, 32 compressor, 34 compressor wheel, 36 turbine, 38 turbine wheel, 40 variable nozzle mechanism, 42 connecting shaft, 44 actuator, 50 exhaust manifold, 52 first exhaust pipe, 54 second exhaust pipe, 55 exhaust treatment device, 56 three-way catalyst, 57 PM removal filter, 58 SCR catalyst, 60 EGR device , 62 EGR valve, 66 EGR passage, 102 engine speed sensor, 104 air flow meter, 106 supercharging pressure sensor, 108 fuel pressure sensor, 200 control device, 201 CPU, 202 memory, L lip portion, T taper portion.

Claims (3)

A combustion chamber formed by being surrounded by a piston, a cylinder head, and a cylinder,
A nozzle for spraying fuel into the combustion chamber,
A three-way catalyst for purifying exhaust gas after combustion in the combustion chamber,
A combustion chamber cavity, which is recessed from the outer peripheral portion of the top surface of the piston and has an opening on the side facing the cylinder head, is formed at the top of the piston.
Let V _{s be} the maximum velocity of the reverse squish flow from the combustion chamber cavity toward the outer peripheral portion of the top surface of the piston at the time of combustion expansion near the compression top dead center, and let V _s be the fuel spray velocity in the vicinity of the opening peripheral edge of the combustion chamber cavity. When V _sp is set, the opening diameter of the combustion chamber cavity and the nozzle are set so as to satisfy the speed ratio condition that the speed ratio V _s /V _sp becomes 0.4 or less under the operating condition where the equivalence ratio is 0.95 or more. A diesel engine with a fixed injection hole diameter. A taper portion that is gradually inclined from a radial outer side to a radial inner side of the piston is formed on the opening peripheral edge portion of the combustion chamber cavity,
The shape of the tapered portion is determined so that the velocity V _s of the reverse squish flow calculated based on a value between the inner diameter and the outer diameter of the tapered portion satisfies the velocity ratio condition. 1. The diesel engine according to 1. When the diesel engine is operated in an operating region where the speed ratio V _s /V _sp is 0.4 or less, the air drawn into the combustion chamber so that the equivalence ratio becomes 1 in the combustion chamber. The diesel engine according to claim 1 or 2, wherein at least one of an amount and a fuel injection amount of the nozzle is adjusted.

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