JP2011247152A - Fuel injection system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the fuel ignition when carrying out of a plurality of times of pilot injections.SOLUTION: The fuel injection system for an internal combustion engine carries out a plurality of times of pilot injections prior to the main injection into a cylinder. At the plurality of times of the pilot injections, the spray reaching distance is shortened gradually by gradually reducing the fuel injection amount for disposing the sprayed fuel in a plurality of areas. The number of the pilot injections executed for each area is made larger for a region with a shorter spray reaching distance.

Description

  The present invention relates to a fuel injection system for an internal combustion engine.

  When the fuel injection amount is injected a plurality of times, the fuel penetration amount of each time is reduced to the limit value of the fuel injection valve, thereby reducing the fuel penetration force and suppressing the adhesion of fuel to the combustion chamber wall surface, A technique for retaining the spray near the center of the cylinder is known (for example, see Patent Document 1).

  By the way, in the case where pilot injection for injecting fuel less than the main injection is performed a plurality of times before main injection into the cylinder of the internal combustion engine, each pilot injection is affected by the previous pilot injection, It is influenced by the condition inside. For this reason, even if the pilot injection amount of each time is the same, the reaching distances may be different from each other, and it may be difficult to retain the air-fuel mixture at a desired position. Further, if the spray stays near the center of the cylinder, a large amount of fuel evaporates near the center of the cylinder, and the temperature near the center decreases locally. There is a possibility that the ignitability is lowered.

JP 2009-167821 A JP 2010-007619 A JP 2006-283679 A JP-A-2005-090401 JP 2002-357152 A

  The present invention has been made in view of the above-described problems, and an object thereof is to further improve the ignitability of fuel when performing a plurality of pilot injections.

In order to achieve the above object, a fuel injection system for an internal combustion engine according to the present invention comprises:
In a fuel injection system of an internal combustion engine that performs pilot injection a plurality of times before main injection into a cylinder,
When performing the pilot injection a plurality of times, the fuel injection amount is decreased stepwise so that the spray reach distance is shortened stepwise so that the sprays are arranged in a plurality of regions, and the pilot injection performed for each region The number of times is increased as the spraying distance is shorter.

  That is, when pilot injection is performed a plurality of times before main injection, the initial pilot injection amount is maximized and the final pilot injection amount is minimized. By reducing the pilot injection amount, the penetrating force of the fuel is reduced, so that the reach distance can be shortened. Therefore, the spray can be arranged in a plurality of regions by decreasing the fuel injection amount in stages. However, since the fuel injection amount is decreased in order to shorten the reach distance, only a small amount of spray is arranged in one pilot injection in a region where the spray reach distance is short. In contrast, by increasing the number of pilot injections in a region where the spray reach distance is shorter, a larger amount of air-fuel mixture can be formed because more spray can be retained in this region. Thereby, since temperature can be raised in a wide range, the ignitability of main injection can be improved.

  In the present invention, the fuel injection amount of each time by the pilot injection can be set within a range in which the relationship between the fuel injection amount and the spray reach distance is determined regardless of the fuel pressure.

  Here, in the fuel injection valve, when the injection amount is relatively small, the spray reach distance changes according to the fuel injection amount regardless of the fuel pressure. On the other hand, when the injection amount increases, the reach distance changes according to the fuel injection amount and the fuel pressure. Therefore, if pilot injection is performed in a region where the spray arrival distance changes according to the fuel injection amount regardless of the fuel pressure, even if the fuel pressure changes, the arrival of the spray is not considered. The distance can be obtained. This facilitates control of the fuel injection amount.

  In the present invention, the fuel injection amount for each pilot injection can be determined based on a physical quantity that correlates with the resistance of the gas in the cylinder to the fuel spray.

  Here, the spray reach distance varies according to the resistance of the gas in the cylinder to the fuel spray. That is, as the gas resistance increases, the spray reach distance decreases. Based on the physical quantity correlated with this resistance, it is possible to obtain the pilot injection quantity for arranging the spray at a desired position. Note that there is a correlation between the state in the cylinder (for example, the pressure or gas density in the cylinder) and the resistance of the gas to the spray. For example, the higher the pressure in the cylinder, the greater the resistance of the gas to the spray. Therefore, since there is a correlation between the state in the cylinder (for example, the pressure or gas density in the cylinder) and the fuel injection amount for disposing the spray at a desired position, the fuel injection is performed based on the correlation. The amount may be determined.

  According to the present invention, the ignitability of fuel when performing a plurality of pilot injections can be further increased.

It is a figure which shows schematic structure of the engine by an Example. It is the figure which showed the state of the spray in a combustion chamber with respect to the injection pattern of pilot injection in the case of performing pilot injection once with respect to each area | region in a combustion chamber. It is the figure which showed the state of the spray in a combustion chamber with respect to the injection pattern of pilot injection at the time of changing the frequency | count of performing pilot injection with respect to each area | region in a combustion chamber. It is the figure which showed the relationship between the fuel injection quantity and the reach | attainment distance of spray. It is the flowchart which showed the control flow of the pilot injection which concerns on an Example.

  Hereinafter, specific embodiments of a fuel injection system for an internal combustion engine according to the present invention will be described with reference to the drawings. Here, a case where the fuel injection system for an internal combustion engine according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.

  FIG. 1 is a diagram showing a schematic configuration of an engine according to this embodiment. An engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having cylinders 2. The engine 1 is configured by connecting a cylinder head 1a and a cylinder block 1b.

An intake port 4 and an exhaust port 5 are formed in the cylinder head 1a. An intake valve 6 and an exhaust valve 7 that move up and down are provided at the boundary between the intake port 4 and the exhaust port 5 and the inside of the cylinder 2. ing. The cylinder head 1a is provided with a fuel injection valve 3 for supplying fuel into the cylinder 2. A fuel supply source 31 that supplies fuel to the fuel injection valve 3 is connected to the fuel injection valve 3. The fuel supply source 31 includes, for example, a pump and a common rail.

  When a drive current is applied to the fuel injection valve 3, the fuel injection valve 3 opens, and as a result, fuel is injected from the fuel injection valve 3 into the cylinder 2. Here, the amount of fuel injected from the fuel injection valve 3 is calculated by a map from the engine speed and the load. Further, the basic fuel injection timing is also calculated from the engine speed and load using a map. These maps are obtained in advance by experiments or the like.

  Further, the cylinder block 1 b includes a piston 10 inserted into the cylinder 2. A combustion chamber 11 that is recessed toward the center of the piston 10 is formed on the upper surface of the piston 10. A pressure sensor 13 for measuring the pressure in the cylinder 2 is attached to the cylinder head 1a.

  The engine 1 configured as described above is provided with an ECU 12 that is an electronic control unit for controlling the engine 1. The ECU 12 controls the operating state of the engine 1 in accordance with the operating conditions of the engine 1 and the driver's request.

  In addition to the pressure sensor 13, various sensors are connected to the ECU 12 through electrical wiring, and output signals from these sensors are input to the ECU 12. On the other hand, the fuel injection valve 3 is connected to the ECU 12 via electrical wiring, and the fuel injection valve 3 is controlled by the ECU 12. The ECU 12 stores various application programs and various control maps.

  When the fuel injection is performed from the fuel injection valve 3 into the cylinder 2, the ECU 12 performs a plurality of pilot injections prior to the main injection. When a plurality of pilot injections are performed, the respective fuel injection amounts (also referred to as pilot injection amounts) are adjusted so that the sprays are arranged in a plurality of regions. In this case, the injection amount is smaller in the pilot injection in which the spray is arranged in the region closer to the fuel injection valve 3 than the wall surface of the combustion chamber 11.

  Next, FIG. 2 is a diagram showing the state of spray in the combustion chamber 11 with respect to the injection pattern of pilot injection when pilot injection is performed once for each region in the combustion chamber 11. FIG. 2A is a view showing the state of spray in the combustion chamber 11 for each injection, and FIG. 2B is a time chart showing an injection pattern of pilot injection. In FIG. 2B, the fuel injection valve 3 is opened when it is ON and closed when it is OFF. In FIG. 2, pilot injection is performed three times per one cylinder per cycle. Then, from the first pilot injection to the third pilot injection, the fuel injection amount is sequentially decreased by shortening the ON period stepwise. For this reason, as the spray of the pilot injection performed later becomes shorter, the reach distance becomes shorter, so that it stays at a position closer to the fuel injection valve 3. Thus, spray can be arrange | positioned to the several area | region in the combustion chamber 11 by reducing pilot injection amount in steps.

  In the first pilot injection, the injection amount is relatively increased so that the spray 81 is arranged in the vicinity of the wall surface of the combustion chamber 11. In addition, the injection amount of the second pilot injection is made smaller than that of the first pilot injection so that the spray 82 is arranged near the middle from the fuel injection valve 3 to the wall surface of the combustion chamber 11. Further, the injection amount of the third pilot injection is made smaller than that of the second pilot injection so that the spray 83 is arranged closer to the fuel injection valve 3 than the spray 82 by the second pilot injection.

That is, since the first pilot injection is relatively large, the reach distance becomes long. Since the amount of the second pilot injection is smaller than that of the first pilot injection, the reach distance is shorter than that of the first pilot injection. Furthermore, since the third pilot injection has a smaller amount than the second pilot injection, the reach distance is shorter than the second pilot injection. Thus, by reducing the fuel injection amount in stages, a plurality of air-fuel mixtures can be divided and arranged between the fuel injection valve 3 and the vicinity of the wall surface of the combustion chamber 11. However, since pilot injection is performed only once for each region in the combustion chamber 11, only a small amount of fuel is injected in the region close to the fuel injection valve 3, so that the range of spray formed is narrowed. The number of pilot injections, the injection amount of each injection, and the injection interval can be mapped in advance in association with the operating state of the engine 1 (for example, engine speed and engine load). Further, the pilot injection amount for each time can be set as described later.

  By the way, when the compression of the engine 1 is further reduced, the amount of temperature increase during the compression stroke is reduced, and the ignitability is accordingly deteriorated. On the other hand, if pilot injection is performed, the temperature in the combustion chamber 11 can be raised in advance, so that the ignition delay of fuel due to main injection can be shortened. Thereby, deterioration of ignitability can be suppressed. However, in the engine 1 with a small displacement, since the diameter of the combustion chamber 11 is small, the spray by the pilot injection reaches the wall surface of the combustion chamber 11 and heat is taken away by the wall surface, so that the temperature of the air-fuel mixture decreases. There is a fear. Further, even if the spray by pilot injection does not reach the wall surface of the combustion chamber 11, the air-fuel mixture is easily disposed in a narrow range, so that the temperature of the combustion chamber 11 may be locally reduced due to latent heat of vaporization. When the temperature of the air-fuel mixture decreases as described above, the effect of performing pilot injection is reduced.

  On the other hand, since the plurality of air-fuel mixtures are divided and arranged between the fuel injection valve 3 and the vicinity of the wall surface of the combustion chamber 11, it is possible to suppress the temperature of the air-fuel mixture from being locally lowered. Can be improved and cooling loss can be reduced.

  FIG. 3 is a diagram showing a state of spray in the combustion chamber 11 with respect to an injection pattern of pilot injection when the number of times of performing pilot injection is changed for each region in the combustion chamber 11. FIG. 3A is a diagram showing the state of spray in the combustion chamber 11 for each injection, and FIG. 3B is a time chart showing an injection pattern of pilot injection. In FIG. 3B, the fuel injection valve 3 is opened when it is ON, and closed when it is OFF. In FIG. 3, six pilot injections are performed per cylinder per cycle. Then, from the first pilot injection to the sixth pilot injection, the fuel injection amount is decreased stepwise by shortening the ON period stepwise.

  In the first pilot injection, the injection amount is relatively increased so that the spray 91 is arranged in the vicinity of the wall surface of the combustion chamber 11. In addition, the second and third pilot injections are performed with an injection amount smaller than that of the first pilot injection so that the spray 92 is disposed near the middle from the fuel injection valve 3 to the wall surface of the combustion chamber 11. The Further, the fourth to sixth pilot injections are performed at the second and third injections so that the spray 93 is arranged closer to the fuel injection valve 3 than the spray 92 by the second and third pilot injections. Less than the pilot injection.

  That is, since the first pilot injection has a relatively large amount, the reach distance becomes long, so that the spray 91 is arranged in the vicinity of the wall surface of the combustion chamber 11. Since the second and third pilot injections are less in amount than the first pilot injections, their reach is shorter than that of the first pilot injection. Therefore, the spray 92 is formed on the wall surface of the combustion chamber 11 and the fuel. It arrange | positions in the substantially middle position with the injection valve 3. FIG. Furthermore, since the fourth to sixth pilot injections are less in volume than the second and third pilot injections, their reach is shorter than the second and third pilot injections, and the spray 93 is It is arranged in the vicinity of the fuel injection valve 3.

Thus, the fuel injection amount is decreased stepwise by shortening the period in which the fuel injection valve 3 is turned on stepwise from the first pilot injection to the sixth pilot injection. For this reason, since the spray reach distance can be shortened step by step, the air-fuel mixture can be divided and arranged in a plurality of regions from the fuel injection valve 3 to the vicinity of the wall surface of the combustion chamber 11.

  And the frequency | count of the pilot injection performed with respect to each area | region of the combustion chamber 11 is increased, so that the area | region where the spray arrival distance is short. Here, in order to shorten the fuel reach distance, it is necessary to reduce the fuel injection amount. As shown in FIG. 2, if the injection amount per one time is simply decreased, the pilot injection amount decreases in the region closer to the fuel injection valve 3, so that a sufficient amount of heat generated by the pilot injection can be obtained. Disappear. For this reason, there is a possibility that the combustion state is deteriorated while traveling in a highland or at a low temperature. On the other hand, as shown in FIG. 3, if the number of pilot injections is increased as the amount of pilot injection per time is decreased, more fuel is disposed even in a short fuel reach range. be able to.

  In FIG. 3, the air-fuel mixture is formed in three regions between the fuel injection valve 3 and the wall surface of the combustion chamber 11, but the same can be considered for two or more regions.

  Next, the relationship between the fuel injection amount and the spray reach distance will be described. FIG. 4 is a diagram showing the relationship between the fuel injection amount and the spray reach distance. FIG. 4 shows the spray reach distance with respect to the fuel injection amount when the fuel is injected from the fuel injection valve 3 only once. The solid line indicates the case where the fuel pressure is relatively high, and the alternate long and short dash line indicates the case where the fuel pressure is relatively low. Here, the relationship between the fuel injection amount and the spray reach distance varies according to the pressure in the cylinder 2. It also changes depending on the number of pilot injections. FIG. 4 shows a relationship in a preset reference state, and the relationship shown in FIG. 4 is corrected by the pressure in the cylinder 2 and the order of pilot injection.

  As shown in FIG. 4, the greater the fuel injection amount, the longer the spray reach distance. Until the fuel injection amount reaches the amount indicated by A, the relationship between the fuel injection amount and the spray reach distance is determined regardless of the fuel pressure (may be the common rail pressure), but the fuel injection amount is A If the amount exceeds the amount indicated by, the relationship between the fuel injection amount and the spray reach is affected by the fuel pressure. That is, the region where the fuel injection amount is smaller than A in FIG. 4 (the region on the left side of A in FIG. 4) is a range where the relationship between the fuel injection amount and the spray reach distance is determined regardless of the fuel pressure. I can say that. In addition, it can be said that the region where the fuel injection amount is larger than A in FIG. 4 (the region on the right side of A in FIG. 4) is the range in which the spray reach distance becomes longer as the fuel pressure is higher. Note that the fuel injection amount indicated by A in FIG. 4 is determined according to the performance of the fuel injection valve 3 and can be obtained in advance by experiments or the like. Further, if the relationship between the fuel injection amount and the opening time of the fuel injection valve 3 is obtained in advance by experiments and stored in the ECU 12, the ECU 12 controls the opening time of the fuel injection valve 3. Thus, the fuel injection amount can be adjusted.

  In this embodiment, the pilot injection amount may be controlled so that the pilot injection amount is equal to or less than the fuel injection amount indicated by A in FIG. Then, the fuel injection amount indicated by A is the maximum value of the pilot injection amount, and is hereinafter referred to as the maximum pilot injection amount A. Thus, by setting the pilot injection amount to be equal to or less than the maximum pilot injection amount A, the energy given to the spray is governed only by the injected fuel amount. Even if the pressure is changed, the pilot injection amount and the kinetic energy depend only on the injection period. For this reason, the initial energy of spraying can be controlled by the fuel injection amount. That is, by setting the pilot injection amount to be equal to or less than the maximum pilot injection amount A, it is possible to eliminate the influence of the fuel pressure on the spray reach distance.

Here, the reach of the pilot-injected fuel also varies depending on the state (for example, pressure or density) in the cylinder 2. That is, since the resistance of the gas in the cylinder 2 to the spray changes according to the state (for example, pressure or density) in the cylinder 2, the reach distance of the spray changes. For example, in a highland where the air density is low, the resistance of the gas in the cylinder 2 is smaller than in a lowland where the air density is high, and therefore, the spray reach distance is long even with the same fuel injection amount. Further, the pressure in the cylinder 2 also changes depending on the crank angle. When pilot injection is performed a plurality of times during the compression stroke, the later the injection timing, the higher the pressure in the cylinder 2, and the shorter the reach distance. Therefore, in the second and subsequent pilot injections, although the reach distance becomes longer due to the airflow generated by the previous pilot injection, the higher the pressure in the cylinder 2, the smaller the degree of reach.

  On the other hand, for example, if a correction value for correcting the relationship in FIG. 4 is obtained in advance by experiments or the like according to the pressure in the cylinder 2, the relationship in FIG. 4 is corrected according to the pressure in the cylinder 2. Can do. And based on the relationship shown in FIG. 4 after correction | amendment, the pilot injection quantity of each time is computable. The pressure in the cylinder 2 is obtained by the pressure sensor 13. Further, since the pressure in the cylinder 2 can be estimated from, for example, the atmospheric pressure, the pressure in the intake pipe, the crank angle of the engine 1, and the like, these relationships may be stored in the ECU 12 in advance.

  Here, in a situation where it is difficult for the fuel to combust, such as at extremely low temperatures or when traveling at high altitudes, the required value of the total amount of pilot injection becomes large, so each time the pilot sprays within the range where the fuel spray does not reach the wall surface of the combustion chamber 11. It is better to increase the injection amount as much as possible. Thus, when the required value of the total amount of pilot injection is large, the first pilot injection must be performed at a relatively early time when the pressure in the cylinder 2 is low, but in the pilot injection performed after that, Since the pressure in the cylinder 2 increases, it is possible to suppress the reaching distance from increasing even if the pilot injection amount is increased. That is, the ignitability can be improved by increasing the pilot injection amount.

  FIG. 5 is a flowchart showing a control flow of pilot injection according to the present embodiment. This routine is executed by the ECU 12 every predetermined time.

  In step S101, a required pilot total amount that is a total amount of required pilot injection amount is calculated. The required pilot total amount is the total amount of pilot injection required per one cylinder cycle, and is determined according to the operating state of the engine 1. For example, the relationship between the engine speed, the engine load, and the required pilot total amount is obtained in advance through experiments or the like and stored in the ECU 12.

  In step S102, a correction value for the spray reach distance is calculated. In this step, a correction value for correcting the spray reach distance based on the gas resistance in the cylinder 2 is calculated. That is, a correction value for correcting FIG. 4 is calculated. Since the correction value has a correlation with the pressure in the cylinder 2, the relationship between the correction value and the pressure in the cylinder 2 is obtained in advance through experiments or the like and stored in the ECU 12.

  In step S103, the first pilot injection amount is calculated. In this step, the pilot injection amount is calculated such that the spray is arranged in the vicinity of the wall surface of the combustion chamber 11 according to the correction value calculated in step 102 and the relationship of FIG. The pilot injection amount is equal to or less than the maximum pilot injection amount A, but may be a pilot injection amount that exceeds the maximum pilot injection amount A in a range where fuel does not adhere to the wall surface of the combustion chamber 11.

In step S104, the second pilot injection amount is calculated such that the reach distance is shorter than the first pilot injection. This second pilot injection amount is an amount that is smaller than the first pilot injection amount, and that the spray reach distance is closer to the fuel injection valve 3 than the spray by the first pilot injection. Here, the reach of the second pilot injection becomes longer due to the airflow generated by the first pilot injection. However, since the pressure in the cylinder 2 is higher at the time of the second pilot injection than at the time of the first pilot injection, the resistance of the gas to the spray becomes larger, so that the reach distance of the second pilot injection is shorter accordingly. Become. Taking these into account, the second pilot injection amount is calculated. In each case, a map for obtaining a correction value for correcting the relationship of FIG. 4 may be stored, and the second pilot injection amount may be calculated based on the correction value. The third pilot injection amount is set to the same value as the second pilot injection amount.

  In step S105, the fourth pilot injection amount is calculated such that the reach distance field is shorter than the second and third pilot injection amounts. The fourth pilot injection amount is smaller than the second and third pilot injection amounts, and the spray reach distance is closer to the fuel injection valve 3 side than the sprays by the second and third pilot injections. Is the amount. In this step, the fourth pilot injection amount is calculated as in step S104. Then, the fifth and subsequent pilot injection amounts are set to the same value as the fourth pilot injection amount.

  In step S106, pilot injection is performed using the calculated pilot injection amount. When the total amount of pilot injection exceeds the required pilot total amount, pilot injection is terminated at that time.

  As described above, according to the present embodiment, the reach of the fuel spray can be controlled with high accuracy. As a result, the fuel can be prevented from adhering to the wall surface of the combustion chamber 11 and the air-fuel mixture can be retained at a desired position. In addition, since the air-fuel mixture can be arranged in a wide range, local temperature reduction can be suppressed, so that ignitability can be improved. Thereby, low compression is attained.

1 Engine 1a Cylinder head 1b Cylinder block 2 Cylinder 3 Fuel injection valve 4 Intake port 5 Exhaust port 6 Intake valve 7 Exhaust valve 10 Piston 11 Combustion chamber 12 ECU
13 Pressure sensor 31 Fuel supply source

Claims (3)

In a fuel injection system of an internal combustion engine that performs pilot injection a plurality of times before main injection into a cylinder,
When performing the pilot injection a plurality of times, the fuel injection amount is decreased stepwise so that the spray reach distance is shortened stepwise so that the sprays are arranged in a plurality of regions, and the pilot injection performed for each region The fuel injection system for an internal combustion engine is characterized in that the number of times is increased in a region where the spray reach distance is shorter.   2. The fuel injection system for an internal combustion engine according to claim 1, wherein the fuel injection amount of each time by pilot injection is within a range in which the relationship between the fuel injection amount and the spray reach distance is determined regardless of the fuel pressure. .   3. The fuel injection system for an internal combustion engine according to claim 1, wherein the fuel injection amount for each pilot injection is determined based on a physical quantity that correlates with the resistance of the gas in the cylinder with respect to the fuel spray.

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