JPH04308356A - Fuel injection device for compression ignition type internal combustion engine - Google Patents

Abstract

PURPOSE:To suppress the generation of combustion noise, NOx, soot and unburnt HC. CONSTITUTION:A fuel storage chamber 14 is formed in a fuel injection valve 6, and a needle 9 is operated by a piezoelectric element 15. The injection pressure based on cylinder internal pressure is set to be less than 400(kg/cm<2>). The diameter of a nozzle hole 8 is set to be more than 0.4m, and the oil droplet Sauter's mean diameter of fuel spay is made approximately between 0.2mm and 1.0mm. Fuel injection is completed in the comparatively early stage of a compression stroke. Upon the completion of fuel injection, completion ignition is performed after fuel spray is diffused in a wide range.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel injection system for a compression ignition internal combustion engine.

0002

[Prior Art] It has been believed that in order to obtain good combustion in a compression ignition internal combustion engine, it is essential to improve the atomization of the injected fuel and reduce the Sauter average particle size of the sprayed oil droplets. There is. This Sauter average particle size becomes smaller as the flow velocity of the injected fuel becomes faster, so the Sauter average particle size becomes smaller as the injection pressure becomes higher. Furthermore, since this Sauter average particle diameter is proportional to the diameter of the nozzle opening, the smaller the diameter of the nozzle opening is, the smaller the Sauter average particle diameter becomes. Therefore, in order to reduce the Sauter average particle size, it is possible to increase the injection pressure and reduce the diameter of the nozzle orifice, but in the past, there was a limit to how high the injection pressure could be, and it was not possible to increase the injection pressure that high. I couldn't. Specifically, in a conventional compression ignition internal combustion engine, the injection pressure is about 300 to 500 (kg/cm2) and the diameter of each nozzle opening is about 0.3 mm at most. The average particle size is approximately 0.05 mm.

[0003] On the other hand, recently, for example,
A common rail injection device as described in Japanese Patent No. 12658 has been developed. In such a common rail type injection device, since the injection pressure can be set to 1000 (kg/cm2) or more, the Sauter average particle size can be further reduced. When the Sauter average particle diameter is small as described above, quite fine fuel particles are inevitably generated during fuel injection, and these particles are gasified to form an air-fuel mixture around the fuel spray near the nozzle opening. When the air-fuel mixture is formed in this way, the air-fuel mixture is immediately ignited, and this serves as a spark that sequentially causes the injected fuel to burn. That is, compression ignition is performed during fuel injection, and a considerable portion of the fuel is combusted by the time the fuel injection is completed.

0004

However, when an air-fuel mixture is formed around the fuel spray in this way, this air-fuel mixture is ignited and combusted all at once. As a result, the combustion pressure suddenly increases, causing not only combustion noise, but also combustion temperature becoming high and a large amount of NOx being generated. Further, when the air-fuel mixture is ignited and burned all at once, the oil droplets in the fuel spray are combusted in the form of oil droplets, resulting in the generation of a large amount of soot and unburned HC.

[0005]

[Means for Solving the Problems] In order to solve the above-mentioned problems, according to the present invention, the injection pressure based on the in-cylinder pressure is increased to approximately 4
00 (kg/cm2) or less, and set the area of the nozzle opening so that the Sauter average particle diameter of the sprayed oil droplets is between approximately 0.2 mm and approximately 1 mm, even during high engine load operation. Compression ignition is caused some time after all fuel injection is completed.

[0006]

[Operation] By advancing the injection timing and increasing the Sauter average particle diameter of the sprayed oil droplets compared to the conventional method, individual oil droplets are evaporated and burned.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is a diesel engine body, 2 is a cylinder block, 3 is a piston, 4 is a cylinder head, 5 is a combustion chamber, and 6 is a fuel injection valve. The fuel injection valve 6 has a nozzle port 8 that opens into the combustion chamber 5 in its housing 7, and a needle 9 that controls opening and closing of the nozzle port 8.
and. A back pressure chamber 10 is formed on the top surface of the needle 9, and the needle 9 is always connected to the nozzle port 8 in the back pressure chamber 10.
A compression spring 11 is inserted which urges the A fuel reservoir 12 is formed around the lower end of the needle 9, and this fuel reservoir 12 is communicated with an annular large-capacity fuel reservoir 14 via a fuel passage 13. On the other hand, a piston 16 driven by a piezoelectric element 15 is slidably inserted into the housing 7, and a pressure control chamber 17 is defined by the tip end surface of the piston 16. This pressure control chamber 17 is communicated with the back pressure chamber 10 via a fuel passage 18. The fuel storage chamber 14 is connected to a pressure control chamber 17 via a check valve 19 that allows flow only toward the fuel storage chamber 14, and the pressure control chamber 17 is connected to a check valve that allows flow only toward the pressure control chamber 17. It is connected to a fuel supply pump 21 via 20. As shown in FIG. 1, the fuel storage chamber 14 is provided between the needle 9 and the piston 16, and therefore, the fuel storage chamber 14 is formed near the nozzle port 8. The piezoelectric element 15 expands in the axial direction when charged with electric charge, and contracts in the axial direction when the electric charge is discharged. The charging and discharging action on the piezoelectric element 15 is controlled based on the output signal of the electronic control unit 30.

The electronic control unit 30 is composed of a digital computer, and includes a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and an input port 35, which are interconnected by a bidirectional bus 31. and an output port 36. The accelerator pedal 37 is connected to a load sensor 38 that generates an output voltage proportional to the amount of depression of the accelerator pedal 37, and the output voltage of the load sensor 38 is connected to an AD converter 39.
The signal is input to the input port 35 via the input port 35. Also connected to the input port 35 is a rotational speed sensor 40 that generates an output signal representing the engine rotational speed. On the other hand, the output port 36 is connected to the piezoelectric element 15 via a drive circuit 41.

Pressurized fuel discharged from the fuel supply pump 21 is supplied into the pressure control chamber 17 via the check valve 20.
The pressurized fuel supplied to the pressure control chamber 17 is supplied into the fuel storage chamber 14 via the check valve 19 on the one hand, and into the back pressure chamber 10 via the fuel passage 18 on the other hand. Therefore, the fuel pressure in the fuel storage chamber 14 and the fuel pressure in the back pressure chamber 10 are equal, and at this time the needle 9 closes the nozzle port 8. When the charge is discharged from the piezoelectric element 15, the piezoelectric element 15 contracts in the axial direction, causing the piston 16 to rise, causing the fuel pressure in the pressure control chamber 17 to drop rapidly, and the fuel in the back pressure chamber 10 to Pressure drops rapidly. As a result, the needle 9 rises, and fuel injection from the nozzle port 8 is started. Note that when the fuel pressure in the pressure control chamber 17 suddenly decreases, pressurized fuel is supplied into the pressure control chamber 17 via the check valve 20, so that the fuel pressure in the pressure control chamber 17 gradually increases. However, during this time, the needle 9 remains open. Next, when the piezoelectric element 15 is charged, the piezoelectric element 15 expands in the axial direction. As a result, the piston 16 descends, causing the fuel pressure in the pressure control chamber 17 to rise rapidly, and at the same time, the fuel pressure in the back pressure chamber 10 to rise rapidly. As a result, needle 9
As the fuel is lowered, fuel injection from the nozzle port 8 is stopped.

As mentioned at the beginning, the Sauter average particle size of the oil droplets in the fuel spray becomes smaller as the flow velocity of the injected fuel becomes faster, and as the diameter of the nozzle opening 8 becomes smaller. If the injection pressure based on the cylinder pressure is ΔP, the flow velocity of the injected fuel is proportional to the square root of ΔP. Therefore, if the diameter of the nozzle port 8 is D, the Sauter average particle diameter is

It is approximately proportional to [Equation 1]. Conventionally, for example, ΔP=40
0 (kg/cm2), the maximum D=0.3mm, so

[Equation 2] is approximately 0.015 (mm·kg/cm2) at the maximum, and the Sauter average particle diameter at this time is approximately 0.05 mm. However, when the Sauter average particle diameter is around this level, a mixture E is formed around the fuel spray D injected from the nozzle port C, as shown in FIG. 4(B).
is immediately ignited. That is, as can be seen from FIG. 4A showing the injection rate Q, when injection is started and a mixture E is formed, ignition is performed by this mixture E during injection. As described above, when such a combustion method is used, combustion noise is generated, a large amount of NOx and soot are generated, and a large amount of unburned HC is generated.

On the other hand, in the present invention, for example, ΔP=1
00 (kg/cm2), the diameter D of the nozzle opening 8 is 0.00 (kg/cm2).
It is assumed to be 4 mm or more. In this case, the diameter D of the nozzle opening 8 is set to 0.
Even if it is 4mm

[Equation 3] is 0.04, and the Sauter average particle diameter at this time is approximately 0.15 mm. The solid line A in FIG. 2 is in the present invention.

[
4], and the dashed line B shows the minimum of

It shows the maximum of [Equation 5]. When the Sauter average particle diameter is increased as in the present invention, the penetration force of the fuel spray increases, so the oil droplets are spread farther and wider than in the past, as shown by fuel spray D in FIG. 3(B). It turns out. Since it takes time for the oil droplets to spread widely in this way, the top dead center TD is
Injection occurs well before C. Air-fuel mixture E formed around fuel spray D when Sauter average particle diameter is increased
Since the amount of fuel is small and the injection timing is advanced, this air-fuel mixture E is not ignited immediately after it is formed. Shortly after the injection is completed, the mixture E and the mixture formed by the evaporation action from the oil droplets are ignited, but since the amounts of these mixtures are small, the combustion pressure does not rise rapidly. The generation of combustion noise will be suppressed. When ignition occurs, the oil droplets are heated and the fuel evaporates little by little from the oil droplets, and the evaporated fuel is gradually combusted. When the fuel evaporates from the oil droplets, the latent heat of vaporization is taken away from the surroundings, so the combustion temperature does not become very high, thus suppressing the generation of NOx. Further, since the oil droplets are widely spread and the fuel evaporated from the oil droplets is sequentially burned, the generation of soot and unburned HC is suppressed.

As described above, the present invention is characterized in that the fuel spray is compressed and ignited after the entire fuel spray has been sufficiently diffused. Therefore, as shown in FIG. The fuel injection timing is determined so that compression ignition is performed after a period of time, that is, a diffusion time α has elapsed. As shown in FIG. 5, this diffusion time α is determined by the Sauter average particle size S. M. The larger D becomes, the longer it becomes. Therefore, this diffusion time α depends on the injection pressure ΔP and the diameter D of the nozzle opening 8
It will be determined from As the diffusion time α becomes longer, the fuel injection timing advances accordingly, and the fuel injection timing is set at about 40 degrees to 100 degrees before top dead center, although it varies depending on the operating condition.

By the way, as mentioned above, ΔP=100(k
g/cm2) and D=0.4 mm, the Sauter average particle diameter is approximately 0.15 mm. However, in order to achieve good evaporative combustion of oil droplets, it is preferable to suppress the evaporation effect of the fuel by making the ratio of the surface area to the volume of the oil droplets a little smaller. Preferably in the range of 1 mm.
Therefore, the oil droplets become quite large. Sauter average particle size is

Since it is not directly proportional to [Equation 6], the diameter D of the nozzle opening 8 is set to 0.
If the diameter is 5 to 0.6 mm or more, the Sauter average particle size is 0.2
mm or more. Note that as the Sauter average particle diameter increases, the penetration force of the oil droplets increases, so the injection pressure ΔP is adjusted so that the oil droplets do not adhere to the inner wall surface of the cylinder bore or the top surface of the piston 3.
needs to be low. Therefore, in the present invention, the injection pressure ΔP is set to 400 (kg/cm2) or less.

As mentioned above, in the conventional compression ignition internal combustion engine, various problems arise due to the small Sauter average particle diameter, and in addition to these, there are the following problems. That is,
In a conventional fuel injection valve, when the needle opens and fuel injection starts, the fuel pressure inside the fuel injection valve temporarily drops, so as shown in Fig. 4(A), the injection pressure drops immediately after the injection starts. It doesn't rise. Therefore, the fuel injected when the injection pressure is high will overtake the fuel injected at the beginning when the injection pressure is low. That is, in FIG. 4(B), the numbers 1, 2, 3, 4, 5, and 6 attached to each fuel spray represent the injection order, so that the fuel injected first is overtaken by the fuel injected next. I know what's going on. However, when the fuel sprays overtake each other in this way, the area where the fuel sprays overlap becomes extremely concentrated, resulting in the problem of generating a large amount of soot and unburned HC.

In order to prevent such overtaking of the fuel spray, in the embodiment according to the present invention, a fuel storage chamber 14 is provided near the nozzle port 8, as shown in FIG. When such a fuel storage chamber 14 is provided, injection is performed at a high injection pressure as soon as the needle 9 opens, and since the fuel storage chamber 14 has a large volume, the injection pressure does not decrease much even during injection. Therefore, the injection rate Q has a sharp rise and fall as shown in FIG. 3(A). In this case, Figure 3
As shown in (B), the fuel spray injected later follows the fuel spray injected earlier, and the fuel spray injected later does not overtake the fuel spray injected earlier. Therefore, since the fuel sprays do not overlap, no over-concentration region is formed, and the generation of soot and unburned HC is thus prevented.

FIG. 6 shows a main routine for executing fuel injection control. Referring to FIG. 6, first, in step 50, the ignition crank angle θb at which compression ignition is performed is calculated. This ignition crank angle θb is stored in advance in the ROM 32 as a function of the depression amount L of the accelerator pedal 37 and the engine speed N, as shown in FIG. Next, in step 51, the injection completion timing θp is calculated by subtracting the diffusion time α expressed in crank angle from the ignition crank angle θb. Next, in step 52, the fuel injection time TAU is calculated. This fuel injection time TAU is determined by the accelerator pedal 37 as shown in FIG.
ROM in advance as a function of the amount of depression L and the engine speed N.
32. Next, in step 53, the injection start timing θa is calculated from the injection completion timing θp and the fuel injection time TAU.

[0017]

[Effects of the invention] Combustion noise, NOx, soot and unburned H
The generation of C can be suppressed.

[Brief explanation of drawings]

FIG. 1 is a side sectional view of a fuel injection valve and a combustion chamber.

[Figure 2] Conventional

[Math 7] and in the present invention

[Math. 8] FIG.

FIG. 3 is a diagram for explaining the injection method in the present invention.

FIG. 4 is a diagram for explaining a conventional injection method.

FIG. 5 is a diagram showing diffusion time.

FIG. 6 is a flowchart showing the main routine.

FIG. 7 is a diagram showing the ignition crank angle.

FIG. 8 is a diagram showing fuel injection time.

[Explanation of symbols]

6...Fuel injection valve 8...Nozzle mouth 14...Fuel storage chamber

Claims (1)

[Claims] [Claim 1] The injection pressure based on the cylinder pressure is approximately 40
0 (kg/cm2) or less and set the area of the nozzle opening so that the Sauter average particle diameter of the sprayed oil droplets is between approximately 0.2 mm and approximately 1 mm, even during high engine load operation. A fuel injection device for a compression ignition internal combustion engine that causes compression ignition some time after completion of all fuel injection.