JPH01200014A - Combustion chamber structure for direct injection type diesel engine - Google Patents

Abstract

PURPOSE:To expedite the mixing of combustion gas and new air and improve the performance of exhaust emission by setting the capacity of a main cavity at what has a specific rate against the capacity of the whole cavity including the capacity of a sub-cavity. CONSTITUTION:A main cavity 2 supplying injection fuel and a subcavity 3 surrounding the main cavity 2 are respectively formed at the top portion of a piston 1. In this instance, the capacity of the main cavity 2 is so set that it is within the range of 70-80% against the capacity of the whole cavity including the capacity of the sub-cavity 3. As a result, at a combustion stroke, combustion gas from the main cavity 2 is made to mix rapidly with the low temperature air of the sub-cavity 3, and the generation of NOx is restrained. Also, fuel spray is prevented from being attached on cavity well surfaces and the generation of smoke is restrained by setting D/Db which is the ratio of the internal diameter D of the main cavity 2 to a bore diameter Db, at more than a predetermined value.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement in the combustion chamber structure of a direct injection diesel engine for the purpose of reducing harmful emissions.

(Conventional technology and its problems to be solved) In order to improve the efficiency and performance of a diesel engine, it is necessary to create a direct injection combustion chamber structure and then increase its compression ratio or supercharging. It is known.

However, with direct injection, there is little time and space for the fuel spray from the nozzle to mix with fresh air, so combustion tends to deteriorate compared to pre-chamber engines, especially in small engines. Unburned components such as fuel are more likely to be discharged.

As a countermeasure to this problem, a method has been proposed in which the combustion in the combustion chamber cavity provided at the top of the piston promotes the flow of sulfur, as seen in, for example, Japanese Utility Model Application Publications No. 61-80324 and No. 173728. This is also true for small engines installed in automobiles, where the cavity diameter itself is small, and the fuel spray from the nozzle collides with the cavity wall before it becomes sufficiently atomized, so it will take until exhaust emissions are sufficiently improved. This has not yet been achieved.

In addition, higher compression ratios and higher turbocharging raise combustion pressure and temperature, so they are effective to some extent in reducing smoke and HC, but on the other hand, they hinder the generation of NOx, so they still meet exhaust emission requirements. There are limits from this point on.

The present invention was made in view of these conventional problems, and improves the exhaust emission performance of direct injection diesel engines by promoting the flow of combustion gas and mixing with fresh air within the cavity and combustion chamber. The purpose is to improve.

(Means for Solving the Problems) In order to achieve the above object, the present invention includes a main cavity for supplying injected fuel, and a heater 11 surrounding the main cavity.
A cavity is formed at the top of the piston, and the volume of the main cavity is set within a range of 70 to 80% of the total cavity volume including the volume of the sub-cavity.

(Function) If the ratio of the main cavity g volume to the total cavity volume (hereinafter referred to as "main chamber volume ratio") is 100%, that is, if no sub-cavity is provided, combustion will occur within the relatively large main cavity. Therefore, smoke and HC are less likely to be generated, but since combustion is concentrated within the main cavity, NOx generation is increased.

On the other hand, if a sub-cavity is provided around it and the main chamber volume ratio is reduced to 70-80%, the combustion from the main cavity will be near TDC (top dead center), as will be described in detail later. During the combustion process, it rapidly mixes with the relatively low-temperature air in the surrounding 1311 cavity, suppressing the production of No, and improving overall air utilization, so there is no increase in smoke or IIC.

According to the applicant's findings, NOx generation occurs mainly between 'rDC and 30° ATDC with respect to the crank angle, and during this period, especially between 10 and 15° ATDC, the combustion gas in the main cavity is Rapid mixing with fresh air effectively reduces NOx.

In the above configuration of the present invention, when combustion is started by fuel injection into the main cavity, the piston is
It is located near l' D C, and the clearance between it and the shilling head is very narrow, and the main cavity and sub-cavity are essentially defined, so combustion mainly occurs in Proceeds only within the main cavity. After that, when the crank angle reaches about 10" ATDC, the clearance increases as the piston descends, and the main cavity and the E9j cavity rapidly communicate with each other. Therefore, the high-temperature, high-pressure combustion inside the main cavity is slowed down. It flows out into the surrounding area as a strong jet stream and rapidly mixes with the fresh air in the sub-cavity.This causes the above-mentioned 10
The effect of rapid mixing of combustion gas and fresh air at ~15” ATDC is achieved, reducing NOx.

Furthermore, setting the main chamber volume ratio within the above range does not have a negative effect on the output or fuel efficiency, and according to the applicant's experience, there is a noticeable effect on the output or fuel efficiency when the main chamber volume ratio is in the range of 100% to 60%. No differences were observed.

(Example) Figures A and B show an example of a piston to which the present invention is applied. In the figure, 1 is a piston, 2 is a circular main cavity formed in the medium heat section of the top surface, and 3 is an annular sub-cavity formed around the piston. The main cavity 2 has a right cylindrical shape with a flat bottom, and the 1M+1 cavity 3 has a substantially U-shape with a rounded bottom.

D in FIG. 1B is the inner diameter of the main cavity 2, and Db is the bore diameter. The value of D/Db affects the smoke density, and this point will be discussed later. Needless to say, the fuel injection unit 7:X (not shown) is provided facing the main cavity 2 to form a direct injection type combustion chamber.

Pistons with such a structure were applied to small pistons with bore x stroke: 85 x 86 (lIlm) and compression ratio: 18 for multiple pistons with different main chamber volume ratios (however, D/Db = 50%). , smoke and NOx when operating at constant air excess rate and rotational speed of 1200 rpm
Figure 2 shows the generation characteristics of . As shown in the figure, when the main chamber volume ratio is 80 to 70%, the smoke concentration hardly increases, and the amount of NOx wandering decreases.

In this way, NOx can be reduced by appropriately setting the main chamber volume ratio, but in addition to this, it is also necessary to set the ratio of the bore diameter to the main chamber cavity inner diameter, that is, D/Db, to a certain degree or larger. This prevents the fuel spray from adhering to the cavity wall surface, thereby further suppressing the generation of smoke.

Figure 3 shows D/
It shows the results of measuring smoke concentration for multiple pistons with different DI+ under the same engine and operating conditions as in Figure 2, and as shown, the smoke concentration was D/Db<50.
It decreases sharply in the 60% range and becomes minimum in the 60% and above range. Therefore, if it is necessary to significantly reduce not only NOx but also smoke concentration, the main chamber volume ratio should be increased to 70 to 80.
% and D/Db should be set to about 60% or more. Note that, since the tendency of HC emissions is similar to that of smoke, the reduction in smoke described above also leads to a reduction in the amount of HC emissions.

Figures A and B on page 54 show other embodiments of the present invention.

When the fuel injection valve has a multi-hole nozzle that injects fuel in four directions, the main chamber cavity 2 is formed in a shape that expands in four directions from the center of the piston in accordance with the spray F in the four directions. However, in the embodiment in which the annular sub-cavity 3 is provided around the main cavity 2, the main cavity 2 has a combustion chamber shape suitable for fuel spray, as described above.
During combustion, there is less unused air in the main cavity 2, resulting in so-called rich combustion with a small excess air ratio, which lowers the combustion temperature, making NO even more effective.
x can be reduced.

(Effects of the Invention) According to the above, according to the present invention, the sub-cavity is arranged around the main cavity having a predetermined volume ratio, and when NOx is most likely to be generated during the combustion process, the sub-cavity is disposed from the main cavity to the surrounding 1711 cavities. This creates a flow of combustion gas and rapidly mixes it with fresh air, increasing the air utilization rate and suppressing smoke and IC emissions, while lowering the combustion temperature and reducing the amount of NOx produced. be able to. Therefore, by applying the present invention, it is possible to promote higher compression ratio and higher supercharging of a direct injection diesel engine, thereby improving its performance and efficiency.

[Brief explanation of the drawing]

t51 Figures A and B are a plan view and a vertical cross-sectional view of a piston showing an embodiment of the present invention, respectively, Figure 2 is a diagram showing the relationship between the main chamber volume ratio and smoke/NOx emissions, and Figure 3 is a diagram showing the relationship between the ratio of the main chamber diameter to the bore diameter (D/Db) and the amount of smoke discharged. Figures A and B are a plan view and a longitudinal sectional view of a piston showing other embodiments of the present invention, respectively. 1... Piston, 2... Main cavity, 3
...Sub-cavity. Patent applicant Nissan Motor Co., Ltd. Fig. 1 A 3- Sub cavity No. 1 Fig. 8 Re・← 1 Datoha A amount → Shio○ H d

Claims (1)

[Claims] 1. A main cavity for supplying the injected fuel and a sub-cavity surrounding the main cavity are formed at the top of the piston, and the volume of the main cavity is in the range of 70 to 80% of the total cavity volume including the sub-cavity volume. The combustion chamber structure of a direct injection diesel engine is characterized by being set within the combustion chamber.