GB2169656A

GB2169656A - Diesel engine piston combustion chamber

Info

Publication number: GB2169656A
Application number: GB08500980A
Authority: GB
Inventors: Ryoichi Ohashi; Hitoshi Inaba
Original assignee: Yanmar Diesel Engine Co Ltd
Current assignee: Yanmar Co Ltd
Priority date: 1985-01-15
Filing date: 1985-01-15
Publication date: 1986-07-16
Also published as: GB2169656B; GB8500980D0

Abstract

Combustion chamber 2 has a plurality of alternately arranged fuel spray collision walls 6 and guide walls 7 including an upstream portion 12 and a downstream portion 11, in the direction of the swirl S, with a boundary 13 therebetween. The radial distance from the centre 8 of the chamber 2 to the boundary 13 is shorter than the radial distance to the other portions 6, 11, 12 and each portion 12 has a greater radius of curvature and circumferential length than each portion 11. The walls 7 may be inclined (25,26, Figs. 11 and 13) and a lip (30, Figs. 15,16 and 19) provided at the outer end of the walls 6. <IMAGE>

Description

SPECIFICATION Diesel engine combustion chamber The present invention relates to the structure of a main combustion chamber formed by a hollow in the top of a piston of a directinjection internal combustion engine.

Conventionally, as shown in Figs. 1 and 2, a main combustion chamber 2 of a directinjection internal combustion engine is, generally, of toroidal shape and is formed by a hollow in the top of a piston 1. As the diameter d of the chamber 2 increases with respect to the inner diameter D of a cylinder fuel sprayed from a fuel injection nozzle 10 has to travel a longer distance I. During low speed, low load driving, if the distance increases from a value Ii to 12, the level of detectable exhaust smell decreases for the same effective compression ration e, a shown in Fig. 3. If the compression ratio is increased, the level of the smell is also improved (i.e.

reduced).

However, if the effective compression ratio is increased (if the capacity of the chamber 2 is decreased) in order to reduce the detectable smell of the exhaust during low speed, low load driving, the maximum power decreases.

Further, if the distance I for the spraying is increased (if the diameter d is increased) for the same purpose, the speed of the jet of fuel which has come from the nozzle decreases.

Moreover, a high compression ratio in combination with a long distance of fuel spray travel causes premature firing during high speed and high load driving and the low force of the fuel jet results in a delay in combustion, so that the maximum power, the colour of the exhaust fumes and the fuel consumption all deteriorate.

A main combustion chamber 2, as shown in Fig. 2A, has already been proposed. A hollow forming the chamber 2 is provided with walls 6 against which the sprayed fuel collides.

These collision walls 6 are disposed symmetrically with respect to the centre 8 of the piston 1.

In this chamber, the fuel jet w flows into the rear of the respective collision walls 6 circumferentially from both sides thereof before the end of the compression process of the engine so that the fuel spray can be combusted sufficiently. However, a swirl (circumferential flow) of suction gas cannot be utilized for mixing the fuel and the air because the swirl of the gas generated in the chamber 2 collides against and is dissapated by inclined walls 6' and 6' at respective sides of the collision walls 6 which project towards the centre 8 of the piston. Further, the fuel does not flow along the inclined walls 6' from the vicinity of the wall 6. Therefore, the flow along the wall 6 is not substantially in the hollow and the film of the fuel does not extend or spread so as to improve the combustion performance.

Japanese patent publications 51-29242, 51-29243 and 51-29244 have disclosed structures in which the fuel spray is not sprayed against corners of the combustion chamber. In these cases, since the radius (r) of curvature of the corner is small (i.e. r/R is in a range from 0 to 0.075: R=radius from the nozzle hole to the collison wall), the fuel spray accumulates after collison. Thus, the speed of vaporization is low, which causes accumulation of dregs of the fuel and deterioration of engine performance.

A Japanese utility model publication 57-168729 and a Japanese patent publication 49-16881 have disclosed structures in which collision surfaces are curved or are designed to reflect the fuel spray. However, in these structures, when a small amount of the fuel is sprayed, the speed of the fuel jet is very small in comparison with that when a large amount of the fuel is sprayed, so that the fuel is hardly reflected at all. Thus the fuel accumulates on the wall which is intended to reflect same, leading, disadvantageously, to uncombusted gas and signficant exhaust smell.

Japanese utility model publication 57-107821, Japanese utility model publication 55-4515, and Japanese utility model publication 57-139631 have disclosed structures in which a small amount of fuel can be perfectly combusted in compressed air during low power driving. However, as a consequence of the shape of the hollow, a swirl or circumferential flow is braked during the compression to such an extent that the sprayed fuel is not actually moved by swirl. Further, according to the utility model publication 57-139631, since the radius of curvature of the wall against which the fuel collides is small, uncombusted fuel is not spread when only a small amount of the fuel is sprayed, so that the gas cannot combust sufficiently.Further, since the area of the opening of the combustion chamber is large with respect to the area of the top surface of the piston, the force of the fuel jet and of the swirl of the fuel in the hollow is disadvantageously reduced.

It is an object of the invention to provide a combustion chamber structure for a direct injection diesel engine in which the detected exhaust smell during low speed, low power driving is reduced whilst the maximum power, the colour of the exhaust, the fuel consumption and other parameters are improved during high speed, high power driving.

With this object in view, the invention provides a diesel engine with direct injection comprising a main combustion chamber having a peripheral wall and a bottom wall formed by a hollow in the top of a piston, the peripheral wall including a plurality of collision walls and guide walls which are disposed alternately to each other in a circumferential direction of the piston, and a fuel injection nozzle having a plurality of nozzle holes disposed in or adjacent to the combustion chamber, the injection nozzle being designed to spray fuel only against the collision walls such that the jet of fuel colliding with these walls flows along the guide walls in one direction to form a swirl, characterised in that each guide wall includes an upstream portion and downstream portion, with respect to the direction of swirl, with a boundary therebetween, in that the radial distance from the centre of the chamber to the boundary is shorter than the radial distance from the centre of the chamber to other portions of the peripheral wall, in that each upstream portion has a longer radius of curvature and a longer circumferential length than each downstream portion so that the inner periphery of the chamber has a pin-wheel-like shape, the upstream portions providing smooth walls along which the fuel flows after collision so as to form a film of the fuel, the downstream portions providing walls along which the fuel flows from the boundaries, and in that the circumferential length of each collision wall is so determined that the fuel sprayed from the nozzle holes collides only with the collision walls.

Other features and advantages of the invention will be more fully explained in the following description of the preferred embodiments of the invention, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of the structure of a combustion chamber of a conventional internal combustion engine of direct injection type; Figure 2 is a schematic plan view of the conventional structure shown in Fig. 1; Figure 2A is a schematic plan view of another conventional structure of combustion chamber; Figure 3 is a graph showing the relationship between the level of detectable exhaust smell, the effective compression ratio and the distance of travel of the fuel spray; Figure 4 is a graph showing the relationship between the effective compression ratio, the maximum power obtained, and the distance of travel of the fuel spray;; Figure 5 is a schematic sectional view of the structure of a combustion chamber of an internal combustion engine of direct injection type according to a first embodiment of the invention; Figures 6 to 9 are schematic plan views of the structure shown in Fig. 5; Figure 10 is a schematic plan view of a structure of a combustion chamber according to a second embodiment of the invention; Figure 11 is a schematic fragmentary enlarged sectional view of a combustion chamber in accordance with a third embodiment of the invention; Figure 12 is a fragmentary plan view of the structure shown in Fig. 11; Figure 13 is a schematic enlarged sectional view corresponding to Fig. 11 and illustrating a fuel spray and a flame; Figure 14 is a fragmentary plan view of a combustion chamber in accordance with a fourth embodiment of the invention;; Figures 15 and 16 are schematic fragmentary sectional views of ledges of respectively different (fifth and sixth) embodiments of the invention; Figure 17 is a graph showing operation characteristics of the embodiment shown in Fig. 14; Figure 18 is a schematic fragmentary plan view of a seventh embodiment of the invention; Figure 19 is a schematic fragmentary sectional view of the embodiment shown in Fig.

18; Figure 20 is a schematic sectional view of an eighth embodiment of the invention; Figure 21 is a schematic sectional view of the embodiment shown in Fig. 20; Figure 22 is a schematic fragmentary enlarged view of part of the embodiment shown in Fig. 21; Figure 23 is a schematic plan view of a ninth embodiment of the invention; and Figure 24 is a schematic sectional view of the embodiment shown in Fig. 23.

Referring to a first embodiment of the invention, as illustrated in Fig. 5, a piston 1 is provided at the top with a hollow which forms a main combustion chamber 2 defined by a peripheral wall 3 and a bottom wall 4. Referring to Figs. 6 and 7, the peripheral wall 3 consists of a plurality of e.g. four collision walls 6 and four guide walls 7 positioned alternately in a circumferential direction of the piston 1. A fuel injection nozzle 10 having four (same as the number of the walls 6) nozzle holes is disposed near the upper limit of the chamber 2. The centre of the injection nozzle 10 coincides with the centre 8 of the chamber 2. As shown in Fig. 6, the injection nozzle 10 is so designed that it sprays the fuel radially only against the respective collision walls 6 so that the colliding fuel flows along the guide walls 7 to form a swirl in the direction of the arrow S.

The distance L1 (Fig. 7) in the radial direction of the piston from the centre 8 of the chamber 2 to the wall 6 is from 0.25 to 0.40 times as long as the inner diameter D of a surrounding cylinder (not shown). If L1/D is smaller than 0.25, the diameter d of the chamber 2 and the distance which the sprayed fuel has to travel become too small so that the fuel cannot combust sufficiently before reaching the walls 6 which results in an increased exhaust smell. If L1/D is larger than 0.40, the thickness of the piston 1 itself, from the inner peripheral wall 3 of the cham ber 2 to the outer periphery of the piston, becomes too small, so that the thermal load becomes too large to actually use the piston.

The circumferential angle of the sprayed fuel from each nozzle hole to the wall 6 is set as 18 to 25 degrees so that the fuel will not be sprayed onto the guide walls 7.

Each guide wall 7 is divided into a downstream portion 11 and an upstream portion 12 in the direction S of the swirl, with a boundary 13 therebetween. Of all wall portions, the boundaries 13 are positioned nearest to the centre 8 of the chamber 2. In other words, the boundaries 13 project towards the centre 8. The distance L2 from each boundary 13 to the centre 8 is from 0.7 to 0.9 times as long as the distance L1 from the wall 6 to the centre 8. The upstream and downstream wall portions 12 and 11 are curvedly hollowed with respect to the circumferential direction of the piston and have radii of curvature r2 and rl respectively. The radius of curvature r2 of the upstream portion 12 is equal to or more than twice as long as the radius of curvature ri of the downstream portion 11 (r2 > =2rl).

The upstream portion 12 may be straight in the plan view in Fig. 6. The circumferential length L3 of the upstream portion 12 is equal to or more than twice as long as the circumferential length L4 of the downstream portion 11 (13~2L4). Thus, the main combustion chamber 2 has a pinwheel-like shape in plan view.

In the embodiment illustrated in Fig. 5, the bottom wall 4 of the chamber is provided with a substantially pyramid-like projection 20.

As shown in Fig. 8, the projection 20 is provided with outwardly extending ridges 21 equal in number to the number of the nozzle holes or walls 6. This projection 20 prevents the sprayed fuel from contacting the bottom wall 4 before colliding with the walls 6, and functions to fill a useless space between the sprayed fuel and the bottom wall 4.

In the chamber 2 of pin-wheel-like shape, the swirl in the direction S (Fig. 6) forms the collided fuel into a film and causes it to flow smoothly. Further, the chamber 2 of the above shape has a small opening area with respect to the area of the top surfacr of the piston 1 which thus prevents reduction of the force of the jet of fuel.

In the chamber 2 having the above distances L1 and L2 in which the relationship L2/L1 is from 0.7 to 0.9, spiral flow of the fuel is produced as indicated by arrows E in Fig. 9. This flow promotes the mixing of the sprayed fuel and the air and brings the fuel above the top surface of the piston 1 to effectively utilize the air above the piston 1.

If L2/L1 is smaller than 0.7, the upstream portions 12 incline too much with respect to the walls 6 and prevent the formation of the ncessary film of the fuel. If L2/L is larger than 0.9, the portions 11 and 1 2 do not effectively incline with respect to the wall 6, so that the power decreases in a similar manner to that shown for 12 in Fig. 4.

Although in the embodiment of Fig. 7, the centres of the radii of curvature (r1) are apart from the centre 8, they may coincide with the centre 8 has shown in Fig. 10.

With a diesel engine having a main combustion chamber according to the invention, during low speed, low power driving, a small amount of fuel can be sufficiently combusted with the compressed air. Also, the effective compression ratio can be an appropriate value and the sprayed fuel is prevented from contacting the bottom of the chamber. During high speed, high power driving, the sprayed fuel collides with the walls and flows smoothly as the film is formed by the swirl.

The structure is also designed to allow the uncombusted fuel to flow above the top of the piston to effectively utilize the air above the piston.

Consequently, the maximum power during high speed, high power driving is increased, and the colour of the exhaust and the fuel consumption are improved.

As mentioned, the present invention can be effectively employed in a diesel engine of a direct injection type.

Other embodiments will be described hereinafter. In the following embodiments, same or similar parts and members bear the same reference numbers as those in Figs. 5 to 10 and those parts and members will not be described in further detail to avoid repetition.

In the embodiment shown in Fig. 11, an upper portion 25 of each guide wall 7 having a height h2 is inclined away from the centre 8 of the chamber 2 at an angle of d02 which is in the range from about 5 degrees to 15 degrees. The lower portion 26 of each guide wall 7 is inclined only slightly at an angle of dO1 which is in the range from about 2 degrees to 5 degrees so as to permit removal of the die for casting the piston.

During low speed, low power driving of this embodiment, a small amount of the fuel sprayed from the nozzle 10 is completely combusted before reaching the walls 6, and the sprayed fuel does not contact the wall 6, so that neither a blue-white smoke nor a detectable exhaust smell are generated.

During high speed, high power driving, the amount of sprayed fuel increases by five to eight times compared to the above, and the jet of fuel F collides with the walls 6 as shown in Fig. 12. The swirl occurring at high speed (50m/sec to 100m/sec) promotes the flow of fuel in the space as well as the fuel in contact with the walls 6, so that the fuel vaporises rapidly and the flame flows out of the chamber 2 along the wall 7 as shown by dotted lines in Figs. 12 and 13 when the piston moves downward.

Thus, with this embodiment including the in dined wall portions 25, the fuel and flame can flow smoothly and effectively from the chamber 2 to above the piston 1 along the inclined wall portions 25, so that the air above the piston 1 can be utilized effectively, thereby increasing the power of the engine. The sprayed fuel F which collides with the walls 6 is divided into fuel which swirls in the chamber 2 and fuel which travels up above the piston 1 along the inclined wall portions 25.

In the embodiment shown in Fig. 14, each collision wall 6 is integrally provided at its upper edge with a ledge 30 having a radial width b corresponding to 1% to 3% of the inner diameter D of the cylinder. These ledges 30 effectively prevent initial and over-rapid flow of the fuel out of the chamber 2 to above the piston 1.

The fuel which collides with the walls 6 flows in accordance with the swirl S through respective sections c including the ledges 30 to sections d without the ledges 30, and then, a part of the uncombusted fuel easily flows from the sections d to the space above the piston 1, so that the air above the piston 1 can be utilized effectively for combustion, and thus a high power can be obtained.

In the illustrated embodiment, each section c including its respective ledge 30 has nearly the same circumferential length as each section d without a ledge 30. The sectional shape of the ledge 30 may be squarish, as shown in Fig. 15, or it may be rounded as shown in Fig. 16.

The ledges 30 can improve the colour of the exhaust and the maximum pressure in the cylinder as shown in Fig. 17 which illustrates characteristics of the structure with and without ledges in connection with the colour of the exhaust, the maximum pressure (Pmax) in the cylinder and the injection timings.

In structures including the ledges 30, when a small amount of the fuel is sprayed during low power driving, it can sufficiently combust without contacting the ledges 30, so that the colour and the smell of the exhaust can be improved in a similar manner to the previously detailed embodiment. During high power driving, a large amount of the sprayed fuel which collides with the walls 6 flows circumferentially with the swirl. In this operation, the ledges 30 initially prevent the fuel flowing out of the chamber 2. Thus, sufficient jet flow is generated, and the fuel can combust sufficiently. Furthermore, even if the injection timing is retarded to control the maximum pressure (Pmax) in the cylinder, the colour of the exhaust can be improved.

In the embodiment shown in Figs. 18 and 19, similarly to the embodiment in Fig. 14, the walls 6 against which the fuel collides are provided at their respective upper ledges with ledges 30. Furthermore, the guide walls 7 are provided with tapered portions 35 which incline away from the centre of the chamber 2 at an angle dO2 so as to facilitate flowing of the fuel out of the upper space above the piston 1.

During high power driving, the ledges 30 prevent the fuel from immediately flowing out from adjacent the walls 6 to the upper space above the piston 1 and encourage the fuel to flow with the swirl S in the chamber 2, so that vaporization of the fuel in the chamber 2 is promoted. The fuel which is vaporized and incompletely combusted in the chamber flows outalong the tapered portions 35 to the space above the piston 1 and is completely combusted.

Therefore, with this structure, even if the injection timing is retarded to reduce the maximum pressure in the chamber, the fuel can be combusted completely and cleanly, so that the colour of the exhaust is improved and high engine performance is obtained.

In the embodiment shown in Figs. 20 and 21, a glow lamp 40 is disposed adjacent to an upstream side (with respect to the direction of the swirl S) of one wall 6 against which the fuel collides.

The glow lamp 40 is inclined, as shown in Fig. 20, at the angle a with respect to the top surface of the piston 1, and is located circumferentially upstream to a centre of one jet of fuel F issuing from the nozzle.

With this structure, flow A (Fig. 22) along the walls 7 in the chamber 2 is changed into a turbulent flow at the downstream position of the glow lamp 40 with respect to the direction of the swirl S as indicated by an arrow X, so that the mixing of the fuel and the air is promoted. This results in improvements in the starting ability of the engine, in its performance during high load driving, and in the colour of the exhaust and the fuel consumption.

Since the chamber 2 of pin-wheel-iike shape according to the invention has a longer distance L1 between the centre 8 and the wall 6 than the conventional structure, the glow lamp 40 can be distant from the nozzle 10. Therefore, it is easy to arrange the glow lamp 40 at the above mentioned position. Furthermore, since the distance between the nozzle 10 and the glow lamp 40 is relatively long, the firingability can be improved, and thus, the starting ability and the combusting performance during low power can be improved.

In another embodiment illustrated in Fig. 23, the centre 8 of the combustion chamber 2 is shifted away from the centre 44 of the piston 1, and the top surface of the piston 1 has a portion 45 which has a narrower radial width than other portions. The fuel injection nozzle 10 is shifted a little towards the narrow portion 45 with respect to the centre 8 of the combustion chamber 2.

In this embodiment, ledges 47 similar to the ledges 30 in Fig. 14 are provided on, e.g., two collision walls 6a which are adjacent to the narrow portion 45. The radius R of the inner periphery of each ledge 47 may be substantially the same as the radius L2 of the boundarys 13 or it may be shorter than that.

With this structure, the following effect can be obtained in addition to the effects obtained by the embodiment illustrayed in Fig. 14. If the ledges 47 were eliminated, a large amount of the fuel would flow out from the chamber 2 above the narrow portion 45 and would attach to a portion 48 (Fig. 24) of a cylinder liner adjacent to the portion 45 without completely combusting. Consequently, a lubricant would be diluted and carbon would accumulate on the liner portion 48 such that the cylinder liner and a piston ring 49 would become worn to a considerable extent. However, in this embodiment, since the ledges 47 effectively prevent the uncombusted fuel flowing out of the chamber to the liner portion 48, the dilution of the lubricant and the accumulation of carbon is prevented, and thus, the durability of the cylinder liner and of the piston ring are improved.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form may be changed in the details of construction and the combination and arrangement of parts without departing from the scope of the invention as hereinafter claimed.

Claims

1. A diesel engine with direct injection comprising a main combustion chamber having a peripheral wall and a bottom wall formed by a hollow in the top of a piston, the peripheral wall including a plurality of collision walls and guide walls which are disposed alternately to each other in a circumferential direction of the piston, and a fuel injection nozzle having a plurality of nozzle holes disposed in or adjacent to the combustion chamber, the injection nozzle being designed to spray fuel only against the collision walls such that the jet of fuel colliding with these wall flows along the guide walls in one direction to form a swirl, characterised in that each guide wall includes an upstream portion and a downstream portion, with respect to the direction of swirl, with a boundary therebetween, in that the radial distance from the centre of the chamber to the boundary is shorter than the radial distance from the centre of the chamber to other portions of the peripheral wall, in that each upstream portion has a longer radius of curvature and a longer circumferential length than each downstream portion so that the inner periphery of the chamber ha a pin-wheel-like shape, the upstream portions providing smooth walls along which the fuel flows after collision so as to form a film of the fuel, the downstream portions providing walls along which the fuel flows from the boundaries, and in that the circumferential length of each collision wall is so determined that the fuel sprayed from the nozzle holes collides only with the collision walls.

2. A diesel engine as claimed in claim 1 wherein the bottom wall of the main combustion chamber is provided with a pyramid-like projection having ridges extending toward the guide walls so that the sprayed fuel from the nozzle holes may not contact the bottom wall before colliding with the collision walls.

3. A diesel engine as claimed in claim 1 or 2 wherein a ledge projecting toward the centre of the combustion chamber is formed at the upper edge of at least one of the collision walls.

4. A diesel engine as claimed in claim 1, 2 or 3 wherein the centre of the main combustion chamber is offset from the centre of the piston such that the top surface of the piston has a portion which is radially narrower than other portions of the top surface, and a ledge projecting toward the centre of the chamber is formed only at the upper end of the collision wall or walls adjacent to the narrow portion of the top surface.

5. A diesel engine as claimed in any preceding claim wherein the guide walls include inclined portions which extend to the top of the piston and are inclined away from the centre of the chamber at an angle in a range from 5 degrees to 15 degrees.

6. A diesel engine as claimed in any preceding claim wherein a glow lamp is arranged in the main combustion chamber adjacent the upstream side of one of the collision walls.

7. A diesel engine with direct injection including a main combustion chamber substantially as hereinbefore described with reference to and as illustrated in Figs. 5 to 9 or Fig. 10, or Figs. 11 to 13, or Fig. 14, or Fig. 15, or Fig. 16, or Figs. 18 and 19, or Figs. 20 to 22, or Figs. 23 and 24 of the accompanying drawings.