Combustion chamber structure of in-cylinder direct fuel injection engine

1. Field of the Invention

The present invention relates to a structure of a combustion chamber for an in-cylinder fuel injection engine in which fuel is injected directly into the cylinder.

2. Background art

Generally, since the stratified charge combustion strategy provides a readily ignitable rich air-fuel mixture in the vicinity of the spark plug with the overall air-fuel ratio retained in the lean condition, the engine employing the stratified charge combustion strategy has an excellent fuel economy performance.

An engine 1 shown in FIG. 8 is an example of an in-cylinder fuel injection two cycle engine intended to realize the stratified charge combustion according to the prior art. The engine 1 comprises a cylinder block 3 having a cylinder 2 , a piston 4 reciprocating in the cylinder 2 and a cylinder head 5 . A combustion chamber is formed by the cylinder 1 , the top surface of the piston 4 and the cylinder head 5 .

A cavity 4 a is formed on the top surface of the piston 4 , a fuel injector 6 is disposed approximately coaxially with the center line CL of the cylinder bore, and a spark plug 7 is disposed slightly diagonally in the vicinity of the fuel injector 6 .

There are provided a booster port 2 a , a scavenging port 2 b and an exhaust port 2 c on the wall surface of the cylinder 2 and these ports are opened and closed respectively by the reciprocating motion of the piston 4 . The booster port 2 a and the scavenging port 2 b are ports for introducing a fresh air including lubrication oil into the cylinder 2 . The exhaust port 2 c is a passage for discharging exhaust gas therethrough and it is provided opposite to the booster port 2 a.

In this type of the combustion chamber, the spark plug 7 ignites the rear end of the fuel spray in a first stage diffusion. The fuel spray in the first stage diffusion is a fuel at a stage of diffusing and not reaching the top surface of the rising piston 4 .

As described before, according to this prior art, the fuel injector 6 is close to the spark plug 7 , so that the rear end of the sprayed fuel is ignited by the spark plug 7 before the sprayed fuel collides with the top surface of the rising piston 4 . This disposition of the spark plug 7 adjacent to the fuel injector 6 incurs a temperature-rise of the fuel injector 6 itself, this causing the malfunction of the fuel injector 6 , an increase of carbon deposits and the like.

Further, the ignition of the fuel spray in the first stage diffusion tends to provide an inadequate atomization in the sprayed fuel and this inadequate atomization causes a wetting in the spark plug 7 , this often leading to a spark plug fouling. Further, supposing a case of the ignition of the fuel spray in a second stage diffusion, that is, a state of the fuel spray after a collision against the piston 4 , since the flow of scavenging acts as carrying the fuel spray away from the spark plug 7 , this strategy of combustion makes it difficult to realize the stratified charge combustion.

It is an object of the present invention to provide a combustion chamber structure of an in-cylinder direct fuel injection engine capable of realizing the stratified charge combustion and it is a further object of the present invention to provide an internal combustion engine free from serious malfunctions like misfires.

FIG. 1 is a side cross sectional view of an in-cylinder direct fuel injection 2-cycle engine according to a first embodiment of the present invention;

FIG. 2 is a partially top view of the engine shown in FIG. 1;

FIG. 3 a is an explanatory view showing a flow of scavenging air when scavenging ports are opened;

FIG. 3 b is an explanatory view showing a state of a fuel injected from a fuel injector toward a piston cavity when a piston comes near the top dead center;

FIG. 3 c is an explanatory view showing a state of a fuel going upward while being mixed with the fresh air;

FIG. 3 d is an explanatory view showing a state of a fuel stratified around a spark plug;

FIG. 4 is a partially top view of an in-cylinder direct fuel injection 2-cycle engine according to a second embodiment of the present invention;

FIG. 5 is a bottom view of a cylinder head of the engine according to the second embodiment;

FIG. 6 is a partially top view of an in-cylinder direct fuel injection 2-cycle engine according to a third embodiment of the present invention;

FIG. 7 is a side cross sectional view of an in-cylinder direct fuel injection 2-cycle engine according to a fourth embodiment of the present invention; and

FIG. 8 is a side cross sectional view of an in-cylinder direct fuel injection 2-cycle engine according to a prior art.

Referring now to FIG. 1, reference numeral 10 denotes an in-cylinder direct fuel injection 2-cycle engine introducing a reverse scavenging flow method. The engine 10 comprises a cylinder block 12 having a cylinder 11 , a piston 13 reciprocating in the cylinder 11 and a cylinder head 14 which forms a combustion chamber together with the cylinder 11 and the top of the piston 13 .

There are provided a booster port 11 a , a plurality of scavenging ports 11 b and an exhaust port 11 c , which open and close by the reciprocating motion of the piston 13 , in the wall of the cylinder 11 . Further, the exhaust port 11 c and the booster port 11 a are disposed in diametrically opposed locations of the cylinder. The exhaust port 11 c is located slightly higher than the booster port 11 a. Further, as shown in FIG. 2, the scavenging ports 11 b are arranged in a bilaterally symmetrical manner with respect to an axis L 1 connecting the booster port 11 a with the exhaust port 11 c. In this case, the booster port 11 a is one of scavenging ports and acts as promoting an inverse tumble flow (see FIG. 3 a ) of the fresh air in the cylinder 11 .

Further, the booster port 11 a is located at approximately the same level as the scavenging ports 11 b and these ports 11 a , 11 b communicate with a crankcase (not shown) formed in the lower part of the cylinder block 12 . Also, an intake port (not shown) communicates with the crankcase to introduce the fresh air including lubrication oil into the crankcase. The fresh air introduced into the crankcase is fed to the combustion chamber 22 through the booster port 11 a and scavenging ports 11 b.

As illustrated in FIG. 1, the top of the piston 13 is shaped convexly and further a concave piston cavity 13 a is formed with a spherical wall surface 13 b on a part of that convex surface. The piston cavity 13 a is shaped round when viewed from top. The center of the piston cavity 13 a is located approximately on the axis L 1 when viewed from top and is slightly offset on the exhaust port 11 c side with respect to a cylinder bore center line CL.

The cylinder head 14 is mounted on the cylinder block 12 and secured thereto by a plurality of bolts (not shown).

On the back surface of the cylinder head 14 , a squish area 14 a is formed along the top surface of the piston 13 and a combustion chamber dome 14 b having a spherical surface is provided at a position opposite to the piston cavity 13 a.

The center of the dome 14 b approximately coincides with the center axis of the piston cavity 13 a , when viewed from top. Further, the diameter of the edge circle of the dome 14 b is designed so as to be slightly larger than that of the edge circle of the piston cavity 13 a.

Further, a spark plug bore 17 is provided in the cylinder head 14 such that an electrode 16 of a spark plug 15 is positioned on the cylinder bore center CL. Further, a fuel injector bore 19 for mounting a fuel injector 18 is provided at the outer periphery edge of the dome 14 b on the exhaust port side 11 c so as to obtain a maximal distance between the spark plug 15 and the fuel injector 18 in the dome 14 b . That is, an injection nozzle of the fuel injector 18 is positioned approximately at a point of intersection of the outer periphery edge of the dome 14 b and the axis L 1 connecting the center of the booster port 11 a with the center of the exhaust port 11 c.

Further, the fuel injector bore 19 is bored such that the fuel is injected toward the booster port 11 a. On an actual fuel injection, since the fuel is injected at the latter period of the compression stroke, the fuel is sprayed along the spherical wall surface 13 b of the piston cavity 13 a.

The spark plug 15 is threaded into the spark plug bore 17 and the fuel injector 18 is mounted onto the fuel injector bore 19 . The fuel injector 18 is controlled by a control section (not shown) so as to inject fuel into the combustion chamber based on the fuel injection amount and the fuel injection time which are optimally determined according to engine loads and engine speeds. Further, a plurality of coolant passages 20 are provided in the cylinder head 14 so as to circulate therethrough for the purpose of cooling the cylinder wall 11 and the combustion chamber.

In thus constituted engine 10 , the fresh air is compressed by the squish area 14 a , the dome 14 b , the cylinder 11 and the piston cavity 13 a of the rising piston 13 and then the fuel is injected towards the compressed fresh air. The sprayed fuel collides against the wall surface 13 b of the piston cavity 13 a , it goes upward along the wall surface 13 b and forms a locally combustible mixture in the combustion chamber. When the spark plug 15 ignites thus formed combustible mixture, the combustion expands to overall mixture layers, the stratified charge combustion being accomplished.

In operation of thus constituted engine 10 , when the piston 13 goes down from the top dead center toward the bottom dead center, first the top of the descending piston 13 opens the exhaust port 11 a to discharge exhaust gas. Next, the piston 13 opens the booster port 11 a and the scavenging port 11 b which are located slightly below the exhaust port 11 c . At this moment, the fresh air compressed within the crankcase is sent into the cylinder 11 through the booster port 11 a and the scavenging port 11 b to fill the cylinder 11 while it squeezes the residual exhaust gas out of the cylinder 11 through the exhaust port 11 c.

This scavenging operation continues due to the inertia force of the incoming fresh air until the booster port 11 a and the scavenging port 11 b are closed by the top surface of the rising piston 13 . When the piston 13 closes the exhaust port 11 c , a reverse tumble flow of the fresh air is generated in the cylinder 11 (FIG. 3 a ).

Further, when the piston 13 continues to rise toward the top dead center after the respective ports are closed, the fuel is optimized in the fuel injection amount and the fuel injection time and is injected toward the piston cavity 13 a of the rising piston 13 (FIG. 3 b ). The sprayed fuel collides with the piston cavity 13 a , travels upward along the surface configuration of the piston cavity 13 a while it is mixed with the reverse tumble flow of the fresh air (FIG. 3 c ), and reaches immediately beneath the electrode 16 of the spark plug 15 to form a locally rich (stratified) and combustible mixture in the vicinity of the spark plug 15 (FIG. 3 d ). At this moment, the skirt of the piston 13 starts to open the intake port to introduce the fresh air.

The piston 13 further continues to rise and comes close to the top dead center. At an optimum timing before the piston 13 reaches the top dead center, the mixture is ignited by the electrode 16 of the spark plug 15 . At this moment, since the rich, combustible mixture is formed around the electrode 16 , the ignitablity is enhanced, thereby a good stratified charge combustion can be secured.

Further, when the combustible mixture is ignited, since the electrode 16 is positioned approximately on the cylinder bore center line CL, the combustion speed of the combustible mixture can be increased and as a result the combustion time can be reduced. Further, since the spark plug 15 is located as far from the fuel injector 18 as possible in the dome 14 b , the fuel injector 18 can be less affected by heat from the spark plug 15 .

When thus ignited mixture is burned in the combustion chamber 22 , the high pressure gas generated by the combustion pushes the piston 13 downward, thereby the aforesaid cycle is repeated. The movement of the piston 13 is transmitted to a crank shaft (not shown) from which the rotational force is produced.

When the engine 10 is operated in a high load condition, the combustion strategy changes from the stratified charge combustion to the homogeneous charge combustion. Under this condition, since the fuel injector 18 is directed to the booster port 11 a and the scavenging ports 11 b , it is possible to inject fuel toward the upstream of the incoming fresh air and as a result it is possible to make more time for diffusion, evaporation and atomization of the injected fuel.

(Second Embodiment)

Next, an engine according to a second embodiment of the present invention will be described by reference to FIGS. 4 and 5.

According to the first embodiment, as described before, since the fuel injector 18 is arranged above the exhaust port 11 c and extends in the same direction as the exhaust port 11 c , the fuel injector 18 is easily influenced by heat.

To solve this problem, the second embodiment contemplates to reduce the influence of heat on the fuel injector 18 by arranging the fuel injector 18 as far apart from the exhaust port 11 c as possible.

Referring to FIG. 4, the piston 13 ′ has a piston cavity 13 a ′ in an offset position on the exhaust port 11 c side with respect to the cylinder bore center CL and on the upper side with respect to the axis L 1 . Assuming an axis L 2 connecting the cylinder bore center CL with a center of the piston cavity 13 a ′, an angle θ contained by the axes L 1 and L 2 is within a range of 0<θ<90 degrees. The angle θ is more preferably 30 degrees or thereabout, in consideration of a degree of freedom in designing a cylinder head 14 ′. The piston cavity 13 a ′ has the same configuration as the piston cavity 13 a of the first embodiment.

FIG. 5 shows an undersurface of the cylinder head 14 ′ which is mounted on the cylinder block 12 . In the drawing, there is provided a dome 14 b ′ in a squish area 14 a ′ of the cylinder head 14 ′. The dome 14 b ′ is positioned opposite to the piston cavity 13 a ′ and its center is located on the axis L 2 meeting the axis L 1 at an angle θ. Further, an injector bore 19 is provided approximately at a point of intersection of the axis L 2 and the outer periphery edge of the dome 14 b ′ such that the fuel spray from the fuel injector 18 threaded into the injector bore 19 is directed to the wall surface 13 b ′ of the piston cavity 13 a ′.

Further, a spark plug bore 17 is provided in the dome 14 b ′ in the vicinity of the cylinder bore center axis CL such that the electrode 16 of the spark plug 15 threaded into the spark plug bore 17 is positioned approximately on the cylinder bore center axis CL. In the drawing, reference numeral 20 denotes coolant passages and numeral 21 denotes bolt through holes for the tightening bolts 21 of the cylinder head 14 ′.

In the combustion chamber thus constituted, in the same manner as in the first embodiment, the reverse tumble flow is generated, the sprayed fuel travels upwards along the spherical wall surface 13 b ′ toward the cylinder bore center axis CL and the rich, combustible mixture is formed in the vicinity of the spark plug 15 . The mixture is ignited by the spark plug 15 to make the stratified charge combustion.

On the other hand, since the centers of the piston cavity 13 a ′ and the dome 14 b ′ are offset from the axis L 1 respectively, the fuel injector 18 can be mounted as apart from the exhaust port lic as possible. This can protect the fuel injector 18 from being heated by the exhaust port 11 c.

Next, a third embodiment of the present invention will be described by reference to FIG. 6 .

Generally, a plurality of scavenging ports 11 b are formed on the wall surface of the cylinder block 12 in a bilaterally symmetrical manner with respect to the axis L 1 . When the actual scavenging flow is visualized, it is appreciated that the scavenging flow sometimes suddenly starts to rotate in a certain direction and sometimes suddenly starts to rotate in a reverse direction. Therefore, the symmetrical structure of the scavenging ports does not always provide a stable scavenging flow around the spark plug 15 .

The third embodiment contemplates to form a stable scavenging flow in the vicinity of the spark plug 15 .

Referring to FIG. 6, a plurality of scavenging ports 11 b are disposed on the upper side of the axis L 1 and a plurality of scavenging ports 11 d are disposed on the lower side of the axis L 1 . The scavenging ports 11 b are in an unsymmetrical relationship with the scavenging ports 11 d. That is, the opening area of the scavenging ports 11 d is larger (or smaller) than that of the scavenging ports 11 b and the discharge direction of the scavenging ports 11 d are different from that of the scavenging ports 11 b.

Accordingly, the scavenging flow introduced into the cylinder 11 from thus unsymmetrically disposed scavenging ports, has a definite directivity and, as a result, when the fuel injector 18 injects fuel toward the scavenging flow, a steady combustible mixture can be formed securedly in the vicinity of the spark plug 15 .

Next, a fourth embodiment of the present invention will be described by reference to FIG. 7 .

In the first embodiment, the piston 13 has the piston cavity 13 a shaped into a spherical surface 13 b . On the other hand, in the fourth embodiment, the piston cavity 13 a ′ is shaped partially into a spherical surface and partially into a steep cliff. That is, the piston cavity 13 a ′ according to the fourth embodiment is shaped into a spherical surface from the outer periphery edge to the central part of the piston 13 and a steep cliff 13 b ′ at the central part of the piston 13 . The steep cliff 13 b ′ has a more definite directivity toward the electrode 16 of the spark plug 15 than the spherical surface 13 b of the first embodiment. That is, since the fuel spray is concentratedly guided by the steep cliff 13 b ′ toward the electrode 16 , a rich mixture can be more easily formed around the electrode 16 . In other words, the magnitude of the stratification of mixture can be intensified.

While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.