GB2294973A

GB2294973A - Two-stroke engine scavenging

Info

Publication number: GB2294973A
Application number: GB9521432A
Authority: GB
Inventors: Masaya Kaneko; Katsuhiko Shirai
Original assignee: Daihatsu Motor Co Ltd
Current assignee: Daihatsu Motor Co Ltd
Priority date: 1994-11-02
Filing date: 1995-10-19
Publication date: 1996-05-15
Anticipated expiration: 2015-10-19
Also published as: IT1278781B1; GB9521432D0; DE19540110A1; ITFI950223A1; ITFI950223A0; FR2726326A1; GB2294973B; FR2726326B1

Description

1 SCAVENGING DEVICE FOR A TWO-STROKE ENGINE 2294973 The present invention

relates to a scavenging device for a two-stroke engine, and more particularly to a scavenging device adapted to a uniflow scavenging method in a multicylinder two-stroke engine.

As known well with a two-stroke engine, there is a uniflow scavenging as one of scavenging methods. The structure of a two-stroke engine in the uniflow scavenging method is fundamentally constructed as illustrated in Fig. 5. (Refer to Published Unexamined Utility Model Application in Japan No. 115529/19B8). As can be seen in a two-stroke diesel engine in Fig. 5, a plurality of scavenging ports 24 are provided at the middle height position of a cylinder liner 23. Fresh air pressurized by a scavenging pump 38 is induced into a cylinder 21 through the scavenging ports 24 as shown by arrows. Subsequently, combustion gas is expelled to an exhaust port 33 by an upstroke of a piston 37 through an exhaust valve 32 which is being opened.

In the uniflow scavenging method, the gas flows in one direction in the cylinder 21 as described above. A multicylinder two-stroke engine designed for such uniflow 2 scavenging has water-cooling jackets 35 at the upper portion of a cylinder block 34, surrounding each of the cylinders 21. A scavenge room 22 is constructed thereunder surrounding each cylinder 21 as illustrated in Fig. 6. Fresh air is induced into the cylinders 21 from the scavenge room 22 through a plurality of the scavenging ports 24 formed around cylinder liners 23. Combustion gas is then expelled through an exhaust port 33 disposed in a cylinder head 36. Each scavenging port 24 is formed with an inclination in order for fresh air to flow in a swirling motion in the cylinders 21.

The scavenging efficiency will be lowered if fresh air flows backwardly in the scavenge room 22 during a successive scavenging procedure of each cylinder 21. In order to prevent such backward flow, it has been proposed to devise a suitable position of fresh air inducing ports or to suitably arrange protrusions in the scavenge room 22.

The Published Unexamined Utility Model Application in Japan No. 115529/1988 shown in Fig. 6 discloses an arrangement of fresh air inducing ports 25 and 26 at both sides of the scavenge room 22 diagonally opposed to each other, and protrusions 27 and 28 are adequately disposed to form a pair of scavenging passages 29 and 30, so that 3 fresh air induced from each port 25, 26 flows in one direction. Partition walls 31 are further provided between the cylinder liners 23, 23 so as to keep fresh air from being stirred, and to prevent backward flow during a successive scavenging procedure of each cylinder 21.

In the structure shown in Fig. 6, the outline of the scavenge room is formed rectangula rly, which causes to form hollow spaces between the cylinders. In such a configuration fresh air tends to flow straight along the inner surface of the scavenge room and not along the circumference of the cylinders. Accordingly, there is difference in amount of supplied fresh air flowing around the scavenge room, the hollow spaces receiving relatively less amount of fresh air. As a result, fresh air is flown in unequally through each scavenging port 24, forming unstable swirls in each cylinder, which incurrs the mixture of combustion gas and fresh air to limit the scavenging efficiency.

Furthermore, since fresh air inducing ports 25, 26 are opened opposed to the cylinder liners 23 disposed at both ends of the scavenge room 22, relatively larger amount of fresh air flows in through the scavenging ports 24 which are positioned opposite to the ports 25, 26. The flow of 4 fresh air thus becomes uneven in the cylinders to lower the scavenging efficiency. There is also a difficulty in disposing the fresh air inducing ports 25, 26 on both sides of the cylinder block in view of its structure.

A primary object of the present invention is to provide a scavenging device in a two-stroke engine which is arranged to let fresh air flow into each of the cylinders evenly through each scavenging port surrounding thereabout in order to improve scavenging efficiency with simpler configuration of a fresh air inducing port.

The present invention is characterized by having a scavenge room surrounding a plurality of cylinders, a plurality of scavenging ports being formed around the cylinder liners, through which fresh air is induced into each cylinder. Protrusions are arranged on both sides of the inner surface of the s.cavenge room between the cylinders, so that a scavenging passage is formed along the outer circumferemce of the cylinders, having equal spaces between the inner surface of the scavenge room and the cylinder liners. A fresh air inducing port is disposed tangentially to the outer circumference of a cylinder at one end. Partition walls may preferably be provided between the cylinders.

An auxiliary fresh air inducing port can be also employed at the other end of the scavenge room parallel to the main fresh air inducing port. A reed valve can be further supplied at the inlet of the auxiliary fresh air inducing port.

In the present invention, the scavenge passage extends along the circumference of the cylinders, uniformly spaced between the inner surface of the scavenge room and the cylinder liners, which leads fresh air to flow peripherally and evenly in the scavenge room. The current is further led to flow in one direction by a fresh air inducing port which is disposed tangentially to the outer circumference of a cylinder at one end. Such one-directional unstirred current supplies each cylinder with fresh air equally through each scavenging port even during a successive scavenging procedure of each cylinder. Preferable swirling currents are then formed in each of the cylinders, which permits highly efficient scavenging with the least mixture of combustion exhaust gas and fresh air. It also permits a simple configuration, as it is unnecessary to provide fresh air inducing ports on opposite sides of the scavenge room.

To prevent a part of the fresh air from passing through bypass passages formed between the cylinders, partition 6 walls can be employed therebetween so as to achieve a complete circumferential current of fresh air along the inner surface of the scavenge room to further improve the above-described efficiency.

If a fresh air inducing port is provided only at one end of the scavenge room, it may cause loss of pressure by air resistance at downstream side of the scavenging passage, increasing work load of a scavenging pump. The scavenging pressure can be decreased by providing an auxiliary fresh air inducing port at the other end of the scavenge room parallel to the main air inducing port.

If the auxiliary fresh air inducing port is further provided with a reed valve, it opens and closes the air inducing inlet according to the amount of work load so that it prevents the influx of fresh air into the scavenging passage from the auxiliary fresh air inducing port when loss of pressure is low at a low-speed operation. Turbulence of air can thus be prevented to improve a scavenging efficiency by forming an optimum swirl with a decrease in the scavenging pressure. The reed valve can also rectify the direction of fresh air induced from the auxiliary fresh air inducing port to flow along the current in the scavenge room, which permits the flexible design for the disposition or the 7 size of the auxiliary fresh air inducing port.

The invention will be further described by way of example only, with reference to the accompanying drawings, wherein:

Fig. I is a transverse section top view showing a construction of a scavenge room in a scavenging device for a two-stroke engine in a first embodiment of the present invention. Fig. 2 is a transverse section top view showing a construction of a scavenge room in a scavenging device for a two-stroke engine in a second embodiment of the present invention. Fig. 3 is a transverse section top view showing a construction of a scavenge room in a scavenging device for a two-stroke engine in a third embodiment of the present invention. Fig. 4 is a transverse section top view showing a construction of a scavenge room in a scavenging device for a two-stroke engine in a fourth embodiment of the present invention. Fig. 5 is a vertical section side view showing a construction of a conventional twostroke engine. Fig. 6 is a transverse section top view showing a construction of a conventional scavenge room in a 8 scavenging device for a two-stroke engine.

A first embodiment of the present invention will be described hereinafter referring to Fig. 1. Reference numeral 10 in Fig. 1 represents a cylinder block of multicylinder two-stroke engine in the uniflow scavenging method. Three cylinders 1 are disposed in the cylinder block, each upper part of which is surrounded by watercooling jackets (refer to Fig. 5), and a scavenge room 2 is formed thereunder. A plurality of scavenging ports 4 are formed around each of the cylinder liners 3 at an adequate interval to open the scavenge room 2 into the interior of the cylinders 1. Fresh air is induced into the cylinders 1 through the plurality of the scavenging ports 4 from the scavenge room 2. The scavenging ports 4 are formed with a tangential inclination in order for fresh air to form a swirl in the cylinders 1.

Protrusions 5 are arranged on both sides of the inner surface 15 of the scavenge room 2 between the cylinders 1, 1. Partition walls 6 are provided between the cylinders 1, 1, having an inclination along the current which flows in through the scavenging ports 4. A scavenging passage 7 is formed along the outer circumference of each cylinder 1, equally spaced between the inner surface 15 of the scavenge room 2 and the 9 cylinder liners 3. A cross-sectional area of a scavenging passage 7 is therefore uniformly made, and the passage 7 extends peripherally around the cylinder liners 3 in the scavenge room 2.

A fresh air inducing port 8 is arranged in a manner to open tangentially to the outer circumference of a cylinder 1 which is disposed at one end of the scavenge room 2. The fresh air inducing port 8 is connected to a scavenging pump for feeding fresh air, and a cylinder head provided with an exhaust valve and a fuel injection valve is connected on the upper surface of the cylinder block 10 (refer to Fig. 5).

Description will now be made on a function of the device of the present invention. Under a state that fresh air is pressurized and fed from a scavenge pump through the fresh air inducing port 8 into the scavenging passage 7 in the scavenge room 2, pistons successively stroke downwardly in each of the cylinders 1 and the scavenging ports 4 are opened. At the same time, an exhaust valve provided in the cylinder head is also opened so that combustion gas in the cylinders 1 is expelled through an exhaust port, and fresh air flows into the cylinders 1 through the scavenging ports 4.

The configuration of the scavenging passage 7 which is formed along the outer circumference of each cylinder 1 in equal width contributes to even influx of fresh air supplied into the cylinders 1 through the scavenging ports 4, in each of which optimum swirls are made. The fresh air inducing port 8 opened tangentially to the outer circumference of a cylinder 1 at one end further assists the current to flow in one direction. A scavenging operation can thus be performed under a state of the least mixture of combustion gas and fresh air to achieve a high scavenging efficiency.

A second embodiment of the present invention will be described hereinafter referring to Fig. 2. In the first embodiment described above, the fresh air inducing port 8 is provided only at one end of the scavenge room 2 which may cause loss of pressure by air resistance at the downstream side of the scavenging passage 7, increasing work load of a scavenging pump. This embodiment is devised to solve such a problem.

As for Fig. 2, the corresponding reference numerals shown in Fig. 1 are given to the structural elements which are identical to those of the first embodiment, and thus only the points of difference between the first embodiment will be described. In this embodiment, as shown in Fig.

2, an auxiliary fresh air inducing port 11 is opened tangentially to the outer circumference of a cylinder 1 which is positioned at the other end of the scavenge room 2 parallel to the main fresh air inducing port 8 provided in the first embodiment. A cross-sectional area b of the auxiliary fresh air inducing port 11 is formed smaller than that of the scavenging passage 7 at a connecting portion. Both of the fresh air inducing ports 8 and 11 are joined and connected to a scavenging pump. In this embodiment, loss of pressure by air resistance is compensated by inducing fresh air along the current from the auxiliary fresh air inducing port 11. As a result, a scavenging pressure of the scavenging pump is reduced to decrease a work load of the scavenging pump. If a crosssectional area b of the auxiliary fresh air inducing port 11 is too large, the flow of fresh air therefrom becomes dominant, hindering an even supply of fresh air through the scavenging ports 4 and may deteriorate the scavenging efficiency.

A third embodiment of the present invention will be described hereinafter referring to Fig. 3. In this embodiment, a reed valve 12 is provided at the inlet of the auxiliary fresh air inducing port 11 described in the second embodiment. The reed valve 12 opens when loss of pressure in the scavenging passage 7 becomes large in a 12 high-speed operation to induce fresh air thereinto, decreasing a scavenging pressure. An optimum swirl can thus be formed to improve the scavenging efficiency. on the contrary, the reed valve 12 closes the inlet of the auxiliary fresh air inducing port 11 when loss of pressure is low in a low-speed operation to prevent a turbulence in the flow of fresh air in the scavenging passage 7. The reed valve 12 can also rectify the direction of fresh air induced from the auxiliary fresh air inducing port 11 to flow along the current in the scavenging passage 7, which permits flexible design for the disposition or the size of the auxiliary fresh air inducing port 11.

A fourth embodiment shown in Fig. 4 corresponds substantially to the first embodiment in accordance with Fig. 1, except that it has no partition walls 6 between the cylinders 1, 1. Bypass passages 16 are thus formed between the cylinders 1, 1, a cross-sectional area of which is smaller than that of the scavenging passage 7. Further detailed description will be omitted, giving the same reference numerals of Fig. 1 to the identical structural elements in Fig. 4.

In this embodiment, a portion of fresh air passes through bypass passages 16 which are formed between the cylinders 13 1, 1 when flowing around in the scavenge room 2. However, the amount of such fresh air is not large, and almost all fresh air circles round along the inner surface 15 of the scavenge room 2. Compared with the first embodiment, fresh air may be less evenly fed through the scavenging ports 4 into each cylinder 1, sufficiently permitting an even supply of fresh air. fourth embodiment is superior to the first embodiment the point that it permits a simple configuration.

yet The on 14

Claims

1. A scavenging device for a two-stroke engine comprising: a scavenge room having a plurality of cylinders disposed therein, a plurality of scavenging ports formed around each of cylinder liners, protrusions arranged on both sides of the inner surface between the cylinders in the scavenge room, a scavenging passage formed along the outer circumference of each cylinder, uniformly spaced between the inner surface of the scavenge room and the cylinder liners, and a fresh air inducing port arranged tangentially to the outer circumference of a cylinder disposed at one end of the scavenge room.

2. A scavenging device for a two-stroke engine as claimed in claim 1, wherein partition walls are provided between the cylinders.

3. A scavenging device for a two-stroke engine as claimed in claim 1 or 2, wherein apart from the fresh air inducing port disposed at one end of the scavenge room, an auxiliary fresh air inducing port is further arranged to open tangentially to the outer circumference of a cylinder disposed at the other end of the scavenge room.

4. A scavenging device in a two-stroke engine as claimed in claim 3, wherein the auxiliary fresh air inducing port is provided with a reed valve.

5. A scavenging device for a two-stroke engine, substantially as herein described with reference to,and as shown in, Figure 1, Figure 2, Figure 3, or Figure 4 of the accompanying drawings.