TECHNICAL FIELD

This invention relates to improvements in internal combustion engines. In particular, this invention relates to exhaust systems for internal combustion engines and to internal combustion engines incorporating such exhaust systems, as well as to a method of operating such exhaust systems.

BACKGROUND OF _. THE INVENTION AND PRIOR ART

A conventional internal combustion engine repeats a cycle of actions in either two or four movements of a piston. The cylinder in a two-stroke engine, for example, has a transfer port and an exhaust port, each of which is opened and closed by reciprocation of the piston. The cylinder in a four-stroke engine, for example, has, instead of the transfer port and the exhaust port, an inlet valve and an outlet valve, each of which is also opened and closed by reciprocation of the piston.

For the sake of simplicity, that stroke of the piston, which results in the exhaust products being scavenged from the cylinder, will hereinafter be referred to as the exhaust stroke. Further, those means through which the fuel mixture enters the cylinder, being the transfer port of a two-stroke engine or the inlet valve of a four-stroke engine, for example, will hereinafter be

referred to as the transfer port.

Furthermore, those means through which the exhaust products leave the cylinder and enter the exhaust system, being the exhaust port of a two-stroke engine or the outlet valve of a four-stroke engine, for example, will hereinafter be referred to as the exhaust port. It should be noted that the exhaust port may further include one or more header pipes.

After the fuel mixture has entered the cylinder through the transfer port and has been compressed and ignited, the exhaust products are scavenged from the cylinder during the exhaust stroke, through the exhaust port.

It is known that the tuning or adjustment of the exhaust system of an engine can affect engine performance. It is possible to tune an exhaust system with regard to the opening of the exhaust port to enhance engine performance.

For example, Australian Patent No. 584087 describes the tuning of an exhaust system of a two-stroke engine in order to establish a low pressure at the exhaust port, when the exhaust port is open and before the transfer port is closed, and a higher pressure at the exhaust port, after the transfer port is closed and before the exhaust port is closed.

The purpose of establishing the low pressure at the exhaust port before the transfer port is closed is to

enhance scavenging of the exhaust products from the cylinder. The purpose of establishing the higher pressure at the exhaust port after the transfer port is closed is to minimise loss of fresh fuel mixture from the cylinder. Tuning in this manner can produce a

"super-charging" effect, giving both greater engine power and improved fuel economy.

In Australian Patent No. 584087, the tuning of the exhaust port is said to be achieved by controlling the temperature of the gas in the exhaust system.

In tuning the exhaust system of a four-stroke engine, a "super-charging" effect may be achieved during valve overlap time when scavenging of the exhaust products from the cylinder is enhanced by negative pressure created at the exhaust port.

It has now been found that excellent tuning of the exhaust system of an engine may be achieved by providing an exhaust chamber which is variable in effective length during use.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improved exhaust system suitable for certain internal combustion engines.

Accordingly, the present invention provides an improved exhaust system for an internal combustion engine, the

exhaust system having an exhaust port, an exit port and an exhaust chamber communicating with said exhaust port and said exit port, characterised in that variable dimension means in said exhaust chamber are adapted to vary an effective dimension of said exhaust chamber during operation of said engine.

The exhaust system of the present invention is particularly suitable for use with two-stroke and four-stroke internal combustion engines. It is further believed that the exhaust system of the invention may also be beneficial in connection with rotary engines.

The effective dimension of the exhaust chamber which is variable in the system of the invention is preferably the length of the chamber between the exhaust port and the exit port. This is achieved, in a particularly preferred embodiment, by providing an inner member located inside the chamber at the exit port end thereof.

Conveniently, in the case of two-stroke engines, the exhaust chamber consists of a cylindrical centre portion with opposite tapered end portions, one tapered end portion leading into the exhaust port and the other tapered portion leading to the exit port.

In this embodiment, the tapered end portions of the exhaust chamber in the system are shaped so that when the exhaust port is opened and a wave is formed in the exhaust gases to travel at high speed along the exhaust

chamber, the chamber causes the exhaust gases to expand, thus creating a negative wave to travel back to the exhaust port while it is still open. (This negative wave provides low pressure at the exhaust port to enhance scavenging of exhaust products from the cylinder. ) At the same time, the original wave, at reduced strength, continues to travel through the exhaust chamber, which is shaped so as to cause the original wave to rebound, forming a positive wave which reaches the exhaust port to provide high pressure, encouraging the return to the cylinder of fresh fuel mixture which may have escaped from the cylinder via the exhaust port, and coordinating with closing thereof.

In another preferred embodiment of the present invention (which is particularly suitable for use in connection with a four-stroke engine), the exhaust chamber has a cylindrical centre portion and opposite end portions, one being tapered, the exit port being located in the tapered end portion, and one or more header pipes leading into the exhaust port being located at the end position opposite the tapered end portion.

The inner member located near the exit port may echo the shape of the tapered portion leading to the exit port or the shape of the header pipe (in the case of four-stroke engines, for example). The shape of the inner member can be important; however, the shape of the tapered

portion and the relationship between the tapered portion and the inner member need simply be suitable to ensure that the inner member maintains its alignment in the exhaust chamber.

The inner member may assume various configurations, including conical and pyramidal, as discussed in greater detail in connection with the drawings, below.

In another embodiment, the variable dimension means include an inner member located at the exhaust port end of the chamber. In this embodiment, the inner member may be similar in format to the inner member described above, located at the exit port end of the chamber.

In yet a further embodiment, there may be an inner member located at each end of the exhaust chamber.

The variable dimension means for varying the effective dimension of the exhaust chamber may operate to effectively shorten the chamber as engine speed increases, and to allow the chamber its maximum effective dimension as engine speed decreases. For example, when the variable dimension means comprise one or two inner members as referred to above, the or each inner member may be caused to move towards the exhaust port by a servo-mechanism or other suitable means in < response to increased engine speed, thus reducing the effective longitudinal dimension of the exhaust chamber. When engine speed decreases, the or each inner member

may be caused to move away from the exhaust port towards the exit port, thus increasing the effective longitudinal dimension of the exhaust chamber.

This invention also provides an internal combustion engine having a cylinder, a piston movable therein, a transfer port, an exhaust port, an exit port, an exhaust chamber communicating with said exhaust port and said exit port, the transfer port and the exhaust port being arranged so as to be opened and closed in accordance with movement of the piston in the cylinder, such that the transfer port is opened during at least part of the time the exhaust port is opened and a pressure differential is established at the exhaust port, characterised in that variable dimension means in said exhaust chamber are adapted to vary an effective dimension of said exhaust chamber during operation of said engine.

Where the engine is a two-stroke engine, the variable dimension means are preferably adapted to vary the effective dimension of the exhaust chamber so as to establish a negative pressure differential at the exhaust port when the exhaust port is opened and before the transfer port is closed, and so as to establish a positive pressure differential at the exhaust port after the transfer port is closed and before the exhaust port is closed.

SUBSTITUTE SHEET

Where the engine is a four-stroke engine, the variable dimension means are preferably adapted to vary the effective dimension of the exhaust chamber so as to establish a negative pressure differential at the exhaust port during the exhaust stroke.

The present invention also provides a method of operating the exhaust system of the invention the method comprising causing said variable dimension means in said exhaust chamber to reduce said effective dimension in response to a predetermined first engine speed range or to maximise said effective dimension in response to a predetermined second engine speed range, so as to create a pressure differential at the exhaust port.

BRIEF DESCRIPTION OF THE DRAWING

By way of example only, some preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein:

Fig. 1 is an elevation of an exhaust chamber suitable for a two-stoke engine;

Fig. 2 is a sectional view of portion of an embodiment of an exhaust system according to the invention;

Fig. 3 is a sectional view of portion of a second

embodiment of an exhaust system of the present invention;

Fig. 4 is a sectional view of a third embodiment of an exhaust system of the invention;

Fig. 5 is a sectional view of a fourth embodiment;

Fig. 6 is a sectional view of a fifth embodiment;

Fig. 7A is a sectional view of a seventh embodiment, with Fig. 7B representing an exploded perspective view;

Fig. 8A is a sectional view of an eighth embodiment, with Fig. 8B representing a partially exploded perspective view.

DETAILED DESCRIPTION OF THE DRAWING

The exhaust system of Fig. 1 comprises exhaust chamber 10 having cylindrical centre portion 11, tapered end portion 12 and tapered end portion 13. Tapered portion

12 communicates with exhaust port 14, while tapered portion 13 communicates with exit port 15.

For convenience, in the remaining drawings, like numbers will be used to identify those parts which correspond with those of Fig. 1.

Referring now to Fig. 2, located within tapered portion

13 of exhaust chamber 10 is inner member 16, provided in a shape of a cone, which closely echoes the shape of

tapered portion 13 and which includes projection 17 into cylindrical portion 11. Inner member 16 is supported within tapered portion 13 by bearing 18. Inner member 16 is caused to move by a suitable servo-mechanism (not shown) in the direction of arrow 23 in response to engine speed.

In Fig. 3, the exhaust system is similar to that in Fig. 2, except that inner member 16 is supported by bearing 19.

The exhaust systems in both Figs. 2 and 3 are suitable for use in exhausts with straight exhaust pipes.

Fig. 4 illustrates an exhaust system suitable for use with a curved exhaust pipe. In addition, inner member 16 in Fig. 4 does not include projection 17 of Fig. 2.

Fig. 5 shows an embodiment suitable for use with tapers.

Fig. 6 illustrates the embodiment having inner member 16 at each end of exhaust chamber 10. Each member 16 may move towards the other in response to engine speed. Alternately, each member 16 may move independently of the other, to permit different effects within chamber 10 and to increase the variations in the effective length 22 of chamber 10.

The exhaust system in Figs 7A and 7B is an embodinient of the invention which is particularly suitable in connection with four-stroke engines. The exhaust system

of Figs 7A and 7B has exhaust chamber 10 having cylindrical centre portion 11 and tapered portion 13.

Located within tapered portion 13 of exhaust chamber 10 is inner member 16, provided in a shape of a pyramid (absent its base) and having projection 17. Inner member 16 closely echoes the shape of tapered portion 13. Header pipes 20 protrude into cylindrical portion 11 at one end 21 thereof and communicate with exhaust port 14 (not shown) at the other end.

Inner member 16 is supported within tapered portion 13 by any suitable means. Inner member 16 is caused to move by a suitable servo-mechanism (not shown) in the direction of arrow 23 in response to engine speed. Thus, when engine revolutions are high, inner member 16 will be located within tapered portion 13 as shown in Fig. 7A. As revolutions decrease, inner member 16 will move towards the position shown in dotted outline in Fig. 7A.

As shown in Fig. 8A and Fig. 8B, located within tapered portion 13 of exhaust chamber 10 are four inner members 16 being joined together at base 25. Each inner member 16 is slidably received within header pipe 20. As may be seen in Fig. 8A, wall 24 of inner member 16 can fit within wall 26 of header pipe 20, and is caused to move therewhithin by a suitable servo-mechanism in the same manner as in the case of the previous embodiment in response to engine speed.

Gap 27 is provided between group of inner members 16 and tapered portion 13 so that each inner member 16 can fit within the opposite header pipe 20.

The exhaust systems illustrated in Figs. 2 to 8 are capable of increasing engine power (the "supercharging effect") without adversely affecting scavenging.

It is to be understood that the embodiments described in connection with the drawings are for the purpose of illustrating the invention and are not to be interpreted as limiting in the scope of the invention. Other variations will be apparent to one skilled in the art and are within the scope of this invention.