WO1997009520A1

WO1997009520A1 - Engine exhaust system

Info

Publication number: WO1997009520A1
Application number: PCT/AU1996/000549
Authority: WO
Inventors: Julian Mark Stannard
Original assignee: Orbital Engine Company (Australia) Pty. Limited
Priority date: 1995-09-04
Filing date: 1996-09-04
Publication date: 1997-03-13
Also published as: AUPN521495A0

Abstract

An exhaust system for a multi-cylinder two stroke internal combustion engine each cylinder having at least one respective exhaust port, said exhaust system including a respective exhaust passage (5, 6, 7) for at least some of the cylinders, each exhaust passage (5, 6, 7) providing communication between the at least one exhaust port of the cylinder with an exhaust duct (12) for delivering exhaust gas from the respective cylinders to the exhaust duct, wherein said exhaust passages (5, 6, 7) are configured for facilitating cross-charging between the respective exhaust ports and arranged relative to the exhaust duct (12) so that there are reduced uneven momentum transfer effects between the exhaust gases flowing through said exhaust passages.

Description

ENGINE EXHAUST SYSTEM This invention relates to the exhaust system of a multi-cylinder engine wherein at least an individual exhaust duct is provided for each cylinder.

It is known that the performance of an engine can be enhanced by establishing pressure pulses in the exhaust system of the engine that assist in the trapping of the fuel and air charge in the respective engine cylinders prior to closing of an exhaust port provided for each cylinder. In particular, the generation of a high pressure pulse, or "plugging pulse" as it is commonly referred to, at the exhaust port prior to closing thereof is desirable to maximise the mass of air, or fuel and air, trapped in the combustion chamber upon closure of the exhaust port. An exhaust system designed to provide such plugging pulses for enhancing the performance of an engine is typically known as a "tuned" exhaust system. Typically, such exhaust systems are designed to provide enhanced engine performance within a certain range of engine operating speeds such that, when operating within the certain range, the exhaust system is said to be "tuning" the engine.

One way of achieving such engine exhaust tuning, particularly in two- stroke cycle engine exhaust systems, is by employing the high pressure pulse generated in the exhaust system by the opening of the exhaust port of one cylinder to "plug" the exhaust port of another cylinder that is about to close. This method of plugging or exhaust tuning is known as and will be hereinafter referred to as "cross-charging". It is understood that in general, this pressure pulse propagates within the exhaust system at the local speed of sound.

In order to achieve this plugging of one exhaust port in response to the opening of another exhaust port, particular dimensional relationships and orientations are required between the individual exhaust passages connecting the respective exhaust ports to the main exhaust passage or passages or tail pipe. In particular, the length of the individual exhaust passages determines the particular range of engine operating speeds for which the exhaust system is tuning the engine. British Patent No. 1357509 in the name of Textron Inc. shows such a tuned exhaust system for a two stroke engine. The exhaust system shows three tubes respectively extending from each engine cylinder. The tubes are substantially equal in length and are joined to an exhaust tube of a specific length to facilitate the said tuning effect.

British Patent No. 1329167 in the name of Outboard Marine Corporation also discloses a tuned exhaust system for a two stroke engine. It has however been observed that exhaust systems designed for this purpose in accordance with known practices, can result in undesirable influences on the engine operation even though the required plugging function is effectively achieved. In particular, it has been found that one or some cylinders of the engine can be more prone to the occurrence of "knock" than other cylinders of the same engine, or may be subject to more severe knock, even when all cylinders of the engine are operating under the same basic conditions. The problem of knock in engines having tuned exhaust system has not been addressed in known systems.

The applicant has observed in their three cylinder engine that when such a "tuned" exhaust passage system is used, under some conditions one of the cylinders of the engine experiences a higher degree of knock than the other cylinders and therefore limits the maximum performance of the engine. Problems of knock may be accentuated, or even created, by the addition of increasing volumes of catalyst and restrictive mufflers downstream of the junction of the exhaust passages associated with the cylinder exhaust ports. Knock may be adequately controlled by retarding ignition, however this results in a corresponding reduction in torque. It is the applicant's belief that in addition to the plugging pulse used to provide a certain engine tuning state, there is also a mass flow of exhaust gas, which has momentum, between the exhaust passages associated with cylinder exhaust ports. It is the applicant's belief that the selective occurrence of knock in one cylinder of a multi cylinder engine is due to the increased pressure and back flow experienced at the exhaust port prior to the closing thereof. This increased pressure and back flow experienced by a particular engine cylinder is thought to be caused by the momentum of the exhaust gas being transferred between respective exhaust passages whilst cross-charging takes place. That is, it is believed that the cross-charging relationship between certain engine cylinders can lead to a greater exhaust gas momentum transfer between exhaust passages than the cross-charging relationship between other cylinders (ie. uneven exhaust gas momentum effects). In particular, where the possible flow path between cylinders which have a certain cross-charging relationship is fairly uninhibited, it is thought that such geometries will lend themselves to the generation of greater pressure at the exhaust port of the cylinder being plugged prior to exhaust port closure than would other cylinder relationships, such as those having a flow path therebetween which incorporates a substantial corner or junction.

As knock is potentially physically damaging to the engine and can compromise the operation of the engine, and as the typical remedy therefor is to reduce the engine power output, it is highly desirable to prevent the occurrence of knock without detracting from the engine performance.

It is therefore the object of the present invention to provide an internal combustion engine having an exhaust passage system which provides for cross-charging of the engine cylinder exhaust ports while avoiding or minimising the above noted problem of the selective occurrence of knock.

With this object in view, there is provided an exhaust system for a multi- cylinder two-stroke internal combustion engine wherein each cylinder has at least one respective exhaust port, said exhaust system comprising a respective exhaust passage for at least some of the cylinders, each exhaust passage providing communication between the exhaust port or at least one of the exhaust ports of the cylinder with an exhaust duct for delivering exhaust gas from the respective cylinders to the exhaust duct, wherein said exhaust passages are configured for facilitating cross-charging between the respective exhaust ports and arranged relative to the exhaust duct so that there are reduced uneven momentum transfer effects between the exhaust gases flowing through said exhaust passages. Each passage preferably delivers exhaust gases into the exhaust duct in at least substantially the same direction of flow.

Preferably, the exhaust passages are configured so that there is little to no momentum transfer effects between the exhaust gases in said exhaust passages. It has been found that the bifurcation angle between the exhaust passages as they become the exhaust duct should be minimised to obtain maximum reduction in knock effects in the cylinders of the engine. It is understood that the pressure in the cylinder at exhaust port closure is the result of acoustic pressure wave action and gas momentum in the manifold. By directing the gas towards the exhaust duct, rather than towards the other exhaust passages, the momentum component of the cross-charging effect is minimised. As such, a maximum bifurcation angle may be appropriate for a given exhaust system. This may vary depending on a number of factors, such as downstream restriction in the exhaust system and total mass flow through the exhaust. To this end, the bifurcation angle between immediately adjacent exhaust passages may be minimised. The bifurcation angle may preferably be less than 65 degrees. More preferably, the bifurcation angle may be less than 45 degrees.

It is however also envisaged that the exhaust passages of the exhaust system could be arranged such that a uniform level of transfer of exhaust gas momentum could occur between related exhaust passages. For example, the exhaust passages would be positioned such that the exhaust gas flow through each passage is directed towards the outlets of the other exhaust passages so that equal exhaust gas momentum transfer effects occur between each related pair of exhaust passages. In this arrangement, each cylinder would experience the same level of knock under such conditions. Any compromise of engine performance to control knock will therefore not unduly effect any one cylinder.

Said exhaust passage may be provided for all of the cylinders of the engine. Each exhaust passage may provide communication between all of the exhaust ports of the cylinder where the cylinder has a plurality of exhaust ports. The arranging of the exhaust passages so that there is little to no momentum transfer of exhaust gases therebetween, and so that preferably each passage delivers the exhaust gas therefrom into the exhaust duct in the same general direction, preferably a direction generally corresponding to the direction of the axis of the exhaust duct, reduces or eliminates the tendency for the exhaust gas from one exhaust passage to pressurise another exhaust passage whilst the exhaust port which communicates therewith is still open.

In particular, there is little or no tendency for exhaust gas momentum transfer effects to increase the pressure and temperature conditions seen at the exhaust port of the cylinder being plugged. It has been found that this assists in reducing or eliminating uneven levels of knock in the cylinders.

Conveniently, each of the exhaust passages have the same length, measured along the centre line of the passage, from the point of communication of the passage with the engine cylinder to the point of discharge of exhaust gas into the exhaust duct. The lengths of the individual exhaust passages are determined in a known manner to provide a required tuned state at certain engine operating conditions. It is however also envisaged that the exhaust passages may differ in length to promote different exhaust tuning conditions or for easier manufacturing or packaging.

The individual exhaust passages are preferably relatively short to provide for exhaust tuning of the engine via cross-charging at higher engine operating speeds. Typically, such higher operating speeds are within the range from 3500 rpm to 5500 rpm.

The exhaust system may have the same number of exhaust passages as the number of cylinders of the engine. In particular, the exhaust system for a three cylinder engine may have three exhaust passages. The gas flow path through each of the passages may be at least substantially equal in length. This facilitates tuning of the engine.

It has been found that the above described arrangement of the exhaust passages does not adversely affect the pressure pulses generated in the exhaust system to achieve the desired plugging effect of each exhaust port and reduces or avoids exhaust gas momentum being transferred from the exhaust gas within an exhaust passage of one cylinder to the exhaust gas within an exhaust passage of another cylinder. Accordingly, uneven gas flow momentum effects between cylinders which have a certain cross-charging relationship are reduced or eliminated. The arrangement has hence been found to result in providing the required plugging effect for each cylinder of the engine without the substantial differential in the tendency of some or one cylinder to be more prone to knock than other cylinders.

The invention will be more readily understood from the following description of one practical arrangement of the engine exhaust system as illustrated in the accompanying drawings. In the drawings,

Figure 1 is a timing diagram showing the plugging sequence of a three cylinder two-stroke engine;

Figure 2 is a schematic view of a prior art exhaust system for use with a three cylinder two-stroke engine;

Figure 3 is a perspective view of the internal volume of the exhaust system according to the present invention for use with a three cylinder engine; Figure 4 is a view of the internal volume of the exhaust system of Figure 3 from the cylinder block side thereof; and

Figure 5 is a cross-sectional view of the exhaust system of Figure 3.

The exhaust system according to the present invention will be described with respect to its use on a three cylinder two-stroke cycle engine. The cylinders have a designated firing order and thereby have a corresponding order for the exhaust plugging pulses to each respective cylinder. For this particular engine configuration having three cylinders 1 ,2,3, the cross-charging relationship between the cylinders of the engine is such that cylinder 1 plugs cylinder 3, cylinder 2 plugs cylinder 1 , and cylinder 3 plugs cylinder 2 and the cylinder firing order is 1 , 2, 3. This is shown in Figure 1 where, for example, the opening of the exhaust port of cylinder 3 (E03) provides a plugging pulse for the closing of the exhaust port of cylinder 2 (EC2) and so on.

Figure 2 is a schematic view showing the exhaust gas flow paths within a prior art exhaust system used on a three cylinder two-stroke cycle engine. From the exhaust port of each cylinder 1 , 2, 3 respectively extend exhaust passages

1a, 2a, 3a for transferring exhaust gases from each cylinder to an exhaust duct

4.

In the sequence of the exhaust plugging pulses referred to above, cylinder 1 plugs cylinder 3. Because of the arrangement of exhaust passage 1a and 3a, the momentum of the exhaust gas flowing in exhaust passage 1 a will pressurise the gas in exhaust passage 3a at a time at which it is still open (see

A). This can increase the in-cylinder temperatures and pressures and can result in a higher possibility of knock occurring within the cylinder 3. It is to be noted that similarly, the exhaust gas flowing from exhaust passage 3a will pressurise the gas in exhaust passage 1 a, however, this will not occur whilst the exhaust port of cylinder 1 is open and hence there will be little to no effect on the in- cylinder temperature and pressure conditions in cylinder 1.

By comparison, because exhaust passage 3a is at an acute angle to exhaust passage 2a, when the exhaust pressure pulse from cylinder 3 is being used to plug the exhaust port of cylinder 2 (see B), exhaust gas momentum transfer effects are less significant. Referring now to Figures 3 and 4, the internal volume of the three exhaust passages 5, 6 and 7 are aligned with respective exhaust ports of each engine cylinder of a three-cylinder engine (not shown). The three exhaust passages 5, 6 and 7 merge with the internal volume of the exhaust tailpipe 12 in substantially the same plane as indicated at 14 in Figures 1 and 2. The respective exhaust passages 5, 6 and 7 are substantially equal in length when measured along a centre line of the exhaust passage volumes, and are contoured to smoothly blend into the tailpipe volume 12 with each of the exhaust passages 5, 6 and 7 extending at the point of junction in the general direction of the axis of the tailpipe 12. Figure 5 is a cross-sectional view of the intemal volume adjacent the point of junction of the exhaust passages 5, 6 and 7 with the exhaust tailpipe 12. The cross-sectional areas of each of the exhaust passages 5, 6 and 7 at the point of junction are substantially of the same magnitude.

The above arrangement of the exhaust system ensures that the exhaust gases flow out through the exhaust passages with minimal or no uneven exhaust gas momentum transfer effects between related exhaust passages, and that the exhaust gases flow out from the exhaust passages in the general direction of the axis of the tailpipe 12. This minimises the uneven exhaust gas momentum effects which may arise between related exhaust passages as discussed with respect to Figure 2. This is particularly evident from a consideration of Figure 4 where it is seen that a fairly acute angle exists adjacent the point 15 at which the exhaust passage volumes 5 and 7 feed into the tailpipe volume 12. This is similarly evident in Figure 3 from a consideration of the acute angle adjacent the point 16 at which the exhaust passage volumes 5 and 6 feed into the tailpipe volume 12. Hence, it is apparent that a transfer of exhaust gas momentum from exhaust passage volume 5 into exhaust passage volume 7 is unlikely due to the acute angle therebetween at the point 15 at which they feed into the tailpipe volume 12. This is true for any combination of exhaust passage volumes 5, 6 and 7.

The above arrangement minimises the momentum transfer effects between the exhaust gases in the exhaust passages. It is however also envisaged that the exhaust pipes be arranged such that a uniform level of transfer of exhaust gas momentum could occur between related exhaust passages. As each cylinder would experience substantially the same pressure and back flow at their respective exhaust ports, each cylinder would therefore experience knock at the same level where conditions are such that such knock would occur. Any compromise of the engine performance to control knock will not unduly effect any one cylinder.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. An exhaust system for a multi-cylinder two stroke intemal combustion engine each cylinder having at least one respective exhaust port, said exhaust system including a respective exhaust passage for at least some of the cylinders, each exhaust passage providing communication between the respective exhaust port of the cylinder with an exhaust duct for delivering exhaust gas from the respective cylinders to the exhaust duct, wherein said exhaust passages are configured for facilitating cross-charging between the respective exhaust ports and arranged relative to the exhaust duct so that there are reduced uneven momentum transfer effects between the exhaust gases flowing through said exhaust passages.

2. An exhaust system according to claim 1 wherein the exhaust passages deliver exhaust gases from the associated cylinders to the exhaust duct in at least substantially the same direction of flow.

3. An exhaust system according to claim 2 wherein said direction of flow generally corresponds to the direction of the axis of the exhaust duct.

4. An exhaust system according to claim 1 or 2 wherein the exhaust passages are configured so that there is little to no momentum transfer effects between the exhaust gases flowing through the exhaust passages.

5. An exhaust system according to claim 1 wherein the exhaust passages are configured so that a uniform level of momentum transfer effects occurs between the exhaust gases flowing through the exhaust passages.

6. An exhaust system according to any one of the preceding claims where iin a said exhaust passage is provided for each said cylinder.

An exhaust system according to any one of the preceding claims wherein the cylinder includes a plurality of ports, each exhaust passage providing communication between all of the exhaust ports of the cylinder.

8. An exhaust system according to any one of the preceding claims wherein the exhaust passages are at least substantially the same length measured along a centre line of the exhaust passage from a point of communication of the exhaust passage with the engine cylinder to a point of discharge of exhaust gases into the exhaust duct.

9. An exhaust system according to claim 8 wherein the length of each of the exhaust passages are such as to provide a required tuned state at predetermined engine operating conditions.

10. An exhaust system according to any one of claims 1 to 7 wherein the length of each said exhaust passage respectively measured along a centre line of the exhaust passage from a point of communication of the exhaust passage with the engine cylinder to a point of discharge of the exhaust gases into the exhaust duct are different.

11. An exhaust system according to any one of claims 8 to 10 wherein the length of each said exhaust passage provides for exhaust tuning of the engine via cross-charging at relatively high engine operating speeds.

12. An exhaust system according to claim 11 wherein the exhaust tuning is provided at engine speeds within the range from 3500 rpm to 5500 rpm.

13. An exhaust system according to any one of the preceding claims wherein the engine includes three cylinders, or a multiple thereof.

14. An exhaust system according to any one of the preceding claims wherein the bifurcation angle between immediately adjacent exhaust passages is minimised.

15. An exhaust system according to claim 14 wherein the bifurcation angle is less than about 65 degrees.

16. An exhaust system according to claim 14 wherein the bifurcation angle is less than about 45 degrees.