Two stroke cycle internal combustion engines

Abstract

A two stroke cycle engine 10 with an exhaust gas treatment catalyst. Separate reducing and oxidizing catalyst portions 126 and 127 are provided to treat different portions of the exhaust gas exhausted during each exhaust period. The portion of the exhaust gas high in NOx, usually that first exhausted upon initial opening of the exhaust port, is directed to be treated by the reducing catalysts 126 and the remainder of the exhaust gas is directed to be treated by the oxidizing catalyst 127.

Description

20 220. The catalyst unit 223 is constructed with internal channels such that gas flow within the catalyst unit cannot occur to a significant extent in a direction parallel to the axis of the engine cylinder 11 i.e. the direction in which the exhaust porto port 15 is exposed by the piston 12. Thus, exhaust gases entering the top 231 of the inboard face 237 of the catalyst unit 223, when the descending piston 12 first exposes the exhaust port 15, cannot exit from the lower portion 232 of the outboard face 238 of the catalyst unit into the manifold 220.

The catalyst unit 23 223 is constructed such that its upper portion as seen in FIG. 2 is loaded primarily with reducing catalyst and its lower portion is loaded primarily with an oxidizing catalyst. This is achieved by having the catalyst unit constructed from a number of sheet elements 241 stacked upon each other, the sheets varying in catalytic activity with the sheet at the top having only a reducing catalyst coating, and those at the bottom having only an oxidising catalyst coating. The sheets toward the centre may having a coating of either or both oxidising and reducing catalyst depending on the nature of the exhaust gas and the required treatment thereof which may vary with different engines.

Each sheet element 241 is cut from catalytic substrate of the generally known physical form shown in FIG. 3, where a flat metallic sheet 81 is bonded to a corrugated metallic sheet 82, leaving channels 83 there between. Due to the curved walls of the exhaust passage 222 adjoining sheet elements are different sizes so as to extend between the opposite walls of the passage at the level the sheet element is located.

The substrate of sheets 81 and 82 is coated with a thin layer of catalytically active material. The upper sheet element 242 has a coating high in rhodium content while the lower sheet elements 243 has a coating high in platinum content. The intermediate sheet elements may have a coating which is a combination of rhodium and platinum, possibly with each sheet element of a different rhodium to platinum ratio, and this ratio may vary gradually between the extremes of the upper and lower elements 242 and 243.

In an alternate arrangement sheet elements 241 with only two different catalyst coatings are required. One group of elements with one catalytic coating of a dominant reducing activity are placed in the upper part of passage 222 and another group of elements with another catalytic coating of a dominant oxidising activity are placed in the lower part. It will be appreciated that this allows significant cost savings over the earlier described embodiment, which uses many different coating chemistries on the sheet elements to provide a gradual transition of catalytic activity, but it will also be appreciated that the abrupt change of catalyst type results in lower overall catalyst performance.

In a further alternative arrangement, the sheet elements 241 are deleted entirely from the lower part of the passage 222 but elements with reducing activity are positioned in the upper part of the passage. Such an arrangement may be supplemented by a separate catalyst with oxidising activity located downstream far enough so that it is contacted by the exhaust gas from other combustion chambers in the engine.

FIG. 4 illustrates another embodiment of the invention. The engine 10 is generally the same as that of FIG. 2, with the only significant difference being the configuration of the exhaust system. The exhaust manifold 120 is divided longitudinally by a web 121 into two manifold cavities 133, 134 which communicate downstream with a common exhaust pipe, and upstream with the upper and the lower exhaust passages 124, 125 respectively.

The passages 124, 125 are separated by a continuation 121a of the web 121 which extends towards the exhaust port 115. The passages 124, 125 contain respective catalyst units 126, 127 through which exhaust gases pass for treatment. The upper catalyst unit 126 has a reducing activity whereas the lower catalyst unit 127 has an oxidising activity.

An exhaust valve 141, which is able to move about the pivot 142, transverse to the exhaust port, is provided to co-operate with the exhaust port 115 in a generally known manner. The exhaust valve 141 does not completely close the port 115 at any stage in the engine's operation, but serves to alter the effective position of the upper edge of the exhaust port thereby by altering the position in the stroke of the piston that the exhaust port commences to open. The valve does not alter its position during each combustion cycle, but is controlled to change in response to engine operating conditions. Such valves and their operation are known to those skilled in the design of modern two stroke engines, and an example is described in Australian patent application No. 57898/86. The valve is shown in FIG. 4 in the position where it is raised to its full extent to give the earliest exhaust port open timing and the maximum open period.

The present exhaust valve 141 as incorporated in the engine has a significant different to those used previously. As in the conventional construction of such a valve, the present exhaust valve 141 has an operating face 143 shaped to present a face to the combustion chamber when in any operational position that is substantially contiguous with the periphery of the exhaust port, with the leading edge 144 adapted to substantially seal against the piston 12. However, the present exhaust valve 141 is different in comparison with those used previously, by the provision in the operating face 143, and close to the leading edge 144, of a single or series of slots 145 passing through the operating face 143 so as to communicate the exhaust port 115 with the upper exhaust passage 124. During operation of the engine, the only way exhaust gases may pass from the combustion chamber into the upper exhaust passage 124 is by way of the slots 145.

In operation of the engine 110, as the piston travels down on its power stroke and commences to open the exhaust port 115, at every position of the exhaust valve 141 the first exhaust gas to escape through the port is by way of the slots 145. This passes into the upper exhaust passage 124 and on through the upper catalyst unit 126. As the piston travels down further it passes the leading edge 144 of the valve and exhaust gas is then able to pass under the valve 141 into the lower exhaust passage 125 and on through the lower catalyst unit 127. At this stage relatively little exhaust passes through slots 145 into upper passages 124. It is also to be understood that even before the lower exhaust passage has been uncovered pressure pulses generated in the upper exhaust passage 124 will cause a reverse flow back through the upper catalyst unit, and this flow pattern may repeat several times during the open period of the exhaust port.

As explained earlier, the first exhaust gases to escape through port 115 are the products of combustion of a rich air/fuel ratio mixture, having a relatively high NOx content, and require a predominantly reduction catalyst treatment. In contrast the exhaust gases expelled later through the port 115 are a lean air/fuel ratio mixture, incorporating fresh scavenging air, and have a relatively low NOx content but a relatively high hydrocarbons content, and require a predominantly oxidation catalyst treatment. The embodiment of FIG. 4 provides a means by which such selective catalyst treatment may be given to successive portions of the exhaust gases in a single engine cycle, while still preserving the port-timing benefits of the exhaust valve.

The catalyst units 126, 127 may be of the general form described for FIG. 2 and 3 wherein sheet elements are stacked face to face with each other. Preferably however, they have a more conventional substrate having a base structure of suitable ceramic or metallic material providing a multitude of passages. The exposed surfaces of the passages in upper catalyst unit 126 are coated with a suitable reduction promoting material, such as rhodium, whereas the exposed surfaces in lower catalyst unit 127 are coated with a suitable oxidation promoting material such as palladium or platinum.

The engine of FIG. 4 may be modified in the following way to achieve beneficial performance in some applications. Rather than venting the upper manifold cavity 133 to the exhaust system by simply uniting its throughput a short distance downstream from the catalyst unit 126 with that from the lower cavity 134, the upper cavity is instead blanked off such that its only opening is by way of catalyst unit 126 to the cylinder. As described earlier, when the first exhaust gases pass through the exhaust port 115 they enter the upper exhaust passage 124 but cannot enter the lower exhaust passage 125. These gases pass through the reduction catalyst unit 126 and into the upper cavity 133 but can go no further. The cavity 133 thus pressurises, and stays so until the piston 12 passes the leading edge 144 of the exhaust valve and exposes the lower exhaust passage 125, whereupon the compressed gases in the upper cavity 133 pass back through the reduction catalyst unit 126, the upper exhaust passage 124 and the slots 145 into the engine cylinder to then pass through the exhaust port into the lower exhaust passage 125' 125 the lower (oxidation) catalyst unit 127 and the lower manifold cavity 134 to the exhaust pipe In this way, the first gases to pass through the exhaust port experience two passes over the reduction catalyst and one pass over the oxidation catalyst, which gives a significantly improved catalyst treatment to that portion of the exhaust which may most require it. The size and configuration of the upper manifold cavity may be adjusted such that its resonance and effort on pressure waves is beneficial to the trapping efficiency of the engine.

In a further modification to the construction as described with reference to FIG. 4, the lower catalyst unit 127 which treats the gas exhausted late in the exhaust open period, being a chemically oxidising gas, may be located in the exhaust system displaced from adjacent to the exhaust port. If desired this lower catalyst unit 127 may be located in the exhaust manifold in a position where it can treat exhaust gases from two or more cylinders of the same engine.

The two stroke cycle engines, as described above with reference to the accompanying drawings, are each provided with a fuel injection system whereby the fuel is injected directly into the engine cylinder. A particularly advantageous fuel injection apparatus and its operation are described in U.S. Pat. No. 4,693,224. It is however to be understood that the present invention is applicable to two stroke cycle engines which do not have direct fuel injection, provided fuel and air are introduced into the combustion chamber in such a way that the fuel distribution in the combustion chamber at combustion and in the scavenging gases introduced after combustion is such that the air/fuel ratio in the exhaust gases passing through the exhaust ports varies significantly during the course of the exhaust port open period. The invention is also applicable to spark ignition and to diesel engines operating on the two stroke cycle.

Claims

1. A method of operating a two stroke cycle internal combustion engine wherein for each combustion chamber there is provided means to supply fuel to the combustion chamber, an exhaust port through which gases pass from the combustion chamber to an exhaust system, and at least one inlet port through which a fresh charge of air enters the combustion chamber, the inlet and exhaust ports being arranged so that the inlet port open prior to the closing of the exhaust port, the method being characterised in that during an exhaust port open period of each combustion chamber a first portion of the gas exhausted from said combustion chamber during the exhaust port open period is directed into contact with a first catalyst means of a first catalytic character, and a subsequent second portion of the gas exhausted from said combustion chamber in the same exhaust port open period is directed into contact with a second catalyst means of a differing catalytic character to the first catalyst means.

2. A method of operating an internal combustion engine as claimed in claim 1 wherein the first catalyst means is of a character to reduce nItrogen oxides in said first portion of gas.

3. A method as claimed in claim 1 or 2 wherein the second catalyst means is of a character to oxidise hydrocarbons or carbon monoxide in said second portion of gas.

4. A method of operating an internal combustion engine as claimed in claim 2 wherein the first portion of the exhaust gas is directed into contact with the first catalyst means after exhaustion from the combustion chamber and before said first portion has mixed with other exhaust gas from the engine.

5. A method as claimed in claim 1 wherein after the first portion of the exhaust gas has been treated by the first catalyst means at least part of said first portion with the second portion of the exhaust gas are directed into contact with the second catalyst means.

6. A method as claimed in claim 5 wherein at least part of the first portion of the exhaust gas after contact with the first catalyst means is subsequently directed back into the combustion chamber and thereafter is directed with the second portion of the exhaust gas into contact with the second catalyst means.

7. A method as claimed in any one of claims 5 or 6 wherein said at least part of the first portion of the exhaust gas after treatment by the first catalyst means is entrained with the second portion of the exhaust gas when directed into contact with the second catalyst means.

8. A two stroke cycle internal combustion engine having for each combustion chamber means to supply fuel to the combustion chamber, an exhaust port through which gases pass from the combustion chamber to an exhaust system, and at least one inlet port through which a fresh charge of air enters the combustion chamber, the inlet and exhaust ports being arranged so that the inlet port opens prior to the closing of the exhaust port, characterised in that there is provided in the exhaust system a first catalyst means of a first catalytic character located to receive a first portion of the gas exhausted from the combustion chamber during an exhaust port open period, and second catalyst means to receive a second portion of the gas exhausted from the combustion chamber subsequent to the commencement in the same exhaust port open period of the exhausting of the first portion, said second catalyst means being of a different catalytic character to the first catalyst means.

9. An internal combustion engine as claimed in claim 8 wherein the first catalyst means is of a reducing character.

10. An engine as claimed in claim 8 wherein the second catalyst means is of an oxidising character.

11. An internal combustion engine as claimed in claim 8, 9, or 10 wherein the second catalyst means is located to only receive exhaust gas exhausted from the combustion chamber after the inlet port has opened and while the exhaust port is open.

12. An internal combustion engine as claimed in any one of claims 8 to 10 wherein the first catalyst means is located in the exhaust port of the combustion chamber with one end thereof substantially contiguous with the periphery of the exhaust port, and said first catalyst means extends from the end of the exhaust port that is first exposed during opening of the exhaust port in the direction in which such exposure progresses.

13. An internal combustion engine as claimed in claim 12 wherein the second catalyst means is located in the exhaust port adjoining the first catalyst means and with one end thereof substantially contiguous with the periphery of the exhaust port.

14. An internal combustion engine as claimed in claim 13 wherein the second catalyst means is located in a passage extending from the exhaust port, the end of the second catalyst means adjacent the exhaust port being spaced downstream therefrom.

15. An internal combustion engine as claimed in claim 12 wherein the first catalyst means is located in a cavity communicating at one end with the exhaust port and closed at the other end, with a chamber formed between said closed end of the cavity and the adjacent end of the first catalyst means.

16. An internal combustion engine as claimed in claim 8, 9 or 10 wherein the first catalyst means is located in a cavity that communicates at one end with the exhaust port, and a valve means is provided at said one end of the cavity, said valve means being operable to varying the timing of the communication between the combustion chamber and the cavity through the exhaust port.

17. An internal combustion engine as claimed in claim 16 wherein said valve means is a member mounted for controlled movement relative to the exhaust port so a portion of said valve means moves in the a direction generally parallel to the axis of the combustion chamber in substantially contiguous relation to the exhaust port, said member having an aperture through said portion to provide said communication with the cavity.

18. A method of operating a two stroke cycle internal combustion engine as claimed in claim 1 wherein the engine is spark ignited.

19. A two stroke cycle internal combustion engine as claimed in claim 8 being a spark ignited engine.

20. A method of operating a two stroke cycle engine as claimed in any claim 1 wherein the fuel is injected directly into the combustion chamber.

21. A two stroke cycle engine as claimed in claim 8 wherein the means to supply fuel is fuel injection means arranged to inject fuel directly into the combustion chamber.

22. A method of operating a two stroke cycle engine as claimed in claim 1 wherein the first portion of the exhaust gases is of a stoichiometric air/fuel ratio or of a reducing character.

23. A two stroke cycle engine as claimed in claim 8 wherein the first portion of the exhaust gases is of a stoichiometric air/fuel ratio or of a reducing character.

24. An internal combustion engine as claimed in claim 17, wherein said aperture is a slot elongated in the direction transverse to the direction of movement of the valve member.

25. An internal combustion engine as claimed in claim 24 wherein the valve member is mounted for pivotal movement to vary the position of the leading edge of said elongated slot in the axial direction of the combustion chamber.

26. An internal combustion engine as claimed in claim 8 or 9 wherein the exhaust system includes first and second exhaust passages communicating with the exhaust port, said first passage communicating with the first catalyst means and the second passage communicating with the second catalyst means, said passages being spaced in the axial direction of the combustion chamber and separated by a wall therebetween extending from adjacent the exhaust port, a portion of said wall at an end close to the exhaust port end being integral with a valve member having an aperture therein, said valve member being arranged so that said aperture provides communication between the exhaust port and the first passage, said valve member and said portion of said wall being mounted at an end of the wall portion remote from the exhaust port for pivotal movement in the direction of the axis of the combustion chamber whereby the valve member and said portion of said wall can be pivotally moved to vary the position of the aperture in the valve member relative to the exhaust port to control the timing of exhaust of gas through the exhaust port to the first passage.

27. An internal combustion engine as claimed in claim 26 wherein the exhaust system is arranged to direct at least part of the first portion of the exhaust gas to pass through the second catalyst means after said part of the first portion of the exhaust gas has passed through the first catalyst means.

28. An internal combustion engine as claimed in claim 8 wherein said first passage is tuned to develop at some engine speeds a pressure wave reflection at the entrance thereof during discharge of exhaust gas through the exhaust port.

29. A two cycle engine of the type having a cylinder with a piston controlled exhaust port having a leading edge, and means for admitting air to said cylinder during an open period of the exhaust port, comprising: means for supplying fuel to the cylinder for mixing with a fresh air charge, means for providing first and second exhaust gas flow paths for gas exhausted through said exhaust port, and means for dividing the gas exhausted from the cylinder to deliver into the first exhaust gas flow path a first portion of the gas exhausted through the exhaust port during an exhaust port open period of each engine cycle and to deliver a second portion of the gas subsequently exhausted through the exhaust port during the same engine cycle into the second exhaust gas flow path, said means for dividing the gas exhausted from the cylinder comprising an arrangement of openings providing fluid communication between said exhaust port and said first and second exhaust gas flow paths respectively with said openings being spaced apart in an axial direction of said cylinder and an opening to the first exhaust gas flow path being positioned to be uncovered by the piston before an opening to the second exhaust gas flow path.

30. A two cycle engine as claimed in claim 29 wherein said first exhaust gas flow path comprises a first exhaust passage including a first catalytic exhaust treatment means for treating exhaust gases delivered to said first exhaust passage.

31. A two cycle engine as claimed in claim 30, wherein said second exhaust flow path comprises a second exhaust passage which bypasses the first catalytic exhaust treatment means and includes a second catalytic exhaust treatment means.

32. An internal combustion engine as claimed in claim 8, wherein said exhaust system comprises a first and second exhaust passage in communication with said exhaust port; said first exhaust passage in communication with said first catalyst means and said second exhaust passage in communication with said second catalyst means; a wall for separating said first and second exhaust passages which are spaced in the axial direction of the combustion chamber, extending from adjacent said exhaust port; a valve means pivotally mounted on an end of said wall, disposed in closest proximity to said combustion chamber, for pivotal movement in the direction of the axis of the combustion chamber so as to vary the position of said aperture relative to said exhaust port so as to control the timing of exhaust of gas through the exhaust port to said first passage.

33. A two cycle engine as claimed in claim 30 wherein said first catalytic exhaust treatment means is a reduction catalyst.

34. A two cycle engine as claimed in claim 29, wherein said means for dividing the exhaust gas includes a wall co-acting with the exhaust port to provide first and second exhaust passages defining said first and second flow paths, respectively.

35. A two cycle engine as claimed in claim 34, further comprising a valve means which is pivotally mounted and co-acts with the exhaust port to adjustably vary the effective position of the port leading edge and thereby control the timing of the beginning of the exhaust of gas through the exhaust port.

36. A two cycle engine as claimed in claim 35, wherein the valve means is pivotally mounted on the wall, and the wall is spaced from the edge of the exhaust port by a predetermined distance in the axial direction of the cylinder.

37. A two cycle engine as claimed in claim 34, wherein said first exhaust passage is tuned to develop a pressure wave reflection at its entrance during discharge through the exhaust port of the exhaust gas to preferentially urge the flow into the second exhaust passage entrance.

38. A two cycle engine as claimed in claim 37, wherein the first exhaust passage entrance is positioned to communicate with the initial discharge of the exhaust gas from the exhaust port to preferentially direct the first exhaust gas flow into the first exhaust passage.

39. A two stroke cycle internal combustion engine having for each combustion chamber means to supply fuel to the combustion chamber, and exhaust port through which gas passes from the combustion chamber to an exhaust system and at least one inlet port through which a charge of air enters the combustion chamber, the inlet and exhaust ports being arranged so that the inlet port opens prior to the exhaust port closing, characterized by said exhaust system comprising:

first and second exhaust passages communicating with the exhaust port;

means arranged to divide the gas exhausted from the combustion chamber so a first portion of the gas exhausted through the exhaust port during an exhaust port open portion of each engine cycle is delivered into said first exhaust passage and a second portion of the gas exhausted through the port during the same exhaust port open portion of each cycle is delivered into said second exhaust passage;

first catalytic exhaust treatment means connected with said first exhaust passage for treating exhaust gases delivered to said first exhaust passage, and

second catalytic exhaust treatment means connected with said second exhaust passage, and means providing fluid communication between an outlet of said first catalytic exhaust treatment means and said second exhaust passage whereby a mixture of gases from said second exhaust passage and from said first catalytic exhaust treatment means can be delivered to said second catalytic exhaust treatment means.

40. An internal combustion engine as claimed in claim 39, wherein said means to divide the exhaust gas includes an open entrance portion between said exhaust port and said first and second exhaust passages with adjacent respective entrances to said passages, said first exhaust passage being tuned to develop a pressure wave reflection at its entrance during discharge through the port of the exhaust gas to preferentially urge the exhaust gas flow into the second exhaust passage entrance.

41. An internal combustion engine as claimed in claim 39, wherein said first catalytic exhaust treatment means is a reducing catalytic means.

42. An internal combustion engine as claimed in claim 39, wherein said means to divide the exhaust gas includes a wall coacting with said exhaust port and having opposite sides respectively bordering said first and second exhaust passages.

43. An internal combustion engine as claimed in claim 39, wherein the first exhaust passage is proportioned to exhibit engine speed resonant characteristics and thereby establish a reversal of the flow of exhaust gas in said first exhaust passage while the exhaust port is open.

44. An internal combustion engine as claimed in claim 39 or 40, wherein an entrance to said first exhaust passage is positioned to communicate with the initial discharge of the first position of the exhaust gas from the exhaust port to preferentially direct the first portion of the exhaust gas into said first exhaust passage.

45. An internal combustion engine as claimed in claims 39 or 40, wherein the exhaust port has a leading edge, and said engine includes valve means arranged to coact with said exhaust port to adjustably vary the effective position of the port leading edge and control the timing of the beginning of the exhaust of gas through the exhaust port.

46. An internal combustion engine as claimed in claims 39 or 40, wherein the exhaust port has a leading edge, said engine including valve means for adjustably varying the effective position of the port leading edge and controlling the timing of the beginning of the exhaust of gas through the exhaust port by co-acting with said exhaust port, and wherein said means to divide the exhaust gas includes a wall, co-acting with said exhaust port, which has opposite sides respectively bordering said first and second exhaust passages and includes valve means integral therewith, said wall being mounted to pivotally move in unison with said valve means, a portion of the wall being spaced from the valve means a predetermined distance adjacent said port to control the timing of the delivery of said first and second portions of the exhaust gas to the respective first and second exhaust passages.

47. An emission control system in combination with a two-stroke cycle engine of the type having a cylinder with a piston controlled exhaust port, means for admitting air during an open period of the exhaust port and means for supplying fuel to the cylinder, said emission control system comprising

first and second exhaust passages connected with the exhaust port,

separating means for delivering into said first passage a first portion of the gas exhausted through the port during an exhaust portion of each cycle and for delivering into said second passage a second portion of the gas exhausted through the port during the exhaust portion of each cycle,

first exhaust treatment means connected with said first passage for treating exhaust gases delivered to said first passage, and

second exhaust treatment means connected with said second passage and with an outlet of said first exhaust treatment means for treating a mixture of gases from said second passage and from said first treatment means,

wherein said first treatment means is effective to cause an exothermic reaction in the first passage gases causing a temperature increase which increases the temperature of the subsequently mixed gases and aids the operation of the second treatment means.

48. A combination as recited in claim 47, wherein said separating means includes an open entrance portion between said port and said passages with adjacent entrances to said passages, said first passage being tuned to develop a positive pressure wave reflection at its entrance during discharge through the port of the exhaust gas portion to preferentially urge the flow into the second passage entrance, the second passage being tuned to delay a positive wave reflection until after that of the first passage.

49. A combination as in claim 48, wherein said first passage entrance is positioned in line with the direction of initial discharge of a first portion of gas exhausted from the port to preferentially direct said first portion of gas into said first passage.

50. A combination as in claim 47, wherein said separating means includes an open entrance portion between said port and said passages with adjacent entrances to said passages and a valve which controls port timing coacting with said entrances to positively direct a first portion of gas exhausted from the port into the first passage and a second portion of gas into the second passage.

51. A combination as recited in claim 47, wherein said separating means includes a wall coacting with said port and having opposite sides respectively bordering said first and second passages.

52. A combination as recited in claim 51 and further comprising a pivotally mounted port timing member coacting with said port to adjustably vary the effective position of a port leading edge and control the timing of the beginning of the delivery of gas to the exhaust passages.

53. A combination as in claim 52, wherein said wall and the timing member are pivotally mounted together and a portion of said wall is spaced apart from said timing member a predetermined distance adjacent said port to control the timing of the period of delivery of exhaust gas to the exhaust passage.

54. A combination as in claim 47, wherein said exhaust treatment means are catalytic converters.

55. A two-stroke cycle engine of the type having a cylinder with a piston controlled exhaust port and means for admitting air during an open period of the exhaust port, said engine comprising

first and second exhaust passages connected with the exhaust port,

separating means for delivering into said fist passage a first portion of the gas exhausted through the port during an exhaust portion of each cycle and for delivering into said second passage a second portion of the gas exhausted through the port during the exhaust portion of each cycle, said separating means including an open entrance portion between said port and said passages with adjacent entrances to said passages,

said first passage being tuned to develop a positive pressure wave reflection at its entrance during discharge through the port of the first exhaust gas portion to preferentially urge the exhaust gas flow into the second passage entrance, the second passage being tuned to delay a positive wave reflection until after that of the first passage.

56. A combination as in claim 55, wherein said first passage entrance is positioned in line with the direction of initial discharge of the first portion of exhaust gas from the port to preferentially direct the first portion into said first passage.

57. A two-stroke cycle engine of the type having a cylinder with a piston controlled exhaust port and means for admitting air during an open period of the exhaust port, said engine comprising

first and second exhaust passages connected with the exhaust port,

separating means for delivering into said fist passage a first portion of the gas exhausted through the port during an exhaust period of each cycle and for delivering into said second passage a second portion of the gas exhausted through the port during the exhaust period of each cycle, said separating means including an open entrance portion between said port and said passages with adjacent entrances to said passages and a valve coacting with said entrances to positively direct the first portion into the first passage and the second portion into the second passage.

58. An internal combustion engine as claimed in claim 8, wherein said exhaust system comprises a first and second exhaust passage; said first and second exhaust passages communicate with said exhaust port; said first exhaust passage in communication with said first catalyst means and said second exhaust passage in communication with said second catalyst means; a first wall disposed between said first and second exhaust passages which are spaced in the axial direction of the combustion chamber, extending from adjacent said exhaust port; a valve mechanism comprising a valve member, a valve wall and an aperture, wherein said aperture provides communication between said exhaust port and said first passage; and said valve mechanism being pivotally mounted on an end of said first wall, said valve mechanism being disposed in closest proximity to said combustion chamber, for pivotal movement in the direction of the axis of the combustion chamber so as to vary the position of said aperture relative to said exhaust port so as to control the timing of exhaust of gas through the exhaust port to said first passage.