WO1995021323A1

WO1995021323A1 - Two-stroke engine cooling system

Info

Publication number: WO1995021323A1
Application number: PCT/AU1995/000044
Authority: WO
Inventors: Kenneth Phillip Seeber; Robert Walter Frew; Mario Charles Muscat; Stephen John Karay
Original assignee: Orbital Engine Company (Australia) Pty. Limited
Priority date: 1994-02-01
Filing date: 1995-02-01
Publication date: 1995-08-10
Also published as: AUPM364894A0

Abstract

A two-stroke cycle internal combustion engine having at least one cylinder (9), an exhaust port in the peripheral wall of said cylinder (9), and a coolant passage about at least a portion of the peripheral wall of the cylinder (9). The coolant passage is arranged to commence below the level of the exhaust port and divides into two first branches (3, 4), one passing to either side of the exhaust port and communicating again at a location above the level of the exhaust port. At least one of the first branches (3, 4) is configured to extend partly about the peripheral wall of the cylinder (9) in a direction away from the exhaust port and then extend back about the peripheral wall in the opposite direction at a level above the exhaust port. Where gas inlet ports are provided in the cylinder wall, one on each side of the exhaust port, each respective first branch of the coolant passage (3, 4) is arranged to extend about the respective inlet port on the side thereof remote from the exhaust port. Alternatively, the respective first branches (3, 4) can pass between the exhaust port and the inlet port on that side of the exhaust port.

Description

TWO-STROKE ENGINE COOLING SYSTEM

This invention relates to a cooling system for a two stroke cycle engine and particularly to a cooling system for multi-cylinder two stroke cycle engines. As the exhaust port of each cylinder of a two stroke cycle engine is normally provided in the wall of the cylinder, there is commonly a substantial heat transfer into the wall of the cylinder adjacent the exhaust port. This disposition of the exhaust port results in a substantial temperature difference between opposing sides of the cylinder wall and gives rise to thermal stress and potential distortion of the cylinder bores and of the engine block. Also, as the piston reverses its direction of movement at each end of the stroke thereof, there is generally a greater time interval available at each end of the piston stroke for heat to conduct from the piston to the cylinder wall. The lower end of the cylinder, corresponding to the position of the piston at bottom dead centre, is adjacent the exhaust port and accordingly, localised cylinder bore "hot spots" can develop in this region. Further, as is known, the upper end of the cylinder adjacent the top dead centre position of the piston is an area where large amounts of heat transfer into the cylinder wall occur due to combustion in the cylinder whilst the piston is at or near top dead centre. In view of the high temperatures of the cylinder wall adjacent the exhaust port, and at the upper and lower end portions of the cylinder, it has previously been known to provide coolant passages in the engine block adjacent the cylinder wall and about the exhaust port to cool some of these known areas of high heat flux. However, prior known multi-cylinder two stroke cycle engines typically have little or no cooling about or adjacent the lower cylinder area corresponding to the bottom dead centre position of the piston. In fact, this is considered not to be critical on some prior constructions of two stroke cycle engines.

In particular, a major use of two stroke cycle engines has been in marine applications, and particularly outboard marine engines which typically have little or no cooling provided about or adjacent the lower cylinder area, including the area corresponding to the bottom dead centre position of the piston. Further in two stroke cycle marine engines, it is common practice not to recirculate coolant within the engine cooling passages, but to continuously draw cooling water from the surrounding water in which the engine is operating which provides a continuous supply of low temperature coolant.

Although these cooling efficiency limitations have to some extent limited the maximum power output of two stroke cycle marine engines, this has not previously been of major concern. However, with greater demand and emphasis being placed on two stroke cycle engines with higher outputs and particularly, such engines having closed circuit cooling systems, more efficient and effective cooling circuits are required to deal with the higher thermal loadings of such engines.

Still further, in addition to the relatively poor overall cylinder coverage offered by such prior two stroke engine cooling systems, the cooling passages of such systems have generally been of a relatively large cross sectional area. This aspect of the design of the cooling passages has also to some extent arisen from manufacturing considerations. Coolant passages of relatively large cross sectional area generally result in slow coolant velocities therethrough. As cooling of the adjacent cylinder walls and adjacent areas of the engine block primarily take place by forced convective heat transfer, this relatively low velocity of coolant reduces the amount of heat that is able to be transferred to the coolant. This results from the fact that the slow flowing coolant is less efficient than a fast flowing coolant in regard to cooling by virtue of forced convection. That is, the Nusselt number, which is a measure of relative effectiveness of a coolant to absorb heat from its surroundings, falls with decreasing coolant velocity.

Also in two stroke cycle engines, in addition to the exhaust port in the cylinder wall, there is usually a plurality of inlet and/or transfer ports spaced about the periphery of the cylinder wall, and transfer passages communicating the inlet and/or transfer ports with the engine crankcase. This multitude of ports in the cylinder wall, and of transfer passages about the cylinder severely curtail the positioning and extent of the coolant passages. This positioning is particularly restricted when inlet and/or transfer ports and associated transfer passages are located on the same side of the cylinder as the exhaust port, as is desirable in current low emission engines such as disclosed in the applicant's United States Patent No. 4886021. It is therefore the object of the present invention to provide a two stroke cycle internal combustion engine having an improved cooling system particularly in regard to the cooling of the cylinder in the vicinity of the exhaust port and lower cylinder wall.

With this object in view, there is provided a two stroke cycle internal combustion engine having at least one cylinder, an exhaust port in the peripheral wall of said cylinder, and a coolant passage about at least a portion of the peripheral wall of the cylinder, said coolant passage commencing below the level of the exhaust port and dividing into two first branches, one passing to either side of the exhaust port and communicating again at a location above the level of the exhaust port, at least one of the first branches being configured to extend partly about the peripheral wall of the cylinder in a respective direction away from the exhaust port and then extend back about the peripheral wall in the opposite direction to said location above the level of the exhaust port.

Preferably, at least two transfer ports are provided in the cylinder wall, one on each side of the exhaust port, and in one embodiment, each respective first branch of the coolant passage is arranged to extend about the respective transfer port on the side thereof remote from the exhaust port. Alternatively, the respective first branches can pass between the exhaust port and the transfer port on the side thereof nearer the exhaust port. Conveniently, two second branches of the coolant passage extend from the area of intercommunication of the first branches above the level of the exhaust port in respective opposite directions about the circumference of the cylinder to the side of the cylinder opposite to the exhaust port location. Preferably, the second branches are arranged to communicate with corresponding coolant passages in a cylinder head fitted to or formed integral with a cylinder block. The second branches may respectively communicate at the side of the cylinder opposite to the exhaust port location with a single passage which communicates with a corresponding cylinder head passage. Alternatively, a plurality of passages may be provided which communicate with a corresponding cylinder head passage or passages.

The first branches are each preferably of a generally U shaped configuration with the arms of the U shaped configuration being in the peripheral direction of the cylinder. The lower arm of each first branch may be arranged to communicate with a common coolant source. The upper arm of each first branch may be arranged to communicate with a common conduit with which the second branches also communicate. The respective second branches preferably include an intermediate portion downwardly displaced with respect to the remainder of the second branch. The intermediate portion may be positioned at a location spaced peripherally from the first branch on the same side of the cylinder with respect to the exhaust port. The intermediate portion preferably extends downwardly to a level below the level of the upper arm of the adjacent first branch.

Alternatively, at least one of the first branches may be configured to extend substantially about the peripheral wall of the cylinder in a direction away from the exhaust port to a point substantially opposite to the exhaust port and then extend back about the peripheral wall in the opposite direction to said location above the exhaust port. In this embodiment, the downwardly displaced intermediate portion of a respective second branch, may be eliminated. However, smaller downward portion(s) could be provided on the upper or lower arm of one or each first branch on one or both sides of the transfer ports to achieve cooling of the cylinder wall in those areas.

If desired, the first branches or an intermediate portion thereof corresponding to the turning point thereof may communicate at said point substantially opposite to the exhaust port. Further, the second branches may also be arranged to communicate at this point if desired. The first branches or the intermediate portions thereof may be arranged to communicate with a common conduit wherein the common conduit may run along one side of a bank of cylinders in a multi-cylinder configuration. The above disclosed coolant passage configuration enables cooling of all relevant areas of the engine cylinder and adjacent portions of the engine block with passages of relatively small cross sectional area so that the flow rate of the coolant through the passages is relatively high with improvement in heat transfer and a resultant increase in heat removal efficiency, that is, a higher Nusselt number results. The arrangement of the coolant passages as proposed herein to control the direction and velocity of coolant flow has the particular objective of improving cooling efficiency in the areas of the engine that are subject to high heat inputs and which are not effectively cooled by prior proposed cooling passage arrangements which are especially deficient in providing a controlled flow of coolant and direct cooling in the region above and around the exhaust port. This is achieved by reducing the cross-sectional area of the coolant passages in general and, in particular, providing portions corresponding to and adjacent the high temperature areas to provide a relatively fast flowing coolant in these areas and a substantially constant cross sectional area of flow in those areas. The direction of flow of coolant through the passage may also be selected to achieve optimum cooling performance. The particular areas requiring cooling and provided with dedicated cooling passage portions or tiers corresponding to those areas are as follows: below the level of the exhaust port, above the level of the exhaust port and at the upper level of the cylinder. That is, the proposed cooling system essentially provides three tiers of cooling passages along the length of the cylinder unlike prior proposed cooling passage arrangements and particularly provides for cooling immediately above the level of the exhaust ports. The proposed cooling system is particularly appropriate for use in parallel cooling systems, particularly when used in multi- cylinder engines.

Accordingly, the invention also provides a method of cooling a two stroke cycle engine wherein coolant is forced in a controlled direction at a predetermined velocity through a coolant passage configured to extend about at least a portion of a peripheral wall of a cylinder in regions below, either side and above the exhaust port, to provide cooling to these regions.

In this way, the present invention may achieve implicit flow and hence greater cooling capacity which is consistent for all cylinders of a multi- cylinder engine. In particular, direct cooling is provided above the exhaust port unlike prior proposed cooling methods.

In a further aspect, the present invention provides a two stroke cycle internal combustion engine having at least one cylinder, an exhaust port in the peripheral wall of said cylinder, and a coolant passage about at least a portion of the peripheral wall of the cylinder, said coolant passage commencing below the level of the exhaust port and dividing into two first branches, one passing to either side of the exhaust port and communicating again at a location above the level of the exhaust port.

If desired, at least one of the first branches may be configured to extend at least partly about the peripheral wall of the cylinder in a respective direction away from the exhaust port and then extend back about the peripheral wall in the opposite direction to the location above the level of the exhaust port. Preferably, at least two transfer ports are provided in the cylinder wall, one on each side of the exhaust port and each respective first branch of the coolant passage is arranged to extend between the exhaust port and a respective inlet port.

Conveniently, two second branches of the coolant passage extend from the area of intercommunication of the first branches above the level of the exhaust port, in respective opposite directions about the periphery of the cylinder, to the side of the cylinder opposite to the exhaust port location.

In this case, at least one of the second branches may extend downwardly to provide cooling to a region above a transfer port, of which desirably two are provided, one on each side of the exhaust port.

The communication of the first branches above the exhaust port is achieved such that a portion of the coolant passage extends above the exhaust port ensuring a controlled flow of coolant in this region in a manner not achieved in the prior art. In addition, efficient cooling may be achieved between the exhaust port and the transfer ports when the coolant passage bridges the gap between these ports thus avoiding the tendency of formation of undesirable "hot spots" in this region. The greater tendency to overheating at the exhaust port favours design of the first branches of the coolant passage such that the branches are arranged closer to the exhaust port than the transfer ports.

The invention will be more readily understood from the following description of one practical arrangement of the coolant passage as applied to a three cylinder two stroke cycle engine and as illustrated in the accompanying drawings.

In the drawings,

Figure 1 is a perspective front view of the cooling passages of one cylinder of a multi-cylinder engine;

Figure 2 is a perspective side view of the coolant passage shown in Figure 1 ;

Figure 3 is a perspective front view of the cooling system of a three cylinder two stroke cycle engine as viewed from the same direction as Figure 1 ; Figure 4 is a perspective side view corresponding to that shown in

Figure 2 and also showing the transfer passage on one side of the exhaust port;

Figure 5 is a perspective side view of a coolant passage in accordance with a second embodiment of the present invention;

Figure 6 is a perspective front view of the cooling passages of one cylinder in accordance with a third embodiment of the present invention; and

Figure 7 is a perspective side view of a portion of an engine block incorporating the coolant passage configuration of the present invention.

A majority of the above referred to drawings depict the coolant passages independently of the engine cylinder block in order to assist in the clarity of the drawings. In fact, the drawings are based on drawings of the water passage cores used in casting a cylinder block which are incorporated in an associated engine block, a portion of which is illustrated in Figure 7.

Referring now to the drawings and, in particular, to Figures 1 , 2 and

3, the lower coolant manifold 1 extends longitudinally along the exhaust port side of the engine block and communicates with respective vertical portions 2, one provided for each cylinder 9. Each vertical portion 2 is located directly below an exhaust port of the cylinder 9 and communicates with respective first branch passages 3 and 4 of generally U-shaped configuration.

The respective first branch passages 3 and 4 are each oriented so that the arms 5 and 7 of the U shaped configuration extend in the peripheral direction about a cylinder 9 of the engine and are interconnected by the bridge portion 6. The first branches 3 and 4 extend away from the vicinity of the exhaust port in the circumferential direction about the cylinder 9 to a location approximately midway of each respective side of the cylinders 9 of the engine. Accordingly, together, the first branches 3 and 4 extend about approximately 40% of the circumferential extent of the cylinder 9. As seen in Figure 4, the transfer passage 19 that conveys air from the engine crankcase to the engine cylinder 9 passes partly about lower arm 5 of the first branch passage 3 and between the upper and lower arms 5 and 7 to communicate with the engine cylinder 9.

The upper arms 7 of the respective first branches 3 and 4 each communicate with an upper coolant manifold 8 via connecting passages 8a. The upper extremity of the upper manifold 8 is adjacent the upper end of the peripheral wall of the engine cylinder 9. Also communicating with the upper manifold 8 are two second branches 10 and 12 which extend in opposite directions about the upper peripheral portion of the cylinder 9 and respectively communicate with the collector portion 16 on the opposite side of the cylinder 9. The collector portion 16 communicates, at the upper end thereof, with ports or passages in the cylinder head of the engine when the cylinder head is installed. The coolant, after passing through the cylinder head, is typically passed through a heat exchanger such as a radiator and then returned to the lower coolant manifold 1 on the exhaust port side of the engine.

The relation of the core shown in Figure 4 to the engine block may be conveniently seen by reference to Figure 7 which illustrates an engine block portion 41 cast with various cavities to enable location of the core within the block. To provide better illustration of the location of the core within portion 41 , transfer passage 19 as shown also in Figure 4, is shown in dotted outline at the approximate position it would take up when the core is located within block portion 41. Exhaust port 70 is shown at the front of the block portion 41. The location of the coolant passage within the block may also be demonstrated by reference to Figure 7. Arm 5 would extend through passage

50 formed in the block portion 41 such that the transfer passage 19 extends about arm 5. Similarly, arm 7 extends through passage 51 and bridge portion 6 extends through cavity 60.

The upper coolant manifold 8 seats in cavity 55 shown at the top of the drawing and the downwardly displaced collector portion 16 extends downwardly through cavity 56 which may be seen to have a portion 56a especially provided for this purpose. Second branches 10 and 12 would extend through passage 57 and 58 respectively.

The exhaust port 70 may be provided with an exhaust valve and cavities 81 are provided to accommodate spigots projecting from the valve member to retain the valve member in position. If desired, the valve members for a bank of cylinders may be retained in position as described in the applicant's co-pending Australian Patent Application No. 22417/92, the contents of which are hereby incorporated by reference.

Alternative designs for the core and block may of course be developed without departing in any way from the scope of the invention.

Referring again to Figures 1 , 2 and 3, it will also be noted that the area of communication between the collector portion 16 and the second branches 10 and 12 respectively, does not extend the full height of the respective second branches 10 and 12, and that communication only exists at the bottom portion 21 of each second branch 10 and 12 as seen in Figures 2 and 4. This relationship ensures that the coolant flows down to the lowest extremity of the respective second branches 10 and 12 in order to enter the lower region of the collector portion 16 before passing upwardly through the collector portion 16 to enter the cylinder head.

In one form of the coolant system, a small cross-over by-pass passage is provided between the second branches 10 and 12 and the collector portion 16, adjacent the upper end thereof (as indicated at 20), to prevent the creation of a stagnant pocket, or gas pocket, in the upper corner of the respective second branches 10 and 12. When the above described coolant system is applied to a multi- cylinder in-line engine, as shown in Figure 3, it is evident that the second branches 10 and 12 of two adjacent cylinders 9 may join in the region between adjacent cylinders as seen at 18 in Figure 3. Similarly, the arms 5 and 7 and bridge portions 6 of the respective U shaped first branches 3 and 4 of two adjacent cylinders 9 may also join in the region between adjacent cylinders 9 of the engine in a similar manner as seen at 17. This arrangement is particularly beneficial where the bore spacing between adjacent cylinders 9 is desired to be minimised and also simplifies the casting process. The respective U shaped first branches 3 and 4 of each cylinder may be designed to extend about substantially the whole, rather than a portion, of the peripheral wall of a cylinder 9 as shown conveniently in Figure 5. In this case, the arms 5 and 7, are extended with the bridge portion 6 formed further about the peripheral wall of the cylinder 9 in a location substantially opposite the exhaust port. The bridge portion 6 interconnects arms 5 and 7 along a vertical axis extending longitudinally along the peripheral wall of the cylinder opposite the exhaust port.

It will be apparent that the other first branch 4 may likewise extend to the side of the cylinder wall opposite the exhaust port. If desired, the bridge portion, corresponding to bridge portion 6, and interconnecting the upper and lower arms of branch 4 may also communicate either directly, or via a conduit running alongside the cylinder 9 on the side opposite the exhaust port, with bridge portion 6. In a multi-cylinder configuration, such a conduit may run along a bank of cylinders communicating with the arms 5, 7 and/or the bridge portions 6 and 30. This alternative embodiment eliminates the need for the bottom portion 21 due to the provision of three distinct tiers of cooling passages about the periphery of the cylinder wall. The downward portion 7a is positioned to provide cooling to the region alongside a transfer port and may alternatively be arranged on the other side of the inlet port nearer the exhaust port. A similar downward portion may be provided for a transfer port on the other side of the exhaust port to that shown in Figure 5.

The above described construction of the coolant circuit of a two stroke cycle engine provides a substantial improvement in the cooling efficiency of the engine particularly in the vicinity of the exhaust port and lower cylinder wall, and generally in the area of the engine on the exhaust port side thereof. The effectiveness of this cooling system is such that it has been found unnecessary to provide for the passing of coolant through the exhaust manifold of the engine in order to obtain the required overall level of cooling in the vicinity of the exhaust ports. The elimination of the need to pass_. coolant through the exhaust manifold substantially reduces manufacturing and assembly costs and also engine weight and enhances the reliability of the system by providing a lower number of potential leakage paths.

In addition, the U shaped configuration and relatively small cross- section area of the first branches 3 and 4, which provide an implicit flow of coolant about the cylinder wall, not only increases the flow velocity of the coolant in the vicinity of the exhaust port and hence improves the heat transfer to the coolant, but also provides cooling capacity both adjacent the exhaust port, notably providing good cooling above the exhaust port, and about the lower peripheral area of the engine cylinder 9, where there is high heat transference from the piston to the cylinder walls during the change in direction of movement of the piston at its bottom dead centre position. Further, as the coolant passages in general are of smaller than normal cross sectional area, they enable the cylinder bores to be located at a reduced centre distance than would otherwise be possible with currently known coolant systems. In addition, the second branches 10 and 12 of the coolant system located in the vicinity of the upper end of the cylinder wall enhance the cooling effects on the cylinder 9 in the area where the piston is located during the reversal of direction of movement at the top dead centre position, that is, the area where there is high heat transference from the combustion chamber and from the piston to the cylinder walls.

In a further embodiment, as shown in Figure 6, cooling is not provided below the transfer ports and, in this configuration, the first branches 3 and 4 pass along the sides of the exhaust port providing effective cooling in the region of the exhaust port and communicate again with the upper coolant manifold 8. This is designed to achieve a flow of coolant at controlled velocity above the exhaust port thereby achieving effective cooling in this region. Further, this arrangement achieves an implicit flow of coolant adjacent the bridge portion 6 between the exhaust port and the transfer ports and serves to eliminate any undesirable "hot spots" in this area. From the upper coolant manifold 8 the coolant flows through second branches 10 and 12 which extend in opposite directions about the upper peripheral portion of the cylinder 9 and respectively communicate with the collector portion 16 on the opposite side of the cylinder 9. The collector portion 16 may communicate with ports or passages in the cylinder head as above described. As shown in Figure 6, there is a minimal extension of the arms 3 and 4 in the direction about the peripheral wall of the cylinder as described by arms 5 and 7 in Figures 1 to 5. However, the coolant passage may be configured such that an arms 5 extends a certain distance about the peripheral wall of the cylinder before turning upwards to skirt the exhaust port. Further as shown in Figure 6, the second branches 10 and 12 may be arranged to extend downwardly along the cylinder 9 such that coolant may be provided above the level of the transfer ports and over a greater portion of the upper end of the cylinder wall.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A two stroke cycle internal combustion engine having at least one cylinder, an exhaust port in the peripheral wall of said cylinder, and a coolant passage about at least a portion of the peripheral wall of the cylinder, said coolant passage commencing below the level of the exhaust port and dividing into two first branches, one passing to either side of the exhaust port and communicating again at a location above the level of the exhaust port, at least one of the said first branches being configured to extend at least partly about the peripheral wall of the cylinder in a respective direction away from the exhaust port and then extend back about the peripheral wall in the opposite direction to said location above the level above the exhaust port.

2. A two stroke cycle engine as claimed in claim 1 wherein at least one of said first branches is configured to extend substantially about the peripheral wall of the cylinder to a point on the side of the cylinder opposite the exhaust port.

3. A two stroke cycle engine as claimed in claim 1 or 2 including two second branches of the coolant passage extending, from the area of intercommunication of the first branches above the level of the exhaust port, in respective opposite directions about the periphery of the cylinder to the side of the cylinder opposite to the exhaust port location.

4. A two stroke cycle engine as claimed in claim 3 wherein the second branches are arranged to communicate with corresponding coolant passages in a cylinder head fitted to, or formed integral with, the cylinder block.

5. A two stroke cycle engine as claimed in claim 4 wherein the second branches may respectively communicate at the side of the cylinder opposite to the exhaust port location with at least one passage which communicates with a corresponding cylinder head passage.

6. A two stroke cycle engine as claimed in any one of claims 1 to 5, wherein the first branches are each of a generally U shaped configuration with the arms of the U shaped configuration being in the peripheral direction of the cylinder.

7. A two stroke cycle engine as claimed in claim 6 wherein the lower arm of each first branch is arranged to communicate with a common coolant source.

8. A two stroke cycle internal combustion engine as claimed in any one of claims 3 to 5 wherein the upper portion of each first branch is arranged to communicate with a common conduit with which the second branches also communicate.

9. A two stroke cycle engine as claimed in claim 3 wherein the second branches include an intermediate portion downwardly displaced with respect to an initial portion of the second branch.

10. An internal combustion engine as claimed in claim 9 wherein said intermediate portion is positioned at a location spaced peripherally from the first branch on the same side of the cylinder with respect to the exhaust port.

11. A two stroke cycle engine as claimed in any one of claims 1 to 10 wherein at least two transfer ports are provided in the cylinder wall, one on each side of the exhaust port, and each respective first branch of the coolant passage is arranged to extend about the respective transfer port on the side thereof remote from the exhaust port.

12. A two stroke cycle engine as claimed in claim 11 wherein the respective first branches pass between the exhaust port and the inlet port on that side of the exhaust port.

13. A two stroke cycle engine as claimed in any one of claims 1 to 12 having adjacent cylinders, at least one branch selected from the group of first branches and second branches being common to said adjacent cylinders.

14. A two stroke cycle engine as claimed in any one of claims 1 to 12 having adjacent cylinders, at least one branch selected from the group of first branches and second branches of one cylinder communicating with at least one branch, selected from the group of first branches and second branches, of the adjacent cylinder.

15. A method of cooling a two stroke cycle engine wherein coolant is forced in a controlled direction at a predetermined velocity through a coolant passage configured to extend about at least a portion of a peripheral wall of a cylinder in regions below, either side and above the exhaust port, to provide cooling to said regions.

16. A two stroke cycle internal combustion engine having at least one cylinder, an exhaust port in the peripheral wall of said cylinder, and a coolant passage about at least a portion of the peripheral wall of the cylinder, said coolant passage commencing below the level of the exhaust port and dividing into two first branches, one passing to either side of the exhaust port and communicating at a location above the level of the exhaust port.

17. A two stroke cycle engine as claimed in claim 16 wherein at least two transfer ports are provided in the cylinder wall, one on each side of the exhaust port and each respective first branch of the coolant passage is arranged to extend between the exhaust port and a respective transfer port.

18. A two stroke cycle engine as claimed in claim 16 or 17 wherein at least one of said first branches is configured to extend partly about the peripheral wall of the cylinder in a respective direction away from the exhaust port.

19. A two stroke cycle engine as claim in any one of claims 16 to 18 including two second branches of the coolant passage extending from the area of intercommunication of the first branches above the level of the exhaust port in respective opposite directions about the periphery of the cylinder to the side of the cylinder opposite to the exhaust port location.

20. A two stroke cycle engine as claimed in claim 19 wherein at least one transfer port is provided in the cylinder wall on either side of the exhaust port and at least one second branch extends downwardly to provide cooling to a region above each transfer port.