SUPERCHARGED TWO-STROKE ENGINE

Abstract: A two-stroke cycle internal combustion engine having two co-axial cylinders (16, 17) separated by a transverse division wall (18) with a piston (20, 21) in each cylinder (16, 17) rigidly coupled by a rod (22) extending through the division wall (18) in sliding sealed relation. The lower piston (21) is coupled by a connecting rod (25) to a crankshaft (13). The upper piston (20) forms a combustion chamber (60) there-above and a first compressor chamber (16a) therebelow, and the lower piston (21) forms a second compressor chamber (17a) there-above. Each of the compressor chambers (16a, 17a) having a valve controlled inlet port (40, 41) to admit ambient air when the pressure in the chamber is below atmospheric, and a valve controlled port (32) is provided to communicate the two compressor chambers (16a, 17a) when the pressure in the second chamber (17a) is above the pressure in the first chamber (16a). The first compressor chamber (16a) supplying compressed air to the combustion chamber (60) in timed relation to the engine cycle.

Description: SUPERCHARGED TWO-STROKE ENGINE This invention relates to internal combustion engines operating on the two-stroke cycle, and particularly to high compression two-stroke engines, including such engines operated with diesel fuels. One of the problems with two-stroke engines is the restriction on the mass of air in the combustion chamber at the time of combustion due in part to low pressure of that air delivered to the engine during the intake stroke. It has been proposed to obtain higher compression pressures by the use of supercharging, however these have limitations, particularly in regard to the fact that they only operate efficiently at relatively high engine speeds. It is therefore the principal object of the present invention to provide a two-stroke cycle internal combustion engine operating with a relatively high compression pressure and which can effectively produce these pressures over the full working speed range of the engine. With this object in view there is provided a two-stroke cycle internal combustion engine comprising first and second co-axial chambers with a division wall therebetween, first and second piston members mounted in the respective first and second chambers to reciprocate in unison therein and coupled to a crankshaft to rotate the crankshaft in response to said reciprocation of the piston members, a cylinder head closing the end of the first chamber remote from the division wall, whereby a combustion chamber is formed between the cylinder head and the first piston member and a first compressor chamber is formed between the first piston member and the division wall, said second piston member forming with the division wall a second compressor chamber, first means responsive to a predetermined pressure differential between the first and second compressor chambers to communicate said chambers to permit gas to pass from the second compressor chamber to the first compressor chamber, and second means to permit gas to pass from the first compressor chamber for admission to the combustion chamber when the pressure in the first compressor chamber is above a first predetermined pressure, said first means being adapted to terminate communication between the first and second compressor chambers when the pressure in the first compressor chamber is at a pressure less than said first predetermined pressure. Inlet ports are provided to communicate the first and second compressor chambers with atmosphere when the pressure in the relevant chamber falls below atmospheric pressure. The inlet ports communicate with the respective compressor chambers through or adjacent to the division wall. In this location, air may enter the respective chambers as the respective piston members commence to move in a direction away from the division wall, that is commence their suction stroke. The inlet ports are valve controlled so as to open at pressures in the respective compressor chambers below atmospheric pressure and at or above atmospheric pressure the ports are automatically closed. The valve may be of the reed, poppet or disc type appropriately spring-loaded. Conveniently, the second means is arranged to communicate the first compressor chamber with a transfer chamber when the pressure in the first compressor chamber is above said first predetermined pressure. The transfer chamber is selectively communicated with the combustion chamber in timed relation to the reciprocation of the first piston member, conveniently by a port in the wall of the first compressor chamber controlled by the movement of the first piston member. Preferably, a third means is provided to permit gas to pass from the second compressor chamber to the transfer chamber when the pressure in the second compressor chamber is a predetermined amount above the pressure in the transfer chamber. The second chamber at the end remote from the division wall may conveniently be in communication with an engine crankcase in which the crankshaft is journalled. In such a construction, fourth means may be provided to communicate the crankcase with the transfer chamber when the pressure in the crankcase is above the pressure in the transfer chamber. The crankcase is provided with an inlet port that will open when the pressure in the crankcase falls below atmospheric pressure. The fourth means may be a port in the wall of the second compressor chamber that is controlled by the movement of the second piston member. The second, third and fourth means may be arranged so that the first and second compressor chambers and the crankcase are respectively communicated with the transfer chamber when the pressure in the transfer chamber is below that in the respective chamber by a relatively small amount, such as 5 to 10 kpa. Conveniently, the first, second, third and fourth means may be valve controlled ports, such as ports controlled by reed, poppet or disc valves, appropriately spring loaded. The engine as described above has multiple compressors to increase the mass of air available for combustion of the fuel. Also due to multi-stage compression of at least some of the air, the pressure of the air available for admission to the combustion chamber may be raised. These increases in mass and pressure of the air are achieved with a relatively simple engine construction and minimal increase in physical size and weight of the engine. The invention will be more readily understood from the following description of one practical arrangement of the engine as illustrated in the accompanying drawing, which is a longitudinal sectional view of a single cylinder engine. Referring now to the drawing, the engine comprises a cylinder 10 mounted on or integral with a crankcase 11 and having a detachable cylinder head 12 fitted thereto. The crankshaft 13 is journalled in bearings 14 and has an eccentric crank pin 15 within the crankcase 11. The cylinder 10 defines a uniform internal bore divided into upper and lower cylinders 16 and 17 formed by the division wall 18. Piston 20 is mounted in the upper cylinder 16 and piston 21 is mounted in the lower cylinder 17. Pistons 20 and 21 are rigidly connected by the rod 22 which is formed integral with the lower piston 21 and has a spigoted end 23 received in a central bore in the upper piston 20 and the co-operating bolt 24. The conventional connecting rod 25 is rotatably mounted on the eccentric journal 15 of crankshaft 13 and is journalled on the bearing pin 26 attached by the bolts 27 to the lower piston 21. It will thus be appreciated that the upper and lower pistons 20 and 21 reciprocate in unison in the upper and lower cylinders 16 and 17 as the crankshaft 13 rotates. Although the pistons 20 and 21 have been shown to be of equal diameter, it is to be understood that it is not essential for these diameters to be equal, and the diameter of one or both may be varied in accordance with the final compression pressure required in the combustion chamber. The upper piston 20 divides the upper cylinder 16 into a combustion chamber 60 above the piston 20 and an upper compressor chamber 16a below the piston 20. Similarly the lower piston 21 divides the lower compressor chamber 17a from the crankcase 11. The division wall 18 is provided with a seal assembly 30 to permit reciprocation of the rod 22 relative thereto, whilst maintaining a seal against the passage of gas under pressure between the rod 22 and the division wall 18. The division wall 18 is also fixed in a sealed relation to the cylinder 10 so as to prevent any uncontrolled passage of gas between the periphery of the division wall 18 and the internal surface of the cylinder 10. A number of spaced transfer ports 32 are provided in the division wall 18 to provide communication between the upper compressor chamber from the lower compressor chamber. These ports 32 are provided with respective reed valves 33 so that gas above a predetermined pressure may pass from the lower compressor chamber to the upper compressor chamber 16 but gas cannot at any time pass in the reverse direction. Three inlet ports each communicating with ambient air are provided. The inlet port 40 communicating with the upper compressor chamber 16a towards the lower end thereof, the inlet port 41 communicating with the lower compressor chamber 17a near the upper end thereof and the inlet port 42 communicating with the crankcase 11. Each of the inlet ports 40, 41 and 42 are each provided with reed valves 40a, 41a and 42a respectively and arranged so that ambient air will only enter the respective chambers when the pressure in that chamber is lower than ambient pressure. There are also provided four transfer ports, and two transfer passages. Transfer port 46 communicates the interior of the crankcase with the transfer passage 45 and transfer port 47 communicates the transfer passage 45 with the lower compressor chamber 17a. Transfer port 47 is opened and closed by the lower piston 21 as it reciprocates in the lower cylinder 17. The transfer port 48 communicates the transfer passage 44 with the lower end of the upper compressor chamber 16a. The final transfer port 49 communicates the transfer passage 44 with the combustion chamber 60 and this communication is controlled by the reciprocation of the upper piston 20 in the cylinder 16The exhaust port 50 is provided in the wall pf the upper cylinder 16 and is opened and closed in response to the reciprocating movement of the upper piston 20. The reed valves 33 associated with the ports 32 in the division wall 18 are set to open at a higher pressure than the reed valve in the inlet port 40 so that on the upstroke of the pistons 20 and 21 air will first enter the upper compressor chamber 16a through the inlet port 40, and air will be transferred from the lower compressor chamber 17a to the upper chamber 16a only when the pressure in the lower compressor chamber 17a is raised sufficiently to open the reed valves 33. This ensures that during each cycle of the engine a proportion of the engines air intake is atmospheric air drawn into the upper compressor chamber 16a. The operation of the engine of the above construction will now be described with the engine commencing in the position with both the upper and lower pistons 20 and 21 in the top dead centre position (TDC). Assuming the charge in the combustion chamber 60 has been ignited, the resulting expansion of the gas therein will cause the pistons 20 and 21 to move downwardly in unison from the TDC position, with the exhaust port 50 and the transfer port 49 closed by the piston 20 and the transfer port 47 closed by the piston 21. The downward movement of piston 20 will initiate compression of the air in the upper compressor chamber 16a with the result that the reed valves 33 in the ports 32 in the division wall 18 and the inlet port 40 will be closed due to the rising pressure. As a result the air below the piston 20 in the upper compressor chamber 16a will be compressed and delivered into the transfer passage 44. That air will be trapped in the transfer passage 44 since the port 49 is closed by the piston 20. At the same time as the piston 20 is moving down, the piston 21 will correspondingly move down and air will enter the lower compressor chamber 17a through the opening of the reed valve in the inlet port 41. During this downward movement the reed valve in the inlet port 42 in the crankcase 11 will be closed as a result of the rising pressure in the crankcase. As the pistons 20 and 21 continue to move downwardly the pressure in the upper compressor chamber and in the crankcase 11 would progressively increase and both will supply air under pressure into the respective transfer passages 44 and 45, via the respective transfer ports 46 and 48. As the pistons continue their downward movement, the piston 20 will reach a position where the exhaust port 50 is uncovered and the combustion gases from the combustion chamber 60 will commence to be exhausted through the exhaust port 50. Also the pressure of the gases in the upper compressor chamber 16a the crankcase 11 will continue to rise and result in a further rise in the pressure of the air in the transfer passages 44 and 45. At the same time, air will continue to enter the lower compressor chamber 17a via the inlet port 41. After further downward movement of the pistons the upper piston 20 will uncover the transfer port 49 and the high pressure air stored in the transfer passage 44 will flow into the combustion chamber 60 above the piston 20. Further downward movement of the pistons will result in the lower piston 21 uncovering the port 47 and air from the transfer passage 45 and crankcase 11 will flow into the lower compressor chamber 17a. The resulting rise in pressure in the lower compressor chamber will close the reed valve in the inlet port 41. Continued downward movement of the pistons will maintain the supply of air at suitable pressures to the combustion chamber 60 and lower compressor chamber 17a. Upon the pistons 20 and 21 reaching the lower most point in their travel (bottom dead centre) substantially all of the air in the upper compression chamber 16a will have been discharged through the transfer passage 44 to the combustion chamber. Also the lower compressor chamber 17a above the piston 21 will be fully charged with air. As the pistons commence to move upwardly in the cylinders, with the port inlet 41 closed the pressure in the lower compressor chamber will commence to rise. The corresponding initial upward movement of the piston 20 will not create any substantial pressure in the combustion chamber 60 as the exhaust port will still be open and thus scavenging of the exhaust gases from the previous combustion cycle will continue. Also during the upward movement of the piston 21 the pressure in the crankcase 11 will drop sufficiently to open the reed valve in the port 42 so that air at ambient pressure will commence to flow into the crankcase. Also during this upward movement the reed valve in the port 40 will open to permit air at ambient pressure to enter the upper compressor chamber 16a below the piston 20. hen the piston 20 has moved upwardly sufficient to close the transfer port 49 and the exhaust port 50, the piston 21 will have raised sufficiently to close the transfer passage 47 and commence to compress, the air in the lower compressor chamber. Also the pressure in the combustion chamber 60 will commence to rise due to the upward movement of the piston 20. When the piston 21 has moved upwardly sufficiently to bring the pressure in the lower compressor chamber to a level sufficient to open the reed valve 33, air will be passed from the lower compressor chamber 17a into the upper compressor chamber 16a. This admission of relatively high pressure air to the chamber 16a will result in compressing of the air already therein so that the reed valve in the inlet port 40 will close. The upward movement of the pistons 20 and 21 will then continue causing the pressure to rise in the combustion chamber 60 and the pressure to rise in upper and lower compressor chambers 16a and 17a. As the piston 20 approaches the TDC position, the charge in the combustion chamber 60 will be ignited and the above described operational sequence will be repeated for each cycle of the engine. The spark plug 61 is provided to effect the ignition of the charge in the combustion chamber 60. In the construction of the engine above described, the location of the transfer ports 47 and 49 relative to the stroke of the respective pistons 21 and 20 determine the timing in the engine cycle of the admission of air from the crankcase 11 to the lower compressor chamber 17a and from the upper compressor chamber 16a to the combustion chamber 60. Because of the relationship between the opening of the transfer ports and the position of the respective piston within its stroke, the pressure of the air available on opening of the transfer ports is related to the position of the transfer ports. Alternatively, or in addition, valves set to open at respective selected pressures may be associated with each of the transfer ports. The engine as illustrated incorporates a spark plug 61 and the required fuel may be supplied to the combustion chamber 60 by a suitable fuel injector which delivers the fuel directly into the combustion chamber 60 or into the transfer passage 44 or 45. Alternatively a suitable form of carburation may be provided to meter the fuel into the air being drawn into the engine through the inlet port 42 in the crankcase and/or through the inlet ports 40 and 41 in the upper and lower compressor chambers, respectively. In a further alternative fuel could be metered by an injection system into the air passing to one or more of the above referred to three inlet ports. In yet a further alternative, the engine may be operated on the diesel cycle with diesel fuel being injected directly into the combustion chamber 60 after the compression of the gas in the combustion chamber has raised the temperature thereof sufficiently to ignite the diesel fuel during delivery. The engine as above described enables the compression pressure in the combustion chamber 60 at the time of combustion to be substantially higher than that normally achieved with a crankcase compression type two-stroke cycle engine. Also the inlet ports 40 and 41 increase the mass of air delivered to the combustion chamber in relation to that which would be delivered in a simple multi-stage compression of the air. This increases the air mass and higher degree of compression improves the overall efficiency of the engine. In addition the provision of the inlet ports 40 and 41 so that ambient air may enter the upper and lower compressor chambers immediately the suction stroke of the respective pistons commence, reduces substantially the "pumping losses" which would normally occur in conventional two-stroke engines. Further the various components and manner of construction of the engine is in accordance with substantially conventional engine manufacturing techniques and the design can therefore be readily adopted by engine manufacturers without major difficulties.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A two-stroke cycle internal combustion engine comprising first and second co-axial chambers with a division wall therebetween, first and second piston members mounted in the respective first and second chambers to reciprocate in unison therein and coupled to a crankshaft to rotate the crankshaft in response to said reciprocation of the piston members, a cylinder head closing the end of the first chamber remote from the division wall, whereby a combustion chamber is formed between the cylinder head and the first piston member and a first compressor chamber is formed between the first piston member and the division wall, said second piston member forming with the division wall a second compressor chamber, first means responsive to a predetermined pressure differential between the first and second compressor chambers to communicate said chambers to permit gas to pass from the second compressor chamber to the first compressor chamber, and second means to permit gas to pass from the first compressor chamber for admission to the combustion chamber when the pressure in the first compressor chamber is above a first predetermined pressure, said first means being adapted to terminate communication between the first and second compressor chambers when the pressure in the first compressor chamber is at a pressure less than said first predetermined pressure. 2. A two-stroke cycle internal combustion engine as claimed in claim 1, wherein at least one of said first and second compressor chambers includes inlet port means adapted to open when the pressure in the compressor chajnber is below atmospheric pressure to permit atmospheric air to enter the chamber. 3. A two-stroke cycle internal combustion engine as claimed in claim 1, wherein respective inlet port means are provided in each of said first and second compressor chambers in or adjacent to the division wall, each said inlet port means being adapted to open when the pressure in the related compressor chamber is below atmospheric pressure to permit air to enter that chamber. 4. A two stroke cycle internal combustion engine as claimed in claim 1, 2 or 3, having a crankcase in which the crankshaft is located and arranged so that the reciprocation of the second piston member creates variations in the pressure in the crankcase, and transfer port means are provided to selectively communicate the crankcase with the second compressor chamber to permit gas to pass from the crankcase to said second compressor chamber. 5. A two-stroke cycle internal combustion engine as claimed in claim 4, wherein the transfer port means includes a port in the wall of the second compressor chamber which is opened and closed by the second piston member as it reciprocates in the second compressor chamber. 6. A two-stroke cycle internal combustion engine as claimed in any one of claims 1 to 5, wherein the first means comprises at least one port and a complementary valve element in the division wall, the valve element or elements being biased to normally close the port or ports and displacable to open said port or ports when the pressure difference between the first and second compressor chambers is above said predetermined level. 7. A two-stroke cycle internal combustion engine as claimed in any one of claims 1 to 5 wherein the second means - 13 - comprises a passage communicating the first compressor chamber with a port in the wall of the combustion chamber, said port being located to be opened and closed by the first piston member as it reciprocates in the first chamber and so it is opened when the pressure in the first compressor chamber is greater than the pressure in the combustion chamber. 8. A two-stroke cycle internal combustion engine as claimed in any one of claim 1 to 5, wherein the first and second piston member are coupled by a rigid member extending throuqh the division wall in sealed relation and slidable relative thereto in the axial direction of the two cylinders, and the second piston member is coupled to the crankshaft by a connecting rod pivotable relative to the crankshaft and second piston member about respective parallel axes transverse to said axial direction of the two cylinders.

Inventors: URQUHART ROBERT, URQUHART, ROBERT
Applicants: URQUHART ROBERT, URQUHART, ROBERT
Application year: 1987
Main classification: F02B33
IPC codes: F02B33