Patent Grant
7093570
The present invention relates to two stroke internal combustion engines and, particularly, to such engines with stratified scavenging.
A particular field of application of the invention is a two-stroke internal combustion engine. One application of the invention is to a small high speed two stroke engine, such as utilized in hand-held power equipment such as leaf blowers, string trimmers, hedge trimmers, also in wheeled vehicle applications such as mopeds, motorcycles, scooters, and in small outboard boat engines. The small two stroke engine has many desirable characteristics, including simplicity of construction, low cost of manufacturing, high power-to-weight ratios, high speed operational capability and, in many parts of the world, ease of maintenance.
Inherent drawbacks of two stroke engines are high emission levels and poor fuel economy due to short-circuit loss of fuel and air charge during the scavenging process. One drawback of the simple two-stroke engine is a loss of a portion of the fresh unburned fuel charge from the cylinder during the scavenging process. In the two-stroke engine, the homogeneous charge enters the cylinder through transfer ports during the scavenging process, when the exhaust port is also open. As such, some of the charge escapes through the exhaust port leading to high levels of hydrocarbons (HC) in the tailpipe. This leads to the poor fuel economy and high emission of unburned hydrocarbon, thus, rendering the simple two stroke engine difficult to comply with increasingly stringent governmental pollution restrictions. This drawback can be relieved by separating the scavenging of the cylinder, with fresh air, from the charging of the cylinder, with fuel. This separation can be achieved by having a buffer medium of air between the fresh charge and the burnt gas, during the scavenging process.
Several concepts and technologies have been proposed or tried to circumvent the short-circuit loss of fresh charge. Among these techniques are direct or indirect fuel injections, stratified scavenging, air-head, air assisted fuel injection, and compressed wave injection. Most of these technologies are either complex, expensive or need more parts. The fuel injection technology is not economical for small engines but air-head scavenging and stratified scavenging are promising.
An air-head scavenging system disclosed in U.S. Pat. No. 6,513,466 consists of an air channel leading into the storage space in the crankcase and has a reed valve. The filling time is very dependent on the pressure difference across the reed valve and is more likely dependent on engine speed and load. This may lead to an optimum performance only at a certain operating range of speed and load. The storage space may become a dead space when charge bypasses the storage space. U.S. Pat. Nos. 4,821,787, 6,112,708, and 6,367,432 describe reed valve controlled air passage in air-head scavenged two-stroke engines. The use of reed valves increases the cost and complexity and the performance is subject to quality of the reed valves. John Deere has used Reed valve controlled charge injection called compressed wave injection in the hand held application two-stroke engines. Again the use of reed in the engine can add cost and complexity to the engine.
It is desirable to have a simple two-stroke engine with fewer parts and that is easy to manufacture and assemble. It is also desirable to have an air volume high enough to improve the delivery ratio and scavenging and have asymmetric air inlet timing.
A two stroke internal combustion engine includes at least one transfer passage in gaseous communication between a crankcase chamber and a combustion chamber of the engine, an air passage through the crankcase to the crankcase chamber and in gaseous communication with a carburetor of the engine, and a rotatable circular disk rotatably connected to a crankshaft of the engine. At least one first rotary shut-off valve is located in a radially outermost section of the circular disk bordered by a periphery of the circular disk and operatively disposed between the transfer passage and the crankcase chamber for opening and closing gaseous communication between the transfer passage and the crankcase chamber. At least one second rotary shut-off valve is located on the circular disk bordered by a periphery of the circular disk and operatively disposed between the air passage and the transfer passage for opening and closing gaseous communication between the air passage and the transfer passage.
In the exemplary embodiment of the two stroke internal combustion engine the first and second rotary shut-off valves are operably located on the on the circular disk to close the air passage to the transfer passage when the transfer passage is open between the combustion chamber and the crankcase chamber and to close off the transfer passage between the combustion chamber and the crankcase chamber when the air passage is opened to the transfer passage. In a more particular exemplary embodiment of the two stroke internal combustion engine the rotatable circular disk is a crank web, the first rotary shut-off valve is a conical cut out sector in a periphery of the crank web, and the second rotary shut-off valve is a notched cut out in the periphery of the crank web. An engine incudes a cylinder having at least one transfer passage that is a channel in a cylinder bore. A top end of the channel opens into a combustion chamber of the cylinder and the lower end opens into a crankcase chamber of the engine. The top end is opened and closed by a piston operably disposed in the cyliner bore, where as the lower end is alternatively opened and closed into the ambient air by a rotary valve, which in one embodiment of the engine is a crank web. When the rotary valve opens the air inlet to the lower end of transfer passage, as the piston is moving upward, a piston passage in a piston skirt of the piston opens a transfer port into the crankcase. The piston passage may be a window in the piston or a special passage with a fluid diode type that will be described later. The crank web also alternatively opens the lower end of the transfer passage into the crankcase. Connection of transfer passage to air and crankcase is alternative and is accomplished by a groove and cut out in the crank web. A main charge is injected into the crankcase in a usual manner either through a piston-controlled inlet, rotary valve, or a reed valve system.
One embodiment of the engine includes quadruplet transfer passage having a lower end of a first transfer passages closest to an exhaust port is alternatively connected to the ambient air by the rotary valve. The top end of the first transfer passage is connected to an adjacent second transfer passage either through a cut out in the piston or directly through a connecting passage at the top between the first and second transfer passages. The quadruplet passage increases the total volume of air and air acts as a buffer medium in both the transfer passages. It also helps clear the fresh charge in the transfer passages from the previous cycle.
By controlling the lower of transfer passage during scavenging asymmetric timing may be accomplished by the use of rotary valve. Thus the lower end of the transfer passage closest to the exhaust port may be shut off early during the end of scavenging process and may also have delayed opening.
A total length of the transfer passage may be increased by having the transfer passage continue into the crankcase as a grove on the crankcase wall. By using the crank web as a rotary valve to open and close the air inlet to lower end of transfer passage and a window or passage in the piston to open and close the top end of transfer passage into the crankcase, asymmetric air inlet timing is achieved. Thus there is no need for reed valves in the engine disclosed herein.
In one embodiment of the engine, the crank web and passage in the piston has been used to effect three-way scavenging in which air enters the combustion chamber ahead of lean air-fuel charge followed by the rich air-fuel charge. In another embodiments of the engine the crank web and the passage in the piston control a rich charge, thus eliminating a reed valve used in John Deere's compressed wave injection engine and completely replacing it with the rotary valve.
The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where:
a is an enlarged view of crankcase inserts as viewed from the side.
a)–17(f) is an illustration of different piston configurations.
Air-head scavenged engines provide a buffer medium of air between the fresh charge and the burned gas during the scavenging process. When the transfer ports open, the air enters the combustion chamber first and is most likely to be short-circuited, in the sense a small fraction of air is lost into the exhaust. The air is inducted into the transfer passage during the intake process, when the piston is ascending. Typically, a reed valve is provided at the top of the transfer passages for inducting only air into top of the transfer passages that stays in the transfer passages to act as a buffer medium. In some instances, piston ports are also provided in place of reed valves. The disadvantage with the reed valves is that it adds parts and are speed sensitive and the performance is subject to quality of the assembly of reeds and reed themselves.
In the exemplary embodiment the rotary valve, which can be a crank web as described in this case, replaces the reed valves. The two-stroke engine described in this embodiment consists of air inlet ports, opened and closed by the crank web cut out in the crank web for gaseous communication between the air inlet ports and the crankcase port at the bottom end of the transfer passages and the transfer ports at the top end of the transfer passages, which are opened and closed by the top of the piston and also by either cut out in the piston or by the passages in the piston. The cut out in the crank web acts as a rotary valve that periodically establishes gaseous communication between the ambient air and the transfer passages. The second cut out provides gaseous communication between the crankcase and the transfer passage. Thus the crank web alternatively communicates bottom end of the transfer passage with the ambient air and crankcase. The two-stroke engine cycle processes determine which way the bottom of transfer passage opens into.
The air inlet port is in gaseous communication with lower end of the transfer passage at appropriate time only. The timing of the gaseous communication between the air inlet port and the transfer passage is controlled by the passage in the crank web (could be groove or counter sunk). The crank web during the scavenging and expansion process shuts off the air inlet port. The lower end of the transfer passage is open and closed to the crankcase at appropriate time by the cutout on the crank web. Thus the crank web acts as a rotary valve to time the flow air into transfer passage from ambient during intake process and opens the transfer passage to crank case during scavenging process. The air in the transfer passage acts as a buffer medium between the charge and the burnt gas to minimize the loss of charge into exhaust and hence lowers the exhaust emission.
As the piston descends, and before the top of the piston opens transfer port 33, the crankcase port 111 at the lower end of the transfer passage 11 is opened by the cut out 244 on the periphery 43 of the crank web 21. The location of leading edge 179 with respect to TDC position determines the start of scavenging process. The opening of the crankcase port 111 can be leading ahead or trailing behind the opening of the transfer port 33 by the piston. The angular length ‘A’ between the leading edge 179 and the trailing edge 178 determines the duration of the crankcase port 111 opened into the crankcase 26. The intake of main air-fuel charge occurs though the inlet port 84 and through the carburetor control valve 585 in a normal way. The opening of the intake port 84 may be delayed with respect to the air inlet port 650. A typical port timing for the exemplary air-head scavenged two-stroke engine is shown in Table 1.
As the piston descends down, it opens the exhaust port 50 first and then the transfer ports 33. When the transfer ports 33 are opened, the air in the transfer passage 11 enters the combustion chamber 30 first ahead of the charge. Thus pure air acts as a buffer medium between the burnt gas and the fresh charge during the scavenging process. Since air enters the combustion chamber first and has the longest path to travel in the combustion chamber, it is the one that is most likely to be lost into the exhaust port 50. Thus air-head scavenging minimizes the loss of fresh charge into the tail pipe and hence lowers the unburned hydrocarbon emission into the ambient. The scavenging duration by the charge may be delayed by delaying the opening of the crankcase port 111. Thus the duration of time for which charge is likely to escape into the exhaust port may be shortened as determined by the angular length ‘A’ of the cut out 244 in the crank web 21. Also, after discharging trapped air into the combustion chamber, the discharge of charge following the air may be momentarily interrupted by shutting off the crankcase port 111 by the crank web. In that case the cut out 244 is made of two segments; a first cut out 244a for the discharge of air through the port 33. After momentarily shutting the crankcase port 111 the second cut out 751 opens the crankcase port 111 for discharge of charge. Descending of piston toward BDC helps build up crankcase pressure when the crankcase port 111 is momentarily shut off. Increased crankcase pressure around BDC position of the piston helps the delayed discharge of charge into the combustion chamber.
The proper functioning of the rotary valve depends on the good clearance between the port and the rotary valve. If the clearance between the two is excessive it may lead to poor sealing. In order to ensure proper seal between the face 550 of the crank web 21 and the crankcase wall, unique inserts 619 and 652 have been used.
During the scavenging process, the transfer ports 33 and 233 open simultaneously or may have staggered timing, where port 233 farthest from exhaust port 50, opens a few degrees ahead of port 33. The air flowing from the transfer port 33 acts as a buffer medium between the charge and the burnt gas, thus minimizing the loss of charge into the exhaust. By virtue of crank web being able to provide asymmetric crankcase port timing, the opening of the crankcase port 619 may be delayed while opening the transfer port 33 ahead of 233 to have a blow down of exhaust gas into the transfer passage 11 without adversely effecting the crankcase pressure. When the air is discharged later during the scavenging process, it may trap a layer of burnt gas between the fresh charge and the air, which ensures better trapping of the charge. This minimizes the loss of charge into the exhaust, which lowers the engine out emission of unburned fuel.
It is also possible in a quadruplet transfer passage system for only the transfer passage 11 closest to the exhaust port to receive air while the transfer passage 211 is not in communication with passage 11. In that case the piston may have a window for gaseous communication between transfer passage 11 and the crankcase 26 during intake of air into the transfer passage 11. The piston with a window is shown in
a) through 17(f) illustrate different piston configurations usable with the exemplary embodiment, described above. In the case of a quadruplet transfer passages the piston 17(e) provides communication between the transfer ports 33 and 233 through an annular piston passage 103 illustrated as an annular groove in the piston. The height of the passage 103 determines the duration of the communication between the ports 33 and 233. Similarly a window 104 illustrated in
c) illustrates a piston passage 612 with a fluid diode 615 which offers resistance for reverse flow of charge into the transfer passage 11 while offering no resistance or minimum resistance for the flow in one direction (toward crankcase). In a quadruplet transfer passage, any combination of the piston configurations may be used. In the sense that the piston may provide gaseous communication during early or late phase of air intake into transfer passages while providing a window or direct passage into crankcase during early or late intake phase of air into transfer passage.
The air and air-fuel control valves can either be a barrel valve type shown in
In
In
The lower end of the transfer passage 11 has a crankcase port 620 that is alternatively in gaseous communication with the ambient air through the cutout 680 on the outside face 550 of the crank web 21 and an air inlet port 650. The crankcase port 620 is also alternatively in gaseous communication with the crankcase 26. The crankcase port 620 is opened into the crankcase 26 by the cutout 753 on the periphery 43 of the crank web 21. The lower end of the second transfer passage 211 is in gaseous communication with the crankcase 26 through a crankcase port 222 (shown in
As the piston 16 moves upward, the top edge of the piston skirt 17 closes the transfer port 33 first, 233 next and then the exhaust port 50. Both the transfer ports 33 and 233 may be closed simultaneously if the transfer port timing is not staggered (in the sense one port opens earlier than the second). After the exhaust port 50 is closed the crank web shuts off the communication between crankcase port 620 and the crankcase 26. As the piston continues to move upward the air inlet port 650 is opened by the cutout 680 and a little later the cutout 680 opens the crankcase port 620, while the section of the crank web has shuts off direct flow of gas between crankcase port 620 and the crankcase 26. However, the top of the transfer passage 11 can be in gaseous communication with the crankcase 26 either 1) directly through passage 102 in the piston (shown in
As the piston continues to move upward, the sub-atmospheric pressure in the crankcase 26 draws air from ambient (outside the crankcase) into the transfer passage 11 through the air inlet passage 88, air inlet port 650, and into the crankcase port 620 shown in
As the piston starts to move downward the charge in the crankcase 26 is pressurized. If the crankcase port 620 is not closed, then the fresh charge may enter the transfer passage 11. However, since the crank web closes the crankcase port, the charge does not enter the transfer passage from the lower end. In a quadruplet type transfer passage and when the air is contained in both the transfer passages 11 and 211, closing the crankcase port 620 prevents the reverse flow of air into the crankcase 26. However, charge may enter the transfer passage 211 through the crankcase port 222. The volume and length of the transfer passage 11 and 211 may be such that even when the charge enters the transfer passage 211, it may not reach the transfer passage 11 as the crankcase port 620 is closed.
In order to completely eliminate the entry of charge into the transfer passage 211, the crankcase port 222 may also be either closed by the crank web or by the piston port, where the piston skirt closes the port 222 until the transfer port 233 is open. The opening and closing of the transfer port in the crankcase (or in the cylinder) has been disclosed in patent application Ser. No. 10/446,393, filing date May 28, 2003 by the same Inventors.
As the piston descends the exhaust port 50 is open first. The transfer port is open next. Since it is the air that is entering the combustion chamber first and has the longest residential time, it is more likely that it is the air that gets short circuited into the exhaust port. Thus the air-head scavenging system minimizes the loss of charge into the exhaust and thus lowers the unburned hydrocarbons in the tail pipe exhaust.
When quadruplet transfer ports are used, most of the air is retained in the transfer passage 11, which is closest to the exhaust port 50. The transfer port 233 farthest from the exhaust port 50 may open first in the case of a staggered transfer ports. In that case, as the top of the transfer port 211 also has some air and it enters the combustion chamber first followed by the charge. The second transfer port 33 may open a few degrees later discharging pure air in front of charge and acts as a buffer medium between the fresh charge and the burnt exhaust gas.
It is possible to open the crankcase port 111 (620) later after the transfer port 33 is open, since the crankcase port is opened and closed by the crank web. Thus an asymmetric timing is possible with the crank web controlled crankcase port system.
In
In
As the piston 16 ascends the piston skirt 17 opens the port 549 and thus establishing gaseous communication between the crankcase 26 and the ambient through the carburetor 547. The rich charge now flows into the charge passage 39 through a one-way valve 36. As the piston continues to ascend the air inlet into the transfer passage 11 and the lean air-fuel charge into the crankcase 26 occurs in a manner described earlier for the engine shown in
The induction of rich charge into the charge passage 39 ends as the pistons begins to descend. The increase in crankcase pressure forces the one-way valve 39 to close. After the blow down of exhaust gas through the exhaust port 50, the scavenging occurs first through the transfer port 33 where air enters the combustion chamber first followed by lean charge. As the piston continues to descend the crankcase port 111 may be closed and about the same time or before, the window 557 on the piston skirt 17 opens port 549 for injection of charge into the combustion chamber 30. Thus the scavenging process occurs in three phases; first the air enters, followed by the lean charge through the transfer port 33 and then the rich charge is injected through the injection port 40. The transfer passage system may be of quadruplet type described earlier and shown in
The carburetor 551 consists of two passages 300 for rich charge and 310 for either only air or very lean charge. The passage 310 opens into the passage 312 in the adapter plate, which communicates into the crankcase through the main inlet port 84. The rich charge passage 300 opens into a charge inlet passage 302, which has a charge inlet port 60 in the crankcase.
One end of the charge passage 39 has a charge injection port 40 opening into the combustion chamber where it is opened by the top of the piston 16 during scavenging and injection process. The charge passage 39 has a section 545 running down into the channel 552 in the cylinder flange 430 that runs around the cylinder 14 and opens into the passage 544 in the crankcase. The passage 544 in the crankcase opens into the crankcase 26 through a crankcase port 41 which is opened and closed by the cut outs in the crank web 21. The rich charge passage 302 that is in communication with the carburetor 551 has a charge inlet port 60 in the crank case. The cut out 45 (556 in
As the piston descends the piston opens the exhaust port 50 first and the scavenging occurs as the transfer ports 33 and 233 are opened. As the piston descends the crankcase port 41 is opened again by the cut out 44 (558 in
The segment 552 of the charge passage 39 may be on the cylinder flange 430 as shown in
The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/533,477 filed on Dec. 31, 2003, and entitled “STRATIFIED SCAVENGED TWO-STROKE ENGINE” which is hereby incorporated by reference in its entirety.
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| Number | Date | Country |
|---|---|---|
| WO 9212332 | Jul 1992 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050139179 A1 | Jun 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60533477 | Dec 2003 | US |
