IES63195B2 - Improvements in or relating to two-stroke engines - Google Patents

Abstract

A crankcase (1) of a two-stroke engine comprising at least one inlet port (10,11) opening into a crankcase chamber (3) and at least one crankshaft member (4,5) rotatable within the crankshaft chamber (3). The opening and closing of the at least one inlet port (10,11) for allowing or blocking the admission of an air-fuel mixture to the crankcase chamber is controller by the shape of the peripheral surface of the or each crankshaft member (4,5) which rotates in front of the or each inlet port. The invention also relates to a two-stroke engine provided with such a crankcase.

Description

Improvements in or relating to two-stroke engines This invention relates to a crankcase of a two-stroke internal combustion engine and, in particular, to an arrangement for controlling the admission of an air and fuel mixture to the crankcase. The invention also relates to a two-stroke invention having such a crankcase.

Conventionally an air and fuel mixture is supplied to a crankcase of a crankcase-scavenged two-stroke engine via a crankcase valve. Typically such a crankcase valve comprises a reed valve which allows the air-fuel mixture to pass into the crankcase during the upward compression stroke but does not allow the mixture to pass out through the valve from the crankcase during the downward power stroke. Rotary valves are also known in which a plate is rotated with the crankshaft about the crankshaft axis to open and close a port for the admission of the fuel-air mixture to the crankcase, the port opening into the crankcase chamber in a direction generally parallel to the crankshaft axis.

The present invention seeks to provide a novel means of controlling the supply of a air-fuel mixture to a crankcase of a two-stroke engine which is reliable in operation.

According to one aspect of the present invention there is provided a crankcase for a two-stroke internal combustion engine comprising wall means defining a crankcase chamber, a crankshaft member which has a peripheral surface and which is rotatable about a crankshaft axis in a plane perpendicular to the crankshaft axis, and inlet means for supplying an air-fuel mixture to the crankcase chamber including an inlet port opening into the crankcase chamber and lying in the said plane, the crankshaft member being dimensioned and arranged so that the said peripheral surface cooperates with said inlet port to open and close the latter on rotation of the crankshaft member for controlling the admission of the air-fuel mixture into the crankcase chamber - 2 through said inlet port.

It has been found that it is not necessary to positively seal the inlet port to block the passage of the air-fuel mixture into the crankcase chamber. The peripheral surface of the crankshaft member passes in front of and obdurates the inlet port and it has been discovered that the flow of the air-fuel mixture through the inlet port is prevented even when the peripheral surface is spaced from the inlet port and the portion of the crankcase chamber immediately surrounding the inlet port. This spacing is not too critical and a typical spacing would be from 1 to 2 mm although larger distances can provide effective obduration of the inlet port. Thus the crankshaft member is able to freely rotate within the crankcase chamber without making sealing contact with the walls of the crankcase chamber and at the same time is able to control the flow of the air-fuel mixture to the crankcase chamber, and to prevent the flow of the introduced air-fuel mixture from the crankcase chamber, through the inlet port by the shape or contour of the peripheral surface of the crankshaft member.

Conveniently the crankshaft member is of generally circular cylindrical form but cut away over an arcuate portion of its periphery. In this case, the peripheral surface of the crankshaft member will typically be of generally circular cylindrical form over a first arcuate section and may be of any convenient non-cylindrical, e.g. generally frusto-conical form, over a cut away second arcuate section. When the first arcuate section of the crankshaft member is positioned in front of the inlet port the flow of the air-fuel mixture to or from the crankcase chamber via the inlet port is prevented. However, when the cut-away arcuate portion of the crankcase member is presented in front of the inlet port, flow of the air-fuel mixture into the crankcase chamber through the inlet port is possible.

Suitably two axially spaced apart crankshaft members, rotatable in different axially spaced apart and parallel planes of rotation, are provided between which a connecting rod is connected, at a position offset from the common rotation axis of the crankshaft members, via a crankpin connecting the crankshaft members. In this case, the inlet means conveniently comprises two separate axially spaced apart inlet ports each lying in a different one of said axially spaced apart planes of rotation, the opening and closing of the two inlet ports being individually controlled by the two crankshaft members. The second arcuate sections of the peripheral surfaces of the two crankshaft members are suitably generally inclined, e.g. of frusto-conical form, in opposite directions to each other so that, as the two admission ports open on rotation of the crankshaft members, the air-fuel mixture introduced through the inlet ports is directed into the space between the two axially spaced apart crankshaft members .

According to another aspect of the present invention there is provided a two-stroke internal combustion engine having a crankcase according to said one aspect of the present invention.

An embodiment of the invention will now be described, by way of example only, with particular reference to the accompanying drawings, in which: Figures 1 to 3 are schematic side sectional views of a crankcase according to the invention of a two-stroke engine showing crankcase members in three different positions, Figure 4 is a schematic view from above of the crankcase shown in Figure 2, and Figure 5 is a schematic sectional side view of a twostroke internal combustion engine incorporating a crankcase according to the present invention.

Figures 1 to 4 illustrate a crankcase, generally designated 1, for a crankcase-scavenged two-stroke internal combustion engine. The crankcase 1 has walls 2 defining a crankcase chamber 3 in which there is positioned a crankshaft comprising two crankshaft members 4 and 5 journalled for rotation about an axis A in bearings 6 and 7, respectively, in the walls 2 of the crankcase. A crankpin 8 of the crankshaft, which connects the two members 4 and 5 together, receives a lower end of a connecting rod 9 which is connected at its upper end to a piston (not shown).

A pair of axially spaced apart ports 10 and 11 are formed in the walls 2 for supplying a carburetted air-fuel mixture via a branched duct 12 to the crankcase chamber 3. In Figure 4, the branched duct 12 is shown detached from the walls 2 of the crankcase but it will be appreciated that in its fully assembled condition the duct 12 would be fixed, e.g. bolted, to the walls 2. The ports 10 and 11 are axially spaced apart the same distance as the crankshaft members 4 and 5 and lie in parallel planes of rotation of the crankshaft members 4 and 5, respectively, which are perpendicular to the axis A.

Each of the crankshaft members 4 (5) is suitably of generally circular cylindrical form having opposite end faces 4a and 4b (5a and 5b) and a peripheral surface 4c (5c) extending therebetween although it is cut away around a part of its periphery. In particular, each crankshaft member 4 (5) is generally of circular cylindrical form over a first arcuate section 4d (5d) extending through an angle of approximately 210° and is of non-cylindrical form, e.g. of generally frusto-conical form, over a second arcuate section 4e (5e) extending through an angle of approximately 150°. It is preferred that the cut-away second sections of the two crankshaft members 4 and 5 are sloped or angled in opposite directions so as to face towards each other for the reasons discussed hereinafter.

In use the crankshaft members 4 and 5 rotate in the crankcase chamber with their circumferential or peripheral surfaces 4c and 5c rotating in front of the ports 10 and 11. When the circular cylindrical first arcuate sections 4d and 5d move in front of the ports 10 and 11, respectively, they are spaced a small distance, typically from 1 to 2 mm (although this distance may not be critical and larger or smaller distances are possible to provide effective obduration of the inlet ports), from the walls of the crankcase chamber immediately surrounding the inlet ports 10 and 11 and block the flow of the carburetted air-fuel mixture into or out of the crankcase chamber 3 even though there is no actual sealing contact of these sections with the walls of the crankcase chamber. When the noncylindrical second arcuate sections 4e and 5e move in front of the ports 10 and 11, respectively, they do not completely block the flow of the carburetted air-fuel mixture into the crankcase chamber 3 and thus the air-fuel mixture is able to flow into the crankcase chamber 3. The oppositely sloping or angled cut away second arcuate sections encourage the two streams of air-fuel mixture entering into the crankcase chamber 3 through the ports 10 and 11 to be deflected towards each other into the space between the axially spaced apart crankshaft members 4 and 5. The air-fuel mixture therefore rapidly enters into this relatively large space and is not unduly obstructed.

It will be appreciated that the crankcase is designed so that in use the ports 10 and 11 are initially opened just after the piston (not shown) reaches its bottom position (see Figure 1) when the second arcuate sections 4e and 5e of the crankshaft members are positioned in front of the ports 10 and 11. As the piston attached to the connecting rod 9 moves upwards, the air-fuel mixture in the combustion chamber (not shown) above the piston is compressed and there is a slight decrease in pressure in the crankcase chamber 3 which causes a slight vacuum to be created therein. Thus the air-fuel mixture is sucked into the crankcase chamber during the piston upstroke through the now open ports 10 and 11 while the second arcuate sections 4e and 5e remain in front of the ports 10 and 11 (see Figures 2 and 4). Shortly before the piston reaches the end of the upstroke, the ports 10 and 11 are closed by the cylindrical first arcuate sections of the crankshaft members being positioned in front of them. At the top of the piston stroke, ignition takes place and the piston is driven downwards. During the downstroke of the piston, at least one exhaust port (not shown) from the combustion chamber is initially uncovered by the piston so that the exhaust gases can exit therefrom. As the piston moves further downwards, at least one intake port (not shown) to the combustion chamber is opened allowing the air-fuel mixture in the crankcase chamber 3 to be pushed through at least one transfer passage (not shown) through the intake port(s) into the combustion chamber. Throughout the downward movement of the piston the first arcuate sections 4d and 5d block the admission of the air-fuel mixture to, and the exit of the air-fuel mixture from, the crankcase chamber 3 via the inlet ports 10 and 11 (see Figure 3) The crankcase 1 described can be fitted to a cylinder block of, or form an integral part of, any conventional type of crankcase-scavenged two-stroke internal combustion engine. By way of example, Figure 5 illustrates a typical two-stroke engine 20 having a crankcase 1 as previously described. In Figure 5, the same reference numerals have been used to refer to parts of the crankcase which are similar to or correspond with parts of the crankcase shown in Figures 1 to 4 and these parts will not be described in further detail. The connecting rod 9 is connected to a piston 21 which slidably reciprocates in a cylinder 22 on turning of the crankshaft. A spark plug 23 is provided at the top of the combustion chamber 24 formed in the cylinder 22 above the piston 21 and at least one exhaust port 25, and at least one intake port 26, open into the cylinder 22. A transfer duct 27 or the like connects the intake port(s) 26 to the crankcase chamber 3. The manner in which the ports 25 and 26 are covered and uncovered by the reciprocating movement of the piston 21 is well known in the art and will not be further described. It will however be appreciated that Figure 5 is only schematic with parts not necessarily depicted with the correct dimensions.

Although it is preferred that a pair of inlet ports 10 5 and 11 are provided, it will be appreciated that only one inlet port need be provided which is opened and closed by one only of the two axially spaced apart crankshaft members. In the preferred case where two inlet ports are provided whose opening and closing are controlled by two crankshaft members, it is possible for the two ports to be angularly spaced from each other with the first and second arcuate sections of the two crankshaft similarly angularly offset.

Claims (5)

1. A crankcase for a two-stroke internal combustion engine comprising wall means defining a crankcase chamber, a crankshaft member which has a peripheral surface and which 5 is rotatable about a crankshaft axis in a plane perpendicular to the crankshaft axis, and inlet means for supplying an air-fuel mixture to the crankcase chamber including an inlet port opening into the crankcase chamber and lying in the said plane, the crankshaft member being 10 dimensioned and arranged so that the said peripheral surface cooperates with said inlet port to open and close the latter on rotation of the crankshaft member for controlling the admission of the air-fuel mixture into the crankcase chamber through said inlet port. 15

2. A crankcase according to claim 1, in which two axially spaced apart crankshaft members, rotatable in different axially spaced apart and parallel planes of rotation, are provided between which crankshaft members a piston rod is connected, at a position offset from the 20 common rotation axis of the crankshaft members, via a crankpin connecting the crankshaft members.

3. A crankcase according to claim 2, in which the inlet means comprises two separate axially spaced apart inlet ports each lying in a different one of said axially 25 spaced apart planes of rotation, the opening and closing of the two inlet ports being individually controlled by the two crankshaft members .

4. A crankcase according to any one of the preceding claims, in which the peripheral surface of the or each 30 crankshaft member is of generally circular cylindrical form over a first arcuate section and is of non-cylindrical form over a second arcuate section.

5. A two-stroke internal combustion engine including a crankcase according to any one of the preceding claims.