GB2145467A

GB2145467A - Two-stroke I.C. engine exhaust ports

Info

Publication number: GB2145467A
Application number: GB08320806A
Authority: GB
Inventors: Alexander Macfadzean; David William Blundell
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-08-02
Filing date: 1983-08-02
Publication date: 1985-03-27
Also published as: GB8320806D0

Abstract

A shutter 12 is arranged to move into an exhaust passage 11 adjacent an exhaust port 10 at or towards the end of the exhaust phase, from the opposite side to the piston 1. Loss of fresh mixture through the exhaust port is thus reduced, since the exhaust port 10 is closed more quickly than it would be by the piston alone. The shutter 12 is reciprocated by a small shaft 14 driven by the main crankshaft 4 at the same speed as the latter but with a phase lag of about a quarter of a revolution. In the position shown in Fig. 1 the piston 1 is being driven downwards and the shutter 12 is fully retracted within its slot 13. About half a revolution later (Fig. 5) the piston 1 is in a similar position, but moving upwards, and the shutter 12 has left the slot 13 and extends about halfway across the exhaust port 10. <IMAGE>

Description

SPECIFICATION Two-stroke engines This invention relates to two-stroke internal combustion engines comprising at least one working chamber connected to an exhaust passage by an exhaust port which is opened and closed by the moving piston which varies the volume of the working chamber.

In a two-stroke internal combustion engine according to the invention, a shutter is arranged to move into the exhaust passage adjacent the exhaust port at, or towards the end of the exhaust phase, from the opposite side to the piston, With this arrangement, the loss of fresh mixture through the exhaust port can be reduced since the exhaust port is closed more quickly then it would be by the piston alone.

Conveniently, the shutter in its fully extended position extends about half way across the exhaust passage, but should not offer any obstruction to the escape of the exhaust gases at the beginning of the exhaust phase when the piston uncovers the exhaust port.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figs. 1 to 6 show six stages in a cycle of movement of the piston of a two-stroke engine.

The drawings show only some of the components of a two-stroke engine. The engine has a piston 1 which is reciprocated within a cylinder 2 to vary a volume V of a combustion chamber defined by the piston 1, cylinder 2 and a cylinder head (not shown). The piston 1 is connected to an output shaft 4 in the usual manner by a crank 5 carrying a crank pin 6 on which is engaged the big end of a connecting rod 7 the little end of which is connected by a gudgeon pin 8 to the piston 1.

The cylinder wall 2 is formed with a transfer or inlet port 9 through which fresh charges of fuel and air mixture can be admitted into the space V when the admission port 9 is uncovered by the piston 1. The fresh charges are delivered under pressure generated for example by crank case compression by the underside of the piston 1 or by a supercharger or turbo charger.

The cylinder wall 2 is also formed with an exhaust port 10 leading into an exhaust passage 11. Communication between the combustion chamber space V and the exhaust passage 11 though the port 10 is controlled by the piston 1.

In accordance wth the invention, a shutter 1 2 is mounted for sliding movement in a slot 1 3 in the top wall of the exhaust passage 11 adjacent the exhaust port 10. The shutter 1 2 is reciprocated by a small crankshaft 1 4 having a crank 1 5 carrying a crank pin 1 6 connected by a connecting rod 1 7 to a pin 1 8 carried by the shutter 1 2. The shaft 1 4 is driven from the shaft 4 at the same speed as the latter, for example by means of a toothed belt (not shown), but with a phase lag of about a quarter of a revolution (in the case of 96,,).

In the position shown in Fig. 1, the piston 1 is being driven downwards by the gas pressure in the space V as a result of combustion of a previously admitted and compressed charge. The piston 1 is on the point of beginning to uncover the upper part of the exhaust port 10. The crank pin 1 6 is in the uppermost portion of its travel and the shutter 1 2 is accordingly fully retracted into its slot 13.

In the position shown in Fig. 2, the crank pin 6 has advanced though 32 and the downwardly moving piston has uncovered about half the exhaust port 10. The shutter 1 2 is still substantially completely within its slot 1 3 offering no restriction during the blow down phase and is thus shielded from the high temperature of the initial exhaust gas flow into the exhaust passage 11. The piston 1 is also on the point of starting to uncover the transfer port 9 to release a further charge into the space V.

Fig. 3 shows the crankshaft and piston at bottom dead centre. The transfer port 9 is fully uncovered and the shutter 1 2 is moving downwards in the upper part of the exhaust passage 11 whilst still leaving the latter sufficiently open for scavenging.

In the position shown in Fig. 4, the crankshaft has advanced 68 after bottom dead centre and the piston 1 has just closed the transfer port 9. Although the piston has only covered about half of the exhaust port 10, the shutter 1 2 is near its lowermost position and the piston and shutter together has nearly closed the exahust port. Thus, the escape of the fresh charge into the exhaust passage is effectively prevented but there is a small amount of exhaust port area still open. This will have prevented sufficient increase in pressure during upward movement of the piston to cause reverse flow of charge into the transfer port.

In the position shown in Fig. 5, the crank pin 6 is at 100" after bottom dead centre and the piston 1 has just covered the exhaust port 10. Thus, between the position shown in Figs. 4 and 5, the shutter 1 2 has been in its almost fully extended position and, in conjunction with the upwardly moving piston, has helped to prevent loss of charge into the exhaust passage 11.

Finally, Fig. 6 shows the position in which the crank pin and piston are at top dead centre. The new charge has been compressed and ignited ready for the next power stroke.

The shutter 1 2 is now about half way on its upward journey ino the slot 1 3. The power stroke continues to the position shown in Fig.

1, by which time the shutter 1 2 is effectively fully retracted into its slot 1 3. The cycle is then repeated.

Claims

1. A two-stroke internal combustion engine comprising at least one working chamber connected to an exhaust passage by an exhaust port which is opened and closed by a moving piston which varies the volume of the working chamber, and further comprising a shutter arranged to move into the exhaust passage adjacent the exhaust port at or towards the end of the exhaust phase, from the opposite side to the piston.

2. An engine according to claim 1, wherein the shutter in its fully extended position extends about half way across the exhaust passage.

3. An engine according to claim 1 or 2, wherein the shutter is arranged to offer substnatially no obstruction to the escape of the exhaust gases at the beginning of the exhaust phase when the piston uncovers the exhaust port.

4. An engine according to any of claims 1 to 3, wherein the shutter is mounted for sliding movement in a slot in the exhaust passage adjacent the exhaust port.

5. A two-stroke internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.