GB1600049A

GB1600049A - Twostroke engine wiht layered charge

Info

Publication number: GB1600049A
Application number: GB1573078A
Authority: GB
Original assignee: Piaggio and C SpA
Current assignee: Piaggio and C SpA
Priority date: 1977-07-21
Filing date: 1978-04-20
Publication date: 1981-10-14
Also published as: FR2398181B1; JPS5423807A; IT1081148B; NL7805349A; DE2817722A1; FR2398181A1

Description

(54) TWO-STROKE ENGINE WITH LAYERED CHARGE (71) We, PIAGGIO & C. S.P.A. a company organized under law of the Italian Republic of Via A. Cecchi 6 - Genova, Italy do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to two-stroke cycle internal combustion engines, in particular those having a pair of cylinders which communicate with a common combustion chamber.

In accordance with the invention there is provided a two-stroke internal combustion engine comprising a pair of cylinders, each of which communicates with a common combustion chamber, a pair of pistons, each of which is slidingly housed in one of the cylinders and faces the combustion chamber, transmission means connecting the pistons together, a compression crankcase communicating with each cylinder through a transfer duct, means for feeding a first one of the crankcases with a fuel-to-air mixture, means for feeding the second crankcase with air, or a mixture having a lower fuel-to-air ratio than that fed to the first crankcase, and at least one exhaust duct opening from the cylinder which communicates with the second crankcase.

One of the crankcases may be fed with air since this permits the formation of a layered charge in the combustion chamber of the engine.

The invention will now be described in detail with reference to the accompanying drawing which is a sectional view of a twostroke internal combustion engine in accordance with the invention.

As shown in the drawing, opposed pistons 1 and 2 are slidably mounted in cylinders 3 and 4 respectively. In this particular embodiment, the two cylinders are aligned along a common axis and are of different diameters.

The pistons 1 and 2 drive crankshaft 7 and 8 by means of connecting rods 5 and 6. The two crankshafts 7 and 8 are connected to one another by means of gears and a toothed-belt 9.

In the wall of cylinder 3 are formed transfer ports 10 which open into passages leading to a crankcase 11 which houses one end of the connecting rod 5. As the piston 1 moves away from the crankcase 11 fuel mixture is drawn in through a carburettor 20 and as the piston 1 returns, the mixture is compressed until the ports 10 are opened by the piston 1 and the mixture escapes into a combustion chamber 21 which is common to both cylinders 3 and 4.

The exhaust port 12 of the engine is formed in the wall of the other cylinder 4, as are a second set of transfer ports 13 through which the cylinder 4 communicates with crankcase 14 surrounding the end of the connecting rod 6. Movement of the piston 2 draws air into the crankcase 14 through an air filter 15 and the air is compressed as the piston 2 moves toward the crankcase 14.

The combustion chamber 21, which receives spark plugs 16, has a volume which is dependent on the minimum distance between the pistons 1 and 2. The movement of the two pistons with respect to one another may be timed to achieve the best possible performance.

In this embodiment, the combustion chamber 21 is cylindrical in shape and has the same diameter as the cylinder 3. The chamber 21 communicates with the cylinders 3 and 4 by means of ducts 18 and 19.

The shape of the combustion chamber may be different, for example, it may spherical so as to obtain the maximum thermal efficiency or to minimise the pollutants present in the exhaust gases. In the embodiment shown in the drawing the chamber is formed by a separate element 17 but it might equally well be formed integrally with the cylinders 3 and 4.

The engine operates similarly to previously-known two-stroke engines having a compression crankcase associated with each piston. However, scavenging is effected, for the most part, with noncarburetted air coming in through the transfer ports 13 and which is introduced into the combustion chamber 21 while the exhaust port 12 is open. Also, the charge of the combustion chamber 21 results from the combination of a rich air and fuel mixture entering through the ports 10 and noncarburetted air entering from the ports 13.

The transfer ports 10 and 13 may be opened simultaneously or the ports 13 may be opened before or after the ports 10. The timing is chosen to produce the best possible compromise between economy of fuel, reduced production of pollutants and increased delivered power. Other factors arising from the geometry of the engine also affect the choice of timing, for example, the position of the exhaust port 12, the ratio of the sizes of the two cylinders 3 and 4, and the position of the sparking plugs.

One advantage of an engine constructed in accordance with the invention is that it is possible to form a layered or gradualcombustion charge, that is, a charge in which the mixture is rich close to the sparking plugs and leaner in the vicinity of the exhaust port. Ignition of the charge begins in the region of the combustion chamber where the mixture is richest and proceeds until ignition is completed in the portion of the chamber, close to the exhaust port 12, which is fed with air only. In addition, air entering through ports 13 tends to prevent the fuel and air mixture, entering through the ports 10, escaping through the exhaust port 12, thus economising on fuel.

The invention can be applied to both controlled ignition and spontaneous ignition engines and to engines which do not have opposed coaxial cylinders. The axes of the cylinders may be parallel or perpendicular to one another or at any angle in between.

The strokes of the pistons can be in such a ratio to one another that inertial forces are minimised and the intake of an air and fuel mixture and of air into the crankcases may be controlled by the pistons, the cranks or by automatic valves.

WHAT WE CLAIM IS: 1. A two-stroke internal combustion engine comprising a pair of cylinders, each of which communicates with a common combustion chamber, a pair of pistons, each of which is slidingly housed in one of the cylinders and faces the combustion camber, transmission means connecting the pistons together, a compression crankcase communicating with each cylinder through a transfer duct, means for feeding a first one of the crankcases with a fuel and air mixture, means for feeding the second crankcase with air, or a mixture having a lower fuel-to-air ratio than that fed to the first crankcase, and at least one exhaust duct opening from the cylinder which communicates with the second crankcase.

2. An engine according to claim 1, wherein the opening and closing of the transfer ducts is controlled by the pistons in the cylinders.

3. An engine according to claim 2, wherein the exhaust duct is controlled by the piston housed in the cylinder communicating with the crankcase fed with the mixture having the lower fuel-to-air ratio.

4. An engine according to any preceding claim, wherein said combustion chamber is formed by a separate hollow body inserted between said cylinders.

5. The engine according to any preceding claim, wherein at least one spark plug is located in the combustion chamber.

6. An engine according to any of the preceding claims wherein the two cylinders are arranged on a common axis.

7. An engine substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. combustion chamber 21 results from the combination of a rich air and fuel mixture entering through the ports 10 and noncarburetted air entering from the ports 13. The transfer ports 10 and 13 may be opened simultaneously or the ports 13 may be opened before or after the ports 10. The timing is chosen to produce the best possible compromise between economy of fuel, reduced production of pollutants and increased delivered power. Other factors arising from the geometry of the engine also affect the choice of timing, for example, the position of the exhaust port 12, the ratio of the sizes of the two cylinders 3 and 4, and the position of the sparking plugs. One advantage of an engine constructed in accordance with the invention is that it is possible to form a layered or gradualcombustion charge, that is, a charge in which the mixture is rich close to the sparking plugs and leaner in the vicinity of the exhaust port. Ignition of the charge begins in the region of the combustion chamber where the mixture is richest and proceeds until ignition is completed in the portion of the chamber, close to the exhaust port 12, which is fed with air only. In addition, air entering through ports 13 tends to prevent the fuel and air mixture, entering through the ports 10, escaping through the exhaust port 12, thus economising on fuel. The invention can be applied to both controlled ignition and spontaneous ignition engines and to engines which do not have opposed coaxial cylinders. The axes of the cylinders may be parallel or perpendicular to one another or at any angle in between. The strokes of the pistons can be in such a ratio to one another that inertial forces are minimised and the intake of an air and fuel mixture and of air into the crankcases may be controlled by the pistons, the cranks or by automatic valves. WHAT WE CLAIM IS:

1. A two-stroke internal combustion engine comprising a pair of cylinders, each of which communicates with a common combustion chamber, a pair of pistons, each of which is slidingly housed in one of the cylinders and faces the combustion camber, transmission means connecting the pistons together, a compression crankcase communicating with each cylinder through a transfer duct, means for feeding a first one of the crankcases with a fuel and air mixture, means for feeding the second crankcase with air, or a mixture having a lower fuel-to-air ratio than that fed to the first crankcase, and at least one exhaust duct opening from the cylinder which communicates with the second crankcase.