GB2272941A

GB2272941A - Two-stroke engine.

Info

Publication number: GB2272941A
Application number: GB9312652A
Authority: GB
Inventors: Reginald Arthur Piper; Eric Arthur Shrubb
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-06-18
Filing date: 1993-06-18
Publication date: 1994-06-01
Also published as: GB9404036D0; GB2279110A; GB2279110B; GB9312652D0

Abstract

A plunger 8 separates inlet air or mixture charge under pressure from exhaust gases except when located adjacent the cylinder head and bridged by recesses or a groove 10 in the cylinder wall. The valve controlled ports 12 may be charge inlet ports and the ports 6 exhaust ports or vice versa. Plunger movement in response to charge supply pressure or piston movement is constrained by a pin 28 which is positioned by a linkage 16 operated by valve gear cams 23, 25 and co-operates with the plunger stem. A pre-combustion chamber (32, Fig. 2) receiving fuel from an injector (34) may open to the groove 10. <IMAGE>

Description

An Engine The present invention relates to an engine, and particularly to a two-stroke engine for use for example in motor vehicles, boats, lawn mowers or the like.

Two-stroke engines have been well known for use in motor vehicles and other applications for many years. They produce a power stroke from each cylinder with every revolution of the crankshaft and so potentially have a higher power to weight ratio than a conventional four-stroke engine, which only produces a power stroke every other revolution.

In one form of construction a two-stroke engine comprises a cylinder which is connected to a crankcase containing a crankshaft. The cylinder is provided with inlet ports for taking in fuel, and outlet ports for expelling exhaust gases. A piston connected to the crankshaft is movable within the cylinder to open or close the ports, and a fan is provided in one or more of the inlet ports to deliver fuel into the cylinder.

When the piston is at its lowest position in the cylinder it is below the inlet ports and the outlet ports. The fan directs fuel into the cylinder above the piston. As the piston rises from its lowest position it obturates the inlet and outlet ports and compresses the fuel. The fuel is made to combust, by provision of a spark in the case of a petrol engine, or as a result of heat and compression in a diesel engine. The gas expansion caused by the combustion forces the piston back down to its lowest position and generates power. This downward movement of the piston first uncovers the exhaust ports, allowing some of the pressurised exhaust gases to leave. The piston then opens up the inlet ports, and remaining exhaust gases are flushed out by fuel delivered by the fan through the inlet ports. Rising of the piston initiates a repeat of the cycle.

A problem with such an engine is that the fan adds to the cost of manufacture, and fuel is lost through the exhaust port when the exhaust gases are being flushed out. This loss of fuel reduces the engine efficiency, and increases environmental pollution.

In an alternative construction, the fuel inlet ports communicate directly with the interior of the crankcase, and fuel is drawn into the crankcase by suction when the piston rises. When the piston returns to its lowest position, it compresses the fuel underneath the piston head, which fuel then passes through a transfer duct from the crankcase into the cylinder to flush out exhaust gases as described above.

This construction eliminates the need for a fan, but it requires hermetic sealing of the crankcase. Fuel is lost in the flushing out of the exhaust gases, and a further problem is that lubrication provided for the crankshaft is liable to end up in the cylinder during combustion. This often produces a characteristic blue smoke that does little for the environment.

A further problem with the constructions described above is that the need for efficient circulation of fuel and exhaust gases means that the ports in the cylinder wall must occupy as large an area as possible. Vertical height for a port orifice is limited by the operation of the engine, so it is necessary to arrange a number of ports around the periphery of the cylinder. A large number of ports increases the likelihood of bore wear and, in a multi-cylinder engine, increases the space needed between cylinders which limits the number of cylinders that can be accommodated in a practically useful engine.

A further limitation with such two-stroke engines is that because the exhaust port is closed after the inlet port it is not possible to turbocharge the engine to improve performance.

According to the invention there is provided a two-stroke engine, the engine including one or more cylinders and, for each cylinder, a piston connected to a crankshaft, and a plunger, the piston and the plunger each being a close fit for the internal circumference of the cylinder body, and the cylinder being provided with one or more lower ports and one or more upper ports, and means for providing fluid communication between the upper and lower surfaces of the plunger when the plunger is located at or adjacent to the top of the cylinder, and wherein the piston and the plunger are independently axially movable within the cylinder.

References to "upper" and "lower" in this specification refer to an engine orientation where the cylinder lies above the axis of the crankshaft.

In a preferred embodiment the upper ports are provided with valves to control the inflow of fuel or air, or the outflow of exhaust gases. In a particularly preferred embodiment the valves used are poppet valves.

The fluid communication means may simply comprise one or more transfer ducts in the form of hollows in the internal wall of the cylinder. Additionally or alternatively the fluid communication means may comprise one or more precombustion chambers to improve fuel mixing or vaporisation prior to combustion. When the engine is a petrol engine, the transfer duct or the pre-combustion chamber is preferably provided with means for igniting the fuel. Any suitable ignition means may be used; for example a spark plug. When the engine is a diesel engine the transfer duct may be connected to a pre-combustion chamber provided with a diesel injector and preferably with a glow plug for cold starting.

When the engine is in use, the piston is operatively connected to a crankshaft by well known methods, and the plunger is operatively connected to one or more cams on an overhead camshaft.

The plunger preferably comprises a circular head which is fixed at or adjacent its centre to a hollow tube with an axis normal to the plane of the head, the tube being slidably mounted on a guide pin. By connecting the guide pin to one or more overhead cams on a camshaft, the plunger may be made to move up or down within the cylinder in response to the movement of the camshaft. In a preferred embodiment the controlling cams are mounted on twin camshafts arranged in a manner similar to that used for operating the valves of an orthodox four-stroke engine.

In a preferred embodiment the plunger is resiliently biased against the guide pin so that the plunger tends to return to a rest position after being retracted or extended. The biasing means may comprise a spring or, preferably, trapped air which provides a pneumatic restoring force.

The engine may be used in two alternative operative modes.

In a first mode the lower ports are used for the expulsion of exhaust gases, and the upper ports are inlet ports for charging the cylinder with fuel and/or air. In an alternative mode of operation the roles of the ports are reversed, with the upper ports being used for the expulsion of exhaust gases, and the lower ports used for charging the cylinder with fuel and/or air.

In both modes of operation the plunger acts to separate the exhaust gases from the incoming charge of fuel, thus reducing the emission of uncombusted fuel to the atmosphere.

Fuel may be supplied to each cylinder by pressure induction, fuel injection, or any other suitable means.

To aid understanding, the invention will now be further described, by way of example, with reference to the following drawings in which: Figure 1 is a part section through a cylinder of one embodiment of an engine in accordance with the invention; Figure 2 is a part section through a cylinder of an engine in accordance with an alternative embodiment of the invention; and Figure 3 is a cross sectional view of a plunger and guide pin arrangement in accordance with the invention.

Figure 1 shows a cylinder of an engine according to the invention. The cylinder body 2 has within it a piston 4 and a plunger 8. The piston 4 is connected to a connecting rod 26 journalled conventionally on a crankshaft 27, and a hollow shaft of the plunger 8 is mounted on a guide pin 28 and is slidable within the cylindrical bore of a housing 18.

Transfer ducts 10, which may be discrete hollows in the cylinder wall or may be joined as a groove around the internal circumference of the cylinder, are located in the wall of the cylinder body 2. A spark plug (not shown) is located in each transfer duct 10. If the transfer ducts 10 are connected to pre-combustion chambers the spark plugs would usually be located in those pre-combustion chambers.

The cylinder has two lower ports 6 and two upper ports 12, the upper ports being opened or closed by poppet valves 14.

The timing of opening of each poppet valve 14 is controlled by means of cams 22 and 24. Auxiliary cams 23 and 25 act on cam followers 35 and 37 to move a rocker bar 20. The rocker bar 20 is connected to the guide pin 28 by a linkage 16 so that vertical movement of the guide pin 28 and the plunger 8 is controlled by the overhead cam mechanism. The cams 22 and 23 are mounted on a common camshaft 31, and the cams 34 and 25 are mounted on a common camshaft 33. A slot 30 is provided in the housing 18 and in the plunger 8 through which the linkage 16 is connected to the guide pin 28. The top and bottom edges of the slot 30 determine the upper and lower boundaries for the plunger 8 for any given position of the guide pin 28.

In one mode of operation, a cycle of the engine operation is as follows. With the piston 4 passing top dead centre (TDC), and ignition having taken place, the piston 4 is forced down the cylinder, generating power and moving the connecting rod 26. When the crankshaft 27 moves to a position approximately 600 before bottom dead centre (BDC) the piston uncovers the lower ports 6, which here function as exhaust ports. Also at this point, the poppet valves 14 open and the plunger 8 starts to move down the cylinder from its retracted position. Movement of the plunger 8 is facilitated by compressed air which has been built up between the plunger 8 and the base of the guide pin 28.

Continued movement of the crankshaft 27 causes the piston 4 to move past BDC and the plunger 8 moves to a point as near to BDC as possible. This movement of the plunger 8 causes filling of the space above the head of the plunger 8 with a fresh charge of fuel, drawn in through the upper ports 12.

At the same time the piston 4 and the plunger 8 co-operate to discharge the exhaust gases through the lower ports 6.

After the crankshaft 27 has moved to a position past BDC it causes the piston 4 to make contact with the plunger 8, which may also be given an initial upward movement by the action of the cam 25. At this moment the poppet valves 14 close and the piston 4 obturates the lower ports, causing initiation of compression of the fuel. The crankshaft 27 now moves to TDC, moving the piston 4 and the plunger 8 together to the cylinder head, when ignition of the fuel takes place. The final upward movement of the plunger 8 brings it into contact with the cylinder head and permits expanding gases caused by the combustion to be transferred to the underside of the plunger 8 via the transfer ducts 10.

The pressure of the expanding gases now forces the piston 4 back down the cylinder, starting the cycle again.

The plunger 8 is thus driven from BDC to TDC by the piston 4 on its compression stroke and from TDC to BDC by air compressed during the compression stroke. The cams 23 and 25 operate via the cam followers 35 and 37 and the rocker bar 20 to determine the position of the plunger 8 at all times, but without subjecting the plunger 8 to excessive stress and wear by providing power to reciprocate the plunger 8.

There are therefore three distinct phases of operation for every one revolution of the crankshaft 27 (taking TDC as oo) 1 The power stroke, from TDC to 1200; 2 The charge and discharge stroke, from 1200 to 2400, where the plunger moves from TDC to its lowest position; 3 The compression and firing stroke, from 2400 to 3600, where the piston makes contact with the plunger and pushes it up to the cylinder head.

The engine shown in .Figure 1 may also be operated in an alternative mode, by reversing the roles of the upper and lower ports, as follows. With the crankshaft 27 passing TDC the piston 4 and the plunger 8 descend together, combustion having taken place above the plunger 8. This movement creates a partial vacuum between the plunger 8 and the guide pin 28. When the crank 26 arrives at 1200 the downward movement of the plunger is arrested by the action of the cams 23, 25. The piston 4 uncovers the lower ports 6, here acting as fuel inlet ports, and the poppet valves 14 open.

The crankshaft 27 moves to 2400 and the plunger 8 is returned to its uppermost position by the action of the cams 23, 25 and the pressure of incoming fuel. The movement of the plunger 8 in this phase discharges exhaust gases through the upper ports 12. The final movement of the crank from 2400 to 3600 is the compression and firing stroke; the poppet valves 14 close off the upper ports 12 and fuel passes through the transfer ducts 10. Combustion takes place above the plunger 8 and the cycle starts again.

Figure 2 shows an alternative embodiment of the invention, in which the transfer port 10 communicates with a precombustion chamber 30. This example shows a Ricardo Comet (TM) pre-chamber for use with a diesel engine, but any other suitable pre-chamber may be used. It comprises. a swirl chamber 32 into which is fitted a fuel injector 34 and a glow plug 36 to heat up the fuel for cold starting of the engine. A pre-combustion chamber for a petrol engine would be provided with a spark plug.

Figure 3 shows how the plunger 8 is mounted around the guide pin 28 within the housing 18. The housing 18 has a port 40 which communicates with the atmosphere via a connector 42.

The hollow stem of the plunger 8 which fits around the guide pin 28 is provided with a hole 44 in its side. When the plunger 8 is extended relative to the guide pin 28 so that the hole 44 is in registration with the port 40 air can pass in either direction and the air in the space 38 between the plunger 8 and the guide pin 28 is at the same pressure as the surrounding atmosphere. When the plunger 8 is pushed upwards the hole 44 is moved out of register with the port 40 and the air in the space 38 becomes compressed. When the guide pin 28 is moved downwards, thus freeing the plunger 8 to move, this compressed air provides a motivating force to move the plunger 8 downwards.

The invention as described may be used in conjunction with a crankshaft, crankcase and lubricating system similar to those of an orthodox four-stroke engine. It may also be used with a twin overhead camshaft, four valve per cylinder arrangement similar to that in general use today.

Because the plunger separates incoming fuel from exhaust gases, loss of fuel through the exhaust ports is reduced compared with a conventional two-stroke engine.

The engine may be constructed by casting two exhaust ports into both sides of the cylinder block which, together with four inlet ports with poppet valves in the cylinder head, permits more efficient breathing.

Because the inlet and exhaust ports are fully segregated by the plunger, supercharging of the engine is possible using turbo or mechanical blowers.

By locating the exhaust ports in the cylinder block below the cylinder head, excessive heat build up in the region of the cylinder head may be reduced. With two exhaust ports on each side of the cylinder block, there is no need for spaces between the cylinders in the engine except to provide for cooling. The engine may therefore retain the same overall length as a comparable four-stroke engine.

Claims

1 A two-stroke engine, the engine including one or more cylinders and, for each cylinder, a piston connected to a crankshaft, and a plunger, the piston and the plunger each being a close fit for the internal circumference of the cylinder body, and the cylinder being provided with one or more lower ports and one or more upper ports, and means for providing fluid communication between the upper and lower surfaces of the plunger when the plunger is located at or adjacent to the top of the cylinder, and wherein the piston and the plunger are independently axially movable within the cylinder.

2 An engine as claimed in Claim 1, wherein the upper ports are provided with valves to control the inflow of fuel or air, or the outflow of exhaust gases.

3 An engine as claimed in Claim 2, wherein the valves are poppet valves.

4 An engine as claimed in any one of the preceding claims, wherein the fluid communication means comprise one or more hollows in the internal wall of the cylinder.

5 An engine as claimed in any one of the preceding claims, wherein the fluid communication means comprise at least one pre-combustion chamber to improve fuel mixing or vaporisation prior to combustion.

6 An engine as claimed in any one of the preceding claims, wherein the plunger is operatively connected to one or more cams on an overhead camshaft.

7 An engine as claimed in any one of the preceding claims, wherein the plunger comprises a circular head which is fixed at or adjacent its centre to a hollow tube with an axis normal to the plane of the head, and wherein the tube is is slidably mounted on a guide pin.

8 An engine as claimed in Claim 7, wherein the guide pin is connected to one or more cams on an overhead camshaft.

9 An engine as claimed in Claim 6 or Claim 8, wherein the controlling cams are mounted on twin camshafts arranged in a manner similar to that used for operating the valves on an orthodox four-stroke engine.

10 An engine as claimed in any one of Claims i to 9, wherein the plunger is resiliently biased against the guide pin so that the plunger tends to return to a rest position after being retracted or extended.

11 An engine as claimed in Claim 10, wherein the biasing means comprises trapped air which provides a pneumatic restoring force.

12 An engine substantially as hereinbefore described with reference to and as shown in any one of the drawings.