GB2279110A - Two-stroke engine - Google Patents

Description

An Engine 2279110 The present invention relates to an engine, and

particularly to a two- stroke engine for use for example in motor 5 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 20 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 30 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, 35 increases the space needed between cylinders which limits SP1253.A2 1 March 1994 3 - the number of cylinders that can be accommodated in a practically useful engine.

A further limitation with such two-stroke engines is that 5 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 10 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 Oupper" 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 35 prior to combustion. When the engine is a petrol engine, SP1253.A2 1 March 1994 the transfer duct or the pre-combustion chamber is preferably provided with means for igniting the fuel. Any suitable ignition means may be used; f or 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.

The cylinder body preferably includes a cylinder liner in 10 which the fluid communication means are provided. However the invention may also be used with a cylinder body of unitary construction.

When the engine is in use, the piston is operatively 15 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 20 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 plunger 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.

SP1253.Al 1 March 1994 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.

The fuel may be supplied indirectly, by means of a precombustion swirl chamber for example, or the fuel may be supplied by direct injection to the combustion chamber.

In a preferred embodiment the fuel is supplied via a conduit in the guide pin and injected directly into the combustion chamber above the plunger. The guide pin here functions as a small diameter injector with a long stem. Fuel may be 25 injected via nozzle holes arranged around the circumference of the lower part of the guide pin. The stem of the plunger is provided with slots or holes through which the fuel is injerpted into the combustion chamber formed between the upper surface of the plunger and the top of the cylinder 30 when the nozzle holes are in register with the slots or holes in the plunger. The slots or holes in the plunger need to have sufficient vertical height to permit spray from the nozzle holes to enter the combustion chamber at commencement of injection and for the duration of the 35 injection. Although it is preferred to inject fuel through SP1253.Al 1 March 1994 the guide pin, it is also possible to inject fuel through one or more separate injectors spaced around the combustion chamber.

In the direct injection embodiment it is preferred that the upper valves, here functioning to open or close exhaust ports, are arranged to present a substantially flat surface to the roof of the combustion chamber. This may be achieved, for example, by the use of vertically mounted poppet valves. Valves may be operated by finger rockers with camshafts mounted in conventional positions. It is also preferred that the upper surface of the plunger is shaped so as to provide a flat bottomed or curved combustion chamber. In its simplest form, this shaping may take the form of a ridge which is provided at the periphery of the upper surface of the circular head of the plunger.

The total swept volume of space beneath the plunger will be greater than that above the plunger because of the presence of the stem of the plunger in the space above the plunger.

This volume differential may assist the achievement of the high compression ratios necessary for diesel engines. The diameter of the stem of the plunger may be chosen so as to achieve a preferred compression ratio for a given volume of combustion chamber.

The direct injection embodiment is particularly of use for diesel fuel injection because the achievement of efficient combustion with direct injection diesel fuel has long been a problem in the art. Turbocharging or supercharging is also possible with this direct diesel injection. However the direct injection embodiment could also be used with petrol or other fuels, and in this case the combustion chamber is provided with appropriate spark producing means.

SP1253.Al 1 March 1994 In order to improve the mixing of fuel and air in the combustion chamber the air may be made to swirl or form a vortex. This may be achieved by the use of transfer ducts which are angled with respect to the longitudinal axis of the cylinder. The tendency of the air to swirl may optionally be aided by the provision of tangentially angled flutes in the ridge at the top surface of the circular head of the plunger, so that air which is f orced out of the transfer ducts is directed over or through the flutes.

The optimum angle for the transfer ducts and for the flutes will depend upon the size and nature of the engine, but the optimum angles may readily be determined by experiment for a given engine. Experimentation will be facilitated if the transf er ducts are formed in a cylinder liner, so that cylinder liners with differently angled transfer ducts may be made and interchanged within a cylinder. Such experimentation may be further facilitated by the use of a cylinder liner which is divided into an upper part and a lower part. Different upper parts may then be formed with differently angled transfer ducts, and each may in turn be tested by seating it on the lower part and evaluating the engine performance as a function of transfer duct angle. In similar manner different tangential angles or different shapes of flute may be tested in the plunger.

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; SP1253.Al 1 March 1994 Figure 2 is a part section through a cylinder of an engine in accordance with an alternative embodiment of the invention; Figure 3 is a cross sectional view of a plunger and guide pin arrangement in accordance with one embodiment of the invention; Figure 4 is a part sectional view through a cylinder of a direct fuel injection engine in accordance with a preferred embodiment of the invention, during various stages in the revolution of the crankshaft; Figure 5 is a perspective view of part of the plunger from the embodiment shown in Figure 4; Figure 6 is a section through the plunger shown in Figure 5; Figure 7 is a perspective view of the upper part of a cylinder body in accordance with an alternative embodiment of the invention; and Figure 8 is a top view of a plunger in accordance with an alternative embodiment of 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 30 26 journalled conventionally on a crankshaft 27, and a hollow stem 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 35 internal circumference of the cylinder, are located in the SP1253.Al 1 March 1994 - 9 wall of the cylinder body 2. A spark plug (not shown) is located in each transfer duct 10. If the transfer ducts jo 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 plunger 8 by a linkage 16 so that vertical movement of 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 24 and 25 are mounted on a common camshaft 33. A slot 30 is provided in the housing 18 through which the linkage 16 is connected to the plunger 8. The top and bottom edges of the slot 30 determine the upper and lower boundaries for the plunger 8.

In one mode of operation, a cycle of the engine operation is 20 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.

SP1253.Al 1 March 1994 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 00):

1 The power stroke, from TDC to 1200; SP1253.Al 1 March 1994 2 The charge and discharge stroke, from 1200 to 2400p 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 24011 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 f inal movement of the crank from 25 24011 to 36011 is the compression and firing stroke; the poppet valves 14 close of f 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 duct 10 communicates with a precombustion chamber 30. This example shows a Ricardo Comet (TN) pre-chamber for use with a diesel engine, but any other suitable pre-chamber may be used. It comprises a swirl 35 chamber 32 into which is fitted a fuel injector 34 and a SP1253.Al 1 March 1994 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 15 upwards the hole 44 is moved out of register with the port 40 and the air in the space 38 becomes compressed. When the cams allow the plunger 8 to move, this compressed air provides a motivating force to move the plunger 8 downwards.

Referring now to Figure 4, this shows a modification of the invention which allows direct fuel injection, for example diesel injection. The various stages of a revolution of the crankshaft 127 are shown. Parts corresponding to parts shown in Figure 1 are here labelled with the same part 25 number incremented by 100.

The engine is basically similar to the engine shown in Figure 1, but the top surface of the plunger 108 is curved so as to form a generally toroidal combustion chamber in 30 cooperation with the inner top surface of the cylinder 102.

The poppet valves 114 are now vertically mounted so that the inner top surface of the cylinder (and of the combustion chamber) is generally flat. The guide pin 128 has a conduit running through it which allows diesel or other fuel to be 35 injected under pressure through nozzle holes in the side of SP1253.Al 1 March 1994 the guide pin and through slots in the side of the stem of the plunger.

In Figure 4a the plunger 108 is in the upper position and 5 all valves are closed or closing. Air which has entered through the lower ports 106 is about to be compressed by the upward travel of the piston 104. The passage of air is here denoted by arrows. Air passes via the transfer ducts 110 into the space above the plunger 108, which will be the combustion chamber.

In Figure 4b the piston 104 has travelled almost to its uppermost position, and air is compressed above the plunger 108. Fuel injection through the guide pin 128 into the combustion chamber has commenced and ignition has commenced.

Because fuel is injected centrally via the guide pin 128 it can be injected substantially evenly in different directions. This may help to produce a more controlled combustion and reduce fknockf.

In Figure 4c the piston 104 is in its uppermost position and the fuellair mixture is at its maximum compression. Combustion of the fuellair mixture takes place. The piston 104 and the plunger 108 are thrusted down the cylinder 102 by the expanding gases from the combustion, as shown in Figure 4d. A partial vacuum is created in the space 138 between the guide pin 128 and the plunger 108. The plunger 108 and piston 104 are now approximately 600 before BDC at the end of the power stroke. All valves are about to open, after which the plunger will be returned to its upper position by the action of the cam linkages similar to that shown in Figure 1. The upward movement of the plunger 108 may be motivated entirely by the cam linkage, or it may receive an additional restoring force from the partial vacuum in the space 138.

SP1253.Al 1 March 1994 In Figure 4e the piston 104 is at BDC and the plunger los is in an intermediate position. The poppet valves 114 are opened by the action of the cams 122 and 124, and exhaust gases are vented through the upper ports 112.

Simultaneously air inhalation takes place through the lower ports 106. The crankshaft 127 moves past BDC and the cycle starts again. Incoming air and exhaust gases are kept separate by the plunger 108, and each revolution of the crankshaft produces a power stroke.

The arrangement for injecting fuel is shown in more detail in Figures 5 and 6. The plunger 108 is provided with a plurality of vertical slots 140 spaced around the stem of the plunger 108. The guide pin 128 is provided with a plurality of nozzle holes 146 which are in register with the slots 140 when the plunger 108 is in or close to its uppermost position. The guide pin 128 is hollow, and it has a needle 142 seated inside and coaxial with the guide pin. The needle 142 is spring loaded so as to press against the bottom of the guide pin 128 and f orm a seal when fuel is not being injected. Fuel is injected between the needle 142 and the inside of the guide pin 128, which is here acting as a conventional fuel injector. The pressure of fuel forces the needle 142 upwards against the spring (not shown) and the fuel is emitted as a spray through the nozzle holes 146 and the slots 140. The dimensions of the nozzle holes 146 may readily be determined by experiment. We would expect that diameters in the ranges 50g to 500g, and particularly 100g to 300g, would be appropriate.

This mode of fuel injection is shown by way of illustration only. Once the concept of injecting fuel centrally via the guide pin is understood a person skilled in the art will be able to devise many alternative ways of achieving this. For example each spray hole may be connected individually to a SP1253.Al 1 March 1994 fuel injection line, and valves and other means may be employed to control the timing and rate of fuel injection.

In order to achieve optimum combustion it is desirable to promote efficient mixing of fuel and air within the combustion chamber formed between the top of the plunger and the top of the cylinder. Figure 7 shows an arrangement of transfer ducts 10 in the cylinder body 2 in which the transfer ducts 10 are angled with respect to the longitudinal axis of the cylinder body 2. when air is forced through the transfer ducts 10 by the action of the piston the angling of the ducts 10 causes the air to tend to swirl or form a vortex which may help to mix fuel and air more efficiently.

Figure 8 shows a plunger 8 which has a ridge around the periphery of the upper surface of the circular disc, for use with direct fuel injection as described above. The ridge is provided with flutes 148 which are tangentially angled so as to enhance the tendency of air which has passed through the angled transfer ducts to swirl.

Although the angled transfer ducts and the tangentially angled flutes in the plunger may be used together they may also be used independently of one another. The optimum angle of the transfer ducts and of the flutes may readily be determined by experimentation.

The invention 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.

SP1253.Al 1 March 1994 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 15 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.

SP1253.Al 1 March 1994

Claims (16)

1 A two-stroke engine, the engine including one or more cylinders and, f or each cylinder, a piston connected to a crankshaft, and a plunger, the piston and the plunger each being a close f it 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 f luid 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 inf low 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.

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.

SP1253.Al 1 March 1994

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 5 is slidably mounted on a guide pin.

8 An engine as claimed in any one of the preceding claims, which has means for providing fuel to the cylinder by direct injection.

9 An engine as claimed in Claim 8, which is adapted to supply fuel through a conduit in the guide pin and through slots or holes in the plunger.

10 An engine as claimed in any one of Claims 6 to 9, 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.

11 An engine as claimed in any one of Claims 7 to 10, 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.

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

13 An engine as claimed in any one of the preceding claims, wherein the fluid communication means are transfer ducts which are angled with respect to the longitudinal axis of the cylinder so as to induce air which is forced through the transfer ducts to swirl.

SP1253.Al 1 March 1994 19 -

14 An engine as claimed in any one claims 7 to 12, wherein the circular head of the plunger is provided with a ridge at the periphery of its upper surface.

15 An engine as claimed in claim 14, wherein the ridge is provided with tangentially angled flutes which tend to cause air which is forced through or over them to swirl.

16 An engine substantially as hereinbefore described with 10 reference to and as shown in any one of the drawings.

SP1253.Al 1 March 1994