GB2030213A - Opposed piston engine - Google Patents

Abstract

Each piston 1,2 is attached to one end of a rocker beam 16,15 which has a fulcrum 14,13 at its opposite end. The rocker beam is connected to a crankshaft 30 via connecting rods 32,31 pivoted at the mid-points of the beams at a distance apart greater than the pins at the beam ends. The fulcrum 14,13 of each beam is mounted in such a way that it can move up and down (towards and away from the piston) as the pistons move in and out. The fulcrums may however be prevented from moving towards or away from each other in the working cycle. The up and down movement causes one of the pistons to lead the other, depending on the direction of crankshaft rotation. Both pistons are at inner dead centre together. <IMAGE>

Description

SPECIFICATION An opposed piston engine Opposed piston engines of the rocking beam type are known, (See our British Patent Specification 1,472,418), which have the piston motion transmitted to the crankshaft via a rocking beam, which has a linkage at one end connecting it to the piston and a fulcrum at the other end which offers restraint to motion parallel to the axis of the piston. In such an engine, it can be arranged that at the inner dead centre (i.d.c.) of the pistons, the connecting rods joining the mid-points of the beams to the crankshaft, are in line. In this condition there is no phase difference between the motions of the pistons and shock loads applied to the pistons are absorbed by shear forces between the crank pins of the crankshaft without bending and excitation of vibrations in the engine frame.

According to the present invention, there is provided an opposed piston engine having cylinder and crankshaft means, and, for each cylinder, two pistons reciprocable in the cylinder, rocker beams each pivoted at one end to the gudgeon pin of a respective piston and at the other end to a fulcrum, connecting rods connecting mid-points of the rocker beams to the crankshaft and being arranged to lie substantially in line when the pistons are at their inner positions, said mid-points being further apart than the fulcrums or gudgeon pins, the rocker beam fulcrums being mounted in a manner which prevents said other ends of the beams from moving towards and away from one another in a direction parallel to the cylinder axis during the working cycle, but allows them to move towards and away from the cylinder axis, in a direction perpendicular thereto.

An advantage of this type of engine is that the pistons can be run out of phase - such that the exhaust piston uncovers the exhaust ports before the air piston uncovers the air ports, without the exhaust piston having to close the exhaust ports after the air piston closes the air ports. However, the pistons are also in phase during combustion when a shock load is applied to the running gear.

To achieve an exhaust piston lead over that of the air piston near the outer dead centre (o.d.c.) position of the piston requires that the mid-point of the rocker beams should no longer be in-line with crankshaft centre. When these mid points are below the crankshaft centre line by an angle '6' then in the direction of rotation from i.d.c. to o.d.c., the exhaust crank occupies a period of 180 - 'O' degrees and that for the air crank 180 + 'û' degrees, thus giving a lead angle of 20 degrees.

The rocker beam is coupled directly to the piston by a bearing giving freedom for the beam to rotate relative to the piston. The fulcrum must therefore be free to rise as the bearing in the piston moves towards and away from the vertical line through the fulcrum.

The fulcrum moves vertically through a distance which is much smaller than the length of the piston stroke. This movement may be accomplished by various structures which give vertical freedom and horizontal rigidity. For example, swing links may be hinged either at the centre line of the engine or to any suitable point along a fixed strut like member which takes the reaction of the piston load. This strut can aternatively be made to extend the whole way between the fulcrums and to support fulcrum shafts.

These shafts may support eccentrics which form floating fulcrum hinge points or aternativelythe whole strut may be free to move vertically and have a horizontal location. A fourth alternative is to have a strut with vertical guide and slide block supporting the fulcrum pins.

The vertical motion required to maintain linear motion over the piston stroke is not enough on its own to give the degree of piston lead required for good port timing and it is proposed also to obtain vertical movement of the small end pin by setting this bearing back relative to the line joining the fulcrums to the gudgeon pin. This positioning of the pin further away from the crankshaft than the line adds a vertical motion to the pin dependant on the set back distance and the angular swing of the beam from i.d.c. to o.d.c.

The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic view, partly in section, of an engine according to the invention; Figure 2 is a detail sectional view of a rocker beam fulcrum; Figure 3 is a port area diagram; Figure 4 is a detail sectional view of a second form of rocker beam fulcrum; and Figure 5 is a detail sectional view of said fourth form of rocker beam fulcrum.

The engine shown in Figure lisa compression ignition engine which has two pistons 1,2 moving in a cylinder 5. An air inlet port 3 and an exhaust port 4 are covered and uncovered by respective pistons as these move backwards and forwards during operatin of the engine. The pistons are connected to rocker beams 15 and 16, and the rocker beams have fulcrums 13 and 14 at their bottom ends. A strut 20 forming a thrust member maintains the fulcrums the same distance apart during operation. The rocker beams are connected to a crankshaft 30 by connecting rods 31 and 32. The crankshaft crankpins overlap as shown.

With the engine shown, and the direction of rotation of the crankshaft indicated by arrow 33, the exhaust port 4 opens before the inlet port 3. This is of considerable advantage in ensuring that all the combustion gases are exhausted from the cylinder.

Figure 3 shows that an area Xis available for the discharge of exhaust gases at a pressure above atmospheric pressure due to the opening of the exhaust port before the inlet port.

Although the arrangement of the pistons, rocker beams and connecting rods shown in Figure 1 is symmetrical, one piston will lead the other because the rotation of the crankshaft destroys the symmetry. If the direction of rotation of the crankshaft is changed, the other piston will lead.

The exhaust port 4 opens at Eo (Figure 3), and then at point A the air ports 3 open and under the pressure of a scavenging device, air enters the lefthand end of the cylinder and displaces the residual exhaust gas from the cylinder. At points Ec and Ac, when the pistons are moving together in the cylinder, the exhaust and air ports close and compression commences. The opening and closing of each port is determined by the same port edge and so there is a large degree of symmetry about its own pistons o.d.c. However by displacing each o.d.c. by an angle '0' from the mean o.d.c. the port area curve for the exhaust ports is advanced by 2'0' earlier than that of the air piston. Port areas displaced in this way are highly advantageous to the efficient operation of the engine.However, if this displacement of piston positions occurs at i.d.c., that is when combustion takes place - the difference in connecting rod loads from the same combustion load is such that the reactions cannot be balanced and an undue load is applied to the structure.

In the structure of Figure 1 the parts of the engine mechanism are shown at their i.d.c. positions.

Symmetry is sufficient for the description to refer mostly to one side only in describing the influence of linkage geometry on the lead of one piston relative to the other.

At i.d.c., the crankshaft centre 8, the crankpin centres 9 and 10 and the small end centres 6 and 7, all lie on a horizontal line so that combustion loads are taken with the working parts all in-line. This avoids bending couples which cause distortion of parts and make noise. To get the exhaust ports to open before the air ports we make the exhaust piston 1 reach its outer dead centre (o.d.c.) position before the air piston 2. Such a condition is given when the connecting rod little end bearings at 6 and 7 drop below the horizontal line. This can be done to some extent by choosng the beam pivoting position F such that the are AB swept by the little end bearing is not at its crest C.However, this arc cannot be moved very far from the crest C without putting the beam at an angle which would give high loads when the conn-rod holds the beam against its inertia at o.d.c. To avoid this condition the fulcrum 14 of the beam 16 is allowed to move vertically, and caused to drop at o.d.c. and so increase the angle between the connecting rod and the horizontal line from a to 0 without increasing the angle of the beam. This can be done by coupling the beam 16 directly to the piston 1 with a normal gudgeon pin, so that as the piston moves in and out, the top end of the beam will follow a straight line. The drop of the fulcrum 14 of the beam 16 from i.d.c. position to o.d.c. position is determined by the position of the centre F.Geometrically a vertical line through F meets the cylinder axis at E and arc GH gives the motion of the gudgeon pin which would be necessary if the eccentric 16 was locked to the beam. In the same manner CB would be the arc of little-end travel. When the eccentric 14 is free, then the distance from H to the cylinder axis is equal to the vertical distance between D and B and is the vertical movement required by the fulcrum end of the beam 16 to keep the gudgeon pin on the cylinder axis. The condition for D to be below B is that the distance from E to the gudgeon pin axis is greater than half the piston stroke.At o.d.c., the little ends 6,7 of the connecting rods lie at positions 11, 12 and offer a greater lead than would have existed had the fulcrum been fixed and the motion HE absorbed by a link between the piston and the beam 16. Furthermore, such a piston linkwould have bearings with too small a motion to maintain a good oil film.

The vertical motion required to maintain linear motion over the piston stroke is not enough on its own to give the degree of piston lead required for good port timing and it is proposed also to obtain vertical movement of the small end pin 6 by setting this bearing back relative to the line FG joining the fulcrums to the gudgeon pin. This positioning of pin 6 further away from the crankshaft than the line FG adds a vertical motion to pin 6 dependant on the set back distance and the angular swing of the beam from i.d.c. to o.d.c.

It is an advantage of rocker beam engines that the engine compression ratio can be changed whilst the engine is running by changing the separation of the fulcrums of the rocking beams. The fulcrum construction is preferably adapted to permit this separation to be changed.

Figure 2 shows a construction where the fulcrums can move vertically, and their separation can be altered. The lower end of a beam 16 surrounds an eccentric 14which runs on pin 17. Rotation on eccentricity 'E' permits rise and fall of the rocker beam. Pin 17 has supports 18 and 19 eccentric to itself by a smaller value 'e' and is rotatable in a rigid strut 20. At the end of supporting pin 19 a square section 21 permits the fitting of a control lever to turn the pin. In the preferred design the eccentricity 'e' is chosen to give the maximum desired change of compression ratio when the pin is turned through about half a turn so that piston reaction loads do not create a substantial torque on the pin.

Other constructions of the fulcrum, where the fulcrum has horizontal rigidity and vertical freedom are shown in Figures 4 and 5. In Figure 4, the strut 20 of Figure 1 is divided at the centre by hinge pin 22 which is rigidly mounted and may have eccentric sections to permit change of compression ratio as described in connection with Figure 2. If, in this case, the strut is not divided then the pin 22 is fitted in a vertical slot in the strut, in order to permitvertica movement of the whole strut without horizontal movement during the working cycle. The strut can then be made as shown on the left of figure (1), the beam (15) pivotting directly on pin 13. The pin 22 of Figure 4 is shown flattened to run in a slot in Figure 1. In Figure 5, the fulcrum is mounted in a vertical slide 23 which runs in a guide in the strut 20. An advantage of th is design is that by incorporating suitable non-return valves, one of which is shown as 24, the vertical motion of the slide 23 can be made to operate as an oil pump feeding into the rocker or through the strut. In this case, the variable compression ratio eccentric is fitted between part 23 and beam 16 in the normal way - as though part 23 were the strut.

The up and down movement of the rocker beam is very small, and may be of the order of 1 mum. It will be appreciated that in the embodiment of Figure 2, and in the embodiment of Figure 4, there will be a small amount of movement in a direction parallel to the cylinder axis when the rocker moves up and down.

However, due to the geometry of the fulcrum construction, movement of the rocker beam ends towards and away from each other when the up and down movement is of the order of 1 mm, will be of the order of 1/1 Omm. So far as this Specification is concerned, this movement parallel to the cylinder axis is considered to be negligible.

Claims (9)

1. An opposed piston engine having a cylinder and crankshaft means, and, for each cylinder, two pistons reciprocable in the cylinder, rocker beams each pivoted at one end to the gudgeon pin of a respective piston and at the other end to a fulcrum, connecting rods connecting mid-points of the rocker beams to the crankshaft and being arranged to lie substantially in line when the pistons are at their inner positions, said mid points being further apart than the fulcrum or gudgeon pins, the rocker beam fulcrums being mounted in a manner which prevents said other ends of the beams from moving towards and away from one another, in a direction parallel to the cylinder axis, during the working cycle but allows them to move towards and away from the cylinder axis, in a direction perpendicularthereto.

2. An engine as claimed in claim 1 wherein each rocker beam is connected to its respective fulcrum via a free-floating eccentric which permits movement of the rocker beam in a direction perpendicular to the cylinder axis.

3. An engine as claimed in claim 2, wherein the free-floating eccentric is in turn journalled on an eccentric shaft, and means are provided for turning the shaft to alter the compression ratio of the engine.

4. An engine as claimed in claim 1, wherein each rocker beam fulcrum is mounted at the end of a hinged strut which permits movement of the rocker beam in a direction perpendicular to the cylinder axis.

5. An engine as claimed in claim 4, wherein the hinged strut is mounted on an eccentric hinge pin, which can be turned to alter the compression ratio of the engine.

6. An engine as claimed in claim 1, wherein each rocker beam fulcrum is mounted in a block slidable in a channel in the engine frame in a direction perpendicular to the cylinder axis.

7. An engine as claimed in clam 1, wherein opposing rocker beam fulcrums are mounted in a strut slidable in the engine frame in a direction perpendicular to the cylinder axis.

8. An engine as claimed in claim 6 or 7, wherein the rocker beam fulcrum is an eccentric which can be turned relative to the block to alter the compression ratio of the engine.

9. An opposed engine piston substantially as herein described with reference to Figure 1 in combination with any one of Figures 2,4 or 5 of the accompanying drawings.