The present invention relates to internal combustion engines and is particularly concerned with the positioning and bearing arrangements for the crankshaft of such engines. The invention relates to that type of engine which comprises two cylinder blocks, each defining one or more cylinders, each cylinder in each cylinder block being opposed to a cylinder in the other cylinder block, the two cylinder blocks being connected together with the interposition of two or more crankcase structures and defining between them a space in which a crankshaft is rotatably supported by at least two main bearings which are at least partially supported by the crankcase structures, a piston mounted to reciprocate in each cylinder, each piston being connected to a crank of the crankshaft (see for example FR-A-1 423 850). Whilst the invention is principally concerned with four stroke engines of spark ignited type, particularly engines for racing cars, it is applicable also to those of two stroke and/or diesel type.
Reciprocating piston engines include one or more pistons which are mounted to reciprocate in respective cylinders and are typically connected by respective connecting rods to a crankshaft accommodated within a crankcase defined beneath the cylinder block. This construction is inherently relative large and heavy. Furthermore, the engine exerts a considerable torque on its mountings during acceleration and has a considerable rotational inertia which can lead to catastrophic failure in the event of the engine seizing during high speed operation.
Engines of the generic type referred to above with the cylinders in the so-called flat configuration are known. The necessary connecting rods and relatively large cranks on the crankshaft are bulky and heavy and this is highly undesirable in racing car engines in which it is always desired to produce more power from an engine of given size and weight, i.e. a more compact engine with an improved power to weight to ratio.
It is an object of the present invention to provide a reciprocating piston engine which is smaller and lighter than known engines and thus has a higher power output per unit weight and per unit volume and a further object to provide such an engine which is perfectly balanced at all times and has no net rotational inertia.
According to the present invention, an internal combustion engine of the type referred to above is characterised in that each opposed pair of pistons is rigidly connected by two walls which afford respective apertures and which define between them a space, that each opposed pair of pistons is connected to two cranks of the crankshaft by two sliders which are accommodated in a respective one of the said apertures and are mounted to slide with respect to the associated wall only in the direction transverse of its reciprocal movement and which define an opening in which a respective one of the cranks is rotatably received, a main crankshaft bearing being supported in the said space between the two walls.
Thus in the engine in accordance with the invention the conventional connecting rods are eliminated and replaced by slider mechanisms of the per se known Scotch Yoke type (see US-A-3 220 390) and pairs of spaced walls connecting opposed pistons which thus reciprocate in unison. The sliders reciprocate laterally in the apertures in the connecting walls and convert the reciprocal motion of the pistons into rotary motion of the crankshaft and this permits the heavy and space-consuming cranks on conventional crankshafts to be replaced by smaller and lighter cranks. The space between the connecting walls accommodates not only a portion of the crankshaft but also a crankshaft main bearing which results in the crankcase being much smaller than is usual. The walls connecting opposed pairs of pistons also inherently occupy less space than traditional connecting rods.
The main crankshaft bearing within the space defined by two connecting walls may be supported solely by the crankcase structures or there may be an additional supporting member whose provision will enable the cylinder heads to be less rigid and thus less heavy. In one embodiment of the latter possibility in which a significant proportion of the crankshaft load is transferred directly to the cylinder heads, thus enabling them to be thinner and lighter, each piston is of annular shape with an axial hole passing through it, the two cylinder blocks are connected together by one or more fastening members, each of which passes through a respective opposed pair of pistons, and each fastening member at least partially supports a main crankshaft bearing in the space between the two associated connecting walls. In this embodiment the fasteners at least partially support or constitute a main crankshaft bearing. This construction results in the engine being more compact and thus lighter and also less subject to vibration. In practice, the fasteners will be connected to the cylinder head which closes the cylinders at the end remote from the crankshaft and the cylinder head may either form an integral part of the cylinder block or be a separate component rigidly connected to it.
It is preferred that each cylinder block defines an even number of cylinders arranged in two parallel rows and that the cylinders of each opposed row are associated with a respective crankshaft and that the two crankshafts are connected together to rotate in synchronism, either in the same sense or, more preferably, in opposite senses. In this embodiment the engine thus has n cylinders, where n is divisible by 4, and the engine may therefore be considered to be constituted by one or more modules, each of which has four cylinders. Two separate crankshafts are provided which counter-rotate and this results in perfect primary balance, perfect secondary balance, no rocking couples and no net rotational inertia.
In the event that the cylinders are arranged in a single row, the cylinder blocks are preferably connected together with the interposition of only two crankcase structures, each main crankshaft bearing being supported either solely by the crankcase structures or partly by the associated fastener member and partly by two crankcase structures. In the preferred embodiment in which the cylinders are arranged in two parallel rows it is preferred, in the case in which the crankcase bearings are supported only by the crankcase structures, that the two lateral crankcase structures and one central crankcase structure and each of the said main crankshaft bearings is supported by a respective portion of a lateral crankcase structure and the central crankcase structure. In the case in which the crankcase bearings are supported partly by the crankcase structures and partly by the associated fastener member, it is preferred that there are two lateral crankcase structures and one central crankcase structure and that each fastening member comprises two separate portions and further that each main crankshaft bearing is supported by two fastening member portions and a respective portion of a lateral crankcase structure and the central crankcase structure, all the said portions being connected together at the crankshaft bearing.
The inlet and outlet valves of the engine may be of conventional type provided in a cylinder head which forms a removable part of the cylinder block but it is preferred that each cylinder contains a cylindrical cylinder liner, the end of which remote from the associated crankshaft is substantially closed by an end wall in which one or more ports are formed, that the free edge of the side wall of the cylinder liner carries gear teeth which engage directly or indirectly with gear teeth on the crankshaft and that one or more inlet apertures and exhaust apertures are formed in that portion of the associated cylinder block which is adjacent to the end wall of the cylinder liner whereby rotation of the crankshaft results in rotation of the cylinder liner about the axis of the associated cylinder and the ports in the cylinder liner move cyclically into and out of registry with the inlet apertures and exhaust apertures in the cylinder block. Thus in this embodiment the engine valves are of rotary type and the movable valve member of all the valves in each cylinder is constituted by a single respective cylinder liner which is rotated continuously about its axis by a geared drive coupled to the crankshaft. This construction results in a considerable simplification and reduction in the number of components as compared to conventional valve actuating mechanisms and the fact that the cylinder liner rotates continuously means that dynamic lubrication of the piston rings is maintained at all times. This construction has advantages which do not require the presence of the features of claim 1 and may therefore be provided in any engine of known type. It is preferred that each cylinder liner carries a hollow bush which is integral with its end wall and extends towards the associated crankshaft and around the associated fastener member, optionally with the interposition of a cylindrical bearing. This bush can thus guide the piston and receive the piston side loads and it can therefore be ensured that the piston does not come into direct contact with the internal surface of the cylinder liner whereby wear and friction are reduced.
In the preferred embodiment the or each crankshaft is provided with an internal oil supply passage extending along its length which communicates with the main crankshaft bearing surfaces via radial passages in the crankshaft.
Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example with reference to the accompanying drawings, in which:-
The engine illustrated in the drawings is effectively an engine module having four cylinders and two crankshafts, and may thus be thought of as a H4 engine. An engine having 8, 12 or even more cylinders may be provided by connecting the appropriate number of such modules end to end and to two common crankshafts. The module includes two opposed cylinder blocks 2, each of which defines two cylinders 4 side by side. Each cylinder 4 receives an annular piston 6, as will be described below, which is connected to the piston 6 received in the opposing cylinder 4 formed in the other cylinder block 2 by two parallel spaced walls 8. The four pistons are thus connected together in two pairs whose axes lie in a common plane. Formed in the centre of each wall 8 is an aperture 10 through which a respective one of the crankshafts 12 passes. Slidably retained within each aperture 10 is a slider constituted by two slippers or half shells 14 which together define a circular aperture in which a crank 16 of the crankshaft is received. In this case the two slippers are connected together by bolts, but these bolts could be omitted and each slipper could be constituted by more than two components. This arrangement constitutes a "Scotch Yoke", which is known per se, and thus as the pistons reciprocate vertically, as seen in Figures 1 and 2, the slippers 14 reciprocate horizontally, as seen in Figure 2, and the crankshaft 12 is rotated. As will be appreciated, there are thus two separate crankshafts, each of which has two cranks which are associated with two pistons. The two crankshafts are connected to respective geared flywheels 17 so that they rotate in opposite senses but in phase. Due to the use of the Scotch Yoke mechanisms which are wholly contained within the cylinder bore diameter the stroke of the pistons is necessarily relatively short and is typically substantially less than their diameter.
The two cylinder blocks 2 are connected together with the interposition of two lateral crankcase structures 18 and a central crankcase structure 20 by means of elongate fasteners. There is one central fastener for each pair of opposed cylinders and it passes through the central hole in the associated pistons 6 and the space between the two walls and corresponding holes in the cylinder blocks and is secured in position by nuts 22. In practice, there may be additional circumferential fasteners around each cylinder but these form no part of the present invention and are not illustrated. Each fastener comprises two rods 24 whose outer end is threaded to receive a nut 22 and whose inner end is part-circular and engages a respective half bearing shell 26. The associated lateral crankcase structure 18 and the central crankcase structure 20 also engage and support a respective one of the half bearing shells 26. The two bearing shells 26 are secured together to define a "flying" main crankshaft bearing which receives and supports the associated crankshaft 12. A crankshaft bearing is thus provided within each connected opposed pair of pistons. The lateral crankcase structures 18 also carry further crankcase bearings 31 at the two ends of the crankshaft 12.
Rotatably received within each cylinder 4 is a cylindrical cylinder liner 32 whose end remote from the associated crankshaft is substantially closed. Formed in the closed end is a coaxial hole around which is a tubular bush 34 which projects through the piston and extends around the associated fastener rod 24. Between this tube and the edge of the aperture in the piston is a cylindrical bearing 33 which transfers piston side thrust to the fastener rod. The free edge of the wall of the cylinder liner carries gear teeth 36 which mesh with a jockey gear 38 which in turn meshes with teeth 40 on the crankshaft. Thus as the crankshaft rotates about its axis the cylinder liner rotates also about its axis. The cylinder liner 32 acts as a rotary inlet and exhaust valve member in which a diametrically opposed pair of ports 42 are formed. Retained between the cylinder liner 32 and the associated cylinder block 2 is a stationary plate 46 in which pairs of inlet apertures 43 and exhaust apertures (not shown) are formed which are in registry with corresponding inlet apertures 48 and exhaust apertures (not shown) in the cylinder block which communicate with the engine inlet and exhaust system respectively. The plate 46 is slightly recessed (not shown) over an area radiating from the inlet apertures which permits high pressure gas to penetrate behind the end of the cylinder liner in a controlled manner and thus to act as a partial force balance and reduce the rotary valve seal loads. Extending through the plate 46 are two spark plugs 47 per cylinder which communicate with the ports 42 at the appropriate times to ignite the fuel air mixture in the ports, which thus serve as combustion chambers, shortly before top dead centre. The cylinder liners perform only half a revolution per cycle of the engine. In use, the ports 42 in the cylinder liner come successively into registry with the inlet apertures 43,48 and the exhaust apertures and thus serve alternately as inlet ports and exhaust ports. Both the inlet and exhaust apertures in the cylinder block are shielded by tubes (not shown) which are spaced from the cylinder block and reduce heat transfer between the inlet air, the exhaust gases and the engine structure.
Each crankshaft 12 has an internal oil supply passage 54 extending along its length which communicates with the main crankshaft bearing surfaces through radial bores 56. In use, pressurised oil is supplied through the passages 54, 56 to lubricate the crankshaft bearings.
In use, perfect "simple harmonic motion" is imparted to the pistons by the cranks through the Scotch Yoke mechanisms and there are thus no secondary out-of-balance forces. The axes of all the pistons of each module lie are coplanar and there are thus no rocking couples. Due to the use of two counterrotating crankshafts, each of which fully counterbalances the inertia of the reciprocating pistons, each engine module is in complete primary balance. The engine also has no net rotational inertia. Due to the Scotch Yokes the acceleration of the piston at top dead centre is reduced compared to an engine employing connecting rods. The rotating cylinder liners are driven in a simple and reliable manner and the central bush attached to the cylinder liner may be used to locate the piston which need thus not touch the cylinder wall. Since the cylinder liner rotates continuously the piston rings are never at rest with respect to it and dynamic lubrication of the piston rings is thus maintained at all times. Supporting the crankshaft bearings by fasteners which pass through the pistons together with the elimination of the conventional connecting rods means that the engine is particularly simple, compact and light.
In the modified construction illustrated in Figure 3 the fasteners 24, bushes 34 and bearings 33 are omitted and the crankshaft bearings are supported only by the crankcase structures. In most other respects the engine is substantially the same as that described above except that the cylinder heads are a little stiffer and thus heavier than previously. This is, however, found to be acceptable and technically satisfactory in many engines, though in, for instance, a high power, turbocharged diesel engine the necessary increase in weight is likely to be unacceptable and the fasteners 24 may thus be necessary.
During the induction stroke of each piston, the subatmospheric pressure within the cylinder coupled with the frictional force exerted on the associated cylinder liner by the piston rings result in a tendency of the cylinder liner to move towards the crankshaft. If this were to happen, even to a very small extent, an unacceptably high leakage of oil into the cylinder could occur. It may therefore be desirable in both the engines described above to provide some means for axially locating the cylinder liners. This may be achieved in many ways but in the engine shown in Figure 3 with no fasteners 24, it is achieved by connecting each cylinder liner 32 to the associated cylinder head 2 by means of a connector 60 which is situated on the rotational axis of the cylinder liner and which is arranged to permit rotational movement but no axial movement of the cylinder liner.