Abstract:
A novel rotary engine has a single toroidal cylinder and a set of pistons on a rotating circular piston assembly. A rotating disk valve perpendicular to the toroid has a cutout position which periodically traverses the chamber to allow passage of a piston therethrough. The geometry of the pistons and the valve are arranged to minimize the residual volume, by altering the geometry of the chamber section formed between valve and piston and/or providing pistons which are mechanically extendible and retractable in synchronization with the opening and closing of the disk valve, to optimize engine efficiency and performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotary engine, and more particularly to an internal combustion engine in which a piston assembly orbits continuously within a toroidal chamber. 
     2. Description of the Prior Art 
     The conventional technology for internal combustion engines is the reciprocating piston engine which has evolved and been refined over a period of some 125 years. That kind of engine is, however, subject to a number of widely recognized, severe limitations and constraints in power generation efficiency. 
     The reciprocating piston engine does not produce rotary motion with a constant torque arm but, rather, uses a crankshaft to convert reciprocating motion of a piston into rotary motion, with the attendant disadvantage of a variable torque arm that is drastically reduced in the top dead centre region of the piston when combustion is initiated. The result is a lack of torque and power and a reduction of engine efficiency. 
     Many attempts have been made to produce a workable “toroidal piston engine” which provides revolving pistons mounted to a central disk to produce the desired constant torque arm, Examples of this kind are to be found in U.S. Pat. No. 4,035,111 (Cronen, Sr.); U.S. Pat. No. 4,242,591 (Harville); U.S. Pat. No. 4,683,852 (Kypreos-Pantazis); U.S. Pat. No. 4,753,073 (Chandler); U.S. Pat. No. 5,046,465 (Yi); U.S. Pat. No. 5,203,297 (Iversen); and U.S. Pat. No. 5,645,027 (Esmailzadeh). 
     In common with all positive displacement combustion engines, the toroidal engine must incorporate means both for compressing the intake charge and for containing the hot expanding gasses that are generated by combustion. In keeping with this principle, previous inventors of toroidal engines have usually made provision for some sort of “valve” to intercept the path of the advancing piston, to retract and so allow the piston to pass by, then to close behind the piston. 
     In this manner, the intake charge is compressed between the advancing piston and the valve blocking its path. The compressed charge is then diverted into a combustion chamber, the valve is briefly opened to allow the piston to pass by, the valve closes and the ignited combustion gases, released from the combustion chamber, expand between the closed valve and the retreating rear face of the piston. Accordingly, each piston is propelled on a circular orbit as it passes through the valve aperture. 
     My study of the prior art, experiments which I have conducted and computer-assisted thermodynamic modelling results have led me to conclude that the reason none of these approaches has achieved commercial success stems from general failure to address a fundamental problem inherent in the operation of toroidal engines, namely, the loss in compression potential and the loss in air mass which occurs between the front face of a piston and a valve intersecting the toroidal chamber in advance of that piston and, likewise, the pressure loss which occurs between the rear face of the piston and the intersecting valve behind that piston. Thus, that air mass between the advancing face of a piston and the intersecting valve which is not diverted into the combustion chamber, but escapes into the toroidal chamber, is “lost” to the useful generation of work. 
     In a toroidal piston engine of this general kind, some mechanism is required for opening and closing a valve seat in advance of and then behind a moving piston to gain the mechanical energy resulting from compression, ignition and expansion. Any such mechanism will take a certain amount of time to open or close and, to that extent, the piston will have travelled further in its angular rotary motion, creating and enlarging a “residual volume” (or, equivalently, “dead volume”). This effect can lead to a loss in compression ratio, a loss in air mass, and concomitant loss of expansion pressure, in turn resulting in significant inefficiency and loss of power. 
     Hitherto, the designers of toroidal engines have apparently acted on the assumption that merely to block the path of the advancing piston with a valve and to trap the intake charge will generate adequate compression, with no loss of air mass, and adequate pressurization of the toroidal chamber. Prior known engines of this kind have never achieved this desired result, however, as each employs one or another intersecting valve opening-and-closing mechanism which is too slow. This results in unacceptably large residual volumes produced ahead of and behind the valve by the rapidly moving pistons. 
     As a specific example, the aforementioned patent to Kypreos-Pantazis discloses a rotating piston internal combustion engine in which the mechanism for opening and closing the toroidal chamber in advance of and behind a piston comprises separating walls adapted to move radially inwardly and outwardly to divide the toroid inner space into sub-chambers. The means to withdraw the separating walls to allow the passage of a piston and thereafter reinsert it is typically a cam coupled mechanically to the central output shaft of the engine to withdraw the walls periodically from the toroid chamber as the shaft and piston assembly rotates, and return springs for reinserting the walls into the toroid chamber. 
     A practical problem with that and with other prior art toroidal engines is that their opening-and-closing mechanisms create significant residual volume between the front and rear of the piston, resulting in entirely unsatisfactory performance. I have employed thermodynamic mathematical modelling to demonstrate the inevitability of the practical failure of toroidal engines using such mechanisms. All of the prior art exemplified in the patent literature employs either planar sliding valves or planar rotating valves, which are required to move in reciprocating fashion owing to the configuration of the toroid. At the high rotational speeds required by an engine cycle, reciprocating mechanisms are very difficult to seal and to maintain. 
     The same thermodynamic mathematical modelling and analysis also revealed a surprisingly drastic improvement in the performance of toroidal piston engines where the residual volumes are contrived to be made as small as possible. Indeed, the dead volume would ideally be zero but as a practical matter, of course, the moving piston and the valve in its closed position must never physically contact each other. 
     The practical conclusion of my analysis is that a toroidal engine of this general kind becomes usefully workable only where the volume in the compression phase of the cycle (between the piston and valve) is physically reduced sufficiently to generate a compression ratio approximating the value achieved in conventional reciprocating piston engines and the loss of air mass is minimized to achieve an efficiency comparable to conventional engine technology. That ratio, in an SI engine, typically lies in the range of between 8:1 and 12:1 or, in the case of the Diesel engine, approximately 18:1. 
     The fundamentally different approach I have taken to improving the performance of toroidal piston engines of this kind is to alter the geometry of the chamber section formed between valve and piston to minimize the residual volumes and thereby attain the very significant improvement in performance which was predicted by the analysis of models. For that reason, I refer to my invention as the “variable geometry toroidal engine” or VGT engine. As discussed below, the aforementioned geometry can be varied by employing a rotating disk valve with an aperture that periodically intersects the toroidal chamber and minimizing the residual volumes between piston and valve. 
     In a first principal embodiment the reduction in the residual volumes is achieved by matching the three-dimensional shape of the piston to the valve opening. According to a second principal embodiment, it is achieved by providing a piston which is mechanically expandible and contractible, to minimize the residual volumes between the piston and the valve just prior to opening of the valve and just following shutting of the valve. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the invention to provide a toroidal engine in which the residual volumes between the piston and the closed disk valve are minimized to achieve superior performance characteristics. 
     It is a further object of the present invention to provide a toroidal piston engine in which the volume between piston and valve in a compression phase of the working cycle is sufficiently small to generate a compression ratio of a value approximating that achieved in conventional reciprocating engines. 
     It is a further object of the present invention to provide an engine as aforesaid which will run smoothly with virtually no vibration. 
     It is a further object of the invention to provide an engine as aforesaid which is compact and which can be built as a gasoline engine running on the Otto cycle or as a Diesel engine by the expedient of reducing the volume of a combustion chamber with an adjustable counterpiston and changing the fuel system to Diesel fuel. 
     It is a further object of the present invention to provide an efficient, pneumatically powered rotary engine for use in environments where combustion is unduly hazardous, as an air motor providing high torque at low rpm. 
     It is a further object of the present invention to provide a rotary motor which can operate as a steam motor with comparable or superior performance to conventional steam turbines but at significantly lower cost of production. 
     It is a further object of the present invention to provide an efficient rotary engine which with a suitable injection system can be built as an engine fuelled by the combustion of hydrogen. 
     With a view to achieving these objects and overcoming the aforementioned disadvantages of prior rotary internal combustion engines, the present invention provides an engine having pistons rotating through a non-circular cross-section toroidal chamber which is intersected by a continuously rotating disk valve having a shutter-like cutout therethrough. Two counter-rotating disk valves may be used to decrease the opening and shutting times still further. 
     The shape of the pistons, the chamber through which they move and the cutout portion of the continuously rotating disk valve, unlike prior art toroidal piston motor arrangements, are designed with a view to minimizing the residual volume, thereby enhancing the compression ratios to levels which are useful in practice. 
     According to a first principal embodiment of the invention, the residual volumes are minimized by having the shape of each piston matched to the non-circular geometry of the toroid and having the trailing and leading edges of each piston formed with a three-dimensional curvature such that the outer surface of each piston remains as close as practicable to the interior walls of the valve cutout as the piston passes through, during operation of the engine. 
     According to a second principal embodiment of the invention, the residual volumes are minimized by providing pistons which are mechanically extendible and retractable, in conformity with the speed of passage of the piston through the disk valve, so as to minimize the residual volumes. 
    
    
     The various advantages and features of the VGT engine according to the present invention will be apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 1 a  are schematic drawings in plan and in part-sectional side elevational view, respectively, of the general arrangement of components in a VGT toroidal piston engine according to the present invention; 
     FIG. 2 is an end view of a selectively shaped piston which may be used in an engine according to the present invention, illustrating the non-circular peripheral contour, with two convex surface portions having different radii of curvature; 
     FIG. 3 schematically isolates details of the toroid, pistons and flat disk valve in a VGT engine of the kind illustrated in FIGS. 1 and 1 a;    
     FIGS. 4 a ,  4   b  and  4   c  are detailed sectional views, sequentially showing the passage of a piston through the cutout portion of a rotating disk valve in a VGT engine according to the present invention, particularly illustrating the novel curvature of a piston over its front and rear faces; 
     FIG. 5 schematically illustrates a variant of the piston used in the VGT engine, which is equipped with a sinusoidal piston ring for improved sealing; 
     FIGS. 6 a  to  6   c  are schematic representations of various alternative sealing arrangements for the central rotating disk carrying the pistons, and of the mounting of a piston to the rotating disk in the VGT engine of FIGS. 1 and 1 a;    
     FIGS. 7 a  to  7   c  schematically illustrate preferred arrangements for the combustion chamber in a VGT engine according to the present invention; 
     FIG. 8 is a schematic illustration of an embodiment of the invention employing rotary combustion chamber valves which operate synchronously with the disk valve, using a timing belt or chain drive arrangement; 
     FIG. 9 schematically illustrates a combustion chamber arrangement for a VGT engine employing multi-spot, partial quantity sequential fuel injection; 
     FIGS. 10 a  and  10   b  schematically illustrate the use in a VGT engine of a toroidal dual radius piston having front and rear faces which may be extended or retracted by operation of a centrally located cam mechanism; 
     FIGS. 11 a  and  11   b  schematically illustrate an alternate, mechanical drive system for an extendible/retractable piston in a VGT engine according to the present invention; 
     FIG. 11 c  schematically illustrates an alternate hydraulic drive system for an extendible/retractable piston in a VGT engine according to the present invention; 
     FIGS. 12 a  and  12   b  schematically illustrate the use of an optional separate boost pressure system in conjunction with the toroidal expansion chamber of a VGT engine; 
     FIG. 13 a  schematically illustrates an arrangement using a direct combustion valve drive; 
     FIG. 13 b  schematically illustrates housing pressurization; and 
     FIG. 13 c  schematically illustrates central lubrication. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The basic co-operating components of the VGT engine according to the invention are to be seen in the views of FIGS. 1 through 4 c.    
     The engine comprises a toroidal chamber  10  within which several pistons  12  rotate in unison. Two, three or four pistons  12  are mounted circumferentially and equiangularly to a disk  14  by means of screws or bolts  11 . FIG. 3 presents a “stripped down” schematic illustration of the relative disposition of toroidal chamber  10 , rotating disk valve  18  and pistons  12  (three in the embodiment illustrated in the drawings). Co-axially oriented with the axis of toroidal chamber  10  is a drive or output shaft  16  for delivery of torque developed by the motor. 
     My novel mechanism for effectively opening and closing a valve in advance of and behind a moving piston comprises a circular disk valve  18  having a cutout portion  19  for passage therethrough of a piston. Disk valve  18  is mounted on a separate actuating shaft  20  at right angles to the axis of output shaft  16 . The edge surface  18 ′ of disk valve  18  is of a concave curvature which conforms to the circularity of rotating mounting disk  14 . As discussed in more detail below, the rotation of disk valve  18  is synchronized with the rotary motion of pistons  12 . 
     Compression is achieved in the VGT engine by the timed intersection of toroidal chamber  10  with rotating disk valve  18 . I have found that a part-circular cutout in a rotating disk can effectively serve as the opening for a rotating valve in a toroidal engine, provided the toroidal cross-section and the pistons are given a “variable geometry” which allows the piston and the solid portion of the rotating valve to approach each other as closely as possible without touching in both the compression and expansion phases. 
     According to a first preferred embodiment of the invention, the “variable geometry” consists in matching the piston contour to the toroidal chamber and the disk valve cut-out. The peripheral shape of a “dual radius” toroidal piston (and of the chamber cross-section which accommodates the piston) is illustrated in FIG.  2 . The nearest practicable approach to flush sealing between the piston and the valve, given the intersecting rotational movements of disk  14  and disk  18  in perpendicular plane, is achieved by having the piston shaped with a curved inner side surface portion  12   a  having a radius R 2  equal to the radius of curvature of rotating disk  18 , and a curved outer side surface portion  12   b  of a smaller radius of curvature R 1  conforming to the interior curvature of the toroidal chamber  10 . 
     The surface portion  12 ′ connecting surface portion  12   a  to surface portion  12   b  may be parallel planar surfaces as illustrated in FIG. 2, or else slightly inwardly convergent, as represented in FIG. 1 a.    
     The “matching” that particularly assists in minimizing the dead volumes, however, is achieved by forming appropriately contoured three-dimensional surfaces at the front and rear faces of both the piston and the disk valve. 
     This is best seen in the views of FIGS. 4 a  to  4   c , the temporal sequence of which is explained in greater detail below. In order to minimize the residual volumes formed between piston  12  and disk valve  18 , the front (leading) face  12   c  of piston  12  and its rear (trailing face)  12   d  are slanted relative to the plane of rotation and three-dimensionally curved to conform to convex front edge surface contour and rear edge surface contour  18   a  and  18   b , respectively, of disk valve  18 . 
     As illustrated in the embodiment shown in FIGS. 1 a  and  1   b , the engine includes a bypass combustion chamber  21  where the majority of compressed air is stored and burned with injected fuel, while a piston  12  bypasses the combustion chamber. A combustion chamber inlet valve  21   a  and a combustion chamber exit valve  21   b  are also synchronized, in their respective opening and closing, with the motion of pistons  12  for opening and closing of transfer passages  21   c  and  21   d , respectively, which joing the combusiont chamber to the cylinder chamber. This synchronization may be effected, for example, by reciprocating connecting rolls  22   a  and  22   b  operatively geared to a gear wheel  16   a  fixed to drive shaft  16  by actuating gears  25   a  and  25   b.    
     The basic working cycle of a VGT engine is analogous to that of reciprocating engines. The compression stroke is effected by the front face  12   c  of the piston and the power stroke by the rear face  12   d.    
     Throughout the figures, the directions of motion of the piston and the disk valve are indicated by arrows P and D, respectively. FIG. 4 a  shows the components just subsequent to compression with the trailing edge  18   b  of the disk valve moving out of the way of advancing piston  12 . Next in temporal order in FIG. 4 b  piston  12  has almost passed through disk valve  18  which is in the process of closing the space behind piston  12  for the power stroke. In FIG. 4 c , the disk valve is closed and the high pressure combustion gasses expand into the space between disk valve  18  and the rear face  12   d  of the moving piston. Additional spark plugs may be placed in the passage to the toroidal cylinder, as at  23   a  in FIGS. 4 a  to  4   c  and/or in the toroidal chamber itself indicated by  23   b . Fuel may also be injected into the transfer passage  21   c  or into the toroidal chamber upstream of the combustion chamber. 
     Air for combustion may be fed through a port  24   a  (FIG. 1 a ) on the toroidal chamber  10  by a blower or charger  26 . Unlike the conventional reciprocating engine, there is no “intake stroke”. The air blown in by charger  26  is compressed once piston  12  has passed air intake port  24   a . Compression occurs in the interior of toroidal chamber  10  because disk valve  18  forms a sealed space between piston and disk. The greater part of the compressed air is stored in bypass combustion chamber  21 , which is sealed off as soon as the intake valve  21   a  and the exit valve  21   b  close. The remainder of the compressed air, in the residual volume, is used later in purging the exhaust gas, once the disk valve  18  opens. Once piston  12  has passed through disk valve  18 , toroidal chamber  10  is sealed off by the closing disk valve, making expansion possible. In the meantime, fuel has been injected into combustion chamber  21  and has been mixed with the air and ignited, readying the combustion gas for the expansion. 
     Combustion chamber  21  is preferably configured as a swirl chamber (described in greater detail below in conjunction with FIGS. 6 a  and  6   b ) and is equipped with its own sparkplug (as in an SI engine), igniting the swirling air-fuel mixture and raising the pressure. As combustion takes place, piston  12  bypasses the combustion chamber through the open disk valve  18 , which then closes behind the piston as in FIG. 4 c.    
     At that point, exit valve  21   b  is opened. The burning air/fuel mixture of the combustion chamber  21  escapes into the toroidal chamber  10  as a high-velocity jet through an orifice of a convergent/divergent nozzle (sometimes referred to as a “Laval nozzle”), best illustrated and described below in connection with FIG. 9. A portion of the fuel can be injected into the toroidal chamber and ignited by the burning fuel jet from combustion chamber  21 , thereby raising the pressure in toroidal chamber  10  against the backside  12   b  of the piston, producing power and torque. 
     The piston which experiences the expansion transfers its power to the disk  14  and the main shaft  16  and drives the next advancing piston which effects the next compression phase and the cycle is repeated. 
     There may be one or more combustion chambers provided on the perimeter of toroidal chamber  10 , each of them having its own associated disk valve for intersection of the chamber. A symmetrical arrangement of such combustion chambers can achieve a more even temperature and less heat distortion. By conventional means, cooling water from the expansion side is ducted to the cooler areas of the toroidal chamber to reduce heat distortion. 
     Exhaust from combustion is vented through on exhaust port  24   b  on the perimeter of toroidal chamber  10 , once the piston which effects the power stroke has passed the exhaust port and causes that port to open The exhaust gases are purged by residual air from the compression stroke which was not captured in the combustion chamber. Instead of being vented to an emission control system, the exhaust gases may be used for turbocharging or a power recovery turbine. 
     Disk valve  18  is rotationally driven by suitable gearing means and/or a timing belt  27  or chain drive for correct synchronization to achieve the above-described compression and expansion phases. Power for the disk valve drive is taken from main shaft  16  on the central disk  14 . As indicated in FIGS. 6 a  to  6   c , the toroidal chamber  10  and the disk valve  18  are provided with suitable lubricated seals  30  to minimize leakage. 
     As illustrated in FIG. 5, the pistons  12  may themselves advantageously be equipped with lubricated sinusoidal piston rings  13  over a constant diameter section of piston  12  to ensure good sealing during the compression stroke and the expansion stroke, and to prevent jamming of piston rings in the disk valve housing area during the by-pass stroke. 
     Proper sealing of the compression chamber and in particular the combustion/expansion chamber in the VGT engine is important. A number of alternative arrangements for sealing the central disk and the piston mounting are illustrated in FIGS. 6 a  to  6   c . Piston rod  15  extends outwardly to join piston  12  (not shown). The rod is secured in place to the upper and lower portions  14   a  and  14   b  of central disk  14  by means of spring-loaded mounting bolts  11 . Central disk  14  rotates with its pistons through the interior of toroidal chamber  10  which comprises an upper toroid shell  10   a  and a lower toroid shell  10   b.    
     The sealing between upper toroid shell  10   a  and upper central disk and between lower toroid shell and lower central disk may be of a number of configurations and materials, depending on the end application of the engine, e.g. grooved labyrinth seals  28  on the perimeter of central disk  14 . A computer model loss study which has been carried out suggests that significant benefits are enjoyed where these grooved labyrinth seals  28  are pressurized, a pressurization which is automatically achieved by the leak air until a steady state pressure has built up. This keeps leakage losses to an acceptable level. Good sealing is achieved by combining the grooved labyrinth seals  28  on the perimeter of the central disk with star-shaped rings  30  which may be made of Teflon where the VGT engine is an air- or steam-motor and of hardened steel where it is an internal combustion engine. The upper and lower toroidal shells  10   a  and  10   b  may also include an abrasive honeycomb-type seal made of superalloy or ceramic materials of the kind conventionally found in gas turbine sealing arrangements. 
     Alternative sealing passage shapes that may be used in particular cases are square wave  32 , triangular  34  or a combination of triangular and sinusoidal  36 . Noted in dotted outline in FIG. 6 c  is an optional spherical mounting for piston-carrying rod  13 . 
     Turning to FIGS. 7 a  to  7   c , the combustion chamber  21  may be equipped with two counterpistons  39   a  and  39   b  respectively moveable by bolts (or helices)  40   a  and  40   b  either manually or electronically using a computer controlled servomotor (not shown), to change the compression ratio, as in the arrangement of FIG. 7 a . This allows for optimal tuning and performance under various speed/load conditions and for improving fuel economy. Moreover, it is possible to operate the engine in a Diesel mode, the adjustment over to Diesel being made while the engine is running or while the engine is shut off. 
     Inlet passage  21   a  to the combustion chamber  21  is positioned at the perimeter of the circular chamber, so that the entering compressed gasses create a swirl in the chamber which continues while a selected quantity of fuel is injected through fuel injectors  41  and ignited by spark plug  42 . The burnt gasses exit chamber  21  through exit passage  21   b  on the opposite side of the chamber, enhancing the atomization and mixing of the air/fuel mixture. 
     An alternative arrangement of combustion chamber is illustrated in FIG. 7 c , in which a single moveable counterpiston  39  is adjusted by screw  40  to tune the combustion characteristics of fuel air mixtures entering through port  21  and ignited by spark plug  43 . 
     FIG. 8 schematically illustrates an embodiment of the invention employing rotary combustion chamber valves  42   a  and  42   b , each having a cutout  43   a  and  43   b  therethrough, with rotary combustion chamber valve  42   a  located at the inlet of the combustion chamber and rotary combustion chamber valve  42   b  at the outlet. A chain drive  44  loops over central sprocket  16   a  which is directly driven by main shaft  16  and passes over both rotary valves  42   a ,  42   b  and an idler sprocket  44  centrally mounted between them for rotation. Combustion chamber valves of the reciprocating plunger type shown in FIG. 1 are preferred for slow running engines, while combustion chamber valves of the rotary flat plate type as shown in FIG. 8 are better suited to fast running engines. 
     A further combustion chamber arrangement, schematically illustrated in FIG. 9, is adapted for a VGT engine employing “multispot”, partial quantity sequential fuel injection. For greater clarity, the inlet and exit valves shown in previous drawings are not included in this Figure. Again, piston  12  is shown in motion in the circumferential direction P through toroidal chamber  10 . Communication between combustion chamber  21 ′ and the interior of toroidal cylinder  10  is through the orifices  21 ′ c  and  21 ′ d  of a convergent-divergent nozzle. A spark plug  45  is positioned in combustion chambre  21 ′ and fuel is injected into the combustion chamber through nozzle  41  a the toroidal expansion chamber itself, through nozzle  41   b , and into the aforementioned orifices through nozzles  41   c  and  41   d . A multispot injection system of this kind, designed to inject portions of the fuel into a number of different locations for the expansion stroke, improves performance in terms of emissions, power, torque and fuel economy at a variety of speed/load conditions. 
     As with all illustrated variants of the basic invention, namely, the use of a continually rotating disk valve in conjunction with a non-circular cross-section toroid chamber, the specific “best” partial fuel quantities are determined by combustion modelling and/or experimental trials. In the arrangement of FIG. 9, injection of fuel starts in combustion chamber  21  and, if required, sequentially continued in the transfer passages (orifices) and/or toroidal chamber  10 . 
     According to a second preferred embodiment of the invention, the “variable geometry” consists in providing a piston which is mechanically extendible to minimize the residual volume. 
     FIGS. 10 a  to  11   c  illustrate such mechanical means for approaching still more closely the ideal of near-zero distance between piston and valve between the compression and expansion strokes. Piston  12 ′ is an extendable/retractable piston which in FIGS. 10 a  and  11   a  is shown schematically in the process of extending, with piston sections  12 ′ a  and  12 ′ b  separating, following closure of the disk valve and commencement of the expansion stroke under the actuation of hydraulic lifter  47 . 
     In the specific arrangement of FIGS. 10 a  and  10   b , push-pull rod  48  undergoes a reciprocating action, as the assembly of hydraulic lifter  47 , bushing  49  and push/pull rod  48  is carried around stationary camming  46  and  48  to induce a reciprocating action on key rod  50 . 
     Under the control of the camming arrangement, piston  12 ′, on commencement of the engine compression stroke following closure of the disk valve in front of the piston contracts in length at the same speed as its circumferential motion through the toroidal chamber, permitting a higher degree of compression. Subsequently, following closure of the disk valve behind piston  10  and commencement of the expansion stroke of the engine, as illustrated in FIG. 10 a , piston  12 ′ extends in length (expands) under the actuation of the hydraulic lifter, again for the purpose of minimizing the space between piston and valve, i.e. the residual volume, during the expansion stroke. 
     In principle, a VGT engine employing extendible/contractible pistons may perform even more efficiently than the “matched” fixed shape piston arrangement, but this will evidently be at the cost of some complexity and added expense of the engine. Again, however, both approaches are intended to reduce the residual volumes in the compression and expansion strokes in the engine in a way not contemplated, much less realized, in previous rotational engines. 
     An alternative camming arrangement for an extendible-piston VGT is shown in FIG. 11 a  which is the same in principle as that of FIGS. 10 a  and  10   b.    
     The retracting and expanding motion of the piston in this arrangement can be achieved either by a double crank mechanism  48   a ,  48   b  and  48   c  inside piston  12 ′, as in FIG. 11 a , or else by a double wedged rod end  50  and spring loaded piston  12  as in FIG. 11 b . In each case, the piston  12 ′ which approaches the disk valve  18  will shorten its length (retraction), thus reducing the volume in front of the disk valve. Similarly, as the piston passes through the open disk valve it commences to expand, i.e. increase its length, and continues to do so after the disk valve has closed behind the piston so, once again, reducing the volume between the (rear) face of the piston and the disk valve. This ensures that the combustion gas pressure impinges immediately on to the piston without first wasting potential for work by filling a large volume. 
     A further variant for effecting the expansion and retraction of the piston  12 ′ in conformity with its speed of passage through the disk valve to minimize dead volume is by hydraulic activation of the expandable/retractable piston as illustrated in FIG. 11 c . Expansion and retraction are effected by the injection (in the direction of arrows O) or withdrawal of hydraulic fluid through passages  51  and  52 . 
     An optional feature of the VGT engine involves the use of a separate boost pressure system in conjunction with the toroid expansion chamber of the VGT engine. Referring to FIGS. 12 a  and  12   b , there is disclosed an expansion boost pressure device which supplies additional pressure to the toroidal expansion chamber  10  after disk valve  18  is closed. This effect reduces the combustion losses which would otherwise occur as the piston keeps moving circumferentially driven by main shaft  16 . The boost pressure device can be either a piston compressor with high compression ratio or any other high pressure vane or roots compressor feeding a charge into the toroidal expansion chamber  10 . In drawing FIGS. 12 a  and  12   b  the booster piston is referenced by number  53  and the boost charge is indicated by arrows B as being fed into the toroidal chamber. Disk valve shaft  16  is geared to a drive system  54  which through crank  56 , drives the booster piston  53  and provides either compressed air only, or an air-fuel mixture. Reference number  59  in FIG. 12 b  indicates a throttle valve for throttling of fuel into the toroidal expansion chamber. 
     FIG. 13 a  illustrates a combustion chamber improvement which may be referred to as “direct combustion chamber valve drive”. The combustion chamber  21  has an intake valve  21  a and an exit valve  21   b  [the former positioned directly behind the latter in this view] which can be driven either from the main shaft  16  about axis  16 A with a speed-increasing gear box, or else directly from the disk valve shaft  20 , eliminating the gear box. Incorporation of such a direct drive, besides obviating the need for a gear box, may also result in a more compact design having fewer parts and lower weight, with higher engine speeds as a possible consequence. Pressurization of the housing of the VGT engine reduces gap losses and thereby enhances fuel economy and power output. 
     A still further improvement for pressurizing the toroidal housing  10  is illustrated in FIG. 13 a . Housing  10  may be externally pressurized alternatively by the admission of supercharger air through shutoff valve V 1 , or by “booster” air through separate booster through shutoff valve V 2 . 
     Illustrated in FIG. 13 c , is means for providing central lubrication to the engine. Lubricant is introduced (arrows L) to a piston  12  through a central passage  60  in the main shaft  16 , a radial passage  60   a  in the main disk  14  to the outer perimeter, and passages  60   b  and  60   c  extending to piston  12 , effecting the dispersion of lubricant through the action of centrifugal force. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments and all suitable modifications and equivalents coming within the scope of the appended claims.