Patent Publication Number: US-2017370218-A1

Title: Piston Type Axial Expander

Description:
FIELD OF THE INVENTION 
     The present invention relates to a piston type axial expander. 
     BACKGROUND OF THE INVENTION 
     Systems for converting heat coming from an engine into mechanical energy are known. Such assemblies are known in particular for converting heat emitted by internal combustion engines, which is dissipated in particular during the expulsion of exhaust gases and is therefore lost. 
     Such assemblies are subjected to demanding weight and size requirements. They thus tend to become more and more compact. 
     These systems operate with a working fluid that is subjected to a Rankine thermodynamic cycle comprising compression, evaporation possibly supplemented with superheating, expansion in an expander and condensation. The expansion within the expander makes it possible to actuate movable members such as pistons so as to obtain mechanical energy. 
     Of particular interest are systems comprising an expander which includes an intake cylinder head for vapor under pressure, comprising an intake opening for the vapor and an expansion zone comprising a plurality of cylinders, wherein a piston sliding in each respective cylinder is connected to a shaft by an inclined plate, each piston being parallel to said shaft. 
     A plurality of valves arranged in the intake cylinder head make possible the alternating intake of vapor into said cylinders, each valve being controlled by a cam arranged on the shaft and cooperating with a lift and return mechanism of each valve. 
     In this type of expander, the pistons can be single-acting, that is having mechanical work occur by expansion of the vapor only on one side, or double-acting, that is having mechanical work occur by expansion of the vapor on both of its sides. 
     Document WO 2005/073511 describes, with reference to  FIGS. 1 and 6 , expanders comprising single-acting pistons. In these expanders, the connection between the valve stem and the cam is lubricated because it is located in a crankcase also containing the inclined plate. However, this configuration has the drawback of requiring very long valve stems, which increases their mass and consequently inertia loads. The return mechanism of the valves—generally a spring—must therefore be sized correspondingly, which further increases the mass and the size of the expander. Moreover, this construction of the valves is hardly compatible with the use of double-acting pistons. Indeed, in the case of  FIG. 1 , the shaft would interfere with the valves and, to avoid passage of lubricant into the cylinder head containing high pressure vapor, it would be necessary to provide special seals, which are expensive and not very reliable at the operating temperatures of the expander. In the case of  FIG. 6 , the positioning of the valves at the periphery of the expander is more favorable to the use of double-acting pistons, but would require an increase of the outer diameter of the expander. 
     Document U.S. Pat. No. 4,262,579 describes another piston type axial expander. This document does not show any confining of the zone extending from the cam to the cylinder in which each piston slides. Consequently, any leakage of vapor through the guides of the valve stems is lost. Furthermore, the intake angles of the vapor expanders are generally very small, which requires extremely stiff valve lifts and requires, for expanders rotating at high speed, extremely stiff valve return springs. These springs being bulky, particularly in the axial direction, they penalize the compactness of the expander. 
     However, in applications intending to carry such a conversion system on a vehicle, it is necessary to minimize the size and the mass of the different components of the system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One aim of the invention is to correct the disadvantages of existing expanders and to design a piston type axial expander which is more compact while still affording good lubrication and making it possible to avoid vapor losses caused by possible leaks. 
     In conformity with the invention, a piston type axial expander is proposed comprising:
         an intake cylinder head for vapor under pressure comprising a vapor intake opening,   an expansion zone comprising a plurality of cylinders, wherein a piston sliding in each respective cylinder is connected to a shaft by an inclined plate, each piston being parallel to said shaft,   a plurality of poppet valves arranged orthogonally to the shaft in the intake cylinder head alternating intake of vapor into said cylinders, each valve being controlled by a cam arranged on the shaft,   a lift mechanism for each valve cooperating with the cam,   a return mechanism for each valve,
 
said expander being characterized in that the intake cylinder head comprises:
   an enclosed and lubricated central zone comprising the cam and the lift and return mechanisms of the valves,   a peripheral zone in which the intake opening leads, extending around the central zone.       

     “Upper” or “lower” means a relative position with respect to the position of the expander during its normal use. 
     “Longitudinal” means a direction parallel to the axis of the shaft. 
     The terms “central” and “peripheral” are defined relative to the shaft axis: the central zone including said shaft axis and the peripheral region surrounding at least part of the central zone. 
     In a particularly advantageous manner, said peripheral region extends only around an upper portion of the central zone. 
     Preferably, said peripheral zone does not comprise a common wall with the lower portion of the central zone. 
     According to one embodiment, the expander further comprises a lubrication circuit for the central zone, said circuit comprising:
         a channel arranged in the shaft and leading into the central zone through a lubricant entry opening, and   a lubricant output opening arranged in a wall of the lower portion of the central zone.       

     Moreover, the central zone advantageously comprises, in its upper portion, a through opening for the vapor in fluid connection with an exhaust zone of the expander. 
     According to a preferred embodiment, the return mechanism of each valve comprises a torsion bar and a return rocker arm extending between said torsion bar and the stem of a respective valve so as to exert a force pressing the head of the valve into the seat of said valve. 
     Preferably, the torsion bar is parallel to the shaft. 
     According to one advantageous embodiment, the return rocker arm is secured to the valve stem by a fork. 
     Preferably, said fork is in contact with the valve tip at two contact points of which the axis is not parallel to the axis of rotation of the return rocker arm, so that the fork induces a rotation of the stem about the axis of said stem. 
     Moreover, the lift mechanism for each valve comprises a pivoting rocker arm secured to a respective valve, said rocker arm, so-called a cam follower rocker arm, cooperating with the cam so as to separate the head of said valve from the seat of said valve. 
     According to one embodiment, the pistons are double-acting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages will be revealed by the detailed description which will follow, with reference to the appended drawings wherein: 
         FIG. 1  is an schematic diagram of a Rankine circuit, 
         FIG. 2  is a perspective view of the outside of an expander according to one embodiment of the invention, 
         FIG. 3  is a cross-section view at the cylinder head of the expander of  FIG. 2 , 
         FIG. 4  is a longitudinal section view of the expander of  FIG. 2 , 
         FIGS. 5 and 6  are perspective view of lift and return mechanisms of a valve according to one embodiment of the invention, 
         FIG. 7  is a perspective view of a particular embodiment of the return rocker arm, 
         FIG. 8  is a cross-section view at the cylinder head of an expander according to another embodiment of the invention, 
         FIG. 9  is a cross-section view at the cylinder head of the expander according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a circuit implementing a Rankine thermodynamic cycle. 
     Shown in this figure is a circuit  1  containing a working fluid, the circuit  1  comprising:
         a pump  2 , typically a pump designed to deliver a flow of working fluid which establishes a pressure of 40 bars on its discharge;   a first heat exchanger  3  which is called hereafter an “evaporator;”   an expander  4 ;   a second heat exchanger  5 , which will hereafter be called a “condenser;”   a bypass device  6 .       

     The pump  2  supplies the evaporator  3  with working fluid in the liquid state under pressure, typically on the order of 40 bars. 
     The evaporator  3  is located in a medium with elevated temperatures, and thus accomplishes heat transfer between this high-temperature medium and the working fluid so that the latter is evaporated and passes into the gaseous state. 
     The working fluid leaving the evaporator  3  is therefore in the gaseous state and under pressure. 
     The working fluid then passes through the expander  4 , wherein expansion occurs. 
     The expansion of the working fluid in the expander  4  involves movable means such as a rotating mechanical shaft, thus allowing the energy of the working fluid to be recovered. 
     The movable means are advantageously coupled to the shaft of the internal combustion engine, so as to re-inject a mechanical torque. The movable means can also be coupled to energy conversion means such as an electric generator, so as to allow conversion of the energy resulting from the expansion into electrical energy. 
     The working fluid leaving the expander  4  is therefore in the gaseous state and at a low pressure, typically on the order of 2.2 bars. 
     The working fluid then passes through the condenser  5 , which is located in a medium having a low temperature so as to accomplish heat exchange between the working fluid and this low-temperature medium to reduce the working fluid temperature, and bring it back to the liquid state. 
     The working fluid leaving the condenser  5  is therefore in the liquid state and at low pressure, on the order of 2.2 bars. It is then re-injected into the circuit  1  by the pump  2  and repeats the cycle previously described. 
     The circuit  1  shown further comprises a bypass device  6  designed to make it possible to take all or a portion of the working fluid upstream of the expander  4  and re-inject it into the circuit  1  downstream of the expander  4 , thus functioning as a short-circuit of the expander  4 . Such a device is often called a “bypass.” 
     Such a bypass device  6  is advantageously used in the initiation phase of the circuit  1  so as to allow the establishment of conditions of temperature and pressure in the circuit  1  and/or so as to achieve a temperature rise in the crankcase  17 , or in the event of needing to modulate the operation of the circuit  1 . 
       FIG. 2  is a perspective view of the outside of an expander according to an embodiment of the invention. 
     In this embodiment, the expander contains double-acting pistons, vapor under pressure being received in two cylinder heads  10  located on either side of the expander. Although it is preferred, this configuration is not limiting and construction with single-acting pistons can be implemented without departing from the scope of the present invention. 
     The expander  4  comprises an intake opening  100  for vapor under pressure in each cylinder head  10 , as well as two exhaust openings  200 . 
       FIG. 3  is a cross-section view at the cylinder head of the expander of  FIG. 2 , in its normal utilization position.  FIG. 4  is a longitudinal section view of said expander. 
     The cylinder head  10  has an intake opening  100  for vapor under pressure. 
     The cylinder head also comprises a central zone  10 A and a peripheral zone  10 B. For reasons explained in more detail below, the peripheral zone  10 B partially surrounds the central zone  10 A. More precisely, the peripheral zone  10 B extends only around the upper portion of the central zone  10 A. 
     The opening  100  leads into the peripheral zone  10 B of the cylinder head. 
     The expander  4  comprises cylinders  110  (three in number on each side of the expander of  FIG. 4 ) which are arranged in an expansion zone, within which a respective piston  111  slides (see  FIG. 4 ). As indicated above, in this figure the pistons are double-acting, although this configuration is not limiting. 
     Each piston  111  is connected to a shaft  40  by an inclined plate  20 , (see  FIG. 4 ), each piston being parallel to the shaft  40 . In the normal utilization position, the shaft  40  extends in a substantially horizontal plane. The plate is arranged in a crankcase  14  containing a lubricant which is in the liquid state under the operating conditions of the expander. The connection between each piston and the inclined plate is therefore lubricated. 
     Each cylinder is in fluid connection with an exhaust zone which leads from the expander through an exhaust opening  200 , for exhausting the working fluid in the expanded state. 
     In a region extending into the cylinder head  10 , the shaft  40  carries a cam  21 . Said cam can form an integral part of the shaft or be manufactured separately from the shaft and then rigidly secured to it. 
     Said cam  21  controls a plurality of valves  12  (three in the embodiment of  FIG. 3 , this number being purely indicative) so as to allow intake, from the peripheral zone  10 B, of vapor under pressure into a respective cylinder  110 . 
     The valves  12  are poppet valves. In a manner known per se, such valves comprise:
         a circular head  120 , performing the functions of a closure member between the intake cylinder head and each cylinder. When the valve is in the closed position, it will press against a seat of valve  101  arranged in the cylinder head to ensure sealing of the cylinder;   a cylindrical stem  121  the diameter whereof is substantially less than that of the head, which provides for vertical guidance of the valve by sliding in a valve guide  102  provided in the cylinder head;   a collar  122  connecting the head  120  to the stem  121  with a high radius of curvature,   a tip  123  located at the end of the stem opposite the head, coupled to the lift mechanism  13  of the valve which cooperates with the cam  21 .       

     The poppet valves are arranged in a plane substantially orthogonal to the axis of the shaft. 
     One advantage of poppet valves is that they are of small size, which makes it possible to minimize inertia loads, thus favoring the compactness of the expander. Moreover, poppet valves are an intake system that requires good lubrication only in the contact between the stem and the guide. The contact between the head and the seat, which is the hottest contact of this mechanism, can content itself with low lubrication, for example a deposition of lubricant circulating with the working fluid of the expander. Mass proportions of lubricant in the working fluid less than or equal to 5% are sufficient. 
     As can be seen in  FIG. 3 , the cam and the lift and return mechanisms of the valves are located in the central zone  10 A of the cylinder head. 
     This zone  10 A is enclosed and lubricated, which ensures good operation of the mechanical connections involved while still avoiding losses of vapor in the event that leaks occur at the valve guides. 
     Indeed, the zone  10 A advantageously comprises a discharge hole  103  for such possible vapor leaks toward the exhaust zone of the cylinders. Thus, the vapor is re-injected into the vapor circuit. 
     Moreover, the lubricant is brought into the central zone  10 A from the crankcase by means of a bore  22  made in the shaft  40 . Said bore extends longitudinally inside the shaft  40  and leads by a radial bore  23  into the zone  10 A. 
     To this end, the lubricant is pumped from the crankcase  14  through channels  150  by an oil pump  15  located at one end of the shaft  40 . 
     The lubricant thus introduced into the central zone  10 A makes it possible to lubricate the different mechanical connections, then it falls by gravity into the lower portion of said zone  10 A. 
     The central zone  10 A also comprises a lubricant output opening  104  arranged in a wall of the lower portion of said zone. Said opening is in fluid connection with the crankcase  14 , so as to allow return of the lubricant to the crankcase. 
     Inasmuch as the lower portion of the central zone  10 A does not have a common wall with the peripheral zone  10 B which is subjected to a high temperature, the lubricant is not uselessly heated prior to its discharge from the central zone  10 A, which prevents it from being degraded. 
     Moreover, the oil pump  15  being located at the end of the expander, ducts can thus be provided in the lower portion of the central zone for feeding the oil pump with lubricant from the crankcase and for feeding, from the oil pump, certain zones of the expander. Thus, ducts (not shown) can also be provided in this lower portion for discharge of the lubricant feeding the rotating seal of the shaft. 
     In the embodiment illustrated in  FIGS. 2 to 4 , the pistons are double-acting and the expander comprises two intake cylinder heads  10  on either side of the expander. The longitudinal bore  22  made in the shaft makes it possible to supply lubricant to the central zone of each of both intake cylinder heads. 
     Alternatively or as a supplement, a duct  103  can be provided between the exhaust zone of one or more cylinders and the central zone  10 A so as to allow flow of the exhaust vapor after expansion toward the central zone  10 A. This configuration allows, in the case where the working fluid is loaded with lubricant, to lubricate the mechanical connections involved in controlling the valves using the lubricant present in the vapor, alone or as a supplement to the lubrication coming from the shaft. 
     A preferred embodiment of the valve lift and return mechanisms will now be described in more detail, with reference to  FIGS. 5 and 6 . 
     The lift mechanism for a valve comprises a rocker arm  130 , so-called a “cam follower rocker arm” pivoting about an axis parallel to the shaft and having a zone  131  in contact with the cam. Said rocker arm is coupled, through a zone opposite to the zone  131 , with the tip  123  of the valve  12  so as to exert a thrust on the valve when the zone  131  is cooperating with the cam  21 . This thrust has the effect of separating the head  120  of the valve from its seat  101 , thus allowing vapor to be received in the corresponding cylinder. 
     The expander further comprises return means for the valve making it possible to exert a force to press the head of the valve against its seat. 
     In the embodiment illustrated in  FIGS. 5 and 6 , this return means comprises a torsion bar  132  arranged longitudinally in the expander. At one of its ends, the torsion bar is rigidly secured to a rocker arm  133 , so-called a “return rocker arm.” At its opposite end, the torsion bar is rigidly secured to a tube  134 , inside which it is arranged. Said tube  134  is itself fixed to the cylinder head. Lifting the valve slightly twists the bar  132 , this torsion inducing in the bar a returning moment. 
     The return rocker arm  133  is secured to the stem  121  of the valve in the vicinity of the tip. The connection between the return rocker arm  133  and the stem  121  is made by a fork  1330  extending to the end of the return rocker arm opposite the torsion bar. The fork  1330  has two parallel arms surrounding the stem  121 , the distance between the arms being equal to the diameter of the stem. 
     The speed of rotation of the cam is in fact high and the opening angle of the valve is very small. Thus the movement of the valve engenders extremely high inertia loads. The mass of the valve must therefore be reduced and inertia must be opposed with a considerable return force. But it is not possible with a compact spring to have sufficient return force for the small lift heights used, without imposing a large spring preload. This preload causes the cam follower rocker arm to rub constantly on said cam, which increases friction in the motor as well as wear. A torsion bar makes it possible to have high stiffness, which makes it possible to avoid the use of a considerable preload. 
     Whatever the means of implementing the torsion bar, this return mechanism is particularly advantageous in that it has great compactness. In fact, with equal stiffness, the torsion bar has smaller dimensions than a coil spring used conventionally as a return means. 
       FIG. 7  is a perspective view of an advantageous embodiment of the return rocker arm, said rocker arm  133  being suitable for use in the return mechanisms shown in  FIGS. 5 and 6 . 
     Contacts C 1 , C 2  between the arms of the fork  1330  and the tip  123  of the valve allow rotation of the stem about its axis. This rotation is induced by a low angle α provided between the axis X of rotation of the return rocker arm and the axis X′ of the contact points C 1 , C 2  of the fork, which generate a different force at each of both contacts. This makes it possible to permanently fit the head of the valve on its seat and thus to limit leaks throughout the life of the expander. 
       FIG. 8  is a cross-section view at the cylinder head of another embodiment of the axial expander according to the invention. Reference symbols identical with those of the preceding figures designate elements having the same functions as in the previous embodiment, and which are therefore not described again. 
     In this embodiment, the return function is not carried out by torsion bars but by springs  140  each arranged between a valve guide and the tip of the respective valve. The springs  140  thus exert a return force on the valve aiming to press it against its seat. 
     This embodiment is however less preferred, because the springs  140  must be very stiff and consequently bulky, which penalizes the compactness of the expander. 
     Moreover, the control laws for lifting the valves are very stiff because each valve is open only over a very narrow angular range (of the order of 70-80°) compared with an internal combustion engine. The lift height of the valves is also small (of the order of 2 mm) with respect to an internal combustion engine. If the lifting speed is too high, the valve is catapulted out of its seat and its inertia makes it difficult for the return mechanism to maintain contact with the cam or, if applicable, on the cam follower rocker arm as in  FIG. 8 . In this case (called “valve floating”) the duration of opening of the valve is no longer controlled (ballistic operation) and shocks when contact is resumed lead to long-term failure of the system. 
     In the embodiment of  FIG. 9 , the pistons are double-acting and the expander comprises two intake cylinder heads  10  on either side of the expander. Each cylinder head  10  has three intake openings  100   a ,  100   b  and  100   c  for vapor under pressure. Each cylinder head also comprises a central zone  10 A and a peripheral zone  10 B in which the intake openings  100   a ,  100   b  and  100   c  lead. This embodiment is particularly advantageous in that it limits the size of the cylinder heads and thus their mass and cost. 
     Finally, it goes without saying that the examples that have just been given are only particular illustrations which are never limiting as regards the field of application of the invention. 
     REFERENCES 
     
         
         WO 2005/073511 
         U.S. Pat. No. 4,262,579