Abstract:
A free piston engine with a linear power generation system with a variable stroke piston and improved construction and control is provided. The free piston engine includes a single double-ended piston located within a cylinder having opposed combustion chambers positioned at opposite ends of the cylinder. The piston is reciprocated within the cylinder between the opposed combustion chambers. The piston stroke is variably adjustable via the combustion cycles of the free piston engine and the linear power generation system such that the top dead centers of each side of the piston are definable. The piston stroke can then be adjusted to meet the optimum working conditions for a respective application.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to free piston engines and linear power generators. More particularly, relating to a free piston engine with a linear power generator having an improved construction and control system where piston stroke is variable. 
   DESCRIPTION OF THE RELATED ART 
   Free piston devices with power generators are known, for example from U.S. Pat. No. 3,629,596, U.S. Pat. No. 3,675,031, U.S. Pat. No. 4,154,200, U.S. Pat. No. 6,953,010, U.S. Pat. No. 6,181,110, and U.S. Pat. No. 6,707,175. Generally, such devices convert the mechanical energy of a reciprocating piston device into electrical energy through various constructs of linear alternators or generators. Such generators can be permanent magnet generators, reluctance generators, linear DC generators, and two and three-coil induction generators. The efficiency of a free piston linear power generator is highly dependent on the design of the free piston engine, the linear generator, and the control and synchronization of the motion of the piston device with the magnetic field of the generator. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a universally applicable free piston engine with a linear power generation system having an improved construction and control is provided. In accordance with the invention, the free piston engine includes a single double-ended piston located within a cylinder having opposed combustion chambers positioned at opposite ends of the cylinder. The piston is reciprocated within the cylinder between the opposed combustion chambers. The piston stroke is variably adjustable via the combustion cycles of the free piston engine and the linear power generation system such that the top dead centers of each side of the piston are definable. The piston stroke can then be adjusted to meet the optimum working conditions for a respective application. 
   In general, in one aspect, a free piston engine with a liner power generating system is provided and includes a housing having a cylinder and first and second combustion chambers at opposed ends of the cylinder. A piston having opposed first and second ends is contained within the cylinder with the first end facing the first combustion chamber and the second end facing the second combustion chamber. The piston is capable of being axially reciprocated between the first and second combustion chambers. The piston is of a material that is permeable to magnetic flux and is electrically insulative. A first coil being carried by the piston and arranged coaxially therewith. A second coil surrounding the cylinder and being coaxially aligned therewith. A source of external electric current at least intermittently connected to the second coil. Combustion is effected in the first and second combustions chambers in an alternating pattern to cause the piston to reciprocate within the cylinder, thereby generating electric current in the second coil. 
   There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. 
   Numerous objects, features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon a reading of the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of descriptions and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
   For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein: 
       FIG. 1  is a schematic partial cross sectional view of the free piston engine with linear power generation system constructed in accordance with the principles of the present invention; 
       FIG. 2  is a schematic diagram of a piston position detecting means for detecting the longitudinal position, velocity and direction of the piston within the cylinder of the free piston engine; 
       FIG. 3  is a schematic cross sectional view of the linear power generation system illustrating magnetic flux generated by a field coil or stator coil passing through the housing of the free piston engine; 
       FIG. 4  is a schematic cross sectional view of a piston in accordance with one embodiment; 
       FIG. 5  is a schematic cross sectional view of the linear power generation system illustrating magnetic flux generated by a field coil or stator coil passing through the housing of the free piston engine and a flux guide bar positioned axially within the cylinder of the housing; and 
       FIG. 6  is a schematic cross sectional view of a piston in accordance with a second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a schematic cross sectional view of a free piston engine with a linear power generation system constructed in accordance with the present invention is shown. The power generation system  10  includes a free piston engine  100  that is integrated into the power generation system. The power generation system  10  further includes a linear motor generator  200 , an electronic control unit (ECU)  12 , first and second feeding units  14  and  16 ; first and second exhaust units  18  and  20 ; and first and second ignition plugs  22  and  24  as ignition units. The ECU  12  executes an overall control for the power generation system  10 , i.e. for the free piston engine  100 , the linear motor generator  200 , the first and second feeding units  14  and  16 , the first and second exhaust units  18  and  20 , and the first and second ignition plugs  22  and  24 . 
   The free piston engine  100  includes a housing  110  having an axial cylinder  112  into which is placed a free piston  114  for reciprocation therewithin. First and second head units  116  and  118  are attached to or formed integral with the housing at opposite ends and cover the open ends of the cylinder  112 . The first and second head units  116  and  118  along with the opposed ends of the cylinder define first and second combustion chambers  120  and  122  respectively. More specifically, the first combustion chamber  120  is defined by the space surrounded by a first end of the piston  114 , the inside wall surface of the housing  110  forming the cylinder  112  and the inward facing surface of the first head unit  116  that is within the periphery of the inside wall surface of the cylinder. The second combustion chamber  122  is defined by the space surrounded by a second end of the piston  114 , which is opposite the first end thereof, the inside wall surface of the housing  110  forming the cylinder  112 , and the inward facing surface of the second head unit  118  that is within the periphery of the inside wall surface of the cylinder. The piston  114  is a double-ended piston and is capable of being axially reciprocated in the cylinder  112  between the first and second combustion chambers  120  and  122 . The piston  114  is free from attachment to any mechanical mechanisms such as a crank arm, shaft or the like. Further, the piston  114  is constructed from composite materials permitting the piston to be lightweight providing faster response, better linear speed and a high reciprocation rate. 
   Each feeding unit  14  and  16  operates to deliver compressed air from a source of compressed air  17  having a known pressure and fuel through the first and second head units  116  and  118  into the first and second combustion chambers  120  and  122  respectively. The ECU  12  operates each feeding unit  14  and  16  to control the timing and the delivery of an appropriate volume of compressed air and quantity of fuel to each combustion chamber  120  and  122 . Each feeding unit  14  and  16  can deliver compressed air and fuel to each combustion chamber  120  and  122  independently, i.e. compressed air can be fed without fuel and fuel can be fed without compressed air. In this manner, the ECU  12  can control the timing and air-fuel mixture that is delivered into each combustion chamber  120  and  122 . Further, which will be described in great detail below, compressed air can be fed into the combustion chambers  120  and  122  to position the piston at a desired location in the cylinder  112 . 
   Each feeding unit  14  and  16  can be fully integrated with each head unit  116  and  118  respectively, partially integrated with each head unit, or a completely separate assembly which interfaces with each head unit. Each feeding unit  14  and  16  includes an air intake valve  124  and a fuel injector  128  which control air and fuel flow into the combustion chambers. The air intake valve  124  and fuel injector  128  can include electromagnetic valves that are operated by the ECU  12  to feed the desired volume of compressed air and quantity of fuel at a desired timing. 
   Each exhaust unit  18  and  20  operate to evacuate each combustion chamber  120  and  122  respectively of compressed air and/or combustion gases. The ECU  12  operates each exhaust unit  18  and  20  to control the activation timing of each unit. Like the feeding units  14  and  16 , the exhaust units  18  and  20  can be fully integrated with each head unit  116  and  118  respectively, partially integrated with each head unit, or a complete separate assembly which interfaces with each head unit. Further, the exhaust units  18  and  20  can include an electromagnetic valve or valves, as exhaust valves  127 , that are operated by the ECU  12  to evacuate each combustion chamber  120  and  122  through an exhaust port at a desired timing. 
   With further reference to  FIGS. 1 and 2 , a piston detecting means  125  is positioned along the longitudinal length of the cylinder  112  and is connected to the ECU  12  to monitor piston position axially within the cylinder. In one embodiment, the piston detecting means  125  can include a plurality of sensor units  126  positioned along the longitudinal length of the cylinder  112  in a spaced relationship. Each sensor unit  126  operates to detect the piston  114  within a particular assigned axial region within the cylinder  112 . Each sensor unit  126  generates an intermittent or constant signal containing information pertaining to the piston position within its assigned axial region. This piston position signal is received by the ECU  12 , where it is processed along with the received signals from the other sensor units  126 . The ECU  12  uses these signals to determine piston direction, velocity and position within the cylinder  112  and operates the first and second feeding units  14  and  16 , the first and second exhaust units  18  and  20 , and the first and second ignition plugs  22  and  24  accordingly to maintain a desired combustion sequence between the first and second combustion chambers  120  and  122  to reciprocate the piston  114  in the cylinder. Each sensor unit  126  can be a pressure sensor which operates to measure the pressure at a discrete location or within a discrete region within the cylinder  112 . The measured pressures can than be used to determine piston position, direction and velocity. The combustion control process of the free piston engine  100  will be further described in more detail below. In the illustrated embodiment, five (5) pressure sensor units  126  are spaced along the length of the cylinder  112  and are numbered P 1  to P 5  in the direction from the first side of the cylinder or first combustion chamber  120  to the second side of the cylinder or second combustion chamber  122 . 
   Turn back to  FIG. 1 , the linear motor generator  200  includes a movable member, as the piston  114 , a stator or field winding  210  and a housing, as the free piston engine housing  110 . The field winding  210  is arranged coaxially with and surrounds the cylinder  112  of the free piston engine. The field winding  210  can be a single coil or multiple coils connected in series. The field winding  210  extends about the longitudinal length of the cylinder  112 . The field winding  210  is connected to an external source of electrical current  220  through the ECU  12 . The ECU  12  operates to energize the field winding  210  with the external source of electrical current  220  intermittently or continuously to create a magnetic field or circuit which flows longitudinally through the housing  110 , through each head unit  116  and  118  and axially through the cylinder  112 , as best shown in  FIGS. 3 and 4 . In the case of multiple coils connected in series, the ECU  12  can operate to energize all or only one particular coil of the field winding  210 . The housing  110  and each head unit  116  and  118  are comprised of a feroceramic material or the like having a desired magnetic permeability which allows the magnetic field or flux to flow easily through the housing and each head unit. 
   With reference to  FIGS. 3 and 4 , the piston  114  is also constructed of a feroceramic material permitting the magnetic field MF to flow through the piston body without a considerable resistance. A secondary coil  212  is integrated with and carried by the piston  114 . The secondary coil  212  is coaxially aligned with the piston  114  and thus with the cylinder  112  and field winding  210  as well. The diameter of the secondary coil  212  is such that the magnetic flux flowing axially along the cylinder  112  is substantially passed through the interior periphery of the secondary coil. The flux of the field winding  210  flowing across the secondary coil  212  induces a current within the secondary coil resulting in the secondary coil generating a magnetic field itself (not illustrated). The secondary coil  212  can be a single or multiple winding coil. In one embodiment, the secondary coil  212  is a single winding coil of a sheet of electrically conductive material having a width greater than its thickness. A second secondary coil  214  or multiple secondary coils can be integrated with or carried by the piston  114  each operating in the same manner as the first secondary coil  212 . 
   Turning now to  FIGS. 5 and 6 , a magnetic flux guide bar  230  can be included, which is positioned coaxially with the cylinder  112  and extends the longitudinal length of the cylinder  112  where it is connected to each head unit  116  and  118  at opposite ends. In this arrangement, the piston  114  includes an axial bore  216  through which the guide bar  230  is passed. The interior diameter of the through bore  216  is greater than the exterior diameter of the guide bar  230  such that no physical contact is made between the piston  114  and the guide bar  230 . The guide bar  230  is constructed of a material having a high magnetic permeability, such as a feroceramic. The guide bar  230  acts to attract the magnetic flux of the field winding  210  flowing axially through the cylinder  112  and to increase the density of the magnetic flux about the longitudinal axis of the cylinder. Increasing the density of magnetic flux of the field winding  210  about the longitudinal axis of the cylinder  112  increases the magnetic flux that is passed through the interior periphery of the secondary coil  212  or coils, which increases the current induced in the coil resulting in a stronger magnetic field generated by the secondary coil. 
   The magnetic flux guide bar  230  can include axially alternating regions of a lower density of ferromagnetic material and a higher density of ferromagnetic material. These alternation regions of densities of ferromagnetic material causes the density of magnetic flux flowing through the guide bar  230  to change between each region, as best shown in  FIG. 5 . In the regions with less ferromagnetic material, the density of magnetic flux reduces resulting in expansion of the flux flow in these regions as indicated at  232 . The alternating density zones of magnetic flux increases the interaction between the magnetic flux of the field winding  210  and the magnetic field of the secondary coil  212  or coils, as will be further described below. 
   Now operation of the power generation system  10  will be explained. The free piston engine  100  can operate as a two-stroke or four-stroke engine. However, only the two-stroke operation of the free piston engine  100  will be explained. While the piston axially reciprocates one time, a compression stroke of compressing and igniting, and an expansion stroke of combustion and exhausting take place in both the first and second combustion chambers  120  and  122 . The compression stroke and expansion stroke being opposite between the first and second combustion chambers  120  and  122 . In other words, as the piston  114  undergoes a compression stroke in the first combustion chamber  120 , the piston undergoes the expansion stroke in the second combustion chamber  122  and vis-versa. These oppositely phased strokes of the piston  114  between the first and second combustion chambers  120  and  122  reciprocate the piston between each combustion chamber. The particular physics of internal combustion is well known in the art and will not be discussed here in great detail. 
   The position of the piston  114  where the first side of the piston is moved the most towards the first combustion chamber  120  is referred to as “first top dead center” and the position of the piston where the second side of the piston is moved the most towards the second combustion chamber  122  is referred to as “second top dead center”. Likewise, the position of the piston  114  where the first side of the piston is moved the most away from the first combustion chamber  120  is referred to as “first bottom dead center” and the position of the piston where the second side of the piston is moved the most away from the second combustion chamber  122  is referred to as “second bottom dead center”. Piston stroke is referred to the linear distance between the first top dead center and the second top dead center. Or in other terms, stroke can be defined as the linear distance between the first top dead center and first bottom dead center and the linear distance between the second top dead center and second bottom dead center. The linear distance between first top dead center and first bottom dead center is referred to as first piston stroke and the linear distance between second top dead center and second bottom dead center is referred to as second piston stroke. An important aspect of the free piston engine  100  of the present invention is the ability to adjust piston stroke, first piston stroke and second piston stroke to provide a free piston engine having variable stroke. It is possible with the construction of the free piston engine  100  of the present invention for the first and second piston strokes to be unequal, i.e. first top dead center and second top dead center may be of different distances. The importance of this variable stroke construction will become apparent with further discussion of the operation of the power generation system  10 . 
   Combustion in each combustion chamber  120  and  122  is effected through control of the first and second feeding units  14  and  16 , the first and second exhaust units  18  and  20 , and the first and second ignition plugs  22  and  24  by the ECU  12 . The ECU  12  utilizes information gained from the piston position detecting means  125  to effect the timing of combustion in each combustion chamber  120  and  122  and the piston stroke. The ECU  12  can operate to adjust the piston stroke, piston velocity and direction by adjusting the timing of the various aspects of internal combustion in each combustion chamber  120  and  122 . 
   Now, one control scheme for effecting operation of free piston engine  100  be described. In this control scheme there are three separate cycles including a priming cycle, and first and second combustion cycles. The priming cycle is only performed once per each instance of engine start. Once the free piston engine  100  is started, the engine cycles between the first and second combustion cycles. Further, each combustion cycle has an initial phase, a transition phase and an end phase during which each sensor unit  126  measures pressure along the longitudinal length of the cylinder  112  to determine piston position, velocity and direction. Changes in measured pressure along the cylinder  112  allows the ECU  12  to determine which phase each combustion cycle is in and to effect the timing of each combustion cycle. 
   During the priming cycle, the ECU  12  operates the first and second feeding units  14  and  16  and the first and second exhaust units  18  and  20  to position the piston  114  from any longitudinal position within the cylinder  112  to a desired position within the cylinder, to index the piston position and to ready the engine to begin the combustion cycles. For example, the priming cycle operates to position the piston  114  at a desired first top dead center position. In the alternative, the piston  114  could be positioned to be at a desired second top dead position. 
   The piston  114  in position at first top dead center, the first combustion cycle is ready to begin. During the initial phase of the first combustion cycle, the first exhaust unit  18  is closed and second exhaust unit  20  is open. The ECU  12  operates the first feeding unit  14  to inject compressed air and fuel into the first combustion chamber  120  and the first ignition plug  22  to ignite the air-fuel mixture. During the initial phase of the first combustion cycle sensor unit P 1  indicates a high pressure and each sensor unit P 2 -P 5  indicate a low pressure. Once the piston  114  begins to move as a result of igniting the air-fuel mixture in the first combustion chamber  120 , the transition phase of the first combustion cycle begins. During the transition phase, the piston  114  translates within the cylinder  112  from first top dead center to second top dead center. As the piston  114  moves from first top dead center to second top dead center, pressure measured at sensor units P 2  and P 3  increases. Once sensor units P 1 , P 2  and P 3  indicate high pressure and sensor units P 4  and P 5  remain to indicate low pressure, the first combustion cycle enters into the end phase. During the end phase, the first exhaust unit  18  opens and the second exhaust unit  20  closes. 
   At this point, the initial phase of the second combustion cycle begins. The ECU  12  operates the second feeding unit  16  to inject compressed air and fuel into the second combustion chamber  122  and the second ignition plug  24  to ignite the air-fuel mixture in the second combustion chamber  122 . During the initial phase of the second combustion cycle, sensor unit P 5  indicates a high pressure and each sensor unit P 4 -P 1  indicate a low pressure. Once the piston begins to move as a result of igniting the air-fuel mixture in the second combustion chamber  122 , the transition phase of the second combustion cycle begins. During the transition phase, the piston translates within the cylinder  112  from the second top dead center to the first top dead center. As the piston  114  moves from second top dead center to first top dead center, pressure measured at sensor units P 4  and P 3  increases. Once sensor units P 5 , P 4  and P 3  indicate high pressure and sensor units P 2  and P 1  remain to indicate low pressure, the second combustion cycle enters into the end phase. During the end phase, the second exhaust unit  20  opens and the first exhaust unit  18  closes. The above described strokes are repeated, so that the free piston engine  100  continues to operate. 
   It is important to note, the end phase and the initial phase of each combustion cycle overlap respectively. For example, as the first combustion cycle begins to enter the end phase, the second combustion cycle begins to enter the initial phase. The time duration of this overlap can be controlled by the ECU  12  resulting in a shorter or longer piston stroke. More specifically, from the above discussion, a high pressure reading from sensor unit P 3  indicates a transition from the transition and the end phase of each combustion cycle. The ECU  12  can operate the free piston engine  100  to maintain sensor P 3  at a high reading for a desired time duration thus delaying the completion of the transition phase. The longer the time duration or the longer the delay, the further the piston will travel in the cylinder before the end phase of the particular combustion cycle is entered and the initial phase of the subsequent combustion cycle is entered. The time duration can be controlled by the ECU  12  to accurately control the top dead center and bottom dead center position of the piston in each combustion cycle, and thus piston stroke. 
   Next the operation of the linear motor generator  200  will be explained. The ECU  12  operates to synchronize energizing the field winding  210  with the external source of electrical current  220  with the reciprocation of the piston  114  in the cylinder  112  during operation of the free piston engine  100 . The energized the field winding  210  generates a magnetic field as discussed above. This magnetic field flows axially through the cylinder  112  and the secondary coil  212  or coils inducing an electrical current in the secondary coil or coils. The energized secondary coil  212  generates a second magnetic field (not shown) opposing the first magnetic field generated by the field winding  210 . Upon relative movement of the secondary coil  212  and the piston  114  with the field winding  210 , a voltage is induced in the field winding in accordance with well known principles, whereby electrical energy can be coupled out. A power generating device is then made, which is based upon the principle of free piston guidance of the piston  114 . 
   The source of external electrical current  220  can be an alternating current (AC) source, a direct current (DC) source. It is also possible to apply a square wave form current source to energize the field coil  210 . In this case the wave form would be synchronized with the motion of the secondary coil carrying piston  114 . 
   A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.