Abstract:
A method according to this invention for starting a free piston internal combustion engine having no crankshaft for controlling movement of the piston, includes providing a combustion cylinder, a piston moveable in the cylinder, and an inlet port opened and closed by the piston as the piston moves in the cylinder, through which inlet port air enters the cylinder. The piston is displaced linearly in the cylinder sufficiently to open the inlet port before admitting fuel to the cylinder to start the engine.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to starting an internal combustion engine. In particular, the invention pertains to providing a pneumatic charge in the combustion cylinders for use in starting a compression ignition or spark ignition free piston engine.  
         [0002]     A free piston internal combustion engine includes one or more reciprocating pistons located in a combustion cylinder and connected to a load. A crankshaft does not mutually connect the pistons. Instead, during operation, each piston reciprocates in response to forces produced by combustion of an air-fuel mixture in a combustion chamber or cylinder without a crankshaft drive connection to another piston. Piston displacement and piston velocity are monitored and controlled by an actuator system employed to correct periodically minor deviations from synchronized piston movement. The actuator system may be used also to reciprocate the piston while starting the engine before combustion of an air-fuel mixture occurs in the cylinder.  
         [0003]     The engine may drive a current carrying conductor in a magnetic field, thereby inducing an electrical current in the conductor and producing electrical power output. Alternatively, the pistons may be driveably connected to a hydraulic or pneumatic pump-motor, which supplies pressurized fluid to a motor that drives a load, e.g., the wheels of a motor vehicle. A free piston engine that produces hydraulic output includes a hydraulic cylinder and a plunger, driven by the engine, for pumping hydraulic fluid from a fluid source to a high pressure fluid accumulator.  
         [0004]     A free piston engine may include only one piston in a combustion cylinder. In such an engine, after combustion occurs the piston is driven away from the cylinder head by the expansion of the fuel-air charge between the piston and head, but an external power source is required to move the piston toward the cylinder head to compress the next fuel-air charge during the compression stroke. Preferably electric energy is used to drive the piston during the compression stroke when the engine produces electric output, and hydraulic or pneumatic energy is used to drive the piston during the compression stroke when the engine produces hydraulic or pneumatic output.  
         [0005]     An alternate arrangement of a free piston engine may include axially-aligned cylinders, an axially inner pair of mutually connected pistons, and an axially outer pair of mutually connected pistons. One piston of each piston pair reciprocates in a first engine cylinder; the other piston of each pair reciprocates in a second cylinder. Each cylinder is formed with inlet ports, through which air enters the cylinder, exhaust ports, through which exhaust gas leaves the cylinder, and a fuel port, through which fuel is injected into the cylinder. Movement of the pistons in one cylinder caused by combustion of a fuel-air mixture there, forces the pistons in the other cylinder to compress subsequently a fuel-air mixture in the second cylinder and to cause combustion of that mixture. In this way the piston pairs reciprocate in the cylinders in mutual opposition, one piston pair moving longitudinal in one direction while the pistons of the other pair move in the opposite direction to compress the mixture is one cylinder. When combustion occurs there, the direction of movement of each piston pair is reversed until the combustion occurs in the other cylinder.  
         [0006]     Because a free piston engine has no shaft connecting the pistons for synchronizing their reciprocation in the cylinders and connecting the pistons to the load, motion of the pistons is controlled by a control system that synchronizes coordinated piston reciprocation, compression and combustion of an air-fuel mixture. While starting a free piston engine, however, the engine pistons must be actuated to produce combustion to a sufficient compression ratio, which depends, at least in part, on the extent to which the pistons move in the cylinder and the temperature of the air-fuel charge in the cylinder. Piston movement during engine starting may be actuation hydraulically, pneumatically or electrically.  
         [0007]     Various techniques have been devised for starting a compression ignition engine. For example, hydraulic or pneumatic motor-pumps, driveably connected to the engine pistons, may be used to reciprocate the pistons while starting the engine under the control of servo-hydraulic valves. These valves are fast acting and provide a high fluid flow rate, but they are expensive and have limited long-term prospects for cost reduction.  
         [0008]     When an engine is stopped, the piston can be at any position in the cylinder. A free piston engine typically has no inlet valves or exhaust valves to control the flow of air and exhaust gas into and from the cylinder. Instead, the inlet is usually pressurized by a turbocharger, and reed valves are opened by fluid entering the cylinder. If the engine is stopped with a piston in the compression stroke, leakage of the air charge from the cylinder through the inlet and exhaust ports and across the piston rings will occur during the shutdown period due to the pressure in the cylinder. This leakage produces a partial vacuum in the cylinder. A free piston engine, however, relies on a compressible charge in the cylinder to provide a pressure force on the piston head resisting the force that moves the piston to the TDC position during the compression stroke. A technique is required to avoid these and related difficulties that arise when starting a free piston engine.  
       SUMMARY OF THE INVENTION  
       [0009]     An engine to which this invention can be applied includes first and second pairs of mutually connected pistons, a first piston of each pair moving in a first cylinder, and a second piston of each pair moving in a second cylinder. Each cylinder has inlet ports and exhaust ports through which fresh air and exhaust gas enter and leave the cylinders, respectively.  
         [0010]     A method according to this invention for starting a free piston internal combustion engine having no crankshaft for controlling movement of the piston, includes providing a combustion cylinder, a piston moveable in the cylinder, and an inlet port opened and closed by the piston as the piston moves in the cylinder, through which inlet port air enters the cylinder. The piston is displaced linearly in the cylinder sufficiently to open the inlet port before admitting fuel to the cylinder to start the engine.  
         [0011]     The method for starting a free piston internal combustion engine having no crankshaft for controlling movement of the piston can be applied also to such an engine that includes axially aligned combustion cylinders, a first pair of mutually connected pistons, and a second pair of mutually connected pistons. A first piston of each pair is moveable in the first cylinder. A second piston of each pair is moveable in the second cylinder. Each cylinder includes an inlet port, through which air enters the cylinder, movement of the pistons closing and opening the inlet ports. The pistons in the first cylinder are moved sufficiently to open the inlet port of the first cylinder before admitting fuel to the first cylinder to start the engine. The pistons in the second cylinder are moved sufficiently to open the inlet port of the second cylinder before admitting fuel to the second cylinder to start the engine.  
         [0012]     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIGS. 1 and 2  are cross sectional views taken at a longitudinal plane through a free piston engine showing schematically the position of piston pairs and combustion cylinders at opposite ends of their displacement;  
         [0014]      FIG. 3  is a schematic diagram of a fluid control system having a controller for operating fluid pump-motors connected to the engine piston pairs for starting the engine; and  
         [0015]      FIG. 4  is a cross sectional schematic diagram of a free piston engine having a single piston reciprocating in each cylinder and an actuator for starting the engine.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     Referring first to  FIGS. 1 and 2 , a free piston engine  10  includes a first cylinder  12  and a second cylinder  14 , axially aligned with the first cylinder, the cylinders being located in cylinder liners or engine blocks  16 ,  17 . A first pair of pistons, inner pistons  18 ,  20 , are mutually connected by a push rod  22 . A first piston  18  of the first piston pair reciprocates within the first cylinder  12 , and the second piston  20  of the first piston pair reciprocates within the second cylinder  14 . A second pair of pistons, outer piston  22 ,  24 , are connected mutually by pull rods  28 ,  30 , secured mutually at the axial ends of pistons  24 ,  26  by bridges  32 ,  34 . A first piston of the second or outer piston pair reciprocates within the first cylinder  12 , and a second piston  26  of the outer piston pair reciprocates within the first cylinder  14 . Each cylinder  12 ,  14  is formed with air inlet ports  36 ,  37  and exhaust ports  38 ,  39 . In  FIG. 1 , the ports  37 ,  39  of cylinder  12  are closed by pistons  18 ,  24 , which are located near their top dead center (TDC) position, and the ports  36 ,  38  of cylinder  14  are opened by pistons  18 ,  24 , which are located near their bottom center (BDC) position. In  FIG. 2 , ports  36 ,  38  of cylinder  14  are closed by pistons  20 ,  26 , which are located near their TDC position, and the ports  37 ,  39  of cylinder  12  are opened by pistons  18 ,  24 , which are located near their BDC position. When the pistons of either cylinder are at the TDC position, the pistons of the other cylinder are at or near their BDC position. Each cylinder is formed with a fuel port  40 , through which fuel is admitted, preferably by injection, into the cylinder during the compression stroke.  
         [0017]     Displacement of the piston pairs between their respective TDC and BDC positions, the extremities of travel shown in  FIGS. 1 and 2 , is coordinated such that a fuel-air mixture located in the space between pistons  18 ,  24  in cylinder  12  and between pistons  20 ,  26  in cylinder  14  is compressed so that combustion of those mixtures occurs within the cylinders when the pistons have moved slightly past the TDC position toward the BDC position. This synchronized reciprocation of the piston pairs is referred to as “opposed piston-opposed cylinder” (OPOC) reciprocation.  
         [0018]     The synchronized, coordinated movement of the pistons is controlled through a hydraulic circuit, that includes fluid motor-pumps check valves and lines contained in a hydraulic or pneumatic block  43 , located axially between the cylinder sleeves  16 ,  17 . Referring next to  FIG. 3 , the control circuit includes a low pressure accumulator  41 , a high pressure accumulator  42 , a motor pump  44  driveably connected to push rod  22 , a motor pump  46  driveably connected to pull rod  28 , and a motor pump  48  driveably connected to pull rod  30 . Push rod  22  is formed with a piston  50  located in a cylinder  51  formed in block  43 . Reciprocation of engine pistons  18 ,  20  causes piston  50  of motor pump  44  to reciprocate. Pull rods  28 ,  30  are each formed with pistons  52 ,  54 , located in cylinders  55 ,  57 , respectively, formed in block  43 . Reciprocation of engine pistons  24 ,  26  causes pistons  52 ,  54  of motor pumps  46 ,  48  to reciprocate.  
         [0019]     When the engine  10  is running, the coordinated reciprocating movement of the engine pistons draws fluid from the low pressure accumulator  41  to the pump motors  44 ,  46 ,  48 , which produce hydraulic or pneumatic output fluid flow, supplied to the high pressure accumulator  42 . The motor-pumps  44 ,  46 ,  48  operate as motors driven by pressurized fluid in order to start the engine, and operate as pumps to supply fluid to the high pressure accumulator for temporary storage there or to supply fluid directly to fluid motors located at the vehicle wheels, which drive the wheels in rotation against a load.  
         [0020]     An electronic controller  56  produces an actuating signal transmitted to a solenoid or a relay, which, in response to the actuating signal, changes the state of a control valve  58 . For example, when the hydraulic system is operating as a motor to move the engine pistons preparatory to starting the engine or while the engine is being started, controller  56  switches valve  58  between a first state  60 , at which accumulator  42  is connected through valve  58  to the left-hand side of the cylinder  51  of pump-motor  44  through line  64 . With valve  58  in the state  60 , the left-hand sides of the cylinders  55 ,  57  of motor-pumps  46 ,  48 , are connected through lines  68 ,  70  and valve  58  to the low pressure accumulator  41 . These actions cause piston  50  to move rightward forcing fluid from pump-motor  44  through line  72  to the right-hand side of the cylinder  57 , and through line  74  to the right-hand side of cylinder  55 . In this way, the first state of valve  58  causes the fluid control system to move engine pistons  18 ,  20  rightward and engine pistons  24 ,  26  to move leftward from the position shown in  FIG. 3 .  
         [0021]     When controller  56  switches valve  58  to the second state  76 , high pressure accumulator  42  is connected through line  68  to the left-hand side of piston  57  of motor-pump  48 , and through line  70  to the left-hand side of piston  55  of motor-pump  46 . This forces engine pistons  24 ,  26  rightward. When valve  58  is in the second state  76 , the low pressure accumulator  41  is connected through valve  58  and line  64  to the left-hand side of cylinder  51  of motor-pump  44 . As pistons  52 ,  54  move rightward, fluid is pumped from cylinders  55 ,  57  through lines  74 ,  72 , respectively, to the right-hand side of cylinder  51 . This causes piston  50 , push rod  22  and engine pistons  18 ,  20  to move leftward.  
         [0022]     To start the engine  10 , before fuel is injected, pistons  18 ,  20  are moved leftward and pistons  24 ,  26  are moved rightward by the actuator system, described with reference to  FIG. 3 , toward the position shown in  FIG. 1 . This causes the pistons to open the inlet ports  36  in cylinder  14 , thereby ensuring that cylinder  14  is filled with a pneumatic charge. Next, pistons  18 ,  20  are moved rightward and pistons  24 ,  26  are moved leftward by the actuator system toward the position shown in  FIG. 2 . This causes the pistons to open the inlet ports  37  in cylinder  12 , thereby ensuring that cylinder  12  is filled with a pneumatic charge. Then, the actuation system reciprocates the pistons with continually increasing displacement, or length of stroke, in each cycle. The increase of piston displacement is accomplished by progressively increasing the magnitude of the pressure applied to actuator during each displacement cycle, or by increasing the length of the period when pressure is applied to the actuator, or by a combination of these actions. In any case, cyclic compression and expansion of the pneumatic charges in cylinder  12 ,  14  are analogous to that of springs, opposing acceleration of the piston masses toward each TDC position. The actuation system provides a force that accelerates the pistons toward each TDC position.  
         [0023]     Pistons  18 ,  24  move rapidly in cylinder  12  due to combustion in cylinder  14 . An engine controller causes a fuel injector to inject an appropriate quantity of fuel into cylinder  12  between pistons  18 ,  24  through fuel port  40 , thereby starting the engine start. The engine continues to run under programmed control with fuel injection being actively controlled by the engine controller.  
         [0024]      FIG. 4  shows a free piston engine  90  that includes a housing  92 , a piston  94  reciprocating in a combustion cylinder  96 , a compression cylinder  98  and a load  100  secured by a shaft  102  to the piston. Air enters the cylinder through air inlet ports  102 , and exhaust gas leaves the cylinder through exhaust ports  104 . Air is carried through inlet ports  102  into combustion chamber  106  when piston  90  nears its BDC position. As piston  90  moves toward its TDC position, fuel is injected into combustion chamber  106  by a fuel injector operating under control of a fuel control system  110 .  
         [0025]     Piston  94  is supported for reciprocal linear displacement in the combustion chamber  106 . An engine starting system for actuating the piston includes an actuator piston head  108  attached to shaft  102  located in cylinder  98  for movement with the piston  94 . Fluid ports  114  and  116  carry pressurized fluid into cylinder  98  from opposite sides of piston head  108 . A pressure force, produced by pressurized fluid in cylinder  98 , causes piston head  108  and piston  94  to move toward the TDC position during the compression stroke. Pressurized fluid entering cylinder  98  through fluid port  116  causes piston head  108  and piston  94  to move toward the BDC position while the engine is being started or if the engine should misfire.  
         [0026]     To start the engine  90 , after an ignition switch is turned ON and before fuel is injected, piston  94  is moved by the actuator system toward the BDC position sufficiently to open the inlet ports  102 , thereby ensuring that chamber  106  is filled with a pneumatic charge. Then, the actuation system causes piston  94  to reciprocate in chamber  106  with continually increasing displacement amplitude in each displacement cycle. The increase of piston displacement is accomplished by progressively increasing the magnitude of the pressure applied to actuator head  108  during each displacement cycle, or by increasing the length of the period when pressure is applied to head  108 , or by applying pressure alternately to both sides of head  108 , or by a combination of these actions. In any case, cyclic compression and expansion of the pneumatic charge is analogous to that of spring, opposing acceleration of the piston mass toward the TDC position. The actuation system provides a force that accelerates the piston toward the TDC position.  
         [0027]     After the piston head  110  reaches a predetermined position in the combustion chamber during this reciprocation cycling procedure, or when a predetermined compression ratio in chamber  106  is reached, or when pressure in compression chamber  106  reaches a predetermined magnitude, fuel is injected into chamber  106  in a suitable volume to produce combustion and to start the engine  90 .  
         [0028]     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.