Abstract:
A method for starting a free piston, internal combustion engine includes supplying an air charge to a space in a combustion cylinder, and reciprocating the piston in the cylinder so that the maximum pressure of the air charge in the space cyclically increases. Air and fuel are cyclically admitting to the cylinder to produce an air-fuel mixture, and spark ignition is used to produce cyclic combustion of the air-fuel mixture. The air-fuel ratio of the mixture is increasing when a maximum pressure in the cylinder occurs within a predetermined period following a TDC position of the piston. Spark ignition is discontinued, and cyclic compression ignition (HCCI) of the air-fuel mixture occurs.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to starting an internal combustion engine. In particular, the invention pertains to a procedure for starting a compression ignition, free piston internal combustion engine. 
   A free piston internal combustion engine includes one or more reciprocating pistons located in a combustion cylinder. No crankshaft is used to connect the pistons, coordinate their reciprocation, or establish the compression ratio of a fuel-air mixture in the cylinder. Instead, each piston moves in response to forces produced by combustion of the air-fuel mixture in the combustion cylinder. Pressure produced by combustion in one cylinder can be used to compress an air-fuel charge in another cylinder. Or an actuating system can be used to compress the air-fuel mixture following the expansion stroke. The actuating system may be used also to reciprocate the pistons while starting the engine before combustion of the air-fuel mixture occurs. 
   Because a free piston engine has no crankshaft connecting the pistons for synchronizing the compression and expansion strokes, a control system is used to synchronize piston reciprocation, compression of the air-fuel mixture and its combustion. Piston displacement, piston velocity, pressure in the combustion chamber, compression ratio, and other engine operating parameters are monitored and controlled by an actuator system, which periodically corrects deviations from the desired coordinated piston movement. 
   While starting a free piston engine, the pistons are displaced by a starter-actuator system preferably using hydraulic, pneumatic, or electromagnetic actuation. Preferably, electric energy is used to actuate the piston when starting an the engine produces electric power output, and hydraulic or pneumatic energy is used to actuate the piston when the engine produces hydraulic or pneumatic output. This invention of cycling the engine while starting in order to eliminate a vacuum condition in the combustion chamber applies to a free piston diesel engine, compression ignition, spark ignition and homogeneous charge compression ignition (HCCI) combustion engines; however, it will be described with reference to HCCI engine operation. When starting the engine, the intake air has a low temperature, but a large compression ratio of the fuel-air charge in the combustion cylinder is required to produce combustion in a compression ignition engine. Therefore, using conventional engine starting techniques, a large magnitude of energy is required to produce the compression ratio required to start the engine, especially under cold starting conditions. 
   If the engine pistons are driven entirely by an actuator, a large magnitude of energy is required to compress a mixture of fuel and air in the combustion chamber, particularly in a compression ignition engine that requires a high compression ratio for self-ignition to occur. A technique is required to avoid the need for a large capacity energy source to start the engine, and to ensure that combustion occurs and is sustained under a wide range of ambient operating conditions. 
   SUMMARY OF THE INVENTION 
   A free piston engine to which this invention may be applied includes axially-aligned cylinders, an inner pair of mutually connected pistons, and an outer pair of mutually connected pistons. One piston of each piston pair reciprocates in a first 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 admitted, usually by injection, 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 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. When combustion occurs, the direction of movement of each piston pair is reversed until the combustion occurs in the other cylinder. 
   When the engine stops, the pistons 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, a turbocharger driven by engine exhaust supplies a pressurized air charge to the cylinder through an inlet port. If the engine is stopped with a piston in the compression stroke, leakage of the air charge from the cylinder through inlet and exhaust ports and across the piston rings will occur during the shutdown period due to the pressure in the cylinder. This leakage can produce a partial vacuum in the cylinder when the piston is cycled during the next engine restart. When the engine is restarted without a sufficient volume of air in each cylinder to provide compression pressure resistance, a piston can collide with the cylinder head or with another piston in the same cylinder because of the air spring provides insufficient resistance to piston displacement. 
   To avoid relying on large hydraulic or pneumatic pressures in the starting actuator, a cyclic starting strategy has been developed. The pistons are reciprocated during starting with a progressively increasing displacement in order to develop a sufficient magnitude of kinetic energy in the pistons and to produce combustion of the fuel-air charge. Energy applied to the piston in each cylinder by a starting actuator and energy recovered from expansion of the compressed charge in the other cylinder before combustion occurs combine to increase the kinetic energy of the reciprocating pistons and steadily to increase pressure in the combustion chamber. 
   The method for starting the engine uses an actuator, such as a hydraulic or pneumatic pump-motor or an electric linear alternator-starter to move the pistons to a position where the inlet ports are opened. This ensures that air is present in the cylinder in a space where fuel will be admitted and combustion will occur. That air space operates as an air spring during the starting procedure to store kinetic energy from the piston by compressing the air charge during the compression stroke and applying force to the pistons during the expansion stroke. The pistons reciprocate in response to the application of force acting against the air spring. The force is a periodic, preferably having a frequency that is the same or nearly the same as the natural frequency of the system that includes the inertia of the pistons and other masses reciprocating with the pistons, and the air spring, the compressible air spring in the combustion chamber. When cylinder pressure or piston displacement reaches a sufficient magnitude, air and fuel are cyclically admitted to the cylinder, and spark ignition is used to produce cyclic combustion of the air-fuel mixture. The air-fuel ratio of the mixture is reduced when a maximum pressure in the cylinder occurs within a predetermined period following a TDC position of the piston. Then spark or glow plug ignition is discontinued (if it was required), and cyclic combustion ignition of the air-fuel mixture occurs. After a sufficient number of engine combustion cycles are completed to create proper conditions for compression ignition HCCI to be established, the engine throttle is fully opened to allow maximum airflow, and fuel delivery is reduced to continue operating with HCCI combustion only. 
   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 
       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; 
       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; 
       FIG. 4  is a cross section taken along a longitudinal plane of an engine and hydraulic motor-pump assembly; 
       FIG. 5  is an isometric view of a portion of the outer surface of the engine of  FIG. 1 ; and 
       FIG. 6  is a partial transverse cross section of the engine of  FIG. 1  taken at the location of a spark plug or a glow plug. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   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  16 ,  17 , surrounded by an engine block. 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 , and 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 shown located near their top dead center (TDC) position, and the ports  36 ,  38  of cylinder  14  are opened by pistons  18 ,  24 , which are shown located near their bottom center (BDC) position. In  FIG. 2 , ports  36 ,  38  of cylinder  14  are closed by pistons  20 ,  26 , which are shown there located near their TDC position, and the ports  37 ,  39  of cylinder  12  are opened by pistons  18 ,  24 , which are shown there 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. 
   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. Combustion of those mixtures occurs within the cylinders, preferably 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. 
   The synchronized, coordinated movement of the pistons is controlled through a hydraulic circuit, which 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. 
   The actuator connects high pressure accumulator  42  alternately to actuator motors  44 ,  46 ,  48  in order to displace the piston pairs  18 – 20 ,  24 – 26  in their respective cylinders  12 ,  14  against the pressure produced in the cylinders during the compression stroke. Preferably the actuator motors  44 ,  46 ,  48  apply force to the pistons when the pistons are at or near the BDC position, and the motors remove the actuating force before the piston reaches the TDC position. The pressure developed in each cylinder during its compression stroke forces the piston away from the TDC position during the expansion stroke. The increase of piston displacement for each piston displacement cycle is accomplished by progressively increasing the magnitude of the pressure applied by the actuator motors 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. 
   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, which drive the wheels in rotation against a load. 
   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, 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 . 
   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. 
   Referring now to the cross section of  FIGS. 4A and 4B , the inner pistons  18 ,  20  are bolted to a hydraulic plunger  82 , which reciprocates with pistons  18 ,  20  within a hydraulic cylinder  84  along axis  86 . When the engine  10  is producing hydraulic output, hydraulic fluid at relatively low pressure is supplied to cylinder  84  from a low pressure rail  88  through a check valve  90 , which opens communication to cylinder  84  when the pressure in rail  88  is greater than the pressure in the cylinder  84 , and otherwise closes communication to prevent flow from rail  88  to the cylinder  84 . Similarly, plunger  82  pumps hydraulic fluid from cylinder  84  through a check valve  92  to a high-pressure rail  94 . Check valve  92  closes to prevent flow from rail  94  to cylinder  84  when pressure in rail  94  is greater than that of cylinder  84 . 
   Pressure sensors  96 ,  98  produce electronic signals representing the pressure in combustion cylinders  12 ,  14 . The signals produced by sensors  96 ,  98  are supplied as input to the electronic controller  56 , which receives other input signals, executes control algorithms that employ information regarding current operation conditions represented by the input signals, and produces output signals for controlling engine throttle valves  128 ,  129 , a fuel supply system, an engine ignition system, and the starter-actuator. Fuel injectors  100  supply fuel to cylinders  12 ,  14  through the fuel ports  40  under programmed control of controller  56 . 
     FIG. 5  shows the location of a spark plug  104 , and  FIG. 6  shows a glow plug  106 , which can be used instead of a spark plug. Either the spark plug or glow plug is located in a wall  108  of each liner  16 ,  17  of combustion cylinders  12 ,  14 . When the spark plug  104  is used, controller  56  produces an output signal that ultimately produces a voltage differential across the spark plug terminals and a spark in the combustion chamber, which ignites the air-fuel mixture there. When the glow plug  106  is used, controller  56  produces an output signal that causes an electric current momentarily to pass through the glow plug creating a hot spot in the combustion chamber that ignites the air-fuel mixture. 
   Air enters the engine through inlet ports  36 ,  37 , which communicate with the output of a turbocharger (not shown). Inlet ports  37  in cylinder  12  are supplied from the turbocharger with air though passages  110 ,  112 ; intake reed valves  114 ,  116 ; scavenge pump inlet ports  118 ; a scavenge pump  120 ; scavenge pump outlet ports  122 ; outlet reed valves  124 ; and an air intake annulus  126 , which communicates with inlet ports  37 . A similar circuit carries air from the turbocharger output to air inlet ports  36  in cylinder  14 . Passages  110  each contain a throttle valve  128 , which opens and closes in response to an output command signal produced by controller  56  while starting the engine. After the engine is started, throttle valves  128  are opened, and the engine operates independently of throttle control or ignition control, preferably with homogeneous charge compression ignition. 
   Before fuel is injected to start the engine  10 , pistons  18 ,  20  are moved leftward and pistons  24 ,  26  are moved rightward by the actuator toward the position shown in  FIG. 1  sufficiently to cause the pistons to open the inlet ports  36  in cylinder  14 , thereby ensuring that cylinder  14  is filled with a pneumatic charge, an air charge. Next, pistons  18 ,  20  are moved rightward and pistons  24 ,  26  are moved leftward by the actuator toward the position shown in  FIG. 2  sufficiently to cause the pistons to open the inlet ports  37  in cylinder  12 , thereby ensuring that cylinder  12  is filled with an air charge. 
   After an air charge is admitted to each cylinder, the actuator reciprocates the pistons producing compression and expansion strokes having increasing piston displacement or stroke, increasing piston speed, increasing peak pressure in the combustion chamber, increasing compression ratio of the air charge, but without allowing piston displacement to open the inlet ducts  36 ,  37 . Cyclic compression and expansion of the air charges in cylinders  12 ,  14  are analogous to the effect of a compression spring located in each cylinder. Compression of the pneumatic charge in a cylinder opposes acceleration of the piston masses toward the TDC position in that cylinder. Expansion of the pneumatic charge in a cylinder assists in accelerating the piston masses toward the BDC position in that cylinder. As the charge in one cylinder is being compressed, the charge in the other cylinder is expanding. Therefore, pressure forces are continually developed that assist the pistons in each cylinder to move alternately toward the TDC and BDC positions in the correct phase relationship. 
   To restart a hot or warm engine, it is expected that only one or two cycles of compression and expansion strokes will be required after admitting the air charges to the cylinders and before subsequent engine starting steps are performed. To start a cold engine, it is expected that about ten such cycles will be required after admitting the air charge and before additional engine starting steps are performed. 
   Next, a volume of fuel to be added to each air charge during a first series of cycles while starting the engine with spark ignition is determined. Throttle valves  128  are used to establish a flow rate of air into the cylinders through the inlet ports  36 ,  37  during a first series of starting cycles. Fuel is admitted to the cylinders through fuel ports  40  such that a stoichiometric mixture of fuel and air, or a mixture that is approximately stoichiometric, is present in the cylinders. Either spark plug  104  or glow plug  106  produces ignition. Combustion of the fuel-air mixture in the cylinders  12 ,  14  at the correct phase relation to the peak pressure occurs. After the engine begins to run under spark ignition, the actuator stops driving the pistons, and the engine operates independently of the starter-actuator. The engine controller causes the fuel injectors  100 ,  102  to inject fuel repetitively in an appropriate quantity of fuel thorough fuel ports  40  into the combustion chambers located between the pistons in each cylinder  12 ,  14 . 
   The peak pressure in each cylinder is monitored by pressure sensors  96 ,  98 . The controller  56  determines whether the peak pressure during spark ignition occurs when the pistons are at the TDC position in the combustion cylinder, or within a predetermined period or distance after the TDC position. The period is preferably about 0.25 ms. after TDC, or a delay comparable to 2° after TDC for a two stroke, crankshaft internal combustion engine supplied with a comparable fuel, such as gasoline. The controller  56  adjusts the spark ignition timing until the peak pressure occurs within an acceptable phase range. 
   When ignition occurs at an acceptable phase relation to the peak pressure, a second series of engine starting cycles begins. During these engine cycles, the air-fuel ratio in the cylinders is reduced by using the throttle valves  128  to increase the air flow rate supplied to the cylinders, or by using the fuel injectors  100 ,  102  to reduce the fuel flow rate to the cylinders, or by using both the throttle valves and fuel injectors to increase the air flow rate and reduce the fuel flow rate. The spark ignition system is turned off by the engine controller  56 . Thereafter, the engine operates preferably with a homogenous air-fuel charge and combustion occurs by compression ignition. After the engine starts and continues to run under programmed control, and an external load can be placed on the engine. 
   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.