US3673999A

US3673999A - Electrical apparatus for initiating combustion in free piston engines

Info

Publication number: US3673999A
Application number: US66215A
Authority: US
Inventors: James G Lacy; John V Byrne
Original assignee: Individual
Current assignee: Tectonics Companies Inc
Priority date: 1970-08-24
Filing date: 1970-08-24
Publication date: 1972-07-04
Anticipated expiration: 1989-07-04
Also published as: AU3259371A; DE2142208A1; GB1357957A; AU461892B2; FR2102376B2; ES394470A2; NL7111552A; FR2102376A2; CA930791A; SE384904B; CS164891B2; DE2142208B2; ZA714718B; BR7105493D0; BE771550R; CH529922A

Abstract

Electrical apparatus is disclosed for initiating and controlling combustion in free piston engines having a power piston freely reciprocating within a power cylinder of the engine and alternately expanding and compressing gases in a combustion chamber within the power cylinder. A velocity probe in the form of a magnet arranged to move with the power piston and a sensing coil fixed to the housing of the free piston engine provides an electrical signal indicating the instantaneous value of the velocity of the power piston of the free piston engine. This electrical signal from the velocity probe is applied to a Schmitt trigger through a suitable electrical connection such as a voltage divider network or preferably a resistor-capacitor and diode network, and the Schmitt trigger provides an electrical timing signal to conventional means for initiating successive combustion power strokes within the engine. Using the electrical apparatus of the present invention, combustion is initiated for each power stroke at a point in time related to the velocity of the power piston, and thus to the time at which the power piston reverses direction, without the necessity of regard to the physical position, i.e., linear location, of the power piston along its path of movement within the power cylinder. Specifically, the Schmitt trigger is set in one condition when the piston velocity increases to a predetermined level as the piston accelerates during the first part of its movement toward a firing position. Then, as the piston slows down while approaching its reversal for the start of the power stroke, the decrease in piston velocity to a predetermined level sets the Schmitt trigger in another condition to cause a spark igniting another combustion initiating action. Additional electrical circuitry is included which may be used to aid in starting free piston engines.

Description

United States Patent 15] 3,673,999 Lacy et al. July 4, 1972 I541 ELECTRICAL APPARATUS FOR 57 ABSTRACT lNlTlATlNG COMBUSTION IN, FREE PISTON ENGINES [72] Inventors: James G. Lacy; John V. Byrne, both of Dublin, Ireland [73] Assignee: Anton Braun, Minneapolis, Minn.

22 Filed: Aug. 24, 1970 [2]] Appl. No.: 66,215

Primary Examiner-Laurence M. Goodridge Attorney-Frederick E. Lange and William C. Babcock Electrical apparatus is disclosed for initiating and controlling combustion in free piston engines having a power piston freely reciprocating within a power cylinder of the engine and al ternately expanding and compressing gases in a combustion chamber within the power cylinder. A velocity probe in the form of a magnet arranged to move with the power piston and a sensing coil fixed to the housing of the free piston engine provides an electrical signal indicating the instantaneous value of the velocity of the power piston of the free piston engine. This electrical signal from the velocity probe is applied to a Schmitt trigger through a suitable electrical connection such as a voltage divider network or preferably a resistor-capacitor and diode network, and the Schmitt trigger provides an electrical timing signal to conventional means for initiating successive combustion power strokes within the engine. Using the electrical apparatus of the present invention, combustion is initiated for each power stroke at a point in time related to the velocity of the power piston, and thus to the time at which the power piston reverses direction, without the necessity of regard to the physical position, i.e., linear location, of the power piston along its path of movement within the power cylinder. Specifically, the Schmitt trigger is set in one condition when the piston velocity increases to a predetermined level as the piston accelerates during the first part of its movement toward a firing position Then, as the piston slows down while approaching its reversal for the start of the power stroke, the decrease in piston velocity to a predetermined level sets the Schmitt trigger in another condition to cause a spark igniting another combustion initiating action. Additional electrical circuitry is included which may be used to aid in starting free piston engines.

11 Claims, 17 Drawing Figures PATEIITEDJUL 4 m2 SHEET 3 [IF 4 VOLTAGE ACROSS TERMINALS 30 AND 32 T-2 VOLTAGE AT COLLECTOR 76 T-z VOLTAGE AT COLLECTOR 92 FIE l AST- 4 FIE 12 I INVENTOR$ ddMES a. may, doll/V 1/. BYE/V5 PNENTEnJum mm 9.979 999 SHEET 4 OF 4 2 42 I f -lO 1 I 172 57 l I F\ I 52 -/17 Z 44 170 85 172 g s l nnn'mm m MEAN VALUE OF VOLTAGE ON c 186 184 ENGINE STARTING Y FROM REST INVENTORS' c/AIMESGLACM c/OH/V If BVQA/E ELECTRICAL APPARATUS FOR INITIATING COMBUSTION IN FREE PISTON ENGINES CROSS REFERENCES The present invention provides improved means for practicing the invention disclosed and claimed in an application Ser. No. 066,385, entitled FREE PISTON ENGINE IGNITION APPARATUS filed of even date herewith by Anton Braun.

BACKGROUND Combustion is initiated by many techniques in prior art free piston engines. The most often used technique is where a direct mechanical link is made to a piston and combustion is initiated by means of this mechanical link at a predetermined linear location of the piston along its normal path of movement. Examples of this technique are where combustion initiating apparatus is directly geared to the piston movement or where the reciprocating piston has a cam surface upon it and a cam follower riding on the cam surface initiates combustion. Other previously used combustion initiating techniques include those where: the piston movement controls the firing gap of the spark device directly and combustion is initiated when the piston approaches sufficiently close to a fixed projection into the power cylinder; where the position of the power piston is magnetically sensed as it approaches a fixed position in the cylinder, and combustion is initiated; and where the fixed minimum volume of the power cylinder is sensed by using it as a resonant chamber, and combustion is initiated when this minimum volume is achieved.

All of these methods have a significant drawback in that combustion is initiated only when the power piston reaches a fixed point in the power cylinder. If the piston does not reach this fixed point, there is no ignition. The piston may not reach this fixed point in the power cylinder for various reasons, including an earlier misfire. Thus, if a misfire occurs and there is not sufficient power developed by the engine to provide the energy necessary to return the power piston to the position necessary to again initiate combustion, the engine will stop since combustion will not thereafter occur.

SUMMARY The apparatus according to the present invention solves this and other problems of the prior art by initiating combustion around the point in the cycle where the power piston within the engine reverses direction, without regard to the physical position of the power piston within the power cylinder. That is, combustion may be initiated at a substantially preselected point in time before the power piston in its cylinder stops and reverses direction, even though the actual physical position of the power piston in its cylinder at this point in time may vary considerably from one piston stroke to another.

This combustion causing apparatus has the advantage that, in the event of a misfire, the engine will only lose power for a few strokes and then regain full power. The engine will regain full power in a few strokes, even if there is insufficient energy, immediately after the misfire, to fully return the power piston to its desired normal operating inner dead point or top-deadcenter position in the power cylinder, because: immediately before the power piston stops and begins to reverse direction, whatever its linear position in the power cylinder may be, combustion will be initiated; this combustion of gases, although less efficient as compared to the correct operating point of the engine, will supply sufficient energy to the power piston to return it to a point closer to a desired inner dead point than achieved during the last stroke; and in a succession of these inefficient power strokes the desired inner dead point of the power piston will again be attained. Thus, in contrast to the prior art, a misfire will not cause the engine to stop and necessitate a restarting.

Briefly, a preferred embodiment of apparatus according to the present invention includes means for sensing the instantaneous velocity of a power piston within the free piston engine. The velocity sensing means includes a magnetic sensing coil attached to the housing of the free piston engine and a magnet connected to move with the power piston. The voltage output of the magnetic sensing coil is proportional to the velocity of the power piston and thus an electrical signal output from the sensing means is proportional to the power piston velocity. The electrical signal output from the sensing means is applied to a Schmitt trigger circuit through a voltage divider network or a resistor-capacitor and diode network, and the Schmitt trigger circuit provides the properly timed signal to initiate combustion.

The voltage signal output from the sensing means first exceeds the preset threshold of the Schmitt trigger circuit and that circuit provides a first signal output which prepares the remaining circuitry for an ignition initiating signal. When the voltage signal output from the sensing means decreases below the preset threshold of the Schmitt trigger circuit, that circuit provides a second signal output which is used to trigger a silicon controlled rectifier of a conventional capacitive discharge ignition arrangement and initiate combustion.

Accordingly, it is a primary object of the present invention to provide improved electrical apparatus for controlling combustion within a free piston engine in a manner related to the time at which the power piston within the engine reverses direction within its cylinder without the necessity of regard to the physical position of the power piston within the power cylinder.

This and further objects and advantages of the present invention will become clearer in the light of the following detailed description of a preferred embodiment of the present invention and from the appended claims.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a diagrammatic view of a free piston engine and the electrical combustion initiating apparatus of the present invention with a velocity probe operated by the free piston engine providing the electrical signal used to initiate combustion;

FIG. la shows an enlarged view of the velocity probe of FIG. 1;

FIG. 2 shows a schematic representation of electrical circuitry operating according to the present invention, with an electrical signal from the velocity probe applied to a Schmitt trigger circuit; 7 t

FIGS. 3 through 6 show electrical waveforms at various points in the electrical circuit of FIG. 2;

FIG. 7 shows a schematic representation of an additional electrical circuit which may be used in conjunction with the electrical circuitry of FIG. 2 to aid in starting a free piston engine;

FIG. 8 shows a diagrammatic representation of a switch shown schematically within FIG. 7;

FIGS. 9 through 12 show waveforms at various points in the electrical circuit of FIG. 2 when used in conjunction with the electrical circuit of FIG. 7;

FIG. 13 shows a schematic representation of additional electrical circuitry which may be used in conjunction with the electrical circuitry of FIGS. 2 and 7 to further aid in starting a free piston engine;

FIG. 14 is a circuit diagram similar to the signal generating and input portion of FIG. 2, but utilizing a preferred resistorcapacitor and diode network with the velocity-responsive transducer to provide an input signal for the Schmitt trigger circuit;

FIG. I5 is a graph showing the relationship of the desired ignition time to the velocity of the power piston and the mean value of the voltage across the condenser of FIG. 14, during operation of the engine; and

FIG. 16 is a graph showing how the voltage across such condenser varies as the engine starts up from rest.

DESCRIPTION In FIG. 1, a free piston engine generally designated as 13 is shown having a power piston 14 and compressor piston 19 inter-connected by shaft 21 to form a commonly moveable power means. Power piston 14 is reciprocally moveable within a power cylinder 23 formed within a housing 25 of free piston engine 13 to alternately expand and compress gases within a combustion chamber 27 defined within power cylinder 23. A permanent magnet generally designated is connected to shaft 21 to move with power piston 14. A magnet member generally designated as is fixed with respect to the housing of free piston engine 13 in the embodiment shown. Magnetic member 20 is formed of a coil of highly conductive wire wound on magnetic material having a relatively high permeability and a relatively low hysteresis and provides an electrical signal output across terminals and 32 from that coil. The electrical signal is applied to the circuitry of FIG. 2 in a manner hereinbelow explained. The circuitry of FIG. 2 provides an output electrical signal to a conventional spark plug 29 in order to initiate combustion within the free piston engine 13. Since magnetic member 20 is designed to be fixed with respect to the housing of the free piston engine in the embodiment shown in FIG. 1 and magnet 15 moves with the power piston 14, magnet 15 and magnetic member 20 reciprocate with respect to each other within free piston engine 13.

In FIG. 1a, permanent magnet 15 is shown having a first pole l6 and a second pole l7 interconnected by a portion 18. Magnetic member 20 is shown including a first arm 22 and a second arm 24. The

arms

22 and 24 of member 20 are attached by a portion 26 so as to override

poles

16 and 17, respectively, of magnet 15. As can be more clearly seen in FIG. 1a, a coil of wire, designated 28, is wound upon portion 26 of member 20 so that the changing flux passing from arm 22, through portion 26, and to arm 24 also passes through coil 28 and induces a voltage across the

terminals

30 and 32 attached to the ends of the coil 28.

Magnet 15 and member 20 together form a movement-related parameter sensing means used with the electronic circuitry of the present invention and yield a signal related to the instantaneous velocity of a power piston of a free piston engine, as will be explained hereinafter. The movement-related parameter which is sensed for generating a signal for the circuitry of the present invention may also be the acceleration of a free piston or the velocity or acceleration of some other moving mass within the engine, a pressure created by a free piston within the engine, a fluid flow produced by the movement of a piston within the engine, the voltage output of a generator run by the free piston engine, or another parameter which is related to the movement of a free piston or other mass within the engine. Thus, for the purposes of the present invention, a movement related parameter is defined to be a parameter which gives an indication of the movement of a free piston or other mass within the engine moving with the free piston, as distinguished from the specific linear position of the piston or other mass. For the purposes of the present invention, a movement-related parameter is to be contrasted with a displacement parameter which directly indicates the position or linear displacement of a free piston along its path of movement. Examples of such displacement parameters are the displacements or positions of parts mechanically linked to a moving piston within a free piston engine and the specific displacement or instantaneous position of a cam follower riding on a cam surface fixed to a moving piston within the free piston engine.

More broadly, such a movement-related parameter is used to provide a controlling electrical signal based on a characteristic of operation which is reversal-time-related, i.e. which has a predetermined relationship to the time at which the power piston ends its compression stroke and is ready to start its power stroke in the opposite direction. At this reversal point, the piston velocity drops at least momentarily to zero. The reversal-time-related signal will thus be used to cause combustion at a point in time which has a desired time relationship to the moment at which the power piston reverses its stroke, even though that moment of reversal may occur at different linear positions of the power piston from one stroke to another.

In FIG. 2, a variable resistor 34 is connected between terminal 30 and ajunction point 36. A resistor 38 is connected between junction point 36 and terminal 32, which also functions as the circuit common or ground. Variable resistor 34 and resistor 38 form the two parts of a conventional voltage divider network with the voltage output from the voltage divider appearing at junction point 36 which is directly connected to a further junction point 37. A first diode 40 has its anode connected to junction point 37 and its cathode connected to a power input terminal 42 which is arranged to connect to a source of positive voltage. A second diode 44 has its cathode connected to junction point 37 andits anode connected to the circuit common which includes terminal 32. Diode 40 prevents the voltage at junction point 37 from exceeding the positive voltage existing on power input terminal 42, and diode 44 prevents the voltage at junction point 37 from decreasing below the voltage at the circuit common. One end of a resistor 46 is also connected tojunction point 37, and the other end of resistor 46 is connected to a base 48 of an N- P-N transistor generally designated as 50. The collector 52 of transistor 50 is connected to power input terminal 42 through a collector resistor 54. The emitter 56 of transistor 50 is connected to the circuit common through an emitter bias resistor 58. An additional transistor generally designated 72 includes an emitter 74 also connected to the emitter 56 of transistor 50 and to one end of emitter bias resistor 58. A collector 76 of transistor 72 is connected to power input means 42 through a collector resistor 78, and a base 80 of transistor 72 is connected to collector 52 of transistor 50 through a resistor 82. Transistor 50, its associated bias circuitry, and transistor 72, along with its associated bias circuitry, form a conventional Schmitt trigger circuit generally designated as 83. A diode 84 has its anode connected to the collector 76 of transistor 72 and its cathode connected to the base 86 of a transistor generally designated 88. Transistor 88 has an emitter 90 connected to the circuit common and a collector 92 connected to power input means 42 through a collector resistor 94. A capacitor 96 interconnects the collector 92 of transistor 88 and a junction point 98. A diode 100 has its cathode connected tojunction point 98 and its anode connected to the circuit common to prevent the voltage atjunction point 98 from decreasing below the voltage of the circuit common.

A pair of input terminals 102 and 104 are also shown in FIG. 2. Terminals 102 and 104 are arranged to connect a conventional 12 to 400 volt DC to DC converter 106 to a source of power, not shown, such as 12 volt battery. Converter 106 includes a positive output terminal 108 and a negative output terminal 110. A silicon controlled rectifier or thyristor 112 has an anode connected to output terminal 108 and a cathode connected to output terminal 110. The gate or trigger of the silicon controlled rectifier 112 is connected to junction point 98. A capacitor 114 has one end connected to output terminal 108 and a second end connected to one end 116 of the primary 118 of a conventional pulse-high frequency ingition transformer generally designated 120 of the type used with capacitor discharge ignitions. The other end 122 of primary 118 is connected to the circuit common. The secondary 124 of transformer 120 has one end 126 connected to the circuit common and another end 128 arranged to attach to the spark plug 29 initiating combustion in the free piston engine or to other known combustion causing means associated with the free piston engine.

OPERATION In operating the electrical combustion initiating apparatus of the present invention, the magnet 15 is attached to move with the power piston of the free piston engine, and thus magnet continually reciprocates with the ower piston. Member is attached to the housing of the free piston engine adjacent the reciprocating magnet 15 such that the

arms

22, 24 of member 20 are aligned with

poles

16, 17 of magnet 15, respectively, as the magnet 15 reciprocates under the member 20. A magnetic circuit is thus completed from pole 17, through arm 24, through portion 26, through arm 22, and to pole 16 as the

arms

22, 24 overlay the

poles

16, 17. The exact spacing of member 20 and magnet 15 depends upon the voltage output desired from coil however, a greater spacing allows for variation in magnetic pole strength of magnet 15 from unit to unit without significantly affecting output. The flux traversing member 20 passes through coil 28 so as to conventionally induce a voltage across terminals and 32 attached to the wire ends of coil 28.

Since the rate of change of flux linking coil 28 is arranged to be proportional to the rate of change of overlay between the poles l6.and l7 and the

arms

22 and 24, the voltage induced in coil 28 is directly proportional to the velocity of magnet 15 with respect to coil 28. This voltage induced in coil 28 is represented in ,FIG. 3. Before the

arms

22, 24 overlay the

poles

16, 17, no voltage is induced in coil 28.

As

arms

22, 24 begin to

overlay poles

16, 17, there is an immediate traversal of flux from pole 17, through arm 24, through a portion 26, through arm 22, and to pole 16. Depending upon the relative velocities of the magnet 15 and the member 20, a step of voltage of a particular amplitude is induced in coil 28. To control the initiation of combustion within the free piston engine, the member 20 is placed near the end of the stroke of the power piston, also termed the topdead-center or the inner dead point of the power piston. This location will insure generation of its desired signals both at the end of a power piston stroke of normal desired length during operation of the engine and also at the end of any power piston stroke of somewhat shorter length which might occur during start-up or as a result of a misfire. This is due to the fact that as shown in FIGS. 1 and 1a, the magnet 15 is substantially longer than the core member 20 on which coil 28 is disposed. Thus, over a substantial portion of the piston stroke including the terminal portion, the electromagnetic coupling between member 20 and coil 28 is relatively constant so that the voltage induced in coil 28 due to relative movement of the member 20 and coil 28 during this portion of the piston stroke is independent of the piston position.

Near the end of the power piston stroke the voltage induced in coil 28 slowly decreases from the initial positive value of this step to zero and slowly increases in a negative direction as the power piston and hence magnet 15 lose velocity, stop, and reverse direction in the free piston engine with respect to coil 28. As poles I6, 17 move from beneath

arms

22, 24, again there is no flux traversing member 20 (and hence no rate of change of flux) andtthe voltage induced in coil 28 drops to zero in a step-like fashion.

The induced voltage represented in FIG. 3 is applied to the electrical circuitry of FIG. 2. The voltage is first reduced by the effect of

voltage divider resistors

34 and 38 such that a fixed proportion of the voltage represented in FIG. 3 appears at

junction points

36 and 37. As is well known to those skilled in the electric arts, the exact proportion of the voltage applied to the voltage divider network through

terminals

30 and 32 that will appear at junction point 16 depends on the relative magnitudes of the

resistors

34 and 38. In fact, by varying resistor 34, the precise timing of the initiation of ignition may be somewhat controlled, as will be hereinafter explained.

The fixed proportion of the voltage appearing atjunction point 36 is applied to Schmitt trigger 83, and in particular to the base 48 of the normally non-conducting transistor 50. The application of the initial step of voltage represented in FIG. 3 at T-1 renders the normally non-conducting transistor 50 conducting by applying a voltage to the base 48 of transistor 50 which exceeds the voltage at the emitter 56 of transistor 50 by more than the normally required base-emitter voltage of a conducting transistor. When transistor 50 begins conducting,

the voltage between the collector 52 and the emitter 56 of transistor 50 begins to decrease rapidly towards a collectoremitter saturation voltage of approximately 0.8 volts for a silicon transistor.

Transistor 72 of Schmitt trigger 83 is normally conducting; however, when transistor 50 begins conducting, the base current for transistor 72 is shunted through transistor 50 and to the circuit common. Since the base bias for transistor 72 is shunted through transistor 50, transistor 72 conducts less current. Since transistor 72 conducts less current, the current through emitter resistor 58 decreases and the voltage across resistor 58 decreases. A decrease in voltage across resistor 58 lowers the voltage at emitter 56 of transistor 50 and thus increases the voltage across the base-emitter of transistor 50 which causes transistor 50 to become more conducting. The circuit then regenerates in the conventional fashion of a Schmitt trigger until transistor 50 is fully conducting. When transistor 50 is fully conducting, the collector-emitter saturation voltage of transistor 50 is approximately equal to the necessary base-emitter voltage of transistor 72; thus there is insufficient voltage remaining across base resistor 82 to provide base current to transistor 72, and transistor 72 is rendered non-conducting.

With transistor 72 in its normal conducting state, the voltage appearing at collector 76 of transistor 72 is just slightly more positive than the circuit common because of the collector-emitter saturation voltage of transistor 72 and the current flowing through resistor 58. When transistor 50 is rendered conducting and transistor 72 is thus rendered non-conducting, the voltage at collector 76 rapidly rises towards the voltage of power input terminal 42. This is depicted in FIG. 4 at time T-l. As the power piston approaches one end of its stroke, and slows towards zero velocity, the voltage provided from coil 28 decreases towards zero, as shown in FIG. 3. At time T-2, the voltage at junction point 36 is no longer sufficient to maintain transistor 50 conducting, and the voltage at collector 52 of transistor 50 begins to rise. This increase in voltage at collector 52 begins to render transistor 72 conducting, and the circuit again regenerates in the conventional fashion of a Schmitt trigger until transistor 50 is again non-conducting and transistor 72 is rendered conducting. Thus, at T2 the voltage at collector 76 of transistor 72 again approaches the voltage level of the circuit common.

By rendering transistor 50 conducting at T-l and non-conducting at T-2, a pulse of voltage is created at collector 76 of transistor 72. This pulse of coltage is applied to diode 84 which is inserted to conventionally provide a voltage level shift between collector 76 of transistor 72 and base 86 of transistor 88. Because of diode 84, the voltage at base 86 more closely approximates the voltage at the common terminal of the circuit when transistor 72 is conducting.

Transistor 88 is normally non-conducting since any base current which may be supplied to transistor 88 through resistor 78 is shunted through the normally conducting transistor 72. However, when transistor 72 is rendered nonconducting at T1 and the voltage at collector 76 of transistor 72 begins to rise towards the voltage appearing at power input terminal 42, transistor 88 is rendered conducting and the voltage at collector 92 of transistor 88 drops from the voltage at power input terminal 42 to approximately the voltage at the circuit common. At T-2 when transistor 72 is again rendered conducting, transistor 88 is rendered non-conducting, and the voltage at collector 92 again rises towards the voltage at power input 42. Thus, transistor 88 has the effect of inverting the voltage pulse appearing at collector 76 of transistor 72, as shown in FIG. 5.

Since capacitor 96 is in series with resistor 94 and the internal gate-to-common impedance of silicon controlled rectifier 112, capacitor 96 is charged to approximately the voltage at power input terminal 42. As the negative going leading edge occurring at the collector 92 of transistor 88 at T-1 is applied to capacitor 96, the voltage across it cannot change instantaneously, as is well-known in the electrical arts, and the voltage at junction point 98 attempts to decrease below the voltage of the circuit common. See FIG. 6. Since diode 100 prevents junction point 98 from decreasing in voltage below the voltage value of the circuit common, at T-l capacitor 96 rapidly discharges through the now conducting transistor 88 and the forward impedance of diode 100. At T-2, the capacitor is arranged to be fully discharged. When the positive going trailing edge appearing at collector 92 of transistor 88 at T-2 is applied to capacitor 96, again capacitor 96 cannot instantaneously change voltage and the positive going step is applied to junction point 98, as shown in FIG. 6.

This positive going step renders diode 100 non-conducting because of its polarity and provides the trigger current necessary to render silicon controlled rectifier 112 conducting.

When silicon controlled rectifier 112 conducts, capacitor 114 and the primary 118 of ignition transformer 120 are connected in series with each other, and there is a rapid oscillatory build-up of current in ignition transformer 120, as in conventional ignition circuits. This rapid increase in current is then applied to the spark plug of the free piston engine by secondary 124 of ignition transformer 120 in a conventional manner.

It may now be seen that varying the resistance of variable resistor 34 of the voltage divider network comprising resistor 34 and resistor 38 varies the exact timing of the initiation of combustion by varying the point at which transistor 50 changes back from a conducting state to its normally non-conducting state. More particularly, if resistor 34 is decreased in value, a larger proportion of the voltage applied across

terminals

30 and 32 will appear across resistor 38 and hence be applied to the input of Schmitt trigger 83. This larger proportion of voltage will delay the point in time at which the voltage applied to the base 48 of transistor 50 will again decrease sufficiently to cause transistor 50 to become non-conducting by the regeneration of Schmitt trigger 83. Thus, decreasing the value of variable resistor 34 will delay the initiation of combustion. Conversely, increasing the value of resistor 34 will cause a decreased proportion of the voltage applied

terminals

30 and 32 to appear across resistor 38, and this decreased voltage will cause transistor 50 to return to its normally nonconducting state at an earlier point in time. Thus decreasing the value of variable resistor 34 has the effect of initiating combustion at an earlier point in time. In effect, the timing can be varied so that combustion is triggered at a desired time interval before the point of reversal or time of zero velocity of the power piston. This triggering point occurs when the electrical signal, which is proportional to the velocity of the power piston in this embodiment, has decreased by a predetermined percentage below its peak value (T-l in FIG. 3), for example to the value shown at T-2 in FIG. 3.

It has been found that in starting a free piston engine, it is useful to bring the power piston from its bottom dead center position to its top dead center position much more slowly than occurs when the engine is running. Since the velocity probe of FIG. 1 is designed to produce the proper output voltage levels qt operating speeds and the voltage levels are designed to decrease in direct proportion to a decrease in operating speed, during the initial starting stroke of the power piston within the free piston engine, the voltage output provided by coil 28 is very nearly zero. That is, the speed of the power piston as it initially moves from its bottom dead center position towards its top dead center position in the power cylinder of the free piston engine is on the order of 1/100 of its maximum normal operational velocity, and therefore the rate of change of flux within coil 28 as magnet passes it is substantially reduced. Thus, during the initial starting stroke of the power piston within the free piston engine, the output voltage provided by coil 28 is very likely to be insufficient to operate Schmitt trigger 83 even if applied directly to base 48 of transistor 50. Moreover, during the first few strokes of the engine, the power piston is continually increasing in maximum speed towards the maximum speed which exists during the normal running condition. During these first few cycles of operation, the output provided from coil 28 is also reduced in maximum amplitude. It has been found that the peak value of voltage during these initial strokes, as divided by resistor 34 and resistor 38, is very likely to be less than the voltage required to operate Schmitt trigger 83 when applied through the voltage divider network, and thus Schmitt trigger 83 may not operate during these first few strokes.

To overcome the problem of the near zero first stroke output of coil 28, the circuitry of FIG. 7 may be used. In FIG. 7, a resistor of a large value is shown connected between power input terminal 42 and ajunction point 132. A capacitor 134 is connected between junction point 132 and the circuit common. The combination of resistor 130 and capacitor 134 are chosen to provide an extremely long time constant, on the order of one-tenth to 1 second. A switch 136 is connected between junction point 132 and input terminal 70. A base resistor 66 is connected between input terminal 70 and a base 68 of a transistor 60. Transistor 60 is connected in electrical parallel with transistor 50 of FIG. 2 such that the collector 62 of transistor 60 is connected with collector 52, and the emitter 64 of transistor 60 is connected to the emitter 56 of transistor 50. A resistor 139 also has one end connected to input 70 and has its other end connected to the circuit common. Resistor 139 is chosen to be of a low value in comparison to resistor 130.

In operation, capacitor 134 charges through resistor 130 and stores the voltage existing at power input terminal 42. During the initial starting stroke of the engine, switch 136 is closed, as will be explained hereinafter, and the voltage existing upon capacitor 134 is rapidly discharged through resistor 139. During the time the voltage across capacitor 134 is being rapidly discharged, a positive pulse is applied to input 70 of transistor 60 as is shown at T-3 in FIG. 9. Since transistor 60 is in electrical parallel with transistor 50 and the voltage pulse provided by discharge of capacitor 134 is of sufficient amplitude to render transistor 60 conducting, Schmitt trigger 83 is caused to operate as heretofore explained with the temporary substitution of transistor 60 for transistor 50. The curves of FIGS. 9 to 12 indicate the voltage and time relationships during this phase of operation as compared to the corresponding voltage and time relationships when the engine is running at full speed as represented in FIG. 3 to 6.

The diagrammatic representation of FIG. 8 indicates the manner in which switch 136 is closed. Switch 136 is in a form of a commonly available reed switch having a glass enclosure 140 and magnetically

actuatable contacts

142 and 144. A magnet 146, different from magnet 15 of FIG. 1, is also mounted to reciprocate with the power piston of the free piston engine sufficiently close to switch 136 to magnetically

actuatable contacts

142 and 144 during a portion of the stroke of the power piston of the free piston engine. A vane 145, composed of magnetic type material, is shown between magnet I46 and switch 136. When the free piston engine is running at the proper speed and it is desired to deactivate the circuit of FIG. 7, vane 145 may be pneumatically interposed between magnet 146 and switch 136 to prevent magnet 146 from closing the

contacts

142, 144 of switch 136. Vane 145 is not necessary, however, since the value of resistor 139 is chosen to be small by comparison to the value of resistor 130 and since the time constant of resistor 130 and capacitor 134 are chosen to be in excess of 0.1 second while the normal stroke time of the free piston engine is in the order of 0.001 second. There is thus insufficient time during a stroke for voltage to build up across capacitor 134 and capacitor 134 is discharged each cycle through resistor 139. Thus after the initial stroke of the engine, transistor 60 is not rendered conducting. Vane 145 may be useful, however, in prolonging the life of switch 136.

It will be realized by those skilled in the art that since the piston movement itself closes switch 136 on the initial starting stroke of the power piston of the free piston engine, the spark will be delayed somewhat from the time switch 136 is closed. Since the power piston is moving so slowly on this initial starting stroke, the slight delay of electrical circuitry does not significantly affect the operation of the engine.

FIG. 13 illustrates additional electrical circuitry which may be used with the circuitry of FIGS. 7 and 8 to further aid in starting the free piston engine to overcome the problem of the reduced amplitude signals from coil 28 during the first few cycles of operation. In FIG. 13, a pair of

control input terminals

150 and 152 are shown. A resistor 154 is connected between control input terminal 150 and junction point 156. A capacitor 158 is connected between junction point 156 and input terminal 152 which is in turn connected to the circuit common. Junction point 156 is further connected to a base 160 of a transistor 162. A collector 164 of transistor 162 is connected to a junction point 36 of FIG. 2 through a resistor 166.

Terminals

150 and 152 are arranged to be connected to an electrical voltage generated by the engine such that the electrical voltage is present in full amplitude only after the engine has reached proper operating speed.

Thus, transistor 162 is non-conducting during starting and resistor 38 alone forms one leg or part of the voltage divider network consisting of resistor 34 and resistor 38 in FIG. 2. A first voltage division ratio is thus provided. After the first few cycles of operation are complete and the voltage provided by coil 28 has reached its proper amplitude, a lesser voltage division ratio may be used. At this point, the electrical voltage generated by the engine has also reached a proper level to render transistor 162 conducting. When transistor 162 is conducting, resistor l66 is placed in electrical parallel with resistor 38 and the value of the resistance of that leg of the voltage divider network is reduced to thus provide the desired lesser voltage division ratio. Thus, in selecting the value of resistor 38, consideration may be given to the proportion required during the first few cycles of operation of the engine. By the proper selection of resistor 166, the voltage division ratio may be decreased for normal operation of the engine.

Of course, if an electrical output is not available from the free piston engine, the switching function of transistor 162 may as well be accomplished by a mechanical switch. Such a switch may be caused to operate from a damped butterfly valve positioned in the exhaust of the free piston engine such that when exhaust indicates the engine is operational, the butterfly valve is caused to open by the passage of the exhaust gases, and the switching function of transistor 162 would be mechanically accomplished.

FIGS. 14 through 16 show details of construction and operation of a preferred resistor-capacitor (RC) and diode network for replacing the voltage divider network illustrated at the left portion of FIG. 2 and described above. As shown in FIG 14, the coil 28, in which electrical signals proportional to instantaneous velocity of the power piston are generated, is connected at one end 32 to the ground or common circuit through parallel paths, one of which includes a condenser 170, and the other of which includes a fixed resistor 171 in series with a variable resistor 172. The other terminal 30 of the velocity sensing coil 28 is connected through a diode 173 to the input 37 of the Schmitt trigger circuit, which is shown in dotted outline at 83.

As in the circuit of FIG. 2,

diodes

40 and 44 are connected between the positive voltage input source 42 and the junction 37 at the input to the Schmitt trigger circuit, and between the junction 37 and the ground or common circuit, respectively.

Diodes

40 and 44 provide essentially the same functions previously described in connection with FIG. 2. Moreover, the diode 44 prevents the application to input terminal 37 of undesired transient signals which might otherwise be generated in the RC part of the network.

FIG. illustrates the relationships of piston velocity and voltage at various points. Thus curve 174 illustrates the instantaneous velocity of the piston starting at point A corresponding to the outer reversal or dead point at which the piston velocity is zero just as the piston reverses to start its compression stroke. The piston velocity then reaches a maximum at point B relatively rapidly during normal operation, as return energy is supplied to the piston in known manner. As the compression stroke continues, and the mixture in the combustion chamber is compressed, the piston velocity decreases from its peak value at B and ultimately reaches zero at point C on the graph, corresponding to the inner reversal point at the end of the compression stroke and the beginning ofthe power stroke. The piston velocity then increases rapidly in the other direction during the power stroke, reaching a maximum at point D and then again reaching zero at the outer reversal or dead point of the piston, shown at E on the graph.

During normal operation, the mean value of the voltage on condenser has an absolute value illustrated by line 176 in FIG. 15 and by its distance 178 from the horizontal axis of the graph. The actual voltage signal across coil 28 will have a value which varies in essentially the same fashion as curve 174, since it is proportional at all times to the instantaneous piston velocity. When that voltage, which is effectively reduced by the negative bias of the voltage across condenser 170, as described below, reaches a maximum corresponding to point !B on curve 174 (or some other preselected value corresponding to a point slightly ahead of point B), the Schmitt trigger circuit will be set in one condition as previously described. When the net effective voltage, as applied at point 37 (FIGS. 2 and 14), drops a predetermined percentage from its peak value (or its slightly lower preselected value), the Schmitt trigger circuit will change to the condition causing a spark in the engine combustion chamber. This firing point is shown at F along the path of movement of the piston in FIG. 15. Line 180 in that figure, and its distance 182 above the axis of the graph represent the desired predetermined decreased velocity (or velocity-responsive signal) at which the Schmitt trigger circuit causes such ignition.

FIG. 16 shows how the mean value of the voltage (shown at 184) across condenser 170 gradually increases in a negative biasing direction as the engine 'starts from rest (at the left of the graph) and reaches normal operation (at the right of the graph). The actual instantaneous voltage across condenser 170 will vary as shown by curve 186, depending on the time constant of the R-C circuit.

When the engine fires initially, as shown at the left end of the graph in FIG. 16, the mean value of the voltage across con denser 170 is zero. The firing point shown at F in FIG. 15 will accordingly be later, in relation to the reversal point of the piston, i.e. farther from the peak velocity point B of the piston and closer to reversal point C than the normal firing point desired during operation. As the engine accelerates to its steady running condition, the mean value of the voltage on condenser 170 will increase in a negative sense as shown in FIG. 16. Thus the spark is progressively advanced.

During operation according to the present invention, we have found that for optimum running, the spark should occur in some cases when the piston velocity has dropped only about l0 percent from the peak velocity which is developed (see point B in FIG. 15) during its compression stroke, i.e. substantially ahead of the point at which the piston velocity drops to zero at the time of reversal of the piston movement as the piston starts its power stroke (see point C in FIG. 15). To obtain such an early signal from the circuit of FIG. 2 would require a relatively high division ratio in the voltage divider network of that circuit and could provide an undesirably low voltage input to the Schmitt trigger circuit. Thus the voltage might be too low to cause a spark. In the circuit of FIG. 14, however, a voltage is developed across the parallel R-C combination which has a mean value proportional to the mean engine velocity. This voltage is added to the velocity signal of coil 28 in a manner which biases the signal from coil 28 negatively. Thus the voltage across the condenser 170, in effect, is subtracted from the voltage signal across coil 28.

Should a misfrre occur, the velocity signal amplitude at coil 28 falls. However, by choosing the time constant of the R-C part of the network in FIG. 14 properly, so that it is equal to or shorter than the mechanical time constant of the engine, the negative bias of the Schmitt input signal resulting from the mean voltage level across condenser 170 will be immediately reduced or removed. Thus even the reduced signal from coil 28 will be sufficient to insure that the signal applied from point 37 to the Schmitt trigger is still sufficient to produce a spark in an effective time relationship to the moment of reversal at the end of the next compression stroke after the misfire.

In one embodiment of the circuit of FIG. 14, condenser 170 has a value of 12 microfareds, while resistor 171 has a value of 2,200 ohms in series with a potentiometer 172 having a maximum resistance of 10,000 ohms.

The operation of the Schmitt trigger circuit in connection with the input network of FIG. 14 is essentially similar to the operation of that circuit as previously described in FIG. 2.

Now that the basic teachings of the present invention have been explained, many extensions and variations will be obvious to one skilled in the art. For example, many configurations of the circuitry of a preferred embodiment will be envisioned to perform according to the present invention; no limitation to this exact circuitry is intended Also, functional blocks other than those shown can be used to perform according to the present invention. No limitation to the precise blocks is intended.

Further, while the preferred embodiment of the present invention has been explained in relation to spark ignition free piston engines, the principles of operation of the present invention may be applied to free piston Diesel engines. In the case of a Diesel engine, a combustion causing means may be a fuel injector system and the electrical circuitry of the present invention may control the fuel pump associated with the fuel injector system. Further, the principles of operation of the present invention may be applied to a stratified charge free piston engine where both the spark ignition and the injection systems are controlled accordingly. The spark ignition system is preferred; however, no limitation to this particular system is intended.

Furthermore, while the operation of the present invention has been explained with the relation to a Schmitt trigger, no limitation to this precise circuit is intended. By reference to a Schmitt trigger, any voltage level responsive trigger means with hysteresis is intended. In fact, a differential amplifier may be used with threshold reference signal applied to one input and the voltage from junction point 37 of FIG. 2 applied to the other input. The Schmitt trigger circuitry has an inherent advantage over a differential amplifier in that, as is well-known to those skilled in the electrical arts, a Schmitt trigger may be designed with hysteresis. Therefore, with as little as a few tenths of a volt hysteresis, once the voltage input to the Schmitt trigger circuit 83 has reached an appropriate level to change the state of the output, noise cannot immediately cause the output to revert to its preceding state. That is, if transistor 50 of FIG. 2 is conducting and the voltage level at base 48 of transistor 50 falls below the voltage necessary to maintain transistor 50 in a conducting state, transistor 50 is rendered non-conducting and there is a change in the output waveform from the Schmitt trigger. Ifa noise pulse follows immediately and causes the input voltage to again exceed the voltage necessary to maintain transistor 50 conducting, the hysteresis of the Schmitt trigger will prevent this noise pulse from again changing the output waveform. If a differential amplifier were used, this noise pulse would again cause the output of the differential amplifier to change states. Since the noise pulse is usually of short duration, the output from the differential amplifier would immediately revert to its alternate state. Thus, a train of pulses could issue from differential amplifier due to noise on the incoming waveform where the hysteresis of a Schmitt trigger would eliminate this effect of the noise.

In summary, the present invention provides improved electrical means for controlling the time at which combustion is initiated during the power piston movement of a free piston engine, by developing an electrical signal having a desired reversal-time-related characteristic, i.e. a desired time relationship to the moment in time when the power piston reaches its reversal point at the end of a compression stroke, even though that reversal point may occur at different physical positions or linear displacements of the piston along its path of movement in the engine. The foregoing specification sets forth the nature and principles of the invention and some of the ways in which it may be practiced.

Now, therefore, we claim:

1. In a free piston engine having a power cylinder, a movable piston assembly including a power piston which is freely reciprocable in said cylinder along a path of movement between desired normal inner and outer piston reversal points and in which a power stroke in one direction toward the outer reversal point immediately follows a compression stroke in the opposite direction toward the inner reversal point, and combustion-causing means for initiating combustion in the cylinder to move the power piston in one direction for each successive power stroke, the improvement comprising electrical sensing means including two relatively movable electrical members operable upon relative movement thereof to produce an electrical sensing signal dependent in magnitude upon the speed of such relative movement, one of which electrical members is operatively connected to the piston, and which members are electrically coupled to each other in a relatively constant manner over a substantial portion, including the final portion, of the movement of the piston during a compression stroke so that an electrical sensing signal is produced by said sensing means determined by the instantaneous value of a movement-related power piston characteristic which has a predeterminable relationship to the moment when the power piston reaches a reversal point at the end of each compression stroke, and which is substantially independent of variations in the actual physical position of the piston along its path at the moment of piston reversal for different power strokes, and electric control circuit means having input means and output means, the input means being connected to receive the electrical sensing signal from the sensing means, and the output means being connected to operate the combustioncausing means by providing a predetermined output signal at said output means, the electric control circuit means also having electric control signal generating means responsive to a predetermined electric sensing signal at the input means for providing said predetermined output signal at the output means at a point in time having a desired time relationship to the moment of power piston reversal at the end of each compression stroke.

2, The free piston engine apparatus of claim 1 having electrical network means connecting the sensing means to the input means of the electric control circuit means, the sensing means having a coil in which the electrical sensing signal is generated, said network means including an R-C section having a resistor and a capacitor connected in parallel with each other and in series with the sensing means coil and also including a diode, the input means of the electric control circuit means having two input terminals, and said network means having means connecting the R-C section, sensing means coil and diode in series with each other across said input terminals.

3. An improved free piston engine apparatus according to claim 1 in which the sensing means includes first and second relatively movable parts, one of which is connected to move with the piston assembly, the electric control signal generating means comprising voltage level responsive trigger means for providing a first output control signal when the voltage level of the electrical sensing signal applied to the input means is below a threshold amplitude and providing a second output control signal when the voltage level of the electrical sensing signal applied to the input means is above the threshold amplitude, said electric control signal generating means providing said predetermined output signal at the output means in response to one of said output control signals from the trigger means, said means for causing combustion including control input means for initiating operation of the combustion causing means when said predetennined output signal is applied to the control input means, and the output means ofthe electric control circuit means being connected to the control input means of the combustion causing means.

4. The free piston engine apparatus of claim 3, wherein the voltage level responsive trigger means comprises a Schmitt trigger having output means, the signal from the sensing means causing a pulse output from the Schmitt trigger output means.

5. The free piston engine apparatus of claim 4, having voltage dividing means connecting the sensing means to the input means of the electric control circuit means, the voltage dividing means comprising: first resistive means; second resistive means; means for connecting the sensing means, the first resistive means and the second resistive means in electrical series; voltage divider output means connected across the second resistive means; and means for connecting the voltage divider output means to the input means of the electric control circuit means and thereby to the Schmitt trigger.

6. The free piston engine apparatus of claim further including an inverting pulse amplifier having an input means and an output means; means for connecting the input means of the inverting amplifier to the output means of the Schmitt trigger; capacitive means for connecting the output means of the inverting amplifier to the electric control circuit output means and thereby to the control input of the combustion initiating means; and diode means having one end connected to the control input of the combustion initiating means and having the other end connected to provide a unidirectional electrical current path to a common point in the electrical circuitry.

7. The free piston engine apparatus ofclaim 5 also including means for aiding in the starting of the free piston engine, comprising in combination: controlled switch means; further resistive means; and connection means for connecting the further resistive means and controlled switch means in electrical series and for connecting the series connection of the further resistive means and the controlled switch means in electrical parallel with the second resistive means for lowering the voltage division ratio of the voltage dividing means when the switch means is closed; the controlled switch means being arranged to be closed upon the normal operation of the free piston engine.

8. The free piston engine apparatus of claim 3 having voltage dividing means connecting the sensing means to the input means of the electric control circuit means, the voltage dividing means comprising: first resistive means; second resistive means; means for connecting the sensing means, the first re sistive means and the second resistive means in electrical series; voltage divider output means connected across the second resistive means; and means for connecting the voltage divider output means to the input means ofthe electric control circuit means and thereby to the trigger means.

9. The free piston engine apparatus of claim 8, also including means for aiding in the starting of the free piston engine, comprising in combination: controlled switch means; further resistive means; and connection means for connecting the further resistive means and controlled switch means in electrical series and for connecting the series connection of the further resistive means and the controlled switch means in electrical parallel with the second resistive means for lowering the voltage division ratio of the voltage dividing means when the switch means is closed; the controlled switch means being arranged to be closed upon the normal operation of the free piston engine.

10. The free piston engine apparatus of claim 1 including switch means operated upon said piston reaching a predetermined position during the initial starting operation of the engine, means responsive to the operation of said switch means for causing operation of said combustion-causing means independently of the value of the signal from said electrical sensing means, and means effective to prevent said switch means from affecting the operation of said combustion-causing means during operation of said engine after combustion has been ini iated.

t 11. The free piston engine apparatus of claim I having electrical network means connecting the electrical sensing means to the input means of the electric control circuit means, the electrical sensing means having as one of said two relatively movable electrical members a coil in which the electrical sensing signal is generated, said network means including means for connecting said coil to said input means of said electric circuit control means and also including capacitive means effective as said engine speed increases to decrease the effect on said circuit control means of a sensing signal of a predetermined magnitude from said coil,

Claims

1. In a free piston engine having a power cylinder, a movable piston assembly including a power piston which is freely reciprocable in said cylinder along a path of movement between desired normal inner and outer piston reversal points and in which a power stroke in one direction toward the outer reversal point immediately follows a compression stroke in the opposite direction toward the inner reversal point, and combustion-causing means for initiating combustion in the cylinder to move the power piston in one direction for each successive power stroke, the improvement comprising electrical sensing means including two relatively movable electrical members operable upon relative movement thereof to produce an electrical sensing signal dependent in magnitude upon the speed of such relative movement, one of which electrical members is operatively connected to the piston, and which members are electrically coupled to each other in a relatively constant manner over a substantial portion, including the final portion, of the movement of the piston during a compression stroke so that an electrical sensing signal is produced by said sensing means determined by the instantaneous value of a movement-related power piston characteristic which has a predeterminable relationship to the moment when the power piston reaches a reversal point at the end of each compression stroke, and which is substantially independent of variations in the actual physical position of the piston along its path at the moment of piston reversal for different power strokes, and electric control circuit means having input means and output means, the input means being connected to receive the electrical sensing signal from the sensing means, and the output means being connected to operate the combustion-causing means by providing a predetermined output signal at said output means, the electric control circuit means also having electric control signal generating means responsive to a predetermined electric sensing signal at the input means for providing said predetermined output signal at the output means at a point in time having a desired time relationship to the moment of power piston reversal at the end of each compression stroke.

2. The free piston engine apparatus of claim 1 having electrical network means connecting the sensing means to the input means of the electric Control circuit means, the sensing means having a coil in which the electrical sensing signal is generated, said network means including an R-C section having a resistor and a capacitor connected in parallel with each other and in series with the sensing means coil and also including a diode, the input means of the electric control circuit means having two input terminals, and said network means having means connecting the R-C section, sensing means coil and diode in series with each other across said input terminals.

3. An improved free piston engine apparatus according to claim 1 in which the sensing means includes first and second relatively movable parts, one of which is connected to move with the piston assembly, the electric control signal generating means comprising voltage level responsive trigger means for providing a first output control signal when the voltage level of the electrical sensing signal applied to the input means is below a threshold amplitude and providing a second output control signal when the voltage level of the electrical sensing signal applied to the input means is above the threshold amplitude, said electric control signal generating means providing said predetermined output signal at the output means in response to one of said output control signals from the trigger means, said means for causing combustion including control input means for initiating operation of the combustion causing means when said predetermined output signal is applied to the control input means, and the output means of the electric control circuit means being connected to the control input means of the combustion causing means.

6. The free piston engine apparatus of claim 5 further including an inverting pulse amplifier having an input means and an output means; means for connecting the input means of the inverting amplifier to the output means of the Schmitt trigger; capacitive means for connecting the output means of the inverting amplifier to the electric control circuit output means and thereby to the control input of the combustion initiating means; and diode means having one end connected to the control input of the combustion initiating means and having the other end connected to provide a unidirectional electrical current path to a common point in the electrical circuitry.

7. The free piston engine apparatus of claim 5 also including means for aiding in the starting of the free piston engine, comprising in combination: controlled switch means; further resistive means; and connection means for connecting the further resistive means and controlled switch means in electrical series and for connecting the series connection of the further resistive means and the controlled switch means in electrical parallel with the second resistive means for lowering the voltage division ratio of the voltage dividing means when the switch means is closed; the controlled switch means being arranged to be closed upon the normal operation of the free piston engine.

8. The free piston engine apparatus of claim 3 having voltage dividing means connecting the sensing means to the input means of the electric control circuit means, thE voltage dividing means comprising: first resistive means; second resistive means; means for connecting the sensing means, the first resistive means and the second resistive means in electrical series; voltage divider output means connected across the second resistive means; and means for connecting the voltage divider output means to the input means of the electric control circuit means and thereby to the trigger means.

10. The free piston engine apparatus of claim 1 including switch means operated upon said piston reaching a predetermined position during the initial starting operation of the engine, means responsive to the operation of said switch means for causing operation of said combustion-causing means independently of the value of the signal from said electrical sensing means, and means effective to prevent said switch means from affecting the operation of said combustion-causing means during operation of said engine after combustion has been initiated.

11. The free piston engine apparatus of claim 1 having electrical network means connecting the electrical sensing means to the input means of the electric control circuit means, the electrical sensing means having as one of said two relatively movable electrical members a coil in which the electrical sensing signal is generated, said network means including means for connecting said coil to said input means of said electric circuit control means and also including capacitive means effective as said engine speed increases to decrease the effect on said circuit control means of a sensing signal of a predetermined magnitude from said coil.