US2871280A - Piezoelectric igniter unit for internal combustion engines - Google Patents

Description

Jan. 27, 1959 J. R. HARKNr-:ss

y PIEZOELECTRIC IGNITER UNIT FOR INTERNAL COMBUSTION ENGINES 4 Sheets-Sheet l Filed Aug. 20, 1956 Jaim/5 JZ. 5dr/misi Jan. 27, 1959 R. HARKNESS `2,871,280

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PIEZOELECTRIC IGNITER UNIT FOR INTERNAL COMBUSTION ENGINES Filed Aug. 20, 1956 4 Sheets-Sheet 4 vthe engine, :such as the engine camshaft.

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PIEZOELECTRIC I'IGNITER YUNIT FOR INTERNAL "COMBUSTION'ENGINES Joseph R. Harkness, 'Milwaukee Wis., assigner to Briggs ySr Stratton Corp., Milwaukee, Wis., a corporation of 'Wisconsin Application August '20, T1956, 'Serial.No. 6074,'880

Claims.` (Cl. ,12S- 148) This invention relates to ignition'tsystems for internal combustion t engines. and refers .more particularly -to van ignition system of the type disclosed-inlatentNo..2,649,- 488, issued August :18, 1953, to'J. R. Harkness, and wherein the spark plug tiring voltage is derivedfrom -a poly crystalline piezoelectric element, .as `distinguished .from

a magneto or battery energizedignitionicoil.

'ln general, the aforesaid A.patent teaches .that a polycrystalline piezoelectric element 4.such as ypolycrystalline metallic -tltanate, mixed with a ceramicbinder and vitri tied into a solid ceramic body is-capable of .generating :a sufficiently .high Vvoltage for satisfactory lcommercially reliable spark .plug ring when .abruptly Amechanically stressed; and, moreover, thatapiezoelectric element of therefore, the cam causes the-.camfollower-to swing the hammer away from the strikervpin and ythen,asaILabrupt step on the cam moves across the follower, .the hammer .is suddenly released to strike .tliezstrilcerpina.forceful blow .in response .to the .strong `bias .acting .upon -the hammer. i

While the operation of an internal .combustion engine equipped with fthe piezoelectricy ignition system .disclosed in the aforesaid patent was generally satisfactory, several annoying problems were presented by=that system. The rst of theseproblerns was that of noisy operation, which resulted from the rapidly recurring'blows of the hammer upon the striker pin. These hammer blows were 'transmitted into all of the metal structure ofthe engineto'the extent that sound was radiated from 'il'the wallsthereof and created a'highly disagreeable and disconcerting rattling noise Whenever the engine'wa's in operation.

Another objection to thepiezoelectric ignitio'nsystem of the aforesaid patent, was that its hammer di'd mot accelerate at "the desired 'high Aspeed when the energy stored in the hammer biasing'spring lwas suddenly released bythe cam, thus creating substantial time'delayanid accompanying spark retardance during the -intervlslhetween release o'f the hammerlbiasing"springand thea'etual impact of the hammer uponthe striker pin. lWhile such time delay has no ill effects at slow engine -speeds,\such as during starting of the engine, itlwas'objeetionablewhen the engine was operated at normal higlrspee'd.

Still another objectionto the piezoelectric ignitionsystem of the aforesaidpatentfresulted from "the factthatits hammer cooking mechanism comprised a-'ca'm with a' long inclined surface bearing upon the hammerfcarrie'd-c'am United States Patent Patented Jan. 2'?, 1959 .follower and leading to an vabrupt step inthe cam "to release the hammer Teach time the abrupt step on the cam moved across the cam follower. Obviously, the cam had to exert a large force upon the hammer carried cam follower in order to overcome the hammer biasing force While imparting cooking motion to the zhammer, to vcarry it away from the strikerpin. The .friction force between the cam `'and its follower during -such cooking of the hammer, of course, was proportional to the hammerfbiasingforce-being exerted upon the cam followerrand was, moreover, greatest when the largest radius portion fof 'the cam rode over the follower. Thus the vfriction torque increased veryrapidly as the cam rotated toward its-step, due to the combinedeffects .of increasing frictional force and the increasing radius at which this greater frictional drag was being exerted. Theresultwas an undesirably large consumption of powerby the ignition system.

The problem outlined above was further complicated by the fact that the cam was connected to its shaft through a ratchet-like unidirectional driving connection which constrained the cam -to rotate with the shaftlin-only one directionof shaft rotation, and disrupted the connection between the cam .and its vshaft :in fthe :event of reverse rotation of the shaft as frequently happens lwhen the enginebacktires'duringstarting. .In the absence-of such'a unidirectional ldriving connection between the cam and its shafhthe h'ammeroperating mechanism including the cam and .-itsffollowen'would almost'certainlybe damaged beyondarepair'in'the event ofengine backring.

:This ratchet-like mechanism, which 'comprisedwseveral small .parts .making'up VAa .relatively complex device, was

,not only-:expensive to manufacture `but wasnever completely free fromthe possibilityzof malfunctionyparticularly after long periods of use.

In general, the object of this invention is vto provide a piezoelectric'ignition system in which all of theobjections discussed above are overcome.

With A4a view :toward effecting a substantial =reduction inrnoise, it is afurther object of thiszinvention to fprovi'cle a piezoelectric igniter of the character described which'is capable of being mounted on the exterior of 'an internal combustion engine as a unit, andtwherein all'ofthe'cornponentseof the igniter are located externally of the rengine crankcase, with the exception of a single drivingrmember by which thefunit maybe operated from ya rotatable-shaft of the engine. fln accordance with this invention, the

components of -the igniter unit vare contained within a sion of-shocks incidental to operation of 'thehammen to the engine walls.

Further according to this invention, the igniter unit includes a drive shaftvjournalled in the housing of the unit and projecting through the cover or resilient disc to-have aresilient.drivingmember fixed thereon yat the-side of the Vcover .remote .from the igniter unit, vwhereby the lonly mechanical connections between the igniter unitandthe engine are "through two resilient members, ,.namely, .the closure disc and the resilient driving member, and whereby the shock .incidental to operation of the hammer ofthe igniter unit will be coniined to the unit. itselfandvmore or less isolated from "the engine walls to minimize noise 'radiation'thereby IIn this connection, it is a more 'specific object of this invention vto provide a piezoelectric igniter unit ofthe characterdescribeid wherein those portions of the igniter 4unit 'housingwhich areheavily stressed during operation 'fofthe'hammer are-provided with considerable cross section so as to have a minimum of resilience, to thus effect i means which operates with negligible friction, so that almost all of the power consumed in operation of the igniter mechanism is that which is directly required for cocking the hammer against its biasing force.

In this connection, it is a more specic object of this invention to provide a piezoelectric igniter of the character described with improved hammer cocking mechanism which is more dependable in operation by reason of the fact that the hammer cock-ing mechanism needs no unidirectional ratchet mechanism such as was previously required in piezoelectric ignition systems of the type hereinbefore referred to.

Still another object of this invention resides in the provision of a piezoelectric igniter of the character described in which the time delay which was inherent in past devices of this nature, is substantially reduced through the expedientof providing a hammer which is of small mass and accelerates more rapidly due to the biasing force thereon when it is released to strike the striker pin.

Other objects of this invention reside in the provision of a piezoelectric igniter unit of the character described, which comprises relatively few and simple parts contained in a compact housing, and which is inexpensive to manufacture and service and dependable in operation,

With the above and other objects in view, which will appear as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described and more particularly dened by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate several complete examples of the physical embodiments of the invention constructed according to the best modes so far devised for Ithe practical application of the principles thereof, and in which:

Figure 1 is a view partly in side elevation and partly in vertical section of a single cylinder' internal combustion engine equipped with the piezoelectric igniter of this invention;

Figure 2 is an enlarged sectional view taken substantially on the plane of the line 2-2 in Figure l;

Figure 3 is an enlarged sectional view taken on the plane of the line 3-3 of Figure l;

Figure 4 is a view similar to Figure l but showing a modified form of the invention;

Figure 4a is a view similar to Figure 4 and illustrating l 'still another embodiment of the invention; and

Figures 5 through 9 are fragmentary views showing the components of the hammer operating mechanism in the relative positions which they assume at several successive stages of one complete operating cycle.

Referring now more particularly to the accompanying drawings in which like reference characters have been applied to like parts throughout the several views, the numeral 5 generally designates the crankcase of a single cylinder internal combustion engine, having a cylinder 6 projecting upwardly therefrom to reciprocably receive a piston 7. As is customary, the reciprocation of the piston is translated into rotation of the crankshaft 8 of the engine through a connecting rod 9.

While it will be understood that this invention is readily adaptable to two-stroke cycle engines as well as to engines having more than one cylinder, it is shown and described by way of illustration in connection with a single cylinder four-cycle engine, wherein the crankshaft S drives a camshaft 14) at one-half crankshaft speed through the customary timing gears 11, and wherein valves are actuated in timed relation to the reciprocation of the piston by means of cams on the camshaft.

At a precise instant during every second revolution of the crankshaft S, the spark plug 12 of the engine must be tired to ignite the combustible mixture compressed by the piston. This instant occurs when the piston is substantially at the top of its compression stroke, and the piezoelectric igniter unit of this invention generally designated 13, provides the necessary spark plug firing voltage at the desired instant. For this reason, the unit is operated from the camshaft 10, through a silent chain C trained about sprockets 14 and 15 on the camshaft and igniter unit, respectively. These sprockets have a one to one ratio so as to properly synchronize the operation of the igniter unit with reciprocation of the piston.

The igniter unit 13 of this invention features a housing 16 which carries all of the components of the igniter mechanism and which is adapted to be mounted on the exterior of one wall of the engine crankcase, beneath one end of the camshaft 10 of the engine. The housing, which is preferably of die cast construction, comprises a main horizontally disposed body portion 16 defining an elongated chamber 17 containing a hammer 18 and hammer operating mechanism generally designated 19. The hammer operating mechanism 19 is contained within one end portion of the chamber 17, just inside a detachable cover 25 which closes said end of the charnber; while the hammer 1S is contained within the other end portion of the chamber, remote from the cover.

The piezoelectric element 2li is mounted in a cylinder 21 joined to and projecting upwardly from the body portion 16 of the housing above the hammer 1S. The cylinder 21 provides a chamber 22 in which the element is contained, and the bottom of the chamber 22 is communicated with the chamber 17 through a bore 23 which opens to the chamber 17 at a point directly adjacent to the hammer 18, and which opens to the interior of the cylinder through a counterbore 28.

The piezoelectric element, which is in the form of an elongated rod, is located endwise between a plug 26 screw threaded into the upper end of the cylinder and a striker pin 27 loosely fitting the counterbore 28 and seating on the bottom thereof. A reduced striker portion 29 on the pin is slidably received in the bore 23 and projects downwardly therethrough for engagement by the hammer.

The piezoelectric element 20, however, does not extend the full distance between the striker pin and the plug, but has its lower end clamped against an electrically conductive adapter electrode 30, and a buffer or cushioning member 31, preferably of plastic material such as nylon is interposed between the adapter electrode and the striker pin. The opposite or upper and lower ends of both the piezoelectric element and of the buffer member are more or less convexly shaped to seat in corresponding concavities in the plug 26, the adapter electrode 30, and the striker pin. lNot only do these concavities in the members which directly engage and endwise confine the piezoelectric element steady the same when the plug 26 is turned inwardly into the cylinder to place the element under initial endwise compression, but they serve to provide a greater area of Contact between the ends of the element and the surface of the plug and adapter electrode, so as to prevent the localization of forces on the element, under the inuence of abrupt me chanical stresses imparted thereto by the hammer, in a way which might damage the element.

The plug 26 at the upper end of the cylinder, of course, provides access to the piezoelectric element, and it also serves as a grounding terminal for the upper end of the element. The lower end of the piezoelectric element is electrically connected with the high tension spark plug lead 32 through a resilient electrically conductive brush 33, in the form of a compression spring,

laterally' engagedv with theA adapter' electrode 3o and mounted in an insulative plug" 34 screw threadedy intothe side wall of the cylinder 2li The insulative plug also supports a suitable terminal 35 vwhich is electrically connected with the brush and which provides" for connection of the spark plug leadv to the igniter unit.

Thev insulating buffer 3'1 which, as stated previously, is preferably of a plastic material? having good electrical insulating properties, prevents short circuiting the upper andlower ends of the' piezoelectric element and, preferably, has a degree of resilience'. The resiliency of the buffer member serves as protection forthe piezoelectric element while, at the same time acting to-v transmit hammer blows on thel striker p'inv to the adapter electrode 30 and thus to the piezoelectric element.

They striker pin, which is more or less looselyencircled by the .walls of the counterbore 28 and the bore 23, is normally clamped against' the'- bottomy of counterbore by the action-of` the-plug 26. The striker pin is also provided 'with an O-ring oil-` seal' 3'6'c`onned in a circumferential groove in the larger diameter portion of the pin and having snug engagement with the wall of thec'ounterbor'e to prevent lubricant in` 'the chamber? 1l7 containing the hammer and its operatingl mechanism from gaining access to the interior of the chamber 22V in which the piezoelectric element is mounted.

Projecting outwardly from the' lower portion of the cylinder 21, substantially at right angles to the cylinder and to the main body portion 16' of the housing, isa tubular extension' 37 formed as part ofthe housing and containing an elongated' torsion bar springV 38 of rectanguiar cross section. At its inner end,A the hollow interior of the extension opens to a counterbore 39 which` crosswise intersects the chambery" 17 and' freely rotatably receives the cylindrical body portion of the hammer 18.

The inner end of the torsion bar projects into the counterbore to have the hammer 18 mounted thereon, and its outer end is non-rotatably received in a rectangular hole in the outer extremity ofthe extension, atV 4d. A pair of bushings 41, which may be of nylon if' desired, are mounted on the torsion bar at each side of the hammer and are freely rotatable in the counterbore 3'9. These bushings cooperate with the hammer to constrain the bar to twisting movement about its longitudinal axis. Caps 42, one pressed over the outer end of the extension and the other into the mouth of the counterbore 39, retain the torsion bar biasing spring against displacement, but can readily be removedto provide for inspection and/ or replacement of worn parts.

The purpose of the torsion bar, of course, is to propel the hammer into engagement with the downwardly projecting striker portion of the striker pin with substantial force whenever the hammer operating mechanism cocks the hammer and then suddenly releases it. When this occurs, the sudden forceful hammer blow upon. the striker pin is transmitted thereby through the buffer member and the adapter electrode 30 onto the piezoelectric element, to cause such abrupt mechanical stressing of the element as will generate the desirable high sparking voltage therein. The manner in which the hammer operating mechanism effects cocking and then sudden releasing of the hammer, will be described at greater length hereinafter. i

The hammer operating mechanism is mounted on a crank pin or eccentric 43 which projects into the charnber 17 in the main body portion of the housing from the enlargedy medial portion 44 of a drive shaft' carried by the igniter housing. The medial portion of the drive shaft is rotatably journalle'd in a bearing 46 carried' by a boss 47 which extends rearwardly fromy the main body portion of the housing and has an adaptor plate 49 ix'ed on its outer end. This adaptor plate is of a size to cover an access hole 50 in the crankcase 5 of the engine, below one end of the camshaft 14), and it serves' to support the igniter housing on the engiueas by means of" cap screws d 51v passing through h'olesin'th'ei platev and threaded into suitable apertures in' the 'crankca'se' wall..

The outer end of'ythe drive shaft projectsinto the crank- 'case from` the boss 47 on the main body portionV ofthe ignit'er housing, through a central hole 52 in the adaptor plate and has' the igniter drive sprocket 15 mountedon its extremity andy drivingly connected' thereto. As will be seen best; from FigureZjY the silent chain C drivingly connects the igniter sprocket' 15 with the camshaft sprocket 1:4l for' rotation in the4 same direction' therewith.

Plhe drive shaft of the' unit is heldk against axial displacement by means of a set screw'5'3 threaded into the boss' 47 onl the housing, andhaving its extremity' projecting through a hole in the bushingy 46 and into' a circumferential groove 54y in the enlarged medial portion 44of the drive shaft. The sprocket'l may bepro'vided'with a square hole 55 in`- its hub torreceive a reduced square end portionL 56 on the drive shaft; andthe sprocket'may be held against axial displacement from a' position engaging theY shoulder at the junction of the portions 44 and 5'6 of the drive shaft by means of a spring clip S7 interengaged between a headed portion on the adjacent extremity of thefdrive shaft and' the face of the sprocket 15 which-isremote from the'igniterunit.

One of the most important features of this invention resides in the fact that both the igniter sprocket 1S and the adaptor plate 49 arel made of a material such as nylon or the like, which though having adequate rigidity, nevertheless embodies a' degree of resilience. By this construction, coupled" with the fact that the hammer and its operating mechanism are'moun'ted externally of and spaced from the engine crankcase,y the sound waves 'created by the blows incidentalv tol forceful impingem'ent ofthe hammer upon thev striker pin during operation of the uniti area't'tenua'ted before reaching the walls of the engine; Without such attenuation, these sound waves would result inthe highly objectional and annoying rattling noise characteristic of past piezoelectric igni'tersystems of the type hereinbefore referred to. The only twol points at which the igniter unit` of this invention is mechanically connected to the internal combustion engine are at the area of the adaptor plate 49 and at the driving member or sprocket 1S, and' both of the igniter unit elements which are involved in this connection are made of a plastic material having some resiliency so as to attenuate sound waves incidental to the blows of the hammerl upon the striker pin and thus quiet the operation of the unit.

While the connection of the igniter unit of this inven tion to' the engine in the manner described serves to minimize the noise incidental to operation of the igniter unit, still further reduction in noise is achievedin accordance with this invention, through the' expedient of constructing those portions of the igniter unit housing which receive maximum stress due to the recurrent hammer blows with the greatest possible stiffness.

The plug 26, for'instance, which bears upon the upper end of the piezoelectric element, is solid metal and has a relatively large diameter and substantial axial length, so as to havea long screw engagement in the upper end portion of the cylinder in which piezoelectric element is mounted. At the opposite end of the piezoelectric element, namely, at the bottom portion of the cylinder 21, the housing of the unit is also provided with extra thick walls surrounding the counterbore 39 in which the hammer 13 is snugly but rotatably mounted.

With the construction described, it will be noted that the reaction forces resultingl from the hammer striking the striker pin are carried directly into the massive and most rigid areas of the housing as represented by the plug 26 and lower end portion of the cylinder surrounding the hammer and striker pin. These sections', of course, .are connected by the rigid thickl walled tubular structure or cylinder 21. Direct radiation of sound waves from the outside of the igniter unitcan be reduced if desired,

by a bag-like covering enclosing the unit and filled with material having low sound conductivity.

Attention is also directed to the fact that the hammer 18, which has a cylindrical body portion, has relatively small polar inertia so that it will quickly respond to the biasing torque Iof the torsion bar spring Whenever it is released from its cocked position by the hammer operating mechanism 19. By holding the polar inertia of the hammer to a minimum, objectionable time delay and consequent spark retardance which was found objectionable in piezoelectric igniter systems of the type hereinbefore referred to is largely eliminated.

Connected with the hammer and preferably integral therewith, is an actuating arm 58 by which the hammer lmay be rocked away from the striker pin and then i thereto.

The underside of the medial portion of the cooking lever is cooperable with a stationary fulcrum 63 provided by a pin in the chamber 17 supported by the walls thereof and parallel to the drive shaft of the igniter unit. The cooking lever is normally biased downwardly, in a counterclockwise direction as seen in Figures l and 5 through 9, to be yieldingly maintained engaged with the stationary fulcrum by means of a curved leaf spring 65. The spring is confined in the chamber 17 with its medial portion substantially encircling the eccentric and its free end bearing upon the upper side of the cooking lever. The lower end portion of the spring is anchored in a well 66 in the bottom wall of the main body portion of the igniter housing just inwardly of the cover Z5. Inasmuch as the leaf spring bears against the inside of the cover and against the upper wall of the chamber 17, it exerts a substantial downward force on the cocking lever but is free to yield in a manner described later, to allow the cocking lever to be lifted oi its stationary fulcrum 63.

Consequently, as the drive shaft rotates, the medial portion of the cocking lever is caused to move on the fulcrum 63 with a combined rocking and endwise sliding action, causing the outer extremity of the lever to describe an arc which is substantially the reciprocal of that described by the eccentrically mounted inner portion of the lever and which arc or orbit, moreover, is intersected by the free end portion of the actuating arm 58 on the hammer. v It is important to note that the fulcrum 63 is spaced from and intermediate the axes of the eccentric and the rocking axis of the hammer carried actuating arm, and that the arm and cooking lever are disposed substantially at right angles to one another. By reason of this arrangement and the relative lengths of the arms, the extremity of the cocking lever is caused t recurrently engage behind the free end portion of the actuating arm and swing it against the bias of the torsion bar spring in the direction to cock the hammer, so as to carry it away from the striker pin, and then abruptly release the actuating arm to permit the hammer to descend upon the striker pin at the proper instant in the cycle of engine operation.

Successive relative positions of the cocking lever and actuating arm during the course of a complete engine cycle are illustrated in Figures 5 to 9, inclusive, wherein the normal direction of drive shaft and eccentric rotation is counterclockwise, and wherein Figure shows the relative positions of the arm and lever immediately after yrelease of the arm to produce a hammer blow upon the striker pin. In this condition kof the parts, the actuating arm lies closest to the fulcrum 63, and the cooking lever is substantially fully endwise projected by the eccentric, with its outer end portion overlying and extending beyond the adjacent end of the arm.

As the eccentric is rotated through the next toward its position shown in Figure 6, it slides the cocking lever endwise along the fulcrum 63, in a direction away from the actuating arm. After rotating nearly v from its position seen in Figure 5, the eccentric retracts the outer end of the cocking lever Ebeyond the plane of the underside of the actuating arm, and the lever begins to swing in a counterclockwise direction on its fulcrum 63 to carry the outer end of the lever downwardly beneath the arm as seen in Figure 7, where the cocking lever is shown fully retracted. The next 90 of rotation of the eccentric, from its Figure 7 position to its Figure 8 position, causes the lever to be projected outwardly, endwise along its fulcrum to thus rock the actuating arm in the hammer :cocking direction, away from the striker pin, `and load the torsion spring. At the same time, the outer end of the lever slides downwardly along the rear of the arm a slight distance away from the outer end thereof, with the lever tilting counterclockwise on its fulcrum 63 through this portion of its cycle. As motion of the free end of the lever continues, due to further rotation of the eccentric, the lever continues to be projected endwise outwardly and to -swing the arm in the direction to complete cooking of the hammer and loading of the torsion spring, but its free end zbegins to slide toward the end of the arm as it rocks clockwise on its fulcrum. Figure 9 shows the positions of the parts when the lever is fully projected by the eccentric and the hammer is fully cocked, and just before the free end of the lever is carried off of the end of the actuating arm. As the eccentric continues to turn past this point, which is about 350 beyond the kposition of the eccentric shown in Figure 5, the lever is rocked or tilted clockwise on its fulcrum 63, and its outer end slides easily from engagement with the actuating arm, permitting the hammer to strike the striker pin a hard sudden blow in response to the action of the torsion spring (Figure 5). The underside of the lever is undercut, las at 70, to assure clearance for the actuating arm 58 when the latter snaps off of the free end of the lever.

If the eccentric should be rotated in the opposite (clockwise) direction, as sometimes happens during starting, or a backfire, the underside of the cooking lever is merely vcarried against the end of the actuating arm and slides thereon, the lever being thereby raised oi the fulcrum 63 against the yielding bias of its spring. The hammer, of course, will remain in its position of engagement with the striker pin so that no sparking voltage can be generated.

With the construction described, the igniter unit is mounted on one end wall of the engine crankcase with the drive shaft of the unit projecting into the `crankcase and parallel to but ya distance below the engine camshaft 10. As shown in Figures 4 and 4a, however, the igniter` unit may also be mounted on that side wall of the engine which is adjacent to the camshaft, with its drive shaft projecting into the crankcase of the engine and disposed with its axis at right angles to the camshaft, but in the same horizontal plane as the axis of the eamshaft.

When the igniter unit is mounted in this manner, its drive shaft may be drivingly connected with the engine in a somewhat simpler manner. In Figure 4, for instance, a bevel gear 72 is fixed on the camshaft and meshes with a companion bevel gear 73 on the square cross section end portion 56 of the drive shaft of the igniter unit. These bevel gears have the same number of teeth so as to assure rotation of the drive shaft of the igniter unit at one-half crankshaft speed.

In the embodiment of the invention illustrated in Figure 4a, the driving connection between the drive shaft of the igniter unit and the engine camshaft is still further simplified through the use of a crown gear 75 on the drive shaft meshing with a gear 76, on the camshaft, and having the same number of teeth. In this case, however, the camshaft gear 76 is provided by one of the timing gears 11, thus making it possible to transmit rotation from the crankshaft to the drive shaft of the igniter unit, through the camshaft, with one less gear than was neces- -sary in the previously described embodiments of the invention.

In both of these modified embodiments, it is important to note that the drive shaft of the igniter unit may `be held against axial displacement from the igniter housing solely by the meshing engagement of the gear on its drive shaft with the camshaft carried gear, thus eliminating the need for the set yscrew 53 used in the embodiment of the invention shown in Figures 1 to 3, inclusive.

From the foregoing description taken together with the accompanying drawings, it will be apparent that this invention provides an improved but simple, compact and dependable mechanism for recurrently imparting abrupt mechanical stresses to a polycrystalline piezoelectric element in timed relation to the angular position of a rotatable part, which mechanism is quieter in operation, has reduced friction and inertia in its moving parts, and has provision for preventing damage to the operating mechanism when the rotatable part turns in the opposite direction from that in which it is intended to go.

What is claimed as my invention is:

1. A piezoelectric igniter unit for an internal combustion engine having a crankcase and a crankshaft driven member therein, comprising: a housing; a piezoelectric element in the housing; a drive shaft rotatably journaled in the housing and having an end portion projecting a distance outwardly from one side of the housing; mechanism in the housing actuated by the drive shaft and operatively connected with the piezoelectric element for effecting sudden change in mechanical stress in the element, said mechanism including a spring which is cyclically alternately loaded and suddenly released in timed relation to rotation of the drive shaft, sudden release of the spring tending to subject the housing to abrupt, noise producing reaction forces; a driving member on said projecting end portion of the drive shaft projectable through an aperture in the crankcase of an engine, to be driven from a crankshaft driven member of the engine; and an adaptor plate fixed on said side of the igniter housing to provide for attachment of the igniter unit to the crankcase of an engine, said plate having a cen- CTL tral aperture therein through which the drive shaft projects and being located on the housing between said driving member and the main portion of the housing, and said adaptor plate being formed of resilient material so as to minimize the transmission of shocks, produced by suddent release of said spring, into the walls of an engine upon which the igniter is mounted.

2. The piezoelectric igniter unit set forth in claim l, further characterized by the fact that said driving member is also formed of a resilient material to further minimize the transmission of said shocks to an internal combustion engine upon which the igniter unit is mounted.

3. The piezoelectric igniter unit set forth in claim l, further characterized by the fact that those wall portions of the igniter unit housing which receive the reaction forces incidental to sudden release of said spring have substantial cross section and rigidity.

4. The piezoelectric igniter unit set forth in claim 3 further characterized by the fact that the piezoelectric element is recurrently subjected to hammer blows by said mechanism upon sudden release of the spring, and wherein the piezoelectric element is mounted in a tubular portion of the igniter housing which is remote from the adaptor plate and has a high diameter to wall thickness ratio, the ends of said tubular portion being joined to housing parts having substantially greater thickness than that of the Wall of the tubular portion.

5. The piezoelectric igniter unit' set forth in claim l, further characterized by: the fact that said housing provides a pair of chambers communicating with one another through a bore; the fact that the piezoelectric element is mounted in one of said chambers between an abutment and a striker pin opposing but spaced from the abutment and having a striker portion thereon projecting through said bore to the interior of the other of said chambers; and by the fact that said mechanism for effecting sudden change in mechanical stress on the piezoelectric element includes a hammer cooperable with said striker portion of the striker pin to impose a forceful hammer blow thereon as a consequence of sudden release of said spring, said mechanism and the hammer being contained in the other of said chambers.

References Cited in the file of this patent UNITED STATES PATENTS 1,945,132 Brown Jan. 30, 1934 2,649,488 Harkness Aug. 18, 1953 2,655,911 Van Russum Oct. 20, 1953 2,717,916 Harkness Sept. 13, 1955

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