GB2134977A - Internal combustion engine and cylinder head therefor - Google Patents

Abstract

A cylinder head of an internal combustion engine carries an overhead camshaft 21 and valves 17, 18 which diverge in a direction from the camshaft towards a combustion chamber with which the valves are associated. Longitudinal centrelines of the valves stems may intersect the camshaft axis. The valve head centres may lie either side of a plume containing the cam shaft axis and a piston diameter or the centre of the head of one valve is located so as not to intersect a plume containing the cam shaft axis and the longitudinal centreline of the other valve (and vice versa). Hair pin 24 or torsion bar (Fig. 8) valve closing mechanisms may be used. <IMAGE>

Description

SPECIFICATION Internal combustion engine and cylinder head therefor In a reciprocating piston internal combustion en gine,there is usually providedforeach combustion chambera plurality of valves,the stems of which diverge in a direction awayfrom the combustion chamber. When these valves are operated by a single overhead camshaft, rockers are required to transmit motion from the camshaft to free ends ofthevalve stems. One advantage of this arrangement is that the valves can have fairly large heads. This contributes to good flow of gases into and out ofthe combustion chamber.

In G.B. 2,005,347, there is disclosed an internal combustion engine with two valves per cylinder. The valves are driven by a single overhead camshaft and the axis of the camshaft is intersected by a longitudinal centreline of each valve stem so that no rockers are required. The cylinderaxes are inclinedtothe horizontal and the camshaft axis is offset upwardly with respect to the cylinder axes. The centre ofthe head of one ofthe valves of each cylinder lies substantially in a plane containing the axes of several cylinders of the engine. The second valve is inclined to the first valve and the centre of its head is spaced from the plane containing the axes of the cylinders.The object of this arrangement is to enable the engine as a whole to be given a large inclination relative to the vertical, with a viewto obtaining a power unit of low height.

According to a first aspect ofthe invention, an internal combustion engine comprises a cylinder head defining valve ports which communicatewith a combustion chamber defined between the cylinder head and a piston, a plurality of poppet valves, one for each of said valve ports, and an overhead camshaft for operating thevalves,the respective paths ofsaid valves, asviewed in a direction along the camshaft, converging with each other in a direction away from the combustion chamber and the valves having respective heads with centres lying on opposite sides of a plane containing the camshaft axis and a diameter ofthe piston.

With this arrangement, a relatively large proportion of the area ofthe cylinder head which is presented to the interiorofthe combustion chamber can be occupied bythevalve heads.

An engine in accordance with the invention preferably has, forthe or each piston, three orfourvalves in a cylinder head which presents to the combustion chamber a nearlyflatface and which is used with a piston presenting towards the combustion chamber a cavity.

The valves can be operated through pushers or tappets by a single overhead camshaft. No rockers or followers ortheir supporting shafts or bearings are necessary, although followers may be used if desired.

An engine according to the invention may comprise morethan one piston and define one cylinderfor each piston. A cylinder head casting may be common to all cylinders of the engine or a plurality of cylinder head castings may be provided. For example, the engine maycomprisetwo banks of cylinders with one cylinder head casting for each bank.

According to a second aspect of the invention, there is provided a cylinder head for an internal combustion engine comprising bearings means defining an axis of rotation of a camshaft and first and second valve guides for guiding respective poppet valves for movement along respective rectilinear paths, each said guide defining a longitudinal centreline of a respective path, and said centrelines, as viewed in a direction along the camshaft axis, diverging in a direction awayfrom the camshaft axis at an angle such that a head of each valve, when fitted in a corresponding one of said guides, does not intersect the plane which contains the camshaft axis and the centreline defined by the other guide.

The cylinder head will generally be formed to present to the combustion chamber an inverse pent-roof face, that is a face comprising two portions in which respective valve ports are formed and which can be regarded as mutually inclined portions which present a reflex angle to the combustion chamber.

There are preferably provided for each combustion chamber a pair of exhaust valves and either one ortwo inlet valves.

One advantage realisedwith the invention concerns the inclination ofthe inlet ducts to the longitudinal axis ofthe cylinder. When two valves are used in a cylinder head which presents a substantially flat face to the combustion chamber, the inlet duct inclination can be favourable, as in the V1 2Jaguar head. In an engine in accordance with the present invention having three or fourvalves associated with the or each cylinder and a single overhead camshaft, the inlet ducts can be similarly inclined, since space is not required to accommodate rockers. Furthermore,the inlet ducts can be substantially straight.

It is preferred that at least a part of the inlet duct, called herein the terminal part, which extends from the inlet portto a position spaced from the inlet port by a distance equal to the length ofthe inlet valve stem is substantially straight. In such a case, we refer herein to the centreline oftheterminal part ofthe inlet duct as the inlet duct axis. In a case where the terminal part of the inlet duct is not substantially straight but has a curved longitudinal centreline, the term "inlet duct axis" used hereinafter means that rectilinear line which is parallel to the chord of the longitudinal centreline of the terminal part and has one half of its length lying on each oftwo opposite sides ofthe centreline.

In an engine in accordance with the present invention, the inlet duct axis for each cylinder is preferably arranged to approach the axis ofthat cylinder in a direction from the cylinder head towards the piston which moves in that cylinder. With this arrangement, air and fuel whichflowthroughthe inlet duct and inlet port enterthe cylinder travelling in a direction inclined towards the centre ofthe cylinder, rather than towards the periphery. This assists charging ofthe combustion chamber.

It is preferred thatthe stem of the inlet valve or of one ofthe inlet valves associated with each cylinder should be inclined at an angle not exceeding 30 , and more preferably not exceeding 25", to the inlet duct axis. Furthermore, It is preftrmd that the stem o-, the inletvalve should enterthe anlet duct atthe side -ithereof which is io nearer to the cylinder axis.

It is also preferred that the stem ofthe or each exhaust valve associated with a cylinder should enter the corresponding exhaust duct at the side thereof nearerto the cylinder axis.

There is preferably provided for each valve a torsion bar spring for urging the valve towards a closes position. The torsion bar springs can be offset laterally from the valve stems and connected with the valve stems by bifurcated fingers. Afinger capable of transmitting force from a torsion bar spring to a valve stem can be accommodated in a space which is insufficient to accommodate a coiled valve spring in the usual way. Thus, the use of torsion springs and fingers enables successive valve stems along the camshaftto be spaced only a small distance apart.

Torsion coil springs can be used in place oftorsion bar springs.

In an engine in accordance with the invention having four valves per cylinder, an angle of inclination ofthe inlet ducts which is more favourable than the inclination achieved in known engines with two valves per cylinder, even when the valves are vertical, can be achieved, since the centres ofthe inletvalves are positioned nearto the perimeter ofthe combustion chamber and the stems of the valves associated with each cylinder are mutually convergent.

As with any other four valve head, the total inlet valve area is increased compared with a head with one inletvalve and one exhaust valve. Other Merits offour valve heads are maintained, for example a small reciprocating weightforthe valves and easier dissipa tion of heatfrom two exhaustvalves rather than one.

There can be used a single central sparking plug or a twin plug arrangement becausethe slight inverse pent-roof shape ofthe preferred head lends itself to a central squish area.

According to a further aspect of the invention, there is provided a cylinder head for an internal combustion engine andfourvalves mounted in the cylinder head for reciprocation along respective paths, wherein each ofsaidfourvalves has a head which differs in diameter from those of the otherth ree valves. With this arrangement, there can be provided valve ports having a large aggregate area.

In this case where there is provided for each cylinder one large inletvalve and one small inlet valve, the small inletvalve can be inclined relative to the large inletvalve and inclined at a relatively large angle to an axis ofthe cylinder. With this arrangement, the main charge will be admitted bythe larger valve and gas entering through the port controlled bythe smaller valvewill promote swirling Of an turbulence in the main charge in the cylinder, whilstavoiding substantial turbulence and swirling in the inlet duct leading to thelargerinletvalve. Itwill be understood that swirling and turbulence in the inlet duct impedes flow ofthe charge into the combustion chamber.

A cylinder head in accordance with the further aspect of the i nvention is preferably used in combination with one or more pistons presenting towards the cylinder head a depression which forms a part of a combustion chamber ofthe kind generally called a bowl-in-piston combustion chamber Particularly in a case where a main inlet duct defined by the cylinder head has a substantially straight terminal portion, good induction of air and fuel into the combustion chamber and good swirl ofthe air and fuel within the combustion chamber can be achieved.

In an engine in accordance with the invention which comprises a rectilinear row of cylinders, it is preferred thatall valve springs associated with that row lie all at the same side of a plane containing the axes ofthe cylinders in that row. In a case where an engine comprises two, parallel rows of cylinr'srs,this arrangement is especially advantageous and all of the valve springs associated with each row preferably lie atthe side ofthat row remote from the other row, thereby leaving an unobstructed space between the rows for inlet ducts.

In known engines having two banks of cylinders with the axes of the cylinders in one bank parallel to one another and inclined tothe axes ofthe cylinders of the other bank,the terminal portion of each inlet duct is generally arranged with its axis inclined at a large angle, typically in the region of 900, to the stem ofthe corresponding inlet valve. With this arrangement, the mixture offuel and airisrequiredtoflowaround a cornerto the inlet port, before entering a combustion chamber. Charging of the combustion chamber is impeded.The difficulty of arranging inlet ducts which do not impede charging ofthe cylinders is more acute with narrow V-engines, that is engines having two banks of cylinders inclined atan angle of 30" or less, and having known valve-operating gear.

The present invention is applicable with particular advantage to V-engines and, more particularly, narrow angle V-engines. The provision of a single overhead camshaft for operating the valves of each bank of cylinders, with the paths of the valves associated with each bank diverging from one another in a direction awayfrom the camshaft, enables substantially straight inlet ducts to be provided to the cylinders of both banks. Thus, good induction can be achieved in an engine which is very compact, forthe aggregate piston area provided and has a very rigid cylinder block.

In a V-engine in accordance with the present invention, the axis of one cylinder is preferably inclined at an angle not exceeding 25" to the axis of another cylinder. The engine may comprise two cylinders only.

Examples of engines embodying the invention will now be described, with reference to the accompanying drawings, wherein:- FIGURE 1 illustrates diagrammatically a crosssection through an engine having four valves associated with each cylinder, the section being in a plane parallel to a longitudinal axis of a cylinder and perpendiculartothe length of a camshaft of the engine; FIGURE 2 illustrates a plan ofthevalve ports associated with one cylinder and defined by the cylinder head ofthe engine of Figure 1; FIGURE 3 illustrates diagrammatically the arrangement ofthe valves associated with a single cylinder of the engine of Figure 1; FIGURE 4 illustrates diagrammatically a modifica tion of the engine of Figure 1; FIGURE 5 illustrates a further modification ofthe engine of Figure 1;; FIGURE 6 illustrates an underside view of a cylinder head of an engine having three valves associated with one cylinder; FIGURE 7 is a view similar to Figure 2 illustrating a furthermodification oftheengineof Figure 1;and FIGURE 8 is an illustration similarto Figure 1 of an engine with cylinders in a V-configuration.

The engines illustrated in the accompanying draw ingsare each reciprocating piston internal combustion engines. Each engine illustrated in Figures 1 to 7 may comprise a single cylinder or a plurality of cylinders, although only one cylinder and parts associated therewith are illustrated. The engine shown hlFigure 1 comprises a cylinder block 10 defining acylinder 11 in which there is provided a piston 12 On the cylinder block there is mounted a cylinder head 13 deRningfourvalve portscommuni- cating with a combustion chamber 14 defined between the cylinder head and the piston. The piston presents a cavityto the combustion chamber. Alternatively, the piston could present a flat face to the combustion chamber.The cylinder head further defines a pairof inlet passages, one of which is shown in Figure 1 at 15, leading to two of the valve ports and a pair of exhaust passages, one of which is shown at 16, leading from the othertwo valve ports. Inletvalves 17 and 19 control the inletports and exhaustvalves 18 and 20 control the exhaust ports.

In the cylinder head 13, there are provided valve guides for guiding the valves along respective paths which, as viewed in Figure 1,are mutually convergent in a direction away from the combustion chamber 14.

Acamshaft 21 is supported abovethe valves and there is interposed between each valve and the camshaft a respective tappetFtwo of which are indicated in Figure 1 by the reference numerals 22 and 23.

The angle atwhich thevalves converge is, in the illustrated example, 150. The resulting inclination of the valve faces with respectto the cylinder head face is 7.5" for each valve. Avery mild form of 'inverse pent-roof' cylinder head combustion face results.

The angle ofthevalvefaces approximates more closelytotheflatfaceofthe head as the valve stem lengthandiorthevalve stem plus tappet length, increases.

In thefourvalve version, the diameters of the inlet valves are chosen so that one is largerthan the other, while the diameters of the exhaustvalve pair are also chosen so that one is larger than the other. The sizes of the inlet valves (though of different size) are preferably chosen such thattheirtotal area of heads hasthe normal relationship to the total area of the exhaust valve heads, viz, between 10% and 20% greater cross-sectional area forthe inlet valves. A line drawn through the centres ofthe inlet valve faces and a line drawn through the centres of the exhaust valve faces are parallel (axes xx, x1x1) when viewed from above (Figure 2).Lines joining the centres of each inlet and its opposite valve are offset (axes W,Y1,Y1). The uppertappetfaces present tangential to the base circle of the camshaft and parallel to the axis of the camshaft. The central axis of each valve stem and the associated tappet intersects the camshaft axis, preferably at right angles. The faces ofthe tappets adjoining the camshaft are inclined, in this example, at7.5 to a plane which is perpendicularto the axis of the cylinder 11.

Alternatively, the heads of the two inlet valves may have the same size and the heads ofthe two outlet valves may have the same size, preferably a size which is smallerthan that ofthe inlet valve heads. The provision, for one cylinder, offourvalves having four respective different diameters is especially advantageous in an engine in accordance with the first aspect ofthe present invention but may also be used in engines which are not in accordance with the first aspect ofthe invention, for example an engine in which some or all ofthe valves are mutually conver- gent in a direction towards the combustion chamber.

"Hairpin" valve springs 24 sited as shown (Figure 1) are used to close the valves. The space required by conventional coil springs with buckettappets is not available due to the close-packed array of a tappet ends (Figure 3). The hairpin springs forthe exhaust valves protrude at right angles to the central axis ofthe head. The hairpin valve springsforthe inletvalves are fitted at an angle to the central horizontal axis of the head in orderto clearthe induction passages which are much more vertically orientated than the exhaust passages in this example. With much "flatter" inlet passages, the inletvalve springs could be orientated in likemannertotheexhaustsprings. In a dimensionally larger cylinder head, it is possibleto use coil springs and buckettappets with this concept.In the three valve version illustrated in Figure 6, the spacing ofthevalve stems can be such that conventional coil springs and bucket tappets can be used, butthe diameter of cylinder head is relevant to such application even with a three valve head, since the bucket tappet is wider than the coil spring and has a diameter much greaterthan the top ofthe type of tappets used in the example of Figure 1. In a case where coil springs are used, even when the cylinder bore is relatively large, it is preferable to provide tappets between the camshaft and the valve stems, in orderto achieve a small angle of inclination between one valve stem and an adjacent valve stem, as viewed in a direction along the camshaft.

An additional feature which allows a choice of inlet passage angles isthefactthatthe camshaft and its bearings and housings are elevated upwards because the tappets length is interposed between the cam and the valve stem. This gives extra transverse space for a down draught inlet passage compared with an arrangementofcamshaftwhich impinges lower down on a bucket tappet and coil spring.

This advantage could be equally applied to a two valve arrangement engine but an incidental advantage is gained by the use of narrow tappets rather than the much wider diameter bucket tappet. So the inward inclination of the valve stems and the use of narrow tappets results in an advantage.

The valves in the illustrated engines cannot accidentallytouch each other at anytime even in extremes ofoverlaptiming because they diverge towards their heads. The cylinder head may be water or air cooled. The inlet passages to suit the valve sizes may be such thatthe larger passage is nearlyvertical.

Thisappliestoengines in accordancewith the invention with two, three and fourvalves per cylinder.

In a case where there are, for each cylinder, a larger inletvalve and a smaller inlet valve, the smaller, higher-gas-speed inlet passage is inclined ata lower angle (for example 45 ), so that it can induce some swirl, whereasthe near-vertical inlet passage is most effective for achieving volumetric efficiency. The camshaft may be driven by any conventional means for example, chain, toothed belt, gears or shaft and bevel. The camshaft itself is such that the lobes serve in order exhaust, inlet, exhaust, inlet and is so formed thatthe smaller inlet valve is opened earlierthan the larger inletvalve is opened.

An engine with this arrangement of valve gear is particularly suited four a motorcycle disclosed in GB Patent No. 1,488,663 with fuel tank positioned under the seat. The entry angle of the inlet passage is especially suitable for use ofthe ducted plenum chamber disclosed in GB 2,088,800. Any extra top height of such an engine is equal to the length of the tappet, for example 36-50mm, and this is no great problem with a short stroke engine. The constructional merits of a cylinder head which presents a flat face to the combustion chamber are very nearly maintained since the valve face cut-outs are shallow (at 7.5 ), althoughthe head isnottotallyflatonthe working face.

Two alternatives are availableforthe mildly inverted pent-roofform of cylinder head face, namely an inverse pent-roofformed by recessed cut outs forthe valve heads, which is shown in Figure 1, and an arrangement such that the working face of the combustion chamber is elevated from the general face ofthe cylinder head to form a protruding inverse pent-roof. This latter arrangement is illustrated in Figure 4. The angles ofthe sides ofthis inverse pent-roof (whether in the cut-outform or in the protruding form) are angled at7.5'fromthe baseflat, ofthe combustion face ofthe cylinder head.This inverse pent-roofform is not symmetrical in side and fore-and-aft views, but the division between the two sides ofthe pent-rooffollows the division shown in the chord "RR" in Figure 2. Itwill be noted that the rectilinear line RR is not parallel to the line joining the centres ofthe exhaust valves and does not pass through the axis ofthe associated cylinder. Alternatively, where larger valves are used the division between the sides ofthe pent-rooftakes the form shown in Figure 7 where it is no longer a straight line division but a curved line necessitated to accommo datetheflats into which the larger ofthe inlet and exhaustvalves are seated.The areas designated S1, S2, S3 and 54 on the combustion face ofthe cylinder head may be used as squish areas with respect to the piston head crown. Alternate sparking plug positions are shown in Figure2 at 25 to 30. Further,thecentral area ofthe combustion face may in certain circumstances be used as a squish area in conjunction with appropriate contour of the piston crown, in which case the sparking p1.ugsoccupyoneofthe peripheral ortwo ofthe peripheral spark plug positions.

An advantage may lie in the selection of differing sizes of valve head diameterforthe pairof inletvalves sincethe higher gas speed from the smaller diameter port may be used to assist in the generation of swirl in the combustion chamber, without promoting swirl or undue turbulence in the inlet ducts upstream of the inlet valves.

The arrangement of valves in a head may be changed from a pair of inlets on one side ofthe chamber and a pair of exhausts on the other, to an arrangement in which the pairs on each side consist of an exhaust valve and an inlet valve i.e. alternate exhaust and inletvalves.

As shown in Figure 3, the fourtappf:s associated with a single cylinder are arranged ir. 3 row along the camshaft,the tappets of the inletvalves being offset slightly to one side ofthe camshaft and the tappets associated with the exhaust valves being offset slightlyto the opposite side ofthe camshaft. As viewed in the direction-towards the plane containing the axes ofthe camshaft and the cylinder, the tappets and valve stems ofthe example illustrated are all parallel to each other. Offsetting ofthe centres ofthe heads of the valves of each pair permits the tappets to be positioned so that they can be operated by a series of cams on a single camshaft.This may be achieved with pairs ofvalves ofthe same diameter but it is advantageous to use valves of different diameters within each pair and, particularly advantageous to use fourvalves, each having a diameter different from that of each of the otherthree valves. The aggregate valve area achieved with this arrangement is greater than in a case where the inlet valves have the same diameter and the outlet valves have a different diameter but are the same as each other in diameter.

As shown in Figure 5, there may be interposed between the camshaft and each valve, in place of a tappet, a pivoted follower31. With this arrangement, thecamshaftcanbenearertothecylinderthan isthe case with the arrangement shown in Figure 1, but a wider space is required in the vicinity of the camshaft to accommodatethefollowersthan is required to accommodatethetappets of Figure 1. An engine having followers as shown in Figure 5 may, in other respects, be identical with the engine illustrated in Figure 1. Alternatively, springs of different form, for example coiled compression springs, may be substituted forthe hairpin-type springs of Figure 1.Further alternative kinds of spring which may be used are torsion barsprings,torsion coil springs, leafsprings and springswith desmodromicoperation.

A cylinder head having three valvesfor a single cylinder arranged in the manner shown in Figure 6 may be incorporated in an engine which, in other respects, is generally the same as that illustrated in Figure 1. In the case ofthe cylinder head illustrated in Figure 6, the two parts of the inverse pent-roof face of the cylinder head intersect on the line RR. The inlet valve lies to one side ofthis line andthe exhaust valves lie to the otherside of the line.

The engine illustrated in Figures 1 and 2 may be modified bytransposing the valves 17 and 19 and their associated inlet passages and by transposing the exhaust valves 18 and 20 and their associated passages.

In each engine in accordance with the invention, there is preferably a single overhead camshaftforthe or each bank of cylinders.

It will be seen that, even in a case where four valves are provided for the or each cylinder, rockers are not requiredfortransmitting motion from the camshaft to the valves. Thus, the valve gear of an engine in accordance with the invention can be relatively simple and have a low inertia. Furthermore, the engine can readily be arranged to have a high compression ratio, even in a case where the stroke of the piston is less than the piston diameter.

In Figure 8,there is illustrated an engine having two cylinders or two banks of cylinders arranged in a V-configuration. Two cylinders only and parts associated with those cylinders are shown. The cylinders are defined by a common block 110 and have respective axes 111 and 112 which are mutually inclined, typically at an angle of 20 . The cylinder axes diverge in a direction towards heads 113 and 114 ofthe engine.

In the case of a two-cylinder engine, a respective head is provided for each cylinder. In a case where there are two banks of cylinders, head 113 may becommonto all cylinders of one bank and head 1 common to all cylinders of the other bank.

The head 113 and parts carried thereby are substantiallythe same as the head 114 and parts carried thereby, any differences arising from thefactthat one head is a mirror image of the other. Accordingly, only the head 113 will be described in detail. This head comprises a lower part 115, an intermediate part 116 and an upper part 117 mounted one upon the other.

The lower part 115 is mounted on the block 110 and, in conjunction with valves carried by the head, closes an upper end of one of the cylinders defined by the block, in which cylinder a piston 132 reciprocates.

The lower part 115 defines four ports which communicate with the associated cylinder. Two only ofthese ports are shown in Figure 8 and, for simplicity of illustration, these are represented as having respec tive centres in a plane which is perpendicular to a camshaft axis 118. In practice, the centres of the four ports would lie in respective different planes perpendicularto the camshaft axis. The lower part 115 carries valve guides in which are disposed poppetvalvesfor controlling the flow of gases th rough the ports. Two only of these valves are shown in Figure 8, namely an inletvalve 119and an outletvalve 120.

The three parts ofthe head 113 collectively define a substantially rectilinear inlet duct 121 leading to the inletportcontrolled bythevalve 119. An upward continuation ofthe inlet duct is defined by a carburettor 122 which is preferably of the form disclosed in my co-pending application corresponding to United Kingdom Application 8235974. The head 113 and the carburettor 122 define a common, rectilinear centreline 123 ofthe inlet duct. It will be noted thatthis centreline, as viewed in a direction along the camshaft axis 118, is convergent with the cylinder axis 111 in a direction from the inlet ducttowards the cylinder.This arrangementfacilitates charging ofthe cylinder with an air/fuel mixture through an inlet port which is situated, as shown in Figure 8, adjacenttothe periphery of the cylinder.

It will be noted that, whilst the centreline 123, atthe corresponding inlet port, extends in a direction which is convergent with the cylinder axis 111,when the engine is viewed along the camshaft axis 118, the longitudinal centreline 1 24 of the inlet valve, as viewed along the camshaft axis, diverges from the cylinder axis 111 in the direction from the inlet duct towards the cylinder. The stem ofthe inlet valve 119 enters the inlet duct 121 atthe side thereof nearer to the cylinder axis 111 and remote from the head 114.

The axis 124 of the valve stem is inclined to the longitudinal centreline 123 of the inlet duct at an acute angle which does not exceed 30 and preferably does not exceed 25 . Accordingly, the relation between the seatforthe valve 119 and the centreline 123 is nearto a perpendicular relation.

The intermediate part 116 ofthe head 113 carries springs for closing each of the valves carried by the head. The springs provided in the engine illustrated in Figure Bare torsion springs and these have respective torsion bars arranged with their longitudinal axes parallel to the camshaft axis 118. The torsion bars which correspond to the valves 119 and 120 repectivelyare identified by the reference numerals 125 and 126 in the drawings. All ofthe torsion bars carried by the head 113 are spaced from the corresponding valves in the same direction, that is a direction away from the head 114and awayfromthe inletduct 121 and any corresponding further ducts defined by the head 113.

Afinger 127 extends from the torsion bar 125, past the stem of valve 120 in close proximity thereto, to the valve 119. Asimilarfinger 128 extends from the torsion bar 126 to the valve 120. Each finger is offset in a direction along the camshaft axis 118 from each other finger associated with the valve springs carried by the head 113. It will be noted that all of the valve springs carried bythe head 113 lie at the same side of the axis 111, that is the side of the axis remote from the inlet ductl2l,the axis 112 and the head 114.

The two valves shown in Figure 8 which are associated with the cylinder in which the piston 132 reciprocates have respective stems with different lengths. Accordingly, the tappets associated with these valves also have different lengths. Thus, the finger 128 engages the valve 120 ata position substantially nearer to the camshaft axis 118 than is the position atwhich the valve 119 is engaged by the finger 127. Positioning some fingers nearertothe camshaft axis than otherfingersfacilitatesaccom- modation of the fingers in the cylinder head. Alternate valves along the camshaft may have the same length as one another, differing from that ofthe intervening valves.

The engine illustrated in Figure may be modified by substituting torsion coil springs for the torsion bar springs shown. Each torsion coil spring may be disposed, substantially entirely, within a bore formed in an associated finger which transmits force to the associated valve. With this arrangement, each finger would be supported by a pair of bearings lying outside the finger and spaced apart in a direction along the camshaft axis 118. It will be understood that the end portion ofeachfingerwhich engages a corresponding valve is bifurcated.

The camshaft 129 is carried by the upper part 115 of the head and a respective tappet is interposed between each valve and the camshaft. Each tappet is co-axial with the corresponding valve stem and intersects the camshaft axis 118, preferably at right angles. Between each tappet and the corresponding valve, there is a valve cap which can be substituted by a cap of different thickness to change the valve clearance. The tappet associated with the valve 119 is identified bythe reference numeral 130andthe corresponding valve cap by the reference 131.To enable a feeler guage to be inserted between a valve cap and the corresponding valve stem, a removable cover (not shown) is provided on the end of the intermediate part 116 ofthe head, to permit access to be gained to the interior ofthe head without removing the upper part 117.

The distance along the camshaft axis 118 between the axes ofthe stems of adjacent pairs of valves is less than the mean diameter of the headsofthesevalves.

Because of the mutually divergent arrangement of adjacent valve stems, the corresponding tappets can be close together at the camshaft.

Thevalvesll9and l20divergefromthecamshaft 129 at an angle which may be in the region of 15".

Respective heads of these valves lie on opposite sides of a plane containing the camshaft axis 118 and a diameterofthe piston 132 which operates in the associated cylinder. In the example illustrated, the heads of both of the valves shown are spaced slightly from this plane. Alternatively, one of the valve heads may intersectthis plane. In any event, the centres of the valve heads lie on opposite sides of the plane containing the camshaft axis and a diameter of the piston.

The valves carried by the head 113 may, when viewed along the cylinder axis 111, have the arrange mentshown in Figure 7 and have the general arrangement illustrated in Figure 3. In this arrange ment, the valve stems are offset from one another along the camshaft axis, the diameter of each ofthe valve heads is different from the diameters of the othervalve heads and each ofthevalve ports lies close to the periphery ofthe cylinder. The valve springs are spaced a substantial distance from the camshaft 129 and therefore do not preventthefourtappets associated with one cylinder being close together at the camshaft.Furthermore, since the springs are all offset to the same side oftheir corresponding valve stems, the springs do not restrict significantly freedom to position the inlet duct 121 so asto achieve maximum efficiency. Itwill be noted that each of the engines illustrated can have substantially straight inlet ducts.

Itwill also be noted that the piston 132 presents towardsthe cylinder head 113a depression which forms a part of a bowl-in-head combustion chamber defined between the cylinder head and the piston. The combination of a substantiallystraight inletduct and bowl-in-head combustion chamber promotes good charging ofthe combustion chamber during the induction stroke. Furthermore, ifthe inlet duct control led by the smaller inlet valve is inclined at a relatively large angletothe axis 111 ofthe cylinder, good swirl of the charge can be achieved in the combustion chamber without swirling being impeded significantly by eitherthe inverted pent-roof shape of the cylinder head orthe piston.

As shown in Figure 8, the inlet duct 121 and the corresponding duct defined by the head 114 are near to each otherandextend in the same general direction. In fact, these ducts converge slightly in a direction awayfrom the block 110. Accordingly, common fuel supply means 133 can be provided. this fuel supply means including a rnain feed pipe disposed between the two inlet ducts and branch pipes leading from that main pipe into the inlet ducts.

Itwill be notedthatthecarburettor 122 can be positioned conveniently somewhat above the upper part il5ofthe cylinder head but nevertheless nearto the inlet port.

The features disclosed in the foreg-:ng description, or the following claims, orthe accomanying drawings, expressed in their specific forms or in terms of a means forperforming the disclosed function, or a method orprocess for attaining the disclosed result, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse formsthereof.

Claims (24)

1. An internal combustion reciprocating piston engine comprising a cylinder head defining at least two valve ports which communicate with a combustion chamber defined between the cylinder head and a piston, a plurality ofpoppetvalves, one for each of said valve ports, and an overhead camshaftfor operating the valves, wherein the respective paths of said valves, as viewed in a direction along the camshaft, converge with each other in a direction away from the combustion chamber and wherein the valves have respective heads with centres lying on opposite sides of a plane containing the camshaft axis and a diameter ofthe piston.

2. A cylinder head for an internal combustion engine comprising bearing means defining an axis of rotation of a camshaft and first and second valve guides for guiding respective poppetvalves for movement along respetive rectilinear paths, each said guide defining a longitudinal centreline of a respective path, and said centrelines, as viewed in a direction along the camshaft axis, diverging in a direction away from the camshaft axis at an angle such that-a head of each valve, when fitted in a corresponding one of said guides, does not intersectthe plane which contains the camshaft axis and the centreline defined bythe other guide,

3.An engineaecording to Claim 1 oracylinder head according to Claim 2furthercomprisingthird and fourth guides and four valves mounted in respective ones of the g uides, wherein each of said four valves has a head which differsin diameterfrom those ofthe other three valves.

4. An engine according to Claim 1 or Claim 3 or a cylinder head according to Claim 2 or Claim 3wherein the cylinder head presents to the combustion cham beran inverse pent-roof face.

5. An engine according to any one of Claims 1,3 and 4 or a cylinder head according to any one of Claims 2 and 4wherein the cylinder head defines a larger inlet port and a smaller inlet port, both of which communicate with the same combustion chamber.

6. An engine according to anyone of Claims 1 and 3 to 5 or a cylinder head according to any one of Clams 2to 5 having four valve portswhich communicatewith the same combustion chamber and fourvalves for respective ones of the ports, wherein the valves have respective longitudinal axes which intersect the cam- shaft at respective positions spaced apart along the camshaft.

7. An engine or a cylinder head according to Claim 6wherein ducts defined bythe cylinder head and leading to said ports diverge in opposite diections from a reference plane which contains the camshaft axis and lies between the axes ofthevalves.

8. An engine or a cylinder head according to Claim 7 wherein there is associated with each of said four valves a respective spring and all of said valve springs lie at the same side of said reference plane.

9. An engine according to any one of Claims 1 and 3 to 8 or a cylinder head according to any one of Claims 2to 8 wherein the cylinder head defines a substantially rectilinear inlet duct leading to one of said ports.

10. An engine according to Claim 1 wherein a longitudinal centreline of each valve associated with said combustion chamber intersects said camshaft, the camshaft being common to all valves associated with the combustion chamber.

11. An engine according to Claim 1 0wherein there arefourvalves associated with said combustion chamber, each ofsaidfourvalves hasa head of a diameter different from the diameters of the heads of the otherthree valves, respective centres ofthe four valve heads are offsetfrom one another in a direction along the camshaft and respective axes of stems of the valves are correspondingly spaced apart along the camshaft.

12. An internal combustion, reciprocating piston engine which defines two cylinders in V-configuration, at least two inlet ducts, one for each cylinder, and at leasttwo exhaust ducts, one for each cylinder, and comprises two camshafts, one for each cylinder, a plurality of valves for controlling the flow of gases into and out of each cylinder and a pluralityofvalves springs for biasing the valves to closed positions, wherein the camshafts are mutually parallel, respective axes ofthe or each exhaust valve and the or each inletvalve associated with one of said cylinders intersect one of said camshafts,thevalvesdiverging from the camshaft,the or one ofthe inletvalves and the or one of the exhaust valves associated with said one cylinder have respective heads with centres lying on opposite sides of a plane containing the axis of the associated camshaft and a diameter of the one cylinderandthe inlet ducts extend th rough respective positions lying between the camshafts.

13. An engine according to Claim 12 wherein respective axes of said cylinders are inclined at an angle not exceeding 30 .

14. An engine according to Claim 12 or Claim 13 wherein said springs are torsion bar springs and the springs associated with each of said cylinders are offsetfrom the corresponding valves in a direction away from the valves associated with the other of said cylinders.

15. An engineaccordingtoClaim 130rClaim 14 wherein said springs are coiled torsion springs, the springs associated with each of said cylinders are offsetfrom the corresponding valves in a direction away from the valves associated with the other of said cylinders, there is provided for each torsion spring a fingerfortransmitting force from that spring to the associated valve and the torsion spring is disposed within a boreformed in the associated finger.

16. An engine according to Claim 14wherein there is provided for transmitting force from each torsion bar spring to the corresponding valve a finger.

17. An engine according to Claim 15 or Claim 16 wherein the fingers associated with adjacent valves engage those valves at positions which are spaced unequallyfrom the camshaft axis.

18. An engine according to Claim 1 wherein said valves define respective axes which intersectthe camshaft axis and wherein the spacing between the positions at which the axes of two successive valves along the camshaft intersectthe camshaft axis is less than the mean diameterofthe headsofthesevalves.

19. A cylinder head according to Claim 2 wherein said centrelines intersect the camshaft axis and the spacing between the positions at which successive centrelines along the camshaft intersectthe camshaft axis is less than the mean diameter ofthe heads ofthe valves.

20. In combination, a cylinder head for an internal combustion engine and four valves mounted in the cylinder head for reciprocation along respective paths, wherein each of said four valves has a head which differs in diameterfrom those of the otherthree valves.

21. An engine substantially as herein described with reference to Figures 1,2 and 3 of the accompanying drawings.

22. An engine according to Claim 21 modified substantially as herein described with reference to Figure 4 or Figure 5 or Figure 6 or Figure 7 ofthe accompanying drawings.

23. An engine substantially as herein described with reference to Figure 8 of the accompanying drawings.

24. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.