This invention relates to an internal combustion engine which may, for example, use lean fuel/air mixtures for fuel efficiency.

Internal combustion engines are known in which a cylinder is provided with two or more valves, most commonly either two or four valves, namely inlet valve(s) and exhaust valve(s). These valves open in a cavity in the cylinder head conventionally known as a combustion chamber. These valves may be directly actuated by the cam(s) of a camshaft. More common is actuation by a rocker arm assembly having respective rocker arms actuated by a cam that is driven by a camshaft rotating in sequence with the crankshaft through a timing chain and driving and driven sprockets.

Typically, such engines are provided with only one spark plug per cylinder, such spark plug optimally being centrally located in the combustion chamber as described - for example - in Indian Patent Application No. 581/DEL/2004. One of the disadvantages of such an arrangement is that the process of combustion from spark initiation to completion, in a single spark plug engine, takes a comparatively long time. At higher engine speeds, the available combustion duration may be insufficient, resulting in incomplete combustion and consequent loss of power, fuel wastage and increase in emissions. These disadvantages are more evident in the case of engines operating with lean fuel- air mixtures.

In contrast to the paradigm in which rich fuel-air mixtures are used to generate high power output, typically in large capacity engines, a lean fuel-air mixture is selected in a lean burn operating paradigm or lean burn mode to reduce fuel operating costs. Such operating costs, even for vehicles like scooters and motorcycles having small capacity engines, are a very significant proportion of vehicle operator income in many countries and can make the difference between having access to a personal vehicle and having no such access. To such vehicle operators, power output is of much less concern than the ability to run a vehicle cheaply and reliably. It is also to be considered that, in many cases, road speeds are low, particularly during urban commuting, and the potential for high power output is of relatively limited importance.

An economic approach to manufacturing and running costs in such lean burn mode vehicles is critical. Such engine can readily be distinguished from engines that are used for higher power output operating with rich fuel-air mixtures Lean burn mode running may be made more efficient by inclusion of two spark plugs, in a counter-intuitive approach which allows appreciable reduction in fuel consumption and fuel costs as indicated in the Applicant's Indian Patent No. 194504. This approach is counter-intuitive because single spark plug small bore engines have previously been considered to allow combustion of lean fuel air-mixtures to a generally acceptable extent.

For optimum operation, insertion of the two spark plugs to specific and optimum locations within the combustion chamber is of much importance yet problems arise because of the difficulty in packaging two spark plugs in a small engine having an SOHC (Single Overhead Cam) type of cylinder head which provides particular space constraints on packaging due to small engine size and, in particular, combustion chamber and cylinder head compactness. Two designs have been considered previously. The two spark plugs are inserted at the sides of an engine cylinder head. In one design, one spark plug is inserted from the timing chain side of the engine (Indian Patent No. 195904) and, due to this a protective sleeve to avoid lubricant falling on the spark plug is used. In a second design, as described in the Applicant's WO 2007080604, the spark plug can be inserted through the dry side of the engine, that is, distal from the timing chain cavity.

The above two applications teach positioning or opening of spark plugs at the periphery of the combustion chamber which improves combustion performance. However, to improve the combustion performance further, the Applicant has found that a construction in which in addition to two peripheral spark plugs, one additional spark plug opening substantially at the center of combustion chamber would be advantageous. However, the positioning of three spark plugs, one of them opening substantially at the center is a challenging task in a shallow and compact combustion chamber unlike earlier designs of combustion chamber.

US 4748946 provides for central location of a single spark plug in the roof of a combustion chamber of the multi-valve (i.e. 4 valve) cylinder head of a multi-cylinder SOHC engine, with a conventional rocker arm assembly for actuation of inlet and exhaust valves, through moving the camshaft from a location extending along a central transverse axis of the cylinder head to substantially offset position. This allows the spark plug to be inserted from the transverse side of the cylinder head, between the inlet valves, and prevents interference of the spark plug with the rocker arm assembly. US 4748946 shows the difficulty in locating a spark plug to open in a central location in the roof of the single spark plug combustion chamber . Furthermore, US 4748946 does not address the issue of packaging multiple spark plugs within the combustion chamber. Nor does it address the particular issues which arise with using and placing a plurality of spark plugs in small bore single cylinder engines. One such problem particularly concerns water cooled engines. In such engines, spark plug resting or seating position for a spark plug must be selected to avoid interference with a water cooling circuit arranged to provide cooling at spark plug and valve locations. This is a challenging process even for a single spark plug.

It is among the objects of the present invention to provide an internal combustion engine with a cylinder head better adapted for enhancing fuel combustion, reducing fuel consumption costs and emissions than the internal combustion engines of the prior art.

With this object in view, the present invention provides an internal combustion engine comprising a cylinder head for a cylinder having a combustion chamber and cylinder bore with a centre longitudinal axis; a cylinder head cover; at least one inlet valve and at least one exhaust valve, said valves being camshaft actuated; and three ignition means comprising a primary ignition means and two secondary ignition means located in the combustion chamber, said primary ignition means extending through an insertion bore through a wall of the cylinder head into the combustion chamber and opening in a central position relative to the combustion chamber; and said secondary ignition means extending through respective insertion bores to open at respective peripheral positions of said combustion chamber wherein said primary ignition means has different dimensions than said secondary ignition means; and said primary and secondary ignition means are located in a shallow and compact combustion chamber.

The engine is designed with a shallow and compact combustion chamber which, in one non-limiting though advantageous exemplary embodiment has a cavity depth in vertical direction of the cylinder head ranging from 8 mm to 12 mm. The person skilled in the art will, with this description, visualise other shallow and compact combustion chamber constructions.

The primary ignition means is so-called because of the importance of a central position of ignition means, hereinafter referred to as spark plug(s), to fuel combustion and fuel efficiency. It may be noted, however, that it has not always been possible to achieve such central position for all engines, particularly small bore engines having a very shallow and compact combustion chamber without making significant changes in engine components, for example increase in size or cavity depth of combustion chamber leading to increased surface to volume ratio, compromise over valve sizes, shifting the position of cam shafts etc..

The primary and secondary spark plugs have different dimensions, particularly for the spark plug mounting. Spark plugs are typically threaded, along a threaded length - typically correspondent with same or similar thread length in a respective spark plug insertion bore - into position in their respective insertion bores within the cylinder head. The threaded portion is the means for fixing the spark plugs into their required location. In such case, the thread length of the primary spark plug is advantageously longer than thread length provided for the secondary spark plugs. Such "extra long" threading allows the primary spark plug seat to be fitted away from the combustion chamber. In a water cooled engine, this allows interference with the water cooling circuit to be avoided and allows accommodation of an efficient water cooling circuit. It also allows the design to retain the use of a common insertion tool for all the plugs in that the hexagonal nut typically used for spark plug insertion can remain of a larger size (as for the other spark plugs) as it is located away from the cooling passages.

Additionally, the insertion bore of the primary spark plug may have larger diameter, than insertion bore diameter for each secondary spark plug, to accommodate a primary spark plug of larger diameter It is important that the spark plugs, particularly the secondary spark plugs, are sized with thread dimension as
small as possible to achieve the required ignition performance. M10 rated spark plugs may be ideal for this purpose. In contrast, the primary spark plug advantageously has different specification, such as M12, if required to optimize ignition performance. Ignition performance is also optimized if the secondary spark plugs are fitted diametrically opposite to each other as this arrangement is associated with optimization of combustion phenomena.

Packaging advantage is promoted by having at least one secondary spark plug passing through a timing chain cavity of the engine, where the engine includes a timing chain. In this case, a vertical plane passing through a central longitudinal axis of the secondary spark plug orients or inclines towards the exhaust valve(s) at an angle with a vertical plane extending parallel to longitudinal axis/axes of a camshaft/s for actuating the valves. Further benefit may be achieved where the tips of both this secondary spark plug and the other secondary spark plug are offset, preferably up to 5mm in the engine dimensioned above, from a transverse plane of the cylinder head towards the region of the exhaust valve(s). The same vertical plane passing through the central longitudinal axis of the secondary spark plug (passing through the timing cavity) may also incline towards a tight side of the timing cavity at an angle with the vertical plane extending parallel to the longitudinal axis of the above mentioned camshaft. This orientation of spark plug towards tight side prevents the timing chain hitting spark plug in case of chain elongation after use.

The invention allows for optimization of the arrangement of spark plugs within the combustion chamber.

The insertion bore of the primary spark plug may enable that spark plug to extend substantially vertically into the combustion chamber, particularly in the case of a dual overhead cam engine embodiment. However, in other cases particularly including single overhead cam engine embodiments, the primary spark plug is preferably oriented, or inclined, at an acute angle p to the centre longitudinal axis of the cylinder bore In either case, the primary spark plug is oriented or inclined to allow accommodation of the secondary spark plug(s) which are also oriented or inclined at an acute angle a to the centre longitudinal axis of the cylinder bore. Angle p is typically smaller than angle a, angle a preferably being equal to or more than 55 degrees as this assists packaging of spark plugs in a shallow combustion chamber thereby optimizing surface area to volume ratio and achieving combustion chamber compactness. The angle a for each of the secondary spark plugs may be different. The actual angle, and placement/orientation of each of the spark plugs within the combustion chamber, is selected to optimize packaging and fuel combustion efficiency.

Three spark plugs may be conveniently packaged in the combustion chamber of both air and water cooled engines, with the same arrangement of spark plugs often being the same for both air and water cooled engines. However, where an engine is water cooled and employs a water cooling circuit, for example in the form of a water jacket, which provides cooling to the three spark plugs and inlet and exhaust valves and notably to the guide and stem portions of the valve elements thereof. The centrally positioned primary spark plug is packaged to avoid interference with the necessary water cooling circuit or jacket. One portion of the water cooling circuit or jacket may cool the secondary spark plugs and a further portion of the water cooling circuit or jacket may cool the primary spark plug.

A seat for the primary spark plug is advantageously positioned at a height above a core of the water cooling circuit to prevent obstruction of its insertion bore with a wall of said water cooling circuit. A seating height of the primary spark plug above an inner wall of combustion chamber of said primary spark plug may be higher than the corresponding height of the seat of each secondary spark plug.

In an embodiment, the water cooling circuit or jacket has portion extending between the valve guides for the inlet and exhaust valves and between the primary and secondary spark plugs. The water cooling circuit or jacket may have a first portion, conveniently forming a circle, ring or annulus, with the primary spark plug insertion bore and inlet valve guide extending through the inside of the portion. The secondary spark plugs would be located outward of this portion yet still provided with cooling by this portion of the water cooling circuit or jacket. The water cooling circuit or jacket may have a second portion, also conveniently forming a circle, ring or annulus located outward of the second portion.

The cooling water circuit may have a branch or portion extending between guides of inlet and exhaust valves and between insertion holes of spark plugs. The water jacket circuit preferably forms two rings, the first ring provides cooling to at least one of said valve guides, conveniently for the inlet valve, and primary spark plug insertion bore which are located within the first ring. Preferably, the second ring provides cooling to the guides of the exhaust valves which are located within the second ring.

Advantageously, a rocker arm assembly for actuating the inlet and exhaust valves is configured to allow space for accommodating the primary spark plug. To that end, the central positioning of the primary spark plug in the above described engine may sometimes require that the rocker arms be redesigned from the conventional planar design to a profile so that the primary spark plug does not interfere with the operation of the rocker arms. A curved profile may sometimes be necessary to enable accommodation of rocker arm in the limited space available. This may, for example, be required where economics (for example through costs of material for the cylinder head and weight which impacts fuel economy and operating costs) dictate that the cam deck location (which may be defined as the height of the cam bearing lower portion from top of the combustion chamber) be a relatively short distance above the combustion chamber. In this case, clearance from the primary spark plug dictates adoption of a short camshaft length in comparison to that conventionally used for an engine of the same bore. However, the "cam deck height" or distance between cam deck (base of an arc of rotation of the camshaft) and combustion chamber may be optimized such that camshaft length approaches or even attains conventional camshaft length as above described though this would impose added material cost or weight or but avoiding other undesirable constraints on the engine without compromising the design of the rocker arms. In this case, rocker arms can be made straight or at least straighter with consequential superior mechanical advantage, in operation of the inlet and exhaust valves, over the curved rocker arm design.

It may be necessary for the rocker arms to have different lengths dependent on whether a rocker arm actuates the inlet or exhaust valve(s). So, for a common position of valves, the roller position coming in contact with a cam lobe is offset with inlet and exhaust valves being driven by different cam lobes.

A preferred engine of this kind is a single cylinder SOHC multi-valve engine operating on the four stroke cycle, whether air cooled or water cooled. In such case, the camshaft may be centrally located relative to the cylinder head. However, the engine is not restricted by number of camshafts. The engine may also be a dual overhead cam (DOHC) engine, for example having two inlet valves and two exhaust valves (though other inlet/exhaust valve arrangements are not precluded).

The engine is conveniently fitted with a pair of inlet and exhaust valves making the engine a four valve engine. The engine is advantageously of cubic capacity between 75 cc and 400 cc. Such engines may be employed as prime movers for operation of two or three wheeled vehicles or other motorized vehicles, for example, motorcycles and other saddle-type vehicles. The engine may also be used in four or multi-wheeled vehicles. The invention also has applicability to engines having cylinder bore equal to or less than 90 mm diameter.

The above described engine allows for the convenient packaging of plural ignition means or spark plugs even in a small bore engine where plural, certainly three or more, spark plugs would normally be used and enables further improvements in fuel economy to be achieved when operating in the cost conscious lean burn mode.

The engine may be fuel injected or carbureted and may be of desired capacity. The engine may be used in various vehicle applications and is especially suited to use in motorcycles.

The internal combustion engine of the invention may be more fully understood from the following description of a preferred embodiment thereof made with reference to the accompanying drawings.

Numeral Description

6 Camshaft
6a Longitudinal Axis of Camshaft 6
14 Cylinder Head
14a Transverse Axis of Cylinder Head 14
14b Longitudinal Axis of Cylinder Head 14

16,16a, b Rocker Arms
17 Cylinder Head Cover
17a Connection Means
18 Rocker Arm Assembly
19 Water Cooling Jacket

21 First Branch of Water Cooling Jacket 19
22 Second Ring of Water Cooling Jacket 19
23 Inlet Valves
24 Exhaust Valves
25 Water inlet
26 Water outlet

23a, 24a Poppet Valve Element of Valves 23, 24
23b, 24b Stem of Valves 23, 24
23c, 24c Valve Guides of Valves 23, 24
30, 31 Camshaft Bearing Walls
32 Bearing of Camshaft Bearing Wall 30
40 Primary Spark Plug
40a Longitudinal Axis of Primary Spark Plug 40
41,42 Secondary Spark Plugs
41a, 42a Longitudinal Axis of Secondary Spark Plugs 41, 42
43 Power Supply
48,49 Insertion Bores of Secondary Spark Plugs 41, 42
50 Insertion Bore of Primary Spark Plug 40
51 Timing chain cavity
53 Tight Side of Timing Chain in Cavity
55 External Opening of Insertion Bore 50
61 Cam of Camshaft 6
100 Internal Combustion Engine
110 Combustion Chamber
111 Roof Surface of Combustion Chamber 110
115 Recess in Roof Surface 111
116a, b Rocker Arms of Prior Art

118 Rocker Arm Assembly of Prior Art
140 Water Cooling Gallery
Numerals Related To Second Embodiment (DOHC)
206,207 Camshafts
214 Cylinder Head
219 Water Cooling circuit
221 First portion of Water Cooling Circuit 219
222 Second portion of Water Cooling Circuit 219
223 Inlet Valves
224 Exhaust Valves
240 Primary Spark Plug
241,242 Secondary Spark Plugs
243 Seat of Primary Spark Plug 240
244, 245 Seat of Secondary Spark Plugs 241, 242
251 Timing Chain Cavity
253 Tight side of Timing Chain in Cavity
254 Inner Wall of Combustion Chamber
255 Core

Fig. 1 is a bottom view of a cylinder head of a single cylinder water cooled four stroke internal combustion engine in accordance with one embodiment of the present invention.

Fig. 2 is a sectional view of the cylinder head of Fig. 1 at section line A-A.

Fig. 3 is a section view of an alternative embodiment of cylinder head to Fig. 2.

Fig. 4 is a bottom view of a cylinder head of a single cylinder air cooled engine.

Fig. 5 is a sectional view of the cylinder head of Fig. 4 taken along section line C-C of Fig. 4.

Fig. 6 is a top view of a rocker arm unit used to actuate inlet and exhaust valves in the engine of the embodiments shown in Figs. 1 to 5.

Fig. 7 is a top view of a prior art rocker arm assembly.

Fig. 8 is a top view of the cylinder head (cover removed) of the water cooled engine shown in Figs.

Fig. 9 is a schematic plan section view showing location of the spark plugs relative to the combustion chamber and water cooling circuit of the engine described with reference to Figs. 1 to 3.

Fig. 10 Is a schematic side view showing location of the valves relative to the combustion chamber of the engine described with reference to Figs. 1 to 9

Fig. 11 is a bottom view of a cylinder head of a single cylinder water cooled engine with dual overhead camshaft actuated valves in accordance with a further embodiment of the invention.

Fig. 12 is a section view of the cylinder head of the engine of Fig. 11.

Fig. 13 is a top view of the engine of Figs. 11 and 12.

Fig. 14 Is a schematic plan section view of Fig. 11 showing location of the spark plugs relative to the combustion chamber and water cooling circuit of the engine shown in Figs. 11 to 13.

Fig. 15 Is a schematic side view showing location of the valves in relation to combustion in accordance with a further embodiment of the present invention.

Figure 16 is a graphical presentation of mass burnt fraction with respect to crank angle for one , two and three plug configuration of combustion chamber.

Figure 17 is a graphical presentation of improvement in fuel consumption in 3 spark plug combustion chamber compared to 2 spark plug combustion chamber.

Referring to Figs. 1 to 5 and 8 to 10, there is shown a cylinder head 14 of a single overhead cam (SOHC) internal combustion engine operating a motorcycle on the four stroke cycle in lean burn mode. That is, engine is operated with the prime concern of achieving fuel economy. To this end, as engine weight influences fuel economy and operating cost, the cylinder head 14 (and as much of the engine as possible) is made of a light weight material, here aluminum alloy. Engine is a water-cooled engine with water being circulated from a water jacket 19 through a water cooling circuit including a branch in the form of gallery 140 in the cylinder head 14 by a water pump (not shown).

The cylinder head 14 has a combustion chamber 110 provided with two inlet valves 23 and two exhaust valves 24, each provided with a poppet valve element 23a, 24a formed with a stem 23b, 24b and being guided by respective valve guides 23c, 24c as shown in Fig. 10. Inlet valves 23 regulate airflow to combustion chamber 110; and exhaust valves 24 regulate release of exhaust gas from combustion chamber 110 at timing governed by camshaft 6 and the four stroke combustion cycle. The included angles of inlet and exhaust valves 23 and 24, for a given set of valve sizes are optimized to minimize compromise to smaller valve size (and lesser power output) as shown in Fig. 10.

The combustion chamber 110 has a pent roof construction and is shallow and compact having a cavity depth in the vertical direction of the cylinder head ranging from 8 to 12 mm.

Three spark plugs 40, 41 and 42 are fitted in the cylinder head 14, the power supply 43 to spark plug 42 being shown in Figs. 2 and 3. Although the other spark plugs 41 and 42 have similar power supply, these are not shown for ease of illustration. Spark plugs 40-42 are threaded into position along their mounting thread length in corresponding respective insertion bores 50, 49 and 48. Respective central longitudinal axes 40a, 41a and 42a of each of spark plugs 40, 41 and 42, as shown in Figs. 2 and 3, are inclined and located substantially in a common plane. Tips of the spark plugs 40, 41 and 42 are located with an offset, up to 5 mm for engine, from a transverse axis ("X-X") of the cylinder head 14. A greater offset would cause reduction in exhaust valve sizes causing performance deterioration or change the combustion chamber shape away from the required compact and shallow construction.

Primary spark plug 40 opens in a central position relative to the roof surface 111 of combustion chamber 110. More specifically, primary spark plug 40 is inserted through passage or insertion bore 50 which extends at an acute inclination angle B to centre longitudinal axis 14b of cylinder head 14. Primary spark plug 40 terminates substantially on the centre longitudinal axis 14b of the cylinder head 14. The central positioning of primary spark plug 40 is achieved in the manner described below. Primary spark plug 40 has an M12 rating and larger thread length than the secondary spark plugs 41 and 42 (each of which are M10 rating).

The combustion chamber 110 may, to a rough approximation in top view, be considered to be divided, by axes X-X and Y-Y, into quadrants of a circle as shown in Fig. 4. The intake valves 23 are located in two neighboring quadrants of the circle, the quadrants forming one semi-circle. Similarly, the exhaust valves 24 are located in two neighboring quadrants of the circle, the quadrants forming the remaining semi¬circle. Secondary spark plugs 41 and 42 are fitted to open at opposite sides of the roof surface of combustion chamber 110. and at positions at the periphery of the combustion chamber 110 Spark plugs 41 and 42, which have M10 rating and thread length (smaller than for primary spark plug 40), are fitted diametrically opposite to each other as this arrangement is associated with optimization of combustion phenomena such that flame fronts originating from each of the spark plugs 41 and 42 move towards each other accelerating the combustion process while reducing emissions and improving fuel consumption. Spark plugs 41 and 42 are selected to have a lower diameter than primary spark plug 40 as this assists in minimizing the height of combustion chamber 110 whilst still enabling a required compression ratio for efficient combustion to be achieved. It is to be noted that attainment of desired compression ratio is not a trivial matter in the compact combustion chambers of small displacement engines, and is further exacerbated when three spark plugs are used as additional volume may also be created as a result of any recess created in the surface of the combustion chamber to accommodate the respective spark plug.
There are further differences in design between primary spark plug 40 and secondary spark plugs 41 and 42. In particular, all spark plugs 40-42 require to be connected or fixed to cylinder head 14. A threaded connection is selected. However, to avoid interference between primary spark plug 40 and water cooling gallery 140, the primary spark plug 40 is provided with a longer threaded portion and threaded diameter than for the secondary spark plugs 41 and 42. This allows the primary spark plug 40 to be connected to the cylinder head 14 away from combustion chamber 110. Specifically, the threaded portion of each of secondary spark plugs 41 and 42 has length 19.05mm and the threaded portion of primary spark plug 40 has greater diameter and a thread length of 26.5mm.

Water cooling jacket 19 has, as shown in Fig. 9, a branch 21 forming a ring circumscribing primary spark plug 40 and its insertion bore 50 which extends through the inside of gallery 140. This provides cooling to primary spark plug 40. Water cooling jacket 19 also has a branch or second ring 22 extending between the inlet and exhaust valves 23 and 24 and between the primary and secondary spark plugs 40 and 42. The insertion bores for secondary spark plugs 41 and 42 are located outward of second ring 22 though close enough to receive cooling. Valve elements and guides of inlet and exhaust valves 23 and 24 are also provided with cooling by rings 21 and 22.

Fig. 1 shows how the spark plugs 40, 41 and 42 lie substantially along a diameter of the combustion chamber 110, this arrangement also being conducive to efficient fuel combustion.

Spark plugs 40 and 41 are located to be distal from, and so as not to extend through the timing chain cavity 53, that cavity being supplied with lubricant. So, no protective sleeve to prevent lubricant fouling of either of the spark plugs 40 and 41 is necessary. A protective sleeve is required for spark plug 42 as this plug extends through the timing chain cavity 53.

Secondary spark plugs 41 and 42 terminate near flush with the roof surface 111 of combustion chamber 110. In contrast, primary spark plug 40 terminates in a recess 115 formed during fabrication of the combustion chamber roof surface 111. This recess 115 forms part of the insertion bore 50 through which primary spark plug 40 extends into the cylinder head 14.

The inlet and exhaust valves 23 and 24 of engine are actuated by rocker arm assembly 18 located under cylinder head cover 17. Cylinder head cover 17 protects the rocker arm assembly 18. Rocker arm assembly 18 has several common features with the rocker arm assembly 118 as described in the Applicant's International Publication No. WO 2007080604, the contents of which are hereby incorporated herein by reference.

Rocker arm assembly 18 is configured to provide a greater space for location of secondary spark plugs 41 and 42 and facilitates the location of spark plugs 40 and 41 distal from the timing chain cavity 51 of engine as shown in Fig. 8. Secondary spark plug 42 passes through a timing chain cavity 53 of the cylinder head 14 and is inclined towards a tight side of the timing chain in cavity 53. This orientation of spark plug 42 towards the tight side prevents the timing chain hitting spark plug 42 in case of chain elongation after use.

Each of the inlet valves 23 and exhaust valves 24 has a respective rocker arm 16 of rocker arm assembly 18, actuated by a cam 61 driven by a camshaft 6. The camshaft 6 is centrally located, relative to centre longitudinal axis 14b of the cylinder bore (Fig. 10), with an axis extending along the centre transverse axis 14a of the cylinder head 14, being journalled to two camshaft bearing walls 30 and 31 forming parts of the cylinder head 14. The bearing 32 of first camshaft bearing wall 30 is shown in Fig. 2. First camshaft bearing wall 30 has a connection means 17a with cylinder head cover 17 and necessary sealing, at this connection means 17a, is provided by a tongue and groove arrangement. The tongue is formed on cylinder head cover 17 and the groove on first camshaft bearing wall 30. Connection means 17a is located a small distance below the centre transverse axis 14a of cylinder head 14.

The primary spark plug 40 is inserted into cylinder head 14 and combustion chamber 110 through insertion bore 50. Primary spark plug 40 and its insertion bore 50 extend at an acute angle p, less than the angle a at which each of the axes of secondary spark plugs 41 and 42, intersect with the centre longitudinal axis 14b of cylinder head 14. The angles a at which the axes of the secondary spark plugs 41 and 42 intersect the centre longitudinal axis 14b of cylinder head 14 are equal.

Insertion bore 50, through which primary spark plug 40 extends, has its external opening 55 located below first camshaft bearing wall 30. Primary spark plug 40 enters the combustion chamber 110 between inlet and exhaust valves 23 and 24.

To prevent interference between primary spark plug 40 and rocker arm assembly 18, the rocker arms 16a and 16b for actuating the inlet and exhaust valves 23 and 24 must have correct shape and design. To this end, and as shown in Fig. 6, rocker arm 16a has partly curved profile (particularly at its inward end) diverging from rectilinear shaped rocker arm 16b of rocker arm assembly 18. Rocker arms 16a and 16b for respectively actuating the inlet and exhaust valves 23 and 24 are also of different length unlike a prior art design of rocker arm unit as shown in Fig. 7 where rocker arms 116a and 116b have the same length.

As a result, for a common position of inlet and exhaust valves 23 and 24, the roller position coming in contact with the cam lobe will be offset.

In an alternative embodiment of cylinder head 14, shown in Fig. 3, camshaft 6 - and cam deck height - the distance, D1, of the base of an arc of rotation of camshaft 6 above combustion chamber 110 - is greater than that, D2, for the cylinder head 14 shown in Fig. 2. In this case, involving optimization over cam deck height, rocker arms may be made straighter with superior mechanical advantage for operation of valves 23 and 24.

The engine may be air cooled, where cylinder head material permits, and this embodiment is illustrated with reference to Figs. 4 and 5. Nearly all features are common; however, the water jacket 19 and cooling galleries 21, 22 140 are omitted. A cylinder head cover 17 would be provided. Transverse axis 14a would again be the axis for the camshaft 61.

The above described engine allows for the convenient packaging of plural, here three, ignition means or spark plugs 40,41 and 42 in a small bore engine where three, or more, spark plugs would not normally be used and enables further improvements in fuel combustion efficiency and fuel economy to be achieved when operating in the cost conscious lean burn mode.

Modifications and variations to the internal combustion engine of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention. In one important modification, the engine may be a dual overhead cam (DOHC) engine as shown in Figs. 11 to 15.

Fig 11 to 15 show a cylinder head 214 of a single cylinder water cooled with dual overhead camshaft actuated inlet and exhaust valves 223, 224 in accordance with a further embodiment of the invention. First camshaft 206 actuates the inlet valves 223 and second camshaft 207 actuates the exhaust valves 224 (as shown in Fig. 15).The longitudinal axis of both the said cam shafts are parallel to each other.

Central spark plug 240 extends substantially vertically into the combustion chamber. Secondary spark plugs 241 and 242 are located in the same arrangement as described above for corresponding secondary spark plugs 41 and 42 of the engine.

Secondary spark plug 242 passes through a timing chain cavity 251 of the cylinder head and is inclined towards a tight side 253 of the timing chain in cavity 251. This orientation of spark plug 242 towards the tight side 253 prevents the timing chain from hitting spark plug 242 in case of chain elongation after use.
The included angles of inlet and exhaust valves 223 and 224, for a given set of valve sizes are optimized to minimize compromise to smaller valve size (and lesser power output). As shown in Fig. 15, included angle for each inlet valve 223 and included angle for exhaust valves , these angles being less than for the single overhead cam engine described above (and referring to Fig. 10).

Water cooling circuit 219 has first and second cooling portions 221 and 222 as shown in Fig. 14. First portion 221 of water cooling circuit 219 cools primary spark plug 240 and inlet valves 223. Second portion 222 of water cooling circuit 219 cools secondary spark plugs 241 and 242 and exhaust valves 224.

Primary spark plug 240 has a seat 243 at a height above the core 255 of water cooling circuit 219 and inner wall 254of combustion chamber []. Location of seat 243 prevents obstruction of spark plug 240 insertion bore with water cooling circuit 219. Secondary spark plugs 241 and 242 also have respective seats 244 and 245, these being at a height above inner wall 254 lower than for seat 243 of primary spark plug 240.

It can be observed from figure 16 that combustion is faster in 2 spark plug configuration compared to one spark plug configuration and combustion is still faster in 3 spark plug configuration as compared to 2 spark plug configuration.

This results into better fuel economy with 3 spark plug configuration which is graphically represented in figure 17.

WE CLAIM:

Claim 1 :An internal combustion engine comprising:

a cylinder head for a cylinder having a combustion chamber and cylinder bore with a centre longitudinal axis; a cylinder head cover; at least one inlet valve and at least one exhaust valve, said valves being camshaft actuated; and three ignition means comprising a primary ignition means and two secondary ignition means located in said combustion chamber, said primary ignition means extending through an insertion bore through a wall of the cylinder head into the combustion chamber and opening in a central position relative to the combustion chamber and said secondary ignition means extending through respective insertion bores to open at respective peripheral positions of said combustion chamber wherein said primary ignition means has different dimensions than said secondary ignition means; and said primary and secondary ignition means are accommodated in a shallow and compact combustion chamber.

Claim 2: :An internal combustion engine as claimed in claim 1 wherein said ignition means are spark plugs; and wherein said primary spark plug has greater diameter and longer thread length for spark plug mounting as compared to diameter and

thread length for said secondary spark plugs.

Claim 3: :An internal combustion engine as claimed in claim 1 or 2 wherein said ignition means are spark plugs accommodated in a shallow and compact combustion chamber having a cavity depth in vertical direction of said cylinder head ranging between 8 to 12 mm.

Claim 4: An internal combustion engine as claimed in any one of the preceding claims wherein said ignition means are spark plugs and a secondary spark plug passes through a timing chain cavity of the cylinder head.

Claim 5: An internal combustion engine as claimed in claim 4 wherein said secondary spark plug has a central longitudinal axis; and a vertical plane passing through said axis inclines towards exhaust valve(s) at an angle with a vertical plane parallel to a longitudinal axis of camshaft for actuating said valves.

Claim 6: : An internal combustion engine as claimed in claim 4 wherein said secondary spark plug has a central longitudinal axis; and a vertical plane passing through said axis inclines towards a tight side of timing cavity at an angle with a vertical plane parallel to a longitudinal axis of camshaft.

Claim 7: :An internal combustion engine as claimed in claim 3 wherein tips of said secondary spark plugs inside the combustion chamber are offset, up to 5 mm, from a transverse plane of said cylinder head towards the region of said exhaust valve(s).

Claim 8: An internal combustion engine as claimed in any one of the preceding claims wherein said ignition means are spark plugs oriented and/or inclined at an acute angle with respect to said centre longitudinal axis of said cylinder bore and wherein an acute angle formed by said primary spark plug with said centre longitudinal axis is smaller than an acute angle formed by each of said secondary spark plugs with respect to said centre longitudinal axis.

Claim 9. An internal combustion engine as claimed in claim 8 wherein the acute angle formed by each of said secondary spark plugs with said centre longitudinal axis of said cylinder bore is equal to or more than 55 degrees.

Claim 10 : An internal combustion engine as claimed in any one of claims 1 to 7 wherein said ignition means are spark plugs and said primary spark plug extends substantially vertically in said combustion chamber and said secondary spark plugs are inclined at an acute angle to said centre longitudinal axis of said cylinder bore .

Claim 11. An internal combustion engine as claimed in claim 10 wherein the acute angle formed by each secondary ignition means with said centre longitudinal axis of said cylinder bore is equal to or more than 55 degrees.

Claim 12: An internal combustion engine as claimed in any of the preceding claims wherein said engine is a water cooled engine.

Claim 13: An internal combustion engine as claimed in claim 12 comprising a water cooling circuit wherein said water cooling circuit cools said three spark plugs and said intake and exhaust valves.

Claim 14: An internal combustion engine as claimed in claim 13 wherein said water cooling circuit has one portion cooling said secondary spark plugs and said water cooling circuit has a further portion cooling said primary spark plug.

Claim 15: An internal combustion engine as claimed in claim 13 or 14 wherein a seat of said primary spark plug is positioned at a height above a core of said water cooling circuit.

Claim 16: An internal combustion engine as claimed in any one of claims 13 to 15 wherein a seating height of said primary plug from an inner wall of said combustion chamber is higher than the seating height of each secondary spark plug.

Claim 17: An internal combustion engine as claimed in any one of claims 2 to 16, wherein said primary spark plug is of M12 specification and said secondary plugs are of M10 specification.

Claim 18: An internal combustion engine as claimed in claim 17, wherein threading length of primary plug is 26.5mm and secondary plugs are of 19.05mm.

Claim 19: An internal combustion engine as claimed in any one of claims 1 to 9 or 12 to 18 being a single overhead cam shaft engine.

Claim 20: An internal combustion engine as claimed in any one of the preceding claims being a dual overhead cam shaft engine.

Claim 21: An internal combustion engine as claimed in any one of the preceding claims 18 having two inlet valves and two exhaust valves.

Claim 22: An internal combustion engine as claimed in claims 1 to 9 or 12 to 19 claims wherein at least one camshaft is centrally located relative to said cylinder head.

Claim 23: An internal combustion engine as claimed in any one of the preceding claims having a cylinder bore less than or equal to 90 mm.

Claim 24: An internal combustion engine as claimed in any one of the preceding claims wherein said combustion chamber is of pent roof type construction.

Claim 25: A vehicle including an internal combustion engine as claimed in any one of the preceding claims.

Claim 26: A vehicle as claimed in claim 24 being a motorcycle or any two wheeled vehicle.