Martin OVERINGTON -- Catalytic Ignition -- Pop. Sci. March 1983 & 2 patents

rexresearch

Martin OVERINGTON
Catalytic Ignition

Popular Science ( March 1983, p. 40 )

Catalytic Ignition makes gas engine act like a diesel
by David Scott

Two-stroke otto cycle engines
US4903482
EP0339969

A two-stroke engine comprises a cylinder (2) accommodating a piston (6) and having an air inlet port (18) and an exhaust port (14,16). The exhaust port (14,16) communicates with an exhaust system (25) which includes a reduction catalyst (R) and an oxidation catalyst (O). The exhaust system (25) includes two exhaust flow paths (24,26) in parallel, the first (24) of which includes the reduction catalyst (R) and the second of which bypasses the reduction catalyst (R), the downstream ends of the two flow paths (24,26) being connected together upstream of the oxidation catalyst (O). The exhaust port (14,16) is controlled by the piston (2) or by poppet valves (32,34) so that as the piston (6) performs its downstroke the initial flow of exhaust gas is substantially through the first flow path (24) and the subsequent flow of exhaust gas is substantially through the second flow path (26).

The present invention relates to two-stroke Otto cycle engines and is concerned with the exhaust system of such engines.

Two-stroke engines include an inlet port and an exhaust port, both of which may comprise a plurality of spaced openings. Whilst the use of poppet valves is known, at least to control the exhaust port, when used in road vehicles such engines do not normally include poppet valves and the ports are usually provided in the cylinder wall and controlled, that is to say opened and closed, by the piston. The exhaust port opens before the inlet port and closes after it and is thus situated higher up the cylinder wall than the inlet port if the engine is in the usual orientation with the spark plug uppermost.

When the engine is performing its working stroke the exhaust port is opened first and a substantial proportion of the exhaust gas is expelled from the cylinder before the inlet port is opened. As the inlet port opens, the inlet charge, namely fresh air, which may contain fuel, enters the cylinder and displaces and replaces the remaining exhaust gases. The inlet port may communicate directly with an external supply of scavenge air or, in the case of an engine with a carburettor, indirectly via the interior of the crankcase. In the latter case, the cylinder is provided not only with an exhaust port and with an inlet or transfer port which communicates with the interior of the crankcase but also with a further admission port which connects the interior of the crankcase to the carburettor via a one-way valve, such as a Reed valve so that air and fuel are admitted to the interior of the crankcase during the upstroke of the piston but can not leave the crankcase during the downstroke of the piston. During the later portion of each upstroke air is admitted to the crankcase from atmosphere and during the later part of each downstroke air is admitted to the cylinder from the crankcase.

A two-stroke engine naturally emits only small quantities of harmful nitrogen oxides (N0x) but due to increasingly strict pollution and emission control regulations it is increasingly difficult to build a two-stroke engine which emits less than the maximum amount of N0x permitted by the stricter regulations. Reduction catalysts are known which reduce the N0x content of exhaust gases, but they are practicable only when the oxygen content of the exhaust gases is low. Unfortunately the oxygen content of the exhaust gases in a two-stroke engine is relatively high for the following reasons:

In order to maximise the efficiency of two-stroke engines it is common to purge residual exhaust gas from the cylinder with the aid of the incoming charge of air and fuel. For this purpose the inlet and exhaust ports are arranged so that there is a period for which they are both uncovered whereby the incoming air and fuel displaces the residual exhaust gas into the exhaust system. However, if this purging is to be efficient it inherently results in a certain proportion of the air and fuel overflowing into the exhaust system, i.e. passing straight through the cylinder without being burnt. The oxygen content of this incoming air represents an additional load on the reduction catalyst and reduces its ability to reduce N0x.

The fuel content of the purge gas which overflows into the exhaust system can be decreased by means of an oxidising catalyst in the exhaust system.

As mentioned above, the ports are generally controlled by the piston but the use of poppet valves whose operation is linked to the crankshaft may be advantageous for certain applications.

It is therefore the object of the present invention to provide a two-stroke engine in which residual exhaust gas may be purged from the cylinder by the incoming charge of air and fuel and whose exhaust system includes reduction and oxidation catalysts but in which the efficiency of the reduction catalyst is not significantly impaired by the presence of oxygen in the exhaust gas.

According to the present invention a two-stroke engine comprising a cylinder accommodating a piston and having an inlet port and an exhaust port, the exhaust port communicating with an exhaust system which includes a reduction catalyst and an oxidation catalyst is characterised in that the exhaust system includes two exhaust flow paths in parallel, the first of which includes a reduction catalyst and the second of which bypasses the reduction catalyst, the downstream ends of the two flow paths being connected together upstream of the oxidation catalyst, and that the exhaust port is so controlled that as the piston performs its downstroke the initial flow of exhaust gas is substantially through the first flow path and the subsequent flow of exhaust gas is at least partly, and preferably substantially, through the second flow path.

Whilst the exhaust port is open for a substantial period of each cycle in a two-stroke engine the present invention is based on the realisation that the majority of the exhaust gas is exhausted in the initial surge as the exhaust port is opened and that this initial surge of exhaust gas contains little or no atmospheric oxygen. This is particularly true when the engine is operating at high loads because the initial surge of exhaust gases is at high pressure. It is also true that the N0x content of the exhaust gases is highest when the engine is at high loads. Once the inlet port has opened the gases within the cylinder will include a certain proportion of oxygen but the flow of gas through the exhaust port at this stage is under a very much lower pressure.

In the engine of the present invention the exhaust port is so controlled, by the piston or by two or more valves which are opened and closed in synchronism with the engine cycle, that the initial surge of exhaust gas, which contains substantially no oxygen, passes through the reduction catalyst which can then reduce the N0x in the desired manner but that the subsequent flow of exhaust gas, which contains a proportion of oxygen from the inlet charge, passes through both flow paths. It will be appreciated that the first flow path has a higher flow resistance than the second flow path because it contains the reduction catalyst and thus when both flow paths are open to the interior of the cylinder the exhaust gas flow is predominantly through the second flow path, i.e. through the oxidation catalyst only and not through the reduction catalyst. The reduction catalyst is thus not additionally loaded by atmospheric oxygen and whilst most of the later portion of the gas flow through the exhaust system does not pass through the reduction catalyst only a minor proportion of the total mass of exhaust gas is involved and it is found in practice that a sufficient proportion of the entire volume of exhaust gas is subjected to the reduction catalyst to enable the emitted exhaust gases to meet the desired emission control standard.

The exhaust port may include one or more openings formed in the wall of the cylinder which are controlled by the piston, that is to say are opened and closed by being uncovered and covered, respectively, by the piston. In a first embodiment of this type in accordance with the invention the two flow paths communicate with the interior of the cylinder through one or more respective openings which are spaced apart in the axial direction of the cylinder, the openings of the first flow path being positioned to be uncovered by the piston before the opening(s) of the second flow path. In this embodiment the first flow path is brought into communication with the interior of the cylinder before the second flow path and thus the entire initial flow of exhaust gas flow through the reduction catalyst. Once the opening(s) of the second flow path have been uncovered also the exhaust gas flows substantially only through the second flow path since it will be appreciated that the flow resistance of the second flow path is less than that of the first flow path since it does not include the reduction catalyst.

In a second embodiment of the present invention the upstream ends of the two flow paths are connected together at a point immediately downstream of the exhaust port, the upstream end of the first flow path being positioned closer to the crankcase of the engine than that of the second flow path and subtending an angle of between 30 DEG and 60 DEG to the axis of the cylinder. It will be appreciated that as the edge of the exhaust port remote from the crankcase is the first to be uncovered by the piston the flow of the exhaust gas has not only a radially outward component but also a component towards the crankcase, that is to say a downward component. In this embodiment, the first flow path is positioned to be generally in line with the flow direction of the initial surge of exhaust gas whereby substantially all the initial surge of exhaust gas flows through the first flow path and thus through the reduction catalyst. Once the remainder of the exhaust port has been uncovered by the piston the subsequent flow of exhaust gas, which includes a proportion of oxygen from the inlet port, is substantially through the second flow path since its flow resistance is lower than that of the first flow path. In a preferred arrangement the upstream ends of the first and second flow paths subtend an angle of substantially 45 DEG and 90 DEG, respectively, to the axis of the cylinder.

It is preferred that the exhaust port comprises one or more series of circumferentially spaced openings in the cylinder wall which communicate with a common exhaust manifold with which the first and second flow paths communicate, the second flow path constituting a single pipe and the first flow path constituting a plurality of pipes substantially in alignment with the initial flow of exhaust gas through a respective opening in the cylinder wall.

It is preferred that the piston crown has a chamfered rim or is domed, that is to say that it is convex, since this is found to facilitate the flow of gas into and out of the cylinder and, in the case of the second embodiment, to ease the flow of the initial surge of exhaust gas into the first flow path.

In a third embodiment of the invention the first and second flow paths of the exhaust system again communicate with the interior of the cylinder through separate openings, which openings are controlled by respective valves which are linked to be operated by the crankshaft of the engine such that the first valve opens before the second valve. Thus in this embodiment the different timing of the exhaust gas flows through the first and second paths of the exhaust system is achieved solely by the provision of timed valves which are linked to the crankshaft and thus opened and closed in synchronism with the engine cycle. The timing of the valves and thus the gap between the opening of the first and second valves may be constant or it may be variable, advantageously by means which are known per se, in dependence on the engine operating parameters to match the catalytic action of the exhaust system to the operation of the engine at any particular time. In practice, the first valve will open between 5 DEG and 70 DEG before the second valve. If the relative timing of the two valves is arranged to be varied as the engine load varies, the gap between the opening of the two valves will be towards the upper end of the range at high load and towards the lower end of the range at low load.

Further features and details of the present invention will be apparent from the following description of three specific embodiments which is given by way of example with reference to the accompanying diagrammatic drawings, in which:

FIGS. 1 and 2 are diagrammatic side views of a two-stroke engine in accordance with the invention, FIG. 1 showing the exhaust port only partly open and FIG. 2 showing the exhaust port fully open;

FIGS. 3 and 4 correspond to FIGS. 1 and 2 and show a second embodiment of a two-stroke engine in accordance with the invention;

FIG. 5 is a view similar to FIG. 4, but on an enlarged scale with the crankcase, crankshaft and connecting rod omitted;

FIG. 6 is a sectional view on the line A--A in FIG. 5;

FIG. 7 is a diagrammatic side view of a third embodiment of a two-stroke engine in accordance with the invention; and

FIG. 8 is a graph showing the rate of exhaust gas flow against the crank angle for an engine in accordance with the invention.

FIGS. 1 and 2 show a crankcase-scavenged two-stroke engine comprising a cylinder 2, through the top of which a spark plug 4 projects and which slidably accommodates the piston 6. The piston 6 is connected by means of a connecting rod 8 to a crankshaft 10 within a crankcase 12. Situated within the side wall is an exhaust port which comprises two peripherally spaced series of openings in the cylinder wall, one series of openings 14 being positioned immediately above the other series 16, as will be described in more detail below. Also positioned in the cylinder wall is the inlet port 18 which comprises a circumferentially spaced series of openings which are positioned slightly below the openings 14. The inlet port 18 communicates with the interior of the crankcase via an inlet line 20. Communicating with the interior of the crankcase are one or more admission ports 22 which communicate with atmosphere via a one-way Reed valve 36 and the engine's carburettor 38.

The exhaust port communicates with an exhaust system 25. Specifically, exhaust openings 14 communicate with a first flow path 24 which includes a reduction catalyst R, typically a porous base of ceramic or metal which is coated with e.g. rhodium, and exhaust openings 16 communicate with a second flow path 26 which bypasses the reduction catalyst. The two flow paths are connected together downstream of the reduction catalyst to form a single exhaust path 28 which includes an oxidation catalyst 0, typically comprising a porous base of ceramic or metal which is coated with e.g. platinum or palladium.

In use, after the spark plug 4 has ignited the fuel/air charge in the cylinder 2 the piston 6 moves downwardly and first uncovers the exhaust openings 14. The high pressure of gas within the cylinder leads to a surge of exhaust gas through the first flow path 24 and thus through the reduction catalyst R. Whilst the piston is moving downwardly it compresses the fuel and air mixture which is present in the crankcase. The piston then uncovers both the exhaust openings 16 and the inlet port 18 and the pressure of the inlet charge in the crankcase 12 results in this flowing rapidly through the transfer passage 20 into the cylinder and thereby displacing the remaining exhaust gases into the exhaust system 25. Due to the fact that the flow resistance of the second flow path 26 is lower than that of the first flow path 24 the majority of the later exhaust gas flow is through the second flow path 26, as illustrated diagrammatically in FIG. 2. During the subsequent upstroke of the piston 6 a fresh charge of air and fuel is drawn into the crankcase 12 through the admission port 22 and the cycle is then repeated.

The engine of FIGS. 3 to 6 (from which the admission port 22 has been omitted for the sake of simplicity) is very similar to that of FIGS. 1 and 2 but instead of the two axially spaced series of exhaust openings there is only a first series of circumferentially spaced exhaust openings 14. The openings 14 communicate with a single exhaust manifold 33 which in turn communicates with the two flow paths. The first flow path 24 constitutes a plurality, in this case three, separate pipes which open through the bottom of the manifold 33 and are positioned circumferentially in positions which correspond to those of the exhaust openings 14. The upstream end of each pipe subtends an angle of about 45 DEG to the cylinder axis. The upstream edge of the opening of each pipe is situated a distance a from the cylinder wall whilst the downstream edge is situated at a distance b from the cylinder wall. The dimension b is preferably approximately equal to the height of the exhaust openings 14 whilst dimension a is preferably in the region of 0 to 0.7b. The height of the exhaust openings 14 may be 50% or more of the length of the piston stroke in the case of a high speed engine, e.g. for a racing motorcycle, but may be very much less, e.g. as little as 10% of the piston stroke, in the case of slower running- engines. The three pipes are joined together a short distance downstream of the cylinder 2 and the exhaust pathway then includes a reduction catalyst R and an oxidation catalyst 0. The second flow path 26 communicating with the exhaust manifold 33 is a single pipe which extends perpendicular to the cylinder axis and bypasses the reduction catalyst. The second flow path 26 joins the first flow path 24 at a position between the reduction and oxidation catalysts. In this embodiment, as in the last embodiment, the piston crown is domed, that is to say convex, and this promotes the flow of the initial surge of exhaust gas into the first flow path 24.

In use, when the piston first uncovers the upper edge of the exhaust openings 14 the flow of the initial surge of exhaust gas has not only an outward component but also a downward component and the gas flow is therefore approximately at 45 DEG to the cylinder axis. The jets of gas flowing through the openings 14 flow substantially straight into the first exhaust flow path 24 and thus through the reduction catalyst. As the exhaust openings 14 are opened further the pressure of the exhaust gas drops and its direction becomes more nearly horizontal and the flow then switches progressively to the second flow path 26.

The engine of FIG. 7 is substantially the same as the engine shown in FIGS. 1 and 2. However, the exhaust port comprises two openings or series of openings 14 and 16 which are positioned at about the same height at the top of the cylinder 2 and which are controlled by respective poppet valves 32 and 34. The poppet valves 32 and 34 are linked to the crankshaft 10 of the engine by any appropriate means, such as a camshaft and push rods of a type well known per se, to be opened and closed as the crankshaft 10 rotates. The connection of the valves 32,34 is such that the first valve 32 opens a short time before the second valve 34.

The operation of this engine will now be described starting from the near bottom dead centre position illustrated in FIG. 3. As the piston moves upwardly the exhaust valves 32 and 34 are initially open and exhaust gases in the cylinder 2 together with a proportion of the inlet charge which has been admitted through the inlet port 18 is displaced into the exhaust system 25. Shortly before the piston passes over and thus closes the inlet port 18 the exhaust valves 32,34 are closed. When the inlet port 18 closes compression begins. Whilst this occurs air is drawn into the crankcase through the carburettor and Reed valve. At or before the top dead centre position of the piston the spark plug 4 is sparked and combustion of the compressed air/fuel mixture in the cylinder results in the piston moving downwardly in its working stroke. As the piston moves downwardly it compresses the inlet charge which has been admitted into the crankcase and a short distance before the inlet port 18 is uncovered the first exhaust valve 32 is opened. This results in a substantial high pressure surge of exhaust gas through the first flow path 24 and this flow is subjected to the reducing action of the reduction catalyst R. As the inlet port 18 is uncovered air in the crankcase is forced through the transfer passage 20 into the cylinder and the second exhaust valve 34 is opened. The inflowing atmospheric air purges substantially all the exhaust gases out of the cylinder and these flow preferentially through the second flow passage 26 since its flow resistance is less than that of the flow passage 24. Whilst a certain proportion of this purged exhaust gas flow will occur through the flow passage 24 and thus through the reduction catalyst the amount involved is very small and thus the reduction catalyst is subjected to only very small amounts of atmospheric oxygen from the inlet charge. When the piston reaches the bottom dead centre position again the above cycle is repeated.

FIG. 8 is a graph which illustrates the rate of exhaust gas flow against crank angle and applies equally to all the embodiments described above. The exhaust ports begin to open at point A and the gas flow rate rises rapidly to a peak value and then begins to fall again as the pressure of the exhaust gas drops. The flow rate has reached a substantially constant value by the time the piston 6 has reached bottom dead centre, which is at point B. The gas flow rate then decreases progressively until it has reached substantially zero at point C at which the exhaust port is closed again. As may be seen from the area under the curve of FIG. 8, the major proportion of the exhaust gas flow is in the initial surge and it is this surge which flows substantially through the reduction catalyst and it is only the latter portion of the exhaust gas flow, that is to say between the points B and C, which contains oxygen and which bypasses the reduction catalyst.

It will be appreciated that an engine in accordance with the present invention need not be of crankcase-scavenged type but that it may also be of the type including a scavenge blower. Whilst the inlet port 18 has been described as being of the type which is covered and uncovered by the piston 6 it may also be of the type which includes a poppet valve and in this event this valve will also be connected to the crankshaft and times to open and close at the appropriate moment.

Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Combustion chamber arrangements in internal-combustion engines.
GB2113759
EP0085258

An internal combustion engine of the spark-ignited piston type has a piston crown of "pent-roof" shape and a cylinder head combustion chamber of corresponding shape. Formed on one inclined face [22] of the combustion chamber [20] is a recess [30] of generally ovate profile, having a plane roof [32] parallel to the face [22] and having a side wall [31] perpendicular thereto. The exhaust valve port [29] opens through one end of the roof of the recess [30] and the spark plug [37] protrudes through the other end. During the compression stroke as the piston [12] approaches top-dead-centre, gas is displaced by the "squish" effect from between the approaching inclined faces [16] and (21] of the piston crown and combustion chamber roof on one side of the apex [18,] [23] and travels across the combustion chamber and into the recess [30] in the other side of the roof thus producing gas movement and turbulence in the recess [30] where ignition takes place.

[0001] This invention relates to spark-ignited reciprocating- piston i.e. engines employing gasolene or other volatile hydrocarbon liquid as fuel, and is applicable both to fuel- injection engines and to carburetor engines, and both to engines with supercharging and to engines with normal induced aspiration.

[0002] To obtain the highest possible output from an i.c. engine it is essential to obtain the highest possible volumetric efficiency and breathing capacity, up to the highest practicable operating speed. This requires the use of large valve sizes or areas. At the same time, modern engines are required to have good fuel economy and low exhaust gas emission levels of unburnt hydrocarbons, carbon monoxide and nitrogen oxides. This demands the use of as high a compression ratio as possible with regard to the octane number of the fuel being used, in the interest of high thermal efficiency.

[0003] The shape and arrangement of the combustion chamber and valve ports associated with each cylinder of the engine is of decisive importance in connection with these and other desiderata. The detailed layout of the combustion chamber must be as compact as is practicable, and disposed in such a way that a high swirl and/or "squish" can be obtained, so as to create a relatively high gas turbulence to assist combustion particularly when operating at high air/fuel ratios (weak mixtures), in the interest of low NOx exhaust emissions and good fuel economy.

[0004] The present invention is concerned with providing certain novel combustion chamber configurations which will make possible improved results in some or all of these respects.

[0005] The invention in its widest concept comprises a spark-ignited piston engine, having a piston crown and cylinder head combustion chamber configuration in which as the piston approaches top-dead-centre during its compression stroke, gaseous charge is displaced across the combustion chamber by a "squish" effect from between approaching opposed faces of the piston crown and combustion chamber roof, respectively, on one side of the chamber into an open recess formed in the roof of the combustion chamber on the other side thereof, the spark ignition taking place in the recess.

[0006] The term "squish" is a known term in the art, which is used to refer to the displacement of a flow of gaseous charge from between opposed surfaces of the piston and cylinder head which approach one another very closely as the piston approaches its top-dead-centre position during its compression stroke, the displaced gas flow being directed into another part of the combustion chamber to create movement and turbulence of the air/fuel mixture which will assist combustion and improve emissions.

[0007] From another aspect, the invention comprises a spark-ignited i.c. engine having a cylinder with a piston whose crown has an upper surface formed by two oppositely-inclined faces which intersect or merge at a level above that of the periphery of the crown, and having a cylinder head formed in its lower surface with a re-entrant combustion chamber whose lower side is a-circular opening in the cylinder head coaxial with the cylinder and of substantially the same diameter, and whose overall . internal shape (apart from the below-mentioned recess) generally corresponds to the external shape of the piston crown, and whose roof is defined by two oppositely-inclined faces, corresponding to and opposed to those of the piston crown, which faces intersect or merge at a level above that of the lower side of the cylinder head, the engine inlet valve port or ports entering the combustion chamber through one of the said faces of the combustion chamber roof, and there being formed in the other face an open recess into which the engine exhaust valve port or ports and a spark plug aperture both open, and in which as in use the piston approaches the top-dead-centre position during its compression stroke its crown enters the combustion chamber and the oppositely-inclined faces of the piston crown closely approach the opposed faces of the combustion chamber roof so that compressed gaseous charge will be displaced laterally from between the face of the combustion chamber roof containing the inlet port(s) and the opposed face of the piston crown by the "squish" effect in a flow which will enter the recess in the other face of the combustion chamber.

[0008] It is to be understood that terms such as "upper", "lower", "above", "below", and the like are used herein to relate to the condition when the engine is orientated with the respective cylinder axis vertical, the mating face of the cylinder head horizontal, and the cylinder head disposed at the upper end of the cylinder with the crankshaft at the lower end thereof, so that the piston reciprocates vertically up and ddwn. However such terms are not to be interpreted as limiting the invention to this orientation in any way, since of course the engine may be mounted and operated in any practicable orientation.

[0009] In a convenient arrangement, the oppositely-inclined faces of the piston crown and of the roof of the combustion chamber may be plane, and may respectively intersect along parallel straight lines which are perpendicular to the axis of the cylinder, the opposed faces of the piston crown and combustion chamber roof being parallel. The inclined faces of the roof of the combustion chamber may be inclined at either equal or unequal angles to the cylinder axis, in a symmetrical or asymmetrical arrangement.

[0010] The recess in the combustion chamber roof is preferably elongate, to accommodate a single exhaust valve port and a spark plug aperture spaced along its major axis. It may have a plane roof, preferably parallel to the inclined face of the combustion chamber in which it is formed. It may have its entire side wall disposed perpendicularly to that face, and in a preferred arrangement its intersection with that face may have a generally ovate profile.

[0011] It will be understood that as the piston approaches the top-dead-centre position during the latter part of its compression stroke, air or fuel/air mixture located between the inclined face of the combustion chamber roof which contains the inlet valve(s) and the opposed face of the piston crown wall will be squeezed by the rising piston between these faces and will be displaced by the well-known "squish" effect in a laterally-moving flow out from between these approaching faces and across the combustion chamber, and this flow of displaced gas will enter the recess in the other inclined face of the chamber to set up gas movement and turbulence therein below the exhaust valve and in the vicinity of the spark plug electrodes, with beneficial effects upon ignition and combustion.

[0012] Should it be required to alter the angle at which the "squish" gas flow enters and/or travels within the recess, to an angle more parallel to the inclined face of the cylinder head in which the recess is formed, various modifications of the combustion chamber or piston crown may be made.

[0013] One such modification is a lip formed along the edge of the recess in the combustion chamber roof nearest to the junction of the inclined faces of the roof, the lip being immediately adjacent to the face containing the recess and projecting into the interior of the recess as an overhang to deflect the "squish" flow downwardly.

[0014] Another modification for the same purpose is an open groove formed in the face of the combustion chamber roof which contains the recess, the groove extending close to and along the length of the edge of the recess nearest to the junction of the oppositely-inclined faces.

[0015] The invention may be carried into effect in various ways, but certain specific embodiments thereof will now be described by way of example only and with reference to the accompanying drawings, in which:-

Figure 1 is a view, in section on the line A-A in Figure 5, showing the upper part of one cylinder and the cylinder head of a piston engine, which may be either a single-cylinder engine or a multi-cylinder engine.
Figure 2 is a view on the arrow X in Figure 1 showing part of the roof of the combustion chamber from below,
Figure 3 is a view in section on the line D-D in Figure 1 through the axes of the inlet valves,
Figure 4 is a view in section on the line E-E of Figure 1 through the exhaust valve axis,
Figure 5 is a sectional plan taken on the line C-C in Figure 1, in the case of the single cylinder head,
Figure 6 is a sectional elevation on the line B-B in Figure 5 through the axis of the spark plug,
Figure 7 is a sectional elevation on the line F-F in Figure 1, in a plane of section through the cylinder axis,
Figure 8 is a sectional plan similar to Figure 5 showing a first modification of the arrangement of Figures 1 to 7,
Figure 9 is a section on the line B-B of Figure 8, similar to Figure 6,
Figure 10 is a view similar to Figure 8 showing a second modification of the arrangement of Figures 1 to 7,
Figure 11 is a section on the line B-B of Figure 10, similar to Figure 6,
Figure 12 is a diagrammatic elevation of another embodiment in which the inlet and exhuast valve stem axes are equally and oppositely inclined to the cylinder axis but the junction line at the apex of the combustion chamber roof is offset from the cylinder axis, and
Figure 13 is a view similar to Figure 12 of another asymmetrical embodiment in which the valve stem axes are unequally inclined to the cylinder axis.

[0016] Figures 1 to 7 show the upper part of the bore of the, or one, cylinder 10 and the cylinder head 11 of a single-cylinder or of a multi-cylinder spark-ignited gasolene engine of piston type, the respective piston being shown in part at 12. Figure 5 shows the single-cylinder version, but in the case of a multi-cylinder head Figure 5 would be amended simply by the removal of the closure metal at the top and bottom of the Figure as drawn, and the extension of the left and right-hand faces with appropriate ports to suit the number of cylinders involved. Similar remarks apply to Figures 8 and 10.

[0017] The axis 13 of the cylinder 10 is shown as vertical in Figures 1 to 7 with the lower face 14 of the cylinder head 11 horizontal. The piston crown 15 projects upwardly, having a known "pent roof" form, its upper surface being defined by two plane faces 16 and 17 which intersect in a horizontal apex line 18 passing diametrally through the axis of the piston at the top of the crown. The two plane surfaces 16 and 17 are downwardly-inclined from the apex line 18 at equal and opposite acute angles, subtending between themselves an included angle less than 180[deg.]. The side surfaces of the piston crown which intersect the oppositely-inclined plane faces on either side of the piston are part-cylindrical.

[0018] The underside of the cylinder head 11 is formed, for the or each cylinder 10, with a circular-profiled re-entrant recess 20, coaxial with the cylinder bore, which recess constitutes a main part of the combustion chamber for that cylinder at top-dead-centre, and whose interior has a shape which generally matches that of the piston crown so that the latter can enter the combustion chamber into close proximity with most of its roof as the piston moves to its top=dead-centre position. Thus the roof of the combustion chamber is also formed by two downwardly-inclined plane faces 21 and 22 which extend parallel to the respective faces 16 and 17 and intersect at an apex line 23 which is parallel to and just above the apex line 18. At top-dead-centre the plane faces 16 and 17 of the piston crown lie very close to the opposed plane faces 21 and 22 of the cylinder head combustion chamber.

[0019] The cylinder head has poppet-type inlet and exhaust valves 25 and 28, and the axes of the valve stems 25A and 26A are perpendicular to the respective plane faces 21 and 22. The cylinder head has twin inlet valves 25 per cylinder as shown in Figure 3, whose valve seatings surround circular apertures formed side by side in the inclined face 21 of the combustion chamber recess 20. The two ducts 26 of the inlet valve ports 27 are cast in the cylinder head as,shown in Figures 1 and 5. A single exhaust valve 28 is used per cylinder, and its exhaust port 29 opens into a recess 30 formed in the cylinder head 11 with its bottom opening through the other plane face 22 of the combustion chamber, the profile of its intersection with that face being generally ovate as shown at 31 in Figure 2. The recess 30 has a plane roof 32 which lies parallel to the face 22, and has a circumferential side wall 33 generated by straight lines perpendicular to the roof 32 and face 22. The major axis of the elongate recess 30 extends generally parallel to the apex line 23, as shown in Figure 2. The exhaust valve port 29 opens into the larger end of the roof of the ovate recess 30, as shown in Figure 2, and its exhaust duct 35 formed in the cylinder head casting extends away from the port29 as shown in Figures 1 and 5. A tapped bore 36 in the cylinder head for a spark plug 37 opens into the smaller end of the roof of the ovate recess 30, as shown in Figures 2 and 6, so that the electrodes of the spark plug 37 lie approximately at the level of the roof 32.

[0020] It will be appreciated that, as indicated by Figure 7, the cylinder head recess 20 and the crown of the piston 12 are planar in sections at right angles to Figure 1, so defining the plane inclined faces 21, 22 and 16, 17. The twin inlet valves 25, which are used to give good breathing at high speeds, are inclined to the cylinder axis at the same angle as but oppositely to the single exhaust valve 28, whose port 29 has a larger diameter than the twin inlet ports 27. The axes of the inlet and exhaust valves are perpendicular to the respective faces 21 and 22, and are inclined at 30[deg.] or less to the cylinder axis. The valves are operated by means of twin overhead camshafts, not shown. However, with some relocation of the spark plug bore 36, a single central overhead camshaft with fingers or rockers to transmit the cam lifts to the valves could alternatively be employed. The inlet ducts 26 of the two inlet valves are separate but diverge from the two closely adjacent circular-section entry holes at the cylinder head entry face, so that a single elongate inlet manifold branch can feed these two entries for the or each cylinder. It is possible to have a single entry at the cylinder head entry face, with the bifurcation of the two inlet ports taking place within the head structure if required.

[0021] If the height of the recess 30 in the plane face 22 of the combustion chamber is h, measured perpendicularly from the face 22 to the roof 32, and being of necessity at least equal to the exhaust valve lift plus the valve head disc thickness, and if the internal length of the recess 30 along its longer axis is L, then the ratio L:h should be within the range 2.7:1 to 4.3:1.

[0022] It is important that the detailed motion of the gaseous air/fuel mixture induced into the combustion chamber through the inlet valves, and there compressed by the piston, should at around top-dead-centre be suitably controlled to facilitate orderly and rapid combustion when a spark occurs at the points of the spark plug, whose energisation is provided for and timed in any orthodox manner. An important factor for obtaining the required charge movement and turbulence is the so-called "squish" previously referred to, namely the gas motion which occurs transversely at one edge of the main combustion space as air or air/fuel mixture is compressed within the cylinder between the approaching faces of the piston crown and the combustion chamber in the head, due to the very small vertical clearance between the piston crown and the roof of the combustion chamber which with good production tolerances can be as low as 1% of the piston stroke. In the illustrated construction, the gas is displaced, as the piston rises, from the left hand side of the combustion space as shown in Figure 1, i.e. from between the opposed faces 16 and 21 and under the inlet valves, and will tend to be projected towards the right in that Figure, into the clearance volume provided under the exhaust valve within the recess 30. The precise orientation of the "squish" gas projection into the recess 30 depends on the geometry, and should be optimised by trial and error to provide good combustion over a wide range of mixture strengths.

[0023] If it is required to alter the direction at which "squish" gas is projected from between the faces 16 and 21 into the combustion chamber 30, to a smaller angle with respect to the faces 17 and 22, various expedients are possible.

[0024] One such expedient is shown in Figures 8 and 9, in which an.integral lip 50 is formed along the edge of the recess 30 nearest to the apex line 23, the lip being located next to the inclined face 22 and projecting laterally into the recess 30. In Figure 8 the ovate broken line 30 shows the outline of the bottom of the recess 30 whilst the inner broken line 50 shows the position of the edge of the lip 50. The lip 50 will have the effect of deflecting the "squish" gas flow to a shallower angle. The lip must have a reasonably substantial section to avoid overheating with the risk of causing pre-ignition.

[0025] Another such expedient is shown in Figures 10 and 11, in which a groove is cast or machined in the - inclined face 22 next to the apex line 23 and between it and the adjacent edge of the recess 30. In Figure 10 the ovate broken line 20 shows the outline of the bottom of the recess 30 whilst the broken loop line 51 shows the position of the groove 51, which as indicated curves around to follow the profile of the recess at each end. The groove 51 will intercept the flow of "squish" gas from between the faces 16 and 21, since the groove is virtually at the apex 23 of the cylinder head recess 20 and will cause the "squish" gas flow to be deflected downwardly.

[0026] In all the embodiments so far described and illustrated the piston 12 and cylinder head combustion chamber 20 have been of the "pent-roof" form with two equally inclined flat surfaces on each, so that the apex of the piston crown and that of the circular recess 20 are both straight lines passing through the central axis 13 of the cylinder and the central axis of the piston which is treated as coincidental with the cylinder axis 13. However this is not essential. Figure 12 shows diagrammatically a possible embodiment in which the piston crown and the cylinder head are asymmetrical, their horizontal apex lines at the intersections of their oppositely inclined faces 16A, 17A and 21A, 22A respectively being offset from the cylinder axis 13, but the axes of the inlet valve stems 25A and the axis of the exhaust valve stem 28A are equally inclined at for example 30<0> to the cylinder axis 13. Thus the faces 16A and 21A are oppositely inclined to the axis 13 at the same angle, 60[deg.], as the faces 17A and 22A, but meet the side wall of the piston at different levels.

[0027] In Figure 13 there is shown diagrammatically another layout in which the axes of the inlet and exhaust valve stems 25B, 28B are respectively inclined at different angles [alpha] and [beta] to the cylinder axis 13. The inclined faces 21B and 22B are still perpendicular to the respective associated valve stem axes, but they slant from a common level at the cylinder head face 14, and intersect asymmetrically at an offset apex line 23B. The faces 16B and 17B of the piston crown are parallel to the faces 21B and 22B respectively, and are similarly arranged, i.e. asymmetrically at different inclinations.

[0028] Moreover it is not essential that the two oppositely inclined plane faces of the pentroof piston crown, and those of the combustion chamber be flat, and it is to be understood that the invention may be utilised in conjunction with variants of this basic pent-roof shape. For example each pair of oppositely-inclined faces might be nonplanar, e.g. shallow convex or concave surfaces each having a large radius of curvature about one axis or two intersecting axes, e.g. part-cylindrical or even part-spherical, these curved surfaces either intersecting at the apex or ridge of the pent-roof, or even merging smoothly into one another at the apex so as to form a shallow part-cylindrical or part-spherical domed roof for the piston and similarly for the combustion chamber. All these and other variations of the basic pentroof piston and combustion chamber shape are to be regarded as being within the scope of the invention and of the following claims.