STATE OF THE ART In today's combustion engines it is generally desirable to increase the output power of the engine. In order to achieve this, there are many different solutions. One method is to decrease the flow resistance in an intake duct, resulting in an increased volumetric filling efficiency. This may however entail a deterioration of the combustion quality. This is a/o caused by the difficulty of creating enough micro-turbulence during the combustion phase at low loads and rotational speeds. At too low turbulence levels, an unacceptably low combustion velocity is obtained, and also excessive cycle-to-cycle variations of the output performance. One method of increasing the combustion quality is to raise the level of the so-called"tumble effect", which is generated in the combustion chamber during the intake or suction stroke.

The tumble effect is defined as a rotating motion of the air volume moving transversely in relation to the cylinder, between the top of a piston and the upper part of the cylinder chamber. In order to increase the tumble effect, the intake duct is normally designed in a special manner, to be described in more detail below, which normally entails an increased flow resistance. This compromise is well known and documented, see for example SAE Technical Paper Series 910477"Effect of Intake Port Flow Pattern on the In-Cylinder Tumbling Air Flow in Multi-Valve SI Engines", by S. Omori et al., February- March 1991.

Fig. 1 illustrates an intake duct 20a for a high flow (denoted by dashed lines) and another one for high tumble (denoted by full lines), which are two known systems, the differences of which will be explained below.

Each one of these systems could be described generally as a combustion system for engines having at least one cylinder 10. The combustion system of the respective system comprises at least one exhaust duct 20b, one intake duct 20a provided with a generally cylindrical wall 22,22a and a cylinder chamber 30 (also called combustion chamber) delimited by a top 100, (also called cylinder head). A centreline 2 la, 21b extends mainly along the centre of the intake duct 20a. Further, the cylinder is provided with a cylinder head (not

shown) having fuel injectors functioning to inject fuel into the intake duct 20a or directly into the cylinder chamber 30. A piston 40, provided with a head 60, also called a crown, performs a reciprocating motion inside the cylinder 10.

Each cylinder is provided with at least one valve 70a for intake and one valve for exhaust 70b. The exhaust valve 70b will not be referred to in the following, and therefore the denomination"valve"will from now on refer to the intake valve 70a. The intake valve 70a is arranged in a guide 72, guiding the valve 70a, where the guide has a longitudinal axle 110 which is angled relative to the top 100 of the cylinder chamber 30. This longitudinal axle touches the centreline 2 la, 2 lb of the intake duct 20a and defines a radius 23a, 23b therebetween, i. e. the radius is tangent to both the longitudinal axle and the centreline. The intake duct 20a is connected to the cylinder chamber 30 by means of a mouth 80 and the valve 70a is arranged to open and close the connection between the intake duct 20a and the cylinder chamber 30. The upper portion of the centreline 2la, 21b, i. e. the portion of the centreline 2 la, 21b located upstream of the longitudinal axle 110 and the radius 23a, 23b, and the longitudinal axle 110, together define a bending angle 25a, 25b. The centreline 21a, b will touch the radius 23a, 23b, provided that the starting point for the radius 23a, b is the same at the end towards the mouth 80.

In the intake duct 20a (dashed contour in Fig. 1) for high flow, the centreline 21a, starting from the mouth 80 of the duct in the cylinder chamber 30, has a preferably maximised radius 23a that touches the longitudinal axle 110 of the valve guide 72. This radius 23a is principally the same as the corresponding curve 24 of the wall 22 of the intake duct, starting from the mouth 80. In order to obtain a maximised radius 23a, the bending angle 25a will be minimised with regard to available upwards space, in a manner allowing the duct to run free of the existing engine components. Existing components are defined as e. g. valve guides, valves, camshaft and seals. The curve of this type of intake duct will be intermittent. By using this type of intermittently curved intake duct, the flow capacity will increase, partly because the duct can adopt a lower angle of attack 27a, b, between the intake duct and the cylinder centreline 130. Another advantage, also contributing to increased flow, is that the air flow can be evenly distributed along the circumference of the valve. In a tumbling duct, the main part of the air flow is directed towards the upper part of the valve circumference. In a so called "filling duct", i. e. a duct for a high flow, the air flow is distributed as evenly as

possible and will use the flow area maximally. However, the tumbling effect will be reduced when this type of duct is used. Furthermore it may be unfavourable to use a small valve angle, normally about 15°-20°, as this can lead to a relatively limited valve size and a high cylinder head, which would require substantial modifications of existing production lines.

In the other case, where the intake duct 20a is highly tumbling, the centreline 21b, starting from the mouth 80 of the duct in the cylinder chamber 30, has a preferably minimised radius 23b that touches the longitudinal axle 110 of the valve guide 72. This radius 23b creates a curve 24a of the intake duct wall 22a, starting from the mouth 80, which then bends shaiply at 24b towards the valve guide 72. In order to obtain a maximal guidance of the flow towards the top side of the valve, the bending angle 25b will be maximised with regard to available downwards space. By using this type of intermittently curved intake duct, the tumbling effect will increase, due to the shape of the duct. However, the flow capacity will be decreased when using such a duct.

Fig. 2 displays a schematic diagram, showing that the relationship between flow and tumbling level is inversely proportional, i. e. linear. The tumbling level may e. g. be defined as the kinetic energy of the tumbling air mass. The flow in the diagram is the flow rate of air. If a comparison is made between the two designs shown in Fig. 1, one will find that if the radius is maximised according to 23 a and the angle 25a is minimised, this will result in a duct having a lower tumbling level and a higher flow capacity. This comparison corresponds to point 10 in Fig. 2. If it were desirable to increase the flow and decrease the tumbling level, this design would in this case be preferable. If another comparison is made between the two designs shown in Fig. 1, where the radius is minimised according to 23b and the angle 25b is maximised, this will result in a duct having a higher tumbling level and a lower flow capacity.

This comparison corresponds to point 20a in Fig. 2.

Further, the crown 60 of the piston 40 may vary somewhat, depending on the application. The simplest form of the crown 60 of a piston 40 is of course a completely flat surface. In order to increase the tumbling effect, the crown 60 of the piston 40 may for example be provided with a bowl-shaped cavity (not shown). This cavity may for example be provided in the direction of the tumbling motion, to support the creation of the tumbling motion. Furthermore, a ridge on the piston may be placed with an offset (not

shown), arranged at an exhaust duct with the intention of gaining a more even flow.

One example of an intake port, intended for achieving a stratified tumbling flow, is described in US-A-5,295,464. Stratified means that the fuel is concentrated for example at the spark plug, so as to allow the use of a lean mixture. Said document also describes how a piston may be provided with a bowl-shaped cavity positioned with an offset.

Another example of how the tumbling effect can be increased is described in US-A-5,115,774. This document describes an intake port provided with a masking adjacent to the mouth of the inlet port. Due to this masking, the tumbling motion inside the cylinder chamber will probably increase. Further, a piston having a bowl-shaped cavity is described.

When using an inlet duct in combination with a piston equipped with a cavity, where both designs contribute to an enhanced tumbling motion, the flow capacity will however decrease, which in turn will cause the engine power output to decrease. The reason for the decrease in flow capacity is that the relationship between the flow and the tumbling motion generated by the inlet duct is linear, i. e. the flow is inversely proportional to the tumbling effect. Consequently, with an increased tumbling motion, a corresponding decrease in flow will be obtained.

As the combustion quality is, to a substantial degree, dependent of the tumbling gas motion that is achieved in the intake duct, and to the breakdown of this motion, it is desirable to try to find a way of increasing the flow capacity at a retained turbulence intensity during combustion in the engine. Moreover, it is desirable to increase the engine power output and at the same time achieve a low fuel consumption with minimal pumping losses, without deterioration of the combustion quality, e. g. by controlling the tumbling motion in such a way as to influence the flow minimally. Pumping losses are defined as the work necessary to introduce air into a cylinder. High pumping losses will entail a lower performance on the engine shaft.

SUMMARY OF THE INVENTION The object of the present invention is to provide a device which will increase the flow capacity and thus the power output, whilst retaining the combustion quality at low loads and rotational speeds.

Another object is to decrease the fuel consumption and lower the pumping losses.

A further object is to be able to provide larger valve angles, which geometrically will allow larger valves, a lower cylinder head and also the possibility of using existing production lines for the manufacture of engines having one or more cylinders.

Still another object is to simplify the manufacturing process for producing the combustion engine, making it both less expensive and more simple than is otherwise common when producing a new combustion engine.

According to the present invention, the above objects are met by providing a combustion system in accordance with claim 1, which describes a combination of two entirely different solutions, a low-tumbling, flow- enhancing intake duct and a piston equipped with a cavity. Through this combination, an advantageous system is obtained, capable of increasing the flow capacity at maintained turbulence intensity during combustion in the engine, decreasing the fuel consumption, achieving a high combustion quality and increasing the power output of a combustion engine.

Furthermore, existing tools and machiner can be used for production, as the valve angle can be maintained as it is, through using an intake duct that is a known design. In this way, the manufacturing process for a cylinder head can also be simplified and thus be made at a low cost.

Preferred embodiments of the device according to the invention are defined in detail in the respective dependent claims.

DESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail, by way of the following description of a preferred embodiment, intended as an example only, with reference to the enclosed drawings, of which: Fig. 1 shows a comparison between two known systems, one with an intake duct for high air flow and another with an intake duct having a tumbling effect; Fig. 2 shows a diagram of the relationship between tumble and flow; Fig. 3 shows a preferred embodiment of the present invention; Fig. 4 shows a cross-section of the piston of Fig. 3; and Fig. 5 shows an enlarged view of the piston of Fig. 3, from above.

PREFERRED EMBODIMENTS

With reference to Fig. 3, a combustion system is shown for engines having at least one cylinder 10. The system includes at least one exhaust duct (not shown), one intake duct 20a having a generally cylindrical wall 22, a cylinder chamber 30 (also called combustion chamber), delimited by a top 100, also called a cylinder head. A centreline 21 extends mainly along the centre of the intake duct 20a. Further, the intake duct is provided with a fuel injector functioning to inject fuel into the intake duct 20a or directly into the cylinder chamber 30. A piston 40, provided with a head 60, also called a crown, perfores a reciprocating motion inside the cylinder 10. Each cylinder is provided with a valve 70 for intake and a valve for exhaust (not shown). The exhaust valve will not be referred to in the following, and therefore the denomination"valve"will from now on refer to the intake valve 70. The intake valve 70 is arranged in a guide 72, guiding the valve 70, where the guide has a longitudinal axle 110 which is angled relative to the top 100 of the cylinder chamber 30. This longitudinal axle touches the centreline 21 of the intake duct 20a. The intake duct 20a is connected to the cylinder chamber 30 via a mouth 80 and the valve 70 is arranged to open and close the connection between the intake duct 20a and the cylinder chamber 30. The upper portion of the centreline 21, i. e. the portion of the centreline 21 located upstream of the longitudinal axle 110 and the point where the radius 23 touches the line 21, and the longitudinal axle 110, together define a bending angle 25. Both the bending angle 25 and the radius 23 will be dependent of each other, as the radius will increase if the bending angle decreases, and vice versa. The straight line 21 passes into a centre curve at the point where the centreline 21 touches the radius 23. According to the invention, the centreline 21 starting from the mouth 80 of the inlet duct in the cylinder chamber 30, has a preferably maximised radius 23, touching the longitudinal axle 110 of the valve guide. In order to obtain a maximum radius 23, the bending angle 25 created between the valve guide 72 and the intake duct 20a upstream of the radius 23 will be minimised regarding available upwards space, in a manner allowing the duct to run free of the existing engine components. Existing components are defined as e. g. valve guides, valves, camshaft and seals. Consequently, this means that the mouth 80 of the intake duct has an intermittent curve 24, coinciding with the valve guide 72, i. e. with the longitudinal axle 110 of the guide, for limiting the tumbling effect inside the cylinder chamber 30, and where the wall runs along the curve 24, from the guide 72 guiding the valve

70, in a direction generally coinciding with an imaginary line extending from the mouth 80 of the intake duct. By using this type of intermittently curved intake duct, the flow capacity will increase, due to the shape of the duct.

However, the tumbling effect will be reduced when this type of duct is used.

To increase the tumbling intensity, energy is required as a vortex is created, and thereby the flow capacity will be reduced in those cases where it is desirable to create a higher tumble inside the cylinder chamber.

Further according to the invention, the tumbling effect is increased inside the cylinder chamber 30, without affecting the flow, by the crown 60 of the piston 40 being provided with a preferably cylindrical cavity 50, as shown in Figs 4 and 5, which is shaped so as to enhance the generation of tumbling in the cylinder chamber 30 during the intake stroke and to contribute to the breakdown of the tumbling into turbulence during the compression stroke. Of course this cavity may take other shapes than cylindrical, such as bowl-shaped or similar. Said cavity may be located on one half of the piston, see Fig. 5, running across the crown 60 of the piston 40.

From Figs. 3,4 and 5 it can further be seen that the piston crown 60 may be provided with a ridge 62 adjacent to the cavity 50. The ridge 62 may run with constant level across the piston crown 60. Furthermore, the piston may be provided with one or more valve pockets 64 to avoid collisions with partly open valves. This type of piston 40 will thus enhance the generation of tumble in the combustion chamber during an intake stroke, at the same time as the piston shape will contribute to a breakdown of the tumbling into turbulence during a compression stroke. Consequently, this means that just as high a tumble intensity, and thereby just as advantageous combustion properties, can be achieved, as when using an intake port with a high tumbling level, at the same time as the flow capacity will be higher than normally allowed by such a port. The port according to the invention can provide a flow capacity increase of at least 10 % compared to what is normally allowed by a port. The more efficient tumbling breakdown will provide decreased rest tumbling and thus lower knock sensitivity, due to less offset of the flame centre (not shown) to- wards that side where the exhausts are discharged. An initially low tumble will contribute to lower cooling and pumping losses. Together, this will lead to a lower specific fuel consumption.

The centreline 110 of the intake duct approaches the top of the cylinder chamber at an angle 27 that may vary from 0° to 180°, preferably

from 20° to 35° and most preferably be 30°, relative to the cylinder chamber centreline 130. The angle 27 should be as small as possible, with regard to available space. The curve to the intake duct may vary between 0-180°. For a modern car engine this curve is usually between 30-90° and most preferably 50°.

A further embodiment form is that the intake duct may be a falling duct, falling in towards the top of the cylinder chamber between the exhaust and intake valves. The same geometrical construction with an intermittent curve is possible also in this case.

Even though the demonstrated embodiments of the present invention have been described in detail with reference to the accompanying drawings, it should be understood that the invention will not be limited to these specific embodiments and that various changes or modifications can be made by someone skilled in the art, without deviating from the scope defined by the following patent claims. For example, one variation would be to use more than one sparkplug, for each cylinder, as well as more than one intake and/or exhaust valve.