Inlet passages can deliver a charge of fuel and/or air into a combustion chamber. A charge-or fuel and air can be introduced into a combustion, chamber of an engine via an inlet passage in a number of ways. In most spark ignition four-stroke engines, an inlet valve opened at the start of an induction stroke (or thereabouts) allows a fuel and air charge to be drawn through an inlet passage and into the combustion chamber by the action of a piston during the induction stroke.

A number of factors are important in ensuring that the combustion process in an engine generates maximum power, is efficient, and limits emissions.

The inlet passage of an engine must be designed to deliver air to a cylinder efficiently whilst generating an in-cylinder flow field which provides suitable levels of in-cylinder turbulence. This turbulence increases flame speed and hence engine thermal efficiency.

It is well known to choose the configuration of an inlet passage in order to achieve desired in- cylinder turbulence. For instance, an inlet passage can be configured to induce the flow of a charge of

fuel and air to form a vortex in the cylinder to improve mixing and atomisation/vaporisation of the fuel in the charge admitted into a combustion chamber.

Two main types of configuration have evolved for producing sufficient mixing and atomisation. Both types make use of an inlet passage whose longitudinal axis is non-linear. The first type, most suited to two valve per cylinder engines, is configured to induce what is commonly known as"swirl"in the atomised charge. The inlet passage is shaped in such a way that the flow of charge on entry to the combustion chamber follows a toroidal path. The direction of flow is tangential to, and bounded by, the walls of the combustion chamber and is generally perpendicular to the longitudinal axis of the combustion chamber. The second type, most suited to four valve per cylinder engines, is configured to induce what is commonly known as a"tumble"in the incoming charge. Tumble occurs when the flow of charge on entry to the combustion chamber forms a vortex with an axis running generally along a transverse plane of the cylinder.

The introduction of swirl or tumble in an inlet charge is especially important when an engine of either configuration is operated at low speed. The rate of induction of the charge into the cylinder may not be sufficient at low speeds to ensure adequate in- cylinder turbulence and in any case more time is available for tumble or swirl to take effect at low speeds. However, the shaping of an inlet passage to induce swirl or tumble can have detrimental effects at high engine speeds. In order to induce swirl or tumble, an inlet passage must be curved. As engine speed increases, the increasing flow velocity of the

charge forces the charge to the outside curved wall of the inlet passage. This can cause the flow to separate from the port floor. This reduces the effective area of the inlet passage and thus hampers delivery of air or fuel and air mixture to a combustion chamber. Thus there is a trade-off between, for instance, configuring an inlet passage to provide high levels of tumble (and therefore a desirable in-cylinder turbulent flow structure) and high volumetric efficiency (the rate of flow of air or fuel/air mixture through an inlet port) which directly affects power of the engine.

At very high engine speeds, the increased velocity of the flow generates high levels of kinetic energy in the charge. This can result in combustion harshness'which can exacerbate a volumetric efficiency problem.

Accordingly, there is a requirement for an inlet port for an internal combustion engine which has the advantages attributable to the effects of swirl or tumble at low engine speeds, but which does not suffer the disadvantages experienced with increasing engine speed.

The present invention provides in a first aspect an internal combustion engine comprising: a combustion chamber ; an inlet passage for delivering air or a mixture of fuel and air to the combustion chamber ; and valve means for controlling flow of the air or the fuel and air mixture into the combustion chamber ; wherein: the inlet passage is at least in part curved in

order to induce swirl or tumble in the air or the fuel and air mixture introduced into the combustion chamber ; characterised in that: flow control means is provided in the inlet passage, which flow control means during flow of the air or the fuel and air mixture to the combustion chamber creates a pressure differential across the flow of the air or the fuel and air mixture in the inlet passage, which pressure differential acts to at least partially counteract those forces acting on the flow of the air or the fuel and air mixture which induce boundary layer separation along an inner boundary wall portion of the curved part of the inlet passage.

In a second aspect, the present invention provides a method of admitting a charge of air or fuel and air mixture into a combustion chamber of an internal combustion engine through an inlet passage which is at least in part curved, the method comprising the step of during flow of the charge into the cylinder, inducing a pressure differential across the flow which at least partially counteracts those forces occasioned in the flow which act to cause boundary layer separation along whatever wall of the inlet passage has the smallest radius of curvature.

The inlet passage will typically be circular in cross-section. Where references are made to curved parts of an inlet passage they should be construed as referring in the case of a circular cross-section inlet passage to a curving of the principal axis of the inlet passage. References to inner wall portions are references to those parts of the wall of the inlet

passage which when viewed in cross-section taken along or parallel to the principal axis of the inlet passage have a radius of curvature smaller than the principal axis. References to outer wall portions are references to those parts of the inlet passage which when viewed in cross-section taken along or parallel to the principal axis of the inlet passage have a radius of curvature larger than that of the principal axis. The flow control means can be incorporated in the inlet passage in the curved portion of the inlet passage or on a straight portion following the curved portion, as convenient.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a front sectional view from above of an inlet passage according to the present invention ; Figure 2 is a perspective sectional view from above of the inlet passage of Figure 1 ; Figure 3 is a rear sectional view from above of the inlet passage of Figures 1 and 2 ; Figure 4 is a rear perspective sectional view from above of the inlet passage of Figures 1,2 and 3 ; Figure, 5 is a view of the inlet passage of Figure 4 illustrating the flow of charge through the inlet port; Figure 6 is a schematic representation of flow through a single valve inlet passage according to the present invention ; Figure 7 is a schematic representation of an inlet passage of the prior art; Figure 8 is a schematic representation of the inlet passage of Figure 7 modified in accordance with the present invention;

Figure 9 is a second schematic view of the modified inlet passage of Figure 8; and Figures 10a and lOb show graphs illustrating the comparative performance of an inlet valve according to the present invention against that of an inlet valve known in the art.

Referring now to Figures 1 to 4, there can be seen a lower section 10 of an inlet manifold according to the present invention. The inlet manifold section 10 is for a four-valve per cylinder spark ignition gasoline internal combustion engine and has two inlet passages 20 and two exhaust passages (not shown) associated with each cylinder.

The inlet passages 20 share a common inlet 30 for receiving a charge of fuel and/or air and each has a port 40 adapted for receiving a valve which is an inlet port of the combustion chamber and through which the charge passes into the combustion chamber. A boundary wall 22 separates the two passages 20.

Each inlet passage 20 is generally circular in cross-section. Each passage 20 is also provided with two raised portions 27 and 28 in the lower part thereof which are raised compared to the surrounding wall.

Raised portions 27 and 28 define between them a gulley 25,26. The raised portions 27 and 28 each have a chosen shape, the purpose of which will be explained later. The edges defining the gulley are sharply defined. The gulley itself may also be shaped.

In use, a charge of fuel and/or air is supplied at inlet 30. The charge may be provided by any conventional apparatus or method known in the art such as, for example, a carburettor or an air intake and fuel injector combination. The opening of an inlet valve at outlet 40 enables the charge to move under the action of a piston during the induction stroke of the engine. The charge is drawn through the inlet passage 20 so as to flow from inlet 30 along a path defined by the longitudinal axis and the boundary wall 22 of inlet passage 20 to outlet 40.

As the flow of charge is drawn through the inlet passage 20, the flow is influenced by the profiles of the shaped raised portions 27 and 28 and the presence of the gulley 26. The influence of the profiles of the shaped raised portions 27 and 28 on the flow of charge is illustrated in Figures 5 and 6. In Figure 5, it can be seen that the flow of charge over raised portions 27 and 28 and gulley 25 in the boundary wall 22 causes vortices 29 to be set up in the flow. The vortices 29 are generated by the local pressure differences set up in the flow in the same way as those which are set up on the tips of aircraft wings where there, is high speed flow over an aerofoil section. The vortices 29 entrain the flow of charge travelling in the direction of the longitudinal axis of the inlet port 20 and have the effect of reducing the boundary layer separation experienced at the lower inner surface of the gulley 26. The vortices increase in size and effect along the longitudinal axis of the inlet port 20 in the direction of flow, countering the boundary layer separation which also increases along the longitudinal axis. Also, as the velocity of the flow of charge increases, so does the effect of the

vortices 29, and accordingly the forces exerted on the flow by the vortices 29 counter those forces experienced by the fuel and air charge which act to cause boundary layer separation. The vortices 29 not only counteract these effects, but additionally assist in promoting beneficial in-cylinder turbulence. The vortices help to ensure that the flow of charge into a combustion chamber of the engine occurs around substantially the entire circumference of the annulus 51 generated by the open inlet valve 50, thus improving volumetric efficiency.

While the present invention maintains the beneficial effects of swirl or tumble at low speeds, the invention can reduce the decrease in flow efficiency (and thereby volumetric efficiency and engine power) attributable to the effects of flow separation at high engine speeds. A side effect can be a reduction in in-cylinder tumble or swirl at high speeds which would reduce combustion harshness.

A schematic illustration of the effects of the raised portions 27 and 28 and gulley 26 is given in Figures 7,8 and 9. Figure 7 shows a cross-section through a curved part of a standard inlet passage 101 designed to provide tumble in an inlet charge (without the benefit of the present invention). The flow of the fuel/air mixture at high velocity as illustrated is mainly in the upper part 101A of the inlet passage 101, i. e. towards the outer boundary wall portion of the inlet passage 101, and thus is over the top of the inlet valve 100 into the combustion chamber 110, there being boundary layer separation of the flow in the lower part 101B of the inlet passage 101, along the inner boundary wall portion 120 of the curved inlet

passage 101. The effective area of the inlet passage 101 is reduced by the boundary layer separation. This leads to a reduction of flow into the combustion chamber.

As can be seen in Figure 8, the presence of the profiled raised portions 27 and 28 (not shown in Figure 8) and the gulley 25, 26 in the inlet passage 20 gives rise to a pressure differential between fluid flowing over the raised portions 27 and 28 and fluid flowing in the gulley 25, 26. The pressure of fluid over portions 27 and 28 is high relative to the pressure of fluid in the gulley 25,26 and this causes the establishment of two vortices 29 (illustrated in Figure 9) which entrain fluid from above the raised portions 27 and 28 and cause the fluid to flow down into the gulley 25,26. This fluid flow sucks down fluid from the upper part 20A of the inlet passage (i. e. away from the outer boundary wall) to the lower part 20B of the inlet passage (i. e. towards the inner boundary wall portion 120 of the curved inlet passage 101) to increase the effective area of the inlet passage 20 as compared with a standard inlet passage 101 (see Figure 7). Even though the raised portions 27 and 28 do reduce the actual area of the inlet passage the increase in the area effective at high speeds outweighs the reduction in actual area of the inlet passage.

The applicant has discovered that the modified geometry of the inlet passage 20 according to the present invention increases the mean flow of charge through the port 20 by 5%, increasing power whilst maintaining low speed tumble to ensure good part-load performance and emissions. The modified passage 20

can also reduce tumble at high engine speeds to reduce combustion harshness. These data have been plotted on the graphs illustrated in Figures 10a and lOb along with the equivalent data obtained for a conventional inlet port well known in the art, to enable comparison.

The preferred embodiment has been described herein as having one gulley 25,26 defined by two raised portions 27,28 in the boundary wall 22 of the inlet port 20. The shape of each of the raised portions is used to generate vortices 29 to counteract the undesirable effects described and any number of designs of gulley 25 and raised portions 27,28 may be incorporated into the boundary wall 22 to counteract these effects. For instance, there may be a plurality of gulleys 25,26 disposed around the inner circumference of the inlet port 20, either at the same longitudinal position or so as to be spaced along the length of the inlet port 20. This may be particularly appropriate where the longitudinal axis is to have many turns or sharp rates of change of direction.

Whilst the inlet passage according to the present invention has been described with reference to a four- stroke spark ignition gasoline internal combustion engine having four valves per cylinder, it will be appreciated that the inlet port is not restricted to use in only this type of engine. The inlet passage is equally well suited for use in two valve per cylinder engines, two stroke engines, compression ignition engines and engines making use of dual fuel technology.