Such a system has previously been described in the Applicant's Patent Application No. PCT/GB96/01602 and will hereinafter be referred to as an intake system of the type defined initially.

Description of the prior art The system described in PCT/GB96/01602 was designed to achieve the same objectives as the present invention but in its practical implementation, it was found not to achieve the desired results. In practice, the degree of charge stratification in many cases was significantly less than expected.

Object of the invention The present invention is concerned with an improvement of the intake system of the type defined initially to achieve a consistent increase in the degree of charge stratification.

Summary of the invention According to the present invention, an intake system of the type defined initially is characterised in that the total volume of the first and second intake passages downstream of the points of minimum through-flow cross- section in the two passages is less than half the swept volume of each engine cylinder.

The invention is predicated on the realisation that the volume of the two passages downstream of the minimum cross- sections acts as a buffer that distorts the expected flow rates. Thus even when the intake valve is closed, the lower pressure in the downstream sections of the passages results in continuing flow past the minimum cross-sections and this air is stored between cycles downstream of the minimum cross-sections. At the beginning of the intake period of the following cycle, this stored air has to be evacuated before the desired throttling effect at the minimum cross- sections of the passages becomes effective. The larger the buffer, the less the throttling effect and the greater the deviation of the actual flow velocities and volumes through the two passages from those calculated for optimum stratification. The volume of the first intake passage downstream of the perforated flow restriction cannot be eliminated completely because a certain recovery length is required to allow diffusion of the high velocity air jets induced at the perforations to a lower uniform velocity within the passage before the flow enters the combustion

chamber. The present invention therefore places a limit on the volume of this section of the first intake passage as well as the volume of the second intake passage to minimise the distortion effect described above.

This ensures that the initial evacuation process is sufficiently short to allow a large contribution from the supply flows at high velocities through the perforated flow restriction and the second intake passage directly during the intake period which produces in the engine cylinder the necessary flow conditions of layered swirling streams of different velocities for optimum charge stratification.

In the preferred embodiment of the invention, the perforated flow restriction is located at the entry end of the first intake passage and a nozzle is located at the exit end of the second intake passage.

The nozzle is advantageously aimed along a line passing through the aperture between the intake valve and its seat when the valve is open, such that a high velocity jet from the second intake passage penetrates directly into the engine cylinder. To promote swirl about the axis of the cylinder, the nozzle may be directed tangentially with the cylinder bore of the combustion chamber.

Conveniently, the volume ratio between the first and second streams is a fixed value between 3: 1 and 1: 3, and the velocity ratio between the first and second streams entering the combustion chamber is a fixed value between 2: 3 and 1: 6.

In order to produce a stratified charge within the engine cylinder, fuel should be introduced only into the low velocity stream from the first intake passage downstream of the perforated flow restriction.

The two separate passages leading to the intake port may be produced by a partitioned intake port, or by casting or placing a tube constituting the second intake passage into the intake port.

Conveniently, the perforated flow restriction comprises a perforated plate movably mounted between a closed position for stratified charge operation in which it is calibrated to reduce the volume flow of the first stream in relation with the volume flow of the second stream, and an open position for homogeneous charge operation in which the volume flow of the first stream is substantially unobstructed.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic section through an intake port taken in a plane parallel to the intake valve stem, Figure 2 is a schematic section through an intake system taken in a plane perpendicular to the intake valve stem, and Figure 3 is a view along the first and second intake passages looking towards the perforated flow restriction.

Detailed description of the preferred embodiments The drawings show an intake port 14 having an intake valve 10 with a stem 11 and a valve seat 12. The intake valve 10 is shown in Figure 1 in its open position in which the intake charge flows over the valve into the combustion chamber as represented by the arrows. A tube 20 is located within the intake port 14 and is secured to a branch 16 of the intake manifold 17 by means of the pivot shaft 34 of a two-position butterfly valve 30. The valve 30 has a butterfly plate with perforations that are designated 32 in Figure 3. The free end of the tube 20 is urged against a wall of the intake port 14 by a spring 24.

The effect of the tube 20 is to divide the intake port 14 into two passages in order to allow separate streams to enter the combustion chamber. The end of the tube 20 is formed as a nozzle 22 to direct a fast air stream tangentially into the combustion chamber to promote swirl, this stream being represented by a long arrow with a hollow head. The other stream represented by the solid arrows is a lower velocity stream that tends to remain in the centre of the combustion chamber and it is into this stream that fuel is injected in order to create a stratified charge. It is important that the two streams should be maintained separate for as long as possible and for this to occur, the intake charge should rotate in the combustion chamber as a solid body. This implies that the two streams must be in a given volume ratio and velocity ratio when entering the combustion chamber.

It should be mentioned at this juncture that alternative constructions may be adopted to direct separate streams through the open intake valve 10 into the combustion chamber, for example, a tube may be cast into the intake port 14 or a vertical partition wall may be cast into the port.

While the first and second intake passages may be connected to separate manifolds as has been described in the earlier mentioned Patent Application PCT/GB96/01602, in the present embodiment both passages are connected to a branch 16 of the same intake manifold 17 having a plenum and a main throttle 18 common to all the branches.

The operation of the intake system to achieve a stratified charge requires the perforated flow restriction 30 to be in the closed position. In this position the dimensions of the perforations are selected by precalibration to achieve the desired relative volume flows in the first and second intake passages. Because the pressure drops at the points of minimum cross-section in both the intake passages are the same, the induced

velocities of the two streams will be the same at these points. In order to achieve a lower velocity stream in the first intake passage, the high velocity jets passing through the perforations 32 must be diffused to a lower uniform velocity before they reach the intake valve 10. It is for this reason that the butterfly valve 30 is positioned at some distance from the intake valve 10. However, the volume between the valve 30 and the nozzle 22, on the one hand, and the intake valve 10, on the other, acts as a buffer volume that fills at the end each intake period and empties at the beginning of the next intake period, distorting the desired velocity ratio in the process. For this reason, in accordance with the present invention, this total volume is maintained as small as possible, consistent with the diffusion of the high velocity jets in the first intake passage, it having been found empirically that this volume should not exceed half of the swept volume of each engine cylinder and more preferably one third of the swept volume.

The initial stratification of the flows from the first and second intake passages on entering the engine cylinder during the intake period is determined by the volume ratio between the two flows. The final stratification of the two flows within the cylinder that may exist at the time of ignition is determined by the velocity ratio and the relative mixing between the two flows during the compression period. For swirling charge motion in the engine cylinder, the flow areas of the perforated plate and the nozzle may preferably be both 25% of the flow area of the open intake valve. This corresponds to a fixed volume ratio of 1: 1 between the two flows and, since the geometric area ratio of the respective flows at the open intake valve is approximately 3: 1, the velocity ratio of the two flows at the open intake valve is also fixed at 1: 3.

In engines where each cylinder has two intake valves supplied jointly by first and second intake passages, the two intake valves may have different opening events and the instantaneous volume ratio between the flows from the first

and second passages may vary at different times during parts of the intake period when the lifts of the intake valves form the effective flow restrictions. Nevertheless, the overall volume ratio between the flows from the first and second intake passages over the complete intake period is still constant for each engine cycle.