This invention relates to internal combustion engines, and in particular to four stroke, throttled, gasoline engines having four poppet valves per cylinder.

It is known that the respective angular timings of poppet inlet and exhaust valves in four stroke engines affect scavenging efficiency, and hence power developed. Valve timings and valve port flow areas affect power, especially if both inlet and exhaust valves are open at the same time, during a "valve overlap" phase. Further, they affect the exhaust emission composition, particularly with respect to unburnt hydrocarbons.

More recently it has been known to employ two inlet and two exhaust valves per cylinder in order to achieve good cylinder scavenging without recourse to a "valve overiap" phase, but high speed performance is inevitably compromised to some extent.

It is desired, in particular, that the composition of exhaust emissions has a low proportion of unburnt hydrocarbons when the engine is operating at low and medium speeds, and at low loads, and it comprises an object of the present invention to provide an engine of a novel construction whereby this requirement is obtained in an advantageous way.

According to the present invention a four stroke internal combustion engine has four poppet valves per cylinder, comprising first and second inlet valves, and first and second exhaust valves, leading individually to each of the inlet valves is a bifurcated part of the inlet port, characterised in that leading individually from each of the exhaust valves is a bifurcated part of the exhaust port, the first inlet valve and the first exhaust valve are actuated individually by cams having timing profiles suited to low and medium engine speeds, the second inlet valve and the second exhaust valve are actuated individually by cams having timing profiles suited to higher engine speeds, included in the passage of the bifurcated part of the inlet port leading to the second inlet valve is an inlet flow control valve, and included in the passage of the bifurcated part of the exhaust port leading from the second exhaust valve is an exhaust flow control valve, and the arrangement is such that the settings of the two flow control valves are changed in unison in response to variations of engine conditions, and

such that the two flow control valves are fully closed when the engine is operating at least at a particular lo or medium speed, and the two flow control valves are fully open when the engine is operating at least at a particular higher speed.

According to another aspect the present invention relates to a method of operating a four stroke internal combustion engine having four poppet valves per cylinder, comprising first and second inlet valves, and first and second exhaust valves, and the method is characterised by operating an engine having a bifurcated part of the inlet port leading individually to each of the inlet valves, and leading individually from each of the exhaust valves is a bifurcated part of the exhaust port, the first inlet valve and the first exhaust valve are actuated individually by cams having timing profiles suited to low and medium engine speeds, the second inlet valve and the second exhaust valve are actuated individually by cams having timing profiles suited to higher engine speeds, included in the passage of the bifurcated part of the inlet port leading to the second inlet valve is an inlet flow control valve, and included in the passage of the bifurcated part of the exhaust port leading from the second exhaust valve is an exhaust flow control valve, and in the method the settings or the two flow control are changed in unison in response to variations of engine conditions, and such that the two

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flow control valves are fully closed when the engine is operating at least at a particular low or medium speed, and the two flow control valves are fully open when the engine is operating at least at a particular higher speed.

The present invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 is a valve event diagram for an engine and suitable for low and medium operating speeds of the engine,

Figure 2 corresponds to Figure 1, but is for higher operating speeds of the engine,

Figure 3 shows a typical valve train mechanism, and the associated part of the cylinder head, associated with a cylinder of an embodiment of a four stroke internal combustion engine, having four poppet valves per cylinder, and in accordance with the present invention, and

Figure is a representation, generally in plan, of the part of the cylinder head shown in Figure 3, together with the inlet and exhaust ports associated with the four poppet valves.

Valve timings to suit a typical engine so that there are desirably low unburnt hydrocarbon emissions, and required tractability , at low and medium engine speeds are indicated by the valve event diagram of Figure 1. As the piston reciprocates, and the engine crankshaft is rotated thereby, and as indicated in Figure 1 at A, the inlet valve opens with the crankshaft 5 degrees before the piston reaches, its top dead centre, (TDC), position. The inlet valve closes, as indicated at B, at 40 degrees after the piston has passed its bottom dead centre, (BDC), position. The exhaust valve opens 50 degrees before BDC as indicated at C; and closes 5 degrees after BDC as indicated at D. There are only short "valve overlap" phases when both the inlet and exhaust valves are open, and curtailed timings due to the suppression of intake and exhaust pressure wave interactions.

Valve timings to suit the same engine so that there is the required tractability, at higher engine speeds, are indicated by the valve event diagram of Figure 2. As indicated at A , the inlet valve opens at 45 degrees before TDC; and, as indicated at B , the inlet valve closes at 65 degrees after BDC. As indicated at C , the exhaust valve opens at 60 degrees before BDC; and as indicated at D , the exhaust valve closes at 35 degrees after TDC. There are relatively longer "valve overlap" phases, and late inlet valve closure, compared with the

valve timings of Figure 1, commensurate with inertia wave effects in both the intake and exhaust systems.

The part of a throttled gasoline, four stroke, internal combustion engine shown in Figures 3 and 4 includes the cylinder head 2, an inlet camshaft 4 actuating first and second inlet valves, respectively, 6 and 7, via the usual tappet and spring system, and an exhaust camshaft 8 actuating first and second exhaust valves, respectively, 10 and 11, via the usual tappet and spring system. Gf the two inlet valves 6 and 7, and the two exhaust valves 10 and 11, only the face of the second inlet valve 7, and only the face of the second exhaust valve 11, are illustrated in Figure 1. The cam associated with the first inlet valve 6 is indicated at 14, and the cam associated with the first exhaust valve 10 is indicated at 16. Unillustrated cams are associated with the second inlet valve 7, and the second exhaust valve 11.

The illustrated cams 14 and 16 have timing profiles suited to low and medium engine speeds, the corresponding inlet and exhaust valve timings, respectively, for the illustrated first inlet valve 6, and the illustrated first exhaust valve 10, being somewhat similar to those indicated on the valve event diagram of Figure 1. The unillustrated cams for the second xnlet valve ana tne second exhaust valve 11 have timing profiles suited to higher engine speeds, the corresponding inlet and exhaust

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valve timings being similar to those indicated on the valve event diagram of Figure 2.

Indicated in Figure 3 is the common inlet port 20 provided in the cylinder head 2 for the two inlet valves 6 and 7. Adjacent to the inlet valves 6 and 7 the inlet port 20 is bifurcated by a partition 22, so that the port is split into two passages each leading individually to an inlet valve. Similarly, a common exhaust port 24 is provided in the cylinder head 2 for the two exhaust valves 10 and 11. Adjacent to the exhaust valves 10 and 11 the exhaust port is bifurcated by a partition 26, so that the port is split into two passages each leading individually from an exhaust valve.

Figure 4 is a representation, generally in plan, of the part of the cylinder head 2 shown in Figure 3. Figure 4 sh ws more clearl than Figure 3 both the arrangement of the two inlet valves 6 and 7, and the two exhaust valves 10 and 11, in relation to the cylinder; and also the arrangement of the inlet port 20 and the exhaust port 24 in relation to these valves.

In accordance with the present invention, and shown in Figure 4, there is provided in the passage 54 of the bifurcated part of tne inlet port 20 leading to the second inlet valve 7, a pivotally mounted, plate type, flow control valve 56; and there is provided in the passage 58

of the bifurcated part of the exhaust port 24 leading from the second exhaust valve 11, a second, pivotally mounted, plate type, flow control valve 60. The flow control valves 56 and 60 are linked together, for example, by mechanical means, and the settings thereof are controlled by πovements of a linkage indicated generally at 62. Movements of the linkage 62, and corresponding changes of the settings of the flow control valves 56 and 60, are under the control of any suitable construction of actuator, indicated generally at 64, operatively coupled to a known form of electronic engine management controller (not shown), and subject to operation in accordance with pre-programmed data. The linkage may have any convenient form. Instead of being coupled to an electronic engine management controller, the actuator may be coupled to the engine throttle (not shown).

In the operation of the engine the inlet and exhaust crankshafts 4 and 8 rotate, respectively, the two inlet cams and the two exhaust cams at half engine crankshaft speed, and all the valves 6, 7, 10, and 11 open and close at the timings determined by their respective cams.

When the plate valves 56 and 60 are closed, the flow of intake charge through the second inlet valve 7, and the flow of exnaust gases through the second exhaust valve 11, are cut off. In response to the pressure drop caused across - the inlet valves by the piston (not shown) moving

on its intake stroke in the co-operating cylinder, the intake charge is accelerated more strongly in the unthrottled passage 66 of the bifurcated part of the inlet port 20. This results in a high gas velocity in the cylinder; and due to the offset of the unthrottled passage 66 and the first inlet valve 6 from the cylinder centre line, there is a strong swirl effect imparted to the inflowing charge in the cylinder. This swirl effect aids in-cylinder turbulence, and enables the ignition advance for optimum combustion to be reduced advantageously, giving good engine thermal efficiency, torque, and fuel consumption.

A similar effect is noted when the piston is moving on its exhaust stroke. The first exhaust valve 10 provides an adequate exhaust time-area integral for blowdown, whilst he absence of "valve overlap" minimises the risk of undesired h drocarbon emissions from the engine due to unburnt charge being sucked directly from the passage 66 of t e bifurcated part of the inlet port 20 into the passage 68 of the bifurcated part of the exhaust port 24.

When the engine's performance corresponds to a predetermined point on the speed/load graph associated therewith, the actuator 64 causes the linkage 62 to be moved, and tne settings of tne flow control valves 56 and 60 are changed so that these valves are fully open. Intake cnarge is no admitted to the cylinder through both the

first and second inlet valves 6 and 7 when these valves are open; and exhaust gases are expelled from the cylinder through both the first and second exhaust valves 10 and

Since the second inlet valve 7 is caused to be open in angular advance of the first inlet valve 6, the gas in the non-bifurcated part of the inlet port 20 accelerates before the first inlet valve 6 opens. Thus, when the first inlet valve 6 does open the mass flow through this valve is greater than otherwise would be the case.

Further, the potential for exploiting the extraction effect of exhaust pressure waves by the effect of the delayed closing of the second exhaust valve 11 is not detracted from by the normal operation of the first exhaust valve 10.

The plate valve 56 on the inlet side of the engine ma be of steel, brass, aluminium, or of a suitable plastics material. The plate valve 60 on the exhaust side of the engine is required to be of a heat resistant steel, or of a ceramic.

Instead of being snapped open to impart a step change in the engine torque curve beyond a certain range of load/speed/throttle setting, the arrangement may be such

hat the plate valves are actuated progressively above a predetermined percentage opening.

The inlet and exhaust valves may be actuated by push rod and/or rocker means if there are provided cams individually actuating each valve.

The plate valves controlling the flow to the second inlet valve, and from the second exhaust valve, could be slideably mounted instead of being pivotably moveable.

The cams actuating the poppet valves each may be mounted individually on a camshaft.

The mechanical linkage may be replaced by any suitable form of actuator; and separate actuators may be provided fcr the two plate valves. In any such arrangement the settings of the two plate valves are changed in the same c cle of the engines, and hence are considered as being changed in unison.

The terms low and medium engine speeds conveniently refer to engine speeds having the mean piston velocity up to 7 metres per second, for engines having bore to stroke ratios in the range 0.8:1 to 1.1:1. The term higher engine speeds conveniently refers to engine speeds having the mean piston velocity in the range 7 to 18 metres per second, with normal bore to stroke ratios as referred to

in the preceding sentence, and a specific output in the range 50 to 90 Kilowatts per litre of swept cylinder volume for naturally aspirated engines.