GB2269857A

GB2269857A - Four-stroke engine with a cylinder wall exhaust port.

Info

Publication number: GB2269857A
Application number: GB9316898A
Authority: GB
Inventors: David William Blundell; Clive Dopson
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 1992-08-14
Filing date: 1993-08-13
Publication date: 1994-02-23
Also published as: GB9316898D0; GB9217345D0

Abstract

A piston controlled exhaust port 16 is provided in addition to the overhead inlet and exhaust valves 12, 13. The port is controlled by a valve 20 which may be a slide (22, figs. 9 to 11) or pivoted (23, Figs. 12 to 14) valve which may close to an extent dependant upon engine operating conditions. The overhead valves may be rotary (Figs. 15 and 16). The exhaust from the port 16 may induce flow through the overhead exhaust valve (Fig. 17). <IMAGE>

Description

FOUR STROKE INTERNAL COMBUSTION ENGINE The present invention relates to a novel four-stroke internal combustion engine.

The majority of four-stroke internal combustion engines have poppet valves for the inlet and exhaust valves for each cylinder. The exhaust valves in the cylinder head are by far the hottest region in the combustion chamber. In multi-valve engines, the exhaust valves can occupy up to 20% of the surface area of the combustion chamber when the piston is at top dead centre.

Typically the poppet valve serving as the exhaust valve will have a head temperature in excess of 8000C at maximum power condition. The exhaust valves thus act to provide a lot of heat to the fresh charge as it is delivered to the combustion chamber. This problem is aggravated since in most cylinder heads the majority of the incoming charge flow is directed over the heads of the exhaust valves during the induction and compression phases of the engine.

The heating of the incoming charge by the exhaust valves is a significant problem. Most of the four-stroke engines available today are limited in their performance by detonation. Detonation occurs when the fresh charge gases in the combustion chamber are so hot that when the charge is ignited by spark the pressure wave propagated throughout the cylinder further increases the temperature of the charge to cause spontaneous ignition of the charge at the periphery of the combustion chamber. This is very disadvantageous. Efficient combustion is achieved when there is progressive combustion in the form of a flame front propagated by the spark. When the engine reaches detonation conditions, detonation can be avoided by increasing the fuel/air ratio, retarding the ignition timing or decreasing the compression ratio.Since the fuel has a high latent heat of evaporation, increasing the fuel/air ratio effectively cools the combustion chamber.

However, the ratio will then move away from the optimum ratio required for best performance/economy. Retarding the ignition or decreasing the compression ratio both decrease the power output of the engine and thus are disadvantageous.

The present invention provides an internal combustion engine operating by the four stroke cycle and having at least one working cylinder with a piston reciprocating therein, wherein first valve means is provided at the cylinder head to control the intake of charge to the cylinder through an inlet port in the cylinder head and also to control the flow of exhaust gases from the cylinder through a first exhaust port in the cylinder head, characterised in that a second exhaust port is provided in the cylinder wall at a point spaced axially along the cylinder from the cylinder head, which second exhaust port is covered and uncovered by the piston in its reciprocating movement and which is connected to the exhaust for the engine.

Whereas a poppet valve is slow to open and flow past the valve is restricted, the ported valve, which is common in two-stroke engines, is very quickly opened by the reciprocating piston and will have an aperture area which is large in comparison with that of the standard exhaust poppet valve in the cylinder head.

In operation of the engine, the hot high pressure exhaust gases after combustion can flow through the exhaust port in the side of the cylinder wall at the end of the expansion stroke of the four stroke engine at the beginning of the exhaust stroke. Thus the port at the bottom of the cylinder deals with the high temperature high pressure combusted gases rather than the cylinder head exhaust valve means. Hence the exhaust valve means at the cylinder head can be kept cooler than in the four stroke engines of the prior art. Hence the engine can be run at speeds and with power outputs that were previously prevented by detonation of the charge.

The provision of the exhaust port in the side of the cylinder wall also has several further advantages. First, the use of the exhaust port cuts down the pumping losses of the engine. As mentioned above, the poppet exhaust valve at the cylinder head is slow to open and the pressure of the combusted gases in the cylinder is still relatively high in the usual four-stroke engine when the piston begins its exhaust stroke. Therefore, the piston has to perform positive work in dispelling exhaust gases from the cylinders in the prior art engine. However, in the engine of the present invention the pressure of the gases within the cylinder is brought very quickly to near atmospheric pressure and therefore the pumping losses are reduced.

A further advantage of the provision of the exhaust port at the bottom of the cylinder is that the exhaust passage can be more effectively cooled, since it is easier to provide passages for flow of cooling water to an exhaust passage in the cylinder block than it is to provide a similar passage in the cylinder head, where there are greater constraints on packaging.

An additional advantage of the engine of the present invention arises since the poppet valve used for the exhaust valve in the cylinder head can be reduced in size. The reduction in size of the exhaust valve will further reduce the heating effect on the incoming charge.

Also the reduction in size of the exhaust valve will allow larger inlet valves or a greater number of inlet valves.

The lower temperatures in the combustion chamber increases the volumetric efficiency of the engine and also the fuel/air mass of each charge.

The present invention also provides the possibility of potential exhaust tuning benefits as a result of the rapid exhaust port opening. All of the above advantages lead to a result that higher compression ratios can be used for a four-stroke engine or poor quality fuels (of lower octane rating) can be used since the combustion chamber temperature is decreased, widening the detonation threshold.

Preferably second valve means is provided in the second exhaust port which controls the opening of the second exhaust port to the cylinder, the second valve means opening the second exhaust port at the end of the expansion stroke and/or beginning of the exhaust stroke of the engine and said second valve means closing the second exhaust port during the compression stroke of the engine and/or during the intake stroke of the engine.

Preferably the second valve means for the second exhaust port comprises a rotary valve rotated in timed relationship to the motion of the piston within the working cylinder.

Rotary valves are ideally suited for use in controlling the exhaust port, since a large quantity of cooling fluid can be provided in the cylinder block to cool the rotary valve. Furthermore, the rotary valve need not rotate at great speed, since the rotary valve would specifically be required to hold the exhaust port open for only about 1000 in every 7200 four-stroke cycle.

In a second embodiment of an engine according to the invention the second valve means for the second exhaust port comprises a guillotine valve mounted in a slot in the cylinder block for reciprocal motion into and out of a cylinder block exhaust passage connected to the second exhaust port and actuating means for reciprocating the guillotine valve in timed relationship with the motion of the piston to control the flow of gas from the working cylinder through the second exhaust port. Preferably the actuating means reciprocates the guillotine valve between a lowermost position and an uppermost position, the uppermost position being varied by the actuating means with engine speed and/or load and/or temperature.

In a third embodiment of an engine according to the invention the second valve means for the second exhaust port comprises a valve member pivotally mounted to oscillate into and out of a cylinder block exhaust passage connected to the second exhaust port and actuating means for oscillating the valve member in timed relationship with the motion of the piston of the cylinder to control the flow of gas from the working cylinder through the second exhaust port. Preferably the actuating means oscillates the valve member between a lowermost position and an uppermost position, the uppermost position being varied by the actuating means with engine speed and/or engine load and/or engine temperature.

The reciprocating guillotine valve or pivotal valve member could be used to vary the length of the expansion stroke of the engine with engine speed and load. The opening of the cylinder to exhaust through the port can be delayed in terms of degrees of motion from top dead centre of the piston for low engine speeds, since a smaller area of exhaust port is required at low engine speeds, since for the same degrees of piston motion there is a greater length of time during each cycle for exhaust. Thus the power output could be improved at low engine speeds.

Preferably the first valve means provided at the cylinder head to control the flow of exhaust gas from the cylinder opens to allow flow in each working cycle only after the piston has uncovered the second exhaust port at the end of the expansion stroke and preferably the first valve means is held open during the exhaust stroke of the engine after the piston covers the second exhaust port.

Preferably the first valve means provided at the cylinder head for controlling the intake of charge to the working cylinder through the inlet port comprises a rotary valve rotating in timed relationship with the motion of the piston within the working cylinder.

Preferably the first valve means provided at the cylinder head for controlling the flow of exhaust gas from the cylinder through the first exhaust port comprises a rotary valve rotating in timed relationship with the motion of the piston within its working cylinder.

The use of rotary valves in the cylinder head can be made possible by the reduction of the heat content of the gases passing through the valves and the lessening of the problem of deformation faced in design of rotary valves.

The use of rotary valves in place of poppet valves could be advantageous since the rotary valves offer favourable flow conditions for the flow of gases into and/or out of the combustion chamber, better than the flow conditions inherent with poppet valves. Furthermore, rotary valves are not speed limited in the same way as poppet valves, the speed of the poppet valves having to be limited to ensure valve train integrity.

In preferred embodiments of an engine according to the invention a cylinder block exhaust passage receives exhaust gas from the second exhaust port, the cylinder block exhaust passage comprising a portion shaped to cause a depression in the pressure of the flow of exhaust gas, the first valve means in the cylinder head connects the first exhaust port to a cylinder head exhaust passage and wherein the cylinder head exhaust passage is connected to the said shaped portion of the cylinder block exhaust passage such that the depression in pressure is used to draw exhaust gas through the first exhaust port in the cylinder head.

Preferably the portion of the cylinder block exhaust passage shaped to cause a depression is a portion of a cross-section which increases in the direction of the flow of the exhaust gases.

Preferably the cylinder block exhaust passage comprises a spout extending into a first conduit of a cross-section larger than that of the spout, the first conduit being joined in the direction of flow of the exhaust gas to a funnel of a cross-section increasing in the direction of flow of the exhaust gas and the funnel in turn being connected to a second conduit of a cross-section larger than the cross-section of the first conduit.

Preferred embodiments of the specification will now be described with reference to the accompanying drawings in which; Figures 1 to 4 show a first embodiment of an engine according to the invention at four different points during its four-stroke cycle; Figures 4 to 8 show a second embodiment of an engine according to the invention at four different points during its four point cycle; Figures 9 and 10 illustrate a third embodiment of an engine according to the invention, both figures showing the engine at the same point in time in its working cycle, but the figure 9 showing a high speed operating condition and the figure 10 showing a low speed operating condition.

Figure 11 shows the third embodiment of the invention illustrated in figures 9 and 10 at the point in a working cycle where the piston begins its compression stroke.

Figures 12 and 13 illustrate a fourth embodiment of an engine according to the invention, both figures showing the invention at the same point in the operating cycle of the engine, but figure 12 illustrating the engine in a high speed operating condition and figure 13 illustrating the engine in a low speed operating condition; Figure 14 shows the fourth embodiment of the invention illustrated in figures 12 and 13 at the point in a working cycle where the working piston of the engine is commencing its upward compression stroke; Figure 15 shows a fifth embodiment of the engine according to the invention; and Figure 16 shows a sixth embodiment of an engine according to the invention.

Figure 17 shows a seventh embodiment of an engine according to the invention.

Referring to figure 1 of the accompanying drawings, the engine of the invention in its most basic form can be seen to comprise a piston 10 which reciprocates within a working cylinder 11. Poppet valves 12 and 13 are provided at the cylinder head.

The poppet valve 12 will be oscillated by cam means (not shown) to control the inlet of fuel and air mixture, usually called charge, from the inlet passage 14.

The poppet valve 13 is controlled by cam means (not shown) to control the flow of exhaust gases from the cylinder 11 to the exhaust passage 15.

A spark plug 17 can be seen in the cylinder head.

It will be appreciated that the cylinder head of the engine of the invention is a standard cylinder head with poppet valves controlling inlet and exhaust.

An exhaust port 16 is provided in the wall of the cylinder 11, which exhaust port 16 is covered and uncovered by the piston 10 during its reciprocal motion.

Figure 1 shows the engine of the invention with the piston at top dead centre and ignition occuring. The poppet valves 12 and 13 shut the inlet and exhaust ports in the cylinder head and the exhaust port 16 is closed off from the combustion chamber portion of cylinder 11 by the piston 10.

In figure 2, the piston 10 has reached bottom dead centre at the end of the expansion stroke and has uncovered the exhaust port 16 in its downward motion. The poppet valves 12 and 13 maintain their cylinder head ports in their closed position, but the high pressure combusted gases in the cylinder 11 can escape to exhaust through the port 16.

The exhaust port 16 is uncovered quickly by the piston 10 in its downward motion and is chosen to be an area sufficient for the pressure in the cylinder 11 to die quickly to near atmospheric.

It will be appreciated that the exhaust port 16 alone deals with the hottest combusted gases of highest pressure. The poppet valve 13 in the cylinder head is closed during the commencement of the exhaust phase.

In figure 3 the piston 10 is moving upward in its exhaust stroke. It will be appreciated that the piston 10 has closed the exhaust port 16 in the side of the cylinder 11. By this time, the poppet valve 13 has opened to enable combusted gases to be forced from the cylinder 11 by the piston 10 out into the exhaust passage 15.

In a standard four-stroke engine, the poppet exhaust valves are opened before the end of the expansion stroke, since poppet valves are generally slow to open and therefore opening must commence before the end of the expansion stroke to allow sufficient area for exhaust during the exhaust stroke.

In the present invention, the opening of the poppet valve 13 can be delayed until after piston 10 has reached bottom dead centre, the limiting timing requirement being that the poppet valve 13 must sufficiently open the cylinder 11 to the exhaust passage 15 by the time that the top of the piston 10 has reached the uppermost lip of the exhaust port 16.

In figure 4 the induction stroke of the engine of the invention is shown, with the poppet valve 12 open to allow flow of fuel/air charge into the cylinder 11 from the inlet passage 14. Obviously, the fuel/air charge is drawn into the cylinder 11 by the piston 10 in its downward motion.

The exhaust valve 13 is shut in the usual manner during the induction stroke, to prevent flow of fuel/air mixture from the cylinder 11 to the exhaust passage 15.

As in standard engines, the inlet passage 14 is angled such that the majority of the fuel/air flow over the poppet valve 12 occurs over the top of the poppet valve (as indicated by the arrows). Thus the flow of mixture passes over the head of the exhaust valve 13.

Since the exhaust valve 13 has not had to deal with the hottest combusted gases (these having been dealt with by the exhaust port 16), the head of the poppet valve 13 will be substantially cooler in comparison with the head of the exhaust poppet valve of a standard engine. Therefore, the incoming fuel/air charge receives less heating during the induction stroke and the problem of detonation is decreased.

It will be seen in figures 1 to 4 that coolant passages 17 and 18 are provided next to the port 16 to receive a flow of cooling oil and/or water. It is far easier to provide sufficient cooling for the exhaust port 16 than for the poppet valve 13, since there is far more room to provide suitable coolant passageways in a cylinder block than in the cylinder head.

The basic engine according to the invention as illustrated in figures 1 to 4 has a disadvantage in that the port 16 will be open to the cylinder 11 during the induction stroke of the engine and therefore fuel/air mixture will be lost through the port at the commencement of the stroke. To avoid this problem it is advantageous to provide suitable valve means in the exhaust port 16 and such an arrangement can be seen in figures 5 to 8.

Most of the components of the engine illustrated in figures 5 to 8 are identical with the components of the engine illustrated in figures 1 to 4. Like components will be given the same reference numerals. The additional feature of the engine of figures 5 to 8 is the provision of a rotary valve 20 which is driven to rotate in timed relationship to the motion of the working piston 10.

In figure 5 the piston 10 is at top dead centre and combustion has just commenced. The rotary valve 20 at such a stage closes the exhaust port 16, but is rotating into an open position.

In figure 6 the piston 10 has cleared the exhaust port 16 and the rotary valve 20 has rotated into a position where it allows flow of gases through the exhaust port 16. The high pressure combusted gases in the cylinder 11 flow out of the chamber 11 through the exhaust port 16 and rotary valve 20 to exhaust. The poppet valves 12 and 13 maintain the cylinder head ports closed.

In figure 7, the top of the piston 10 has cleared the uppermost lip of the exhaust port 16 and the exhaust valve 13 in the cylinder head has opened sufficiently to allow the motion of the piston 10 upwardly within the cylinder 11 to dispel combusted gases from the cylinder.

These combusted gases will be at near atmospheric pressure. It will be seen that the rotary valve 20 is rotating into a position where it closes the exhaust port 16.

In figure 8 the piston 10 is moved down in the induction stroke of the engine. The poppet valve 12 is open to allow fresh charge to be drawn into the cylinder.

Throughout the induction stroke the poppet valve 13 maintains exhaust passage 15 closed apart from the usual valve overlap to allow scavenging of the combustion chamber when the piston is in the top dead centre region of piston motion and the rotary valve 20 maintains the exhaust port 16 closed throughout the induction stroke.

Therefore, no incoming charge is lost to exhaust through port 16.

The rotary valve 20 will maintain the exhaust port 16 closed during the compression stroke of the four stroke engine, so that no fuel/air mixture is forced out of the exhaust port 16 by the upward motion of the piston 10 during the compression stroke.

Instead of using a rotary valve 20 as shown in figures 4 to 8, a guillotine valve 21 can be used as shown in figures 9 to 11. The guillotine valve 21 will be used in the same way as the rotary valve 20; opening the exhaust port 16 at the end of the expansion stroke of the engine and during the commencement of the exhaust stroke of the engine and the guillotine valve 21 (as shown in figure 11) maintaining the exhaust port 16 closed during the intake and compression strokes of the engine.

The guillotine valve 21 slides in and out of the slot 22 provided in the cylinder block to open and close the exhaust port 16. Suitable actuating means (not shown) is used to reciprocate the guillotine valve 21 within the exhaust port 16. The actuating means is chosen such that the guillotine valve 21 always has the lowermost position shown in figure 11, fully closing the exhaust port 16.

However, the actuating means chosen such that the uppermost position of the guillotine valve 21 is variable with engine speed.

Figure 9 shows the uppermost position of the guillotine valve 21 for a high speed condition of the engine. The valve 21 is fully retracted into the slot 22 and the exhausting of combusted gases from the cylinder 11 commences as soon as the piston 10 uncovers the exhaust port 16.

In figure 10, the guillotine valve 21 in its uppermost position extends into the exhaust passage leading from the port 16, but does not fully close the passage. This uppermost position of the guillotine valve 21 in its reciprocating motion will be chosen for low engine speeds, to effectively delay (in terms of degrees after top dead centre) the opening of the exhaust port 16.

The opening of the exhaust port 16 is delayed for low engine speeds to increase the length of the effective expansion stroke of the engine. The motion of the guillotine valve 21 should as a general rule be chosen such that the integral of the aperture of the port 16 with time during each cycle is roughly constant for all engine speeds. At low engine speeds there is a greater time for exhausting combusted gases than at high engine speeds, since the crankshaft will take a greater time to rotate through a specified number of crankshaft degrees at low engine speeds than at high engine speeds. Thus at low engine speeds, the aperture area of the ports 16 can be decreased in comparison with the port area needed for high engine speeds, whilst the required time area for exhaust is maintained.

Preferably the motion of the guillotine valve 21 is controlled such that it provides no impedance to fluid flow during the upward motion of the piston 10 in its exhaust stroke.

Instead of using a guillotine valve 21, a trapping valve 23 can be used as shown in figures 12 to 14. The trapping valve 23 is pivotally mounted for oscillation into and out of a recess in the cylinder block, to open and close the exhaust port 16.

As shown in Figure 14, the trapping valve 23 fully closes the exhaust port 16 during the intake stroke of the engine before the piston reaches the upper level of port 16 and maintains the ports closed during the remainder of the induction period of the engine. Preferably the trapping valve 23 is shaped with a curved front face, to match the exterior of the piston and provide a good seal.

Whilst the lowermost position of the trapping valve 23 in its oscillating motion in each cycle is always as shown in figure 14, the actuating means (not shown) for oscillating the trapping valve 23 is chosen such that the uppermost position of the trapping valve 23 is variable with engine speed.

The reasons for varying the uppermost position of the trapping valve has been discussed before with reference to the movement of the guillotine valve 21. At high engine speeds, the uppermost position of the trapping valve 23 is chosen such that the valve 23 is fully retracted within its recess in the cylinder block so that combusted gases can be exhausted from the cylinder 11 through the exhaust port 16 as soon as the piston 10 uncovers the exhaust port 16 (figure 12).

At low engine speeds, the uppermost position of the trapping valve 23 is chosen such that trapping valve 23 extends into the exhaust port 16, to delay (in terms of degrees of crankshaft rotation) the opening of the exhausting of combusted gases from the cylinder 11 through the exhaust port 16 (figure 13). Thus, the expansion stroke of the engine is increased.

In a preferred embodiment of the invention as shown in figure 15, the cylinder head inlet valve is a rotary valve 30. Since the port 16 is available in the cylinder for exhausting of combusted gases, the the poppet valve 13 can be reduced in size and can permit room for the use of a rotary valve 30 to control the inflow of air/fuel charge into the cylinder 11. The use of rotary valve 30 for the inlet cylinder valve is beneficial for many well known reasons, e.g. the fact that the motion of the rotary valve is not limited in terms of speed in the same way that a poppet valve arrangement is speed limited by the need to maintain valve train integrity.

Indeed, rotary valves could be used for both inlet and exhaust at the cylinder head as shown in figure 16 where a rotary inlet valve 30 is used and a rotary exhaust valve 31. It has always been hard practically to use rotary valves for the exhaust valves in the cylinder head of an engine to date, since they have had to deal with very high temperature and high pressure gases and the high temperatures and high pressures have caused distortion.

Furthermore, it is very hard to cool rotary valves at the cylinder head due to the lack of space available.

However, in the present invention the hottest and highest pressure combusted gases are taken out of the cylinder 11 by the exhaust port 16 and therefore the rotary valve 31 at the cylinder head need only deal with lower temperature and lower pressure combusted gases and therefore use of a rotary valve is made practical.

The engine of the invention could be beneficially provided with an exhaust ejector arrangement and an embodiment of an engine of the invention incorporating such an arrangement as shown in figure 17.

The piston and cylinder arrangement of the embodiment of figure 6 is the same as the arrangement illustrated in figures 1 to 4 and therefore the same reference numerals are used in figure 17. The exhaust passage 16 of the embodiment comprises a spout 45 which protrudes into a cylinder 41 of larger diameter. The cylinder 41 has one closed end and an end open either directly to atmosphere or to a suitable conduit to atmosphere. The cylinder 41 has three portions of different shape; the spout 45 extends into a first cylindrical portion 42, which continues downstream in the exhaust stream by a funnel 43 of a diameter which increases in the downstream direction of the exhaust stream and the funnel then continues in a cylinder portion 44 of a diameter larger than the diameter of the cylindrical portion 42.

The exhaust passage 15 is connected to the portion 42 of the cylinder 41 so that both are in fluid communication.

The flow of exhaust gas through the cylinder 41 from the piston controlled port 16 is a pulsating flow and generates a large depression in the portion 42 of the exhaust tube 45. The depression helps scavenge the cylinder by encouraging exhaust flow past poppet valve 13 into the exhaust passage 15.

Whilst the exhaust ejection arrangement is shown in figure 17 in use with the embodiment of the engine illustrated in figure 1, it could equally well be used with the engine embodiments of any of the figures.

The present invention in all of its embodiments is beneficial for many reasons as discussed in the preamble of the specification.

Claims

CLAIMS:

1. An internal combustion engine operating by the four stroke engine cycle and having at least one working cylinder with a piston reciprocating therein, wherein first valve means is provided at the cylinder head to control the intake of charge to the cylinder through an inlet port in the cylinder head and also to control the flow of exhaust gas from the cylinder through a first exhaust port in the cylinder head, characterised in that a second exhaust port is provided in the cylinder wall at a point spaced axially along the cylinder from the cylinder head, which port is covered and uncovered by the piston in its reciprocating movement and which is connected to the exhaust for the engine.

2. An internal combustion engine as claimed in Claim 1 wherein second valve means is provided in the second exhaust port which controls the opening of the second exhaust port to the cylinder, the second valve means opening the second exhaust port at the end of the expansion stroke and/or beginning of the exhaust stroke of the engine and said second valve means closing the second exhaust port during the compression stroke of the engine and/or during the intake stroke of the engine.

3. An internal combustion engine as claimed in Claim 2 wherein the second valve means for the second exhaust port comprises a rotary valve rotated in timed relationship to the motion of the piston within the working cylinder.

4. An internal combustion engine as claimed in Claim 2 wherein the second valve means for the second exhaust port comprises a guillotine valve mounted in a slot in the cylinder block for reciprocating motion into and out of a cylinder block exhaust passage connected to the second exhaust port and actuating means for reciprocating the guillotine valve in timed relationship with the motion of the piston to control the flow of gases from the working cylinder through the second exhaust port.

5. An internal combustion engine as claimed in Claim 4 wherein the actuating means reciprocates the guillotine valve between a lowermost position and an uppermost position which is varied by the actuating means with engine speed and/or load and/or temperature.

6. An internal combustion engine as claimed in Claim 2 wherein the second valve means for the second exhaust port comprises a valve member pivotally mounted to oscillate into and out a cylinder block exhaust passage connected to the second exhaust port and actuating means for oscillating the valve member in timed relationship with the motion of the piston of the cylinder to control the flow of gases from the working cylinder through the exhaust port.

7. An internal combustion engine as claimed in Claim 6 wherein the actuating means oscillates the valve member between a lowermost position and an uppermost position which is varied by the actuating means with engine speed and/or engine load and/or engine temperature.

8. An internal combustion engine as claimed in any one of the preceding claims wherein the first valve means provided at the cylinder head to control the flow of exhaust gas from the cylinder opens to allow flow in each working cycle only after the piston has uncovered the second exhaust port to allow flow of exhaust gas from the cylinder and the first valve means is held open during the exhaust stroke of the engine after the piston covers the second exhaust port.

9. An internal combustion engine as claimed in any one of the preceding claims wherein the first valve means provided at the cylinder head for controlling the intake of charge to the working cylinder through the inlet port comprises a rotary valve rotating in timed relationship with the motion of the piston within the working cylinder.

10. An internal combustion engine as claimed in any one of the preceding claims wherein the first valve means provided at the cylinder head for controlling the flow of exhaust gases from the cylinder through the first exhaust port comprises a rotary valve rotating in timed relationship with the motion of the piston within its working cylinder.

11. An internal combustion engine as claimed in any one of the preceding claims wherein a cylinder block exhaust passage receives exhaust gas from the second exhaust port, the cylinder block exhaust passage comprising a portion shaped to cause a depression in the pressure of the flow of exhaust gas, the first valve means in the cylinder head connects the first exhaust port to a cylinder head exhaust passage, and wherein the cylinder head exhaust passage is connected to the said shaped portion of the cylinder block exhaust passage such that the depression in the flow of exhaust gas in the cylinder block exhaust passage is used to draw exhaust gas through the first exhaust port in the cylinder head.

12. An internal combustion engine as claimed in Claim 11 wherein the portion of the cylinder block exhaust passage shaped to cause a depression is a portion of a cross-section which increases in the direction of the flow of exhaust gases.

13. An internal combustion engine as claimed in Claim 12 wherein the cylinder block exhaust passage comprises a spout extending into a first conduit of a cross-section larger than the spout, the first conduit being joined in the direction of flow of the exhaust gas to a funnel of cross-section increasing in the direction of flow of the exhaust gas and the funnel in turn connected to a second conduit of a cross-section larger than the cross-section of the first conduit.

14. An internal combustion engine substantially as hereinbefore described with reference to and as shown in the accompanying drawings.