This invention relates to an internal combustion engine. BACKGROUND ART

In for example a petrol driven internal combustion engine, a petrol and air mixture enters the cylinder during an induction (or intake) stroke and fills the piston swept working volume. During the compression stroke which follows the mixture is compressed into a relatively small volume. Compression is accompanied by a rise in temperature which should not in a petrol engine by sufficient to cause spontaneous ignition. The compressed mixture is then ignited by a sparking plug. The heat released on combustion causes further expansion which drives the piston during a working stroke and the spent gases are then expelled in an exhaust stroke.

The fuel-air mixture ignites in the vicinity of the sparking plug and a flame front spreads throughout the combustion chamber which is defined between the piston head, cylinder wall and cylinder head at ignition.

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While ideally combustion would be completed smoothly and very rapidly at maximum compression, in practice ignition is not timed in relation to completion of the compression stroke so that maximum pressure from combustion is achieved at an optimum point in the engine cycle.

The speed of combustion depends on the fuel, the ratio of fuel to air in the combustion mixture, temperature, compression, and other factors. For a given engine compression ratio and a selected fuel there is an optimum fuel to air ratio for smooth rapid combustion.

A mixture richer in fuel would give quicker combustion but may cause self ignition orknocking and would be less economic in fuel consumption.

A mixture poorer in fuel, may lead to a too slow combustion resulting in inefficient engine operation and in extreme cases to difficulties in achieving fuel ignition.

In general it is desired to operate an engine at as high a compression ratio as possible since an increase in compression results in increased engine power output and the thermal efficiency is improved, but, for a given fuel, tendency to knocking or auto-ignition increases as compression ratio increases and places an upper limit on compression ratio.

There is thus necessarily a compromise between high engine efficiency obtainable by increasing compression

ratio and the selection of fuel mixtures which provide rapid combustion but which have a tendancy to premature ignition at high compression and which are less economic to use than poor fuel mixtures closer to a stochiometric optimum.

In a diesel engine it is usual to draw air into the cylinder, and compress the air as a result of which the air is heated to a temperature of approximately 700-900 C. Only then is a metered quantity of diesel fuel injected into the cylinder. The fuel ignites spontaneously but combustion occurs after an injection lag of about 1/1000 sec, because fuel droplets must first mix internally with the air. If the combustion is incomplete and unburnt fuel accumulates sudden preignition or "knocking" occurs.

Although diesel engines operate at much higher compression ratios than do spark ignition engines, a similar compromise is involved between selection of fuel/air ratio and compression. DISCLOSURE OF THE INVENTION

According to one aspect the invention consists in an internal combustion engine comprising: a main combustion chamber, a preliminary combustion chamber in communication with the main combustion chamber, a valve for isolating the preliminary combustion chamber from the main chamber.

means for igniting or causing self ignition of a charge of fuel-air mixture while isolated in the preliminary combustion chamber, and means for opening or permitting to open the isolating valve to release ignited fuel from the preliminary chamber to the main chamber thereby to ignite another charge of fuel-air mixture compressed in the main combustion chamber.

For preference the fuel-air mixture ignited in the preliminary combustion chamber is a rich mixture in comparison with the fuel-air mixture in the main combustion chamber and is less highly compressed than the mixture in the main combustion chamber.

The present invention thus enables combustion to be initiated utilizing one fuel mixture and compression combination selected for optimum ignition and to utilize another fuel mixture and compression combination selected for optimum combustion having regard to engine efficiency and economy. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only with reference to the accompanying drawings wherein:

Figs. 1 to 6 show schematically an engine according to the invention at successive stages of an engine cycle.

Figs. 7 and 8 show schematically a plan of the cylinder head of the engine of Figs. 1-6.

Fig. 9 shows schematically the cylinder head arrangement in a two stroke engine according to the invention. DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to. Figs. 1-6 there is shown an internal combustion engine according to the invention at various stages in an engine cycle.

The engine comprises a piston 10 reciprocating in a cylinder 11 having a cylinder head 12 provided with main fuel inlet port 13 including main inlet valve 14 controlling a fuel-air mixture flow from main line 15 to main chamber 20 and is provided with a main exhaust port 16 having an exhaust valve 17 controlling outflow to exhaust pipe 18.

Cylinder head 12, the walls of cylinder 11, and reciprocating piston 10 define therebetween a main combustion chamber 20, the volume of which varies during an engine cycle.

Cylinder head ^' 12 is provided with a preliminary combustion chamber 30 which communicates with main combustion chamber 20, via communication port 31 which may be opened or closed by means of isolating valve 32. Preliminary combustion chamber 30 may be provided with fuel or a fuel-air mixture via a subsiduary fuel line 35 and subsidiary inlet port 33 which is opened or closed from line 35 by subsidiary inlet valve 34.

According to one embodiment of the invention, the ^" engine is a petrol driven four stroke engine. In this

example the preliminary combustion chamber is provided with a sparking plug 40 and operates as follows:

During an induction stroke (Fig. 1) a fuel-air mixture is admitted to the main combustion chamber 20 at main inlet port 13 via open main inlet valve 14. Another fuel-air mixture is admitted to preliminary combustion chamber 30 via subsidiary inlet port 33 subsidiary inlet valve 34 and isolating valve 32 being open. The latter fuel mixture is preferably a richer mixture than the fuel admitted at main inlet port 13 which may be a poor mixture or in some cases may be air.

At the end of the induction stroke the piston commences a compression stroke.

During the compression stroke (Fig. 2 and Fig. 3) the main inlet valve 14 is closed and the subsidiary inlet valve 33 is closed.

During the first part of the compression stroke (Fig. 2) isolating valve 32 remains open and the fuel mixture in main combustion chamber 20 and in preliminary combustion chamber 30 is compressed.

During the later part of the compression stroke (Fig. 3) isolating valve 32 closes. A first charge of fuel-air mixture is thus isolated in preliminary combustion chamber 30, while the fuel mixture in main combustion chamber 20 (second charge) continues to be compressed. At completion of the compression stroke the fuel-mixture in the main chamber is thus at a higher

compression than the first charge fuel mixture isolated in preliminary combustion chamber 30. The fuel isolated in the preliminary chamber 30 is also a richer mixture than that in the main chamber in the preferred case.

At a predetermined point in the engine cycle at or near completion of compression, the first charge of fuel in the preliminary combustion chamber is ignited. Isolating valve 32 is then opened permitting the ignited fuel in preliminary combustion chamber 30 to ignite the second charge of fuel mixture in the main chamber 20.

Piston 10 is thus driven downwards in a power stroke (Fig. 5) during which isolating valve 32 closes. The cycle is completed with an exhaust stroke (Fig. 6) during which exhaust valve 17 opens and spent gas is driven from main chamber J20.

The apparatus enables the fuel mixture in preliminary combustion chamber 30 and the compression of that fuel to be optimized for a smooth and rapid combustion while at the same time permitting the fuel mixture and compression in the main chamber 20 to be selected for efficiency and economy.

While a usual petrol engine has a compression ratio in the range of 4:1 to 10:1, a petrol engine according to the present invention could have a higher compression ratio, for example of 15:1 or more. That is because the fuel in preliminary combustion chamber 30 is under a low co-πpression. Accordingly fuel rich mixture can be ignited in that chamber to produce a smooth rapid

combustion without knock. Upon opening of isolation valve 32 the combustion is transferred efficiently to the highly compressed but poor mixture in main chamber

20. That fuel can be a relatiavely weak mixture and slower burning because ignition takes place on a broad front, and being weaker can be under higher compression without risk of preignition.

If desired, fuel mixture may be admitted to the preliminary chamber 30 during the power stroke (after valve 32 has closed again and with valve 39 open) . The early admittance of fuel mixture may assist cooling of valve 32.

Figs. 7 and 8 illustrate various locations for isolating valve 32 on the cylinder head.

It will be understood that the invention described with reference to Figs. 1 to 6 need not be a spark ignition engine but may be a diesel engine.

In that case during an induction stroke air is sucked from line 35 via subsidiary inlet valve 33 and isolation valve 32 into main chamber 20 and air is sucked into the main chamber 20 via main inlet valve 13. During the induction stroke fuel is injected into the preliminary chamber to provide a weak mixture in the main chamber which is of a composition below the limit of self ignition. During the compression stroke, at a predetermined compression (equivalent to a compression ratio of, for example, 15 1-18:1) the valve separating

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chambers 20 and 30 is closed. While compression of the charge in the main chamber 30 continues, a charge of fuel is injected into the preliminary chamber by means not shown in Figs. 1-6 and self ignition occurs in the preliminary combustion ^' chamber 20. When ignition has progressed sufficiently, valve 32 opens and the weak mixture at a higher pressure in the main combustion chamber is thereby ignited. If desired ignition in the main chamber may be augmented by further fuel injection into the main chamber.

If desired, fuel may be injected into the preliminary chamber during the power stroke at which time valve 32 and valve 39 is closed.

Although described above with reference to a four stroke engine, a two stroke engine may be constructed to operate according to the invention. Fig. 9 shows schematically the piston head and upper cylinder arrangement for a two stroke engine. The same numerals are used to identify parts thereof as are used to identify corresponding parts of Figs. 1 to 6. The drawing of Fig. 9 does not show the inlet and exhaust* ports which may be as usual for a two stroke engine. Valve 32 closes during the compression part of the cycle as previously described.

It will be understood that it is not essential in the case of a spark ignition engine to admit fuel to preliminary combustion chamber 30 via a second fuel line

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35 and inlet valve 34. If preliminary combustion chamber 30 communicates with the main chamber 20 but is otherwise sealed, then as fuel mixture admitted to 'the main chamber via inlet valve 13 during induction may be introduced and compressed into preliminary combustion chamber 30 via valve 32 during compression. Greater advantage will usually be obtained, however, if the fuel mixture in the preliminary combustion chamber is faster burning and at a lower compression, than the fuel mixture in the main chamber as is achieved with subsidiary fuel inlet to the preliminary combustion chamber.

It will also be understood that the fuels admitted to the main and the preliminary combustion chambers may be different in kind as well as different in enrichment. For example an ethanol-air mixture may be admitted via main valve 13 and compressed in the main chamber 20 and a petrol-air mixture might be admitted via subsidiary valve 34 and ignited in the preliminary combustion chamber 30.

The respective fuel-air mixtures may be compressed each to a respective pressure which assures the highest efficiency for the whole cycle. At a selected time in the engine cycle the fuel in the preliminary chamber is ignited. Isolating valve 32 is then opened permitting a stream of the burning first fuel to ignite the second fuel.

A further advantage of the invention is that in preferred embodiments one engine may efficiently utilize a range of different fuels with relatively simple adjustment. If desired a glow plug may be used in the preliminary combustion chamber with certain fuel-mixtures.

While the invention has been described with reference to piston engines it is equally applicable to the case of a rotary engine.

In the case of a rotary engine having a single rotor, the rotor may act as a valve isolating the preliminary combustion chamber and in a two rotary engine of the Wankel type the preliminary combustion chamber may be defined between one rotor and the engine case while the main combustion chamber is defined between the other rotor and the case.

The preliminary combustion chamber should preferably be. of a small volume in comparison with the main combustion chamber and should permit a streamlined and unobstructed through-flow. That is to say it should be as free as possible from dead spaces and pockets in relation to through-flow.

All the valves may be driven from one cam shaft, or respective values may be driven from different cam shafts, or valve 32 may be adapted to open against for example spring pressure when pressure within the preliminary combustion chamber due to ignited gases

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exceeds a predetermined pressure or a combination of such valve operating means may be employed.

As will be apparent to those skilled in the art from the description hereof, modifications to the design of the respective combustion chambers, valve operation and timing elements may be required in modifying existing engine designs to conform with the invention and may be made without departing from the scope of the invention herein described.

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