The present invention refers to the technical field of in- ternal combustion spark ignition engines.

The aim of the invention is to confer to the internal com¬ bustion engine a multifuel capability combined with maxi¬ mum thermodynamic efficiency corresponding to each particu¬ lar fuel used. The multifuel characteristic will be ex- tremely convenient during the phase of petroleum derivates substitution by alternative fuels. b) BACKGROUND ART

The thermodynamic efficiency of an internal combustion en¬ gine is basically an increasing function of both the com- pression ratio and of the expansion ratio.

In conventional engines, due to design features, these two above mentioned ratios have equal values, and because of this it is usual to consider the thermodynamic efficiency as being a function of the compression ratio only. On the actual engine the influence on the two ratios are distinct. The conversion of heat into mechanical work is exclusively accomplished during the expansion of the hot burned gases which form the thermodynamic medium, and this conversion is as much complete as the expansion ratio in— creases.

The increase of the compression ratio leads also to the in¬ crease of efficiency by raising the energy level of the gases before the combustion. In the internal combustion SI engines which employ volative fuels, or gaseous fuels, the compression ratio is limited by the apparition of an abnormal combustion phenomenon known as detonation, or popularly as knock. The start of detonation depends mainly on the anti-knock characterijstic of the fuel and on the value of the compres- sion ratio.

Due to the fact that in conventional engines the expansion ratio is the same as the compression ratio, the highest

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possible value of the expansion ratio will depend on the limit imposed by the compression ratio.

The thermodynamic cycle which results from the distinction of a higher expansion ratio from a lower compression ratio is know as a "more-complete-exapansion" cycle whose descrip tion can be found in good text-books on combustion engines. Two procedures to achieve the "more-complete-expansion" cy¬ cle are well known. One of them consists of an earlier closing of the intake valve, thus reducing the effective intake stroke.

This procedure creates a very low gas pressure at the end of the intake stroke and consequently conduces to a lower final compression pressure rather than to a lower compres¬ sion ratio, accordingly to the anti-knock value of a par- ticular fuel.

Another way consists in the substitution of the convention¬ al crankshaft by other mechanical device which gives longer stroke for the expansion and the exhaust, and shorter stroke ^' for the intake and the compression. This mechanical device may consist of a shaft with two cams actuating on the piston movement. This cams are designed in such a way that they produce two different strokes. c) DISCLOSURE OF INVENTION The invention on which this call of privilege is based, pr sents a way to combine a conveniently high expansion ratio with an effective compression ratio suitable to the anti¬ knock value of a particular fuel.

The invention includes also the possibility of an easy and fast adjustament of the effective compression ratio ac- cording to the requirements of different fuels.

The multifuel characteristic requires, besides the compres¬ sion ratio change, a controlled adjustament of the fuel/air ratio at the carburetor. Because this procedure is well known and can be performed by needle jets, it is not con- sidered in this invention.

The present invention makes use of intake gas recirculation to adapt a geometric compression ratio, equal to the expan-

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sion ratio, into a variable effective compression ratio. In order to perform the intake gas recirculation, the cyl¬ inder head has a third valve actuated by a cam added to the conventional camshaft system, or by a second camshaft. The third valve will be called recirculation valve. The recirculation valve will be opened by the corresponding cam at the beginning of the compression stroke thus allowing that a part of the charge of fresh gases leaves the cylin¬ der. While the valve remains open, there will be no compres_ sion of gases buth there will be a discharge of fresh gases through the recirculation valve.

At one preestablished part of the compression stroke the recirculation valve is closed by the corresponding cam, and at that point the effective compression of the remaining gases begins at a value corresponding to the fuel used. The unburned gases which leave the cylinder through the recirculation valve are readmitted into the intake piping either of the same cylinder or of another one. The device to perform the above described operation is called INTAKE GAS RECIRCULATION - IGR -. The IGR referred to in this invention constitutes a simple and efficient way for a practical realization of the "more-complete-expan¬ sion" cycle,readily adaptable to different fuels. The IGR procedure to vary the effective compression ratio ... is completely different, from the principle and mechanics of the two procedures cited in the background art section. Only the IGR offers the versatility of both easy and fast adaption of the effective compression ratio. In order to allow a change of the endurance of the open po- sition of the recirculation valve, with the consequent var¬ iation of the effective compression ratio, the actuating cam must have distinct profiles along the longitudinal di¬ rection of the camshaft. These profiles actuate selectively according to an external control. The intake gas recirculation through the recirculation valve should not be confused with a third valve generally used in stratified charge engines, when a rich combustible

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mixture is introduced into a prechamber where these gases are ignited by a spark plug. The resulting flames in the prechamber ignite the mixture of fresh gases in the main chamber. In this case the third valve has no influence on the value of the compression ratio or of the expansion ratio, and consequently on the thermodynamic efficiency op¬ timized for different fuels.

Also, the intake gas recirculation - IGR - should not be confounded, with exhaust gas recirculation - EGR - where part of the burned and expanded gases are recycled into the intake system. This procedure aims the reduction of pollu¬ tion and/or improving fuel economy.

An additional favorable effect of IGR is the improvement of •the homogeneity of the air/fuel mixture, which favors the quality of combustion and permits the use of leanermixtures. When the fuel used is hydrated alcohol, the IGR permits a large variation in the proof value of the alcohol, consider ing that different water contents in the alcohol produce different anti-knock values, and therefore require differ- ent compression ratios.

All the factors influenced by the IGR combined with the highest possible effective compression ratio result in a maximum overall efficiency, and consequently minimum fuel consumption. d) BRIEF DESCRIPTION OF DRAWINGS

The mechanical system shown at figures 1 and 2 referring to the present invention consists of a cylinder head (1) , with spark plug (2) , two conventional intake and exhaust valves (3) , a recirculation valve (4) with the correspond- ing actuating mechanism (5) which assures the operation of the recirculation valve (4) .

The basic engine (6) formed by the cylinder block, pistons, connecting rods, crankshaft, camshaft, valve trains and accessories, remains unchanged to the conventional SI com- bustion engine.

The recirculation valve (4) is actuated by the cam (7) . The outlet of the valve (4) is connected by means of a recircu

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lating tube (8) to the intake tube (9) of the engine. The cam (7) has a proper shape to give the actuating movement to the recirculation valve (4) in order to maintain the valve (4) opened during the compression stroke at a piston movement from position (9) - bottom dead center - to anoth¬ er preselected position (10) from where the desired effec¬ tive compression starts until position (11) - top dead cen¬ ter-.

The expansion of the gases corresponds to the displacement of the piston from the position (11) corresponding to the TDC to the position (9) corresponding to the BDC. e) BEST MODE OF CARRYING OUT THE INVENTION

As the basic engine is a conventional one, it will be nec¬ essary that the design and construction of a new cylinder head be according to this invention, and that the design and construction of an actuating device for the recircula¬ tion valve be according to this invention as well. The new cylinder head and the new valve actuating mechanism can be adapted to any existing SI engine, or on a new en- gine design. f) INDUSTRIAL APPLICABILITY

The invention is directly applicable in the existing auto¬ motive engine industry, or on SI engines for other purposes. With the possibility of a compression ratio adjustament to different specific fuels of distinct octane numbers, com¬ bined with expansion ratios as high as 1:20 will give a spectacular increase in efficiency. This fact can easily be proved through the "more-complete-expansion" cycle, theory.