USA patent 6,199, 519 discloses a free-piston, two-stroke auto-ignition internal combustion engine, which utilises relatively high compression ratios, short burn durations, and homogeneous fuel/air mixtures in conjunction with low equivalence ratios. The engine includes an electrical generator having a linear alternator with a double-ended free piston that oscillates inside a closed cylinder. Fuel and air are introduced in a two-stroke cycle fashion on each end, where the cylinder charge is compressed to the point of auto- ignition without spark plugs. The piston is driven in an oscillating motion as combustion occurs successively on each end. This leads to rapid combustion at almost constant volume for any fuel/air equivalence ratio mixture at very high compression ratios.

A disadvantage of the known engine is that it utilises fossil fuels, which cause polluting emissions, albeit at levels lower than conventional internal combustion engines. An exhaust system therefore has to be provided and the two-stroke cycle results in a relatively high fuel consumption.

OBJECT OF THE INVENTION It is therefore an object of the present invention to provide a free piston internal combustion engine with which the aforesaid disadvantage can be overcome or at least minimised.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a free piston internal combustion engine comprising: a cylinder defining at least one combustion chamber; -at least one inlet to the chamber for combustible fluid ; and a piston disposed inside the chamber and freely reciprocatingly movable inside the chamber on combustion of the combustible fluid ; the engine being adapted to run on combustible fluid including predominantly hydrogen and oxygen, the arrangement being such that an outlet for exhaust gas is optional.

The applicant foresees that, instead of an exhaust outlet, the chamber may include a liquid outlet for draining liquid, which may form during the combustion of the fluid.

The piston divides the combustion chamber into first and second secondary combustion chambers disposed on opposite sides of the piston.

Each secondary chamber may therefore be provided with its own inlet for combustible fluid, the arrangement being such that the combustible fluid is combusted on each stroke of the piston.

The arrangement may be such that, for the first chamber, each stroke of the piston includes a fluid intake phase and a subsequent combustion phase.

The arrangement may further be such that for the second chamber, the corresponding stroke of the piston includes an implosion phase coinciding with the intake phase of the first chamber and vice versa.

The free piston engine may include an induction current generating device comprising a conductive coil and an inductive body of magnetic or magnetisable material movable relative to the coil to induce an electrical current in the coil.

In one embodiment, the inductive body is provided by the piston.

Alternatively, the inductive body may be connected to and movable with the piston.

Further, the inductive body may be connected to the piston via a connecting shaft.

Each chamber may be provided with a spark plug.

The spark plugs may be provided with electrical current from the induction current generating device.

Alternatively or in addition, the engine may be provided with a battery connected to the induction current generating device and/or the spark plugs.

The piston may be connected to a crank means comprising a crankshaft connected to a flywheel via a crank.

The crankshaft may extend through a slot defined in a side wall of the cylinder.

Said fluid inlet may be connected to a source of combustible fluid via an inlet tube comprising a tube of insulating material such as Teflon or the like surrounded by a negatively polarised conductive braid, the arrangement being such that charged hydrogen particles are forced along the inlet tube towards the inlet.

According to a second aspect of the invention there is provided a vehicle including a free piston internal combustion engine according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further by way of non-limiting examples with reference to the accompanying drawings wherein: figure 1 is a longitudinal sectional view of a free piston internal combustion engine according to a first embodiment of the invention with a piston in a top dead position ; figure 2 is the same view as that of figure 1, with the piston intermediate the top dead and lower dead positions; and figure 3 is a longitudinal sectional view of a free piston internal combustion engine according to a second embodiment of the invention also with its piston in the top dead position.

In the drawings, like components are given the same numbers.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to figures 1 and 2, a free piston internal combustion engine according to a first embodiment of the invention is generally designated by reference numeral 10.

The free piston engine 10 comprises a cylinder 12 defining a combustion chamber 14; and a piston 18 disposed inside the chamber 14. The piston 18 divides the combustion chamber 14 into two secondary combustion chambers 14.1 and 14.2 respectively, disposed on opposite sides of the piston 18.

The secondary combustion chambers 14.1 and 14.2 are each provided with an inlet 16.1 and 16.2 for combustible fluid ; and a spark plug 20.1 and 20.2, respectively. The free piston engine 10 is also provided with a battery (not shown) connected to the spark plugs 20.1 and 20.2.

The piston 18 is freely reciprocatingly movable inside the chamber 14 on combustion of the combustible fluid, between a top dead position (TDP) shown in figure 1 and an opposite lower dead position (LDP). In figure 2, the piston 18 is shown intermediate the TD and LD positions.

The piston 18 is further connected to a crank means 22 comprising a crankshaft 22.1 connected to flywheel 22.2 via a crank 22.3. The crankshaft 22.1 extends through a slot 24 defined in a side wall 26 of the cylinder 12.

Each inlet 16 is connected to a source (not shown) of combustible fluid containing predominantly hydrogen and oxygen, via an inlet tube 28. The inlet tube 28 comprises a tube of insulating material such as Teflon or the like surrounded by a negatively polarised conductive braid (also not shown), the arrangement being such that charged hydrogen particles are forced along the inlet tube 28 to the inlets 16.

In use, when the piston 18 moves from the LDP to the TDP, inlet 16.1 is opened to fill the first secondary combustion chamber 14.1 with combustible fluid, during a first or fluid intake phase of movement of the piston 18 to the TDP. As will become evident below, this first phase coincides with an implosion phase in the second secondary combustion chamber 14.2. Subsequently, while the piston 18 is moving towards the TDP, the inlet 16.1 is closed and the spark plug 20.1 fired to combust the combustible fluid in the first secondary combustion chamber 14.1. This drives the piston 18 further towards the TDP during a combustion phase.

Owing to the nature of the combustible fluid, the combustion phase is followed by an implosion phase, which pulls the piston 18 backwardly to the LDP, while the second inlet 16.2 is opened to allow combustible fluid to flow into the second secondary chamber 14.2. This in turn constitutes a first fluid intake phase of the piston's movement towards the LDP. Subsequently, the inlet 16.2 is closed and the second spark plug 20.2 fired to combust the combustible fluid in the second secondary combustion chamber 14.2, thus driving the piston 18 further towards the LDP during a combustion phase. Owing to the nature of the combustible fluid, the combustion phase in the second secondary combustion chamber is followed by an implosion phase in the second secondary combustion, which pulls the piston 18 back towards the TDP. The above procedure is repeated rapidly and in close succession to drive the piston 18 reciprocatingly at a relatively high speed between the LDP and the TDP.

This reciprocal movement of the piston 18 is used to drive the crankshaft 22.1, which in turn rotates the flywheel 22.2 via the crank 22.3.

Referring to figure 3, a free piston internal combustion engine according to a second embodiment of the invention is generally designated by reference numeral 10B.

The engine 10B is similar to the engine 10, with the exception that the engine 10B includes an induction current generating device 30. Instead of a crank means 22, the device 30 includes a conductive coil 32; and an inductive body 34 of magnetic or magnetisable material. The inductive body 34 is located inside the coil 32 and connected to the piston 18 via a connecting shaft 36. The inductive body 34 therefore moves reciprocatingly inside the coil 32, when the piston 18 moves as hereinbefore described, to induce an electrical current in the coil 32. In accordance with an embodiment, which is not shown, the piston itself can provide the inductive body, with the coil being wound around the cylinder substantially as described in the above USA patent 6,199, 519.

The current, which is generated by the current generating device 30, can be used for a plurality of purposes. For example, the spark plugs 20.1 and 20.2 in each combustion chamber 14.1 and 14.2 can be supplied with electrical current

or the battery can be charged. Further for example, electrical motors (not shown) can be driven.

It will be appreciated, that owing to the fact that water is the major combustion product and no harmful exhaust gases are generated during combustion, no exhaust system is necessary. The applicant foresees that all that would be required is a liquid outlet comprising a water trap 38; a drain 40, extending from the trap 38; and a valve 42 disposed in the drain 40.

It will be appreciated further that variations in detail are possible with a free piston internal combustion engine according to the invention without departing from the scope of the appended claims.