AN IMPROVED FREE-PISTON ENGINE

FIELD OF THE INVENTION

This invention relates to internal combustion engines. More particularly although not exclusively it discloses an improved form of free-piston engine.

BACKGROUND OF THE INVENTION

With known prior art free-piston engines such as those described by M. Goertz and L. Peng in March 2000 SAE Paper 2000-01-0996, entitled FREE-PISTON ENGINE ITS APPLICATION AND OPTIMIZATION, and Galileo Research, Inc. at www.galileoresearch.corn, 1999 entitled FREE-PISTON ENGINE-GENERATOR TECHNOLOGY the gas enters the combustion chamber via intake slots through the wall of the cylinder sleeve. This is typical of the method used on most conventional two stroke internal combustion engines. The disadvantage of such intake arrangement is that as the piston rings slide over the intake slots (twice during each stroke) the radial support area is reduced and a slight ring deformation occurs. The deformation results from the elasticity of the unsupported ring material when subjected to radial forces imposed by gas pressure and the pre-tension in the rings. This deformation accelerates the wear rate of the rings and cylinder sleeve and is partly responsible for abandonment of the two stroke engine in modern passenger cars.

It is also known to provide a valve-in-pistσn arrangement in a reciprocating piston crankcase engine as described in Australian patent application 63021/99 by E. Wechner.

The motion of the piston in the free-piston engine is largely controlled by the second- order mass-spring system comprised of the piston mass and the spring effect of the combustion chamber gases. For a given stroke, the operating frequency of a free- piston engine is therefore affected by the mass of the piston. As with all cyclic energy conversion devices, a higher operating frequency leads to higher power density (so long as the increased frequency does not have a detrimental effect on other aspects of the system). By minimizing the mass of the piston, the natural frequency of the piston can be increased. Therefore, a higher operating frequency can be achieved with multiple smaller free-piston modules (preferably four), rather than one larger unit.

An additional benefit of using four free-piston modules in parallel is that the forces and moments of the individual modules can be cancelled to achieve dynamic balance. In other words, a fully balanced free-piston engine requires 8 cylinders (consisting of 4 free piston modules, each with 2 cylinders). However, this leads to large number of parts (i.e. 8 exhaust heads, 8 cylinder liners, 8 piston heads).

SUMMARY OF THE INVENTION

It is therefore an object of this invention to ameliorate the aforementioned disadvantages and accordingly an internal combustion engine is disclosed having a housing with at least one cylinder with electricity generating stator means fixed relative thereto, a piston arranged in said cylinder for cycles of reciprocating compression and power strokes between positions at top dead centre and bottom dead centre, inlet valve means for introducing an air or fuel mixture into said cylinder prior to said compression strokes, outlet valve means for the expulsion of exhaust gases following said power strokes and said piston being linked to a linear actuator for

-3- movement therewith so that during operation of said engine the reciprocating strokes of said piston and linear actuator with respect to the stator means generates usable electrical energy and wherein said engine further includes spring means acting on the piston to bias said piston towards top dead centre on said compression strokes.

Preferably said inlet valve means is located in said piston and comprises a portion of the head thereof.

It is further preferred that the spring biasing means comprises a coil spring acting between underside flanges of said piston and a base of said housing.

It is further preferred that the spring coil cross-section varies, whereby said spring is adapted to exert a force on said piston which increases exponentially during displacement toward bottom dead centre.

It is further preferred that said coil cross-section increases along the length of the spring extending away from said piston.

It is further preferred that the coils of said spring are flattened to provide a contact area between adjacent coils.

BRIEF DESCRIPTION OF THE DRAWINGS

The currently preferred embodiment of the invention will now be described with reference to the attached drawings in which:

Figυre 1 shows a cross-sectional schematic view of a single cylinder free- piston engine along the centre axis of the cylinder with the piston at bottom dead centre, and

Figure 2 shows a cross-sectional schematic view of a free-piston engine along the centre axis of the cylinder with the piston at top dead centre.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to figure 1 the main components of the engine are the cylinder block or housing 1, cylinder IA, cylinder head 2, piston 3, linear actuator 4, electricity generating stator 5, biasing spring 6 and reed valve 7.

As with prior art free-piston engines the cylinder preferably fires in the two stroke cycle and the resulting reciprocating linear motion is converted into electrical energy by means of relative movement between the linear actuator and stator assemblies.

In accordance with the currently preferred form of this invention the inlet valves comprise poppet valves 8 which are located in the head of the piston 3.

In figure 1 the piston 3 is shown at the end of the expansion or power stroke in cylinder IA. Both the inlet valves 8 and exhaust valves 9 are thus open to enable the two stroke gas exchange or scavenging process to take place. The intake gas 10 for this scavenging process was compressed in the linear actuator compression chamber

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11 during the preceding expansion stroke. The pressure obtained for the intake gas 10 is sufficient to open the inlet valves S in the piston 3 against both the force of valve springs 12 and the opposing kinetic force from deceleration of the valve mass 8 at the end of the poλver stroke. The incoming pressure of this gas 10 assists the evacuation of the exhaust gas through the exhaust or outlet valve 9 and port 13. There is a control solenoid 14 in the cylinder head 2. This opens the exhaust valve 9 for selected variable time periods to optimise the efficiency of the gas exchange at a given power consumption. For example, at low power consumption only a small amount of exhaust gas is evacuated through the valve 9. This in turn limits the entry of intake gas 10 to the mass required to maintain the desired idle speed of the engine. Such arrangement releases a minimum amount of pressure in the cylinder combustion chamber 15 during the gas exchange process to reduce pumping losses. At maximum power the valve 9 is held open long enough to evacuate substantially all of the exhaust gas. This allows the maximum mass of fresh intake gas 10 to enter the combustion chamber. As with the prior art valve-in-piston engine the inlet valve 8 is held closed during the subsequent compression stroke against the opposing kinetic forces of deceleration by gas pressure in the cylinder combustion chamber 15. In this bottom dead centre position of figure 1 at the end of the power stroke the biasing spring 6 is completely compressed between the piston underside flanges 15A and the base 15B of the cylinder housing and adjacent coils 15C of said spring are in contact to prevent further downward displacement.

As an alternative to the above described arrangement for the inlet valves 8 a kinetic valve actuator mechanism may be fitted as described in the applicant's pending Australian patent application 2007901265 filed 9 March, 2007.

As an alternative to the above described arrangement for the inlet valves 8 a kinetic valve actuator mechanism may be fitted as described in the applicant's pending Australian patent application 2007901265 filed 9 March, 2007.

The piston 3 is shown by figure 2 in the ignition position at top dead centre after having completed a previous compression stroke. This compression stroke was effected with the potential energy stored in the biasing spring 6 at a bottom dead centre position as shown in figure 1. During this compression stroke fresh intake gas 17 was drawn by the linear actuator 4 in through the inlet 16 and the reed valve 7 into the compression chamber 1 1. During the next expansion or power stroke of piston 3 after ignition this gas 17 will be compressed in chamber 1 1 to comprise the subsequent intake gas charge 10 for the combustion chamber 15 of cylinder IA. Potential energy will also again be stored in the biasing spring 6 for the succeeding compression stroke.

The linear actuator 4 is equipped with gas seal 18 to facilitate its function as a compressor piston for the gas exchange process. This eliminates the need for a external intake gas charging device. Between the electricity generating stator 5 and the linear actuator 4 there is also a cylindrical sleeve 19 which provides a dynamic mating surface for the gas seals 18. This sleeve 19 should be electrically non- conductive, non-magnetic and sufficiently thin to avoid adverse effects on the generating process. Suitable material may include ceramics or high temperature composite plastics which may be either deposited on the surface or pressed into the stator 5.

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Preferably the biasing spring 6 is composed of steel coils 15C which increase in cross- section as shown along the length of said spring from the piston to the cylinder base. This produces an exponentially increasing force on the piston as first the upper smaller cross-sectional and then the larger lower cross-sectional coils compress together. In order to provide a stable load bearing contact surface between adjacent coils they are preferably flat sided as shown at 19A and 19B.

Other components of the preferred embodiment as shown in the drawings are as follows:

20 Stator Housing

21 Electricity Generating Coils

22 Permanent Magnets

23 Linear Actuator Bearings

It will be thus be appreciated that this invention at least in the form of the embodiment disclosed provides a novel and useful improvement to free-piston internal combustion engines. Clearly however the example disclosed is only the currently preferred form of the invention and a wide variety of modifications may be made which would be apparent to a person skilled in the art. For example the shape and configuration of the valves and linear actuator gas compressor may be changed according to engine design requirements. Also, while the engine described has only one cylinder the invention could be extended to multiple cylinder designs.

Expressions such as "upward", "upper", "lower", "top" and "bottom" etc. refer to the described device as illustrated in one of many possible operational configurations and therefore are not to be read as necessarily limiting.