AU703335B2 - Low compression ratio internal combustion engine - Google Patents

Description

Technical Field The present invention relates to rotary machines and more particularly to internal combustion engines having linear piston motion producing a rotary output.

Background Art This invention is related to developments by the present applicant in connection with International Patent Applications PCT/AU89/00275; PCT/AU91/00224; PCT/AU94/00529 and PCT/AU94/00569 the contents of which International patent applications are incorporated herein by reference and nominated as split-cycle arrangements.

As herein defined a "split cycle" arrangement is a rotary machine of the general form shown in US patents 5146880 and 5279209 but not limited to the specifics of the embodiments as shown in those specifications. In particular, a machine having a primary axis and comprising: a plurality of radially reciprocable pistons disposed radially of said primary axis; and a circular array of lobed shafts constrained for orbital motion about said primary axis, each shaft being rotatable about a respective secondary axis parallel to the primary axis, the shafts being rotatably driven by drive means at a rate being a predetermined proportion of their orbital rate, and the planes of the lobes lying approximately in the radial plane of the pistons, and wherein during the rotation and orbit of the shafts and reciprocation of the pistons each piston is connected with at least one lobe for rotation and orbit of the shaft in unison with reciprocation of that piston, or, in an alternative, a machine having a primary axis and comprising: a plurality of radially reciprocable pistons disposed radially of said primary axis; and a circular array of lobed shafts constrained for orbital motion about said primary axis, each shaft being rotatable about a respective secondary axis parallel to the primary axis at a rate being a predetermined proportion of their orbital rate, and the planes of the lobes lying approximately in the 7 g radial plane of the pistons, and wherein during the rotation and orbit of the shafts and reciprocation of the pistons each piston maintains substantially continuous contact with at least one lobe throughout each cycle of reciprocation of that piston; or a machine having a primary axis and comprising: a plurality of radially reciprocable pistons disposed radially of said primary axis; and a circular array of lobed shafts constrained for orbital motion about said primary axis, each shaft being rotatable about a respective secondary axis parallel to the primary axis at a rate being a predetermined proportion of their orbital rate, and the planes of the lobes lying approximately in the radial plane of the pistons, and wherein during the rotation and orbit of the shafts and reciprocation of the pistons each piston maintains substantially continuous contact with at least one lobe throughout each cycle of reciprocation of that piston, and further wherein there is a transition without substantial time delay, between each successive cycle of reciprocation of each piston defined by the period between contact and separation of respective successive lobes and said piston and wherein said pistons are arranged in pairs, the pistons of each said pair pumping fluid from one of the other in response to piston reciprocation so as to maintain substantially asynchronous reciprocation of the pistons of each pair is hereinafter referred to as a "split-cycle" machine." By employing lobed shafts as the means of transmission of linear piston motion to rotary output motion a wide range of varying piston motions can be achieved simply by varying the shape of the lobes. A particular advantage arises in that by appropriate designing of the profiles of the lobes on the shafts the pistons can be controlled in such a manner as to vary their dwell time at any position between top dead centre and bottom dead centre as well as at those limits of piston motion.

A particular advantage arises where the piston dwell time is extended at its top dead centre position to facilitate combustion of the fuel charge while the piston remains at that location to enable maximum energy transmission to the piston and thereby achieve the best possible conversion of combustion energy to engine output power and torque.

In previous disclosures rotary machines of this form have contemplated the use of 50mm diameter pistons having a 6mm stroke.

Typically, the combustion chamber, being the volume above the piston at top dead centre within the cylinder and head of the engine or machine, has been sized at 2mm high with a 50mm bore.

With a 50mm diameter piston and a 6mm stroke the swept volume is approximately 11.8cc for each cycle of motion of the piston while the head volume of the cylinder with the piston at top dead centre is approximately 4000 cubic millimetres. The effective cubic capacity of the cyclinder is eg 10.3cc being the swept volume of the cylinder after closing the exhaust pot slit in the cylinder's sidewall as the piston travels toward TDC. *0 0* WO 96/17160 PCT/AU95/00787 2 Disclosure of Invention Conventional engine technology leads to a combination of an oversquare engine design coupled with a high compression ratio in order to achieve improved engine output while the present inventive development has found that by reducing the compression ratio coupled with an appropriately designed dwell time at top dead centre an improved engine output can be achieved. The dwell time is readily modified by varying the shape of the lobes on the lobed shafts of the split cycle engine.

In one example where the compression ratio was changed from 4 to 1 to 1.75 to 1 a marked increase in engine performance was achieved.

In the case of an embodiment with a 50mm diameter piston and a 6mm stroke with a 2mm high combustion chamber it was varied so as to provide an 8mmn high combustion chamber at top dead centre the compression ratio had thereby changed from 4.1 to 1.75 to 1 which was coupled to a dwell at top dead centre equivalent to 720 of crank rotation in a conventionally cranked engine. In this embodiment a marked increase in engine performance was achieved.

An analysis of the described embodiment shows that the volume of fuel and air charge in the lower compression ratio engine provided a charge volume at the top dead centre of approximately 16,000 cubic millimetres with a compression ratio of 1.75 to 1 as compared with 4000 cubic milliimetres in the engine with a compression ratio of 4 to 1 while the cylinder pressure at top dead centre was reduced from 80 pounds per square inch for the 4 to 1 compression ratio engine to 50 pounds per square inch for the engine with a 1.75 to 1 compression ratio. The ratio of the charge volume for the 1.75:1 compression ratio engine to that of the 4:1 engine is 7:4 being the ratio of the combined cylinder and head volumes with their pistons at bottom dead centre.

An analysis of the functional operation of the differences between these versions shows that considerably more fuel energy is supplied per firing of the lower compression ratio engine as compared to that with the higher ratio and provided that the additional fuel energy is efficiently converted to apply force to the piston then a greater torque is achieved at the engine output shaf than from the higher compression ratio engine.

The present invention has established that by a combination of control of the piston dwell time during combustion of the fuel charge a substantial lowering of the compression ratio relative to that of conventional cranked piston engines results in a previously unanticipated increase in engine torque.

The range of compression ratios contemplated by the present invention is from 1:1 to 4:1 in the environment of an internal combustion engine constructed in accord with the invention disclosed in one or more of our aforementioned International patent applications.

A low compression ratio engine of the first aspect of the invention enables improved power output frown the engine but the lack of a squish effect due to low compression and lack of turbulence requires extended dwell of each piston at top dead centre for a complete burn of each fuel charge.

To control combustion more effectively and achieve a substantially complete burnout of each fuel charge it is preferred to mount a high intensity I:" combustor adjacent each cylinder to take either a full or partial fuel/air charge to be ignited to drive the piston.

In another aspect the present invention contemplates a low: compression engine of the kind described above comprising a high intensity combustor or pre-combustion chamber having a combustion volume adjacent each cylinder of the engine wherein a fuel/air mixture of a combustible charge is ignited in the combustion volume so that the charge expands into the cylinder as it burns within the combustion volume to apply pressure to the piston within the cylinder. Combustion in the combustion volume of the high intensity combustor commences slowly as expected under low compression conditions and intensifies during the piston dwell at TDC. As the piston moves away from TDC the combustion gases expand into the cylinder through a venturi increasing velocity and turbulence of those gases. The speed of the flange front increases and substantially complete combustion is achievable.

In an embodiment of this aspect. stratified charges may be added to the cylinder housing the piston. Such charges could include such as a low grade

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fuel and air mixture. atomised water or excess air or other additives as understood by those skilled in the art.

The present invention has determined that by a combination of control of the piston dwell time during combustion of the fuel charge and a substantial lowering of the compression ratio relative to that of conventionally cranked piston engines there arises a previously unanticipated increase in engine torque.

Brief Description of Drawings The present invention will now be described by way of example with reference to the accompanying drawings. in which:- Fig. 1 is a schematic cross-sectional view of single cylinder-piston of an existing engine of the type with which the present invention is concerned. Fig. 2 is a schematic cross-sectional view similar to Fig. 1 but configured with a compression ratio in accord with the present invention.

Fig. 3 is a plan view of the head of an engine in accord with an: embodiment of a second aspect of the invention: Fig. 4 is a radial cross-sectional view IV-IV of the head and upper cylinder of Fig. 3: and Fig. 5 is a pictorial representation of another embodiment of the second aspect of the present invention.

*555 Best Modes In both Figs. 1 and 2 the piston 20 drawn in solid lines is shown at TDC while that in dashed lines is the piston at BDC. The stroke of the piston 20 in Figs 1 and 2 is the same at 6mm and the cylinder wall slit ports for intake and exhaust have not been depicted.

In the embodiment of Figs. 3 and 4. a high intensity or pre-combustion chamber having a combustion voltage 10 is mounted atop head 11 and is connected with the cylinder 12 housing piston 13 via venturi 14. Pressurised fuel and air is injected into volume 10 via passage 16 while a spark plug is mounted to volume 10 via threaded bore 15. Cylinder 12 contains an exhaust slit 17.

Fig. 5 is a schematic representation of another embodiment of the second aspect of the invention where like parts are similarly numbered to their equivalents in Figs. 3 and 4. The operation of this embodiment which shows a piston 13 and cylinder 12 mounted to a split cycle linear to rotary power transmitter 30 is similar to Fig. 4 with high intensity combustion chamber volume 10 being fitted with a spark plug 31 and fed with supercharged air via inlet 32 and fuel via injector 33.

a *aaft* WO 96/17160 PCT/AU95/00787 Upon ignition of a relatively low flash point fuel, such as gasoline fed by injector 33 into volume 10 the flame front and burning mixture passes through venturi 14 into cylinder 12. Cylinder 12 is charged with a larger charge of a lower grade. higher flashpoint fuel via injector 35 and supercharged air via inlet 34 at, say. 20 psi. That charge of lower grade fuel.

such as diesel, is ignited by the burning mixture entering via venturi 14.

Such an arrangement enables low grade fuels to be burnt at low compression ratios while still achieving substantially complete combustion.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are. therefore. to be considered in all respects as illustrative and not restrictive.

Claims (7)

1. A split cycle internal combustion engine having a plurality of pistons reciprocable within corresponding cylinders and having an extended piston dwell at the top dead center, wherein each piston and cylinder has a bore and stroke operative to provide a compression ratio in the range of greater than 1:1 to 4:1.

2 An engine as claimed in claim 1, wherein a combustible mixture is fed to cylinders of the engine under pressure greater than atmospheric.

3. An engine as claimed in claim 1 or 2, comprising a pre-combustion chamber adjacent each cylinder of the engine, each said pre-combustion chamber being adapted to feed combustion products to a respective cylinder.

4. An engine as claimed in claim 3, wherein the combustion products from each combustion chamber are fed into each respective cylinder to ignite a fuel/air mixture of which the fuel is of a lower grade, higher flashpoint than the fuel ignited in the pre-combustion chamber.

An engine as claimed in claim 4, wherein compressed air is fed into each cylinder as part of the fuel/air mixture.

6. An engine as claimed in any one of the preceding claims, wherein the dwell of each piston at top dead centre is equivalent to 700 of crank rotation in a conventionally cranked internal combustion engine.

7. A split cycle internal combustion engine as claimed in any one of the preceding claims substantially as hereinbefore described with reference to the accompanying drawings. Dated this fourteenth day of January 1999 SPLIT CYCLE TECHNOLOGY LIMITED Patent Attorneys for the Applicant: F B RICE CO