A TWO-STKCKE CYCLE ENSINE

DESCRIPTION: ^" This description refers mainly to a single- cylinder motor, this containing all the essential features .of the invention, being applicable to multi-cylinder formats.

Fig's a. and b. of diagram 1 are respectively a sagittal sectio and a plan view of the basic motor, including the gas-flow (indicated by thick arrows ^ ) which characterises the transfer/scavenge phase of the 2-stroke cycle of operation.-

The piston (A), being reciprocally mounted ( in this example to a conventional crankcase assembly via a connecting rod and gudgeon pin (K)), is depicted at the bottom of it's stroke, within the cylinder (W) . The region (F), in conjunction with a conventional 2-stroke type sealed crankcase as illustrated, or alternative mechanism, and being equipped with an induction valve mechanism (in this example taking the form of piston porting (I) ) ,comprises a scavenge charge pump, charge delivery being made through the piston crown via the PISTON TRANSFER VENTURI TUBE (B) , then radially into the combustion chamber in the cylinder head (C), and from thence, down through the annular sectioned working displacement (E), purging the exhaust gases radially through exhaust ports (G), distributed around the cylinder walls.

Specific to Fig, a., the exhaust gases are projected from the elongated exhaust ports (G) tangentially into a circularly sectioned gas expansion and collection chamber (H). Smooth entry of the exhaust gases into this manifold is facilitated by maintaining the lineal velocity of the gases throughout the length of the ports (G), such flow resistance as is provided by these ports also contributing to the flow regime in the

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working displacement (E) . Thus Fig.a..illustrates the ports (G) having a convergent cross-section along the flow path, this form being applicable to the toroidal scavenge belt shown in plan 1, Fig. b.

As illustrated in drawing lb.,the chamber (H) is of a basically toroidal form, surrounding the cylinder, and having one or more outlets to any external exhaust system. The exhaust gases follow a roughly helical path through (H).

Returning to drawing la.,and the motor's operation: the piston ^■O transfer venturi tube (B) is arranged to slide, as the piston ascends, into a secondary bore (D) in the upper part of the cylinder head (C), such conjunction creating a suitably gas- tight seal between regions (F) and (E) (thereby permitting the autonomous functions of compression and expansion in (E) and (F)^ ) and furthermore, functioning as a sleeve valve controll¬ ing the transfer of air and/or combustible charge from (F) to . (E). The secondary bore (D) will hereafter be referred to- ^r as the counterbore .

Regarding Practical Implementation:

O The piston (A) . having the transfer venturi tube (B) formed integrally, has a full skirt (i.e. having no cutaways) and is fitted with one pinned,, plain compression ring. The piston crown is -of a naturally rigid form, not requiring additional reinforcement.

In order to facilitate the clean and precise entry of (B) into the counterbore, the piston fits the main bore closer than normally. To prevent siezure, due to thermal expansion, the piston -is made from a material having a suitably lower thermal expansion coefficient than that of the bore.

60 Although other possibilities exist, the combination of a steel

piston and an aluminium (alloy) bore offers these advantages:-

- Expansion coefficients well matched to typical temperature rises.

- Ease of manufacture with ready fabrication of the pisto Low overall motor weight.

- High combustion temperature capability.

Steel's high tensile strength minimizes the cross-, sectional thickness of (B), and hence the width of the annular recess in (C), and consequently, the scavenge -JO flow disturbance at this point.

Improved wear characteristics; the altered ratio of hard/soft metal sliding surfaces (together with the reduced piston play) favours a lower wear rate, such wear occuring mainly on the piston. Thus, whereas conventional motors wear typically faster in the bore, subsequently requiring re-boring and the fitting of a_ oversize piston, this motor may well only require periodic replacement of the piston(s).

The gudgeon pin (K) is carried in webs traversing the piston gO interior and being connected to the piston skirt. These webs' design minimizes disruption of the gas flow through the piston. Again, the use of steel enables a minimal cross-sect¬ ion.

Multiple exhaust ports (G), distributed equally, circu ferent- ially about the cylinder, permit a large total port area with minimized port height and transfer phase duration. These parts are separated by webs (T), which provide lands in the bore to support the piston ring(s) and structural support to the motor.

Optimum exhaust port height is approximately of the piston ^Q stroke, permitting almost 50% greater maximum torque than typical modern 2 stroke motors. Circumferential induction porting (I), of similiar design to the exhaust porting, offers superior scavenge pump volu¬ metric efficiency, obviating the need for supplementary valving, such as unreliable reed or disc valves.

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Drawing 2 refers to an aircooled, vertical, single-cylinder application, fitted with a conventional crank assembly and a flywheel/ magneto.

( ) - is a one-piece aluminium alloy casting incorporating; the cylinder-head and counterbore, the main cylinder barrel (through to the crankcase mounting flange and including all ports), the.top half of the expansion chamber (H), the webs (T) (Drawing lb.), and cooling fins (Z).

An induction manifold, (M) directs air to the ports (I ⁾ . ⁽ Drawing la.). Structural webs are incorporated into (M) and coincide vertically with the webs (T). (These webs do not provide ring lands in the bore, these being provided by the cylinder spigot on (L) ).

Throu rtstuds (V) are housed within the webs (T), securing the cylinder assembly (L) to the crankcase, and sandwiching the lower half of the expansion chamber (H),and (M) .

Twin spark plugs (S) are provided for ignition.

The cylinder-head core (R) is further described by Fig. a. , Drawing 3 , which is a sagittal section of the cylinder head assembly.

(R) - The cylinder-head core, having a series of deep, closely- spaced circumferential grooves on its outer surface, and being mounted by means of an integral flange, functions in conjunct¬ ion with (B) as a labyrinth seal, fulfilling the requirement set out in lines 42 - 47 incl.

The seals' action is that of a series of flow constrictions interspersed with small expansion chambers (the grooves),the number of stages being proportional to the depth of insertion of (B) into (D). It is analagous to the action of an electri¬ cal, multi-stage EC filter, in which capacitors represent the grooves; resistors,, the constrictions; Vj _n , the compression

and combustion pressur es ; I o ,u, t, , the seal leakage :

I30 Contact between (B),(R) and (D) is incidental to the seal's operation, and should be fairly minimal. Cold clearance be¬ tween (B) and (D) is approximately twice that between the piston and main bore as is that between (B) and (R).

The mechanical requirements for (R) are fairly undemanding as a are the thermal demands. Ceramic suggests itself for product¬ ion and would also thermally insulate the fuel carrying parts housed within and on top of (R).

N.B. Plastics based on tetra fluoroethylene, or similiar compounds, are NOT recommended because of the extreme inde-- f 4-0 struetibili-ty and- carcinogenicity of the monomers, and the impossibility of preventing their release into the environment if used in motors.

The following describes a LIQUID FUEL ASPIRATION SYSTEM for a motor as described. The design introduces the system of power control by strict regulation of the volumetric portion of the combustion chamber occupied by combustible charge, the rest of the chamber containing only incombustible charge.

The air supply to the scavenge pump is normally un-throltled; hence, disregarding the reduction of volumetric efficiency. 0 at high speeds, a constant gas volume ( a little less than the piston's displacement) is delivered, at every reciprocation of the piston, to purge the working displacement, (E). This, and the relative lack of turbulent interaction between the spent and fresh charges, improves the purging at all power levels and produces a relatively constant and maximised absolute charge compression pressure at ignition (regardless of i.m.e.p.) In conjunction with the maintenance of a constant fuel/air ratio in the combustion region(s), the following results _:

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- Constantly optimal combustion conditions; _j β - Minimized exhaust hydrocarbon emissions;

Raised fuel efficiency; - Reduced misfiring at low power levels (2-strokes); Constancy of combustible mixture during acceleration or deceleration obviates the need for auxiliary mixture compensation.

The following refers to Drawing 3:

The manner in which the combustion chamber is divided into sectors containing combustible and inert charges is shown in Fig. b., each half of which illustrates half of two typical atomiz- 170 ation patterns, as they would appear looking up the cylinder at the head.

The annular jet assembly (J), generates these patterns auto¬ matically, the angular width of the combustion sector being determined simply by the rate of fuel flow permitted by the fuel regulator valve (0), (this valve being linked to an ex¬ ternal speed control) .

The jet assembly (J) comprises a hollow annular portion (Y) , which receives fuel from a central hub via hollow support - webs (N). (Y) has a series of spraying holes drilled through 120 its outer flared skin, which are sized to regulate fuel flow

(and hence the fuel/air mix), and which, are subjected to a depression (relative to the fuel pressure within (Y) )due to the acceleration of the scavenge charge through (B), and the action of a fuel pressure modulator(further reference).

Note that during the initial moments of the scavenge charge transfer, the piston tube (B) has not withdrawn from the counter¬ bore (D), sufficiently to expose the outer surface and spray¬ ing holes of (J) to any depression. This ensures that this initial highly-turbulent flow (generated close to (D) ) remains ) -\0 fuel free, and creates a "cushion" of pure air to better main¬ tain the autonomy, of the spent and fresh charge regions.

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Returning to the spray patterns generated by (J); At low flow rates, all fuel entering (¥) will be drawn from the spraying-holes closest to the bottom of (J) (as in the lower h of Fig. b.), while at higher flow rates, the fuel level within (Y) rises, permitting its escape from more holes (as in the upper % of Fig. b.).

' Although not the only possibility, the motor plan shown, having two diametrically opposed combustion sectors, each with its

' ZOO own ignition source, seems sensible. Apart from ensuring complete combustion throughout, this layout balances out the explosive side thrusts applied to the piston.

A fuel pressure modulator maintains the instantaneous fuel supply pressure to the regulator valve (0), (and thence jet as- assembly (J) ) at approximately that of the gas charge in the scavenge pump (F). The modulator is divided into two chambers by a flexible diaphragm (P). The upper chamber (Q), is coupled to the scavenge pump chamber (F) by means of an external pipe. The lower- chamber, (U), is filled with fuel 2.lO through a non-return valve (e.g. a port controlled by a diaphragm, (P), and a light spring) when the pressure in (F) is negative. As this pressure rises so the valve is closed and such pressure is coupled to the fuel in (U) through the diaphragm.

During boththe compression and expansion phases of the 2- stroke cycle, the fuel in the modulator remains static be¬ cause it and the jet (J) are subject to very nearly equal pressures. However, during the transfer phase, the gas flow ^* in (B) results, in a lower pressure at (J) than is being 2 0 applied to the fuel in the modulator, and consequently, a flow of fuel to (J) .

This modulator also has limited pumping and pressure reduction capabilities. Hence typical small plant applications would not necessarily require a separate fuel pump and/or pressure regulator.

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Note: Assuming the external pipe joining (Q) to (F) to be fully unconstrictive, the pressure phase lag due to this pipe's length ( in the order of a millisecond), does not affect unduly the modulator's operation. The deliberate ^ O introduction of restrictions in this pipe can be used to - control fuel/air mixture.

Miscellany:

In a motor as described herein, lubricating oil cannot be introduced via the fuel, but must be metered and pumped as directly as possible to the crankpin bearing, from whence it may splash feed the bore and main bearings. The counterbore is lubricated by oil centrifuged from the scavenge charge as it passes this point.

The fact that the oil is not diluted by fuel, in the crank- 2_Η_J case, aids lubrication, and must reduce the amount of oil lost through atomization.

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