GB2499639A - Internal combustion opposed-piston barrel engine - Google Patents

Abstract

An internal combustion engine, preferably diesel, has at least three pairs of cylinders (six cylinders) 36 to 41 and a drive shaft 11, each cylinder having two opposed pistons 13a, 13b therein, with piston heads facing each other and piston rods of the pistons coupled to the drive shaft through respective opposed cams (12a, 12b, fig.6). The opposed cams (12a, 12b) are profiled (fig.11) such as to ensure that the motion of the pistons in cylinders 39, 40, 41 sum algebraically to zero (fig.12) as do the motion of the pistons in cylinders 36, 37, 38 thus ensuring that piston inertia forces are balanced out thereby reducing vibration and noise. The engine may have more than three pairs of cylinders where the pairs of cylinders are diametrically opposite cylinders and wherein the number of the said pairs of cylinders is a prime number.

Description

DIESEL INTERNAL COMBUSTION ENGINE

This invention relates to a diesel internal combustion (I.C.) engine.

Diesel engines have several advantages over carburetted petrol engines. Spark-distributors, magnetos or sparking plugs are a source of unreliability in petrol engines. The higher specific energy content of diesel fuel means that a greater endurance can be experienced with a full fuel tank. Also, the higher combustion temperature and pressure of the diesel engine results in a higher efficiency, hence lower fuel cost and environmental impact.

The present invention relates to an opposed-piston diesel engine of the barrel type wherein the cylinders lie parallel to and concentric with the drive shaft and the drive from the pistons to the drive shaft are by means shaped cams.

In a conventional internal combustion engine with the cylinders arranged in-line or in a V-formation, the combustion forces impose high levels of stress between the cylinder heads and the crankcase. These forces can be alleviated by matching the weights of the reciprocating masses such as pistons and connecting rods so that the inertia forces arising from the acceleration of these masses partly offsets the combustion forces. However these forces can only be partly in balance and then only at one engine speed and at other engine speeds the forces will be unbalanced and thus a strong crankcase is required.

This disadvantage also applies an opposed piston engine having crankshafts. Referring firstly to Figure 1, a Jumo-type diesel engine is diagrammatically illustrated. Manufactured by the German Junkers company during the 1930s and 1940s, the Jumo diesel engine was a two-stroke aero internal combustion engine with two opposed pistons per cylinder. The pistons came together in the centre of the cylinder at Top Dead Centre (TDC), 1, for fuel injection and combustion, and were furthest apart at Bottom Dead Centre (BDC), 2, for exhaust gas removal and fresh air charging of the cylinders. Each piston drove a crankshaft at opposite ends of the engine, 3 and 4. The two crankshafts were linked by a gear-train 5 to drive a single propeller shaft 6. The major advantages of the design were

1) The combustion forces acted equally on both pistons in opposite directions and the pistons moved with equal but opposite accelerations during the power stroke thereby eliminating the majority of unbalance forces.

2) The air inlet 7 and exhaust gas exit 8 were effected through ports in the extremities of the cylinders which were uncovered as the l

pistons reached the end of the power stroke. Thus conventional poppet inlet and exhaust valves and their associated rocker-arm and tappet assemblies with the associated lubrication, wear and maintenance, were eliminated. Further advantage was gained from the fact that the inlet air was supplied under pressure from a shaft-driven or exhaust gas-driven turbocharger, thus facilitating scavenging of exhaust gases.

Sections (a) to (f) of Figure 1 show the cylinder and pistons at different parts of the cycle.

The placing of the inlet and exhaust ports was such that as the power stroke ended, the exhaust port opened first, allowing exhaust gases to rush out. (a). As the cylinder pressure fell to near atmospheric pressure, the inlet port opened, admitting air at turbo-charger pressure, to scavenge the cylinder of remaining exhaust gases, (b). After the pistons reached Bottom Dead Centre (BDC), on the return stroke, the inlet port closed first, then the exhaust port, and the air was compressed to high pressure, (c, d). As the pistons approached Top Dead Centre (TDC), (e), fuel was injected at high pressure into the space between the pistons and ignited spontaneously, causing the pistons to be driven back again with great force, to drive the crank-shafts, (f).

The Jumo design was produced with six cylinders in-line, to produce very smooth running, but it had three disadvantages;

1, The gear trains to the propeller shaft were expensive, caused transmission losses and added weight.

2, In poppet valve diesel engines the valve opening and closing times can be altered by the shape of the cams which drive them, and the exhaust valve can be closed before the inlet valve, allowing gas at turbo-charger pressure to fill the cylinder before cutting off the inlet gases, thus enabling a concentrated charge of air, which can sustain a larger fuel charge and hence higher power output per stroke. In the Jumo engine the inlet port closes first, so the inlet cylinder pressure is close to atmospheric pressure, and power output per stroke is lower.

3, Whilst the piston acceleration forces of the opposed pistons are balanced, the forces acting on the pistons due to combustion are transferred via the crankshafts to the engine crankcase and as they act in opposite directions they require a very strong construction.

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Figure 2 diagrammatically illustrates another known internal combustion engine, the barrel engine, in which the cylinders 10 lie parallel to the drive shaft 11 and impart rotary motion to the shaft 11 by means of an angled cam 12 mounted on it. The angle of slant of the cam 12 is such that the distance between the extremes of the face of the cam 12 as it rotates, is equal to the stroke of the piston 13. The position of the axial face of the cam 12 varies sinusoidally with the shaft angle. The ends of the pistons 13 push against the angled face of the cam 12, causing it to rotate. Such engines have been made in petrol- ignition or diesel form, with two or four strokes to the firing cycle.

Barrel engines have been built with pistons at one end as in Figure 2, or both ends such as is illustrated in Figure 3. The inlet and exhaust gases enter and leave via conventional poppet valves 14 in the cylinder head 15. The design has the advantage of a small frontal area, which is attractive for aero-engine, low-deck omnibus and marine applications. It also has the advantage of the fact that the drive is direct to the shaft 11, without gearing, and that a plain cylindrical shaft 11 is used, without expensive cranks. There are no connecting rods or big-end or little-end bearings, although there are bearing surfaces 13c required between the end of the pistons 13 and the cam 12. The pistons 13 engage with the cam 12 with a shoe, with roller bearings or with ball-ended sockets as shown in Figure 3.

In addition the cam 12 does impart side forces to the pistons 13, and does require complex machining during manufacture. In practice the cam may have two or more identical cycles of axial variation per revolution as illustrated in Figure 4, in which case there will be two or more firing strokes per revolution of the shaft 11.

This design has the advantage in that the number of moving parts is kept to a minimum and provides improved reliability over internal combustion engines fitted with poppet valves and the associated camshafts, push-rods and rocker arms.

As diagrammatically shown in Figure 5 several cylinders 10 may also be arranged in a barrel arrangement around the shaft 11, comparable to the chambers of a revolver firearm.

According to another existing invention (Reference 1, Redrup and Redrup, March 1955) there is provided an internal combustion engine comprising at least two cylinders each with two opposed pistons therein, with piston heads of the pistons facing each other, piston rods of the pistons being coupled to a drive shaft of the engine through two opposed cams. The two opposed cams have sinusoidal cam profiles. And therefore the performance is similar to the earlier Jumo engine,

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except that the forces on the cams due to combustion and the piston inertia forces are equal and opposite and cancel out. Thus a lightweight casing can be used, but the drive shaft must be dimensioned to carry these axial forces.

According to another existing invention (Reference 2, Renegar, October 1979) there is provided an internal combustion engine comprising at least two cylinders each with two opposed pistons therein, with piston heads of the pistons facing each other, piston rods of the pistons being coupled to a drive shaft of the engine through two opposed cams. The two opposed cams have preferably non-sinusoidal cam profiles. The two opposed cams also preferably have different profiles near the Bottom Dead Centre position to optimize inlet and exhaust gas flow. Furthermore the cams can be shaped to hold the pistons at or near the Top Dead Centre position for sufficient time to enable combustion to be completed at constant-volume thereby improving the thermal efficiency of the engine by making the engine operate on the well-known Otto Cycle as used in the conventional internal combustion engine instead of the usual constant-pressure cycle of a diesel engine.

This previous invention (Renegar) also has the advantage that the cams may be shaped so that the inlet and exhaust ports may be opened and closed at different times to enable the combustion cycle to be optimized, thereby ensuring that a higher thermal efficiency may be achieved than that by conventional sinusoidal profiles. However this previous invention has a serious disadvantage.

The disadvantage of this previous Renegar-type invention is that the non-sinusoidal movement of the inlet-end and the exhaust-end end pistons produce unbalanced accelerations of the pistons at and near Bottom-Dead-Centre and hence unbalanced axial forces leading to excessive vibration. This problem can be partially corrected by the addition of extra balance shafts and weights, thereby detracting from the inherent simplicity and reliability of the design and adding extra weight.

According to another existing invention (Reference 3, Fairney, April 2009) there is a solution to the problem of unbalanced forces in engines wherein the cylinders number four or multiples of four cylinders. This is achieved by modifying the cam profiles so that the unbalanced accelerations of the pistons near bottom dead centre on one pair of pistons are compensated for by the introduction of additional accelerations of the pistons near top dead centre in the other pair of pistons.

The invention described in Reference 3 is however only applicable to engines wherein the total number of cylinder pairs is a multiple of two. It cannot be used on engines wherein the number of cylinder pairs is an odd number. Hence

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engines comprising 3 or 5 or 7 or indeed any prime number M of pairs of cylinders cannot be balanced in this way.

The present invention solves this problem by the use of cam profiles that are designed to ensure accurate balance of engines having a prime number of pairs of cylinders without detracting from the overall improvement in engine thermal efficiency.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to Figures 6 to 10 of the accompanying drawings, in which: -

Figure 6 is a diagrammatic illustration of one form of the previous Renegar diesel internal combustion engine, having two cylinders.

Figure 7 is a diagrammatic illustration showing an idealized working cycle for the previous Renegar type of engine;

Figure 8 is a diagrammatic illustration showing a practical working cycle for the previous Renegar type of engine;

Figure 9 shows an illustration of a typical six-cylinder engine of the present construction providing the features which give improved axial balance of the pistons within the cylinders of the engine.

Figure 10 shows the piston motions of one cylinder of the six-cylinder engine of the present construction shown diagrammatically in Figure 9.

Figure 11 shows the motions of the pistons in the cylinders of the six-cylinder engine of the present construction shown diagrammatically in Figure 9.

Figure 12 shows the residual unbalance motions in the three cylinders represented in Figure 11.

Referring to Figure 6 which shows one form of the previous Renegar engine which uses the two-stroke opposed piston concept, with the cylinders 10 arranged diametrically opposite each other around the drive shaft 11, and with the pistons 13a and 13b acting on two cams 12a and 12b at the front and the rear of the engine respectively.

Because the pistons 13a and 13b in each cylinder act in opposition, there is no net axial force on the shaft 11 due to combustion gases. Thrust bearings 17 are used, which are rated to carry the axial thrust from the drive shaft 11 or other driven load (not shown). Each cylinder 10 fires once per revolution. The pistons

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13 in each cylinder are 180 degrees out of phase with the other. A simple cam disc or protrusion 18 on the shaft 11 initiates the fuel injection to each cylinder, once per revolution, either by direct mechanical drive or by triggering an electronic signal.

Induction of pressurised air and exhaust of combustion gases are through respective ports 19, 20 in the cylinder walls, as in the Jumo design, and there are no poppet valves or gear drive train, except from one end of the shaft 11, as shown at 22 for auxiliaries such as high-pressure fuel pump, oil pump and super-charger. Turbocharging may alternatively be achieved from an exhaust-gas driven turbine 21. The cam 12a is coupled to the pistons 13a which control the inlet ports 19 whilst the cam 12b is coupled to the pistons 13b which control the exhaust ports 20,

Features common to the previous and the present construction include the inlet and exhaust timing.

In the prior Jumo design, the inlet port must open after and close before the exhaust port, as the piston movement bears a fixed relationship to the crank-shaft rotation.

With the present construction and in the previous Renegar-type invention the inlet cam 12a and exhaust cam 12b do not have the same profile and preferably non-sinusoidal profiles are used for each. During the firing stroke, both ports 19, 20 are closed, and it is advantageous for the cams 12a and 12b to have the same profile for this part of the stroke, to equalize axial inertia forces.

Referring to Figure 7, the idealised profiles for the inlet and exhaust cams are shown. At A, the pistons are close together at Top Dead Centre for fuel injection and combustion. Only when combustion is complete do the pistons separate for the power stroke as the gases expand. (B). Toward the end of the stroke the exhaust port opens first. (C) in Figure 7. As the exhaust gases leave, the cylinder pressure falls rapidly. As it falls below supercharger or turbocharger outlet pressure, the inlet port opens, D. This allows air at super-charger pressure to sweep into the cylinder and scavenge out remaining exhaust gases. In most two-stroke engines, some exhaust gases remain in the cylinder, diluting the air and reducing output power. In the present Renegar arrangement, the port timing is such that the cylinder 10 is scavenged by more than its own volume of air, before the exhaust port 20 closes, thus removing all exhaust gases (E). When the exhaust port 20 closes, air continues to enter and reaches super-charger or turbo-charger pressure until the inlet port 19 closes. The air is then compressed by the approaching pistons F, until at or near Top-Dead-Centre, fuel is injected at very high pressure, and spontaneous combustion takes place again, A.

Referring to Figure 6 fuel injection is initiated by a shaft-mounted cam disc or

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projection 18 operating directly on the injector or triggering an electronic signal to the fuel injection control system. Output power is varied by varying the duration of fuel injection and intake airflow. This ability to vary port timing gives an inherent advantage over the Jumo design, with higher power output per cylinder displacement, and higher fuel efficiency.

With the previous Renegar-type invention the idealized cam profiles are as shown in Figure 7a. The net axial motion of the inlet-end and exhaust -end pistons is shown in Figure 7b. It is seen that during period A the pistons are stationary at the Top Dead Centre position. Also, during the inlet and exhaust cycles there is different axial motion of the two pistons. It is this net motion that results in axial vibration and noise.

The instantaneous opening and closing of inlet and exhaust ports is not practicable, and more realistic practical cam profiles are shown in Figure 8. The inlet and outlet piston motions are shown in Figure 8a as 23 and 24 respectively as a function of the cam phase angle which varies from 0 degrees to 360 degrees for one complete firing cycle of the engine. In Figure 8b the piston motions are superimposed to show the difference clearly.

In this manifestation of the Renegar engine the motion starts at the Top Dead Centre position 26 where fuel has been injected and combustion has taken place. The inlet and exhaust pistons 13a and 13b move with preferably but not necessarily equal sinusoidal motions to the Bottom Dead Centre position 27. The exhaust piston 13b continues with preferably but not necessarily a sinusoidal motion to the Top Dead Centre position at 28. The exhaust piston 13b remains in this position for duration 29 and returns to the start position at 26 for the cycle to repeat. From the position 27 the inlet piston 13a is delayed by an angle 30 to a point 31. The inlet piston 13a then follows a preferably but not necessarily sinusoidal motion to the Top Dead Centre point 28 where it stays for duration 29 before commencing the next cycle.

It will be seen from Figure 8 that the positions of the inlet port 19 and the exhaust port 20 are such that the exhaust port 20 opens at a position 32 which is earlier than the position 33 at which the inlet port 19 opens thus allowing exhaust gases to escape before pressurised air from a supercharger or turbocharger enters the cylinder to scavenge away any remaining exhaust gases. It will also be seen from Figure 8b that the exhaust port closes at a position 34 which is earlier than the position 35 at which the inlet port closes thus allowing the inlet air to pressurise the cylinder. This phasing of the opening and closing of the inlet port 19 and the exhaust port 20 is a result of the delay 30 in the motion of the inlet piston 13a. The difference in the motion of the inlet piston 13a and the exhaust piston 13b is shown at 25 and this is responsible for an axial vibration of the Renegar type of engine.

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Additional features relate to shaft bending and engine speed.

In this two-cylinder version of the Renegar engine as shown in Figure 6 there are bending moments on the cams which are transferred to the shaft and hence to the casings via the bearings. The bearings and casings therefore have to be strong enough and stiff enough to carry these bending loads with ease.

In the present invention is provided a means to eliminate these bending forces and enable the bearings and casings to be made of a light construction. Figures 9 and 10 illustrate a typical embodiment of the present invention consisting of the case when M equals 3 wherein there are three diametrically opposite pairs of cylinders but the present invention is applicable to any engine wherein M has a value of any prime number exceeding unity.

In Figure 9 the end view of the cylinder casing shows the disposition of the three pairs of diametrically opposed cylinders. In the isometric view only three cylinders are shown for clarity, 39, 40 and 41, adjacent to each other in the engine casing and disposed at 60 degrees around the drive shaft 11. In this construction there are two identical cycles of the cam profile around each cam, thus the pistons 13a and 13b in one cylinder 41 are shown at the Top Dead Centre position and the pistons in cylinders 39 and 40 are shown at intermediate positions. The driving forces from these illustrated pistons impose bending moments on the shaft, but these are counteracted by the pistons in the cylinders 36, 37 and 38 which are diametrically opposite cylinders 49, 40 and 41 respectively thereby eliminating any bending loads on the shaft 11.

The four-lobed cams produce two cycles of axial movement per revolution as shown previously in Figure 4. This arrangement gives two firing cycles per engine revolution, which means that for a given power output shaft speed will be halved. This is particularly advantageous for a diesel engine used for driving a propeller for a marine vessel or aeroplane but not limited to these applications.

Diesel engines are most effective at high cylinder firing rates, whilst for water-borne vessels and aircraft the maximum permitted shaft speed is determined by the propeller tip-speed approaching a speed at which cavitation occurs in the case of a vessel or the speed of sound in an aircraft. In principle a larger number of cycles of movement can be used on the cams to reduce engine speed further. If S is the number of firing strokes per second per cylinder, and L is the number of cam lobes, the shaft speed N is given by

N = 120xS /L revolutions per minute.

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The present invention improves on the previous Fairney invention and extends it to engines wherein the numbers of equally-spaced opposite cylinder pairs M can be any prime number such as 3, 5, 7, etc.

Figure 9 shows one manifestation of the engine of the present invention. The invention is applicable to any engine having a prime number M of pairs of opposed cylinders arranged around a central drive shaft 11. The Figure shows a construction in which M = 3 and which incorporates three pairs of diametrically opposed cylinders 36, 37 and 38 and 39, 40 and 41 in which cylinder 36 is diametrically opposite cylinder 39 and cylinder 37 is diametrically opposite cylinder 40 and cylinder 38 is diametrically opposite cylinder 41. The motions of the pistons in cylinders 39,40 and 41 are described and these are identical to the motions of the pistons in cylinders 36, 37 and 38 respectively.

Figure 10a shows the uncorrected difference motion of the pistons 13a and 13b in cylinder 41 wherein the net unbalance motion is shown at 25.

According to the present invention there is an engine comprising three pairs of diametrically opposite cylinders in each cylinder of which the piston motions 13a and 13b have two corrective components 42 and 43 as shown in Figures 10b. According to the present invention these corrective motions 42 and 43 have a value equal to one half of the value of the unbalanced motion 25 and are displaced from the motion 25 by +120 degrees and -120 degrees respectively on the cam phase axis and these corrective motions are applied in a negative sense to the original difference motion 25.

In Figure 10c is illustrated the total difference motion 46 between pistons 13a and 13b with the corrective components 42 and 43 added to the original motion 25.

According to the present invention the corrective motions 42 and 43 can be divided between the motions of the inlet piston 13a and the exhaust piston 13b in an arbitrary manner subject to the condition that the difference between these separate motions is equal to the required corrective motions 42 and 43.

Figure 10d shows one manifestation of the application of these corrective motions 42 and 43 in which the apportionment of the corrective motions 42 and 43 between the pistons 13a and 13b is equal and opposite. The amended motions of the inlet pistons 13a and the exhaust piston 13b are shown at 44 and 45.

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According to the present invention in this manifestation where M = 3 and in which has three pairs of diametrically opposite cylinders the motions of the pistons in cylinders 39 and 40 are identical to the amended motions 44 an 45 but differ by +120 degrees and -120 degrees of the cam phase angle respectively.

According to the present invention Figure 11a, b and c shows the amended motions 46, 47 and 48 for the pistons contained in the three cylinders 39, 40 and 41 and the net motions for the pistons therein respectively. The pistons in the diametrically opposite cylinders 36, 37 and 38 have identical motions to those in cylinders 39, 40 and 41 respectively.

According to the present invention Figure 12d shows the algebraic sum of these net motions in all six cylinders which because of the methodology comprising the present invention is equal to zero.

According to the present invention an engine in which the algebraic sum of the net piston unbalanced motions is zero there is no net acceleration force transferred from the cams 12a and 12b to the drive shaft 11 and thus axial vibration arising from the motion of the pistons 13a and 13b is eliminated.

According to the present invention in the general case where there are a prime number M pairs of diametrically opposed cylinders

Then (M-1) corrective motions are applied and each such corrective motion has a magnitude of

1 /(M-1)

times the original net motion 25 and such corrective motions are spaced by

360/M degrees along the axis of the cam phase angle profile.

It will be appreciated that the present construction has considerably fewer moving parts than a conventional petrol engine, previous cam engines or the Jumo diesel engine. This makes for easier construction, improved reliability, lower weight and lower maintenance costs.

It will be appreciated that the present construction is intended to provide an engine with high performance, greater economy and a reduced number of moving

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parts with a consequential reduced cost and weight and potential high reliability.

It will be appreciated that the combination of the two opposed-pistons per cylinder, two-stroke diesel design with the cam-driven barrel-engine design to minimize moving parts, should reduce cost and weight and improve efficiency.

It will be appreciated that the use of non-sinusoidal cam profiles can optimize cylinder charging, combustion, and power per firing stroke.

It will be appreciated that the use of cam profiles which compensate for the piston accelerations at and near Top Dead Centre and at and near Bottom Dead Centre in an engine with a multiple number of cylinders can eliminate net axial force on the drive shaft arising from piston accelerations.

It will be appreciated that the use of multiple cam profiles around the cam periphery can optimize the number of firing strokes and engine revolutions and improve engine balance particularly but not exclusively for aero engine or marine engine applications.

References

(1) UK Patent No. 726,64; Redrup and Redrup, March 1955.

(2) UK Patent No. 2,019,487; Renegar, October 1979.

(3) UK Patent Application No. GB2453131A April 2009.

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Claims (4)

CLAIMS What we claim is>

1) An internal combustion engine of the barrel type having three pairs of cylinders lying parallel to the drive shaft and positioned symmetrically around the said drive shaft in which cylinders are fitted opposed pistons which each engage with a separate cam mounted on the said drive shaft and which cams have profiles generated substantially as shown in Figure 11 and Figure 12 in which the algebraic sum of the accelerations of the pistons in all six cylinders is zero..

2) An internal combustion engine substantially as in Claim 1 having two or more identical profiles around the periphery of the cams.

3) An internal combustion engine substantially as in Claim \ having pairs of diametrically opposite cylinders wherein the number of tti^ said pairs of cylinders is any prime number in excess of unity.

4) An internal combustion engine substantially as in Claim 3 having two or more identical profiles around the periphery of the cams.

4) An internal combustion engine substantially as in Claim 2 having two br more identical profiles around the periphery of the cams.

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Amendments to the claims have been filed as follows

What we claim is:-

1) An internal combustion engine of the barrel type having at least three pairs of cylinders lying parallel to the drive shaft and positioned symmetrically around the said drive shaft in which cylinders are fitted opposed pistons which each engage with a separate cam mounted on the said drive shaft and which cams have different and asymmetrical profiles formed such that algebraic sum of the accelerations of the pistons in all six cylinders is zero..

2) An internal combustion engine substantially as in Claim 1 having two or more identical profiles around the periphery of the cams.

3) An internal combustion engine substantially as in Claim 1 having pairs of diametrically opposite cylinders wherein the number of the said pairs of cylinders is any prime number in excess of unity.