GB2453131A

GB2453131A - Internal combustion opposed-piston barrel engine

Info

Publication number: GB2453131A
Application number: GB0718776A
Authority: GB
Inventors: William Fairney
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-09-26
Filing date: 2007-09-26
Publication date: 2009-04-01
Anticipated expiration: 2027-09-26
Also published as: GB0718776D0; GB2453131B

Abstract

An i.c. engine, preferably diesel, has at least two cylinders 10 and a drive shaft 11, each cylinder having two opposed pistons 13 therein, with piston heads 13a, 13b facing each other and piston rods of the pistons coupled to the drive shaft through respective opposed cams 12a, 12b. The opposed cams 12a, 12b are profiled such as to ensure that the motion of the pistons 13 in one cylinder at and near Bottom-Dead-Centre (BDC) is balanced by the motions of the pistons 13 in another cylinder at and near Top-Dead-Centre (TDC), thus ensuring that piston inertia forces are balanced out thereby reducing vibration and noise. Furthermore the profiles of the cams 12a, 12b at and near Top-Dead-Centre are such that whilst the opposed pistons 13 are held together at a fixed separation until combustion is substantially complete they continue to move along the cylinder bore thereby eliminating dead motion and the associated breakdown of lubrication which can lead to wear and overheating.

Description

DIESEL INTERNAL COMBUSTION ENGINE

This invention relates to a diesel internal combustion (IC.) 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.

This 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 on 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 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 turbo-charger, 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 in let 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.

3, 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.

5, 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.

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 13a 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-or lithe-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 ball-ended sockets, or with a shoe, or with roller bearings 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 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, rather like 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, axcept 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 dimentioned to cany 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 of 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 two disadvantages.

One 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.

A further disadvantage of this previous Renegar-type invention is that holding the pistons stationary at Top-Dead-Centre (Dead Motion) can result in a breakdown of the oil film between pistons and cylinder walls, thereby reducing lubrication and leading to excessive wear and overheating.

The present invention solves these two problems by using cam profiles that are carefully tailored to ensure accurate balance without detracting from the overall improvement in engine thermal efficiency, and at the same time to ensure that whilst the pistons are held together at Top-Dead-Centre, they continue to move along the cylinder wall thus maintaining the oil film and lubrication.

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 is a diagrammatic illustration showing a practical working cycle for the engine of the present construction with features added to provide improved axial balance of the pistons and to eliminate dead motion of the pistons; Figure 10 shows an illustration of a typical four-cylinder engine of the present construction incorporating the features of the present engine designed to provide improved axial balance of the pistons; Figure 11 shows an enlargement of part of Figure 9 illustrating the cam profiles used to eliminate dead motion of the pistons; Figure 12 shows the operation of the engine of the present construction incorporating features to prevent dead motion of the pistons at or near top dead centre.

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 13 acting on two cams 12 at the front and the rear of the engine respectively.

Because the pistons 13 in each cylinder act in opposition, there is no net axial force on the shaft 11 due to combustion gases, so light thrust bearings 17 can be 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 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 and exhaust 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. Turbo-charging may alternatively be achieved from an exhaust-gas driven turbine 21. The cam I 2a is coupled to the pistons 1 3b which control the inlet ports 19 whilst the cam 1 2b is coupled to the pistons I 3c 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 referred to above, the cams 12 do not have the same profile. Non-sinusoidal profiles are used for each, and the inlet and exhaust cams 12a, 12b, have different profiles.

During the firing stroke, both ports 19, 20 are closed, and it is advantageous for the cams 12 to have the same profile for this part of the stroke, to equalize axial thrust forces.

Referring also 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). Towards 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 super-charger or turbo-charger pressure, the inlet port opens, 0. 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 arrangement, the port timing is such that the cylinder 10 is scavenged by more than its own volume of air, before the exhaust port closes1 thus removing all exhaust gases. 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, E, until at or near Top-Dead-Centre, fuel is injected at very high pressure, and spontaneous combustion takes place again, A. Fuel injection is initiated by a shaft-mounted cam disc or 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 7. The net axial motion of the inlet-end and exhaust -end pistons is shown in the lowest part of the Figure. It is seen that during period A the pistons are stationary at the Top-Dead-Centre position where oil film breakdown can occur leading to wear and noise. Also, dunng 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, but it is seen that there is still net axial motion.

The features of the present construction to eliminate this vibration-inducing motion are presented below.

It will be appreciated that the motions of the pistons 1 3a and 1 3b in the two cylinders lOin Figure 6 are identical but displaced by 180 degrees of shaft rotation.

In the present construction he motion of the pistons around the inlet and the exhaust ports at Bottom-Dead-Centre in one cylinder are mirrored by motions at Top-Dead-Centre of the pistons in the other cylinder, and vice-versa. These motions are designed to take place such that combustion is completed at maximum compression, thereby ensuring that maximum thermal efficiency is achieved. It will be appreciated that the compensating motion of the pistons at and near Top-Dead-Centre may be apportioned to either piston provided the sum of the motions of the two pistons mirrors and opposes the sum of the motions of the two pistons which are at or near Bottom-Dead-Centre.

Figure 9 diagrammatically shows these cam profiles which are designed so that mathematically the net acceleration of all four pistons is zero at any instant in time.

The axial acceleration forces of each piston are absorbed by the cam to which it is connected and the cams absorb all the forces to produce no net axial acceleration force on the drive shaft.

Additional features relate to shaft bending and engine speed.

In this two-cylinder version of the present construction, whilst the net acceleration forces on the cams are zero 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 strong enough and stiff enough to carry these bending loads with ease.

The present invention therefore also proposes a means to eliminate these bending forces and enable the bearings and casings to be made of a light construction. Figure 10 shows a four-cylinder arrangement of the engine. In this figure only two sets of pistons are shown, for clarity, adjacent to each other in the engine casing, i.e. at ninety degrees around the casing. In this construction there are two cycles of the cam profile around each cam, thus one set of the displayed pistons (30) is shown at the Top-Dead-Centre position and the other set of pistons (31) is shown at the Bottom-Dead-Centre position. The driving forces from the pistons shown impose bending moments on the shaft, but these are counteracted by the pistons in the cylinders diametrically opposite.

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 = l2OxS IL revolutions per minute.

However, only certain combinations of number of cylinders and lobes will give vibration-free operation. Because the pistons are the only reciprocating masses, and these are opposed in pairs and shaped to mirror the motion at both Top-Dead-Centre and Bottom-Dead-Centre positions, negligible vibration arises from this source.

It will be appreciated that in the case of a four or eight cylinder-engine, or indeed any multiple of four cylinders, the pistons which are instantaneously approaching Bottom-Dead-Centre will be balanced by the pistons approaching Top-Dead-Centre and that no net axial acceleration force will exist, and that no bending moment will exist on the shaft.

The features which relate to eliminating dead motion of the pistons and thereby improving lubrication of the pistons whilst at or near top dead centre are shown in Figures 11 and 12.

In all piston-powered internal combustion engines the pistons reach Top-Dead-Centre and reverse their motion. In conventional engines there is a moment when the piston speed is zero but the rapid reversal enables the oil film between the piston and the wall of the cylinder to be maintained and lubrication to be continuous. In the barrel engine of the previous invention where the pistons are held at Top-Dead-Centre for up to 60 degrees of shaft rotation (Dead Motion) the oil film can break down leading to excessive wear and overheating.

In the present construction of an opposed piston engine the motions of the two cams at and near Top-Dead-Centre are as shown diagrammatically in Figure 11. The compensating motion has been apportioned equally to the two pistons so that they move together for the period of fuel injection and combustion.

As shown in Figure 12 the cams are shaped such that at maximum compression the pistons I 3a and I 3b come together a short distance to one side of the central cylinder position 23 where the fuel injector 24 is located. (Figures 12a and 12b).

The crowns 25 of the pistons are hollowed out such that the central volume between pistons contains inlet air at the volume suitable to the designed compression ratio.

The piston crowns contain one or more slots 26 in the side through which fuel is injected. (Figure 12b). The cams are shaped such that as the fuel is injected 24 the pistons move continuously along the cylinder but remain close together and hence maintain the same combustion volume following the Otto Cycle of combustion. (Figure 12c). The slots 26 in the crowns 25 are of sufficient depth to enable fuel injection to take place as the pistons traverse the cylinder in unison. When fuel injection is completed, typically after about 30 degrees of shaft rotation, the pistons continue to move together with constant combustion volume, until combustion is completed, typically after a further 30 degrees of shaft rotation. (Figure 12d).

Once combustion is completed the cams are shaped such that the pistons move apart on their power stroke (Figure 12e), continuing to follow the cam profiles which are designed to eliminate unbalanced inertia forces as described in the first part of the present construction claim. This present construction therefore endures that whilst constant combustion chamber volume is maintained at or near Top-Dead-Centre during fuel injection and combustion, the cams are shaped such that the reversal of piston motion is instantaneous and there is no stationary period (Dead Motion) in which the oil film can break down and cause high friction, overheating and losses.

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 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 mirror the piston accelerations at and near Top-Dead-Centre and at and near Bottom-Dead-Centre eliminates net axial force 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.

It will be appreciated that the use of cam profiles in a barrel-type engine having one or more cam profiles around the periphery of the cams can be designed to simultaneously improve engine balance and eliminate dead motion in such an engine operating on the Otto or similar cycle thereby improving lubrication and reducing friction losses.

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

(2) UK Patent No. 2,019,487; Renegar, October 1979. i0

Claims

What we claim is:- 1) The use of cam profiles which mirror the accelerations of pistons at or near Top-Dead-Centre to those of pistons at or near Bottom-Dead-Centre in an opposed-piston barrel-type engine to eliminate the net axial force arising from piston accelerations.
2) The use of such profiles as in Claim 1 in a barrel-type engine having one or more cam profiles around the periphery of the cams to optimize the number of finng strokes and of engine revolutions and to improve engine balance.
3) The use of such profiles as in Claim 1 in a barrel-type engine having one or more cam profiles around the periphery of the cams and having four cylinders or a multiple of four cylinders to optimize the number of firing strokes and of engine revolutions and to improve engine balance.
4) The use of cam profiles in a barrel-type engine having one or more cam profiles around the periphery of the cams to eliminate dead motion in such an engine operating on the Otto or similar cycle thereby improving lubrication and reducing friction losses.