GB2367328A

GB2367328A - I.c. engine with opposed pistons and cam surfaces to transmit the piston movements

Info

Publication number: GB2367328A
Application number: GB0022669A
Authority: GB
Inventors: William Fairney
Original assignee: FAIRDIESEL Ltd
Current assignee: FAIRDIESEL Ltd
Priority date: 2000-09-15
Filing date: 2000-09-15
Publication date: 2002-04-03
Also published as: GB0028803D0; GB0121762D0; GB0028801D0; GB2369859A; GB0022669D0

Abstract

The internal combustion engine, eg a two-stroke diesel engine, has at least two cylinders 10 arranged around a drive shaft 11, each cylinder having two opposed pistons 13 therein, with piston heads 13b, 13c facing each other. The piston rods are coupled to the drive shaft through respective opposed cams 12a, 12b which preferably have non-sinusoidal cam profiles and also preferably have different profiles to optimise inlet and exhaust gas flow. The piston rods may engage the cams via rollers or sliders. There may be four or six cylinders arranged in a barrel formation around the drive shaft. The profiling of the cams 12a, 12b may be such that each cylinder is scavenged by more than its own volume of air before its associated exhaust port closes.

Description

DIESEL INTERNAL COMBUSTION ENGINE

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

Diesel engines have potentially several advantages for aero engines. Magnetos and sparking plugs are such a source of unreliability in petrol aero engines that aviation authorities insist on duplication. Also, the higher specific energy content of diesel fuel means that more payload can be carried. The higher combustion temperature of the diesel engine results in a higher efficiency, hence lower cost and environmental impact.

Referring firstly to Figure 1, a Jumo diesel engine is diagrammatically illustrated. Manufactured by the German Junkers company during the 1930's, 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 advantage of the design lay in the fact that an 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 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 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-deadcentre (TDC) (e), the 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 inline, to produce very smooth running, but it had two 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, 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.

Figure 2 diagrammatically illustrates another known internal combustion engine, in which the cylinders 10 lie parallel to a drive shaft 11, and impart rotary motion to the shaft 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 each piston 13.

The axial face of the cam 12 varies sinusoidally with the shaft angle. The piston ends 13a push against the angled face of the cam 12, rotating it. Such engines have been made in petrol-ignition or diesel form, with two or four strokes to the firing cycle.

Engines have been built in the past with pistons at one or both ends, such as is illustrated in Figure 3.

The inlet and exhaust gases enter and leave via conventional poppet valves 14 in each cylinder head 15.

The design has the advantage of a small frontal area, which is attractive for aero-engine applications. It also has the advantage of the fact that the drive is direct to the shaft 11, without gearing, and that a plain

W 4 cylindrical shaft is used, without expensive cranks. There are no connecting rods or big and little-end bearings, although there are bearing surfaces required between the ends 13a 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 16 as shown in Figure 4.

In addition the cam 12 does impart side forces to the pistons 13, and does require quite complex machining during manufacture. In practice it may have two or more cycles of axial variation per revolution as illustrated in Figures 5a, 5b and 5c. Figure 5a shows a crosssection and Figures 5b and 5c show profiles of the cam 12.

As diagrammatically shown in Figures 6a and 6b several cylinders may also be arranged in a barrel arrangement around the shaft, rather like a revolver.

According to the present invention, 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 to optimise inlet and exhaust gas flow.

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 7 to 11 of the accompanying drawings, in which: Figure 7 is a diagrammatic illustration of one form of the present diesel internal combustion engine;

Figure 8 is a diagrammatic illustration, similar to Figure 1, showing the working cycle of the present engine; Figure 9 shows a series of illustrations of a typical two-cylinder engine of the present construction, in which Figure 9a is a perspective view; Figure 9b is a front view with parts broken away; Figure 9c is a side view; Figure 9d is a plan view; Figure 10 illustrates a cam and roller design for the present engine, with Figure lofa, Figure lOb and Figure 10c showing one form of cam and roller design, whilst Figure 10d shows a modification; and Figures lla, lib and lie illustrate a modification of a slipper design of cam and roller assembly, with Figure lie being a diagrammatic section taken along line A-A in Figure lib.

The present construction uses the 2-stroke opposed piston concept, but with cylinders 10 arranged in a barrel 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, as shown in Figure 7. Because the pistons 13 in each cylinder act in opposition, there is no net axial force on the shaft 11, so light thrust bearings 17 can be used, which are rated to carry the axial thrust from a propeller (not shown) or other driven load. Figure 7 is a diagrammatic arrangement, showing a two-cylinder design, with two-lobed cams 12, i. e. 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 18 on the shaft 11 operates a fuel injector to each cylinder, once per revolution. 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 12a is coupled to the pistons 13b which control the inlet ports 19 whilst the cam 12b is coupled to the pistons 13c which control the exhaust ports 20.

Features specific to 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, as illustrated in Figure 8 for example, the cams 12 do not have to have the same sinusoidal profile; indeed non-sinusoidal profiles can be used for each, and the inlet and exhaust cams 12a, 12b, can 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 equalise axial thrust forces, but towards the end of the stroke the exhaust port opens first (position (a) in Figure 8). As the exhaust gases leave, the cylinder pressure falls rapidly. As it falls below super-charger pressure, the inlet port opens (position (b) ). 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 closes, thus removing all exhaust gases.

When the exhaust port 20 closes (c), air continues to enter and reaches super-charger pressure as the inlet port 19 closes (d). The air is then compressed by the approaching pistons, until just before Top Dead Centre, fuel is injected at very high pressure, and spontaneous combustion takes place (e). Fuel injection is initiated by the shaft-mounted cam disc 18 operating on the injector. 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.

Additional features relate to shaft bending and engine speed.

The arrangement shown in Figure 7 is not ideal from a balance point of view. The driving forces from the pistons 13 impose bending moments on the shaft 11 which will produce a rotating unbalance exciting force. By using cylinders 10 on opposite sides of the shaft 11, and cams 12, with four lobes, i. e. , with two cycles of axial movement per revolution as shown previously in Figure 5, these cylinders 10 will fire at the same time, and the axial thrust components will be equal and eliminate bending moments on the shaft 11. In addition 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. Diesel engines are most effective at high cylinder firing rates, whilst for aircraft, the maximum permitted shaft speed is determined by the propeller tipspeed approaching the speed of sound. 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 = 120S/L rpm.

However, only certain combinations of number of cylinders and lobes will give vibration-fLee operation.

Because the pistons are the only reciprocating masses, and these are opposed in pairs, negligible vibration arises from this source. The rotating cams are almost mirror images of each other and so primary balance is achieved.

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 and lower maintenance costs.

Outline calculations indicate that a two-cylinder engine of about 100 HP (74,570 W) would have a bore of 80mm and a double stroke of 170mm. In a horizontal layout, typical arrangements would be as in Figures 9a9d. In this case an exhaust-driven turbo-charger 23 is

shown. In Figure 9d one of the cams 12c is sectioned to show the Top Dead Centre section.

By standardising on a single cylinder and cam size, with the cylinder centre-lines on a standard diameter, output can be increased in stages by adding further cylinders whilst minimising casting and machining costs.

A water-cooled arrangement is shown, with cooling water in cast ducts 24 in the casing, and in direct contact with the cylinder liners 25.

Even with nominal 200 HP (149, 140W) 4-cylinder and 300 HP (223, 710W) 6-cylinder designs, the overall dimensions can be very similar to those of the 2-cylinder arrangement shown in Figure 9.

Further features of the present construction relate to the cam or roller arrangements.

Diesel engines have extremely high cylinder pressures and as a result, high thrust forces and bearing pressures. Hence the cam and roller design has to have fine tolerances to minimise free play. The cam, as well as experiencing the axial force from the piston, experiences sideways forces, and these have a reaction force on the piston. Thus the cylinder walls and piston slider experience lateral forces. The lubrication system should allow for this. Figures lOa to lad illustrate a cam and roller design that provides a combination of rolling and sliding contact. The rollers 26 are carried on a slider 27 which is a precision fit in a slider housing 27a. Furthermore the use of roller bearings 28 within the roller 26 will give best precision and lowest losses. Lubrication channels 29 are provided in the

slider from the housing, with oil directed onto the bearing surfaces.

From Figure 10 it can be seen that with the parallel roller design there will be a scrubbing action due to the rotation (arrow A in Figure lofa) of the cam at different radiuses on the roller.

Figure 10d shows an improved arrangement in which tapered rollers 26A are used, with a taper angle such that all contact is purely rolling. However, this will result in a radial reaction force and lubrication channels are provided in the surface of the slider to ensure low friction forces.

Alternative arrangements to the cam and roller assembly are possible. One design used for barrel engines has ball-ended pistons which slide in an annular slot in the cam, and this could be used for the diesel engine. However care would be required in the lubrication system, as access is difficult with this arrangement.

An alternative arrangement is simple slipper design, with sliding surfaces, as shown in Figures lla to lie. A nose 30 of the slipper nearest the piston 13 will experience the full driving force of the piston, and lubrication arrangements are similar to those for the roller design. The oil is fed via a slot 31 in a slider of the piston, and hollow ducts 32 through the slider, to the nose of the slipper, emerging at the leading edge just ahead of the line of contact 33. Fine grooves 34 in the surface of the cam 12 and the slipper 35 concentrate oil flow at the point of peak bearing pressure. An outermost face of the slipper 36 only carries the force

required to compress the air on the compression stroke, and this is much less than the power stroke force, but a suitable lubrication channel 37 is provided. As with the roller design, oil is forced onto the cam 12 during the power stroke, and is directed onto a cylinder liner 10A during the compression stroke, thus ensuring good piston lubrication. An oil scraper ring 38 ensures that excessive oil does not enter the cylinder chamber, whilst an additional oil duct 39 leads oil to the side of the slider to allow for the side-thrust.

This arrangement gives lowest cost and fewer moving parts, but despite using hardened surfaces, wear rates on the slipper and cam may prove high.

It will be appreciated that the present construction is intended to provide an engine with high performance, greater economy, and 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 minimise moving parts, should reduce cost and improve efficiency.

With the use of non-sinusoidal cam profiles, this optimises cylinder charging, combustion and power per firing stroke.

The use of multiple cam lobes optimises the number of firing strokes and engine revolutions, particularly for aero-engine applications and improves engine balance.

The use of taper-roller bearings minimises wear between cams and rollers, whilst the option of using a simple slipper design reduces the number of moving parts.

Finally, the use of tailored lubrication channels in the sliders reduces friction arising from axial, radial and side-thrust forces.

Claims

1. 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.

2. An engine according to claim 1, wherein the two opposed cams have non-sinusoidal cam profiles.

3. An engine according to claim 1 or 2, wherein the two opposed cams have different profiles to optimise inlet and exhaust gas flow.

4. An engine according to claim 1,2 or 3, wherein the cylinders are arranged in a barrel formation around the drive shaft.

5. An engine according to any one of the preceding claims, wherein one of the cams controls at least one inlet port and the other cam controls at least one exhaust port.

6. An engine according to any one of the preceding claims, wherein profiling of the cams is arranged such that each cylinder is scavenged by more than its own volume of air before its associated exhaust port closes.

7. An engine according to any one of the preceding claims, wherein each cam has an associated roller arrangement by which the cam is coupled to its piston rod.

8. An engine according to any one of claims 1 to 6, wherein each cam has an associated slider arrangement by which the cam is coupled to its piston rod.

9. An engine according to any one of the preceding claims, and being a two-stroke engine.

10. An engine according to any one of the preceding claims, and having four cylinders and associated pistons and cams.

11. An internal combustion engine, substantially as hereinbefore described, with reference to Figures 7 to 11 of the accompanying drawings.

11. An engine according to any one of claims 1 to 9, and having six cylinders and associated pistons and cams.

12. An engine according to any one of the preceding claims and being a diesel engine.

13. An internal combustion engine, substantially as hereinbefore described, with reference to Figures 7 to 11 of the accompanying drawings. Amended claims have been filed as follows CLAIMS

1. 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, one cam controlling at least one inlet port and the other cam controlling at least one exhaust port, each

cam having a non-sinusoidal profile and the two opposing cams having different profiles to tv optimise the inlet and exhaust gas flow.

2. An engine according to claim 1, wherein the two opposed cams are profiled such that each cylinder is scavenged by more than its own volume of air before its associated exhaust port closes.

3. An engine according to any one of the preceding claims wherein the two opposed cams are profiled such that the exhaust port of each cylinder closes before the inlet port thereby enabling full supercharger or turbocharger pressure to be attained in each cylinder.

4. An engine according to any one of the preceding claims wherein each piston has an associated slider arrangement by which the associated cam is coupled to its piston rod.

5. An engine according to any one of the preceding claims wherein the bearing surfaces between each of the pistons and the cylinder walls and of the two opposed cams and its associated pistons are cooled and lubricated by channels in each piston which direct oil onto the bearing faces of the cylinders, cams and pistons.

6. An engine according to any one of the preceding claims, and having four cylinders and associated pistons and cams.

7. An engine according to any one of the preceding claims, and having six cylinders and associated pistons and cams.

8. An engine according to any one of the preceding claims wherein the cylinders are arranged in a barrel formation around the drive shaft.

9. An engine according to any one of the preceding claims and being a two-stroke engine.

10. An engine according to any one of the preceding claims and being a diesel engine.