GB2369859A - I.c. engine with opposed pistons and cam surfaces to transmit the piston movements - Google Patents

Abstract

An internal combustion engine, preferably diesel, has at least two cylinders (10) and two drive shafts (11a, 11b), each cylinder having two opposed pistons (13) therein, with piston heads (13a, 13b) facing each other and piston rods of the pistons are coupled to the drive shaft or shafts through respective opposed cams (12a, 12b). The opposed cams are profiled such as to ensure that the opposed pistons are held at the top dead centre postion until combustion is substantially complete.

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. In addition aircraft require high short-term power outputs for takeoff and climb, with sustained economic output at lower powers in the cruise. The ability of the diesel engine to produce a high torque at low speed results in lowere noise emissions.

Diesel engines also have similar advantages for seagoing vessels. In particular certain vessels require high reliability and economy in the cruise, coupled with fast response and high short-term power output capability for manocuvring in port.

The spark-ignition engine has the advantage over the conventional diesel engine in that ignition takes place very rapidly at the point of highest compression. In the conventional diesel engine fuel continues to be burnt well into the power stroke, reducing the thermal efficiency of the engine. There is considerable advantage to be had if a diesel engine can be made to approximate to the combustion cycle of the spark-ignition engine.

Referring firstly to Figure 1, a Jumo diesel engine is diagrammatically illustrated.

Manufactured by the German Junkers company during the 1930s, the Jumo diesel engine was a two-stroke aero internal combustion engine with two opposed pistons per cylinder. The pistons 13 came together in the centre of the cylinder 10 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 rotate a drive shaft 11 coupled to a single propeller 6. The major advantage of the design lay in the fact that the air inlet 7 and exhaust gas exit 8 were effected through ports l 9 and 20 respectively in the extremities of the cylinders 10 which were uncovered

as the pistons 13 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 turbocharger, thus facilitating scavenging of exhaust gases.

Sections (a) to (i:) of Figure I show the cylinder and pistons at different parts of the cycle. The placing of the inlet 19 and exhaust 20 ports was such that as the power stroke ended, the exhaust port 20 opened first, allowing exhaust gases to rush out. (a). As the cylinder pressure fell to near atmospheric pressure, the inlet port 19 opened, admitting air at turbocharger pressure, to scavenge the cylinder 10 of remaining exhaust gases. (b). After the pistons reached bottom-dead-centre (BDC) 2, on the return stroke, the inlet port 19 closed first, then the exhaust port 20, and the air was compressed to high pressure (c, d). As the pistonsl3 approached top-dead-centre (TDC)1, (e), the fuelwasinjected25 et high pressure into the space between the pistons 13 and ignited spontaneously, causing the pistons 13 to be driven back again with great force, to drive the crankshafts 3 and 4. (f).

The Jumo design was produced with six cylinders in-line, 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 turbocharger 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 the 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 pistons 13 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 cylindrical shaft is used, without expensive cranks. There are no connecting rods or big and little-end bearings, although there are bearing surfaces 13c required between the pistons 13 end 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 where there are four lobes 17 on the cam 12.

As diagrammatically shown in Figures 6a and 6b several cylinders l O may also be arranged in a barrel arrangement around the shaft 11, rather like a revolver.

According to the invention described in UK Patent Application Number 0022669.6 and illustrated diagrammatically herein in Figure 7, there is provided an internal combustion engine comprising at least two cylinders l O each with two opposed pistons l 3a, 1 3b therein, with piston heads of the pistons 1 3a, 1 3b facing each other, the piston rods

of the pistons being coupled to a drive shaft 11 of the engine through two opposed cams 1 2a, 1 2b respectively.

The two opposed cams 12a, 12b have non-sinusoidal cam profiles. The two opposed cams 12a, 12b also have different profiles to optimise the inlet port 19 and exhaust port 20 opening and closing sequence. In particular by enabling the inlet ports 19 to remain open after the exhaust ports 20 have closed the pressure in the cylinders 10 may achieve turbocharger 27 or supercharger outlet pressure.

The mass of air in a cylinder of an internal combustion engine with opposed pistons becomes fixed when the inlet ports and outlet ports become closed by the pistons as they return from the Bottom Dead Centre (BDC) position. The volume of air at this point is the uncompressed volume. When the pistons reach the Top Dead Centre (TDC) position, the volume of air is at the compressed volume. The ratio of the uncompressed volume to the compressed volume is the Compression Ratio.

It will be appreciated that in an internal combustion engine with the opposed piston arrangement with the driveshaft linked through two opposed cams as described above the pistons are directly coupled to the drive shaft of the engine through the opposed cams and that the rotation of the drive shaft is synchronised to the two opposed cams and that the pistons meet at the Top Dead Centre (TDC) position for every compression stroke of the engine and that the compression ratio of the engine is thereby fixed. It will also be appreciated that because the opposed pistons operate on the opposed cams at a fixed radius the torque generated by this arrangement will be greater than that from a conventional crankshaft engine design, and that power will be generated at a lower speed.

According to the present invention there is provided an internal combustion engine comprising at least two cylinders 10 each with two opposed pistons 13a and 13b therein, with piston heads of the pistons 1 3a and 1 3b facing each other, piston rods of the pistons being coupled to the drive shaft 1 1 of the engine through two opposed cams 1 2a and 1 2b.

The opposed cams 1 2a and 1 2b have non-sinusoidal profiles and each control at least one inlet port 19 and at least one exhaust port 20 respectively. The two opposed cams 1 2a and 12b have different profiles such as to control the inlet 19 and exhaust ports 20 asymmetrically thereby enabling the gas flows through the inlet ports 19 and exhaust ports 20 to be optimised and to thereby enable increased power output to be obtained from the engine.

In particular the two opposed cams 1 2a and 1 2b are profiled such as to hold the two opposed pistons 1 3a and 1 3b at the top dead centre position 1 until combustion of all the fuel is completed. 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 Figure 8 of the accompanying drawings, in which: Figure 8a shows a typical sparkignition engine combustion cycle with a compression ratio of 10.

Figure 8b shows a typical diesel engine combustion cycle with a compression ratio of 10.

Figure 8c shows a typical diesel engine combustion cycle with a compression ratio of 20.

Figure 8c shows a typical combustion cycle for the present invention with a compression ratio of 20.

A typical spark-ignition engine has a maximum compression ratio of about 10, because the fuel-air mixture in the cylinder is likely to ignite prematurely during the compression stroke at higher compression ratios. This is due to the temperature rise which occurs during compression and the premature ignition can lead to engine damage. As the piston approaches top dead centre a spark initiates combustion which leads to rapid fuel burn and a powerful subsequent power stroke. Thus all of the heat energy is added at or just after top dead centre. The resulting high pressure, as shown in Figure 8a is available to drive the piston down for the majority of the power stroke. Figure 8a shows how the cylinder pressure

varies with cylinder volume as the piston moves through its complete cycle. The area within the curve 21 represents the energy available per cycle. The area below the curve 22 represents the lost energy, and because of the high area within the curve, this cycle. known as the Otto cycle, has a high efficiency.

A conventional diesel engine, operating at the same compression ratio of 10 would have a lower efficiency. Because fuel injection commences near top dead centre in a diesel engine to prevent spontaneous ignition during the compression stroke, there is a time delay whilst the fuel is injected and atomised. Thus combustion is not entirely completed even when the piston is well down its power stroke, and peak combustion pressure is not available as in the spark-ignition engine. In practice the pressure in a conventional diesel engine stays almost constant during the power stroke, as shown in Figure 8b. The energy available per cycle 23 is reduced, and hence efficiency is lower than in a comparable spark-ignition engine. In addition because combustion is still taking place well into the power stroke, partially-burnt fuel is usually emitted with the exhaust gases.

In practice diesel engines will not operate reliably at compression ratios as low as 10, because of irregular spontaneous ignition. Instead a lower limit of about 14 is used, and more usually, compression ratios in the range 20 to 25 are used. These higher compression ratios result in a higher power output 24 and efficiency as shown in Figure 8c because of the higher pressures achieved, and typical diesel engines are considerably more efficient than spark-ignition engines. However, because of the higher combustion pressures achieved, conventional diesel engines experience much higher crankshaft forces and have to be made much stronger and hence heavier, than spark-ignition engines.

In the present construction, the opposed cams 1 2a and 1 2b are profiled such that the heads of the opposed pistons 1 3a and 1 3b are held together at the top dead centre position 1 for the duration of the fuel injection period, and for some time longer until combustion is complete. The pistons 13a and 13b are then released,and the full combustion pressure is available for the power smoke. Thus the present invention combines the high efficiency of the Otto cycle with the high efficiency of the high-compression ratio diesel engine to produce an engine of even higher power output 26 and efficiency as shown in Figure 8d. In addition,

because combustion is substantially completed before the power stroke commences, emissions of partially burnt fuel are much reduced.

In the present construction the opposed cams 12a and 12b have profiles for the power stroke which are mirror images of each other, and therefore the forces generated on the cams 12a and 12b are equal. The tangential forces are in the same direction and drive the drive shaft 11 around to produce the motive power for the propeller 6 or other load. The axial forces on the cams 1 2a and 1 2b however are equal and opposite and balance each other out along the drive shaft 11. Thus a heavy engine casing is not required and a diesel engine may be constructed which is considerably lighter than a conventional diesel engine of the same power output and the lack of unbalance forces during the power stroke results in an engine with lower noise emissions..

It will be appreciated that the combination of the two opposed pistons per cylinder, two crankshaft, two-stroke diesel engine with the opposed cam design having cam profiles such as to replicate the Otto combustion cycle enables a diesel engine to be built with higher efficiency and power output, lower weight, lower emissions and reduced noise by generating full power at a lower speed.

Claims (5)

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 arranged such as to hold the pistons at the top dead centre position until fuel combustion is substantially complete.

2. An engine according to claim 1, and being a two-stroke engine.

3. An engine according to any one of the preceding claims and having three or more cylinders.

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

5. An internal combustion engine, substantially as herein before described, with reference to Figure 8 of the accompanying drawings.