GB2265942A - Split cycle I.C.engine. - Google Patents

Abstract

A split cycle internal combustion engine comprises a compression unit (1) and an expansion unit (2) in which the fuel is injected into the working space of the expansion unit (2) towards the end of the exhaust stroke and hot compressed air is subsequently admitted to initiate combustion. Preferably, the compression and expansion units (1; 2) each comprise two high pressure cylinders (24, 25; 27, 28) in conjunction with one low pressure cylinder (26; 29) in which the low pressure crankshaft (31; 81) rotates at twice the speed of the high pressure crankshaft (30; 80).

Description

INTERNAL COMBUSTION ENGINE The invention relates to compression ignition internal combustion engines, that is, engines in which fuel is mixed with air that has been compressed so that its temperature is raised sufficiently to ignite the fuel.

In conventional compression ignition internal combustion engines, the air is compressed in a cylinder and the fuel is injected into the space between piston and cylinder head a few degrees of crank angle before top dead centre. In this arrangement there is always a delay in the ignition of the fuel spray which results initially in a very rapid rise in pressure when combustion commences. This rapid pressure rise necessitates a very robust construction and also contributes significantly to the noise output of the engine. Problems also arise in ensuring complete combustion of the fuel spray to avoid undesirable smoke emissions. These effects restrict the maximum speed of the engine and limit the maximum power output. It is an . object of this invention to overcome these difficulties.

The present invention is embodied in a split cycle engine, that is, an engine in which a compression unit and an expansion unit are mechanically coupled together and in which an air charge is compressed in the compression unit and then fed via a transfer passage to the expansion unit, heat being supplied to the air charge by burning fuel in either the transfer passage or the working space of the expansion unit. Such a split cycle is widely used in gas turbines but it is also known to use positive displacement devices for both compression and expansion units. Unlike a conventional compression ignition engine, a split cycle engines operates on a constant pressure cycle and hence peak pressures are closely controlled.

Positive displacement devices of the type described may be either rotary or conventional piston and cylinder units.

The compression unit normally includes an inlet valve which is open for the entire suction stroke and a discharge valve which is open for only a short time at the end of the compression stroke when the compressor pressure is approximately equal to the pressure in the transfer passage. This valve is conventionally an automatic valve which is retained shut by a light spring and the pressure in the transfer passage and is opened by the compressor pressure. However, this type of valve tends to be noisy, unreliable and unable to operate at high compressor speeds. It is another object of the invention to avoid the use of automatic valves.

The expansion unit also normally includes two valves, being an exhaust valve which is open for the entire exhaust stroke and an admission or "cut-off" valve which admits the compressed air charge into the cylinder and is normally open for only a limited rotation of the output shaft. The "cut-off" ratio is then defined as the ratio of the expansion unit volume at the time the admission valve closes to the maximum volume of the expansion unit.

Both compressor discharge valve and expansion unit admission valve are open for very short periods of time which limits the maximum speed of the engine. It is a further object of the invention to provide a simple compound compression and expansion arrangement which extends substantially the time available to operate these valves.

According to one aspect of the invention there is provided a split cycle internal combustion engine adapted to drive a load, the engine comprising a compression unit for inducting and compressing air; an expansion unit for combusting fuel and air compressed in the compression unit; and a transfer passage communicating between the compression and expansion units, the compression unit including valve means for discharge of compressed air to the transfer passage and the expansion unit including means for the injection of fuel and valve means for the admission of compressed air from the transfer passage, the engine including means whereby fuel is injected towards the end of the exhaust stroke during the period when the pressure in the expansion unit is substantially at atmospheric pressure.

In one embodiment, the engine includes power output control means operatively connected firstly to the admission valve means to vary the opening period thereof and secondly to the fuel injection means to control the quantity of fuel injected in dependence on the opening period of the admission valve, whereby the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when the discharge valve means begins to open.

In an alternative embodiment, the engine includes power output control means operatively connected firstly to the fuel injection means and secondly to the admission valve means to vary the opening period thereof in dependence on the quantity of fuel injected, whereby the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when the discharge valve means begins to open.

The pressure in the compression unit working space as the discharge valve begins to open can vary due to a number of reasons, for example changes in altitude.

The engine may also be used in conjunction with a turbocharger and the degree of boost will affect the compressor pressure. However, it can be shown that the pressure in the working space of the compression unit when the discharge valve begins to open is a fixed multiple of the absolute pressure at the compression unit inlet. In the first embodiment above, where the power output of the engine is controlled by varying the period of opening of the admission valve of the expansion unit, the input to the control system is the difference between the pressure in the transfer chamber and a fixed multiple of the pressure at the compression unit inlet, and the output adjusts the quantity of fuel injected until the input to the control system is zero.

The compression and expansion units each preferably comprise compound units. For example, the compound units may each comprise two high pressure cylinders connected to a common crankshaft and a low pressure cylinder, the low pressure crankshaft rotating at twice the speed of the high pressure crankshaft. In such an arrangement, ports may communicate between the upper region of the low pressure cylinder and the middle region of the high pressure cylinders of each compound unit.The pistons in the high pressure cylinders of each compound unit may be arranged to be driven in antiphase and the low pressure and high pressure crankshafts of each unit may be parallel with each other and with the crankshafts of the other unit, the crankshafts of the respective units may be mechanically coupled so that corresponding crankshafts rotate at the same speed, the cylinders of each unit may be co-planar with each other and with the cylinders of the other unit, and the pistons in the one unit may be arranged to move in phase but in the opposite direction to the corresponding pistons in the other unit.

Thus, a split cycle engine according to the invention may include a compression unit comprising two first cylinders with pistons driven in antiphase from a first crankshaft and a second cylinder carrying a piston driven by a second crankshaft which is geared to the first crankshaft so that the second crankshaft rotates at twice the speed of the first crankshaft, ports uncovered at mid stroke by the first piston and connecting to the second cylinder initially when the second piston is at its top dead centre position, and inlet and delivery valves in the head of the first cylinders, and an expansion unit comprising two first cylinders with pistons driven in antiphase from a first crankshaft and a second cylinder carrying a piston driven by a second crankshaft which is geared to the first crankshaft so that the second crankshaft rotates at twice the speed of the first crankshaft, ports uncovered at mid stroke by the first piston and connecting to the second cylinder initially when the second piston is at its top dead centre position, and admission and exhaust valves in the head of the first cylinders, in which the first and second crankshafts of the compression unit are parallel to the first and second crankshafts of the expansion unit and the crankshafts of the compression unit are mechanically coupled to the crankshafts of the expansion unit so that corresponding crankshafts rotate at the same speed, the first and second cylinders of the compression unit are respectively co-planar with the first and second cylinders of the expansion unit, and the pistons in the first and second cylinders of the compression unit move respectively in phase with but in the opposite direction to the pistons in the first and second cylinders of the expansion unit so that inertia forces are balanced.

The invention also includes a method of operation of a split cycle internal combustion engine including a compression unit, an expansion unit and a transfer passage communicating between the compression and expansion units, in which fuel is injected into the working space of the expansion unit towards the end of the exhaust stroke during the period when the pressure in the expansion unit is substantially at atmospheric pressure. Preferably, the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when the discharge valve controlling entry to the transfer passage from the compression unit begins to open.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which Figure 1 is a schematic block diagram of a split cycle engine embodying a first method of controlling the power output of the engine; Figure la is similar to Figure 1 but shows an alternative method of controlling the power output; Figure 2 is a section through a compression unit; Figures 3a, b, c and d are schematic representations of the sequence of events during one cycle of the compression unit of the engine; Figure 4 is a section through an expansion unit; Figures 5a, b, c, d and e are schematic representations of the sequence of events during one cycle of the expansion unit of the engine; Figure 6 is an end view of the engine; .

Figure 7 is a section through the engine on the line C-C of Figure 6; Figure 8 is a section through the engine on the line D-D of Figure 6; Figure 9 is a side view of the engine in the direction of arrow A; Figure 10 is a side view of the engine in the direction of the arrow B; Figure 11 is a section on the cranked line E-E of Figure 9; Figure 12 shows the mechanism for adjusting the cut off ratio of the admission valves of the expansion unit; and Figure 13 shows an admission valve of the expansion unit to an increased scale.

The elements of the engine and their functions will now be described with reference to Figure 1 and Figure la.

The engine consists of a compression unit 1 and an expansion unit 2 which are mechanically coupled together and here shown for simplicity mounted on a common shaft 3. The compression unit 1 takes in air from the atmosphere through inlet passage 4 and inlet valve 5 and discharges compressed air through discharge valve 6 into the transfer passage 7.The compression ratio of the compression unit is assumed to be 20:1; that is, if we assume an atmospheric pressure of 101 Kilonewtons per square metre (14.7 p.s.i.) the pressure in the transfer passage assuming adiabatic compression will be 6695 Kilonewtons per square metre or 944 pounds per square inch and assuming an input temperature of 2930K (200C) the temperature will be 970 K. Compressed air begins to be admitted to the expansion unit 2 via variable cut-off admission valve 8 when the working chamber of the expansion unit is near its minimum volume. The compressed air mixes with the fuel charge, already introduced via injector 9 which is fed from injector pump 10 driven by shaft 3 and, since the incoming air charge has a sufficiently high temperature, combustion is initiated.Admission valve 8 remains open and compressed air continues to be admitted as the volume of the working chamber increases. In order to maintain constant pressure in the transfer passage, it is necessary that equal volumes of compressed air are supplied by the compressor unit and removed by the expansion unit.

Since the volume of the combustion products is dependent on the amount of fuel suppled, admission valve 8 needs to remain open for a longer period as fuel supply is increased so that the volume of compressed air fed from the transfer passage to the expansion unit remains constant. When admission valve 8 shuts, the working chamber of the expansion unit continues to increase in volume, expanding the combustion products until, at is maximum volume, exhaust valve 13 opens and exhaust gases are expelled via passage 14. Just before the working chamber of the expansion unit reaches minimum volume, exhaust valve 13 shuts and a fresh charge of fuel is introduced via injector 9.

The power output of the engine can be controlled by operation of pedal 11 which may vary the setting of cut off valve 8 as shown in Figure 1. Alternatively pedal 11 can vary the quantity of fuel supplied as shown in Figure la.

It is desirable that when discharge valve 6 opens the pressure in the compressor working space should be as close as possible to the pressure in the transfer passage to minimise noise and energy losses. This is effected by control system 16. This control system is fed with the difference between the pressure in the compressor cylinder when the discharge valve opens and the pressure in the transfer passage. It is not easy to measure the cylinder pressure directly but since the compression ratio is constant, in this case 20:1, and compression is adiabatic the cylinder pressure will be equal to the inlet pressure times 20 to the power of 1.4 (1.4 equals the ratio of the specific heats of air at constant pressure and constant volume.), that is, 66.3.

Transfer passage pressure is measured by transducer 15 and compression unit inlet pressure by transducer 17.

These signals are processed by control system 16 and the output either adjusts the amount of fuel injected by injector pump 10 as shown in Figure 1 or adjusts the cut-off ratio as shown in Figure la until there is no pressure difference when discharge valve 6 opens.

In order to start the engine, an electrically driven starter motor (not shown) can be used. During starting it may be necessary to provide a heater 12 in transfer passage 7 or glow plugs in the combustion chambers of the expansion unit to ensure that combustion is initiated.

Referring now to Figure 2, the cylinder block of a compression unit is shown generally at 40 and carries three cylinders 24, 25 and 26. Cylinders 24 and 25 carry pistons connected to crankshaft 30 and cylinder 26 carries a piston connected to crankshaft 31.

Crankshaft 30 and crankshaft 31 are geared together via gears 32 and 33 so that the rotational speed of crankshaft 31 is twice that of crankshaft 30. Ports 34 and 35 connect cylinders 24 and 25 to cylinder 26.

The operation of the compression unit will now be described with reference to Figures 3a, b, c and d. In this diagrammatic representation, cylinders 24 and 25 are shown on either side of cylinder 26 so that the action of the interconnecting ports 34 and 35 can be better understood.

Piston 41 reciprocates in cylinder 24 and is connected to the crank shown diagrammatically at 42 by means of rod 43. Piston 44 similarly reciprocates in cylinder 25 and is connected to crank 45 by rod 46. Cranks 42 and 45 are carried on crankshaft 30 (Figure 2) and are arranged so that pistons 41 and 44 are driven in antiphase.

Piston 47 is connected by rod 48 to crank 49 and reciprocates in cylinder 26. Crank 49 is carried on crankshaft 31 (Figure 2) which rotates at twice the speed of crankshaft 30. The phase of the gears 32 and 33 is arranged so that, when either piston 41 or 44 is at top dead centre, piston 47 is at bottom dead centre as shown in Figures 3a and 3d and, when piston 47 is at top dead centre, pistons 41 and 44 are at mid stroke as shown in Figure 3b.

Cylinders 24 and 25 carry discharge valves 50 and 51 which connect to the transfer passage 7 and inlet valves 52 and 53 through which atmospheric air may be drawn into the cylinder. Cylinder 26 may carry an additional inlet valve 54.

In this example the swept volume of cylinder 26 is assumed to be four times the swept volume of cylinder 24 or cylinder 25.

Figure 3a shows pistons 41 and 47 at their bottom dead centre positions just commencing to move upwards.

Cylinders 24 and 26 are filled with the air charge which has been drawn in during the previous stroke.

Valves 50, 52 and 54 are closed and the air is compressed and transferred from cylinder 26 to cylinder 24 as. the pistons move upwards. Piston 44 is at the top of its stroke and as it moves downwards inlet valve 53 opens and a fresh air charge begins to be drawn into cylinder 25.

In Figure 3b, piston 47 has reached the top of its stroke and all the air in cylinder 26 has been transferred to cylinder 24. Port 34 is now closed and compression continues in cylinder 24. Meanwhile the inlet stroke has been proceeding in cylinder 25 and piston 44 is about to uncover port 35. Piston 47 is about to descend and inlet valve 54 begins to open.

Figure 3c shows the suction stroke continuing in cylinders 25 and 26. In cylinder 24, piston 41 is 600 from top dead centre, that is, one quarter of its stroke remains. Thus, since cylinder 26 is four times the size of cylinder 24, the air charge has been compressed to one twentieth of its original volume and the pressure in cylinder 24 will be equal to the pressure in the transfer passage. Discharge valve 50 now opens and the compressed charge is delivered to the transfer passage. It should be noted that discharge valves 50 and 51 need to have a diameter of only about one quarter of the diameter of inlet valves 52 and 53.

Figure 3d shows the completion of delivery from cylinder 24. Piston 41 is at top dead centre and discharge valve 50 has closed. Suction is now complete in cylinders 25 and 26 and suction valves 53 and 54 are also closed. One half of a cycle has now been completed. Figure 3d is the same as Figure 3a except that piston 41 in Figure 3d is in the position that piston 44 occupied in Figure 3a and vice-versa. The sequence of events just described is therefore repeated with the functions of cylinders 24 and 25 transposed.

Referring now to Figure 4, the cylinder block of an expansion unit is generally at 82 and carries three cylinders 27, 28 and 29. Cylinders 27 and 28 carry pistons connected to crankshaft 80 and cylinder 29 carries a piston connected to crankshaft 81.

Crankshaft 80 and crankshaft 81 are geared together via gears 78 and 79 so that the rotational speed of crankshaft 81 is twice that of crankshaft 80. Ports 37 and 38 connect cylinders 27 and 28 to cylinder 29.

The operation of the expansion unit will now be described with reference to Figures 5a, b, c and d. As in the case of Figure 3, cylinders 27 and 28 are shown on either side of cylinder 29 so that the action of the interconnecting ports 37 and 38 can be better understood. The arrangement of cylinders and pistons for the expansion unit is similar to that for the compression unit.

Piston 61 reciprocates in cylinder 27 and is connected to the crank shown diagrammatically at 62 by rod 63.

Piston 64 similarly reciprocates in cylinder 28 and is connected to crank 65 by rod 66. Cranks 62 and 65 are carried on crankshaft 80 (Figure 4) and are arranged so that pistons 61 and 64 are driven in antiphase.

Piston 67 is connected to crank 69 of crankshaft 81 by rod 68 and reciprocates in cylinder 29. As in the case of the compression unit, crankshaft 81 rotates at twice the speed of crankshaft 80 and the phase of the gears 78 and 79 is arranged so that, when either piston 61 or 64 is at top dead centre, piston 67 is at bottom dead centre and, when piston 67 is at top dead centre, pistons 61 and 64 are at mid stroke.

Cylinders 27 and 28 carry variable cut-off admission valves 70 and 71 which admit compressed air from the transfer passage 7 (Figure 1) and exhaust valves 72 and 73. An additional exhaust valve 74 may be carried by cylinder 29. Fuel injectors 75 and 76 are fitted to cylinders 27 and 28 respectively.

As in the case of the compression unit, cylinder 29 has four times the swept volume of cylinder 27 or 28.

Figure 5a shows pistons 61 and 67 at their bottom dead centre positions just commencing to move upwards.

Cylinders 27 and 29 are filled with exhaust gases from the previous stroke. Exhaust valves 72 and 74 are opening and the exhaust gases are being swept from the cylinders. Piston 64 is at top dead centre. The clearance volume in cylinder 28 contains the fuel charge which was introduced via injector 76 towards the end of the previous stroke and retained exhaust gases which have been compressed. A substantial proportion of the charge has vaporized. Variable cut-off valve 71 now begins to open. The pressure in the clearance volume of cylinder 28 when admission valve 71 opens is slightly lower than that of transfer passage 7 and hence intense turbulence is produced as hot compressed air rushes into cylinder 28 and combustion is initiated.

Figure 5b shows combustion continuing. Admission valve 71 is still open and will remain open for a period dependent on the quantity of fuel injected. The size of valve 71 is chosen so that turbulence is maintained to ensure complete combustion. It is estimated that the diameter of valve 71 may be df the order of one quarter of the diameter of exhaust valve 72.

Initially, combustion takes place at a high fuel-air ratio which progressively reduces during the time valve 71 is open. Exhaust continues in cylinder 27 and 29 via exhaust valves 72 and 74.

In Figure 5c, piston 67 is at top dead centre, all the exhaust gases in cylinder 29 have been expelled and exhaust valve 74 has closed. Pistons 61 and 64 are at mid stroke and both ports 37 and 38 are closed.

Admission valve 71 has closed and expansion has been taking place in cylinder 28. As piston 64 continues to move downwards, port 38 is uncovered and, since piston 67 is also moving downwards, expansion continues in both cylinders 28 and 29. Meanwhile piston 61 continues to move upwards to expel the exhaust gases.

Figure 5d shows expansion almost complete in cylinders 28 and 29. Piston 61 is approximately 300 from its top dead centre position. Exhaust valve 72 has now closed and fuel is injected via injector 75.

Figure 5e shows the completion of the expansion stroke in cylinder 28 and 29. Exhaust valves 73 and 74 are opening and the exhaust stroke is commencing. Piston 61 is at top dead centre and admission valve 70 is about to open to initiate combustion .in cylinder 17.

Figure 5e is the same as figure 51 except that piston 61 in figure 5e is in the position that piston 64 occupied in Figure 5a and vice-versa. The sequence of events just described is therefore repeated with the functions of cylinder 27 and 28 transposed.

The mechanical construction of the engine will now be described with reference to Figures 6-13. Figure 6 is an end view of the engine. A compression unit is shown generally at 40 and an expansion unit is shown generally at 82. The compression and expansion units are mounted together in a common structure 100. The crankshafts of the high pressure cylinders of both the compression unit and the expansion unit are geared to the crankshafts of their respective low pressure cylinders, in the case of the compression unit via gearwheels 32 and 33 and in the case of the expansion unit via gearwheels 78 and 79.

The compression unit low pressure crankshaft 31 is geared via gearwheels 90 and 91 to the low pressure crankshaft 81 of the expansion unit so that compression and expansion crankshafts rotate at equal speeds.

Camshaft 98, which operates the inlet and discharge valves of the high pressure compression unit cylinders, is driven by belt 92 from crankshaft 30. Camshaft 99, which operates the inlet valve of the low pressure compression unit cylinder, is driven by belt 93 from crankshaft 31.

Similarly in the case of the expansion unit, camshaft 96 is driven by belt 94 from crankshaft 80 and camshaft 97 is driven by belt 95 from crankshaft 81. Camshaft 96' is driven from camshaft 96 via a differential mechanism and forms part of the mechanism to provide a variable cut-off ratio.

Figure 7 shows the arrangement of the high pressure compression unit cylinders 24 and 25 and the high pressure expansion unit cylinders 27 and 28. Piston 44 in cylinder 25 is driven by crank 45 and connecting rod 46 and piston 41 in cylinder 24 is driven in antiphase to piston 44 by means of crank 42 and connecting rod 43. Similarly for the expansion unit, pistons 64 and 61 are driven in antiphase by cranks 65 and 62 and connecting rods 66 and 63 respectively. Pistons 44 and 64 and pistons 41 and 61 move in phase but in opposite directions so that the assembly is dynamically balanced.

Camshaft 98 carries cams 102 and 104 which operate respectively the inlet valves 52 and 53, which connect to the inlet manifold 105,.and cams 101 and 103, which operate discharge valves 50 and 51 which connect to the transfer passage 7. Camshaft 96 carries cams 114 and 115 which respectively operate exhaust valves 72 and 73 which connect with the exhaust manifold 135. Camshaft 96 also carries cams 108 and 109 which form part of the variable cut-off admission valves not shown in this figure. Fuel is fed to cylinders 27 and 28 by injectors 75 and 76 respectively.

Figure 8 shows the arrangement of the low pressure compression cylinder 26 and the low pressure expansion cylinder 29. Cylinder 26 carries piston 47 driven from crankshaft 31 via crank 49 and connecting rod 48 and cylinder 29 carries piston 67 driven from crankshaft 81 via crank 69 and connecting rod 68. Pistons 47 and 67 move in phase but in opposite directions so that the assembly is dynamically balanced. Crankshaft 81 carries flywheel 118, and serves as the output shaft of the engine. Cams 134 and 116 respectively operate admission valve 54 and exhaust valve 74.

Figure 9 is an end view of the compression unit and shows the arrangement of cams and camshafts. As can be inferred from the positions of cams 101 and 103 with respect to their cylinders, the inlet valves operated by cams 104 and 102 are of substantially larger diameter than the discharge valves operated by cams 101 and 103.

Figure 10 is an end view of the expansion unit showing the arrangement of the cams and camshafts. The exhaust valves are directly operated by cams 114, 115 and 116.

Variable cut-off admission valves are operated from the centres of the differential levers 112 and 113. The cut-off ratio can be varied through differential mechanism 107 which connects camshaft 96 to camshaft 96'.

Figure 11 shows the relationship between the high and low pressure cylinders. It also shows air filter 106 and silencer 117.

The operation of the differential mechanism will now be described with reference to Figure 12. To camshaft 96' is secured a gearwheel 122 and to camshaft 96 is secured a similar gearwheel 119. Gearwheel 122 meshes with a first idler gear 121, which in turn meshes with a second, similar, idler gear 120, which in turn meshes with gearwheel 119. First idler gear 121 is carriea on a first arm 126 which rotates freely about camshaft 96' and holds idler gear 121 in mesh with gearwheel 122.

Second idler gear 120 is carried on a second arm 124 which rotates freely about camshaft 96 and holds idler gear 120 in mesh with gearwheel 119. The free ends of arms 126 and 124 are connected by a link 125 each end of which pivots about the same axis as one of the idler gears 121 and 120. The link 125 holds the two idler gears 121, 120 in mesh. To the arm 124 is secured a regulator lever 123 which rotates, with the arm 124, about the camshaft 96.

Since camshaft 96 is driven from crankshaft 80 in a clockwise sense, camshaft 96' will be driven via idler gears 121 and 120 to rotate at the same speed as camshaft 96 but in an anti-clockwise sense.

It can be shown that adjustment of the rotational position of arm 124 produces a change in the rotational position of camshaft 96' relative to the rotational position of camshaft 96 which is equal to twice the sum of the angular changes in position of arms 124 and 126.

The adjustment of arm 124 is conveniently produced through lever 123 by means of the power setting pedal 11.

The construction of the variable cut-off admission valve is shown in Figure 13. The valve is operated by two similar cams 108 and 110. Cam 108 is carried on camshaft 96 which is driven at crankshaft speed in an anti-clockwise direction. Cam 110 is carried on camshaft 96' and driven via the differential mechanism also at crankshaft speed but in a clockwise direction.

The valve 70 is driven from the centre point of differential lever 112 through sliding member 127 which is carried on the valve stem 129 and pivotted on differential lever 112.

The valve 70 is subject to the input pressure which tends to open it. It is therefore provided with a balancing piston 133 which is slightly larger than the area of the valve to provide the restoring force for valve closing. This is augmented by the spring 128.

The balancing piston 133 is supplied with compressed air from the transfer passage via the aperture 131.

Sliding member 127 and differential lever 112 are kept in contact with cams 108 and 110 by means of spring 132. Adjusting nuts 130 are provided so that, when cam 110 is at maximum lift and cam 108 at minimum lift, there is minimum clearance between the face of sliding member 127 and the top of adjusting nut 130.

The operation of the variable cut-off mechanism will now be described. Cam 108 governs the opening of the valve and, prior to its beginning to operate its end of the differential lever 112, cam 110 has reached maximum lift and closed the clearance between the end face of sliding member 127 and the adjusting nut 130.

Cam 108 continues to have maximum lift for approximately 1800 and closing of the valve is effected by cam 110 which is similar in shape to cam 108 but is phase controlled by the differential mechanism. Thus the closing time of the valve is determined by the differential mechanism setting.

The other admission valve 71 is driven in an exactly analogous manner to valve 70.

Discharge valves 50 and 51 of compression unit 40 are also preferably balanced valves.

A split cycle engine according to the invention provides the low noxious emissions and fuel economy of a Diesel engine with the refinement and driveability of a spark ignition engine. The engine uses a constant pressure cycle, thus avoiding high peak cylinder pressures, and consequently can be much more lightly constructed than a conventional compression ignition engine. Furthermore, the fact that fuel is injected into the cylinder of the expansion unit before the end of the exhaust stoke, while the pressure in the cylinder is substantially at atmospheric pressure, enables injector design to be simplified. The fuel and the residual exhaust gases are then compressed; a substantial proportion of the fuel vaporizes and the temperature of the retained exhaust gases is raised.

When admission of the hot compressed air charge first takes place, intense turbulence is created. Initial combustion takes place under very fuel-rich conditions, the fuel/air ratio progressively decreasing as admission continues. Intense turbulence continues throughout combustion because of the high velocities which can be tolerated through the admission valve with negligible losses.

Half the number of injectors are required for engines according to the invention compared with an equivalent four stroke conventional engine.

The use of compound units, in which two high pressure cylinders operate in conjunction with a low pressure cylinder, the low pressure crankshaft rotating at twice the speed of the high pressure crankshaft, enables much higher effective rotational speeds to be used, thereby increasing substantially the power output from a given cylinder displacement.

In engines according to the invention, all valves are positively mechanically operated; compounding increases the time available to operate these valves by a factor of up to five times and also reduces the torque variations over one cycle by a factor of about four times, enabling a small flywheel to be used and providing better low speed performance. The reduction in torque needed to start the engine results in a smaller starter motor and battery requirement.

The arrangement of compression and expansion units provides a compact engine which is convenient for packaging in a vehicle. It also provides complete dynamic balance. Low fuel/air ratios and the retention of exhaust gases result in very low emissions.

Claims (10)

1. A split cycle internal combustion engine adapted to drive a load, the engine comprising a compression unit for inducting and compressing air; an expansion unit for combusting fuel and air compressed in the compression unit; and a transfer passage communicating between the compression and expansion units, the compression unit including valve means for discharge of compressed air to the transfer passage and the expansion unit including means for the injection of fuel and valve means for the admission of compressed from the transfer passage, the engine including means whereby fuel is injected towards the end of the exhaust stroke during the period when the pressure in the expansion unit is substantially at atmospheric pressure.

2. An engine according to Claim 1, including power output control means operatively connected firstly to the admission valve means to vary the opening period thereof and secondly to the fuel injection means to control the quantity of fuel injected in dependence on the opening period of the admission valve, whereby the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when- the discharge valve means begins to open.

3. An engine according to Claim 1, including power output control means operatively connected firstly to the fuel injection means and secondly to the admission valve means to vary the opening period thereof in dependence on the quantity of fuel injected, whereby the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when the discharge valve means begins to open.

4. An engine according to any preceding claim, in which the compression and expansion units each comprise compound units.

5. An engine according to Claim 4, in which the compound units each comprise two high pressure cylinders connected to a common crankshaft and a low pressure cylinder, the low pressure crankshaft rotating at twice the speed of the high pressure crankshaft.

6. An engine according to Claim 5, in which ports communicate between the upper region of the low pressure cylinder and the middle region of the high pressure cylinders of each compound unit.

7. An engine according to Claim 5 or Claim 6, in which the pistons in the high pressure cylinders of each compound unit are arranged to be driven in antiphase and the low pressure and high pressure crankshafts of each unit are parallel with each other and with the crankshafts of the other unit, the crankshafts of the respective units are mechanically coupled so that corresponding crankshafts rotate at the same speed, the cylinders of each unit are co-planar with each other and with the cylinders of the other unit, and the pistons in the one unit are arranged to move in phase but in the opposite direction to the corresponding pistons in the other unit.

8. A method of operation of a split cycle internal combustion engine including a compression unit, an expansion unit and a transfer passage communicating between the compression and expansion units, in which fuel is injected into the working space of the expansion unit towards the end of the exhaust stroke during the period when the pressure in the expansion unit is substantially at atmospheric pressure.

9. A method according to Claim 8, in which the pressure in the transfer passage is maintained substantially equal to the pressure in the compression unit when the discharge valve controlling entry to the transfer passage from the compression unit begins to open.

10. A split cycle engine substantially as herein described with reference to and as illustrated in the accompanying drawings.