US3550569A - Step-piston two-stroke engines - Google Patents

Description

United States Patent Inventor Bernard Hooper Maybank House, Hope St. Wordsley, Stout-bridge, Worcester, England 798,688

Feb. 12, 1969 Dec. 29, 1970 Jan. 3, 1969 Great Britain No. 422/69 Appl. No. Filed Patented Priority STEP-PISTON TWO-STROKE ENGINES 19 Claims, 11 Drawing Figs.

11.8. CI Int. Cl

Field ol'Search .III .IIImQI 123/71, 655

References Cited UNITED STATES PATENTS 880,704 3/1908 Wood Coles 1,040,299 10/1912 Fite .1:

1,182,494 5/1916 Leech 123/71 1,813,730 7/1931 Famam el al.. 123/71 2.923.281 2/1960 Martin 123/71 Primary Examiner-Wendell E. Burns Attorney-Kurt Kelman ABSTRACT: The specification discloses a tw0-stroke internal combustion engine having one or more cylinders each containing a stepped piston, each cylinder and piston having a pumping part of larger diameter and a working part of smaller diameter, the working part of the or each cylinder being charged from the pumping part of the cylinder or one of the 'cylinders. Each cylinder has a receiver associated therewith into which the charge is received from said pumping part and from which it is delivered to said working part, the volume of the receiver being at least three times the swept volume of said pumping part and the sole valve means between the or each cylinder and its associated receiver being formed by port means in the working part of the cylinder, the piston itself and recess means in the piston.

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2. Description of the Prior Art Such engines have previously been proposed because they do not rely on crankcase compression for charging the working part of the or each cylinder but are charged by a charge delivered from the pumping part of the or each cylinder and since the charge does not pass through the crankcase it is possible in such engines to use lubrication techniques which have been highly developed in connection with four-stroke engines and are in conventional use therein rather than mixing the lubricating oil with the fuel.

Difficulties have previously been encountered in obtaining good volumetric efficiency in such engines and it is an object of the invention is to provide an engine having improved operating characteristics over those in se.

SUMMARY OF THE INVENTION According to the invention we provide an engine including one or more cylinders each containing a stepped piston, the or each cylinder and its contained piston having a pumping part of larger diameter and a working part of smaller diameter, a receiver associated with the or each cylinder and arranged to receive, through port means in the working part of its associated cylinder, a charge from the pumping part of said cylinder. and to deliver, through said port means, the charge to the working part of said cylinder, recess means in the piston in said cylinder and which cooperate with the port means to allow and control entry of the charge into the receiver, the piston also being arranged to cooperate with the port means to allow and control entry of the charge into the working part of said cylinder from said receiver; and means to control the entry of the charge into the pumping pan of said cylinder; there being no valve means other than that formed by the cooperation of 'the port means and the piston and the recess therein between the receiver and the working or pumping parts of said cylinder, the or each receiver being isolated from the crankcase of the engine and arranged so that the total volume of the or each receiver plus that of any passages between the receiver and the cylinder or cylinder or cylinders with which it communicates is atleast three times the swept volume of the pumping part of the cylinder, or of one of the cylinders if more than one, with which the receiver communicates.

The term receiver" is used herein to means a volume or space into which the charge is delivered from the pumping part of the or each cylinder of the engine thus the receiver need not be a single vessel but can be made up of a number of vessels each connected to its associated cylinder or cylinders. Conveniently in a single cylinder engine, the receiver can comprise two chambers connected to the cylinder.

In the operation of, for example, a single cylinder engine embodying the invention, upon the expansion stroke a charge is induced into the pumping part of the cylinder while the exhaust gas escapes from the working part of the cylinder, the port means being closed during the majority of this stroke. At the end of the stroke, the receiver is allowed to deliver its charge into the working part of the cylinder thus scavenging it and, during the compression stroke, a charge which is now trapped in the pumping part of the cylinder is transferred through the recess means to the receiver while a charge which has previously-been delivered to the working part of the cylinder is compressed in said part. The cycle then continues as before.

Two arrangements of multicylinder engine may be provided. in the first arrangement each cylinder of the engine communicates with a receiver which is separate from the receivers communicating with the other cylinders, In this arrangement, the total volume of receiver associated with each cylinder plus that of any passages between the receiver and the port means of its associated cylinder is at least three times the swept volume of the pumping part of the cylinder.

In another arrangement of multicylinder engine the or each receiver communicates with at least two of the cylinders and the total volume of the common receiver plus that of any passages between the common receiver and the port means in the cylinders with which it communicates is at least three times the swept volume of the pumping part of one of said cylinders,

Either of the foregoing arrangements of multicylinder engine may be used when the volume of the or each receiver is fixed and invariable. However, I have found that if the volume of the or each receiver can vary in a given manner, this gives advantageous results. However, the use of a variable volume receiver is only applicable to a single cylinder engine in which the cylinder is associated with a receiver or to a multicylinder engine in which each cylinder is associated with its own receiver which is separate from the receivers associated with the other cylinders. In every case the or each receiver is isolated from the crankcase of the engine.

According to this feature of the invention there is provided, for the or each cylinder, an associated receiver which is isolated from the engine crankcase and communicates only with its associated cylinder and a movable wall of the or each receiver and arranged so that, over at least the lower part of the operative speed range of the engine, as the charge is received in the receiver the volume of the latter increases and so that as the charge is delivered from the receiver the volume of the latter decreases, the total volume comprising the minimum volume of the receiver plus that of any passages between the receiver and the port means being at least three times the swept volume of the pumping part of the cylinder with which the receiver is associated.

I have found that the amount of charge retained in the receiver for delivery to the working part of the cylinder can be increased, as compared with engines embodying the invention buthaving the or each receiver of fixed volume, by providing each receiver with a movable wall so that the volume of the receiver can vary such that the volume increases as the charge enters the receiver and decreases as the charge is delivered from the receiver. I have found that this arrangement gives increased volumetric efficiency.

The movable wall preferably takes the form of a diaphragm and various arrangements may be used to move the diaphragm.

There are two main possibilities, thus either the movable wall can have a movement imposed thereon which gives the desired effect or the movable wall can be subject to a substantially constant pressure which is less than the maximum pressure in the receiver but greater than the minimum pressure therein so that as the charge enters the receiver and the pressure increases the wall will be deflected to increase the volume of the receiver and as the charge is delivered the movable wall will be deflected by said constant pressure to decrease the volume of the receiver.

In the first arrangement, the movable wall may be located between the crankcase of the engine and the receiver and be subject to changes of pressure in the crankcase due to the reciprocation of the piston in the cylinder associated with the receiver. It is apparent that if the invention is applied to a multicylinder engine it will be necessary to divide the crankcase into a number of compartments each corresponding to one of the cylinders and arranged so that the pressure in each compartment moves the wall of the receiver associated with the cylinder corresponding to the compartment.

Where a steady pressure is applied to the movable wall, such pressure being between the minimum and maximum pressures in the receiver, the pressure may be derived from a spring, from the pressure in the crankcase of the engine or from a separate air supply which may be bled off some convenient point of the engine i.e. from the pumping part of the cylinder. Thus if the crankcase is sufficiently large, although the pressure therein will vary there will be a mean pressure and if this is between the maximum and minimum pressures in the receiver, it may be used to bias the movable wall thereof.

Where the movable wall has movement imposed thereon from the crankcase, for example, the amplitude of this movement will decrease as the speed of the engine increases so that the imposed movement may only be effective at the lower part of the operative speed range of the engine but this will assist in engine starting and will increase efficiency in the lower speed ranges of the engine.

I have found that if said total volume is less than three times said swept volume there results difficulty in starting and a severe restriction in power. The receiver has to be of sufficient volume to prevent a significant proportion of the charge from being forced to return immediately into the pumping part of the cylinder.

Thus the large volume of the receiver as compared to the volume of the pumping part of the cylinder with which it is associated acts to damp the flow of gas out of the receiver as the pumping part of the piston returns towards its bottom dead center, position and before the port means is closed by the piston wall. If the receiver volume is too small a high pressure will be generated in the receiver with consequent considerable loss of charge through the port means as the piston moves from top dead center. By damping this outflow of charge more of the charge remains in the receiver to be delivered into the working part of the cylinder when the port means is uncovered by the working part of the piston.

It is preferred that said total volume shall be between three and twelve times said swept volume. I have found that if said total volume is greater than twelve times said swept volume, although the power output of the engine is not seriously restricted there occurs difficult starting, backfiring into the receiver and also an excessive space requirement for the receiver. Moreover, in petrol engines and other spark ignition engines it is necessary to keep the receiver volume as low as possible due to the danger that backfiring into the receiver may ignite the stored charge.

For optimum results it is preferred that said total volume is between four and six times said swept volume.

In order to secure maximum power from the engine it is desired to tune the volume of the receiver and said passages, where provided, with respect to the speed of the engine. Preferably, therefore, the difference between the maximum and minimum pressures in the or each receiver during operation of the engine is a maximum at a speed S which lies between 0.5 N and 0.9 N where N is the speed at which the engine develops its maximum power. This is the requirement for a general purpose engine. It is desired to make said difference a maximum since this will mean that the maximum charge will have been transferred from the receiver since the higher the maximum pressure in the receiver during operation the greater will be the charge introduced into the receiver and the less the minimum pressure in the receiver during operation the greater will be the amount of the charge which has been delivered from the receiver.

For a sports, or racing engine the criterion is rather different and in such a case it is preferred that the difference between the maximum and minimum pressures in the or each receiver during operation of the engine is a maximum at a speed S which lies between 0.7 N and 1.1 N where N is the speed at which the engine develops its maximum power.

Preferably, the port means of the or each cylinder each consists of a plurality of ports and each port of said plurality is arranged to be traversed by a charge both as it enters the receiver and as it is delivered therefrom. In other words, each of the ports in the working part of the or each cylinder provides a path for the charge to enter the receiver and also a path for the charge to be delivered from the receiver into the working part of the cylinder. In a single cylinder engine there cylinder and in such an arrangement the; ports of the port I means are arranged so as to provide uniflow scavenging.

The invention will now be described in detail by way of example with reference to the following drawings in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a section through an engine embodying the invention with the piston shown at top dead center, the halves of the drawing being taken in two planes at right angles to each other;

FIG. 2 is a view similar to FIG. 1 but with the piston shown a l l5 fter top dead center;

FIG. 3 is a view similar to FIGS. 1 and 2 but with the piston shown at bottom dead center;

FIG. 4 is a view similar to FIGS. 1 to 3 but showing the piston at 320 r top dead center;

FIG. 5 is a sectional view of an arrangement constituting a second embodiment of'the invention and including mechanically operated inlet and exhaust valves;

FIG. 6 is a graph indicating the effect of the change in volume of the receiver;

FIG. 7 is a graph showing the effect of tuning the air receiver;

FIG. 8 is a view similar to FIG. 1, but showing an engine having a receiver with a movable wall;

FIG. 9 is a view of the engine of FIG. 8 with the piston at bottom dead center;

FIG. 10 is a detail view showing the movable wall controlled by a spring; and

FIG. 11 is a detail view showing the movable wall controlled by an air bleed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 to 4, the engine there shown comprises a crankcase casting 10 having formed therein the pumping part 11 of a stepped cylinder, the working part 12 of the cylinder being formed in a cylinder casting 10a; the pumping part 11 is of greater diameter than the working part 12. Reciprocable in the cylinder is a piston indicated generally at 13 and having a working part 14 which is reciprocable in the working part 12 of the cylinder and a pumping part 15 which is reciprocable in the pumping part 11 of the cylinder.

A combustion space 16 is provided at the top of the cylinder into which protrudes a sparking plug 17. An exhaust port 18 is formed in the wall of the working part 12 of the cylinder which communicates with an exhaust passage 19 and an exhaust pipe 20. An inlet passage 21 is formed in the lower part of the cylinder casing and contains a reed valve comprising two blade members 22 which are made of springy metal and which in their normal positions engage a center bar 23. The ends of the members 22'remote from the bar 23 are attached to a wedge shaped member 24. The inlet passage 21 communicates with an inlet pipe 25 which in turn is connected to a carburetor, not shown.

The piston is connected by means of a connecting rod 26 to the crankpin 27 of a crankshaft. The working part 14 of the piston is provided with piston rings 28 and the pumping part 15 of the piston is provided with a piston ring 29.

Formed in the casting 10 are two incremental receiver volumes one of which is indicated at 30, the other volume being at l to the volume shown in the FIG. when the engine is viewed in plan. The receiver volume 30 communicates by means of a passage 31 with a port 32 in the wall of the working part of the cylinder, the receiver volumes thus communicate with the working part 12 through two ports 32 which together constitute port means;

The piston is provided with recesses indicated generally at 33 in its surface. A lower part of the recess indicated at 34 is formed in the pumping part of the piston, theupper part of the recess is provided with a deep portion 35 in the working part of the piston into which the gudgeon pin boss 36 protrudes. The piston is provided with a recess such as 33 communicating with each port 32.

The operation of the engine described above is as follows. In FIG. 1, the engine is shown at top dead center and it will be assumed that the charge has been ignited and that the receiver volumes 30 are full of a fresh charge which they have obtained in a manner to described. As the piston begins to move down from its ,top' dead center position, the exhaust gases flow out into the exhaust passage 19 as indicated by the arrows 37 in FIG. 2. The pumping part 15 of the piston also creates a depression on the pumping part 11 of the cylinder so that a charge is indicated by the arrow 38 flows into the pumping part of the cylinder forcing the members 22 apart so that the pumping part of the cylinder becomes filled with the charge. It will be notedthat at this time the upper part 39 of the piston wall is opposite to the ports 32 so that the charge in the receiver volume 30 is not able to enter the working part 12 of the cylinder.

When the piston has reached the bottom dead center as shown in FIG. 3, the part 39 of the piston wall has uncovered the ports 32 so that a charge flows from the receiver volume 30 into the working part 12 of the cylinder as indicated by the arrow 40. The two ports 32 are arranged to give loop scavenging and to assist in the exit of the exhaust gases indicated by the arrows 37.

It will be noted from FIG. 3 that the members 22 are moving together again as the depression in the pumping part 11 of the cylinder decreases. As the piston begins to move up from bottom dead center, the members 22 which constitute an inlet valve, close so that the charge is trapped in the pumping part of the cylinder. As the piston moves up, the charge is compressed in the pumping part of the cylinder and simultaneously the charge in the working part 12 of the cylinder is also compressed. Just before the piston reaches its top dead center position it will be seen that therecess 33 comes into alignment with the ports 32 leading to the receiver volume 30 so that the charge trapped in the pumping part of the cylinderwhich has been compressed flows as indicated by the arrow 41 into the receiver volume 30. When the piston reaches its approximate top dead center position the charge is ignited as before and the cycle is repeated.

It will be seen that during each cycle a charge is compressed in the pumping part of the cylinder and delivered to the receiver volumes and is delivered from the receiver volumes to the working part of the cylinder. It will further be seen that although there is a valve constituted by the members 22 controlling the entry of the charge into the pumping part of the cylinder there is no valve other than that formed by the cooperation of the port 32 with the recesses 33 and the part 39 of the piston wall, between the receiver volumes and the pumping part or the working part of the cylinder.

l have found that tov obtain the best results the total volume of the receiver must bear a relation to the swept volume of the pumping part of the cylinder with which the receiver is associated. I have found that the total volume of the two receiver volumes with the passages such as 31 leading up to the ports such as 32 must, for optimum performance, be at least three times the swept volume of the pumping part of the cylinder. The reason for this is as stated above.

FIG. 6 is a graph showing the relation between the brake horsepower developed by the engine and the number of times the total volume of the receiver and passages is of the swept volume of the pumping part of the cylinder with which the receiver is associated. The curve is indicated at 42 and it will be seen that the optimum range starts to the right of a line 43. The region 44 between the line 43 and the initial portion 45 of the line 42 is a region of reduced air consumption causing low performance of the engine and poor starting. The upper part of the range is defined by the line 46 which is equivalent to the receiver volume, with the passages, being twelve times the swept volume of the pumping part of the cylinder. I have found that beyond this line. there is a region of instability causing backfiring into the receiver and poor starting.

In petrol engines and other engines using spark ignition it is essential to keep the receiver volume as low as possible due to the danger of a backfire into the receiver igniting the stored charge. It is therefore desired where possible to choose a ratio lying between the lines indicated at 47 and 48 i.e. where the total volume of the receiver and its passages is between four and six times the swept volume of the pumping part of the cylinder. It will be seen that within this range the maximum performance will nearly be attained while the receiver volume can be kept comparatively small thus assisting to keep the power-to-volume ratio small.

I have also found that it is advantageous to tune the receiver and passages to provide a resonant system having a natural frequency so arranged that the maximum difference between the maximum and minimum pressure in the receiver occur in a predetermined speed range.

FIG. 7 is a graph showing the effect of turning the air receiver system. The graph shows the receiver pressure against the crank position and the curve indicated at 49 is a typical curve of the pressure variation in the receiver which will reach a maximum as indicated at 50 when the charge flows into the receiver and a minimum such as indicated at 51 when the charge has flowed out of the receiver. It will be apparent that for the maximum charge to be transferred the difference as indicated by the line 50a should be a maximum since the greater the maximum pressure the more charge will have been forced into the receiver and the less the minimum pressure the less charge will have remained in the receiver at the end of the flow of charge from the receiver. A preferred form of curve is that indicated generally at a 50a having a maximum at 53 and a minimum at 54, so that it will be seen that there is a gain between the maxima 50 and 53 and also a gain between the minima 51 and 54. It is preferred that this optimum curve 52 shall occur at a speed S which, in a case of a general purpose engine, is between 0.5 N and 0.9 N where N is the speed at which maximum power is developed. For a sports or racing engine it is preferred that the optimum curve 52 shall occur at a speed S between 0.7 N and L] N where, as before, N is the speed at which maximum power is developed.

The volume transferred should be as large as possible to get the maximum power and in fact the curves of FIG. 7 can be developed on an oscilloscope and the air receiver system tuned accordingly.

Various modification can be made to the invention thus far described which is applicable to a multicylinder engine. Such a multicylinder engine may comprise a number of cylinders each having their own receiver so that each cylinder will act substantially as the engine described above. In this arrangement, the receiver associated with each cylinder will have the relation described above, i.e. the total volume of the receiver plus its passages will be between three and twelve times the swept volume of the pumping part of the cylinder.

In an alternative arrangement of multicylinder engine, two or more of the cylinders thereof may share a common receiver. In this arrangement, the volumeof the common receiver plus the passages between the receiver and the cylinders with which it communicates will be between three and twelve times the volume of the pumping part of one of such cylinders. As mentioned above this arrangement is only possible where the volume of each receiver is fixed.

FIG. 5 shows an arrangement in which there are provided mechanically operated inlet and exhaust valves. The operation of the engine is otherwise as described above. Referring to FIG. 5, the engine comprises a cylinder casing 53 having a cylinder comprising a working part 54 and a pumping part 55.

A piston 56 is reciprocable in the cylinder and has a working part 57 and a pumping part 58. Two incremental receiver volumes 59 are provided which communicates through passages 60 with ports 61 in the wall of the working part of the cylinder 54. The exhaust from the combustion chamber 62 is controlled by a poppet exhaust valve 63 operated by a cam shaft, not shown. A sparking plug is indicated at 64. The inlet of the charge into the pumping part 55 of the cylinder is controlled by an inlet valve 65 which is also operated from the cam shaft.

Operation of this engine of FIG. is as described above in relation to the engine of FIGS. 1 to 4 except for the mechanical operation of the valves as compared with the automatic operation of the reed inlet valve and the use of the piston controlled ports for exhaust.

Referring now to FIGS. 8 and 9, the engine there shown is identical to that shown in FIGS. 1 to 4 except for the arrangement of the receiver. Parts of the engine of FIGS. 8 and 9 which correspond to parts of the engine of FIGS. 1 to 4 are given identical reference numerals as are corresponding flow arrows. The difference between the two engines lies in the construction of the receiver.

Thus a receiver volume 70 is formed in the crank case casting 10 between a cover plate 71 and a flexible diaphragm 72 which is sandwiched around its edge between an abutment 73 on the crankcase casting and a mounting ring 74. The diaphragm is movable between the position shown in FIG. 8 and the position shown in FIG. 9. When the diaphragm is in the position shown in FIG. 8 the volume of the receiver 70 is at a maximum while when the diaphragm is in the position shown in FIG. 2 the volume of the receiver is at a minimum.

The receiver 70 communicates with the working part 12 of the cylinder by means of the passage 31 leading to the port 32 in the wall of the working part 12. l

The operation of the engine is substantially similar to the operation of the engine described in relation to FIGS. 1 to 4. Thus referring to FIG. 8 a charge has been compressed in the pumping part 11 of the cylinder and is being delivered through the recess 35 into the passage 31 and the receiver 70 as indicated by the arrow 41. The diaphragm 72 will be in the position shown in FIG. 8 since the pressure of the charge being introduced into the receiver 70 will be greater than the pressure in the crankcase 75 of the engine, this latter pressure being low since the piston is in its top dead center position.

The receiver thus has its maximum volume and is able to receive a maximum amount of the charge indicated by the arrow 41. If the charge in the combustion space is now ignited, the piston will start to move towards its bottom dead center position and as shown in FIG. 9 upon approaching the bottom dead center position the upper part 39 of the sidewall of the piston will uncover the port 32 and allow the charge to escape from the port into the working part 12 of the cylinder, this escaping charge being indicated by the arrow 40. The exhaust gases will pass through the exhaust ports 18 as indicated by the arrows 37. The diaphragm 72 will now move to the position shown in FIG. 9 due to an increase in the pressure in the crankcase 75 due to the descent of the piston to its bottom dead center position and will thus decrease the volume of the receiver thus ensuring that the charge is efficiently expelled from the receiver.

As has been described above, the total volume comprising the minimum volume of the receiver 70 plus the volume of the passage 31 must be at least three times the swept volume of the pumping part 11 of the cylinder. This comparatively large volume of the receiver and its associated passage is required to damp the flow of the charge so as to prevent a significant proportion of the charge flowing back into the pumping part of the cylinder as the piston commences to descend from the position shown in FIG. 8. I have found that the combination of this large receiver volume with a movable wall such as provided by the diaphragm 72 does improve the volumetric efficiency of the engine. The movement of the diaphragm will be imposed upon it by movement of the piston in the crankcase 75, so that it will be the required movement i.e. so that the volume of the receiver will be at a maximum when the piston is in its top dead center position and the volume of the receiver will be a minimum when the piston is in its bottom dead center position. As the speed of the engine increases the amplitude of the movement of the diaphragm will decrease and at higher speeds there may be a mean pressure in the crankcase which will be between the maximum and minimum pressure in the receiver so that changes in volume of the receiver will take place but not to the same amplitude as shown in FIGS. 8 and 9.

Various modifications may be made to the invention which has been specifically described in relation to the single cylinder engine.

If desired the diaphragm may be subjected to a substantially constant pressure by a spring or by a separate air feed bled off the engine at a convenient position, for example from the pumping part of the cylinder. The former arrangement is shown in FIG. 10 where the diaphragm 72 is acted upon by a spring 76 and the latter arrangement is shown in FIG. 11 where the diaphragm 72 closes a chamber 77 formed between the diaphragm and a wall 78 of the crankcase, the chamber 77 being connected by a pipe 79 and passages 80 and 81 to the pumping part 12 of the cylinder. If necessary a nonretum valve may be provided in the pipe 79 and a control valve may be provided in the pipe 79 communicating with the chamber 77 to control the value of the pressure in the chamber. The spring force or pressure force on the diaphragm will be such as to be between the maximum and minimum pressure in the receiver thus allowing the receiver volume to change substantially in the manner as described above.

If the crankcase is sufficiently large then the pressure therein will be substantially, constant and can be arranged to be between the maximum and minimum pressure in the receiver.

The invention is also applicable to multicylinder engines where each cylinder has its own receiver as described above and if it is desired to use a crankcase pressure to operate the movable wall then it will be necessary to partition off the crankcase into a number of sections each section corresponding to one of the cylinders.

The swept volume of the pumping part of the or each cylinder may be equal to or different from the swept volume of the working part of the cylinder. If the swept volume of the pumping part is greater than swept volume of the working part then the scavenging is improved if loop scavenging is employed. If uniflow scavenging is employed supercharging may be obtained. This feature is applicable both to petrol and diesel engines.

It will be seen that the invention provides an engine which is simple but which has good volumetric efficiency.

Iclaim:

1. A two-stroke engine comprising: a crankcase; at least one cylinder containing a stepped piston, the cylinder and piston each having a pumping part of larger diameter and a working part of smaller diameter; a receiver associated with the cylinder; port means in the working part of the cylinder and communicating with the receiver; the receiver being arranged to receive a charge from the pumping part of said cylinder and to deliver, through said port means, the charge to the working part of said cylinder; recess means in the piston and which cooperates with the port means to allow and control entry of the charge into the receiver, the piston also cooperating with the port means to allow and control the entry of the charge into the working part of said cylinder, the port means, the piston and the recess therein constituting the sole valve means between the receiver and the working and pumping parts of said cylinder; and means to control the entry of the charge into the pumping part of said cylinder, the receiver being isolated from the crankcase of the engine and the total volume of the receiver plus any passages between the receiver and the cylinder with which it is associated being at least three times the swept volume of the pumping part of said cylinder.

2. A multicylinder engine according to-claim 1 including a plurality of receivers and wherein each cylinder communicates with a receiver which is separate from the receivers communicating with the other cylinders.

3. A multicylinder engine according to claim 1 including at least one receiver, each of which communicates with at least two cylinders said total volume of each receiver being at least 3 times the swept volume ofthe pumping part of one of said cylinders.

4. An engine according to claim 1 wherein the receiver communicates only with a single associated cylinder and wherein the receiver has a movable wall arranged so that. over at least the lower part of the operative speed range of the engine. as the charge is received in the receiver the volume of the latter increases and so that as the charge is delivered from the receiver the volume of the latter decreases, said total volume comprising the minimum volume of the receiver plus that of any passages between the receiver and the port means and being at least three times the swept volume of the pumping part of the cylinder of which the receiver is associated.

5. An engine according to claim 4 wherein the movable wall is in the form of a diaphragm.

6. An engine according to claim 5 wherein the diaphragm has a movement imposed thereon.

7. An engine according to claim 6 wherein the diaphragm is located between the crankcase-of the engine and the receiver and is subject to changes of pressure in the crankcase.

8. An engine according to claim 5 wherein the diaphragm is subject to a substantially constant pressure which is less than the maximum pressure in the receiver but greater than the minimum pressure therein.

9. An engine according to claim 8 wherein said substantially constant pressure is derived from a spring.

10. An engine according to claim 8 wherein said substantially constant pressure is derived from the pressure in the crankcase.

11. An engine according to claim 8 wherein said substantially constant pressure is derived from a separate air supply bled off the engine.

12. An engine according to claim 1 wherein said total volume is between three and twelve times said swept volume.

13. An engine according to claim 1 wherein said total volume is between four and six times said swept volume.

14. An engine according to claim 1. wherein the difference between the maximum and minimum pressures in the receiver during operation of the engine is a maximum at a speed S which lies between 0.5 N and 0.9 N, where N is the speed at which the engine develops its maximum power.

15. An engine according to claim 1 wherein the difference between the maximum and minimum pressures in the receiver during operation of the engine is a maximum at a speed 5 which lies between 0.7 N and H N, where N is the speed at which the engine develops its maximum power.

16. An engine according to claim 1, wherein the port means of the cylinder consists of a plurality of ports and wherein each port of said plurality is arranged to be traversed by a charge both as it enters the receiver and as it is delivered therefrom.

17. An engine according to claim 16, wherein the ports are arranged so as to provide loop scavenging in the cylinder.

18. An engine according to claim 16, including a mechanically operated exhaust valve in the cylinder and wherein the ports of said cylinder are arranged so as to provide uniflow scavenging.

19. An engine according to claim 1, wherein the difference between the maximum and minimum pressures in the receiver during operation of the engine is a maximum at a speed S which lies between 0.5 N and 1.1 N, where N is the speed at which the engine develops its maximum power.

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