US2408031A - Free piston internal-combustion engine - Google Patents

Description

Sept. 24, 1946. is. LBE'ME Y 2 4 8,03

FREE PISTON INTERNAL-COMBUSTION ENGINE Filed Feb. 16, 1945 2 Sheets-Sheei 1 Se t. 24, 1946. E. s. BE ALE 2,403,031

FREE PISTOITIHTERHAL-QOIBU3TION ENGINE Filed Feb. 16, 1945 2 Shets-Sheet 2' S: Q 7 V $5 MQ E2 J V v 1 Q r2 E g E Q (mu E mm 5 V S (Q & \MM 7 m @k m/rslvrorc fre l Jfiularf lansolowne flea/ -& 7-77.

Patented Sept. 24, 1946 Z,4d8,03l

FREE PISTGN INTERNAL-COMBUSTION ENGINE Evelyn Stewart Lansdowne Beale, Staines, England, assignor to Alan Mimtz & Company Limited, Hounslow, England, a company of Great Britain Application February 16, 1943, Serial No. 476,041 In Great Britain February 2, 1942 14 Claims.

This invention relates to free-piston internal- COIl'llbllStlOll engines of the kind in which the free-piston assembly consists of a power piston rigidly connected to a piston of larger diameter serving as a compressor piston or as a pneumatic cushion piston or as both. The free-piston assembly is moved through the in-stroke, namely the one that effects compression in the combustion chamber, at least in part by energy accumulated in the pneumatic cushion during the preceding expansion stroke. The cylinders in which these pistons reciprocate are coaxial, and, as they are of different diameters, the clearances between piston and cylinder," required to allow for thermal expansion, are larger for the larger cylinder. Furthermore, as it may be necessary to make the larger piston of an aluminium alloy and the smaller piston of cast iron, the difference in the clearances may be accentuated. Consequently, particularly in an engine in which the axis is horizontal, the piston assembly is apt to tilt in relation to the cylinders in starting from cold, but when the engine is hot and the clearances are taken up, the tilt is eliminated. An object of this invention is to provide an improved arrangement which overcomes such tilting when the engine is cold.

Such engines are usually started by setting the free-piston assembly at its outer dead point and thereafter quickly admitting a charge of air (or other gas) under relatively high pressure into the cushion cylinder. The starting air is released into the cushion at atmospheric temperature. This air then does work on the piston so that its temperature falls. Thus the mean temperature of the cushion air is lower than the temperature of the cushion cylinder walls. After the engine has been running for a short time, the air in the cushion will have become heated until its mean temperature is approximately that of the cylinder walls. Such immediate rise of mean temperature necessarily involves a rise of mean pressure in the cushion. A further object of this invention is to provide an improved arrangement of cushion in which the effect of this pressure rise on starting is at least partly compensated for.

A further object is to provide an improved arrangement for cooling the crown of the piston that operates in the internal-combustion cylinder.

An embodiment of the invention will be described, by way of example, with reference to the accompanying drawings, as applied to a symmetrical type of two-stroke free-piston gas-generator.

known manner.

The term gas-generator means a combination of internal-combustion engine and air-compressor in which air from the compressor is used to scavenge and supercharge the internal-combustion engine, the gaseous product of the generator being a mixture of combustion products and air at such a temperature and pressure that it is capable of driving a turbine or other elastic fluid motor which converts the energy of the gaseous product into a mechanical power.

In the drawings:

Fig. l is a diagrammatic side elevation of the gas-generator, partly in section on the line I-l in Fig. 2,

Fig. 2 is a sectional end elevation, the righthand half being on the line 2Rr-2R in Fig. 1, and the left-hand half on the line 2L2L in Fig. 1.

Fig. 3 is a side elevation of the middle portion of the same gas-generator with a part of the casing broken away, and

Figs. 4, 5 and 6 are graphs illustrating the operation of this gas-generator.

The generator includes a power cylinder H1 in which operate opposed power pistons H and HA, the cylinder being provided with exhaust ports l2 controlled by the piston I IA and with transfer ports i3 controlled by the piston H. The power cylinder Ill is disposed within a casing l4 forming a scavenge air receiver for compressed air, which is employed to scavenge the power cylinder. The exhaust ports discharge to a motive gas delivery connection 36.

The power pistons form respectively parts of two identical free-piston assemblies operatively connected together by a synchronising train in A combined air-compressor and cushion cylinder i5 is arranged co-axially with the power cylinder and attached to one end wall it of the air receiver. The outer end of the cylinder :5, the diameter of which is substantially larger than that of the power cylinder It), is closed by a cover it which carries a rigid hollow cylindrical guide 58 projecting towards and co-axially with the power cylinder.

The power piston H is constituted by a crown and a cylindrical portion or belt 26 which is provided with circumferential grooves for the piston rings and which bears on the power cylinder wall. This piston closes the inner end of a cylindrical sleeve l passing through a gland IS in the end wall IS. The internal diameter of the sleeve l9 substantially exceeds the external diameter of the guide H3. The external diameter of the sleeve I 9 is substantially less than that of the ring-carrying belt 20 of the piston II. The outer end of the sleeve H! has a bore 2i slidably bearing on the guide I8 and an external annular piston 22 co-operating with the compressor and cushion cylinder iii. The inner portion of the cylinder (i. e. that portion nearer the power cylinder) forms the air-compressor chamber, air being drawn in, during the out stroke, through valves such as 23 in the cylinder wall and discharged, during the in stroke, to the receiver, through valves such as 24 in its end wall IS. The outer portion of the cylinder I 5 forms the first cushion compartment, while the space within the guide 18 and the sleeve I 9 of the piston assembly forms the second cushion compartment.

The guide I8 has hollow walls forming a jacket 25 in which a cooling fluid is circulated, and the inner surface of the guide is provided with fins 26 adapted to assist in cooling the cushion air therein. A pressure-equalising port 2'! is provided through the guide near its outer end, communicating between the two cushion compartments. The inner end of the guide is provided with an orifice 26 adapted to direct a jet of air against the under side of the head of the power piston l I during the in stroke. An inlet poppet valve 29 for starting air is provided in the outer end cover I! opening into the first cushion compartment.

The starting apparatus is of the standard Pescara type, and it includes a starting air chamlbflr 30 mounted on the cover I! and provided with a cylinder 3| in which operates a piston 32 directly connected to the valve 29, which is adapted to be held on its seat by pressure within the chamber 30. The outer end of the cylinder 3| communicates by a pipe 31 with a 3-way valve 33, which is adapted to close the pipe 31, or to open it to atmosphere, or to put it in communication with a compressed-air reservoir 34. A non-re turn valve 35 in the piston 32 allows air to pass from the cylinder 3i to the chamber 30. The

diameter of the piston 32 exceeds that of the valve The displaced volume in and the compression ratio of the first cushion compartment in the outer portion of the cylinder I5 are larger than those of the second cushion compartment within the sleeve I9 and the guide l8.

The other end of the engine is identical with that already described, the parts of the other end being denoted in the drawings by the same refer ence numerals with the addition of A.

The synchronising mechanism constraining the two free-piston assemblies to move equally and oppositely is of the rack-and-pinion type (Figs. 2 and 3). Racks 3'! and 31A are rigidly connected, by rods 38 and 38A, to the pistons 22 and 22A respectively. These racks slide in guides 39 and 39A and mesh with a common pinion d0 fixed on a shaft 4!. This shaft is journalled in a bearing 42 on the cylinder i0 and in a cover 43 closing an aperture in the casing I4. A cam 44 on the shaft 4| operates a fuel-injection pump 45 supplying a fuel-injection nozzle 41 through a pipe 46. Another pair of racks 3i and 37A, symmetrically opposed, with reference to the axis of the cylinder ID, to the racks 31 and 3'iA, are connected to the pistons 22 and 22A respectively similarly to the racks 31 and 31A (one of the coupling rods appears in Fig. 2 and is denoted by 38). These racks mesh with a common pinion 40' fixed on a shaft 4|, which may be employed to actuate auxiliary mechanism in a casing 68.

Ducts 43 and 49A open into the first cushion compartments and lead to control means (not 4 shown) for varying the quantity of air contained in the cushion.

This gas-generator is started as follows. The control valve 33 is'operated to admit air from the reservoir 34 (which is charged by any convenient means not shown) to the cylinders 3! and 3IA, whence it passes, through the non-return valves, to the starting air chambers 3e and 30A, holding the starting Valves on theirseats. When these chambers are charged, the control valve 33 is shut. The free-piston assemblies are now moved to their outer dead points in known manner by means not shown in the drawings and the control valve 33 is operated to vent the cylinders 36 and 3 EA to atmosphere. This release of the air pressure on'the outer faces of the starting valve pistons, such as 32, allows the pressure of the air in the starting air chambers acting on the inner faces of these pistons to open the starting valves, so that the starting air is almost instantaneously discharged into the first cushion compartments. where it acts on the pistons 22 and 22A and starts the engine. While the engine is running there will be a certain leakage of scavenge air from the receiver between the power cylinder and the sleeve l9 to the exhaust ports if during a part of each cycle, but as in any case the motive gas contains a large quantity of excess air, such leakage is immaterial.

Although the equalising ports 27 and 2lA could be of large area, it is preferred to make them of restricted area for various reasons.

Firstly, owing to the pressure drop developed across the cooling jet orifice 28, some air passes from the second to the first cushion compartment that would otherwise assist to raise the velocity of the jet of air cooling the power piston crown. For this reason, the smaller the-equalising port is made, the stronger the cooling jet becomes. A high velocity in the cooling air jet entails an increased amount of work, and whereas the work done per stroke varies approximately as the square of the velocity the heat-exchange co-eflicient is only approximately proportional to the velocity. For this reason, the size of the orifice 28 is selected to limit the air velocity. For example, the maximum pressure drop across this orifice may be between /2 lb. and 2 lbs. e. g. 1 lb. per sq. in.

Secondly, flow of air under the pressure difference between the two cushion compartments represents a pumping loss and performs no useful function when normal conditions are established. It is therefore economical to reduce such loss.

Thirdly, a suitable restriction of the equalising port facilitates compensation for the rise of mean pressure, already referred to, that occurs in the cushion'on starting the engine. Fig. 4. shows the temperature variation of the cushion air plotted as ordinate on a base of free-piston displacement between the inner and outer dead points, denoted by IDP and ODP, in a known free-piston engine. Curve S denotes the starting conditions, starting air being admitted at atmospheric temperature T1 and expanding down to a temperature T2 with a mean temperature Tm. Under stable running conditions, if the same weight of air remains in the cushion, the temperature variation will be as shown by curve R, from T1 to T2, the mean being Tm which is substantially higher than Tm. Fig. 5 shows the corresponding pressures, the curve S denoting the starting conditions with the range P1 to P2, and the curve B the running conditions with the range P1 to P2. The compression ratio in the power cylinder for a given outer dead point varies rather widely with the cushion pressure, and ,in one particular case investigated it was found that, if no air were released from the cushion while the same indicated work was maintained in the power cylinder (i. e. an approximately constant quantity of fuel was injected at each cycle), the compression ratio in the power cylinder would rise from about to 1 to about 20 to 1, owing to the rise of temperature of the cushion air after the engine was started.

If the equalising port is small, since'the whole of the starting air is released into the first cushion compartment, the compression ratio in the power cylinder during the initial cycles will be substantially higher than if the equalising port had been large. Consequently, when a small port is used, the compression ratio in th power cylinder tends to fall as the mean pressures in the cushion compartments become equalised. If the area of the equalising port is suitable, the initial rate of this fall in compression ratio can be made about equal to the initial rate of rise of this compression ratio due to the rise of temperature of the cushion air on starting.

In some designs it may be possible to select the relative areas and clearance volumes of the cushion compartments in such a way as to make the fall in compression ratio due to equalisation of cushion pressures equal to, or slightly larger than, the rise in compression ratio due to temperature rise of the cushion air, so that complete compensation is attained. However, in other designs the range of adjustment of the compression ratio in the power cylinder due to equalisation of cushion pressures is substantially less than the range due to the temperature effect. In such cases it is therefore necessary to provide in known manner additional means for maintaining the compression ratio substantially constant. Such means may automatically release a part of the cushion air, or automatically reduce the quantity of fuel injected which in turn brings the outer dead point inwards. Such controls are likely to suffer from a certain time lag, whereas the pressure equalisation has an immediate efiect and reduces not only the speed but also the range of operation required of the automatic control means. This is shown in Fig. 6 in which compression ratio r is plotted as ordinate on a time base. Curve A shows how the compression ratio would rise if there were no leakage of air from the first cushion compartment, and curve B shows how it would fall if there were no temperature rise of the cushion air but the equalising port were operative. Curve C shows how the two effects combine to maintain the compression ratio nearly constant from the time of the first cycle to to the time t1 when the automatic control gear begins to take effect.

An alternative way of making use of this equalisation effect is to provide in known manner other automatic control means which provide full compensation for the temperature rise of air in the cushion. In this case the effect of releasing the Whole of the starting air into the first cushion compartment is to make the compression ratio in the power cylinder substantially higher on the starting stroke than it will be after the cushion pressures have become equalised. This may be useful in ensuring reliable ignition of fuel on the starting stroke when the air charge in the power cylinder will be colder than in subsequent strokes.

I claim:

1. A free-piston internal-combustion engine having a power cylinder, a second cylinder coaxial with and having a diameter larger than that of said power cylinder, a free-piston assembly including a power piston having a ring-carrying belt and rigidly connected by a sleeve to a second piston having a central bore 2|, said piston 00- operating respectively with said cylinders, and a cylindrical guide rigid with the outer end of said second cylinder and extending co-axially within said sleeve, the internal diameter of said sleeve substantially exceeding the external diameter of said guide, and said free-piston assembly being guided at one end by said belt bearing in the bore of said power cylinder and at the other end of said central bore bearing on said guide, which prevents said second piston from being in the bore of said second cylinder.

2. A free-piston internal-combustion engine having a power cylinder, a second cylinder coaxial with and having a diameter larger than that of said power cylinder, said second cylinder having an inner end wall at its end nearer to said power cylinder and an outer end wall at its end farther from said power cylinder, a cylindrical guide extending co-axially within said second rigid cylinder from said outer end wall, and a free-piston assembly including a power piston having a ring-carrying belt bearing in said power cylinder, a second piston co-operating with said second cylinder and having a central bore bearing on said cylindrical guide, and a cylindrical sleeve rigidly connecting said pistons together and passing through a gland in said inner end wall, said sleeve having an internal diameter which exceeds the external diameter of said guide and said sleeve having an external diameter which is less than the bore of said power cylinder.

3. A free-piston internal-combustion. engine having a power cylinder, a pneumatic cushion, a free-piston assembly comprising a power piston .co-operating with said power cylinder and a cushion piston forming part of said cushion, said assembly being movable through the stroke that effects compression in said power cylinder at least in part by energy accumulated in said cushion during the preceding expansion stroke in said power cylinder, and the gas space enclosed in said cushion being divided into two compartments communicating with each other by a restricted passage, an inlet for compressed starting gas opening into one of said compartments which has its volume varied by movement of said free-piston assembly, and valve means controlling said inlet.

4. A free-piston internal-combustion engine having a power cylinder, a pneumatic cushion, a free-piston assembly comprising a power piston co-operating with said power cylinder and a cushion piston forming part of said cushion, said assembly being movable through the stroke that effects compression in said power cylinder at least in part by energy accumulated in said cushion during the preceding expansion stroke in said power cylinder, and the gas space enclosed in said cushion being divided into two compartments communicating with each other by a restricted passage, and said compartments having their volumes varied by movement of said free-piston assembly with difierent .efiective cushion piston areas respectively, an inlet for compressed starting gas opening into the one of said com- 7 piston area, and valve means controlling" said inlet.

5. A free-piston internal-combustion engine having a power cylinder, a pneumatic cushion, a free-piston assembly comprising a power piston co-operating with said power cylinder and a cushion piston forming part of said cushion, said assembly being movable through the stroke that effects compression in said power cylinder at least in part by energy accumulated in said cushion during the preceding expansion stroke in said power cylinder, and the gas space enclosed in said cushion being divided into two compartments communicating with each other by a restricted passage, and said compartments having different ratios of clearance volume to volume swept upon movement of said free-piston assembly, an inlet for compressed starting gas opening into the one of said compartments that has the smaller ratio of clearance volume to swept volume, and valve means controlling said inlet.

6. A free-piston internal-combustion engine having a power cylinder, a pneumatic cushion, a free-piston assembly comprising a power piston co-operating with said power cylinder and a cushion piston forming part of said cushion, said assembly being movable through the stroke that efiects compression in said power cylinder at least in part by energy accumulated in said cushion during the preceding expansion stroke in said power cylinder, and the gas space enclosed in said cushion being divided into a first and a second cushion compartment communicating with each other by a restricted passage, an inlet for compressed starting gas opening into said first cornpartment, and valve means controlling said inlet, said first compartment having a larger effective cushion piston area and a smaller ratio of clearance Volume to volume swept upon movement of said free-piston assembly than said second compartment.

'7. A free-piston internal-combustion engine having a power cylinder, a cushion cylinder '00- axial with and having a diameter larger than that of said power cylinder, a cover closing the end of said cushion cylinder farther from said power cylinder, a hollow cylindrical guide extending coaxially within said cushion cylinder from said cover, a free-piston assembly including a power piston co-operating with said power cylinder, an annular cushion piston co-operating with said cushion cylinder and said guide, and a sleeve rigidly connecting said pistons together, the space between said annular piston and said cover constituting a first cushion compartment and the space contained by said guide, said sleeve and said power piston constituting a second cushion compartment communicating with said first compartment by a restricted passage, and means for admitting a charge of starting air under pressure to said first compartment.

.8l A free-piston internal-combustion engine having a power cylinder, a cushion cylinder coaxial with and having a diameter larger than that of said power cylinder, a cover closing the end of said cushion cylinder farther from said power cylinder, a hollow cylindrical guide extending coaxially within said cushion cylinder from said cover and having an inner end wall provided with an orifice, a free-piston assembly including a power piston co-operating with said power cylinder, an annular cushion piston co-operating with said cushion cylinder and said guide, and

' a sleeve rigidly connecting said pistons together, the space between said annular. piston. and saidcover constituting a first cushion compartment and the space contained by said guide, said sleeve and said power piston constituting a second cushion compartment, said orifice being placed to direct a jet of gas onto the under side of the head of said power piston, and the space within said hollow guide communicating with said first cushion compartment by a restricted duct, and means for admitting a charge of starting air under pressure to said first compartment.

9. A free-piston internal-combustion engine having a power cylinder, a cushion cylinder coaxial with and having a diameter larger than that of said power cylinder, a cover closing the end of said cushion cylinder farther from said power cylinder, a hollow cylindrical guide extending coaxially within said cushion cylinder from said cover and provided with a jacket for a cooling fluid, said guide having an inner end wall provided with an orifice and said guide having a restricted passage opening out of the outer part of the space within it and leading to the outer part of said cushion cylinder, a free-piston assembly including apower piston co-operating with said power cylinder, an annular cushion piston co-operating with said cushion cylinder and said guide, and a sleeve rigidly connecting said pistons together, the space between said ans nular piston and said cover constituting a first cushion compartment and the space contained by said guide, said sleeve and said power piston constituting a secondcushion compartment, said orifice bein placed to direct a jet of gas on the under side of the head of saidpower piston, and means for admitting acharge of starting air under pressure to said first compartment.

10. A free-piston internal-combustion engine having a power cylinder, cushion cylinder coaxial with and having a diameter larger than that of said power cylinder, a cover closing the end of said cushion cylinder farther from said power cylinder, a hollow cylindrical guide extending co-axially within said cushion cylinder from said cover, a free-piston assembly including a power piston co-operating with said power cylinder, an annular cushion piston co-operating with said cushion cylinder and said guide, and a sleeve rigidly connecting said pistons together, the space between said annular piston and said cover constituting a first cushion compartment and the space contained by said guide, said sleeve and said power piston constituting a second cushion compartment communicating with said first compartment by a restricted passage, and means for admitting a charge of starting air under pressure to said first compartment, the throttling effect of said restricted passage being such that, after starting, the initial rate of fall in compression ratio in' said power cylinder due to equalisation of mean pressures in said compartments is approximately equal to the initial rate of rise of said compression ratio due to the rise of temperature of the cushion gas. a

11. A free-piston internal-combustion engine having a power cylinder, a cushion cylinder coaxial with and having a diameter larger than that of said power cylinder, a cover closing the end of said cushion cylinder farther from said power cylinder, a hollow cylindrical guide extending co-axially within said cushion cylinder from said cover and provided witha jacket for a cooling fluid, a free-piston assembly including a power piston co-operating with said power cylinder, an annular cushion piston co-operating with said cushion cylinder and said guide, and

a sleeve rigidly connecting said pistons together, the space between said annular piston and said cover constituting a first cushion compartment and the space contained by said guide, said sleeve and said power piston constituting a second cushion compartment communicating with said first compartment by a restricted passage, and an inner end wall on said guide having an orifice for directing a jet of gas from the interior of said guide onto the under side of the head of said power piston.

12. A free-piston internal-combustion engine havin a power cylinder, a second cylinder coaxialwith and having a diameter larger than that of said power cylinder, a hollow cylindrical guide extending co-axially within said second cylinder, said guide having an external diameter less than the bore of said power cylinder, a power piston bearing in said power cylinder, an annular piston bearing on said guide and cooperating with said second cylinder, and a sleeve rigidly connecting said pistons together and having substantial clearances from said guide and said power cylinder, said sleeve and said power piston forming with said guide a compression chamber, and said guide having a hollow wall for the circulation therein of a fluid for cooling the contents of said compression chamber.

13. A free-piston internal-combustion engine having a power cylinder, a cushion cylinder oo- 10 axial with and having a diameter larger than that of said power cylinder, a hollow cylindrical guide extending co-axially within said cushion cylinder, a free-piston assembly comprising an annular piston bearing on said guide and 00- operating with said cushion cylinder to form a cushion chamber, a power piston bearing in said power cylinder and a sleeve rigidly connecting said pistons together, the space within said guide and said sleeve constituting a second cushion chamber, and means including a communication between said chambers for varying the gas content of both of said chambers.

14. A free-piston internal-combustion engine having a power cylinder, a cushion cylinder coaxial with and having a diameter larger than that of said power cylinder, a hollow cylindrical guide extending co-axially within said cushion cylinder, a free-piston assembly comprising an annular piston bearing on said guide and cooperating with said cushion cylinder, a power piston bearing in said power cylinder, a sleeve rigidly connecting said pistons together and having a clearance from said guide, and control eans operable for admitting a charge of compressed starting gas directly into the outer portion of said cushion cylinder, a communication being provided between said outer portion and the interior of said sleeve for transferring gas between said outer portion and said interior.

EVELYN STEWART LANSDOWNE BEALE.

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