US2554762A - Free piston type internalcombustion compressor - Google Patents

Description

May 29, 1951 R. J. WELSH ET AL 2,554,762

FREE PISTON TYPE INTERNAL-COMBUSTION COMPRESSOR Filed Jan. 22, 1946 72v Sheets-Sheet 1 May 29, 1951 l R. J. WELSH ET AL 2,554,762

FREE PIsToN TYPE INTERNAL-coususuou COMPRESSOR Filed Jan. 22, 194e 2 sheets-sheet 2` N Y y Q I .hul xr 4l- "1 l y Q g Q: I I Q Q l u) v u \g 'N E" u; INvENToRs:

ROBERT J. WELSH 'Q ALEC B. FITZHEBBERT ATTORNEYS lPatented May 29', '1951 FREE Pls'roN TYPE INTERNAL.- YCOMBUSTIC Ny @vergessen Robert James Welsh and Benjamin Fitz Herbert, Rugby, England, assignors to The Englislr Electric Company land. a British @mary Application January 22, 1946, Serial No. 642,612 In` Great Britain Januarylz; Y

This invention relates to. a free piston type internal combustion `compressori. e. .a device wherein a compressor piston is directly ,connected to or combined with an internal combustion en..- gine piston and wherein the motion of these pistons is not constrained by coupling through connecting rods land ,cranks to rotating members. There are usually .two opposed engine pistons reciprocable towards and away from veach other in .one cylinder. The engine is .usuallyone 'which operates on a two-.stroke compression-ignition Cycle, some .or allof the air from the compressor serving to. scavenge and `charge the engine. cylinder.

A device of this kind may be used as a gasgenerator to supply another piece of apparatus such as a gas turbine which .may .have as its Working fluid either (i) solely air coming direct from the compressor or (ii). solely the exhaust from the engine cylinder (comprising the vproducts. of the combustion in .the engine cylinder mixed with the .scavenging .air supplied to the engine by .the compressor) or (iii) amixture of the engine exhaust with air coming .direct from the compressor. The second of these three alter natives is most usual..

A free-piston internal .combustion operated compressor according to the invention .has .a single-actingcompressor with a closed idle space, and means vare provided which tend, under normal running conditions, to equalize vthe mean effective pressure on the idle side of .the vcom-.- presser piston with .the .pressure .at the .inlet to the compressor cylinder. The invention will .be described With reference to` the accompanying drawings, in which Fig. l show adiagrammatic elevation, l.partly in section on the axis, ofladevice embodying the invention arranged as a gas generator `for a gas. turbine, .while Fig. -2 is a diagramof .the pressure in the idle space depending on the position of the equalis'ing means.

The body of the Afree piston type internal combustion compressor shown in Fig. 1 comprises a single engine cylinder I, a pair of compressor cylinders 2 at opposite ends thereof andafpair of buffer cylinders 3 at the .eXtreme outer ends of cylinders 2. Within the engine cylinder I a pair of opposed engine-pistons ll -reciprocatfsynchronously in opposition to each other. Eachof these pistons 4 formsa free piston unitrwith one of the compressor pistons dreciprocating in a cylinder 2 and with one .of the buier pistonsG reciprocating in a cylinder 3.. The headof each of the compressor Vcylinders 2 is providedl with atleast one non-return inlet valve .I2 and at Limited. Londre least .one nou-return discharge Yalve .|3- The inlet. .vane 11.2. puts .the .Compressor space .21.1 ef Compressor .cylinder 2. in. @Ommufneaton with inlet .chamber ilu., while the. discharge valve .li puts this .compresser space 2.a in eommunloatien with charge. .chamber 150i` The .dischalee chamberv Ito .isoontinuou with ,eonduit l5. which transfers-the. .compressedai-r .to `the annular .Spese lh surrounding .the ring of inlet. .ports la in the engine ,cylinder t., Theexhaust .conduit i6 from the. .engine .cylinder .l carries. .the mixture of scavenging .air vand combustion products to. toed a gas .turbine ,25.

Fuel is. .injected into the. cylinder .I by fuel injectors. `9, supplied .through fuel pipe ll... tributor Ill and 'fuel pipes .IJrom the fuel injection pump. l0. .quantity ,or fuel injected for each .cycle of the. engine vis determined by the position of its. .fuel rack Illa. This fuel. rack iS. controlled through rods .3 I. and b ell cranks v3i: by the governor 28 of the turbine 25, so as .t0 decrease the fuel .supply ,per .cycle when the turbine speed. rises. At the ,same v.time the turbine gouernorl .controls throttles 32 .in the inlet vconduits, JA .so as to. .tend .to .close these throttles whentherturbine speed rises.

.While .the `.pistons .are melting an out-stroke under `the .influence ofthepressure .due .to combustion .in .engine cylinder l, the air trapped in compressor space :2av is compressed and partially dischargedrthrough. .valve t3. The air trappedin buffer cylinder .-3 .is'notl discharged, but is compressedand'servee, .together with the residual compres-sed air in compressor space 2a, .to return the-pistons sc that :they make an in-stroke and compress thelairinenginecylinder .I

yEach .'ofpthe compressor .cylinders 2 is singleacting, jthat. is to say,'only..the space 2a on one side'mthe outer si'clee-.ofl compressor piston 5 is uscdlto supply .compressed air. vThe spaceb on the .other sidevis an'idle space. .If this idle .space 2121 .were .left ofpenlto .the atmosphere, considerable energy Wouldbeexpended every cycle in pumping .air in and out. of it. .In this .arrange-V menti .this yidle space .is closed so that, .during normally running, .the air trapped therein is compressed on .every in-s'troke and expands .on every '.outstroke. xIf .the Vpresence in this idle space-2b at theend'of .the out-stroke were made equal to that in the Ycompressor' inlet chamber Ma, the eiect .of the. .compression in .the idle spacefh.duringtheinestroke would be to oppose the action .of .bu-ierpiston 6*. If, .on the other hand, .the `pressure inlthis idler space 2b at the end ofitheinestroke were made equalto that faccinea the compressor inlet chamber Ida, the effect of the expansion in the idle space 2b during the out-stroke would be to assist the action of buier piston 6 on the following in-stroke.

This opposition'or assistance to the action of the buffer piston cannot altogether be offset simply by changing the size of the buffer cylinder, because the pressure/stroke characteristic of the idle space will be quite different from that of the buffer cylinder and the variations with varying position of outer dead centre due to change in working conditiom will therefore tend to become unmanageable. l

In the present arrangement, these difliculties which arise when the idle space 2b is closed are overcome by the provision of means which en sure that the net work done on any single inward or outward stroke is zero. sult, the mean effective pressure in the idle space is made substantially equal to the pressure atthe compressor inlet, which pressure, ifa throttle is employed as at 32, will usually be below atmospheric. The point on the compression/ expansion curve of the idle space where this mean effective pressure equals the compressor inlet pressure will be situated at approximately mid-stroke, consequently the terminal pressures corresponding to the inner and outer dead point positions will be above and below the compressor inlet pressure respectively. Therefore during an outward stroke and from the inner dead point to approximately mid-stroke, positive work will be done on the idle face of the compressor piston while from mid-stroke to the outer dead point, negative work will be done on it. By proper dispositioning of the parts it is possible to make the negative Work equal the positive work so that the net work done is zero. During an inward stroke the order will be reversed but the net work done will again be substantially equal to zero. v

Thus, if the relationship between stroke and pressure in the idle space be represented by a diagram after the fashion of an ordinary indicator card, then the compression and expansion line of the idle space would be so .disposed in relation to the atmospheric line that if a perpendicular were dropped from the atmospheric line at each end of the stroke then the area of the indicator diagram above the atmospheric line at one end of the stroke would be equal to the area of the indicator diagram below the atmospheric line at the other end of the stroke. This will be explained more in detail with reference to Fig. 2.

In the arrangement shown in Fig. 1, a port 50 in the wall of cylinder 2 is connected to the compressor inlet chamber Ida through a non-return valve I. The port 50 is situated near mid-stroke of the inner edge 5b of the skirt of compressor piston 5 so that on an outward stroke it is closed by this piston during the whole of the travel from the inner dead point to approximately mid-stroke (hereinafter termed the inner half-stroke) and is opened to the idle space 2b during the remainder of the stroke (hereinafter termed the outer half-stroke). During an inward stroke, the port 5E) will of course be open to the idle space 2b during the outer half-stroke and closed during the inner half-stroke. The precise position of port 5U will be as nearly as possible opposite the point which is reached'by the edge 5b of the piston skirt at the instant in the piston stroke when, under normal working conditions, the instantaneous pressure on the idle side of the piston 5 is equal to the mean effective pressure To achieve this rey 4 thereon taken over the whole stroke. During the inner half stroke when port 50 is closed by piston 5, the pressure in idle space 2b is above that in inlet chamber Ida; but during the outer half stroke when port 50 is uncovered, the pressure in idle space 2b is lower than that in inlet chamber [4a and the non-return valve 5I therefore remains closed. If, however, the quantity of air in idle space 2b at any time is above the proper value, so that the mean effective pressure exceeds the pressure in the inlet chamber 14a, then non-return valve 5l will open for a short period after port 50 is uncovered, and will permit the excess air to escape into inlet chamber Illa. In order to increase gradually the quantity of air in idle space 2b when it is below the proper Y value, one or more small leaks are provided, un-

less the natural leakage is adequate. Such leaks v are indicated (a) at 52 in the non-return valve 5I, (b) at 53, leading to atmosphere, and (c) at 54 leading into the scavenge air space 15b. Only one of these leaks will normally be provided, and in that case it may take any of the positions 52, 53, 54.

Operation An additional non-return valve 55 is arranged to lead from the idle space 2b into the scavenge air space |517. During normal running of the compressor, the maximum pressure in idle space 2b does not exceed the discharge pressure of the compressor, which is also approximately the pressure present in scavenge air space |5b, and this valve 55 therefore remains closed. During starting, however, the discharge pressure of the compressor cylinder may be approximately equal to atmospheric pressure. Before each attempt to start the engine, it is usually barred to the end of an out-stroke, and an in-stroke may then be initiated by admitting compressed air to buffer cylinder 3. The idle space 2b may be full of air at atmospheric pressure before the in-stroke commences. During the first half of the instroke, until port 50 is covered, air will escape from the idle space through port 5U and non-return valve 5I into inlet chamber Ma. If port 50 is large, therefore, the pressure in the idle space 2b when port 50 becomes covered will be approximately atmospheric. Thereafter, however, it will tend to rise and to produce an unwanted compression pressure which will oppose the action of the starting air admitted to buffer cylinder 3. The non-return valve 55 relieves this pressure during this stroke, and similarly relieves it to a gradually decreasing degree during successive strokes as pressure is gradually built up in the scavenge air space 15b. The release of air from the idle space through relief valve 55 is not fully compensated by inward leakage through leaks 52, 53, 54 until a considerable number of cycles of engine operation have occurred. During this time, the idle space has a mean effective pressure substantially below the pressure in inlet chamber Ida and therefore assists the buffer cylinder in returning the pistons on their instroke. When normal running conditions have been reached, the valve 55 will remain closed, and the port 50 and non-return valve 5| will operate in the manner previously described to neutralise the elect of the air trapped in the idle space 2b. The balance of the return energy provided by the buffer cylinder 3, and the residual compressed air in the space 2a will not therefore be disturbed as would otherwise be the case.

The, effect of the position of the port 50 is illustrated in Fig. 2, parts (a), (b) and (c) of which represent the indicator diagrams obtaining in the idle space when (a.) the port 50 in the compressor cylinder wall is situated too near to the inner dead pointi. D. P.) position of the piston, (b) when the port is in the correct desired position near to mid-stroke and (c) when it is displaced towards the outer dead point (O. D. P.) position.

Every time the port 59 connects the idle space 2b to the inlet chamber the pressures in the two will be at least approximately equalised, therefore the initial pressure pi of the subsequent compression or expansion, according as to whether the piston is moving inwards or outwards, will be the pressure in the inlet chamber. From (a) it will be seen that with the port situated towards the I. D. P., the compressive or positive work, l, done in the idle space is much less than the expensive or negative work, and that the mean eiective pressure pm represented by the dotted horizontal line lies well below the inlet pressure pi. With (b) the position of the port has been correctly chosen and in this case the mean effective pressure 'pm coincides with the pressure pi at the inlet, with the result that the positive and negative work areas -1- and are equal. (c) represents the case when the port 56 is situated too far towards the outer dead point (O. D, P.) position (and here the positive area, of the diagram is much in excess of the negative area, The mean eiective pressure pm in this case lies well above the inlet pressure p1.

These diagrams show therefore that there is only one point at which the port 50 can be situated to meet the requirement that it is in operation at or about the instant when the pressure in the idle space corresponds to the mean effective pressure.

What we claim as our invention and desire to secure by Letters Patent is:

A free piston internal combustion engine operated compressor or gas generator comprising an engine cylinder, at least one single acting ported compressor cylinder, at least one free piston assembly adapted to reciprocate in the said cylinders and comprising an engine piston and a compressor piston, a closed idle space formed by the said compressor piston and cylinder on the nen-working side of the compressor piston, a compressor inlet duct, a port in the wall of the said ported "compressor cylinder adapted to be uncovered by the compressor piston at a point corresponding substantially to mid-stroke, a duct connection between said port and said inlet duct and an outwardly opening automatic non-return energy equalizing valve adapted to close the said port against the air inlet pressure when the said pressure is greater than that in the said idle space, a scavenge chamber intermediate the said idle space and the said engine cylinder wherein the pressure during normal running of the compressor or gas generator remains continuously above the maximum pressure in the said idle space but wherein the pressure during starting of the compressor or gas generator is below said maximum, a connection between said idle space and said scavenge chamber and a non-return starting relief valve in said connection, said starting relief valve being adapted to permit ow in the direction from the said idle space to said scavenge chamber and thereby to relieve compression pressure in the said idle space during starting.

ROBERT JAMES WELSH.

ALEC BENJAMIN FITZ HERBERT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,046,631 Janicke July '7, 1936 2,083,680 Anderson et al June 15, 1937 2,086,162 Janicke July 6, 1937 2,086,163 Junkers July 6, 1937 2,108,890 Janicke Feb. 22, 1938 2,178,310 Pescara Oct. 31, 1939 2,434,280 Morain Jan. 13, 1948 FOREIGN PATENTS Number Country Date 257,995 Great Britain Sept. 9, 1926

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