BOUNCE CHAMBERS FOR MULTI-CYLINDER LINEAR ENGINE COMPRESSORS

Abstract: This invention is a multi-stage compressor (1,5) embodying a stepped piston (5) in a stepped cylinder (1) and con­ nected through a coaxial rod (4) to the piston (3) of an engine. Control means (10) in a passageway or a conduit extending from the second (5e) or third stage (5f) discharge of the compressor or any other source of pressure to a single bounce chamber (9), responds to pressure in the bounce chamber (9) to increase or reduce gas pressure in the chamber, to balance the energy and work quantities acting in opposite directions to provide optimum operation of the compressor (1,5) at var­ ying pressures.

Description: This invention is concerned with a multi-stage engine compressor of the free floating piston type for the compression of gases. In certain applications, such compressors are used to compress gas from a varying pressure on the suction side of the compressor through several compression stages to a substantially varying discharge pressure. In a three stage compressor, the suction pressure may vary between 15 psia and 30 psia (103x10³ Pa and 207x10³ Pa) while the discharge pressure varies between 2000 psia and 3600 psia (13790x10³ Pa and 24821x10³ Pa) which requires the compression section to work with compression ratios from as low as 67:1 to as high as 240:1. This may result in substantial differences in the energy balance between the engine section and the compressor section and in the work balance between the power stroke and the return stroke of the engine compressor assembly. This problem of balancing these energy and work quantities is considerably aggravated by the fact that the compression work done in the first stage compressor may vary at a substantially different rate from that of the second and any subsequent stages under the varying intake and discharge pressures. In addition to these specific problems there are the basic problems of multi-stage compressors, whether they be of the free floating piston or of the crank-driven type, of arranging the compressor piston faces in such a way that the maximum gas forces are kept to a minimum. In the crank-driven compressor one or more stages are often divided into two and the vectorial sum of the products of the piston faces and their respective maximum pressures are arranged to be zero to keep maximum stresses and maximum bearing loads of the machine to a minimum. In compressors of the free floating piston type the balance of the forces and related work quantities which are done on or by the piston can be more important than their absolute value in that a given work balance is greatly affected by the choice and location of the individual piston faces. This may result in excessive pressures in the bounce chamber of such a machine or require more than one bounce chamber or even require more than one engine/compressor piston assembly, for such an engine/compressor to operate satisfactorily throughout its entire pressure and load range and preferably for a number of different gases. To deal with these complex problems many different solutions have been proposed. In US Patent No. 2,241,957, for instance, it is proposed to provide two separate oppositely moving motor or engine piston assemblies one of which carries an energy accumulator or balancing piston and the other carries all the compressor pistons whose non-active faces (back faces), of which are least one is subjected to a pressure higher than atmospheric pressure, act as additional energy accumulators, each providing the balancing work of the corresponding compressor section. An object of the present invention is to provide a compact and strong multi-stage engine compressor with a minimum number of parts and adapted to operate through a wide range of operating conditions and suitable for a number of different applications without having to changes its major components. Only one engine piston is needed in this compressor. According to the invention, a free piston internal combustion engine-driven compressor comprises an engine piston, working in an engine cylinder and coupled to drive a compressor piston working in a coaxial compressor cylinder containing a gas compression chamber and a negative bounce chamber, means providing a limited gas flow to the bounce chamber in said cylinder, and a gas flow passageway from a source of control gas pressure to control means arranged to variably control gas flow out of said bounce chamber in response to variations in the pressure in said source of different gas pressure, characterised in that the compressor is a multi-stage compressor having a plurality of interconnected coaxial compressor pistons working in a plurality of respective coaxial compressor cylinders, one of the compression stage cylinders contains the bounce chamber and another compression stage provides the source of control gas pressure. Such an arrangement provides a multi-stage engine compressor of the free piston type requiring a small number of parts and of smaller than usual size and which can be adapted to keep the engine or power section at near optimum operating conditions during substantially varying suction and discharge pressures and pressure ratios. Another advantage is that the engine can be arranged to operate at near optimum conditions during a part load requirement. Furthermore, the arrangement is versatile in that it does not require restricted configurations of piston and cylinder design to meet different sets of conditions or applications but only needs merely slight changes in the control elements, such as the diameter of a small control valve piston and/or the strength of a valve spring. One form of compressor according to the invention will now be described in more detail by way of example and with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of a compressor according to the invention; Figures 2 to 6 are force-stroke diagrams of the engine section, the bounce section, the first compression section, the second compression section and the third compression section of the compressor of Figure 1; and Figure 7 is an enlarged view of a control device connected to a negative bounce chamber. In the compressor shown in Figure 1, an axially elongated stepped housing 1 contains a single reciprocally movable piston assembly 2 comprising the engine or power piston 3 working in an engine or power cylinder provided by the housing and directly interconnected by a piston rod 4 to a single compressor piston assembly 5 of the multi-stage stepped piston type working in a stepped compressor cylinder also provided by the housing 1. In the preferred embodiment of the invention, all compressor stages of the stepped piston compressor are single acting and act in the same direction. Only one back face of the compressor piston assembly is a working face in that it provides the bouncer piston of the single accumulator or bouncer section and is preferably the back face of the first stage compressor piston. It acts in the opposite direction to provide the work balance during both the power and the return strokes, which is necessary to have the engine or power section operate at or near its optimum operating conditions. In Figure 1, a three stage compressor is shown as the preferred embodiment. In Figures 2 to 6 the force-stroke diagrams of respectively the engine section, the bouncer and the three compression stages are shown, representing, by the areas under their respective curves, the relative work of each section during both the power and the return stroke, friction work and scavenge work having for clarity's sake been neglected. The work for each section during the return stroke is represented by the areas under the broken lines while the work for each section during the power stroke is represented by the areas under the solid lines. For the desired operation of the engine compressor, the engine or power section has to operate with a certain compression ratio, determining the position of the left ordinate of the curve, the right ordinate being determined by the desired fuel input into the engine and the suction and discharge pressures of the compressor, the equality of the power and the return strokes being obtained by providing the appropriate pressure in the balancing or bouncer section, whose absolute work during the power and return strokes is essentially the same. The appropriate bounce pressure during the return stroke of the piston assembly must result in the area under the solid line of the Figure 3 diagram being equal to the difference of the sum of the areas under the broken lines of the diagrams in Figures 4, 5 and 6 and the area under the broken line of Figure 2. Similarly, during the power stroke of the piston assembly and during steady state operation of the engine compressor, the same area under the solid line of the bouncer pressure diagram in Figure 3 must and will be equal to the difference of the area under the solid line of the Figure 2 diagram and the sum of the areas under the solid lines of the diagrams in Figures 4, 5 and 6. For different gases and different suction and discharge pressure levels and different pressure ratio and part load requirements, as well as for different numbers of stages of the engine compressor, the appropriate bounce pressure may be supplied in a different fashion and from different sections or even from the atmosphere. Similarly, to obtain the most desirable results, the relative piston diameters and compressor clearance volumes may be selected so as to obtain the best overall characteristics. We now consider an engine compressor, as shown in the accompanying drawings, for the compression of natural gas from suction pressures varying between approximately 15 psia and 30 psia (103x10³ Pa and 207x10³ Pa) to discharge pressures varying between approximately 2000 psia and 3600 psia (13,790x10³ Pa and 24,821x10³ Pa), for the charging of compressed natural gas (CNG) cylinders for use in vehicles operating on natural gas as a fuel. It is possible to run such an engine compressor through the above pressure ranges at an essentially constant and optimum engine compression ratio and with the maximum bouncer pressure in the bounce chamber remaining within very close range of the first stage discharge pressure, which is a very efficient condition similar to that in a double-acting compressor stage. These desirable results were obtained with the rather simple control system shown in Figures 1 and 7 in which a small control flow of gas through a conduit or passageway 6 from the first stage discharge side is bled through an orifice unit 7 and check valve 8 into a negative bounce chamber 9, the maximum pressure in the bounce chamber being regulated by a control device 10 which is operated by gas from a gas pressure source, such as the discharge side of the second stage through conduit 11 and, if desired, may bleed the excess control gas to the atmosphere or into the suction or inlet line or conduit to the first stage compressor chamber 5d. In the control device 10 (see Figure 7), a compressor valve 10a responds to the difference between the pressure in the bounce chamber 9 and the discharge pressure of the second compression stage in lines 11 and 14 to increase the pressure in the bounce chamber with increasing pressure in line 11 and reduce it as the pressure in line 11 goes down. In operation, when the engine drives piston 3 to the right hand position as shown in Figure 1, the first stage compressor piston 5a forces gas from the first stage compressor chamber 5d through an outlet check valve into line 6, and thence through orifice member 7 and check valve 8 into the bounce chamber 9, as well as through branch line 12, a gas intercooler 13 and an inlet check valve into the second stage compressor chamber 5e. Simultaneously, gas is driven from the second stage compressor chamber 5e by the second stage compressor piston 5b through an outlet check valve and line 11 to the controller device 10, as well as through branch line 14 and gas intercooler 15 to the third stage compression chamber 5f where the gas is simultaneously being further compressed by the third stage compressor piston 5c. The valve 10a of the control device 10 in Figure 7 determines the maximum pressure in the negative bounce chamber 9 automatically depending on the 2nd stage discharge pressure, thereby increasing the negative bounce pressure as the 2nd stage pressure increases and vice-versa. The two springs 10d and 10c acting oppositely on the valve plunger of the control device 10 may be adjusted to cause the control device to increase or decrease the bias of the pressure balance in the compressor. The nominal flow of one 3-stage engine compressor model built according to the present invention is between 20 and 35 scfm (566 and 991 standard litres per minute) at a safe speed of only approximately 1000 cycles per minute depending on whether the engine is carburetted or fuel injected, and the dimensions of the machine are 18″ x 20″ x 80″ (46cm x 51cm x 203cm) and its weight approximately 600 - 700 lbs (272 - 318kg). The energy consumption of the machine in compressing natural gas is of the order of 30% below that of the most widely used and commercially available units and its cost is substantially less. Comparable free piston engines of only the single-stage type are substantially more complex and at least three times the weight with much higher manufacturing cost. A conventional three stage compressor of this type would be of even greater weight and cost. While the machine described above has proven, through testing, to be a practical, robust, cost effective and efficient compressor, it should be well understood that changes in the particular arrangement described may be made without departing from the scope of the invention. In particular, the engine compressor may be equipped with a balancing mechanism e.g. as described in my US Patent No. 3,853,100 and illustrated in Figure 3 of that patent, or may include other known refinements.

Claims: 1. A free piston internal combustion engine-driven compressor comprising an engine piston (3), working in an engine cylinder and coupled to drive a compressor piston (5a) working in a coaxial compressor cylinder containing a gas compression chamber (5d) and a negative bounce chamber (9), means (6,7,8) providing a limited gas flow to the bounce chamber in said cylinder, and a gas flow passageway (11) from a source of control gas pressure to control means (10) arranged to variably control gas flow out of said bounce chamber in response to variations in the pressure in said source of different gas pressure, characterised in that the compressor is a multi-stage compressor having a plurality of interconnected coaxial compressor pistons (5a-5c) working in a plurality of respective coaxial compressor cylinders, one of the compression stage cylinders contains the bounce chamber and another compression stage (5a) provides the source of control gas pressure. 2. A compressor as defined in Claim 1, wherein said control means also variably responds to varying pressure in said bounce chamber. 3. A compressor as defined in Claim 1 or Claim 2, wherein said bounce chamber is on the opposite side of the first stage compressor piston (5a) from the first stage compression chamber (5d). 4. A compressor as defined in any preceding claim, wherein said compressor has more than two stages of compression chambers (5d-5f) and compressor pistons (5a-5c). 5. A compressor as defined in any preceding claim, wherein said control means comprises a valve (10a) that is spring biased in opposition to said control gas pressure. 6. A compressor as defined in any preceding claim, wherein all the compressor and engine pistons are directly connected to one another to form a single unitary coaxial piston assembly (2). 7. A compressor as defined in any preceding claim, wherein said source of gas pressure is the compression chamber (5e) of a higher pressure stage than the stage that includes the bounce chamber. 8. A compressor as defined in any preceding claim, wherein there is a bleed line (11a) from said bounce chamber variably controlled by said control means. 9. A compressor as defined in any preceding claim, wherein there is only one bounce chamber. 10. A compressor as defined in any preceding claim, wherein all compressor stages perform their compression and discharge work simultaneously during a single power stroke of the engine piston.

Inventors: BRAUN, ANTON
Applicants: BRAUN, ANTON
Application year: 1987
Main classification: F04B17
IPC codes: F04B25, F04B35, F02B71