GAS COMPRESSORS

Abstract: In a piston and cylinder air compressor unloading of which is achieved by an unloader valve 7 connecting an additional volume (27) to the compressor clearance volume, a further valve (34,32) is provided in the cylinder head assembly which closes off the compressor delivery port (10) during unloading intervals whereby regenerative purging of an air dryer does not result in appreciable depletion of pressure which is required to be maintained in the extra volume (27).

Description: This invention relates to gas compressors and relates more especially but not exclusively to piston and cylinder air compressors in which the ultimate pressure of the delivered compressed air is limited by introducing additional clearance volume. An early compressor of which the delivered pressure is controllable by introducing additional clearance volume is described in the Specification of GB-A-257848. In that disclosure a clearance volume is formed in a delivery passage of the compressor and a controllable composite delivery valve is switchable from a normal delivery valve mode to a mode in which the clearance volume is directly connected to the compression chamber. In such an arrangement when the clearance volume is formed as part of the normal delivery passage of the compressor the clearance volume pressure in the off load condition cycles about a value near the delivered pressure with the accompanying heat dissipation. This compressor is also quite cumbersome in its construction. Another more recent design of a compressor is described in the Specification of United Kingdom Patent Application No. 8717707 wherein an additional clearance volume is provided which is connectable to the compression chamber of the compressor only upon unseating of a spring biassed unloader valve which is seperate from the inlet and delivery valves of the compressor. In the off-load condition the pressure in the compression chamber therefore cycles about a pressure which is somewhat above compressor inlet pressure. Whatever precise detailed features such a compressor may have, it may conveniently be supplied with air from a turbo charger. In that way it is possible to enhance the stored pressure or amount of stored air. This can be particularly useful in relation to compressors for compressed air braking systems of vehicles in which the compressor and turbocharger can each be fitted to the engine of the vehicle. A turbo charger is often fitted to an engine also to enhance the engine performance in known manner. In order moreover to ensure long and reliable operating life of a compressed air braking system over a wide range of temperatures, often well below zero, it is now also common to include an air dryer between the compressor and reservoirs to be charged with compressed air. Such air dryers are rechargeable by controlled purging from time to time with dry air and this usually involves temporary venting during intervals when the compressor is unloaded. With such an air dryer and unloading system as outlined above, it will become apparent that purging of the air dryer can result not only in substantial venting of the mentioned additional volume which is connected to the compressor cylinder when unloaded but in the case of a turbo charger being used, also as the turbo charger input may be substantially reduced, engine performance may be affected and the present invention seeks to provide an improved compressor which is not only compact in design but has means for avoiding this shortcoming. According to the present invention there is provided a gas compressor having a piston which reciprocates in a cylinder alternately to induce gas into a compression chamber via an inlet valve and to compress induced gas for delivery via a delivery valve to a delivery port and unloading valve means operable in response to a control signal to change from a normal unoperated state to an operated state in which the compression chamber is connected thereby to an additional clearance volume whilst disconnecting it from the delivery ports for interrupting delivery of the compressed gas when a predetermined delivered gas pressure is attained characterised in that the inlet valve, the delivery valve and the unloading valve means are contained in a cylinder head assembly of the compressor said unloading valve means comprising an unloader valve and a further valve between the delivery valve and the delivery port and said additional clearance volume comprises a discrete volume which for the normal inoperative state of the unloading valve means is separate from the delivery valve and the delivery port. Said further valve is preferably operable by the same control signal as said unloader valve. Said additional clearance volume is preferably included within said cylinder head assembly. Said additional clearance volume may be arranged to be connected with a volume which exists between the delivery valve and the further valve when the further valve is closed. In order that the invention may be clearly understood and readily carried into effect, the same will be further described by way of example with reference to the accompanying drawing. Referring to the drawing a reciprocating piston air compressor has a cylinder indicated at reference 1, having a bore 2 and a piston 3 sealingly slideable therein. The cylinder 1 is provided with a valve plate 4 with delivery and inlet valves 4a and 4b together with a cylinder head assembly comprising a main portion 5 and a cover portion 6 which is separable from 5 for ease of manufacture, assembly and subsequent maintenance of an unloader valve in the form of a poppet valve denoted by reference 7. The cylinder head has an induction chamber 8 communicating air from an inlet port (not shown) to a region 9 above the piston via the inlet valve 4b. Delivered air under pressure from the region 9 passes through the delivery valve 4a to a delivery port 10 for supplying compressed air via an air dryer 11. The air dryer 11 has a purge volume 11a which receives dry air and dry air is also supplied via pipe 12 to utilisation means such as reservoirs of a brake system. The pressure in the line 12 is sensed by a governer device 13 which as will be seen later applies an unload signal to the compressor and a purge signal to the air dryer. The governer device is typically a governer type D2 as marketed by Applicant and described in their Technical Pamphlet 4/002. The compressor cylinder head portion 6 has a control input port 14 which communicates with the larger area 15 of a sealingly slideable stepped piston 16 with seals 17 and 18. An annular region between seals 17 and 18 is vented via a small passage 26 to atmosphere. An opposing face 19 of the piston 16 is engageable with an inward end 20 of a stem 7a of the unloader valve 7 which is located in a valve guide 21 and has a head 22 engageable with a seat 23 formed in the cylinder head portion 5. The valve head 22 is, therefore, located in the compression chamber formed by region 9 and the area presented by the head 22 of the valve to the pressure in the region 9 lies between the area of 19 and the area of 15. The valve 7 is biassed into the closed position shown by a frusto-conical spring 24 acting between the guide 21 and a collet 25 retained on the stem in an annular groove by a suitable circlip or cotter 29. Incorporated in the cylinder head assembly there is provided an additional clearance volume by virtue of a region 27 which communicates via apertures 28 in the valve guide 21 and unseated valve 7, with the region 9 above the piston. The passage 15 referred to above also communicates with the right hand end of a piston 29 carrying seals 30 slideable in a bore 31 which is axially aligned with the delivery port 10. Delivery port 10 is formed by a threaded insert 33 of the cylinder head assembly the inner end 32 of which insert is abuttable by an enlarged end 34 of of piston 29 in the manner of a further valve. Piston 29 is guided to achieve this by a tubular rod-like extension 35 freely slideable in a guide 36 of insert 33. A spring 37 is provided as shown to urge the piston 29 into the position shown rendering the further valve normally open for delivery of compressed air. The bore 38 of tubular rod-like extension 35 communicates with the annular space between seals 30 of piston 29. The threaded insert 33 permits convenient insertion or removal of the further valve 34 as may be required for servicing thereof. The air dryer 11 may now be briefly explained and suffice it to observe that 11 comprises a main body 41 having an input port connected to receive air from delivery port 10. The delivered compressed air follows the direction as illustrated by solid arrows up around the inner surface of a desiccant cannister 42 and down through a body 43 of granular desiccant to the purge chamber 11a and also via a check valve 44 to the line 12 for charging reservoirs (not shown). A purge valve 45 is provided in main body 41 and when this receives a control signal from governor 13, dry air stored in purge volume 11a is metered back through the desiccant in the direction of the broken arrows to atmosphere through valve 45. The operation of such air dryers is well known and does not require to be described further except to observe that one result of such purge valve action can be to cause the pressure at the delivery port 10 of the compressor to descend towards atmospheric pressure. In operation of the shown system, the compressor operates in a conventional manner drawing air in via induction chamber 8 and delivering compressed air via the delivery port 10 into air dryer 11 and volume 11a. Upon attainment of a selected pressure the governer device 13 applies a pressure signal to the control port 14. The pressure delivered by the governer approximates to the pressure in the mentioned reservoirs and this acts on the upper surface of the piston 16 to thereby urge the unloader valve member 7 against the action of spring 24, and the pressure in chamber 9 acts on the area of 22, such that the unloader valve is rapidly unseated. Opening of this valve connects the extra clearance volume, provided by region 27, directly to the volume of the chamber 9. The volume of 27 is approximately twenty times the minimum volume of chamber 9, typically 100 cubic centimetres. Under these conditions the maximum pressure attained in the chamber 9 immediately drops to a value which is appreciably less than the former delivery pressure and the similar pressure in the reservoir 11, so that the compressor now ceases to deliver compressed air. Due to the action of piston 3 there are then cyclic pressure fluctuations accompanied by flow of air backwards and forwards, via the unseated valve 22, between the regions 9 and 27 as the piston 3 cyclicly varies the volume 9. The pressure signal applied by governor device 13 to port 14 is also applied to the air dryer purge valve 45 so that the compressed air pressure in the air dryer main body 41 collapses rapidly towards atmospheric pressure and metered purging commences of the desiccant 43 by dry air from volume 11a. The control pressure at port 14 at commencement of unloading and purging is also communicated via passage 15 to the right hand side of the piston 29. Because the left hand side of member 34, being communicated via port 10 with the air dryer body 41, is now subject to near-atmospheric pressure the piston 29 can rapidly move from the position shown and into substantial sealing engagement with the inner end of insert 33. Member 34 is then held in that position against the action of spring 37 by the pressure on its annular right hand side and the pressure in line 15. This has the result of isolating from the delivery port 10 and the air dryer body 41, the region between the delivery valve 4a and the delivery port. The trapped pressure in the additional volume 27 is thereby not vented via the delivery valve 4a and delivery port 10, the air dryer purge valve. The air dryer is thereby able to be purged in a normal manner without adversely affecting the advantageous features of compressor unloading provided by an unloader valve 22, 23 and so called "dead volume" 27. Turbo charger pressure drop is also prevented. When the compressed air pressure in line 12 is again depleted to a predetermined value due to brake applications or other uses of the compressed air supply stored in a reservoir (not shown) the governor device 13 removes the control pressure signal at port 14, the unloader valve reverts to the condition shown and the spring 37 now overcomes the pressure on the annular right hand side of member 34. This member 34 therefore also reverts to the position shown and the compressor resumes normal working supplying compressed air via the air dryer to line 12. If the compressor is supplied via a turbo charger it may be desirable to reduce the pressure which is trapped in the "dead volume" 27, when the compressor is unloaded, to a level less than that which would otherwise be present. This can readily be achieved by connecting the dead volume 27 via a suitable passage of the cylinder head to the region between the further valve and the delivery valve. Such a passage is shown by reference 47 and it will also be seen that this can readily be closed by closure means carried by the piston 34 in the normally operating position.

Claims: 1. A gas compressor having a piston (3) which reciprocates in a cylinder (1) alternately to induce gas into a compression chamber (9) via an inlet valve (4b) and to compress induced gas for delivery via a delivery valve (4a) to a delivery port (10) and unloading valve means operable in response to a control signal to change from a normal unoperated state to an operated state in which the compression chamber is connected thereby to an additional clearance volume (27) whilst disconnecting it from the delivery port for interrupting delivery of the compressed gas when a predetermined delivered gas pressure is attained characterised in that the inlet valve, the delivery valve and the unloading valve means (7, 34) are contained in a cylinder head assembly of the compressor said unloading valve means comprises an unloader valve (7) and a further valve between the delivery valve and the delivery port and said additional clearance volume comprises a discrete volume (27) which for the normal inoperative state of the unloading valve means is separate from the delivery valve (7a) and the delivery port (10). 2. A gas compressor as claimed in claim 1, characterised in that said additional clearance volume is contained within said cylinder head assembly. 3. A gas compressor as claimed in claim 1 or 2, characterised in that said further valve comprises a valve member (34) movable along an axis of the delivery port (10) into sealing engagement with an annular surface (32). 4. A gas compressor as claimed in claim 3, characterised in that said delivery port (10) has an axial guide (36) for a part (35) of said further valve. 5. A gas compressor as claimed in claim 3 or 4, said control signal being a fluid pressure signal and characterised in that said further valve member (34) has a piston portion (29) sealingly slideable in a cylindrical bore (31) of the cylinder head said piston portion (29) being subject to said fluid pressure control signal. 6. A gas compressor as claimed in claim 3, 4 or 5 characterised in that said delivery port has a screwthreaded insert (33) of said cylinder head assembly permitting insertion and removal of said further valve. 7. A gas compressor as claimed in any of claims 1 to 6 characterised in that a passage (47) is provided via which the said additional volume (27) is connected to a region between the delivery valve (4a) and said further valve (32, 34) when said further valve is closed. 8. A gas compressor as claimed in claim 6, characterised in that said further valve (32, 34) carries closure means to close said passage (47) when said further valve is opened.

Inventors: CHRISTMAS, MICHAEL C.
Applicants: BENDIX LIMITED
Application year: 1989
Main classification: F04B49
IPC codes: F04B49