IMPROVEMENTS RELATING TO GAS COMPRESSOR APPARATUS

Abstract: In a single cylinder piston and cylinder air compressor, transmission noise and stresses are reduced by a valve arrangement (16,21) driven by an excentric (14) of the compressor crankshaft to vent the compression chamber at or near its minimum volume thereby reducing the cyclic torque reversal which occurs.

Description: This invention relates to gas compressor apparatus and relates especially but not exclusively to reciprocating piston compressors which are required to be driven by a prime mover through a gear train. In a vehicle compressed air braking system it is usual to provide one or more compressed air reservoirs which are chargeable through suitable check valves and/or protection valves by a compressor driven from the engine via a train of gears. The compressor may either be provided with means such as an unloader valve operated by a governor for unloading it when the reservoirs are fully charged to a preset pressure or the compressor may be disengaged from the engine by means of a clutch. When a compressor is working into a near-fully charged reservoir it suffers from a torque reversal following the top of each compression stroke of the reciprocating piston and this torque reversal may introduce repetitive high stresses in the gear train and associated shaft splines which can cause noise and even limit their useful life. It has already been proposed to reduce torque reversal in gas compressors by providing vent passages which are uncovered by the piston of the compressor when the piston attains its uppermost position thereby to reduce the pressure of the small volume of air trapped in the compression chamber at that point. An alternative proposal has been to arrange the connecting rod action to uncover a small vent port in the piston to achieve the same result. Such proposals are made in U.K. Patent Specification No. 2158888. In practice such methods have hitherto been found to be unsatisfactory and the present invention seeks to provide an improved means for reducing the effects of torque reversal in a compressor. According to the present invention there is provided a gas compressor apparatus including a housing having bearing means carrying a crankshaft and a reciprocating compressor member cyclically operable by the crankshaft to increase and decrease the volume of a compression chamber to induce gas via an inlet valve and to drive it under compression through an outlet valve and characterised by a pressure relief valve operable by means of an eccentric cam of the crankshaft to open a venting passage to vent the compression chamber in the proximity of a point of minimum volume thereof. In a preferred example of the invention the venting passage connects the compression chamber to the crankcase of the compressor. In an example of the present invention to be described the compressor may have unloading means for unloading the compressor by connecting an additional volume to the compression chamber and includes inhibiting means to prevent the operation of the relief valve whilst the compressor is unloaded. In a further example of the invention said relief means vents the compression chamber in the proximity of a point of minimum volume thereof into a closed volume at or near a point of minimum volume of the compression chamber, and means for unloading the compressor operates by connecting the compression chamber to said closed volume. In order that the invention may be more clearly understood and readily carried into effect, the same will be further described by way of example with reference to the accompanying drawing which illustrates a single cylinder air compressor incorporating the invention. Referring to the drawing, the compressor has a crankcase and cylinder casting denoted by reference 1, provided with a mounting flange 2 for mounting the compressor on to the crankcase of an engine of a heavy vehicle. The compressor is, therefore, intended to be driven through a train of gears (not shown) connecting the engine to the driven end 3 of the crankshaft of the compressor. The crankshaft runs in main bearings 4 and 5 and has a crankpin 6 connected by a connecting rod 7 and a gudgeon pin 8 to a piston 9, which is thereby able to reciprocate in a bore 10 of the compressor cylinder. The cylinder is provided with a valve plate 11 and a cylinder head 12, details of which are not shown but it should be appreciated that the valve plate 11 comprises reed-valves for intake and delivery via respective input and output ports in the head 12. Between the bearing 5 and the adjacent crankweb 13 of the crankshaft, there is provided an eccentric machined cam surface 14 against which a lower end 15 of a rod-like valve member 16 is urged by a spring 17. The rod-like valve member 16 is provided with an annular seal 18 retained in the crankcase casting by a washer 19 held in position by spring 17. The rod 16 is thereby sealingly slideable in a bore 20, drilled alongside the cylinder bore 10 and the upper end 21 provides for opening and closing of a venting passage 22 from the compression chamber 10a of the compressor cylinder. The passage 22 communicates with the crankcase volume via a valve assembly 23 bolted to the side of the compressor cylinder. The valve assembly 23 has a passage 24 which registers with the passage 22 and communicates via a closure element 25 and a passage 27 with a chamber 26 connected to the crankcase volume via a drilling in the housing as denoted by reference 27a and shown dotted. The closure member 25 has two seals 28 and 29, one each side of 24 and provides a seat 30 for a small check valve member 31 urged into engagement therewith by a light spring 32 resting against a piston 33 with an upward extending stem 33a. The underside of the piston 33 is connected to a control port 34 such that when sufficient pressure is applied to port 34, the piston stem 33a moves upwards against 31 to maintain the check valve closed against its seat 30 for reasons to be described below. Referring to the operation of the compressor, the crankshaft 3 is driven through the drive means comprising a train of gears and operates in the normal way to supply compressed air via check valves or protection valves to reservoirs of a vehicle braking system. However, the eccentric cam surface 14 is such that is operates to allow the rod 16 to move downwards under the influence of spring 17 to uncover the vent passage 22 at or near the point of minumum volume for the compression chamber 10a above piston 9. The otherwise trapped high pressure in the compression chamber is thereby vented via passage 22, closure member 31 of check valve 33, chamber 26 and passages 27 and 27a into the crankcase volume of the compressor. By virtue of such venting reverse torque which otherwise is applied to the crankshaft at commencement of the downward stroke of the piston is substantially reduced in magnitude. In a typical compressor arrangement, the compressor may be provided with an unloader valve which interconnects the inlet and outlet chambers to interrupt the operation of the compressor when the reservoirs are fully charged. In these conditions, the crankshaft continues to rotate off-load absorbing relatively little power from the engine. An alternative method for unloading the compressor is to provide an unloader valve in the cylinder head of the compressor which conects the compression chamber to an additional clearance volume. Such an arrangement is described for example in the Specification of European Patent Application No. 0254524 In that case, a variable volume of air is enclosed within the combined compression chamber and an additional clearance volume also provided in the cylinder head. In order to prevent compressed air being despatched from the additional clearance volume, via the passage 22 when a valve such as 16 is incorporated in the compressor, the piston 33 is signalled by an unloader signal pressure at port 34 from the governor at the same time as the unloader valve is signalled to connect the compression chamber to the additional clearance volume. The effect of this is to maintain the check valve member 31 closed against seat 30 as described above so that continuing actuation of the rod 16 during the unloaded condition 1 of the compressor has no effect. In the embodiment of the invention described above there is provided valve means which vents the compression chamber of the compressor into the crankcase volume at or near the point of minimum volume. In the case of a compressor in which unloading is accomplished by means of an unload valve for connecting an additional clearance volume torque reversal may be reduced by means of a small valve such as 16 but which connects the compression chamber not to the crank case but to the additional clearance volume at or near the point of minimum volume. In such an arrangement it is desirable to ensure that the vent passage remains open during a sufficient part of the down-stroke of the piston to allow the pressure in the extra volume to reduce before and in readiness for the next compression stroke. An advantage of this arrangement is that it is not now necessary to provide a valve such as provided by seat 30 and valve member 31. Moreover, energy inparted by the compressor piston to air in the extra volume is in part returned to the system during each induction stroke. The effect is therefore to reduce the magnitude and widen the reverse torque characteristic of the compressor. In yet another alternative for a compressor in which unloading is accomplished by means of an unloader valve for connecting an additional clearance volume, torque reversal may be reduced by arranging the unload valve to be operated on every cycle at or near the point of minimum volume of the compression chamber. This maybe readily achieved by means of a suitable overhead valve rocker arrangement operated by a push rod activated by the eccentric cam surface 14. The unload valve thereby also operates as the pressure relief valve, in accordance with the invention.

Claims: Gas compressor apparatus including a housing having bearing means carrying a crankshaft (3) and a reciprocating compressor member (9) cyclicly operable by the crankshaft to increase and decrease the volume of a compression chamber (10a) to induce gas via an inlet valve and to drive it under compression through an outlet valve and characterised by a pressure relief valve (21,22) operable by means of an eccentric cam (14,15) of the crankshaft to open a venting passage (22) to vent the compression chamber (10a) in the proximity of a point of minimum volume thereof. Gas compressor apparatus as claimed in claim 1, characterised by said pressure relief valve comprising a rod-like valve member slideable in a bore in the housing to cover or uncover the venting passage (22) communicating with the compression chamber (10a). Gas compressor apparatus as claimed in claim 1 or 2 characterised by further valve means (23) operable by a compressor unload signal to inhibit venting via the venting passage (22). Gas compressor apparatus as claimed in claim 3, characterised by said further valve means (23) being a fluid pressure operable valve operable by an unload signal at a control port (34). Gas compressor apparatus as claimed in claim 1, 2, 3 or 4 characterised by said vent passage being between the compression chamber and a crank case volume. Gas compressor apparatus as claimed in claim 1 or 2 characterised by the apparatus having means for compressor unloading by connecting the compression chamber to an additional closed volume and said venting passage connecting said pressure relief valve (21,22) connecting said compression chamber to said additional volume. Gas compressor apparatus as claimed in claim 1 characterised by said pressure relief valve comprising valve means which is operable also by a compressor unload signal to connect said compression chamber to an additional closed volume.

Inventors: DURRANT, JEREMY JAMES
Applicants: BENDIX LIMITED
Application year: 1988
Main classification: F04B39
IPC codes: F04B39