GB2158888A - Reciprocating gas compressor - Google Patents

Abstract

In a reciprocating piston air compressor torque reversal forces are limited by means of a pressure relief valve which limits the maximum pressure attainable at top dead centre of the reciprocating piston (7). The relief valve may be in the form of interengaging ground surfaces (35,36) between the piston (7) and an extension (18) of a connecting rod (10) which allow air to flow off from the working chamber (17) when passages (37, 38) through the surfaces register. Alternatively the relief valve may comprise a passage connected to the working chamber and uncovered by the piston near top dead centre. <IMAGE>

Description

SPECIFICATION Improvements relating to gas compressor This invention relates to gas compressors and relates especially but not exclusively to reciprocating piston compressors which are required to be driven by a prime mover through clutches.

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 an engine-driven compressor. 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 driving 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 clutch and associated shaft splines which can limit their useful life.

Accordingly, the invention, therefore, seeks to reduce the torque reversal.

According to the present invention there is provided a gas compressor including compressor drive means rotary transmission means between the drive means and a compressor, the compressor having a reciprocating compression member cyclicly operable to increase and reduce the volume of a compression chamber to drive gas induced via an inlet valve under compression through an outlet valve and pressure relief means operable in the proximity of the point of minimum volume of a compression chamber to limit the pressure in said chamber such as to reduce torque reversal in the transmission means.

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 drawings of which: Fig. 1 illustrates some compressor torque characteristics, Fig. 2 illustrates one embodiment of a compressor modified in accordance with the present invention Figs. 3, 4, 5 and 6 illustrate alternative embodiments of the invention.

Referring to Fig. 1, the curve of Fig. 1 (a) is a typical theoretical curve of the torque required in driving a single cylinder compressor in an off load condition through a complete cycle. The first half of the curve corresponds to the acceleration of the reciprocating masses of piston connecting rod and crank from topdead-centre (TDC) towards bottom-dead-centre (BDC), the torque passing through zero where the acceleration is zero and becoming negative as the piston decelerates towards BDC.

Likewise, the second half of the curve corresponds to the return from BDC (180 ) to TDC.

This curve assumes zero friction throughout.

When the compressor is placed on load, as when charging a reservoir against a charged pressure of (say) 8 BAR, the theoretical curve changes in shape as shown in the curve of Fig. 1(b).

The effect of the pressure in the compressor cylinder acting on the piston and via the connecting rod on to the lever arm of the crankshaft causes a significant positive torque on the compression stroke just before TDC (360 ).

Residual pressure trapped in the compression chamber at its minimum volume acts on the piston at the commencement of its downward movement and causes the extra torque reversal at point 'X'.

The curve of Fig. 1 (b) is still considerably simplified in that it not only assumes zero friction but it also tends to underestimate the cylinder pressure due to restricted flow rate through the delivery valve. The result of these further factors is illustrated by the curve of Fig. 1 (c) which represents a measured torque characteristic and it will be seen that that the forward and reverse torque peaks are appreciably amplified. It is these peaks which can result in onerous strains on any transmission means such as splines and a clutch between an engine and a compressor.

Referring to Fig. 2 the single cylinder compressor shown in sectional view therein has a crankcase and cylinder 1 with cooling fins 2.

The engine-driven crankshaft reference 3 is carried in conventional ball bearing races 4 and 5, the assembly being effected by means of a bearing plate 6. A conventional connecting rod 10 drives reciprocating piston 7 in cylinder 8 by means of a gudgeon pin 9. The cylinder is provided with a cast metal cylinder head 11 between which and the cylinder a valve plate 1 2 is bolted down, the valve plate 1 2 carrying an inlet valve 1 3 and an outlet valve 1 4 leading to inlet and outlet ports 1 5 and 16 respectively, in the cylinder head.

In the TDC position as shown a small volume 1 7 exists above the piston head the piston being provided with piston compression rings 18 and an oil scraper ring 1 9. In accordance with the present embodiment of the invention, drillings such as indicated at 20 are provided in the cylinder wall each with drillings 20, 21 and slots 22 in order to provide communication passages between volume 1 7 and the crankcase volume beneath the scraper ring when the piston approaches TDC.In this way the passages are effective at least partially to vent the trapped pressure in volume 1 7 as the crankshaft moves over TDC position thereby at least partially relieving the crankshaft and the transmission means of the substantial reverse torque referred to above in relation to Fig. 1(c). Typically the passages from volume 1 7 may be chosen to provide a reduction of the onerous reverse torque without appreciable unacceptable loss of efficiency of the compressor and a typical curve generally as shown in Fig. 1 (d) may be looked for.

It will be seen that the effect of the added passages 20 is also to be taken account of as appearing from several degrees before TDC to several degrees after TDC but this is the region where minimal compression of volume 1 7 occurs. In an alternative the bores and passages 20 may be replaced by a number of narrow grooves extending down from volume 1 7 to just below the scraper ring. Instead of relying upon the piston clearance for venting the passages 20 into the crankcase, the piston may be provided with an extra ring groove and a drilling connecting same to its inner space.

In yet another alternative, the passages or grooves in the cylinder bore may line up at TDC with spaces between one or more of the piston rings.

In yet another alternative such as shown in the section of Fig. 3, the pressure relief valve means for partially venting chamber 1 7 at the TDC position may comprise a further hard rubber.vaive member as shown at 31 carried in the piston and operable to open a small passage 32 by virtue of a cam action provided by a relief 33 at the small upper end of the connecting rod into which a protected end 34 can move at the TDC position. That this valve can open also at BDC is, of course, of no practical consequence.

Referring to Fig. 4 the single cylinder compressor which is shown in sectional view therein has a principal component with reference numerals corresponding to those of the compressor illustrated in Fig. 2 conventional ball-bearing races 4 and 5 and is drivable through a bearing plate 6 which provides access to the interior of the crankcase for assembly purposes and bearing replacements.

A connecting rod 10 drives the reciprocating piston denoted by ref. 7 in its cylinder bore 8 via a gudgeon pin 9. The cylinder of the compressor is provided with a cast metal cylinder head 11 between which the cylinder a valve plate 1 2 carrying an inlet valve 1 3 and a outlet valve 14 leading to the respective inlet and outlet ports 1 5 and 1 6 in the cylinder head.

In the TDC position of the crankshaft, connecting rod and piston (as shown), a small volume 1 7 exists upon the piston head. In accordance with the present embodiment of the invention the upper end 1 8 of the connecting rod 10 is extended to form a valving member having a cylindrical surface 35 sealingly engageable with a complimentary cylindrical surface 36, ground in the underside of the piston head. A small aperture 37 in the head of the piston is thus opened at TDC into a passage 38 of the valving member. The latter passage 38 communicates with an annular groove 39 in the gudgeon pin 9 providing access through a radial hole 40 with an interior volume 41 of the gudgeon pin 9.As will be appreciated from Fig. 5, (which shows the piston at TDC) when the crankshaft 3 passes from TDC through BDC the interior of the gudgeon pin is briefly reopened to the compression chamber above the piston to read mist stored energy at the commencement of each compression half cycle.

In the arrangement of Figs. 4 and 5 the efficiency of the compressor is quite critically dependent upon the seal between the complimentary surfaces 35 and 36 and Fig. 6 shows a modification wherein the surface 35 of Figs.

4 and 5 is replaced by a hard wear-resistent foil 43 carried by a resilient tapered rubber memher 42 which is pressed into a wedge shaped recess 44 formed in the extension 1 8 of the connecting rod. By this means and with the sufficient flexibility of the resilient metal foil the accuracy of matching of the cylindrical surfaces 35 is far less critical as in Figs. 4 and 5 and subsequent lapping thereof against the complimentary surface 36 is accommodated by the resilience of the rubber wedge.

It is by no means essential that the passage 38 shall communicate with the interior of the gudgeon pin. If desired, this passage may be drilled to break out into the crankcase volume.

In yet a further alternative embodiment of the invention means operable in the proximity of the point of minimum volume of the compression chamber of the compressor to reduce the peak pressure attainable may be arranged to operate in accordance with the pressure itself. Typically such means may comprise a suitable spring biassed check valve.

In a variant upon the foregoing proposals the compressor may be provided with a small chamber into which air in the compression chamber is released by the pressure relief means and air trapped briefly in said small chamber may then be discharged into the compressor inlet on the sensing indication stroke. Such a small chamber could be formed for example between a pair of inlet reed valves.

By virtue of the invention a torque characteristic as shown in Fig. 1 (d) may be expected wherein the inherent torque reversals of the compressor are appreciably reduced and the life of unloading clutches and associated transmission means may be appreciably increased.

Claims (11)

1. A gas compressor including compressor drive means, rotary transmission means between the drive means and the compressor the compressor having a reciprocating compressor member cyclicly operable to increase and decrease the volume of the compression chamber to drive gas induced via an inlet valve under compression through an outlet valve and pressure relief valve means operable in the proximity of a point of minimum volume of said compression chamber to limit the pressure in said chamber and thereby limit torque reversal in the transsmission means.

2. A gas compressor as claimed in Claim 1 wherein said reciprocating member comprises a piston and the pressure relief means comprises a passage communicating between the compression chamber and a point which the piston passes during a compression stroke.

3. A gas compressor as claimed in Claim 1 said recirprocating compressor member carrying a valve device operable from a closed to an open condition in the proximity of said point of minimal volume.

4. A gas compressor as claimed in Claim 3 said valve device being operable by a connecting rod.

5. A gas compressor as claimed in claim 4 said valve device comprising a resilient member actuable by a cam action of a connecting rod.

6. A gas compressor as claimed in claim 4 the valve device being formed between an arcuately moveable end of the connecting rod engageable with a cylindrical surface of said member.

7. A gas compressor as claimed in Claim 6 said end of the connecting rod being provided with a hard-surfaced resilient insert.

8. A gas compressor as claimed in Claim 1 said relief means comprising a biassed check valve set to a limiting pressure.

9. A gas compressor as claimed in any preceding claim said pressure relief means being operable to discharge pressure from said pressure chamber into a small ancilliary volume.

10. A gas compressor as claimed in claim 9 wherein said ancilliary volume is formed between a pair of inlet valves in the compressor.

11. A gas compressor substantially as described herein with reference to any one of Figs. 2, 3, 4, 5, 6 of the accompanying drawings.