GB2319569A

GB2319569A - Unloading gas compressors

Info

Publication number: GB2319569A
Application number: GB9624348A
Authority: GB
Inventors: J J Durrant
Original assignee: Knorr Bremse Systeme fuer Nutzfahrzeuge GmbH; Knorr Bremse Systems for Commercial Vehicles Ltd
Current assignee: Knorr Bremse Systeme fuer Nutzfahrzeuge GmbH; Knorr Bremse Systems for Commercial Vehicles Ltd
Priority date: 1996-11-22
Filing date: 1996-11-22
Publication date: 1998-05-27
Also published as: GB9624348D0

Abstract

A piston and cylinder air compressor is provided with a governor pressure operated unloader valve which provides venting from the compression chamber 6 of the compressor for interrupting the supply of compressed air therefrom when not required, the unloader valve comprising a pressure responsive valve member 15 biased towards its closed position by a spring 18 and also by pressure from the compression chamber 6 admitted to a region 20. To open the valve, governor pressure is supplied to a chamber 23 so as to act over a pressure responsive area opposed to the spring 18 and the pressure in the region 20. Pressure is admitted to the region 20 via a one-way valve 26 which substantially isolates said region from the dead volume of the compressor. During off-load periods the pressure in the region declines, resulting in an improvement in the off-load efficiency.

Description

IMPROVEMENTS RELATING TO GAS COMPRESSORS This invention relates to gas compressors and relates more especially to air compressors fitted with unloader valves which when signalled operate to connect a compression chamber to atmosphere or to provide interconnection of pairs of cylinders.

In air compressors of a type fitted to and driven by heavy vehicle engines for supplying compressed air to reservoirs of compressed air braking systems, it is common to include an unloader valve to be signalled in response to a desired reservoir pressure being attained. Typically, in a piston and cylinder compressor, such a valve can be fitted in the cylinder head of the compressor and arranged to open against the force of a spring when signalled by a pressure signal from a governor to connect the compression chamber to the inlet chamber or to another cylinder so as to interrupt supply of compressed air to a delivery port and the reservoirs. Unloader valves of such type are described, for example, in German Offenlegungsschrift 2918482.

As further described in the Specification of European Patent No. 0240278 such earlier forms of integral unloader valve had been attended by energy losses manifested as additional heating which accompanies cyclic unseating of the unloader valve during operation. This shortcoming is reduced by providing a pressure responsive area in the valve subject to pressure in a region which is connected via a small passage to the compression chamber of the compressor. This pressure responsive area is arranged to act in a sense to assist the force of the spring which acts in a closing direction of the valve.

The object of the present invention is to improve the efficiency of a compressor utilising such an unloader valve.

According to the present invention there is provided a gas compressor comprising a reciprocating member forming a movable wall of a compression chamber for cyclically increasing and decreasing the volume thereof to induce and deliver compressed gas to a delivery port an unloader valve integral with the head of the compressor and including a biased valve member acting in a sense to close a vent passage communicating with said chamber, said unloader valve being operable by a fluid pressure signal acting on a pressure responsive area thereof in a sense to counteract the bias and thereby to vent said compression chamber during compression strokes of said member said valve having a region connected to receive pressure from the compression chamber via a one-way passage the pressure in said region acting on a further pressure responsive area in a sense to assist the bias.

In a preferred embodiment of the invention said further pressure responsive area is at least as large as the area presented to the pressure in said compression chamber by the valve member in the closed position thereof.

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 of which, Fig. 1 illustrates a part sectional perspective view of a single cylinder piston air compressor and, Figs. 2a and 2b illustrate an unloader valve on an enlarged scale in open and closed states respectively.

Referring to Fig. 1, this illustrates the upper part of a single cylinder piston-type air compressor, with the crankcase and crankshaft assembly not shown. The crankshaft is pivotally connected to a piston 2 via a gudgeon pin 3 and a connecting rod, the upper end of which is indicated by reference 1. The piston 2 is thereby reciprocable in a bore 4 of a cylinder 5 alternatively to increase and decrease the volume of a compression chamber 6 of which the piston 2 forms a movable wall. The compressor is also provided with a valve plate 7 carrying a delivery reed valve 8 and an inlet reed valve 9. The delivery reed valve 8 provides a one way communication from the compression chamber 6 through the valve plate to a delivery port 10 of a cylinder head 11 and the inlet valve 9 provides a one way communication between an induction or inlet chamber 12 through a passage 13 in the valve plate. The cylinder head and valve plate are assembled to the cylinder by conventional bolts denoted by ref. 14.

The cylinder head 11 is further provided with an unloader valve illustrated on a larger scale in Figs. 2a and 2b.

This comprises a valve member 15 having a seal 15a sealingly slideable in a bore 16 into and out of abutment with the upper surface 7a of the valve plate 7 to open and close a passage 17 between the compression chamber 6 and the inlet chamber 12. This valve member has an annular pressure responsive area 23 provided by a larger diameter part of the valve member between seals 15a and 15b subject to control pressure from a governor at a control port 27 of the compressor and acting in a sense to oppose the bias of a spring 18 when a desired reservoir pressure is attained.

The valve member also has a further internal seal 15c slideable on a concentric cylindrical stop 24 retained by an upper end cover 22. Additionally, the pressure responsive member is provided at its lower end with a one-way internal passageway 19 which communicates with an interior region 20 presenting pressure responsive area 20a which acts in a sense to enhance the bias effect of spring 18 in the presence of pressure in region 20. One-way valving of passage 19 is provided by a check valve 26 having a stem axially guided in 24 and with light bias spring 27. The whole assembly of the unloader valve is retained together by the mentioned closure member 22 with suitable retaining bolts in tapped drillings (not shown).

Considering the operation of the compressor and the included unloader valve, the reciprocation piston 2 draws air from the inlet chamber 12 and compresses it for delivery to the port 10. At the same time fluid pressure builds in region 20 via the one-way passage 19. When the pressure in the reservoir or reservoirs being supplied via the compressor delivery port 10 has attained a desired pressure value, a pressure signal applied by a governor device to the control port 27 and acting on the pressure responsive annular control area 23 of the unloader valve, is sufficient to lift the valve member 15 against the bias of spring 18 as shown in Fig. 2a. The vent passage 17 of the valve plate is therefore uncovered and the compression chamber 6 is connected to the inlet chamber 12 which is at approximately atmospheric pressure. On a downward stroke of the piston 2, whilst air is induced via the inlet valve on the next compression stroke of the piston 2 this air may be expelled during the next upward stroke with minimum energy absorption via the open passage 17.

Ignoring for the present the inner area 20a and assuming that a small control signal pressure drop occurs at the control port 27, the unloader valve tends to close as shown in Fig.

2b during induction strokes and to cyclically partially reopen on compression strokes, such cyclic reopening being caused by recovering delivered pressure enhanced by compression chamber pressure acting on the valve closure area of passage 17. The result of such partial reopening can be substantial energy dissipation and over-heating. By including the region 20 charged by compression chamber pressure via passage 19 and acting in a sense to assist the bias of spring 18 on the unloader valve, such over-heating can be appreciably reduced or prevented. Preferably the additional area 20 is at least larger than the area of the valve member covering the vent passage 17 which is subject to compression chamber pressure in the closed position thereof.

By additionally providing the one-way valve comprising check valve member 26 and spring 27 cyclic discharge of high pressure from the region 20 is substantially prevented. The region 20 is thereby prevented from contributing to the dead volume of the compressor after being charged up by a number of on-load cycles of the compressor. On the other hand such rapidly achieved accumulation of air pressure in region 20 during on-load period via the passage 19 and one-way valve 26 is insufficiently so sealed in as to prevent its decline during a few cycles off-load. The result is in improved compressor on-load efficiency to the extent that the compression chamber is substantially isolated from the region 20 and piston strokes draw more air via the inlet chamber than otherwise.

Claims

1. A gas compressor comprising a reciprocating member forming a movable wall of a compression chamber for cyclically increasing and decreasing the volume thereof to induce and deliver compressed gas to a delivery port an unloader valve integral with the head of the compressor and including a biased valve member acting in a sense to close a vent passage communicating with said chamber, said unloader valve being operable by a fluid pressure signal acting on a pressure responsive area thereof in a sense to counteract the bias and thereby to vent said compression chamber during compression strokes of said member said valve having a region connected to receive pressure from the compression chamber via a one-way passage the pressure in said region acting on a further pressure responsive area in a sense to assist the bias.

2. A gas compressor substantially as described herein with reference to Figs. 2a and 2b of the accompanying drawing.