GAS COMPRESSORS

Abstract: A single cylinder air compressor is provided with a valve arrangement (19, 20) in the crankcase which impedes breathing of the crankcase (11) during induction strokes to create crankcase pressure elevations to expel lubricating oil from a low point of the crankcase via a small passage (18).

Description: This invention relates to gas compressors and relates more particularly to gas compressors which utilise a reciprocating piston in a cylinder, according to the preamble of claim 1. A common form of gas compressor relies upon one or more reciprocating pistons in cylinders. Such gas compressors are used to charge reservoirs of compressed air braking systems of heavy road vehicles. The bearings of such a compressor are usually lubricated by oil under pressure from the lubrication system of a vehicle engine upon which it is drivingly mounted. One problem encountered is that accumulation of oil drained into the compressor crankcase from the bearings can cause over-lubrication of the cylinder bores and resultant oil carry-over into the system can be excessive. Such oil carry-over can be detrimental to the brake system and is prefereably to be removed or prevented. One obvious way to reduce such oil carry-over is to limit the flow of oil to the compressor bearings but this can seriously impair the working life of the compressor. In the Specification of DE-A-3401998 there is described a single cylinder and piston air compressor drivingly mounted on the crankcase of an internal combustion engine in which bearings of the crankshaft of the compressor are lubricated by oil-mist from the engine crankcase itself. The oil-mist is introduced and returned via inlet and return passages through one-way reed valves by virtue of crankcase pumping action of the compressor piston. In such an arrangement the compressor bearings can easily be starved of lubrication when starting up from cold and this can result in reduced compressor bearing life. Moreover, as explained in the above document, normal breathing of the compressor crankcase is preferably eliminated to maximise the crankcase pressure fluctuation for the pumping action. Accordingly, since substantial crankcase pressure elevations can occur during induction strokes of the piston the tendency for oil-mist to travel past the piston and appear as oil carry-over is enhanced. This is disadvantageous as referred to above and the present invention has as an object to reduce these shortcomings. The present invention thus seeks to provide an improved gas compressor wherein oil carry-over can be substantially reduced without undesirably reducing the oil-flow to the bearings. According to the present invention there is provided a gas compressor comprising a crankcase and cylinder assembly having a rotatable crankshaft with a crankpin connected to a piston to reciprocate the piston in the cylinder for alternatively effecting induction and compression strokes said crankshaft rotating in a bearing supplied with lubrication by a source of lubricant and said crankcase being provided with a sump into which lubricant can drain from said bearing a sump outflow passage for lubricant in the sump and a breather passage having valve means which restricts said breather passage creating crankcase pressure during induction strokes to scavenge lubricant therefrom via said outflow passage characterised in that said outflow passage is a free passage and the inner end thereof is below a predetermined level of lubricant in the sump and said valve means (19, 21; 23) is timed by cooperation of an outer surface (20) of a crank web (21) with the inner end of the breather passage (19) to restrict said breather passage during predetermined portions of said induction strokes to create sufficient crankcase pressure elevations to scavenge lubricant via said outflow passage. 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 a part-sectional view of a piston and cylinder air compressor in accordance with the invention, and Fig. 2 illustrates a side view thereof. Referring to Fig. 1 a single cylinder piston compressor shown in sectional form therein comprises a cylinder and crankcase casting 1 provided with a valve plate 2 and a cylinder head 3. The casting 1 has a cylinder bore 4 within which a piston 5 is reciprocated by means of a crankshaft 6 connected to the piston via a connecting rod 7 and a gudgeon pin 8. The crankshaft is rotatable in respective bearings 9 and 10 which are pressure fed from the right-hand end of the crankcase wherein an oil pressure chamber 11 is provided. This chamber is pressure fed with oil from an engine to which the compressor is mounted by a suitable flange 12. The inner end 13 of the crankshaft is provided with a drive pinion (not shown) engageable with a gear train in the crankcase of the engine. The oil-ways providing connection between the oil chamber 11 and the big-end bearing and the left hand main bearing, are denoted by broken outlines 14 and 15 and it will be appreciated that in operation, oil flow leakage will run down into the lower part of the crankcase which is shown to be closed by a suitable cover plate 16 and as shown in Fig. 1 and Fig. 2, there is provided a small upwardly directed oil flow passage 18 communicating below a predetermined level with the sump in the lower part of the crankcase. Furthermore, at an appreciably higher point in the crankcase there is provided a somewhat larger air breather passage denoted by reference 19 (Fig. 2) and shown dotted in Fig. 1. The angular position of this air passage 19 in relation to the crankshaft rotation is chosen so that it is substantially obscured by close proximity of an outer face 20 of the crankweb counterweight 21 during predetermined portions of induction strokes of the crankshaft and piston assembly. Since passages 18 and 19 are the only breathing passages for the crankcase, the inner end of the passage 19 and outer face 20 of the crankweb 21 comprise a timed valve which can impede the breathing of the crankcase over a predetermined portion of the compressor induction stroke during the crankshaft rotation cycle. As shown in Fig. 2 the direction of rotation of the crankshaft is denoted by an arrow 22, the crankweb 21 being shown in dotted outline. It is seen moreover that at a suitable angular position, moving away from top dead centre after completion of a compression stroke, the surface 20 of the counterweight of the crankweb is about to obscure the inner end of the passage 19. The effect of this is to immediately impede the formerly free flow of air into and out of the crankcase through passage 19 so that the further downward movement of the piston is now able to create a suitable pressure elevation which acts in a sense to drive oil above the plate 16 out into the gear housing upwards via the small passage 18. By such means, the level of drained-out oil in the crankcase of the compressor is effectively limited to a level determined by the position of the passage 18. The sizes of the passage 18 and the larger passage 19 are selected, along with the range of angular position of the crankshaft for which the passage 19 is closed during induction strokes, to ensure that any tendency for oil build-up in the crankcase is prevented. On the other hand pressure elevations in the crankcase are minimised. This is advantageous as it is undesirable to create more than minimal cyclic pressure elevations in the crankcase. Such elevations may not only reduce the efficiency of the compressor but as discussed earlier they can tend to drive lubricant upwards past the piston into the compression chamber of the compressor and result in oil carry-over in the compressor output. It will be appreciated that the present invention relies upon the production of acceptable crankcase pressure elevations to drive out surplus oil from pressure fed bearings. In the case of a single cylinder compressor this is effected by closing a valve during an induction stroke but in the case of multiple cylinders with a common crankcase volume it may be necessary to arrange the relative cyclic positions of the pistons such that there is an interval of net reducing crankcase volume and to arrange for the valve device to close during that particular interval. This will inevitably still occur during an induction stroke of at least one such cylinder.

Claims: A gas compressor comprising a crankcase and cylinder assembly (1) having a rotatable crankshaft (6) with a crankpin connected to a piston (5) to reciprocate the piston in the cylinder for alternatively effecting induction and compression strokes said crankshaft (6) rotating in a bearing (9, 10) supplied with lubrication by a source of lubricant and said crankcase being provided with a sump into which lubricant can drain from said bearing a sump outflow passage (18) for lubricant in the sump and a breather passage (19) having valve means which restricts said breather passage creating crankcase pressure during induction strokes to scavenge lubricant therefrom via said outflow passage characterised in that said outflow passage (18) is a free passage and the inner end thereof is below a predetermined level of lubricant in the sump and said valve means (19, 21; 23) is timed by cooperation of an outer surface (20) of a crank web (21) with the inner end of the breather passage (19) to restrict said breather passage during predetermined portions of said induction strokes to create sufficient crankcase pressure elevations to scavenge lubricant via said outflow passage. A gas compressor as claimed in claim 1 characterised by said sump outflow passage (18) being directed upwardly from said predetermined level.

Inventors: CARVER, NIGEL JAMES, DURRANT, JEREMY JAMES, OLIVER, PATRICK RONALD
Applicants: BENDIX LIMITED
Application year: 1987
Main classification: F04B39
IPC codes: F01M1, F01M11, F04B39