OIL PRESSURE

Wanganui Chronicle, Volume 72, Issue 291, 7 December 1929, Page 13

OIL PRESSURE

CORRECT DEDUCTIONS PROM GAUGE READINGS Articles on lubrication have caused motorists to ask what is the correct pressure that should be shown on the oil gauge when the engine is turning over at a given speed. There is no correct pressure. The fact that this question is usually asked in warm weather might suggest that, if thought were taken on the subject. When summer days and long drives thin the oil, the pressure registered on the gauge must always drop off, but that does not mean that the engine is not getting lubrication just as efficiently as in winter, when pressure readings are higher. Here is something about oil pressure, and the causes of the fluctuation on the gauge.

In a “pressure feed” engine lubri. cation system the pump, driven by the engine, takes oil from the crank case supply, and forces it through a tube or channel in the engine base from which branch tubes or channels lead to the several crank shaft bearings and other lubrication points. Connected to this main oil pipe or header is the tube connecting to the pressure gauge on the dash. There is also usually connected into the main oil pipe a spring-controlled valve—the oil relief valve or by-pass. This valve automatically opens when the oil pressure acting in the piping reaches a certain predetermined value, and when it thus opens part of the oil pumped flows through it and takes a short cut back to the reservoir, without passing through bearings or taking part in lubrication at all. Sufficient oil flows through the bypass to reduce the pressure to the point for which it is set. Among the factors determining the oil pressure indicated by the gauge are:—Engine speed; the quality of oil as a viscosity or body; oil tempera. ture; clearance at bearings through which oil flows back into the reservoir after serving as a lubricant and the setting of the relief-valve spring. 1 Other things being equal, oil presI sure increases with pump speed—that is, with engine speed. With oil in viscous condition, it will increase to the pressure at which the relief valve is set; but with thin oil it may not rise as high as this, reaching only the value of the back pressure or frictional resistance of flow through the bearings. Variations in Pressure At any given pump speed pressure will be proportional to the natural viscous quality of the oil in use and to its degree of coolness. Pressures will run high in proportion as bearings have slight clearances, and low in proportion as bearings will usually depend on the setting of the relief valve or by-pass (oil regulator) spring. So long as the oil gauge shows pressure, the pump is obviously in operation, forcing oil through the system; put when there is no pressure indicated, with engine running briskly, presumably lubrication has entirely ceased for some reason, and the engine should not be run until oil flow is restored.

Even at idling speeds some slight pressure should be shown. In starting an engine on a summer morning, after it has been idle for some time and the oil has had time to cool, pressure will at first probably rise to the relief valve setting, at least when the engine is accelerated; but as the hot part of the day arrives and the oil warms up unusually and becomes very thin, gauge indications drop off gradually but persistently, until, even at speeds, they run noticeably low—very much lower than the driver is accustomed to in ordinary moderate weather. It is very natural for him to fear that these low pressure readings mean that his engine is not receiving sufficient lubrication.

However, if his engine is supplied with an oil.as “heavy” as advised by the manufacturer for midsummer driving, and it has not become diluted, and there is nothing wrong with the engine’s cooling system, the situation is normal, and there is no cause for alarm.

As a matter of fact, the rate of oil flow through all bearings is unusually ample, on account of its exceptional fluidity. All oil pumped is flowing through the bearings. The other side of the picture is the case of the engine started in an unheated garage on a zero morning. Assuming that the pump can move the oil at all, the gauge usually goes up to its maximum reading immediately and indicates the pressure at which the relief valve is set to open, usually remaining at this point until the engine is capable of running “unchocked” or some time after.

This is a dubious situation, particularly if suitable oil is not in use, for so far as the gauge indicates, practically all the oil pumped may be diverted through the by-pass back into the sump and only an insufficient amount be flowing through the bearings. Very gentle operation of the engine is more nearly normal, in order to forestall damage to cylinder walls.