Positive Displacement Pumps - The Gear Pump
Having established that a positive displacement pump is the only practical way of supplying lubrication to our engine, the question remains as to which of these many types is most suited. And to understand this it is necessary to examine the engine’s demands on the oil.
In any high performance unit the oil has two primary functions: to lubricate (obviously) and to cool. For more mundane applications we can also include things like ‘to protect against corrosion’ or ‘ to minimise the build up in deposits’ but since I am assuming that the oil won’t be left in the sump for long let’s just stick to these two.
In lubricating the engine, conventional theory suggests that there has to be oil flow at a sufficient pressure to fill the void between the journals and the bearing shells and take care of the amount leaking away at the edges. To this must also be added a small amount bled off to the cylinder head to feed the cam bearings, tappets etc. While this also takes some of the heat away with it, there will also most likely be a series of oil spray jets feeding directly off the main oil gallery directed at the underside of each piston to keep these cool. The exact details of how much and the distribution of the flow will depend upon many parameters, including things like, the bearing clearances, the relative temperatures and, of course, the viscosity of the oil.
Thus the demand is for a supply of oil at more or less a constant flow rate and delivery pressure for a given engine speed. As engine speeds increase so will the flow losses, but this will be more than made up by the output of our positive displacement pump, assuming it is geared directly to the speed of the crankshaft. Any oil over and above that required will be deposited back in the sump or oil tank by the pressure relief valve. Oil pumps best suited to this kind of duty have traditionally been gear pumps, the gerotor pump or occasionally in certain circumstances, the lobe pump.
Gear pumps, or perhaps more exactly external gear pumps, consist of a pair of meshing gears, one driven with the other one idling and they work on the principal of oil being trapped in the void between successive teeth and the outside of the casing, as the gears rotate. Since there is line contact between the gear teeth and the clearance at the sides is very small, there is little or no room for the oil to leak away and so the oil entering the cavity is that delivered to the outlet port. Thus for every rotation of the gear a fixed volume of oil is transferred from the intake port to the outlet. A common fitment on all engines at one time, these would be installed in the sump of an engine and driven from a skew gear off the camshaft, which at the other end of the shaft, could often be found the distributor.
Still widely available, cheap to make and very reliable, these pumps can also be readily found in external dry sump pumps. However in recent years partly as a result of cavitation at higher flow rates this type of pump is giving way to the gerotor pump.
We’ll look into this and the other pump sometimes used, the lobe pump, next month.
Written by John Coxon.