20. The most common type of oil distribution device is a simple orifice which directs a metered amount of
oil onto its target. These jet orifices are positioned as close to the target area as possible to overcome the
possibility of the local turbulent environment deflecting the jet of oil. The smallest diameter of a jet
orifice is 0.04 inch which allows a flow of 12 gallons per hour when operating at a pressure of 40 lb. per
sq. in. The use of restrictors upstream can reduce the flow rate if required.
|Fig. 8-5 Principle of a gear pump.|
21. All engines transfer heat to the oil by friction, churning and windage within a bearing chamber or
gearbox. It is therefore common practice to fit an oil cooler in recirculatory oil systems. The cooling
medium may be fuel or air and, in some instances, both fuel-cooled and air-cooled coolers are used.
22. Some engines which utilize both types of cooler may incorporate an electronic monitoring system
which switches in the air-cooled cooler only when it is necessary. This maintains the ideal oil temperature
and improves the overall thermal efficiency.
23. The fuel-cooled oil cooler (fig. 8-7) has a matrix which is divided into sections by baffle plates. A large
number of tubes convey the fuel through the matrix, the oil being directed by the baffle plates in a series
of passes across the tubes. Heat is transferred from the oil to the fuel, thus lowering the oil temperature.
24. The fuel-cooled oil cooler incorporates a bypass valve fitted across the oil inlet and outlet. The valve
operates at a pre-set pressure difference across the cooler and thus prevents engine oil starvation in the
event of a blockage. A pressure maintaining valve is usually located in the feed line of the cooler which
ensures that the oil pressure is always higher than the fuel pressure. In the event of a cooler internal
fault developing, the oil will leak into the fuel system rather than the potentially dangerous leakage of fuel
into the oil system.