engine. Several scavenge oil pump elements can be used  to accommodate the larger capacity of oil and air mix. The  scavenge elements have a greater pumping capacity than the  pressure element to prevent oil from collecting in the bearing  sumps of the engine. In most oil tanks, a pressure buildup is desired within the  tank to ensure a positive flow of oil to the oil pump inlet.  This pressure buildup is made possible by running the vent  line through an adjustable check relief valve. The check  relief valve is usually set to relieve at about 4 psi, keeping  positive pressure on the oil pump inlet. If the air temperature  is abnormally low, the oil may be changed to a lighter  grade. Some engines may provide for the installation of an  immersion-type oil heater. The pumps may be one of several types, each type having  certain advantages and limitations. The two most common  oil pumps are the gear and gerotor, with the gear-type being  the most commonly used. Each of these pumps has several  possible configurations. The gear-type oil pump has only two elements: one for  pressure oil and one for scavenging.  [Figure 6-32]  However,  some types of pumps may have several elements: one or more  elements for pressure and two or more for scavenging. The  clearances between the gear teeth and the sides of the pump  wall and plate are critical to maintain the correct output of  the pump. A regulating (relief) valve in the discharge side of the pump  limits the output pressure of the pump by bypassing oil to the  The oil pump is designed to supply oil under pressure to the  parts of the engine that require lubrication, then circulate the  oil through coolers as needed, and return the oil to the oil  tank. Many oil pumps consist of not only a pressure supply  element, but also scavenge elements, such as in a dry-sump  system. However, there are some oil pumps that serve a single  function; that is, they either supply or scavenge the oil. These  pump elements can be located separate from each other and  driven by different shafts from the engine. The numbers  of pumping elements (two gears that pump oil), pressure  and scavenge, depend largely on the type and model of the  Oil Pump Scavenge element Shear section Relief valve Pressure element From main bearings and coupling assembly To accessories case To oil filter From accessories case From supply Pressure oil Scavenge oil Figure 6-32.  Cutaway view of gear oil pump.  6-23