Reciprocating Engine Lubrication Systems – Airflow Controls

in Lubrication and Cooling Systems

By regulating the airflow through the cooler, the temperature of the oil can be controlled to fit various operating conditions. For example, the oil reaches operating temperature more quickly if the airflow is cut off during engine warm-up. There are two methods in general use: shutters installed on the rear of the oil cooler, and a flap on the air-exit duct. In some cases, the oil cooler air-exit flap is opened manually and closed by a linkage attached to a cockpit lever. More often, the flap is opened and closed by an electric motor.

Figure 6-13. Floating control thermostat.

Figure 6-13. Floating control thermostat. [Click image to enlarge]

One of the most widely used automatic oil temperature control devices is the floating control thermostat that provides manual and automatic control of the oil inlet temperatures. With this type of control, the oil cooler air-exit door is opened and closed automatically by an electrically operated actuator. Automatic operation of the actuator is determined by electrical impulses received from a controlling thermostat inserted in the oil pipe leading from the oil cooler to the oil supply tank. The actuator may be operated manually by an oil cooler air-exit door control switch. Placing this switch in the “open” or “closed” position produces a corresponding movement of the cooler door. Placing the switch in the “auto” position puts the actuator under the automatic control of the floating control thermostat. [Figure 6-13] The thermostat shown in Figure 6-13 is adjusted to maintain a normal oil temperature so that it does not vary more than approximately 5° to 8 °C, depending on the installation.


During operation, the temperature of the engine oil flowing over the bimetal element causes it to wind or unwind slightly. [Figure 6-13B] This movement rotates the shaft (A) and the grounded center contact arm (C). As the grounded contact arm is rotated, it is moved toward either the open or closed floating contact arm (G). The two floating contact arms are oscillated by the cam (F), which is continuously rotated by an electric motor (D) through a gear train (E). When the grounded center contact arm is positioned by the bimetal element so that it touches one of the floating contact arms, an electric circuit to the oil cooler exit-flap actuator motor is completed, causing the actuator to operate and position the oil cooler air-exit flap. Newer systems use electronic control systems, but the function or the overall operation is basically the same regarding control of the oil temperature through control of the air flow through the cooler.

Figure 6-14. Dual oil cooler system.

Figure 6-14. Dual oil cooler system.

In some lubrication systems, dual oil coolers are used. If the typical oil system previously described is adapted to two oil coolers, the system is modified to include a flow divider, two identical coolers and flow regulators, dual air-exit doors, a two-door actuating mechanism, and a Y-fitting. [Figure 6-14] Oil is returned from the engine through a single tube to the flow divider (E), where the return oil flow is divided equally into two tubes (C), one for each cooler. The coolers and regulators have the same construction and operations as the cooler and flow regulator just described. Oil from the coolers is routed through two tubes (D) to a Y-fitting, where the floating control thermostat (A) samples oil temperature and positions the two oil cooler air-exit doors through the use of a two-door actuating mechanism. From the Y-fitting, the lubricating oil is returned to the tank where it completes its circuit.