The oil cooler return line is the pipe used to return the cooled oil from the cooler to the engine oil pan. The principle of operation is as follows:

1. Cooling the oil: The oil cooler is usually located in the engine cooling system and cools down the overheated oil through contact with the cooling medium (e.g. water or air). Cooling the oil improves the thermal stability of the engine, extends the life of the oil, and protects the engine components from overheating damage.

2. Oil return pipe connection: The oil return pipe of the oil cooler connects the outlet of the cooler and the inlet of the engine oil sump. The cooled oil flows from the cooler back to the engine oil sump through the return pipe.

3. Gravity and Pressure Driven: The oil cooler return line is normally driven by gravity and pressure inside the engine to return the oil. When the oil cooler is in the higher position, gravity will drive the cooled oil to flow back to the engine oil pan along the return line. At the same time, the oil pressure inside the engine will also push the oil flow to ensure smooth and rapid oil return.

4. Oil passage design: The design of the oil return pipe usually takes into account the oil flow and resistance. Reasonable oil channel design can minimize the resistance and pressure loss in the oil return process, ensuring the efficiency and stability of oil return.

The oil return pipe of the oil cooler returns the cooled oil from the cooler to the engine oil sump driven by gravity and the oil pressure inside the engine. It plays a key role in the engine cooling system, ensuring the cooling and circulation of the oil and maintaining the normal operation and life of the engine.