The temperature of lubricating oil (LO) supplied to an engine requires close control due to:

• Elevated LO temperatures increase the oxidation rate, doubling it for every 10°C rise. Oxidation produces acidic compounds and insoluble sludge that foul the engine components.
• At high temperatures, if water is present in the oil film, a tin oxide layer may form on white-metal bearings, resulting in hard, black or grey corrosion.
• LO temperatures between 25-40°C encourage microbial growth when water is present, especially when the engine is in a laid-up condition.
• LO viscosity is temperature-dependent, and fluctuations can affect the oil’s load-carrying capacity, leading to inadequate lubrication.
• Low LO temperature can cause thermal shock, while high LO temperature can lead to overheating, both of which increase the risk of piston cracking.
• Uncontrolled LO temperature may lead to engine slowdowns or shutdowns.

The main engine lubricating oil cooling system uses cascade control. In this case, the two main variables that influence the oil temperature are the engine load and the sea cooling water inlet temperature, which forms two loops: An outer loop that measures the engine oil inlet temperature and passes the information for further processing to a controller which is called the master or primary controller and an inner loop that measures the seawater inlet temperature and passes the information to a second controller called the slave or secondary controller. The secondary controller processes the signals from the primary controller and the secondary sensor and sends an appropriate signal to the 3-way valve to control the oil temperature at the inlet to the engine. When the seawater temperature changes, an immediate signal will be sent to the slave controller for adjustment of the 3-way valve even before the actual oil inlet temperature begins to change. The response is, therefore, faster.