ENGINE COOLING CIRLUITS Most marine engines utilize one or more cooling-water circuits. A closed freshwater cooling circuit is always used for cooling the engine jacket water. A second circuit is also required on turbocharged and aftercooled engine arrangements when colder water is used in the aftercooler. Cooling of the marine transmission oil is accomplished using either engine jacket water, aftercooler water, or a separate cooling circuit, depending on the model of marine transmission and/or the engine cooling arrangement. Jacket water temperatures are maintained high enough by water temperature regulators to provide efficient engine operation. Maximum jacket water temperature limits are controlled by the size of the coolers and the flow of coolant. The closed jacket water system consists of engine water jacket, the circulating pump, water temperature regulator, oil cooler, engine-mounted expansion tank, and cooler. The jacket water pump has sufficient capacity to maintain proper flow through the engine while circulating water through a heat-exchanging circuit with moderate line resistance. An increase in pipe diameter is required when external resistance reduces water flow below the required minimum.