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Marine engine cooling system notes
Typology: Lecture notes
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The function of the cooling system is to carry away excess heat in order to prevent the engine’s metal from melting and the lubricating oil from breaking down. The cooling system must maintain a relatively uniform temperature at any engine speed, load and varying ambient temperatures
Of the total amount of heat created by the combustion process: 1/3 is transformed into useful work 1/3 is removed in the exhaust gases 30 – 35% is left in the engine
Low engine temperatures can cause: Engine oil to turn to sludge Build up of deposits which reduces the amount of coolant flow Condensation in the L/O system, contaminating the oil causing premature wear Poor combustion Reduced fuel economy Reduced oil viscosity Lower combustion temperatures resulting in unburnt fuel, carbon build- up and excessive black smoke.
A coolant is defined as the liquid medium that circulates through the cooling system to absorb excess heat. There are several types of coolants: Plain Water – causes rust and corrosion Hard Water – leaves scale deposits on the heating surfaces that reduce the cooling effect Distilled Water - can cause corrosion Ethylene glycol or propylene glycol – manufacturer’s choice when used with corrosion inhibitors
There are two types of cooling systems used in modern diesel engines: Air Cooled Systems Liquid Cooled Systems
Air is supplied by forward movement of the engine and/or by a fan or blower. The air cools the engine by transferring the heat to the atmosphere. Air cooled engines are fitted with cooling fins The fins are short and they direct the flow of air around the cylinder and they slow down the surrounding air.
The two major types of liquid cooling systems are: Open System Closed System (or a dual system)
The closed system is normally used in transportation and construction equipment. In smaller engines, the coolant is circulated by a water pump over the jackets, and coolers, through the thermostat to the radiator. The radiator removes the heat from the coolant. When the engine is cold the coolant bypasses the cooler and the radiator.
For a large diesel engine, the cooling water system can have a variety of configurations. Here: Fresh water coolant is pumped around the system. The fresh water is cooled by sea water. Coolant (F.W.) enters bottom of block. Heat is transferred to the coolant (F.W.). Coolant (F.W.) exits through cylinder head. If the engine coolant (F.W.) is cool, the coolant is pumped back to lube oil cooler and engine block. If coolant (F.W.) is too hot a thermostatic valve opens, directing the how water to a cooler. Here, raw water (in this case sea water) is used to remove heat from the coolant (F.W.) being used in the engine. Raw water (S.W.) is sent overboard. Coolant (F.W.) is circulated back to engine block.
The following system is divided into two separate systems for fresh water. It also has a raw water side.
Jacket water leaves engine. Passes through raw water circulated cooler. It then enters the jacket water circulating pumps. Pumped through block, heads, and turbocharger. Expansion tank allows for expansion and make-up water. Vents led from engine to expansion tank. Heater allows for pre-heating of engine prior to starting.
Two raw water pumps. Cools both fresh water systems(piston
Coolers for lube oil, jacket water, and piston cooling. All circulated by raw water. Raw water cools turbo air.
Freshwater system has a high temperature and low temperature circuit. The high temperature fresh water circulates through the main engine and auxiliary engines. Temperature is regulated by the 2, three way valves shown in the drawing. The low temperature fresh water circulates through the lube oil, auxiliary equipment, air and other coolers. Raw water circulates central coolers. As freshwater temperatures fluctuate there is mixing of HT and LT freshwater sides.
